Classification and genesis of stockwork molybdenum deposits

Economic Geologq Vol. 76. 1981, pp 844-87:3

Classification and Genesis of Stockwork Molybdenum Deposits GERHARD WESTRA

Exxon Minerals Company, 2425 N. Huachuca Drive, Tucson, Arizona 85705

AND STANLEY B. KEITH

Bureau of Geology and Mineral Technology, 845 North Park Avenue, Tucson, Arizona 85719

Abstract

Molybdenum deposits are classified by using magma series chemistry expressed as K57.5 (i.e., the K20 content at 57.5% SiO2 in a magma series); the F, Nb, Rb, and Sr contents of the source pluton; and the abundance of F and Sn in the hydrothermal system. By plotting K57.s values against the Nb, Rb, and F contents of the granitic source pluton, a natural dividing line emerges separating igneous rocks with KsT.s values below 2.5 from rocks with Ks7.5 values above 2.5. This metallogenically significant break forms the basis for our proposed classification.

No molybdenum deposits have been found in calcic magma series. Calc-alkaline stockwork molybdenum deposits are associated with calc-alkalic and high K calc-alkalic magma series (K575• < 2.5). The peraluminous source pluton, as a rule, contains less than 20 ppm Nb, between 100 and 800 ppm Sr, and from 100 to 850 ppm Rb. Molybdenite grades in the hydrothermal system rarely exceed 0.25 percent and fluorine is only weakly enriched in the phyllic zone. Tungsten in the form of scheelite may be common, but tin is absent. Stock and plutonic deposit types are distinguished by taking into account depth of formation and hydrothermal and fluid inclusion characteristics. Examples of stock type calc-alkaline molybdenum deposits include Kitsault and Hall, whereas Endako and Adanac represent plutonic calc-alkaline molybdenum deposits.

Alkali-calcic and alkalic molybdenum stockwork deposits are related to metaluminous to peraluminous granitic differentiates of high K calc-alkalic (K•?.•> 2.5), alkali-calcic, and alkalic magma series. Source plutons contain from 25 to in excess of 250 ppm Nb, 200 to 800 ppm Rb, less than 125 ppm Sr, and less than 0.2 percent TiO2 and are enriched in F, Sn, and Mo. The hydrothermal system commonly contains molybdenite grades above 0.80 percent and is characterized by intense K metasomatism and an abundance of fluorite and/or topaz. Tungsten is common and occurs as huebnerite or wolframite. The deposits are enriched in tin. The alkali-calcic to alkalic deposit category has been subdivided into: (1) transitional deposits cogenetic with granitic differentiates of high K calc-alkalic (Ks•.•> 2.5) and alkali-calcic magma series characterized by moderate enrichment in Nb, Rb, and F (Questa, Mount Hope); (2) Climax-type deposits associated with highly differentiated granites of alkali-calcic magma series (e.g., Climax, Urad-Henderson, Mount Emmons); and ($) alkalic deposits found with alkali-calcic and alkalic magma series. The alkalic molybdenum deposits are subdivided into (a) deposits cogenetic with metaluminous quartz-deficient syenitic and monzonitic stocks (e.g., Nogal Peak) and (b) deposits related to granites (e.g., Malmbjerg).

Calc-alkaline molybdenum deposits occur in continents and older island arcs in volcano- plutonic arcs associated with plate convergence. Climax-type deposits are found in areas of crustal relaxation and extension above the deepest part of a subduction zone. Alkalic deposits are present in areas of back-arc spreading associated with converging plate margins, in intracratonic rifts, and in rifts associated with the opening of oceanic basins. Magmatism responsible for the formation of calc-alkaline molybdenum stockwork deposits may be a product of partial fusion of subducted oceanic lithosphere. Partial melting of enriched upper mantle in response to pressure release in areas of back-arc spreading and intracratonic rifting could form magmas related to Climax-type and alkalic deposits. We propose that the chemical differences between source plutons and hydrothermal systems in the two categories indirectly reflect the phlogopite stability field in the subducted slab and the overlying mantle wedge. We also propose that, based on the data presented, molybdenum contained in all stockwork deposit types is of subcrustal origin.

The validity of the proposed classification has been tested by plotting radiometrically dated molybdenum deposits of the western United States on maps showing the distribution and magma chemistry of arc magmatism for the appropriate time span. The location of the deposit accurately predicts the type of deposit as determined independently by igneous trace element data and hydrothermal system characteristics. Our findings suggest that at any time during

844

STOCKWORK Mo DEPOSITS: CLASSIFICATION AND GENESIS 845

the development of the magma arc each molybdenum deposit category has its unique position within the arc. Geochemical data presented also show that a continuum exists between calc- alkaline molybdenum stockwork deposits and porphyry coppers. Calc-alkaline molybdenum deposits grade into Climax-type deposits through the intermediate transitional deposit type. No deposits transitional between alkalic porphyry copper deposits and Climax-type molybdenum deposits are known to us.

The striking differences between calc-alkaline and Climax-type molybdenum deposits are a consequence of original magma chemistry, molybdenum concentration levels in the parent magma, and the behavior of molybdenum in the melt and hydrothermal system. The high K, F, Rb, and low Ti and Fe contents of granitic rocks associated with Climax-type deposits create conditions ideal for molybdenum concentration, initially through crystal fractionation and liquid state thermogravitational diffusion and later by means of volatile transfer or the formation of a hydrous alumina-deficient K-rich silicate melt. Following vapor saturation of this residual melt, molybdenum will be introduced into the hydrothermal system. Decom- position of the K silicate component in response to cooling and a pH decrease results in addition of potassium to the hydrothermal fluid or the wall rocks and the precipitation of quartz and molybdenite. The striking correlation between added K20 and MoS• grades in the deposits lends credence to this concentration mechanism. No quantitative correlation between fluorine and molybdenum introduced in the hydrothermal system has been documented. In our es- timation, the importance of fluorine principally lies in its capability of altering magma characteristics that help to enhance molybdenum concentration processes. Differentiates of calc- alkaline magma series have lower initial molybdenum concentrations and this factor combined with the higher Ti and Fe levels, and the less extreme K and Rb and low F contents, prohibits extreme molybdenum concentration during the late magmatic stage. Consequently, most calc- alkaline molybdenum deposits are less intensely mineralized.

The relationship between magma chemistry and metallogeny as exemplified by the metallogeny of molybdenum deposits may be extended to include other metals and could prove to be a valuable exploration tool.

Introduction

STOCKWORK molybdenum deposits represent the most important source of molybdenum in the world. The general characteristics of this deposit type have been reviewed by Clark (1972), Soregaroli and Sutherland Brown (1976), Woodcock and Hollister (1978), and Hollister (1978a and b). The locations of molybdenum deposits and prospects in the Western Cordillera of North America are shown in Figure 1.

This communication presents a classification of molybdenum deposits based on magma chemistry. The general characteristics of the various deposit types and the relationship between deposit type and magma chemistry are developed for the western United States. Finally, we examine the influence of magma chemistry on molybdenum concentration mechanisms in the melt and in the hydrothermal system and outline tentative genetic models. The inter- pretations presented are speculative and undoubtedly will require modification as additional information regarding molybdenum deposits, magma generation, and molybdenum chemistry becomes available. Wes- tra assembled the geochemical and geological data and developed the proposed classification scheme. Keith documented the time-space development of arc magmatism in the western United States and the relationship between depth of magma generation and fluorine content in the magma.

Geological and geochemical studies of a large number of porphyry copper-molybdenum deposits en-

abled Lowell and Guilbert (1970) to develop an empirical alteration-mineralization model which has since been expanded to include stockwork molybdenum deposits. Sutherland Brown (1969, 1976) proposed a morphological classification of porphyry systems, including stockwork molybdenum deposits, based on the degree of complexity of the deposit. Hollister (1978b) used the metal and alteration distribution patterns and the shape of the ore zone to distinguish hood and zoned molybdenum deposits. Ney and Hollister (1976) combined the morphological approach with a subdivision based on the chemistry of the cogenetic igneous phases to delineate three groups of porphyry systems: (1) calc-alkaline porphyry copper deposits (Cu-Mo); (2) alkalic porphyry copper deposits (Cu-Au); and ($) calc-alkaline molybdenum deposits (Mo). These three groups are subdivided into plutonic and stock types depending on pluton size, shape, and depth of emplacement. Wright and Mutschler (1979) used the chemistry of the source pluton to subdivide molybdenum stockwork deposits into granodiorite systems and granite systems. Sillitoe (1980) distinguished subduction-related molybdenum deposits associated with quartz monzonites and rift- related deposits cogenetic with alkali granites.

The Relationship between Magma Chemistry and Molybdenum Stockwork Deposits

Molybdenum deposit classifications in existence are empirical in nature and based on some combination

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TINTINA BOSWELL RIVER LOGTUNG GLADYS LAKE ADANAC MOLY-TAKU MOUNT HASKIN QUARTZ HILL AJAX TIDEWATER ROUNDY CREEK KITSAULT BELL MOLY

MT. THOMLINSON GLACIER GULCH SERB CREEK LUCKY SHIP RED BIRD ENDAKO SALAL BOSS MOUNTAIN MOUNT COPELAND GEM TROUT LAKE COXEY AND CASCADE MT. TOLMAN CHILCO BIG BEN SILVER OIKE BALD BUTTE CANNIVAN GULCH IMA THOMPSON CREEK WHITE CLOUD LITTLE FALLS CUMO MAJUBA HILL BUCKINGHAM SPRUCEMONT MT. HOPE PARADISE PEAK MOREY PEAK PINE NUT HALL PINE GROVE NYE

URAD-HENDERSON CLIMAX

RED MOUNTAIN REDWELL BASIN MT. EMMONS CUMBERLAND PASS MT. AETNA

QUESTA-GOAT HILL-LOG CABIN NOGAL PEAK TItREE RIVERS STOCK CAVE PEAK OPODEPE

FIG. 1. Stockwork molybdenum deposits and prospects in the Western Cordillera of North America (modified from Woodcock, 1979). Deposit classification: Mega deposit, >250,000 tons Mo; large deposit, 100,000-250,000 tons Mo; moderate deposit, 50,000-100,000 tons Mo; small deposit and prospects, <50,000 tons Mo (after Woodcock and Hollister, 1978). The following sources have been used for molybdenum deposits mentioned in the text: Climax (Wallace et al., 1968, 1978; Steininger, 1971; Surface et al., 1978); Urad-Henderson (MacKenzie, 1970; Ranta et al., 1976; Wallace et al., 1978); Mount Emmons (M. W. Ganster, pers. commun., 1979); Redwell Basin (Sharp, 1978); Questa (Ishihara, 1967; Clark, 1968; Car- penter, 1968; G. Dunlop et al., unpub. data, 1979; M. S. Bloom, in prep.); Nogal Peak (Thompson, 1968); Three Rivers stock (Giles and Thompson, 1972); Cave Peak (Sharp, 1979); Opodepe (Leon and Miller, 1979); Buckingham (Blake et al., 1979); Hall (Wright, 1976; Cameron, 1980), Majuba Hill (MacKenzie and Bookstrom, 1976); Mount Hope (Missallati, 1978); Pine Grove (W. J. Tafuri, pers. commun., 1980; Abbott and Williams, 1981); Big Ben (S. D. Olmore, unpub. data, 1979); Cannivan Gulch (Schmidt and Worthington, 1977; Schmidt et al., 1979); Thompson Creek (Lain6, 1974; Schmidt et al., 1979); Ima (Rostad, 1971); Cumo (Shannon, 1971); Mount Tolman (W. C. Utterback, unpub. data, 1979, and pers. commun., 1979); Quartz Hill (Stephens, 1979; Hudson et al., 1980); Adanac (White et al., 1976); Glacier Gulch (Kirkham, 1969; Bright and Jonson, 1976; M. S. Bloom, in prep.); Kitsault (Woodcock and Carter, 1976; Woodcock and Hollister, 1978; Steininger, 1979); Roundy Creek, Ajax (Woodcock and Carter, 1976); Endako (Kimura and Drummond, 1969; Drummond and Kimura, 1969; Kimura et al., 1976; M. S. Bloom, in prep.); Boss Mountain (Soregaroli, 1975); and Logtun (anonymous, 1980). Deposits not shown include Mount Pleasant (Dagger, 1972; Pouilot et al., 1978); Cumobabi (Pazour, 1980); Malmbjerg (Kirchner, 1964); Tyrny Auz (Laznicka, 1976); East Kounrad (Smirnov, 1977); Compaccha (Hollister, 1978b); and Bordvika (Geyti and Sch6nwandt, 1979).

846


of deposit characteristics, associated igneous rock types, and tectonic setting. In this paper we use the recently documented relationship between magma chemistry and metallogeny (Keith, 1979a and b; Sil- litoe, 1974b; Lawrence and Wood, 1980; Baskina, 1980) as a basis for a petrochemical classification scheme of molybdenum deposits which incorporates the potassium content of the magma series with which the deposit is associated, the maior and minor element chemistry of cogenetic igneous phases, and the enrichment levels of fluorine and lithophile elements in the hydrothermal system.

The magma series nomenclature used in this paper is shown in Table 1. A magma series refers to a series of contemporaneous igneous rocks that are genetically related by differentiation from a parent magma commonly basaltic or gabbroic in composition. The chemistry of a magma series can be graphically represented by Harker variation diagrams which plot weight per- centages of major element oxides against silica content and classified by means of Peacock's (1981) alkali-lime index, i.e., the weight percentage SiO2 at which Na20 q- K•O equals CaO. This paper adopts Keith's (1978) modification of Peacock's classification to include calcic, calc-alkalic, high K calc-alkalic, alkali-calcic, and alkalic magma series (Table 1). In this scheme each magma series is defined by its range of K•O values at a constant SiO• content. For con- venience we have chosen to use the KzO values at 57.5 percent SiO2, i.e., the K57.5 value.

Stockwork molybdenum deposits are associated with siliceous intrusives ranging in composition from granodiorite to granite. High-grade deposits are typically related to differentiated granitic stocks and this distinction has formed the basis for classifying molybdenum deposits into granodiorite and granite systems (Wright and Mutschler, 1979; Mutschler et al., 1981). However, granite differentiates of various magma series may have very similar major element

T^•LE 1. Magma Series Nonmenclature (after Keith, 1978)

Depth to Peacock Magma series inclined seismic K.•75 1951) alkali- (this paper) zone in km range lime index

Calcic 80-120 0.4-1.2 62-68 (calcic)

Calc-alkaline 120-220 1.2-2.4 56-62 calc-alkalic)

High K calc-alkalic 220-260 2.4-$.0 57-59 calc-alkalic)

Alkali-calcic 260-$90 $.0-4.4 52-58 (alkali-calcic)

Alkalic >890 4.4-6.0 45-52 (alkalic)

chemistry (Figs. 2 and 8) and mineralogical compositions, thus invalidating this characteristic as a sole basis for classification. In contrast, minor and trace element contents of granite differentiates of various magma series show significant differences. We will show that specific trace elements (F, Nb, Rb, and Sr) can be used to classify igneous rocks associated with different types of molybdenum deposits.

The relationship between magma series chemistry and fluorine content is demonstrated by the increase in fluorine content in magmatic biotite as a function of K57.5 (Fig. 4). The fluorine content in biotite is con- trolled by the HF activity in the melt (Carmichael et al., 1974) which also increases with increasing K57.5 values. The fluorine content within a magma series increases with increasing differentiation and this effect is accentuated in magmas with high K57.5 values. The marked increase in fluorine in granites in a magma series with K57.5 > 2.5 corresponds to a well- defined increase in fluorine and tin content of mo-

lybdenum deposits associated with these granites and can be applied to distinguish molybdenum deposit types.

The fundamental metallogenic importance of K57.5 -- 2.5 is reinforced by taking into account trace element concentration levels in various magma series. Figures 5 and 6 show Nb and Rb contents in unaltered granitic source rocks of molybdenum deposits and document the marked increases in Nb and Rb content

in granites of magma series with K57.5 > 2.5. Figure 7 shows the Rb and Sr contents of unaltered granitic rocks associated with molybdenum deposits. Two dis- tinct groups of deposits are apparent: deposits associated with granitic rocks high in Rb and low in Sr and with a K575 in excess of 2.5 and deposits characterized by granitic rocks with high Sr and moderate Rb contents and a K57.5 value less than 2.5. This chemical subdivision is further strengthened by considering Mo and Sn concentrations in various magma series. High Mo (5-70 ppm) and Sn (8-80 ppm) concentrations are present in alkali-calcic high fluorine rhyolites (e.g., Christiansen et al., 1980), whereas calc-alkaline rhyolites and granites contain less than 2 ppm Mo and 8 ppm Sn.

The magma series chemistry expressed as K57.5 and the F, Nb, Rb, and Sr concentration levels of the cogenetic igneous rocks uniquely define source rocks associated with various types of molybdenum deposits irrespective of tectonic setting. Based on these cri- teria, we distinguish two major molybdenum deposit categories: calc-alkaline molybdenum stockwork deposits associated with calc-alkaline and high K calc- alkalic magma series with K57.5 •< 2.5 and alkali-calcic and alkalic molybdenum stockwork deposits related to granitic differentiates of magma series with K57.5 > 2.5. Calc-alkaline deposits can be subdivided into stock type and plutonic by taking into account depth

848 G. WESTRA AND S. B. KEITH

// • // "ALKALIC" •//• t/ ß

o Glacier Gulch

ß Thompson Creek

ß Mt Pleasant

• White Cloud

•- Cormivan Gulch

ß Mnl Emmons

Adonac

Ouesta

•' Henderson

+ Rialto

x Three River Stock

• Mnt. Hope

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Quartz Hill

5 6 7 8 9 I0 II 12 13 14 15 t6 17

Mol % AI203

FIG. 2. Molecular Na20 + K20 vs. Ai2Oa contents in unaltered igneous rocks associated with stockwork molybdenum deposits. Note the high alkali content of quartz-deficient igneous rocks related to aikalic molybdenum deposits. Also shown are the general compositional ranges of igneous rocks associated with various types of molybdenum deposits as defined in the text. Data from Merriam and Anderson (1942), Missallati (1975), Mutschler et al. (1980), Mutschler et ai. (1981), Butler and Vanderwiit (1955), Ishihara (writ. commun., 1979), Hudson et al. (1980), White et al. (1976), Bright and Jonson (1976), White (writ. commun., 1980), Thompson (1968), Giles and Thompson (1972), Dagger (1972), Lain• (1974).

of formation, alteration features, and fluid inclusion characteristics.

Alkali-calcic and alkalic molybdenum stockwork deposits are subdivided into three types based on the petrochemistry of the source pluton: (1) transitional deposits associated with high K calc-alkalic (K57.5 > 2.5) and alkali-calcic magma series; (2) Climax-type deposits related to alkali-calcic magmas; and (8) alkalic molybdenum deposits associated with alkali-calcic and alkalic magma series. Based on the chemistry of the source pluton--especially the Na20 + K20 content (Fig. 2)--the alkalic deposits can be further subdivided into: (a) syenite- and monzonite-related deposits and (b) granite-related deposits. This classification scheme is outlined in Figure 8 and the most important characteristics of each deposit type are listed in Table 2.

Our proposed classification of molybdenum stockwork deposits is based on fundamental chemical differences between the magma series and is not the result of a variable degree of magma differentiation. Implicit in our classification is the assumption that at any time during the development of a volcano-plutonic arc, each molybdenum deposit type can form at only one position in the arc. This assumption will be tested in a later section.

The Relationship between Porphyry Copper and Stockwork Molybdenum Deposits

Calc-alkaline stockwork molybdenum and porphyry copper deposits to a large extent occupy the same position within a magmatic arc and can occur in the same area. Based on the Cu and Mo contents

of the deposits (Fig. 9) and the Rb, Sr (Fig. 7), and whole-rock geochemistry of the associated igneous rocks, a continuum can be shown to exist ranging from Mo-rich end members (Endako) through Mo- Cu deposits (Mount Tolman, Copaquire, and Mineral Park) to Mo-rich porphyry copper deposits (Brenda and Sierrita).

Porphyry copper deposits are associated with quartz dioritic to quartz monzonitic stocks of calcic to alkali- calcic magma series (Fig. 10). The high Sr and low to moderate Rb contents (Fig. 7) suggest that the magma is relatively undifferentiated. High molyb- denitc concentrations do occur in some porphyry copper systems but are typically associated with late magmatic granitic to pegmatitic differentiates (e.g., Copper Creek, Copper Basin, Cumobabi).

Calc-alkaline stockwork molybdenum deposits are commonly found in quartz monzonitic, granitic, and alaskitic differentiates of calc-alkaline and high K


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14

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08

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ß CALC-ALKALIC

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F ALKALIC

ø56ø 62. 64 66 68 70 7• 74 76 78 8O

Weight percent S•O 2

FIG. 8. The AI•Oa/K•O + Na•O + CaO (in mole percent) ratio vs. SiO• content (in weight percent) of unaltered igneous rocks associated with eale-alkalie, transitional, Climax-type, and alkalie stockwork molybdenum deposits as defined in the text. For data sources see Figure 2.

calc-alkalic magma series. The igneous rocks have moderately high Rb contents and much lower Sr contents (Fig. 7). Chlorine is the dominant halogen in hydrothermal fluids in calc-alkaline molybdenum

and porphyry copper systems (Berzina and Sotnikov, 1977).

Molybdenum deposits transitional between the calc-alkaline and Climax type are cogenetic with granitic differentiates of high K calc-alkalic and alkali- calcic magma series. These igneous rocks are enriched in Rb and have high Rb/Sr ratios (Fig. 7). The hydrothermal system is anomalous in fluorine commonly contained in fluorite. Examples include Questa, Mount Hope, and Glacier Gulch.

Igneous rocks associated with Climax-type molybdenum deposits are highly differentiated granites of the alkali-calcic magma series characterized by high Rb and very low Sr contents (Fig. 7). The hydrothermal system is characterized by an abundance of fluorite and/or topaz, and fluorine is the dominant halogen in the hydrothermal fluid (e.g., Munoz, 1980). Although alkalic porphyry copper-gold deposits are known (Barr et al., 1976), no deposits transitional between Climax-type and alkalic porphyry copper deposits are known. The relationship between various porphyry deposit types is shown schematically in Figure 10 and the main characteristics are listed in Table 2.

Cult-Alkaline Molybdenum Stockwork Deposits Calc-alkaline molybdenum stockwork deposits are

genetically related to calc-alkaline and high K calc- alkalic magmas. To our knowledge, no known molybdenum deposits are found in calcic magma series. The parent magma, which often crystallizes as a batholith, ranges in composition from quartz diorite to granite with granodiorite and quartz monzonite most common. Molybdenum mineralization is spatially and temporally related to peraluminous (Fig.

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_

x

5000 I0 000 15,000 ZO,000 25,•00 • '5 CO0 40,000 45,000 90 OOC

VLUORINE IN 16NEOUS BIOTITES

(IN PPM)

FIG. 4. Relationship between fluorine in igneous biotite and magma series chemistry expressed as K57s (i e, percentage K20 at 57.5% SiO2 in the magma series). Data compiled from Dodge and Moore (1968), Dodge et al. (1969), Dodge and Ross (1971), Banks (1974), Parry and Jacobs (1975), Kesler et al. (1975), and Jacobs (1976).


ALKALI- CALClC

HIGH K CALC-ALKALIC K 57 5>25

HIGH K CALC-ALKALIC K 575(25

CALC-ALKALIC

(7)

•, (r}

(4}

• (4}

ciN.ClC

0 20 40 60 80 I00 120 140 160 180 200 220 240 260 260

ppm Nb

FIc. 5. Relationship between the chemistry of the magma series expressed as K575 and the niobium content of weakly altered and unaltered granitic and rhyolitic igneous rocks genetically related to stockwork molybdenum deposits. Number in parentheses indicates number of deposits from which analyses were used. Nb analyses by XRF. No molybdenum deposits are known in calcic magma series, and niobium content of granitic to granodioritic rocks in calcic magma series (Hine and Mason, 1978) is shown for comparison only.

3), silica-rich, leucocratic granites, alaskites, and aplites of near-eutectic composition (Soregaroli and Sutherland Brown, 1976) which represent the final products of magma differentiation (Hudson et al., 1980). Most plutons associated with calc-alkaline molybdenum deposits intruded shales, schists, gneisses, and quartz-bearing batholiths. Calcareous rocks are present at the Cannivan Gulch, Logtun, and Tyrny Auz deposits. Doming of wall rock (Kitsault), formation of sheeted contact zones (Hall), and the presence of breccia pipes (Boss Mountain, Quartz Hill) attest to forceful intrusion of the magma.

Using Sutherland Brown's (1976) morphological classification talc-alkaline molybdenum deposits can

ALKALll

ALKAU-CALCI(

HIGH K CALC-ALKALIC K 575>25

HIGH K CALC-ALKALIC K 575•<25

CALC-ALKALIC

NO DATA

me (2)

(5)

(6)

0 200 400 600 800 I000 1200

ppm Rb

FIG. 6. Relationship between the chemistry of the magma series expressed as Ks?.•and the rubidium content of unaltered and weakly altered granitic and rhyolitic rocks associated with stockwork molybdenum deposits. Number in parentheses indicates number of the deposits from which analyses were used. Rb analyses by XRF. No molybdenum deposits are known in calcic magma series and rubidium content of granitic rocks is shown for comparison only.

be subdivided into stock (i.e., his phallic type) and _

plutonic types. Stock-type molybdenum deposits are associated with crudely circular composite porphyritic stocks which are commonly less than 1,500 m in diameter. The ore zone may have formed at depths ranging from 1,000 to 2,000 m (Sutherland Brown, 1976, fig. 1). Examples include Kitsault and Boss Mountain in British Columbia, and Hall and Buck- ingham in the United States.

Plutonic molybdenum deposits are found within a comagmatic batholith and can be cogenetic with either porphyritic or equigranular igneous phases. The depth of ore formation is estimated at $,000 to 5,000 m (Sutherland Brown, 1976, fig. 1). Examples include Endako and Adanac in Canada, and Thomp- son Creek in the United States.

The tenor of molybdenum mineralization in calc- alkaline molybdenum deposits tends to be signifi- cantly lower than in Climax-type deposits. Important general features of calc-alkalic molybdenum deposits are listed in Table 2.

Tectonic setting

All known calc-alkaline molybdenum stockwork deposits occur within a magmatic arc in a convergent plate margin setting, but no deposits have yet been found in a young island-arc setting. Figures 12 through 16 show the relationship in space and time between arc magma chemistry and calc-alkaline molybdenum deposits in the western United States. Magma chemistry, published geologic information, and unpublished trace element data suggest that Opodepe, Hall, Buckingham, Thompson Creek, White Cloud, and Mount Tolman are calc-alkaline molybdenum deposits.


ß olmax ty•e motybde•um deposits- •- "Transitional '* molybdenurn deposits ß Oalc-alkaline malybdenurn cleposfls

ß MP'Mount Pleasant

7'00 z• Porphyry copper deposit m continental plate mar(]m sett•n•

600 • • o Porphyry copper deposit in •sland arc sellin(] /

ioo

o ioo ;•oo 300 400 5oc) 600 700 8oo 900 iooo ilOO iL;oo 13oo 14oo 15oo

ppm Sr

FIG. 7. Rubidium and strontium contents of unaltered igneous rocks associated with stockwork molybdenum and porphyry copper deposits. Data from Dagger (1972), Laughlin et al. (1969), Armbrust et al. (1977), Chivas (1978), Ford (1978), Kesler et al. (1977), Mason and MacDonald (1978), Olade (1976), Moore (writ. commun., 1979), and G. Westra (unpub. data). Data points of stockwork molybdenum deposits have been grouped according to the Ks?s value of the magma series.

Limited trace element and whole-rock chemical

data and published geologic information indicate that most Canadian molybdenum deposits should be included in the calc-alkalic category. The deposits, which occur in the Omenica, Intermontane, and Coast Crystalline belts of the Canadian Cordillera, formed during five metallogenic epochs; 140 m.y., 100 to 110 m.y., 70 m.y., 50 m.y., and 8 m.y. ago (Christopher and Carter, 1976). The younger deposits are clearly related to arc magmatism, but the complex older plate tectonic history (Monger et al., 1972)

makes a direct correlation between subduction and

formation of older molybdenum deposits more tenuous.

HFdrothermal alteration

The molybdenum ore zone in calc-alkaline deposits can occur within the batholith (Endako), at the top of the batholith (Mount Tolman), or be spatially related to smaller satellitic stocks above (Quartz Hill) or adjacent to (Thompson Creek, Trout Lake) the main batholith. Depending on the level of erosion,

Stock type (Kttsaalt) CALC-ALKALINE MOLYBDENUM STOCKWORK DEPOSITS •

( K575 = I 5-2 5) •Plutonlc type (Endoko)

K575' 2: 5 LOW FLUORINE

Trans•honal DeposHs ( Mount Hope) • K575.2.5 _•.0

/•Chmox- type Depostts(Climax) ALKALI-CALCIC P- ALKALIC MOLYBDENUM• K 30 44

STOCKWORK DEPOSITS • 575' ' - ' (K575 = 2 5-60) •

• •Syenlte • monzonlte

(Malmbjerg)

FIG. 8. Proposed stockwork molybdenum deposit classification scheme.


z z H•


0.40

o 20

B,4

057 _•1 Er lsbercj 3ompac•a

ß Chmax-type molybdenum deposits

ß Alkalic molybdenum deposds

ß Oatc-alkahne porphyry. rnolybdenurn-copper and copper-malybdenum deposits m continental plate rnarcjm seti'mcj

• Calc-alkahne porphyry copper deposit in island arc setting

[] Alkalic porphyry copper-cjold deposit

5 Number refers to deposit in Fiejure I

i 33

I-5

ß44

,rneba b•

• ßCopoq ....

ß26

COntl•tal East Min•al Po• B,n•om EI T•m•te ßChuqulcamala ß ß Bercj

ß Sieurira •1• • Pe ombres ß H Oh o I Scha•t& •E•cjdad •ut!e•e•rlY) Brenda i m n Cr• • • Islo• • Son Manuel r,mO C • &Omsput•a Yondera• Pmn$o x • •r

Lor•x• Valley • EJy

Gmb ...... • • •,p,l• •wtn • Aft•n S,•mne •v•Afias• •Ha•una •Morco•r •11es , - • •Co•lb•- ? CoDer Mo•niomn

o.• o'.• 0.3 o;4" .... 0'5 "'" 6.6 0.7 0.8

% Copper

115

FIG. 9. Hypogene copper and molybdenum grades in porphyry copper and stockwork molybdenum deposits. Data derived from tonnage and grade figures published in trade iournals and company annual reports. Some deposits, e.g., Mount Tolman (26), are shown more than once, reflecting different cut-off limits.

the ore zone in a stock-type calc-alkaline deposit forms an inverted cup, arcuate in cross section, or an annular ring (Kitsault). The ore zone is commonly localized along the top and sides of the cylindrical pluton (Kitsault; Opodepe and Hall). The presence of breccia pipes can strongly influence ore distribution in stock-type deposits (Boss Mountain). Ore distribution in plutonic calc-alkaline deposits is commonly irregular and cannot be related to a single igneous phase. Ore controls in plutonic deposits include major structures (Endako and East Kounrad) and intrabatholitic igneous contacts (Adanac).

Alteration assemblages and distribution patterns are reviewed by Clark (1972), Soregaroli and Suth- erland Brown (1976), Woodcock and Hollister (1978), and Hollister (1978a and b). Most deposits show a potassic core, a quartz-sericite-pyrite-(pyrrhotite) zone, an outer and upper(?) argillic zone, and a propylitic halo. Alteration zoning is usually described in terms of changing K+/H + ratios in a cooling hydrothermal fluid (Hemley and Jones, 1964). The core of the hy-

drothermal system may consist of a stockwork of paragenetically early barren quartz _ K-feldspar veins (Kitsault) or a zone of barren potassic alteration (Hall). K-feldspathization in calc-alkaline molybdenum deposits is much less intense than in Climax- type deposits. In plutonic deposits hydrothermal K- feldspar is present in and around quartz veins (En- dako, East Kounrad). In the Quartz Hill and Adanac deposits small additions of late magmatic K-feldspar in the igneous rock are the only signs of K-feldspar metasomatism.

In all calc-alkaline deposits the highest molybdenite concentrations are found at the outer edge of the potassic zone and within the inner quartz-sericite- pyrite zone. The quartz-sericite-pyrite halo is well developed at Buckingham and Mount Tolman, poorly developed at Kitsault and Endako, and absent at Adanac. The pyrite content ranges from I percent at Endako, to 1.6 percent at Kitsault, to g percent at Buckingham, to between g and 5 percent at Mount Tolman. At Kitsault and Buckingham pyrrhotite is


Porphyry Copper Molybdenum Deposits

"Calc- a lkalic"t ype "AI kalic"t ype Cu-¾o.

iCe-,alil C•-• C•I • ii "Alkalic" type

Calc-alkalic- type Climax- ty•{

l •' C -JF'-- CA •K- CAJ•--- AC Low Fluorine I High Fluorine in Biotite • • •---•'Ensialic Back-Arc Spread in

O L- 800 .... I OO" ':,¾66'•: ( ..................... ,•o•: - '

• . • ................... :•... •:: ...................................................... .•.:.•H•.•K•::.•::.•:.:•;:..•.:+:..•: '.'.•-'-':•?•:-:-:-..... • • • "::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 2 0 0 •::•??:•?:;::•::•::•?:•?:•?:;::;?:•?:;:?:;::• ::•;.:.:.•:•-• L....•; • :•.:•.•::.:.:•.• .:.:.•

================================================================================================================================================= :...:n:.•:•:...:...•:::::::::: •0 =========================================================================================================================================================================================================================================================== :::::::::::::::::::::::: ::::::::::::::::::::::::::::::::

Distance Km

FIG. 10. Schgmatic representation of the relationship between stockwork molybdenum deposits, porphyry copper deposits, and arc magma chemistry in a convergent plate margin setting. Schematic geotherm configuration after TSksSz et al. (1971). Shaded area outlines zone in which phlogopite is no longer stable. Note that the stability limit of phlogopite in the mantle is strongly dependent on the thermal configuration one adopts. The upper stability limit of phlogopite as shown is based on experimental data by Wendlandt and Egglet (1980) and a geotherm configuration (not shown) proposed by Anderson et al. (1978).

common in the quartz-sericite zone within biotite hornf els.

Element distribution in the hydrothermal system

Calc-alkaline molybdenum stockwork deposits contain a central (Buckingham; Compaccha, Peru) or annular (Kitsault, Hall) molybdenite zone which may be surrounded by a chalcopyrite zone with hypogene copper values ranging from 80 to >1,000 ppm (Kit- sault, Buckingham, Hall, Opodepe). Anomalous tungsten concentrations occur as scheelite at Kitsault and

Boss Mountain and as powellite at Adanac (Soregaroli and Sutherland Brown, 1976). Skarns associated with calc-alkaline molybdenum deposits at Cannivan Gulch, Thompson Creek, and Trout Lake may contain economically significant scheelite concentrations. In the Logtun property, tungsten and molybdenum mineralization with a WOa/MoS2 ratio of 2.5 occurs in metasediments and two quartz monzonite stocks. Similar molybdenite-bearing porphyry tungsten deposits occur in the USSR (Tyrny Auz) and in China (Yan et al., 1980). Scheelite is the dominant tungsten mineral in calc-alkaline molybdenum deposits, whereas huebnerite is predominant in Climax-type deposits. Tin is present only in trace amounts.

Although some fluorite is present in most calc-alkaline molybdenum deposits, the level of fluorine enrichment is comparable to that in porphyry copper deposits and no correlation exists between fluorine and molybdenum concentrations in calc-alkaline porphyry systems (Lain(•, 1974). The highest fluorine concentrations are present in the phyllic zone (Lain(•, 1974). Unaltered igneous rocks in both porphyry copper and molybdenum stockwork systems are low in fluorine. The lack of significant fluorine enrichment in the hydrothermal system permits a clear distinction between calc-alkaline molybdenum deposits and Cli- max-type and alkalic deposits.

Weakly developed peripheral zinc, lead, and silver halos occur at Hall and Compaccha. At Kitsault paragenetically late lead-zinc-silver-bismuth mineralization is superimposed on the ore zone.

Climax-type Molybdenum Stockwork Deposits

Climax-type molybdenum deposits produce close to half of the world's molybdenum. In view of their great economic importance, this deposit type has been studied in detail. Examples of Climax-type molybdenum stockwork deposits include Climax, Urad- Henderson, Mount Emmons, and Pine Grove. A


group of transitional-type deposits intermediate between typical Climax-type deposits and calc-alkaline molybdenum systems includes Questa, Mount Hope, and Glacier Gulch. Important characteristics of Cli- max-type deposits (including the transitional type) are listed in Table 2.

Climax-type deposits are cogenetic with metaluminous and peraluminous (Fig. $) alkali granites high in silica (•75% SiO2) and potash (•5% K20). Granite pegmatites, aplites, and aplitic quartz porphyries of near-eutectic composition characterize deeper igneous phases (Ishihara, 1967; Clark, 1972; Ganster, 1976). The unaltered cogenetic igneous rocks are enriched in Rb (200-800 ppm) and Nb (25-200 ppm) and depleted in calcium, strontium (commonly •25 ppm), Ba, and Ti (•0.2% TiO•). Unaltered granite at Henderson (Desborough et al., 1978) is enriched in Sn, Ta, U, Th, and Y. Felsic tuffs comagmatic with the igneous complex at Pine Grove are enriched in Rb, Y, Mo, U, Th, and Nb (J. D. Keith, 1979); high silica alkali-rich rhyolites contemporaneous with the magmatic event at Pine Grove are also anomalous in Th, Be, Sn, Ta, Li, Cs, F, and CI (Butt et al., 1980). Igneous trace element characteristics are typical of magmatism in a back-arc spreading environment (Pearce and Gale, 1977).

Stocks associated with Climax-type deposits intrude shales, quartzites, schists, gneisses, quartz-bearing igneous rocks, and volcanics. Forceful intrusion is indicated by doming (Glacier Gulch, Climax, Mount Emmons) and by the presence of ring dikes, cone sheets, and radial dikes (Climax and Urad-Hender- son). The intrusive complex is roughly circular in shape and rarely exceeds 2,000 m in diameter. Cli- max-type deposits at Urad-Henderson, Pine Grove (J. D. Keith, 1979), Mount Hope, Redwell Basin, and Questa occur in a volcanic setting, and comagmatic ash flows are found at Mount Hope and Pine Grove. The molybdenum ore zone probably formed at depths ranging from 600 (Urad) to •4,500 m (the Lower orebody at Climax).

Tectonic setting

Climax-type molybdenum deposits in the western United States are found associated with several alkali-

calcic fluorine-rich rhyolite complexes that formed between $5 and 10 m.y. ago. Many workers argue for regional ensialic interarc or back-are extension during all or much of this time span (e.g., Scholtz et al., 1971; Thompson and Burke, 1974; Elston, 1976; Stewart, 1978, 1980). The comparatively relaxed state of stress in the lithosphere is directly related to maior changes in plate motion and drastic lowering of convergence rates near 40 m.y. (Coney, 1972) which resulted in a rapidly increasing angle of subduction (Keith, 1978). The genetic relationship between alkali-calcic

magmatism above the deepest parts of subduction zones and the subduction process remains unclear (Barker, 1977; Robin and Tournon, 1978; Cameron et al., 1980), but magmatism at Climax and Hen- derson was probably still related to subduction (Lip- man et al., 1972). The emplacement of the Climax stock between $0 and 24.5 m.y. ago (Surface et al., 1978) and a fluorine-rich rhyolite stock at Nathrop 28-29 m.y. ago (Christiansen et al., 1980) heralds the onset of crustal extension in Colorado between $0 and

28 m.y. ago (Eaton, 1979; Tweto, 1979). This period of crustal relaxation during the transition to an ex- tensional structural regime was highly favorable for the formation of Climax-type molybdenum deposits. The initial development of the northern part of the Rio Grande rift 26 m.y. ago (Lipman and Mehnert, 1975) was followed by the formation of the Questa deposits in the Red River graben (Clark, 1968). The deposits are related to alkali granites typical of the bimodal basalt-rhyolite association (Christiansen and Lipman, 1972) commonly found in areas of crustal extension.

Mid-Tertiary Climax-type deposits in the Great Basin of Nevada and Utah are also related to alkali-

calcic fluorine-rich rhyolites (Christiansen et al., 1980) of the bimodal suite which extruded prior to and during basin and range extension. A close spatial and genetic relationship between crustal extension, fluorine-rich rhyolites, Climax-type molybdenum deposits, fluorite deposits, and mid- to late Tertiary bimodal magmatism is evident (Fig. 11; el. Lamarre and Hodder, 1978; Shawe, 1976). Sillitoe's (1980) rift- related molybdenum deposits are, in part, equivalent to the Climax-type deposits distinguished in this paper.

Element distribution patterns

Molybdenite in Climax-type molybdenum deposits forms a discrete ore shell with grades commonly in excess of 0.$ percent MoS•. Molybdenite gradients tend to be very sharp (Bright, 1974); MoS• increases from 100 to 1,000 ppm within 100 m and from 1,000 to 2,000 ppm within 60 m are documented at Hen- derson (Wallace et al., 1978). Several ore shells may be present, each shell associated with a separate phase of magma crystallization; shells can be stacked (Cli- max, Urad-Henderson, Redwell Basin) or can occur in close proximity though related to separate apophyses (Redwell Basin, Mount Emmons). The shape of the ore zone reflects the geometry of the heat source and commonly forms an inverted cup, arcuate in cross section and elongate or circular in plan. The ore zone overlaps with the upper contact of the causative intrusion (Upper and Lower orebod- ies at Climax, Henderson, Mount Emmons) or may


be found in excess of hundred meters above this con-

tact (Ceresco orebody at Climax, Wallace et al., 1968, fig. 7). Grade distribution may also be influenced by local structures (the Main Fissure at Urad), by regional structures (Questa), or by the development of cone sheet fractures (Mount Emmons). A close spatial relationship exists between molybdenite distribution and potassium enrichment (Ranta et al., 1976, fig. 4). The strong fluorine enrichment of parts of the hydrothermal system sets Climax-type deposits apart from calc-alkaline molybdenum deposits. The highest fluorine concentrations are found directly above the molybdenite ore zone (Henderson, Questa), but strongly anomalous fluorine values also coincide with and extend several hundred meters above the ore zone.

Anomalous tungsten concentrations in the form of huebnerite-wolframite or rarely as scheelite (Glacier Gulch) occur above the molybdenite ore zone (Climax Upper orebody, Questa, Glacier Gulch, Pine Grove) or coincide with the ore zone (Climax Lower orebody, Mount Hope). Tin, possibly as cassiterite, is present at Climax (Wallace et al., 1968), Henderson, Redwell Basin, Questa, and Mount Hope. The tin halo is poorly defined and may coincide with the tungsten halo or occur within a base metal halo well above the ore

zone. A strong tin halo straddles the 0.01 percent MoS2 contour at Henderson (Bright, 1974).

Base metal distribution patterns in Climax-type deposits are complex. A zone 330 to 660 m above the ore zone contains high zinc (in Fe-rich sphalerite) and manganese (in rhodochrosite or spessartine), strongly anomalous silver, and weakly anomalous copper, lead, bismuth, and tin values (Henderson, Bright, 1974; Redwell Basin; Mount Hope). Paragenetically late base metal concentrations may occur in or below the ore zone (Wallace et al., 1968, 1978) and are found adiacent to late dikes (Smith, 1976). A discrete chal- ocopyrite zone can be present 100 to $00 m above ore or coincide with the zinc-silver-lead halo (Sharp, 1978; Ranta et al., 1976).

Hydrothermal alteration Climax-type molybdenum deposits commonly show

a well-developed alteration pattern that includes a potassic core, a quartz-sericite-pyrite zone, an outer and upper argillic zone, and a propylitic halo. Dif- ferences from calc-alkaline deposits include the more intense K-feldspar alteration and the abundance of fluorite in all, and topaz in some, deposits. Carbonates, including calcite, ankerite, and rhodochrosite, are present in the argillic zone at Henderson and Pine Grove and are abundant at Mount Hope. At Mount Emmons and Mount Hope the ore zone is underlain by a quartz-magnetite zone, whereas at Climax and Henderson the deep igneous phases show greisen al-

teration along fractures. The total sulfide content ranges from 1 to $ percent in the ore zone. The pyrite content in the sericite-pyrite zone varies from 6 to 10 percent at Henderson to 0.5 percent at Pine Grove.

A generalized vein paragenesis shows the following sequence: (1) barren quartz veins, (2) quartz-K-feldspar-biotite _ molybdenite veins, ($) quartz-molyb- denitc veins with K-feldspar selvages, (4) quartz-pyrite-molybdenite veins with sericite selvages, and (5) quartz-base metal-fluorite veins (Kamilli, 1978; M. S. Bloom, in prep.).

Alkalic Molybdenum Stockwork Deposits

Alkalic molybdenum stockwork deposits are associated with silicic and subsilicic alkalic magmatism in zones of crustal extension above the deepest part of a subduction zone (West Texas), in intracratonic rift zones (the Oslo graben, Geyti and Scht•nwandt, 1979), and in areas of rifting and/or hot spot activity related to the opening of an oceanic basin (East Greenland magmatic province).

Economically insignificant molybdenum mineralization is cogenetic with predominantly metaluminous (Fig. $) silica-saturated but quartz-deficient monzonite and syenite plutons in south central New Mexico (Thompson, 1968; Giles and Thompson, 1972). The igneous rocks contain between 60 and 65 percent SiO2 and are strongly enriched in alkalies (Fig. 2), Mo, Nb, and F (Giles and Thompson, 1972). These stocks were emplaced during a period of tectonic relaxation immediately preceding the formation of the Rio Grande rift (Elston, 1976).

Important alkalic molybdenum stockwork deposits are related to alkali granites that were emplaced in areas of predominantly quartz-deficient magmatism. Examples include Cave Peak, Texas; Mount Pleasant, New Brunswick; and Malmbjerg, Greenland. Sparse trace element data on cogenetic igneous rocks (Dag- ger, 1972) suggest similarities with intraplate magmas, i.e., high Rb, Nb, K•O, and low Sr contents (Pearce and Gale, 1977). Molybdenite deposits associated with granitic rocks in these subsilicic provinces are very similar to Climax-type molybdenum deposits but occur in a variety of tectonic settings. Molyb- denitc mineralization at Cave Peak appears to be cogenetic with a biotite granite porphyry phase which occurs below a brecciated quartz monzonite-quartz latite plug. Fragments of slightly older fluorine-rich syenite are present in a mineralized breccia pipe. The deposit is located in a setting similar to the Kenya rift (Barker, 1977), and the relation between igneous activity and active subduction is tenuous. The alkalic rocks in this part of the Trans-Pecos zone probably formed by fractional crystallization of a mafic magma reservoir of mantle origin (Barker et al., 1977). The


EXPLANATION

Area in wWch fluorine-rich igneous

rocks occur ( F >.- O. I percent)

Area with an abundance of fluorine-rich

igneous rocks and/at F>•0.2 percent

Outline of areas with fluorite deposits

500 MILES 500 KILOMETRES

FIG. 11. Distribution of stockwork molybdenum deposits (Woodcock, 1979), fluorine-rich igneous rocks, and fluorite deposits in the western United States (after Shawe, 1976, figs. 8 and 9). Symbols used for molybdenum deposits are explained in Figure 1.

Malmbjerg deposit (Kirchner, 1964) in eastern Green- land is cogenetic with a leucogranite which is part of an episode of alkalic magmatism (Bearth, 1959) dated at $0 m.y. ago (W. Lodder, pers. commun., 1979). Magmatism may be genetically related to the opening of the northern part of the North Atlantic (Talwani and Eldholm, 1977) or may reflect the presence of a mantle hot spot. Within the deposit inter- mineral sodalite carbonatite and trachyte dikes suggest that during mineralization a subsilicic alkalic magma chamber was present at depth.

Mo-W-Sn-Bi mineralization associated with Car-

boniferous alkali-calcic to alkalic felsic magmatism occurs at Mount Pleasant in New Brunswick and is

tentatively included in this class. The Bordvika prospect is related to an intrusion of Permian alkali granite in a cauldron setting in the intracratonic Oslo graben, a failed arm of a triple junction north of Denmark (Burke and Dewey, 1975). The granite is comagmatic with a syenite ring dike within the cauldron.

End members of alkalic molybdenum deposits may be represented by the Mount Copeland mine near Revelstoke, British Columbia, where rich segregations


and disseminations of molybdenite were mined in nepheline syenite pegmatite, aplite, and the enclosing nepheline-bearing rocks (Soregaroli and Sutherland Brown, 1976), and by molybdenum-rich carbonatites (Deans, 1966).

Alkalic molybdenum deposits contain abundant fluorite (Cave Peak, Malmbjerg) and topaz (Mount Pleasant) and are strongly enriched in tungsten as huebnerite-wolframite (Cave Peak, Mount Pleasant). Important characteristics are listed in Table 2, but data on this deposit type are scarce.

Distribution of Magma Series and Molybdenum Deposits in Space and Time

The great majority of stock work molybdenum deposits are associated with magmatism in convergent plate margins. The relationship between magma series chemistry and molybdenum deposit types can best be demonstrated in the western United States by virtue of its large number of known molybdenum deposits (Fig. 11) and detailed documentation of the time-space evolution of magmatism (Gilluly, 1965; Christiansen and Lipman, 1972; Lipman et al., 1972; Coney and Reynolds, 1977; Keith, 1978). By using the major element geochemistry of more than 200 radiometrically dated igneous suites in the western United States, Keith (1978 and in prep.) was able to outline the position of parallel belts of calc-alkaline through alkalic magma series from 120 m.y. to the present. Known molybdenum deposits range in age from 87 to 17 m.y. In Figures 12 through 16 we have superimposed the locations of radiometrically dated molybdenum deposits on the appropriate magma chemistry map for the western United States. It is important to realize that in constructing the magma chemistry belts, the chemistry of igneous rocks associated with molybdenum deposits has not been con- sidered. Therefore, a comparison between magma series chemistry and the location of a molybdenum deposit should allow us to predict the type of molybdenum deposit we are dealing with!

Figure 12 shows the location of the Thompson Creek deposit (87-86 m.y., Schmidt et al., 1979) in the context of magma activity and magma chemistry between 105 and 80 m.y. The predicted calc-alkaline nature of the deposit is confirmed by its association with an equigranular granodiorite-quartz monzonite stock (Schmidt et al., 1979) low in fluorine (Lain6, 1974), Rb, and Nb and high in Sr.

Between 80 and 60 m.y. ago (Fig. 18) the belts of igneous activity and magma chemistry shifted east- ward. Numerous molybdenum deposits, including Trout Lake, Mount Tolman, White Cloud, Cannivan Gulch, Buckingham, Hall, and Opodepe formed within a zone of calc-alkaline to high K calc-alkalic magmatism between 76 and 54 m.y. ago. A limited

amount of published geologic information and the trace element geochemistry of igneous rocks present at Trout Lake, Mount Tolman, Cannivan Gulch, Buckingham, and Hall support a calc-alkaline classification for these deposits. Within the same calc- alkaline belt, major porphyry copper deposits formed at this time in Arizona and Sonora.

From 60 to 40 m.y. ago magmatism was largely restricted to the northwestern United States (Fig. 14) and characterized by high K calc-alkalic and alkali- calcic chemistry. Information regarding molybdenum deposits and prospects shown is scarce but, based on the position with respect to the magma belts, Cumo appears to be a calc-alkaline deposit; Ima and Bald Butte are possibly transitional-type deposits; and Big Ben may be a Climax-type occurrence. No trace element data are available to us, but the presence of significant amounts of fluorite and huebnerite at Ima (Rostad, 1971) and of an "alkali granite" at Big Ben (S. D. Olmore, unpub. data, 1979) tends to support such classification. In British Columbia the porphyry molybdenum cluster near Kitsault is also associated with high K calc-alkalic magmatism active between 48.•3 and 58.5 m.y. ago (Soregaroli and Sutherland Brown, 1976).

Between $6 and 17 m.y. ago (Figs. 15 and 16), very large molybdenum deposits formed in the western United States. Climax-type deposits including Cli- max, Urad-Henderson, Mount Emmons, and Pine Grove are genetically related to granitic differentiates and all coincide with areas of alkali-calcic magmatism regardless of present-day crustal thickness (Fig. 17) or the presence of a gravity low (Fig. 18). Transitional deposits include Questa in an alkali-calcic magma province and Mount Hope in a high K calc-alkalic magma series. Alkalic deposits associated with syenite (Three Rivers stock) and monzonite (Nogal Peak) occur in a belt of alkali-calcic magmatism and the alkalic Cave Peak deposit is genetically related to a biotite granite (Sharp, 1979) within a quartz-deficient to subsilicic alkalic province (Barker, 1977).

The close correlation between magma series chemistry and type of molybdenum deposit regardless of tectonic setting, geologic environment, and crustal thickness in a magmatic are as complex as the western United States provides strong support for the proposed classification and indicates that it is built on fundamental metallogenic parameters inherent in the parent magma chemistry.

The Relationship between Magmatism, Molybdenum Deposits, and Subduction

The close spatial and temporal relationship between plate subduction, arc magmatism, and the formation of porphyry copper and stockwork molyb-


denurn deposits in the western United States suggests that here magmatism and molybdenum stockwork deposits are fundamentally subduction related. The shifting magma chemistry patterns of the western United States have been linked to changes in subduction angle with time (Keith, 1978).

The dominant influence of arc magma chemistry on the metallogeny of molybdenum deposits indicates that the origin of these magmas has an important bearing on the genesis of the deposits. The presence of similar magma series in convergent continental and island-arc plate margins implies that the ultimate source of the magma lies in the mantle. Two magma sources can be envisioned, i.e., the subducted slab or the overlying mantle wedge.

The close correlation between the K20 content of the magma series and the depth to the Benioff zone may reflect partial melting of subducted oceanic

105-80 m.y.

FK;. 12. Spatial distribution of magmatism and stockwork molybdenum deposits in the western United States between 105 and 80 m.y. ago. References used in reconstruction of the magmatic arc will be given in Keith and Dickinson (in prep.). Thompson Creek age date from Schmidt et al. (1979). In Figures 12 through 16 the outlines of arc magma chemistry zones define the extent of major igneous activity at that time. Small igneous centers that lack published whole-rock geochemistry to define the magma series chemistry have been omitted. Basin and range extension has been removed.

•. Q,r--ALKALI-CALClC /• h HIGH- K CALC-AIdKALIC

ß 67-5• to ALKALI* tALC[It

/ ,

o •oo•. •;,, '. 80-60 m.y. ,

FI(;. 13. Spatial distribution of magmatism and stockwork molybdenum deposits in the western United States between 80 and 60 m.y. ago. Age dates: Trout Lake (H. C. Boyle, pets. commun., 1979); Mount Tolman (W. C. Utterback, pers. commun., 1979); Cannivan Gulch (Schmidt et al., 1979); White Cloud (Armstrong et al., 1978); Buckingham (T. J. Theodore and E. H. McKee, pets. commun., 1980); Mineral Park (Mauger and Damon, 1965); and Opodepe (Leon and Miller, 1979). For legend, see Figure 12.

lithosphere at successively greater depths. Magma generation would depend on the thermal regime in and above the slab, the mineralogical composition of the subducted material, and the amount of water released during dehydration. Experiments show that most hydrous minerals are dehydrated above 150 km (Wyllie, 1973), but potassium, fluorine, and minor amounts of water may be carried in phlogopite to depths in excess of 250 km (Fyfe and McBirney, 1975).

Porphyry systems--including calc-alkaline molybdenum stockwork deposits--are associated with calc- alkaline magmatism derived by partial fusion of subducted oceanic lithosphere (Sillitoe, 1972) at depths ranging from 120 to 240 km. The melt will undergo some modification on its way to the surface (Stern, 1974; Best, 1975; Wyllie, 1973, 1979), but copper and molybdenum may be largely derived from the subducted slab (Sillitoe, 1972).


o •oo • 52 - 45 m.y.

FIG. 14. Spatial distribution of magmatism and stockwork mo- iybdenum deposits in the western United States between 52 and 45 m.y. ago. Age dates: Big Ben (S. D. Olmore, unpub. data, 1979); Bald Butte (Rostad, 1978); Ima (Armstrong et al., 1979); and Cumo (Rostad, 1971). Age dates for the Red Mountain and Tuzo Creek prospects in British Columbia taken from Christopher and Carter (1976). For legend, see Figure 12.

With increasing depth, the breakdown of phlogopite in the subducted slab may yield a hydrous fluid containing H•O, CO•, and F, which upon introduction into the overlying mantle wedge triggers partial melting of mantle peridotitc (Fyfe and McBirney, 1975; Best, 1975). This small melt fraction will be strongly enriched in K, F, and incompatible elements, including Nb and Rb. Introduction of this melt into the upper mantle may result in the addition of phlogopite, incompatible elements (Best, 1975), molybdenum (Uzkut, 1974), tin (Sillitoe, 1974a; Pearce and Gale, 1977), and volatiles including fluorine. Partial melting of the enriched upper mantle in response to doming and pressure release (Bailey, 1974) below a thick sialic crust or due to the development of a thermal diapir associated with mantle convection in a zone of ensialic back-arc spreading (Elston, 1976; Anderson et al., 1978) produces alkali-calcic and alkalic magmas. Magma differentiation and possible involvement of deep crustal material may yield fluorine-rich high silica alkali rhyolites and granites associated with Climax-type and alkalic molybdenum

deposits. These magmas will be dominated by mantle- derived components and closely resemble intracratonic rift-related alkaline magmas. We propose that the contrasting magma chemistry of calc-alkaline and alkali-calcic and alkalic igneous rocks is a direct consequence of the stability field of phlogopite within the subducted slab and the overlying mantle wedge. In this model, magma series with K57.5•< 2.5 are largely derived from subducted oceanic lithosphere and are little modified by interaction with the mantle wedge. Consequently, these parent magmas are low in fluorine and incompatible elements. Magma series with K57.• • 2.5 contain a significant mantle component and are the result of magma formation due to phlogopite breakdown in the subducted slab at depths in excess of 240 km and in the overlying mantle wedge at depths probably in excess of 175 km (An- derson et al., 1978; Wendlandt and Eggler, 1980). Hydrous melts formed in the slab at depths in excess of 240 km will be greatly modified on their way to the surface by interaction with the small melt fraction enriched in incompatible elements, potassium, and

•• CALC-ALKALI• / •/ • -- , ///

•o• 35-25 m.y.

FIG. 15. Spatial distribution of magmatism and stockwork molybdenum deposits in the western United States between $5 and 25 m.y. ago. Age dates: Mount Hope (Roberts et ai., 1971); Edwards and McLaughlin 1972); Urad-Henderson (Ranta et al., 1976); Cli- max (Surface et al., 1978); and Cave Peak (Sharp, 1979). For legend, see Figure 12.


could have been the source material for molybdenum found in these deposits.

Upon closer inspection of the data at hand this conclusion is no longer warranted. Major molybdenum deposits are found in areas with crustal thick- nesses ranging from 50 to <20 km (Fig. 17) and gravity lows do not always reflect thick continental crust (e.g., the Great Basin, Thompson and Burke, 1974). We are impressed by the strong control magma chemistry exerts on the metallogeny of the molybdenum deposit that ultimately forms, and propose that molybdenum, fluorine, and the incompatible elements found in igneous rocks and associated hydrothermal systems in molybdenum stockwork deposits are largely mantle derived. The following observations support our hypothesis:

1. Initial strontium isotope ratios range from 0.7055 (Kitsault, Giles and Livingstone, 1975) to 0.7069 (Questa, Laughlin et al., 1969) and are similar to those found in igneous rocks associated with porphyry copper deposits (Moorbath et al., 1967) for which a deep crustal or upper mantle origin is proposed. Studies reported by McCarthy and Groves (1979) have shown that initial strontium isotope ratios in excess of 0.710 can be the result of fractional crystallization of a

FIG. 16. Spatial distribution of magmatism and molybdenum stockwork deposits in the western United States between 25 and 20 m.y. ago. Age dates: Majuba Hill (MacKenzie and Bookstrom, 1976); Pine Grove (W. J. Tafuri, pets. commun., 1980); Questa (Ishihara, 1967; Clark, 1972); Mount Emmons (M. W. Ganster, pers. commun., 1979). For legend, see Figure 12.

volatiles that is present in the mantle wedge above the Benioff zone (Fig. 10).

Source of metal

25

Economic molybdenum deposits occur within areas of sialic continental crust although important prospects have been found in older island arcs (Sulawesi, Sillitoe, 1980; Japan, Ishihara, 1978). No molybdenum deposits have been found in young island arcs, and porphyry copper deposits found in this setting are characteristically low in molybdenum (Kesler, 1978). Several investigators (King et al., 1970; Wood- cock and Hollister, 1978) have stressed an apparent relationship between thick sialic crust (Fig. 17), pro- nounced gravity lows (Fig. 18), and large high-grade molybdenum deposits. These authors imply that continental sialic crust is the main source for molybdenum contained in molybdenum stockwork deposits. Indeed, Wallace and co-workers (1978) suggest that molybdenum-enriched Precambrian rocks found in the vicinity of the Climax and Henderson deposits

40

\

FIG. 17. Relationship between spatial distribution of stockwork molybdenum deposits and thickness of sialic crust (after Smith, 1978) in the western United States. Symbols used for molybdenum deposits are explained in Figure 1.


FIG. 18. Relationship between the spatial distribution of molybdenum stockwork deposits and regional gravity patterns in the western United States. Gravity data after Eaton et al. (1978) and Woodcock and Hollister (1978, fig. 2). Symbols used for molybdenum deposits are explained in Figure 1.

magma with a low initial Sr isotope ratio within a closed system.

2. The very high initial strontium ratio of Precam- brian gneisses in the Colorado-New Mexico area (0.76-0.85, Hedge, 1974) argues against even minor assimilation of upper crust ancient sialic basement and associated molybdenum in granite magma coeval with Climax-type deposits.

$. Sulfur isotope data at Questa (Field, 1966) and Kitsault (Giles and Livingstone, 1975) indicate an homogenized mantle sulfur source.

4. In the western United States a striking relationship exists between magma chemistry and the presence of Climax-type deposits regardless of present- day crustal thickness.

5. The enrichment of alkali-calcic and alkalic magmas in incompatible elements, including molybdenum, tin, and fluorine, is independent of the initial strontium isotope ratio, tectonic setting, or crustal thickness.

6. The occurrence of very high molybdenum concentrations in some carbonatites (Deans, 1966) indicates that the mantle is locally highly enriched in molybdenum.

7. Significant molybdenum enrichment does take place during fractional crystallization of mantle-de-

rived basic magmas, as shown by the presence of molybdenum mineralization associated with granitic differentiates of basalts in oceanic islands (Iceland, Jancovic, 1972).

8. Highly peraluminous two-mica granites were emplaced in the western United States during periods of extreme slab flattening and occur in the same areas where earlier molybdenum stockwork deposits were formed (Fig. 14). However, these crustally derived wet granites are extremely depleted in lithophile elements and fluorine (Keith and Reynolds, 1981).

The Genesis of Stockwork Molybdenum Deposits Most students of stockwork molybdenum deposits

believe that molybdenum introduced into the hydrothermal system is derived from a crystallizing magma (Ganster, 1976; Wallace et al., 1978; Kamilli, 1978; Lamarre and Hodder, 1978; Soregaroli and Suther- land Brown, 1976). A review of magma generation processes, the physicochemical characteristics of magmas, and the influence of magma chemistry on crystallization may help to explain the observed differences in the various types of molybdenum deposits and aids in developing a speculative genetic model.

Mol•tbdenum contents in igneous rocks The molybdenum concentration in igneous rocks

increases with increasing Ks7.s content between magma series. At constant Ks7 •, the molybdenum content increases with increasing alkali and SiO2 contents during differentiation within a magma series (Keith, unpub. data). The molybdenum content of a residual melt will depend on the initial molybdenum content of the parent magma. Alkalic rocks are strongly enriched in molybdenum and the concentration level increases with increasing silica-undersaturation and increasing alkali content (Table 8; Uzkut, 1974). Mo- lybdenum values in carbonatites range from 10 to 100 ppm but may get as high as 0.1 to 0.8 percent Mo (Deans, 1966; Uzkut, 1974).

Mol•tbdenum concentration processes during magma consolidation

In a previous section we have suggested that the initial molybdenum enrichment in alkali-calcic and alkalic magma series may be the result of mantle wedge enrichment above the deepest part of a subduction zone. However, even in these enriched magmas the level of molybdenum concentration is rarely sufficient to form a molybdenum deposit. Following intrusion in the crust, additional molybdenum enrichment must take place within the magma chamber. Several processes may concentrate molybdenum at the top of a magma chamber, e.g., magma differentiation by fractional crystallization, liquid-state thermogravitational diffusion, volatile transfer following vapor saturation, and magma convection.


TABLE 8. Molybdenum Content in Igneous Rocks (after Uzkut, 1974)

Arithmetic

Range in ppm mean in ppm

Dunite 0.1-1.$ 0.$4 Peridotite 0.2-1.$ 0.6 Gabbro 0.1-1.8 0.78 Diorite 0.2-1.6 0.9 Quartz diorite 0.2-$.2 1.2 Granodiorite 0.2-$.2 1.7 Granite 0-280 2.4

Syenites-nepheline syenites 1.0-200 4 Carbonatites 10-100 (max. 0.2%) 50 Basalt 0.2-$.2 1.4 Alkali basalt • 5-11 Andesite 0.2-8.2 1.7 Rhyodacite-dacite 0.8-8.1 1.9 Rhyolite 0.6-5.0 2.8

• 10 samples

McCarthy and Groves (1979) have demonstrated that prolonged crystal fractionation of a quartz monzonite magma can result in the formation of a highly differentiated granite melt enriched in volatiles and lithophile elements. Molybdenum concentration in the melt during fractional crystallization is governed to a large extent by molybdenum speciation (Uzkut, 1974). Molybdenum may occur in the melt as cation (Mo +4, Uzkut, 1974) or as a molybdate anion complex (e.g., MoO• 2, Gevorkyan, 1968). Based on data presented by Gevorkyan (1968), Uzkut (1974, p. 108- 109) suggested that, in melts with a combined Na q- K content of less than 4 to 5 percent; molybdenum occurs as Mo +4 and can substitute for Ti +3, Fe +3, and AI +• in early crystallizing minerals like magnetite, sphene, and ilmenite. This behavior may explain the lack of stockwork molybdenum deposits associated with igneous rocks more basic than granodiorite and the lack of molybdenum deposits in rocks of calcic magma series. In granodioritic, quartz monzonitic, and granitic melts, molybdenum is present as a molybdate anion complex and will be concentrated in the residual melt during fractional crystallization (Uzkut, 1974). Molybdenum enrichment in the residual melt is favored by a high initial molybdenum concentration, a high alkali content, and low iron and titanium concentrations, i.e., by magma compositions found in Climax-type molybdenum deposits.

Liquid-state thermogravitational diffusion (Shaw et al., 1976; Hildreth, 1979; Smith, 1979) allows migration of elements to take place in response to the thermal and gravitational field within a water-undersaturated stationary layer I to 2 km thick at the top of a convecting magma column. A hood zone may form which is enriched in Nb, Mo, Ta, W, U, Th, Sn, Rb, and volatiles including F, and depleted in Mg, Sr, and Ba (Hildreth, 1979). The documented trace

element enrichment patterns in these silicic rocks are similar to those found in igneous rocks in Climax-type deposits. Enrichment factors are higher in melts with a low solidus temperature (Ludington et al., 1979), in melts containing in excess of 74 percent SiO2, in more alkalic melts, and in central vent magma chambers (Hildreth, 1979). Hildreth (1979) and Ludington et al., (1979) attribute molybdenum concentrations found in some molybdenum deposits solely to liquid- state thermogravitational diffusion in the magma chamber, although molybdenum enrichment factors documented in natural systems (e.g., the Bishop Tuff, Hildreth, 1979) are orders of magnitude lower than those needed to produce a Climax-type molybdenum deposit.

Following vapor saturation in the magma, numerous constituents including molybdenum may be par- titioned into the vapor phase (Holland, 1972; Kilinc and Burnham, 1972). The gravitational rise of the vapor phase and the rapid diffusion of magma constituents through the vapor phase can result in the concentration of volatiles, alkalies, and incompatible elements at the top of the magma column (Kennedy, 1955; Jahns and Burnham, 1969) and the introduction of these components in an overlying hydrothermal system (cf. Burnham, 1979).

Magma convection has also been proposed (Whit- ney, 1975) as a means to concentrate metal- and sulfur-bearing hydrothermal fluids derived from a large magma volume at the top of the magma chamber.

The effectiveness of molybdenum enrichment and the additional concentration and release of molybdenum from the magma into the hydrothermal system are strongly dependent on the physicochemical characteristics of the magma and its crystallization history.

The effect of alkali content: Igneous rocks associated with Climax-type and alkalic molybdenum deposits contain high concentrations of potassium and rubidium. Experimental studies have shown that these high alkali levels will drastically lower the solidus temperature of the melt (Glyuk and Anfiligov, 1974; Glyuk et al., 1977; Glyuk and Trufanova, 1977). This effect is enhanced by volatile components such as lithia and fluorine (Wyllie and Tuttle, 1961, 1964; Wyllie, 1979). Tuttle and Bowen (1958) have suggested that, during final crystallization of these volatile-rich granitic magmas, an alkali-rich hydrous rest melt might form. Depending on its chemical composition this alkali-rich melt would continue to crystallize quartz and .K-feldspar down to temperatures as low as 400øC. Stemprok (1974) and Isuk (1976) have shown experimentally that molybdenum is highly soluble in hydrous alkali silicate melts at near- solidus temperatures and it appears feasible that under favorable conditions molybdenum contained in the magma could ultimately be concentrated in a


late-magmatic hydrous K-rich rest melt. Experimen- tal data (Isuk, 1976) indicate that cooling of this melt and a decrease in pH will result in precipitation of quartz and molybdenite.

The effect of volatiles: Granitic magmas associated with molybdenum stockwork deposits are commonly volatile rich. The relatively shallow intrusion level of the stock indirectly suggests that the water content of the magma during intrusion probably did not exceed 4 percent (cf. Burnham, 1967; Tuttle and Bowen, 1958). Textural and volumetric relations of water-saturated melt pockets in the Primos Border phase at Henderson (Ganster, 1976)--taking into account the high f.F (Munoz, 1980) of the hydrothermal fluid--indicate that the water content of the parent magma probably did not exceed 8 percent. Fluorine- poor calc-alkaline magmas probably contain less than 1.5 percent water (Maalde and Wyllie, 1975).

The presence or absence of high fluorine concentrations in the parent igneous rock and the hydrothermal system is one of the main differences between calc-alkaline molybdenum deposits and the alkali-calcic and alkalic molybdenum deposits. High fluorine concentrations influence the crystallization history and reduce the viscosity (Kogarko, 1974; Burt et al., 1980) of the granitic magma, thus enhancing molybdenum concentration processes. Experimental data regarding the HF-granite system show that at high fluorine levels:

(1) the solidus may be depressed by as much as 150øC (Glyuk and Trufanova, 1977), which allows for highly effective thermogravitational enrichment of molybdenum prior to water saturation (Ludington et al., 1979);

(2) the crystal/melt ratio is very low even directly above the solidus (yon Platen and Winkler, 1961; yon Platen, 1965), creating conditions favorable for the development of very high fluid pressures during final crystallization (Kadik et al., 1975) and the formation of a zone of stockwork fracturing (Phillips, 1978); and

(8) the water solubility in the melt drastically increases (Koster van Groos and Wyllie, 1968).

At high alkali and fluorine, and low silica concentrations, volatiles may be retained in the magma (Kogarko, 1974). This may be the reason why no known stockwork molybdenum deposits occur in subsilicic alkaline rocks. In the SiOz-rich melts found with Climax-type deposits, HF is released into the vapor phase (Kogarko, 1974). Due to its high faF, the vapor phase will also contain high concentrations of SiOz, K-Na-Al-fluorides, and trace elements including Nb, Zr, and $n (Bailey, 1977; Ryabchikov et al., 1974; Ludington et al., 1979). At greater depth this fluorine- enriched magma may exsolve a residual immiscible fluoride melt (Glyuk and Trufanova, 1977) which,

upon cooling, reacts with the igneous host to produce a zone of greisen alteration.

In summary, high concentration levels of K, Rb, and F, and low concentrations of Fe and Ti in a silica- rich alkali granite magma of the Climax-type favor efficient concentration of molybdenum in the upper part of the magma chamber by crystal fractionation and liquid-state diffusion. Late in the crystallization sequence these magmas may generate a residual hydrous K- and SiOz-rich fraction in which much of the molybdenum contained in the parent magma will be concentrated. The chemistry of calc-alkaline magmas is much less favorable for molybdenum concentration processes.

Molybdenum chemistry in the hydrothermal environment

Molybdenum concentrated in a highly differentiated near-eutectic granitic residue may be introduced into the hydrothermal system by one or more of the following mechanisms: (1) a molybdenum-rich vapor phase associated with a late-magmatic water- saturated K-rich silicate melt (Isuk, 1976); (2) vapor transport of oxymolybdate or halogen compounds (Arutyunyan, 1969; $hcherba, 1970; Glemser and Wendlandt, 1968); and (8) as molybdate, oxymolybdate, and thiomolybdate complexes in hydrothermal fluids (Khitarov et al., 1967; Smith et al., 1980).

Isuk (1976) demonstrated experimentally that the vapor phase associated with molybdenite-bearing water-saturated K-silicate melt can contain high concentrations of potassium, silica, and molybdenite and may transport molybdenum under weakly alkaline conditions. He proposed that this molybdenite-rich vapor phase plays an important role in the formation of molybdenum deposits.

Field observations (Zies, 1929; Yoshida et al., 1972) and thermodynamic calculations (Krauskopf, 1957, 1964) suggest that at high temperature molybdenum can also be transported in the vapor phase as hy- droxide and oxymolybdate species. Estimates of molybdenum complex stabilities based on thermodynamic extrapolations (Smith et al., 1980) indicate: (1) molybdenum concentrations in the hydrothermal fluid may reach several thousand parts per million at 850øC and the solubility decreases drastically between 850 ø and 800øC; (2) molybdenum is largely transported as HMoO• with lesser amounts contained in HzMoO4 and MoOaF-; and (8) fluorine is not nec- essary for molybdenum transport in hydrothermal solutions. The limited role of halogen and oxyhalide complexes in molybdenum transport in the hydrothermal system is a consequence of their narrow stability ranges which are defined by conditions rarely encountered in nature (Arutyunyan, 1969).


The relationship between potassium and mol•tbdenum enrichment in stockwork mol•tbdenum deposits

Potassic alteration takes place before, during, and after molybdenite mineralization in most molybdenum stockwork deposits (e.g., Climax). A close spatial relationship between molybdenite mineralization and potassium enrichment is found at Climax, Urad-Hen- derson, Questa, Mount Emmons, Redwell Basin, in several Russian deposits (Berzina and Sotnikov, 1977), and in most Canadian deposits (Soregaroli and Suth- erland Brown, 1976). In several deposits high concentrations of disseminated molybdenite occur in K- feldspar-rich aplite and pegmatite phases, and almost pure K-feldspar masses containing up to 7 percent MoS2 were found in the Urad deposit (Wallace et al., 1978). Late magmatic intergrowths of massive molybdenite and K-feldspar occur in the Roundy Creek deposit. These observations show that potassium and molybdenum enrichment are closely related during final magma crystallization. A comparison of whole- rock analyses of unaltered and potassically altered igneous rocks at Henderson (W. H. White, writ. commun., 1980) and Questa (Ishihara, 1967) shows that potassium enrichment is not accompanied by an enrichment in alumina. Therefore, the late magmatic phase responsible for potassium metasomatism must have been K rich and alumina deficient and is prob-

ably similar in nature to late magmatic alkali silicate melts suggested by Tuttle and Bowen (1958). Exper- imental data and field evidence support the hypothesis that such late magmatic hydrous K-rich silicate melts play a significant role in concentrating molybdenum.

The enrichment model predicts a close correlation between the amount of introduced potassium and molybdenum grade which, in fact, can be demonstrated (Fig. 19). An estimate of the maximum average molybdenite grade of the deposit can be ob- tained by converting the amount of introduced K20 into K-disilicate and using the experimentally determined MoS• solubility in the K-disilicate-H•O system (Isuk, 1976). The experimental data for the K-disilicate-H•O system are used as an approximation of the far more complex natural residual melt systems. For example, the potassic zone at Henderson contains 2 percent introduced K•O (White, writ. commun., 1980), the equivalent of 4.6 percent K-disilicate which, if saturated, contained 12.5 percent MoS2 at 650øC and 680 bars (Isuk, 1976). By assuming that all molybdenite is deposited within the ore zone, the maximum average grade of the deposit is calculated to be 0.575 percent MoS• (i.e., 0.046 X 12.5), which compares closely with the published grade of 0.49 percent MoS• (including a significant tonnage of >0.6 percent MoS•, Wallace et al., 1978).

Similar calculations based on added K•O at Kitsault

Adanac

Kltsoult

Thompson Creek ß

Ouesta Goat

Henderson

0. l% 0;>% 05% 04% 0.5%

Average % MoS Z Content •n Ore Zone

FIG. 19. Relationship between potassium enrichment and molybdenite grades in molybdenum stockwork deposits. Data from W. H. White (writ. commun., 1980), S. Ishihara (writ. commun., 1979), Wood- cock and Carter (1976), White et al. (1976), and Lain/• (1974).

866 ½. WESTRA AND S. B. KEITH

(Woodcock and Carter, 1976) indicate a maximum grade of between 0.14 and 0.28 percent MoS2. Pub- lished ore grades range from 0.192 percent MoS2 to 0.25 percent MoS• (Soregaroli and Sutherland Brown, 1976). Calculated grades at Adanac range from 0.17 to 0.18 percent MoS• compared to a published grade of 0.16 percent MoS• (Soregaroli and Sutherland Brown, 1976).

Genesis of calc-alkaline molybdenum stockwork deposits

Calc-alkaline molybdenum stockwork deposits are associated with calc-alkaline and high K calc-alkalic are magmas. Parent magmas are low in fluorine (Fig. 2; Lain•, 1974), molybdenum (Uzkut, 1974; Hudson et al., 1980), and incompatible elements and contain less than 1.5 percent water (Maal•Se and Wyllie, 1975).

Following emplacement of the granodiorite or quartz monzonite magma in the crust, fractional crystallization may produce a highly differentiated silicic granite melt, low in marlcs and enriched in volatiles, potassium, and molybdenum. If this differentiate sep- arates from the main magma chamber and intrudes to with I to :3 km of the surface, a stock-type molybdenum deposit may form. The lack of comagmatic extrusive rocks associated with calc-alkaline molybdenum deposits suggests the magma had a low water content or was emplaced at rather deep levels in the crust. Following vapor saturation of the magma column, explosive pressure release and the formation of a zone of stockwork fracturing signal the start of a complex magmatic-hydrothermal convective system. A central zone dominated by magmatic hydrothermal fluids at lithostatic pressure is surrounded by a large meteoric convection system under hydrostatic pressure (of. Cunningham, 1978). Molybdenum released from the magma was probably concentrated in a potassium-rich hydrothermal fluid at near-magmatic temperature (el. M. S. Bloom, in prep.) above the water-saturated surface in a convecting water-undersaturated magma column (el. Whitney, 1975; Westra, 1979). In the hydrothermal system molybdenum is transported mainly as HMoO• (Smith et al., 1980) and deposited in response to adiabatic de- compression (M. S. Bloom, in prep.), cooling, boiling, or mixing with meteoric waters. Molybdenite mineralization at the Buckingham deposit (Blake et al., 1979) took place during circulation of nonboiling or "slightly" boiling moderately saline fluids (4-12 equiv. wt % NaCI) at temperatures between :300 ø and 400øC and a hydrothermal fluid pressure of less than :300 bars within a zone of mixing of meteoric and magmatic hydrothermal fluids. Late base metal veins formed at temperatures between 260 ø and $50øC in the presence of moderately saline fluids. Due to the low initial molybdenum concentration in the parent

magma and the ineffective concentration mechanisms in the magma, the final molybdenite grade in the hydrothermal system will rarely exceed 0.25 percent MoS•.

A plutonic mot•ybdenum stockwork deposit may form when the granitic differentiate is trapped within, or at the top of, the batholith. Following vapor saturation of this melt, molybdenum, volatiles, and potassium might be concentrated at the top of the magma by upward diffusion (Kennedy, 1955; Jahns and Burnham, 1969). Wall-rock permeability may be increased by stockwork fracturing or tectonic causes (Endako), but the high confining pressure in these deep-seated systems (Sutherland Brown, 1976) precludes boiling of the magmatic hydrothermal fluid, as indicated by fluid inclusions (M. S. Bloom, in prep.). Molybdenite precipitates in response to cooling (Smith et al., 1980) and a concurrent pH decrease. However, high wall-rock temperatures in the previ- ously consolidated batholith and the lack of meteoric circulation result in gentle thermal gradients. Con- sequently, molybdenite is dispersed in a very large rock volume and grades tend to be low (0.10-0.15% MoS2). The lack of boiling (Endako; M. S. Bloom, in prep.) precludes effective H•S fractionation into the vapor phase and, consequently, the pyrite halo is weakly developed (Endako) or completely lacking (Adanac) in plutonic calc-alkaline molybdenum deposits.

Genesis of Climax-type molybdenum stockwork deposits

In high-grade Climax-type molybdenum deposits, molybdenum is enriched by a factor of 1,000 to 2,000 over background values. We propose that these extreme enrichment factors are the result of a multi-

stage enrichment process which started in the mantle. Partial melting of an enriched upper mantle pro-

duced alkali-calcic and alkalic magmas enriched in incompatible elements and volatiles. Subsequent differentiation and possible involvement of deep crustal material may result in the formation of a fluorine- rich, high silica, and K-rich granitic melt. Following intrusion, these water-undersaturated (cf. Burnham, 1967) silicic magmas may form rather large magma chambers (Isaacson and Smithson, 1976) at depths ranging from 2 to 6 km. High water, alkali, and fluorine contents drastically lower the solidus temperature of the magma, thus enhancing liquid state diffusion of Mo, Sn, F, Nb, and Rb toward the top of the magma column (Hildreth, 1979; Ludington et al., 1979). The high volatile and alkali contents increase the water solubility in the magma and allow for the formation of a residual hydrous K-rich silicate melt (Tuttle and Bowen, 1958) rich in fluorine (Koster van Groos and Wyllie, 1969). Molybdenum present in the melt is likely to be concentrated in this K-rich melt.


Upon water saturation, vapor pressures at the top of the magma column will rise, ultimately exceeding lithostatic confining pressure (Berzina and Sotnikov, 1977; Ganster, 1976; Kamilli, 1978; M. S. Bloom, in prep.). Confining pressure release results in the formation of a zone of stockwork fracturing (Phillips, 1975). Excess pressures sufficient to exceed lithostatic confining pressure can only be generated at confining pressures of 500 to 1,000 bars (Kadik et al., 1975), i.e., at depths ranging from •1.9 to $.8 km in accord with estimated depths of formation of the ore zones at Climax (y$.6 kin) and Henderson (1.6 km, Wallace et al., 1978). The hydrothermal fluid in equilibrium with the molybdenum-rich residual melt is itself rich in molybdenum (Isuk, 1976). As the final melt fraction crystallizes to quartz and K-feldspar, excess potassium is introduced into the wall rock or carried away in hypersaline molybdenum-rich (Kamilli, 1978; M. S. Bloom, in prep.) and fluorine-rich (Berzina and Sotnikov, 1977; Munoz, 1980) magmatic hydrothermal fluids which are trapped at near-solidus temperatures in early barren quartz veins at pressures equal to or above lithostatic confining pressure (Kamilli, 1978; M. S. Bloom, in prep.). The fluorine fractionation behavior (Koster van Groos and Wyllie, 1969) suggests that the initial HF-rich fluid phase represents only a small fraction of the volatiles contained in the melt. Following a reduction in confining pressure, a much larger volume of less saline (Kilinc and Burn- ham, 1972) magmatic hydrothermal fluid will be released which is responsible for the main mineralizing event (Hall et al., 1974; Kamilli, 1978; M. S. Bloom, in prep.). The core zone of the hydrothermal system will be dominated by magmatic hydrothermal fluids at near-lithostatic pressures separated from a zone of convecting (Cathies, 1977) meteoric water at hydrostatic pressure (Cunningham, 1978; Westra, 1979) by a zone of throttling. Mass balance calculations (Wal- lace et al., 1968) indicate that large magma volumes must have been involved in the formation of the

Climax deposit. The most likely process to supply the large volume of hydrothermal fluids, the molybdenum inventory, and the thermal energy to form Cli- max-type systems may well be magma convection.

Molybdenum in the hydrothermal fluid is predominantly transported as HMoO•- (Smith et al., 1980). Molybdenite precipitated in the presence of moderately saline fluids (0.7-12 equiv. wt % NaC1) at Cli- max (Hall et al., 1974) and from highly saline fluids ($0-65 equiv. wt % NaC1) at Henderson (Kamilli, 1978), Questa, and Glacier Gulch (M. S. Bloom, in prep.) at temperatures ranging from 600 ø to 300øC and pressures fluctuating between lithostatic and hydrostatic. The relative scarcity of vapor-rich inclusions suggests that boiling occurred only locally and was an intermittent phenomenon. The spatial separation of zones of highest fluorine and highest mo-

lybdenum concentrations in many Climax-type deposits and the lack of fluorine enrichment in molybdenum deposits of the calc-alkalic type indicate that fluorine is not essential in molybdenum transport.

At Henderson boiling took place within the orebody but not above it (Kamilli, 1978). Boiling of the magmatic hydrothermal fluid in these deposits pro- duced an acid vapor phase enriched in HF, HaS, and COa and a high HF/HC1 ratio (Berzina and Sotnikov, 1977). The zone of boiling is characterized by a high quartz content and replacement of feldspars by fluorite and/or topaz (Smirnov, 1977). Extensive mixing of magmatic and meteoric hydrothermal fluids took place in the quartz-sericite-pyrite and argillic zones (Hall et al., 1974), but within the ore zone magmatic fluid pressures equal to, or greater than, lithostatic pressure effectively restricted influx of meteoric fluids to the edge of the zone of stockwork fracturing (cf. Cunningham, 1978). The importance of meteoric waters for the formation of the molybdenite ore shell appears minor (Wallace et al., 1978; Kamilli, 1978). Base metal and manganese anomalies, however, could form by mixing of meteoric waters containing Pb, Zn, Ag, and Mn--metals easily leached from the wall rocks (Ellis and Mahon, 1964, 1967)--and the HaS- enriched condensed vapor phase above the zone of boiling. Reaction with wall rock and dilution of the vapor condensate with meteoric waters (White et al., 1971) increases the K+/H + ratio and results in the formation of an argillic zone outside the quartz-sericite-pyrit.e zone. The increasing K+/H + ratio may be reflected in an inner kaolinite zone and an outer

montmorillonite zone (as documented, for example, at Climax, Steininger, 1971). The pH increase may also result in precipitation of carbonates in the argillic zone (White et al., 1971). Both the argillic and propylitic zones form in the presence of meteoric fluids of low salinity (Hall et al., 1974).

Following maximum expansion of the magmatic hydrothermal fluid domain, the temperature and pressure of the fluids are likely to decrease in response to a decrease in the magma convection rate or due to magma crystallization. Meteoric waters (salinity 0- $ equiv. wt % NaC1) will encroach upon the inner alteration zones precipitating late base metal-quartz- fluorite veins at 200 ø to $00øC. Ultimately these fluids will occupy the core of the system (Hall et al., 1974). At this time the fluid pressure throughout the hydrothermal system will have dropped to hydrostatic (Berzina and Sotnikov, 1977; Kamilli, 1978). The sequence of events as described above may be repeated several times, resulting in overlapping or spatially separated pulses of molybdenite mineralization (Cli- max and Urad-Henderson, Wallace et al., 1968, 1978), each cogenetic with a separate episode of magma crystallization. The high water solubility in the melt, its low solidus temperature, and its low crys-


tal/melt ratio favor crystallization of only a small volume of water-saturated melts without interfering with magma convection at depth. Liquid-state thermogravitational diffusion will rapidly re-establish trace element gradients in the magma column (Hil- dreth, 1979) and, following water saturation at the top of the column, a new phase of stockwork fracturing and hydrothermal activity may ensue.

In summary, the extreme enrichment of molybdenum in Climax-type deposits is the result of (1) highly efficient crystal fractionation in a molybdenum-enriched alkali-calcic parent magma, (2) concentration of most of the molybdenum present in the enriched silicic magma within a late magmatic hydrous K-rich-silicate melt, and ($) the release of hydrothermal fluids with high molybdenum concentrations into a hydrothermal system with very sharp thermal gradients resulting in (4) complete molybdenum removal from the hydrothermal fluids within a relatively small rock volume (i.e., the ore zone).

Genesis of alkalic molybdenum stockwork deposits

The probability of a molybdenum stockwork deposit forming in rocks of alkalic composition is largely dependent on the silica, volatile, alkali, and halogen contents of the magma. Only in the subordinate quartz-rich magma phases within an alkalic province will vapor saturation be a common phenomenon. Following vapor saturation of the magma column, the development of the hydrothermal system follows the general outlines described for Climax-type deposits. If vapor saturation does not take place, molybdenum contained in the melt will crystallize as late magmatic disseminated molybdenite so commonly found in syenites and nepheline syenites.

Conclusions

1. Molybdenum stockwork deposits can be classified by using the chemistry of the associated magma series expressed as K57.5 (i.e., the K20 percentage at 57.5% SiO2 in the magma series). Two main molybdenum deposit types are proposed: (a) calc-alkaline molybdenum stockwork deposits related to granodioritic to granitic differentiates of calc-alkalic to high K calc-alkalic magma series with K57.• •< 2.5, and (b) alkali-calcic and alkalic molybdenum stockwork deposits related to granitic differentiates of alkali-calcic and alkalic magma series with K57.5 > 2.5. Addi- tional subdivisions can be made based on the chemical

characteristics of the source pluton and the concentrations of lithophile elements and fluorine in the hydrothermal system. The resulting classification is shown in Figure 8 and the main characteristics are listed in Table 2.

2. Molybdenum stockwork deposits are found in magmatic arcs associated with convergent plate margins (calc-alkaline deposits), in zones of ensialic back-

arc spreading above the deepest parts of a subduction zone (transitional, Climax, and alkalic type deposits), in zones of initial continent break-up (alkalic type), and in intracratonic rift zones (alkalic type).

$. The metallogenic characteristics of molybdenum deposits are highly dependent on the magma series chemistry, are somewhat influenced by the chemistry of the cogenetic igneous pluton, but are largely independent of the local crustal thickness and the geological environment in which the pluton is emplaced.

4. The arc magma chemistry, especially the incompatible element and fluorine concentrations, is strongly influenced by the phlogopite stability field in the subducted slab and the overlying mantle wedge.

5. Geochemical characteristics and isotope data of igneous rocks genetically related to molybdenum deposits support a mantle origin for the magma, molybdenum, fluorine, and sulfur.

6. The high fluorine concentration in the magma series with K•7.5 > 2.5 strongly enhances molybdenum enrichment processes by suppressing crystallization, lowering the solidus temperature, and increasing the water solubility in the magma, thus promoting the formation of a late magmatic hydrous K-rich silicate melt. In contrast, low fluorine activity in calc-alkaline magma series does not depress the solidus temperature to the point where hydrous K-rich silicate melts can easily form.

7. The close correlation between molybdenum and potassium introduced into the hydrothermal system lends support to a late magmatic concentration process involving a hydrous alumina-deficient K-rich silicate melt, as proposed by Isuk (1976).

8. No firm correlation exists between fluorine and

molybdenum concentrations in molybdenum deposits, and we conclude that fluorine is not essential for transport of molybdenum in the hydrothermal system.

9. The close relationship between metallogeny and magma chemistry in volcanic arcs as demonstrated by molybdenum stockwork deposits may be expanded to include other metals and has the potential to develop into a powerful exploration tool (Keith, 1979a; Lawrence and Wood, 1980).

Acknowledgments

We are greatly indebted to M. L. Fellows and T. C. Patton of the Exxon Minerals Company for their encouragement and critical reviews of various ver- sions of the manuscript. The paper has benefited from critical reviews by J. M. Proffett of the Anaconda Copper Company, M.P. Martineau of the British Petroleum Company, Dr. R. W. Hodder of the Uni- versity of Western Ontario, and anonymous reviewers of this magazine. Additional comments by W. Lodder


of Amax Exploration, J. C. Wilson of the Anaconda Company, and J. T. Abbott of the Homestake Mining Company proved helpful.

Dr. S. Ishihara of the Geological Survey of Japan kindly made available revised whole-rock analytical data of the Questa area. G. Dunlop of Molycorp pro- vided us with unpublished information of the Questa deposits, and W. Lodder supplied us with published and unpublished information regarding the Malm- bjerg deposit. D. E. Ranta and W. H. White of the Climax Molybdenum Company and M. S. Bloom of Monash University allowed us to quote from their unpublished works. W. J. Moore of the U. S. Geolog- ical Survey allowed us to use unpublished Rb-Sr data of the Bingham district.

Stimulating discussions with Mike Martineau, Jeff Abbott, Reese Ganster, Bill Utterback, Gary Dunlop, Dave Rife, Vaughn Surface, and Bill Tafuri helped to shape G.W.'s ideas regarding molybdenum stockwork deposits. Useful discussion and encouragement from Bill Dickinson, Stephen Reynolds, and John Guilbert greatly aided S.B.K.'s extension of the K-H relationship to magma chemistry and metallogeny. We hasten to add that the views and concepts presented in this paper are not necessarily shared by these individuals.

We would like to express our gratitude to the man- agement of the Exxon Minerals Company for pro- viding drafting and secretarial assistance and for per- mission to publish this paper. Special thanks are extended to Sara Lehman for typing numerous ver- sions of the manuscript and to Richard Dupont for drafting the illustrations.

January 1, 1980; March 10, 1981

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