Granites Alterados 2

Exploration and Mining Geology, Vol. 19, Nos. 12, p. 1322, 2010 2010 Canadian Institute of Mining, Metallurgy and Petroleum.

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Abstract Alkali/alumina and MgO/alumina molar ratio plots graphically portray both chemical and mineralogical changes accompanying potassic, phyllic, argillic, chloritic, and alunitic alteration of quartz monzonites and granodiorites hosting porphyry (as well as lode and greisen) ore deposits. The molar ratio plots can be used to identify different types of alteration. In most cases, the identi-fication based on molar ratios coincides with petrographic data. In those instances where the molar ratio and petrographic identifications do not agree, the mineralogy might need to be re-examined. Hydrothermal alteration studies using trace elements could benefit from the application of comple-mentary alkali/alumina molar ratio plots. 2010 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved.

Key Words: Alteration, alkali/alumina molar ratios, granitoids, porphyry deposits.

Sommaire Le diagramme des rapports molaires alkali/alumine et MgO/alumine illustre claire-ment les changements chimiques et minralogiques qui accompagnent les altrations potassique, phyllique, argillique, chloritique, and alunitique des monzonites quartz et des granodiorites encais-sant les gtes de type porphyrique, ainsi que des veines et des greisens qui leur sont associs. Les diagrammes de rapports molaires peuvent tre utiliss pour identifier divers types daltration. Dans la plupart des cas, lidentification base sur les rapports molaires est en accord avec celle base sur les donnes ptrographiques. L ou il y a dsaccord entre les rapports molaires et les donnes ptro-graphiques, il peut tre ncessaire de rexaminer la minralogie. Les tudes de laltration hydro-thermale accompagnes dune tude des lments traces auraient avantage utiliser un diagramme des rapports molaires alkali/alumine. 2010 Canadian Institute of Mining, Metallurgy and Pe-troleum. All rights reserved.

1 Department of Earth Sciences, Laurentian University, Ramsey Lake Road, Sudbury, Ontario, P3E 2C6. Corresponding Author: E-mail: [email protected]

Alkali/Alumina Molar Ratio Trends in Altered Granitoid Rocks

Hosting Porphyry and Related Deposits

J.F. DAVIES1, and R.E. WHITEHEAD1(Received November 10, 2009; accepted November 30, 2009)

14 Exploration and Mining Geology, Vol. 19, Nos. 12, p. 1322, 2010

Introduction

Several methods of assessing the nature and intensity of hydrothermal alteration utilize various element ratios. The most common of these involve the use of immobile trace elements such as Ti or Zr as standards against which the levels of mobile elements such as Na, K, and Mg are measured. Early examples using immobile trace elements are Gresens equations (Gresens, 1967) and Pearce element ratios (Pearce, 1968). Adaptations of these methods have been widely used in the study of alteration associated with VHMS deposits (e.g., MacLean, 1990; Barrett and Mac-Lean, 1991; Barrett et al., 1991; MacLean and Hoy, 1991; Stanley and Madeisky, 1994).

Alkali/alumina molar ratio plots employ oxide ratios in which Al2O3 serves as the denominator in place of Ti or Zr. Such plots have the advantage of portraying, in a simple and direct way, both chemical and mineralogical patterns on the same diagram. For example, a single diagram can illustrate the entire plagioclase series, K-feldspar, biotite, muscovite, and kaolin (Fig. 1a). The diagram might be fur-ther expanded by plotting molar K2O/Al2O3 against molar MgO/Al2O3 to accommodate such minerals as chlorite and phlogopite.

Alkali/alumina molar ratio diagrams have been used to portray the compositions of both unaltered and altered host rocks for several types of mineral deposits. Examples include felsic volcanic rocks hosting massive sulfide (VHMS) deposits (Davies and Whitehead, 2006), and Se-dex deposits (Davies and Whitehead, 1994).

The present study examines the application of these dia-grams to granitoid intrusions hosting porphyry and related deposits.

The essential features of alkali/alumina molar ratio dia-

diorites should plot within or near the wedge-shaped area defined by the line joining albite (Na2O/Al2O3 value of 1.0) to K-feldspar (K2O/Al2O3 value of 1.0) and the line join-ing oligoclaseandesine (Na2O/Al2O3 value of 0.6) and K-feldspar (K2O/Al2O3 value of 0.90). Figure 1b shows that the compositions of several Cenozoic granitoid intrusions mostly plot within, and the remainder lie adjacent to, the designated wedge-shaped area (after Davies and White-head, 2006).

Molar Ratios and Porphyry Deposits

Alteration assemblages associated with porphyry de-posits are potassic (biotite-K-feldspar), phyllic (quartz-sericite), intermediate argillic (kaolinite, montmorillonite), and advanced argillic (dickite and alunite in addition to kaolinite, montmorillonite).

Outside the boundaries of porphyry mineralization, low intensity propylitic alteration (chlorite, albite, epidote, and calcite) is analogous to low-grade greenschist regional metamorphism. Propylitic alteration yields no well defined molar ratio patterns because of the variable composition of the altered rock resulting from differing amounts of alkalis added to or removed from the rocks.

Summary accounts of alteration of granitoid rocks host-ing porphyry deposits might be found in Creasey (1966) and Lowell and Guilbert (1970). The following types of reaction are involved:K-feldspar to sericite: 1.5KAlSi O + H 0.5KAl Si O (OH) + K3 8 + 3 3 10 2 (1)

Sericite to kaolinite: 2KAlSi O (OH) + 2H + 3H O Al Si O (OH) + 2K3 10 2 + 2 2 2 5 43 (2)

Fig. 1. Na2O/Al2O3 vs. K2O/Al2O3 molar ratio diagrams: a. Alkali/alumina molar ratios of feld-spars, micas, and clay minerals; alunite plots at the same point as muscovite; clay minerals and other aluminous nonalkali-bearing minerals such as many chlorites, epidote, and topaz lie at the origin on this diagram; b. Molar ratio plot of unaltered Cenozoic granitoid intrusions (after Davies and Whitehead, 2006). Abbreviations: Kspar = K-feldspar.

grams are shown in Figure 1a, where molar K2O/Al2O3 for biotite and K-feldspar range from 0.9 to 1.0. The molar ratio value of 0.33 is the ratio of muscovite and alunite; illite is slightly less than 0.33. The Na2O/Al2O3 molar ratios range from 1 for albite to 0 for Na-free anorthite. The full range of values for plagioclase is given in Table 1. Minerals such as kaolin, chlorite, and epidote plot at coordinates (0, 0).

Included on the diagram are sev-eral joins representing commonly oc-curring mineral pairs, such as albitemuscovite and albiteK-feldspar. The join connecting the Na2O/Al2O3 value of 0.6 (oligoclaseandesine boundary) and K-feldspar (or biotite) is selected as appropriate for the lower limit of felsic volcanic rocks and granitoid in-trusions in that the plagioclases gener-ally range from albite to oligoclaseandesine. Consequently, most samples of unaltered rhyolites, rhyodacites, granites, quartz monzonites, and grano-

Alkali/alumina Molar Ratio Trends in Altered Granitoid Rocks Hosting Porphyry and Related Deposits J.F. Davies and R.E. Whitehead 15

case of the Sibert deposit, molar ratios calculated from only four bulk samples trace a progressive decrease in both Na2O/Al2O3 and K2O/Al2O3 values.

Bingham Porphyry Copper DepositNumerous studies of the Bingham porphyry copper

deposit have yielded a large amount of information and chemical data on hydrothermal alteration of the host Oligo-cene equigranular quartz monzonite and quartz monzon-ite porphyry, the two main phases of the Bingham stock (Moore and Nash, 1974; Bray et al., 1975; Lanier et al., 1975, 1978; Moore, 1978; and references within). The Bingham stock is part of a composite intrusion which also includes the Last Chance quartz monzonite stock and the Phoenix quartz monzonite dike. The Bingham stock is also cut by a number of latite and minette dikes. The possible significance of these dikes is considered below.

The copper orebody is hosted by the quartz monzonite porphyry and adjacent parts of the surrounding equigranu-lar quartz monzonite. The porphyry, which forms the core of the Bingham stock, is the most highly altered phase of the stock and is considered to have been the main conduit of the hydrothermal fluids responsible for mineralization and accompanying hydrothermal alteration.

Almost all of the Bingham stock has been altered to some extent. According to Lanier et al. (1978), the best analogue for the original composition of the equigranu-lar quartz monzonite might be the unmineralized and un-altered Last Chance quartz monzonite, which consists of orthoclase (30%) and plagioclase (30%), with the remain-der being augite, amphibole, and biotite.

Lanier et al. (1975, 1978) reported well-defined mineral-ogical alteration zoning in the equigranular quartz mon-zonite. The zoning is co-axial with respect to the quartz monzonite porphyry and copper orebody. Between 2200 and 3400 feet from the porphyry contact, hydrothermal ac-tinolite after augite comprises about 16% of the rock, and is accompanied by about 11% chlorite, which also replaces augite and, in part, hydrothermal actinolite. Lanier et al. (1975) reported about 5% magmatic phlogopite in the ac-tinolite-chlorite zone.

Quartz-orthoclase-phlogopite alteration extends out-ward for 2200 feet from the contact of the equigranular quartz monzonite with the quartz monzonite porphyry. Within this zone, up to 28% hydrothermal phlogopite re-places actinolite (Lanier et al., 1975, 1978). Hydrothermal orthoclase occurs here as rims replacing plagioclase, as ir-regular patches enveloping Na-rich plagioclase, and as re-placement perthites (Lanier et al., 1978).

Contrary to Lanier et al. (1975, 1978), Moore and Nash (1974) claimed that hydrothermal orthoclase is not a prom-inent constituent of the equigranular quartz monzonite, of which the quartz-orthoclase-phlogopite zone is a part. These contradictory statements underscore the problem of distinguishing between hydrothermal and magmatic feld-spars (and quartz) on the basis of textural interpretations.

Modal analyses by Lanier et al. (1975, 1978) re-veal a continuous decrease in plagioclase/total feldspar from the peripheral actinolite-chlorite zone toward and

Sericite to alunite: KAl Si O (OH) + 4H + 2SO KAl(SO ) (OH) + 3SiO3 3 10 2 + 4 4 2 6 22 (3)

Muscovite to topaz: 2KAl Si O (OH) +4H +4.8F

3Al SiO [(OH) F ] + 2K + 3 3 10 2

+

2 4 0.2 0.8 2+ 33SiO + 2.8(OH) + 2H O2 2

(4)

Albite (in Na-plagioclase) to K-feldspar: NaAlSi O + K KAlSi O + Na3 8 + 3 8 + (5)

Albite to muscovite: 3NaAlSi O + K + H KAl Si O (OH) + 3Na + 6SiO3 8 + + 3 3 10 2 + 22 (6)

Albite to Na-montmorillonite: 1.17NaAlSi O + H

0.5Na Al Si O (OH) + 1.67SiO3 8

+

0.33 2.33 3.67 10 2 22+ + Na

(7)

Na-montmorillonite to kaolinite: (8)Na Al Si O (OH) + 3H O

3.5Al Si O (OH) + Na +0.33 2.33 3.67 10 2 2

2 2 5 4+ 4SiO 2

Experimental data from Hemley and Jones (1964) show that the sequence K-feldspar sericite kaolinite results from progressive decreases in aK+/aH+ as a consequence of increasing aH+. Consequently, mixed assemblages con-taining, for example, K-feldspar and sericite or sericite and kaolinite are common. However, assemblages such as K-feldspar and clay minerals are generally excluded ex-cept in cases of extreme disequilibrium or when supergene processes are superimposed on the original hydrothermal assemblages.

In the following sections, data from various porphyry deposits are presented. It is important to point out that the petrographic descriptions, the identifications of the altera-tion assemblages, and the whole-rock analytical data from which the molar ratios were calculated, were all taken dir-ectly from the relevant referenced publications. The molar ratio patterns for each deposit are then compared with the interpretations presented by the original authors.

The chemical analyses used in the paper have been lim-ited mainly to papers published up to the 1980s because more recent papers contain few published whole rock an-alyses. It might be said that some of the deposits described in this paper also have too few samples. However, in the

Table 1. Approximate Molar Na2O/Al2O3 Values for the Plagioclase Series

Plagioclases Molar Na2O/Al2O3Albite 1.00.8

Oligoclase 0.80.6Andesine 0.60.33

Labradorite 0.330.12Bytownite 0.120.05Anorthite 0.050.00


clase-phlogopite alteration assemblages display consider-able overlap with one another and also with the least al-tered samples (Fig. 2b,c). This overlap is not unexpected because alteration of primary pyroxenes and amphiboles to actinolite and chlorite does not involve significant change in the alkali content of the rocks. However, many of the ac-tinolite-chlorite and quartz-orthoclase-phlogopite samples have molar K2O/Al2O3 ratios that overlap with the most altered assemblage (quartz-orthoclase-biotite; Fig. 2b,c). Considerable K2O must have been added to both the actino-lite-chlorite samples and the quartz-orthoclase-phlogopite alteration zone; this additional K2O most probably exists as hydrothermal orthoclase and phlogopite. If this is the case, the data would support the interpretation of hydrothermal alteration by Lanier et al. (1975, 1978).

The value of molar ratio plots in the study of alteration at Bingham resides in their ability to indicate possible chan-ges in mineralogy not readily recognizable by petrographic

examination alone. In the case of actinolite-chlorite altera-tion, the increase in K2O/Al2O3 and decrease in Na2O/Al2O3 relative to the samples of least altered rock might be a re-sult of K2O alteration of plagioclase to orthoclase (reaction 5), as well as the growth of hydrothermal phlogopite. The distinction between primary and secondary (hydrothermal) feldspars is not always a simple matter in altered rocks, and might be easily overlooked.

Several papers published in 1997 document the pres-ence of latite and minette dikes that cut the Bingham stock (Chesley and Ruiz, 1997; Deino and Keith, 1997; Keith et al., 1997). Deino and Keith (1997) suggested that minette magmas might have played a role in the petrogenesis of the ore-related intrusions at Bingham. Although this has no direct bearing on the central theme of the present paper, the somewhat unusual Mg-rich quartz-orthoclase-phlogopite alteration (Fig. 2c) might be related to minette intrusions and coeval shoshonite lavas.

into the quartz-orthoclase-phlogopite zone. Both quartz and total ferromag-nesian minerals, largely phlogopite, are greater in the quartz-orthoclase-phlogopite zone than in the peripheral actinolite-chlorite zone.

The quartz monzonite porphyry, which forms the core of the Bing-ham stock, has undergone extensive potassic (biotite-orthoclase) altera-tion, which is more or less coincident with the copper orebody (Moore and Nash, 1974; Moore, 1978). About 25% hydrothermal biotite is present mainly as aggregates of small flakes pseudo-morphing earlier ferromagnesian min-erals. In places, irregular intergrowths of orthoclase and quartz permeate the aplitic groundmass of the porphyry.

Pervasive sericite alteration was superimposed on the potassic zone of the quartz monzonite porphyry and to a lesser extent on the adjacent quartz-orthoclase-phlogopite zone of the equigranular quartz monzonite. Sericite partly replaces plagioclase and biotite but only to a minor extent; it has not affected orthoclase. Sericite consti-tutes about 5% or less of the rock and does not greatly affect K2O/Al2O3 val-ues compared to the effects of biotite and orthoclase.

Chemical analyses of the samples used by Lanier et al. (1975, 1978) to determine the zones of alteration dis-cussed above are plotted on alkali/alumina and MgO/alumina diagrams in Figure 2. The alkali/alumina char-acteristics of the least-altered Bingham samples are shown in Figure 2a. The actinolite-chlorite and quartz-ortho-

Fig. 2. K2O/Al2O3 vs. Na2O/Al2O3 molar ratios of least altered (a) and (b) variously altered Bing-ham equigranular quartz monzonite and quartz monzonite porphyry. c. K2O/Al2O3 and MgO/Al2O3 molar ratio plot of the same samples as in a and b. Least-altered samples are omitted from b; they would plot over the actinolite-chlorite and some of the quartz-orthoclase-phlogopite samples. Ab-breviations: act = actinolite, alt = altered, bio = biotite, chl = chlorite, monz = monzonite, orth = orthoclase, phlog = phlogopite, qmp = quartz monzonite porphyry, qtz = quartz.


Granite-Mo SystemsData compiled by Mutschler et al. (1981) on granites

hosting molybdenite deposits in the western U.S. illustrate the alkali/alumina molar ratio characteristics of a variety of alteration assemblages associated with many porphyry deposits. The host rocks of the molybdenum deposits are epizonal granodiorites, granite, and rhyolite porphyry of early Cenozoic age.

Only the granite and rhyolite porphyry intrusions are considered here; chemical data for the granodiorites are lacking. The phenocrysts in the porphyries are predomin-antly quartz and alkali feldspars, many of which are perthitic. Groundmass feldspars are sodic plagioclase and non-perthitic orthoclase.

Mutschler et al. (1981) report whole-rock analytical data from various Mo deposits in which they designate altera-tion assemblages as (a) moderate potassic, (b) strong potas-sic, (c) moderate quartz-sericite, (d) strong quartz-sericite, (e) intermediate argillic, and (f) strong argillic; these are shown in Figure 3. The molar ratio values correspond well for assemblages (a) to (d), representing first, K+ metasomat-ism to produce moderate potassic alteration (a), and then to strong potassic alteration (b), followed by H+ metasomat-ism to produce assemblages (c) to (f) in which H+ is added and alkalis are removed from the altered rock. Quartz and sericite of assemblages (c) and (d) can be produced by the addition of H+ to and removal of K+ from K-feldspar (reac-tion 1) or alternatively by the addition of both K+ and H+ to and the removal of Na+ from plagioclase (reaction 6).

Mutschler et al. (1981) do not comment on whether K-feldspar or plagioclase (or both) were involved in the pro-duction of the quartz-sericite assemblages. They do, how-ever, note that the argillic assemblages resulted from the replacement of plagioclase by clay minerals such as mont-morillionite, kaolinite, pyrophyllite, and dickite (some ex-amples are given in reactions 7 and 8).

Designation of the argillic assemblages as strong and intermediate is perhaps puzzling. Except in the presence of alunite, which is absent, the K2O/Al2O3 values for as-semblage (e) do not suggest moderate argillic alteration, nor does the presence of quartz and sericite. The explana-tion might reside in how Mutschler et al. (1981) classify strong, viz. 25 to 75 volume percent of the index clay minerals. Allowing for this, the molar ratio values cor-respond well with the description of the alteration assem-blages.

Zijinshan Copper-Gold Deposit, ChinaThe Zijinshan epithermal Cu-Au deposit occurs within

Jurassic biotite granites intruded by a Cretaceous dacite porphyry pipe. Both rock types display similar alteration and mineral zoning according to So et al. (1998). Only the granites and their ores are discussed here.

Phyllic alteration (sericite-quartz) is developed in the lower and outer parts of the deposit. Argillic alteration (mainly quartz-dickite) forms a narrow band between the phyllic and overlying alunite zone. Alunite alteration (quartz-alunite) overlies the argillic zone, is the largest of the alteration assemblages, and is host to most of the high-

sulfidation ore (mainly digenite and enargite). Alunite is described as replacing both sericite and dickite. The low-er phyllic and argillic zones contain pyrite, chalcopyrite, bornite, sphalerite, and galena.

The high sulfidation ores and alunite alteration are over-lain by a Au-bearing silica capping of quartz and opal re-sulting from extreme acid leaching. Inward and upward toward the center of mineralization, the spatial sequence of alteration is phyllic argillic alunite and silicic. This sequence parallels the paragenetic sequence as well as the change from low-grade Cu-Pb-Zn-Mo ores to high-sulfida-tion Cu ores, to dominantly Au ores in the silicic zone.

The sequence of alteration at Zijinshan is illustrated on the molar ratio alkali/alumina diagrams (Fig. 4). The se-quence weakly altered phyllic argillic (dickite) alunitic is common in porphyry deposits. Extreme acid leaching and production of a silicic capping is less com-mon. The molar ratio plot in Figure 4b reveals unusually high molar Na2O/Al2O3 values in the silicic zone, values which are not readily apparent from a cursory scan of the weight percent oxide values. Examination of weight per-cent oxides shows that the absolute amount of Na2O de-creases from 0.45 wt.% in the alunite zone (average of 5 samples) to 0.32 wt.% in the silicic zone (average of 2 samples); however, Al2O3 deceases more substantially, from 12.77 wt.% in the alunite zone (average of 5 samples) to 1.48 wt.% in the silicic capping (average of 2 samples), indicating that Al2O3 was diluted almost 8-fold in the ex-tremely altered silicic zone, compared to a factor of only 1.5 for Na2O. This is puzzling because Na2O is much more mobile than Al2O3. So et al. (1998) offer no explanation

Fig. 3. K2O/Al2O3 vs. Na2O/Al2O3 molar ratio plot of alteration in various Cordilleran intrusions hosting Mo deposits. The letters a to f indicate: a = moderate potassic alteration, b = strong potassic alteration, c = moder-ate phyllic alteration, d = strong phyllic alteration, e = moderate argillic alteration, f = strong argillic alteration.


for this incongruity. In any case, the unusually high Na2O/Al2O3 molar values only become readily apparent when the whole-rock analyses are converted to molar ratios.

Sibert Porphyry Cu-Mo Deposit, FranceThe Sibert deposit at Rhone, France, occurs in a porphy-

ritic granite that has been variably altered over an area of 1 by 2 km (Beaufort and Meunier, 1983). Although not a major deposit, it constitutes an uncomplicated example of alteration trends as portrayed by alkali/alumina molar ratio diagrams.

Early narrow quartz-orthoclase-pyrite-chalcopyrite-(molybdenite) veins were accompanied by pervasive po-tassic alteration. Transecting the potassic alteration are narrow (12 cm) quartz-sericite-pyrite veinlets bordered by phyllic alteration envelopes containing white micas re-placing the orthoclase, plagioclase, and biotite of the ear-lier potassic alteration.

Although the alteration envelopes are only 1 cm or less wide, where the quartz-sericite veinlets are abundant and form an interconnecting network, the phyllic alteration is pervasive over areas of several square meters.

Beaufort and Meunier (1983) described and interpreted zoning around the quartz-sericite veinlets in the follow-ing manner. In the outermost zone, orthoclase and biotite are mostly unaffected and plagioclase is only partially

replaced by sericite. Within the intermediate zone plagio-clase has been completely replaced by sericite, biotite has been partially replaced by phlogopite and sericite, where-as K-feldspar is only partially replaced by sericite. In the innermost envelope, closest to the veinlets, only quartz and sericite are present. This was interpreted as K-feldspar and phlogopite of the intermediate zone having been complete-ly replaced by sericite.

Figure 5 illustrates the molar ratios calculated from bulk whole-rock analyses reported by Beaufort and Meunier (1983). The plot traces the progressive increase of phyllic alteration from the early potassic assemblage to the inner-most quartz-sericite zone. The pattern portrays a continu-ous decrease in molar Na2O/Al2O3 and K2O/Al2O3 toward the innermost phyllic zone. However, this pattern cannot be reconciled with the interpretation based on field and petrographic evidence presented by Beaufort and Meunier (1983).

If the transition from the early potassic phase to the outer phyllic assemblage involved only alteration of pla-gioclase to sericite, leaving K-feldspar unaffected, K+ ions must have been added from the fluid. The decrease in K2O/Al2O3 suggests that this could not have been the case. From where, then, were the K+ ions obtained? Alteration of K-feldspar to muscovite might have released K+ ions (reaction 1) in sufficient quantities to be only partially consumed in the alteration of plagioclase (reaction 6). The remainder of the K+ ions would have stayed in the fluid phase. The end result would be a decrease in both Na2O/Al2O3 and K2O/Al2O3 in the solid phase of the outermost phyllic envelopes. This scenario, involving the alteration of both K-feldspar and plagioclase, is consistent with the molar ratio pattern. The process suggested by Beaufort and Meunier (1983), requiring addition of K+ ions from an external source is not consistent with the molar ratio pattern.

A second problem at Sibert concerns the composition of the intermediate phyllic assemblage. Beaufort and Meunier (1983) claimed that all of the plagioclase had been altered to sericite in the intermediate phyllic assemblage. Further-more, X-ray analysis of mica flakes did not reveal the pres-ence of paragonite in samples from the intermediate zone. The absence of plagioclase and paragonite imply the ab-sence of Na2O. Yet Figure 5 reveals a Na2O/Al2O3 value of about 0.25, suggesting the presence of unaltered plagio-clase.

The two discrepancies between molar ratio patterns and petrographic data might be attributed to misidentification of the feldspars, a not unlikely possibility considering the degree of sericitization imposed on these minerals. How-ever, this can only be determined by re-examination of the samples studied by Beaufort and Meunier (1983). In that context, it is perhaps worth noting that the study by Beau-fort and Meunier (1983) was concerned mainly with the composition and structure of the micas and not with the feldspars.

San Rafael Tin Deposit, PeruCassiterite lodes of the San Rafael stock are confined

to major shear zones within Late Oligocene peraluminous

Fig. 4. K2O/Al2O3 vs. Na2O/Al2O3 molar ratio plot of alteration at the Zi-jinshan Cu-Au deposit, China, showing alteration trends: a. weakly al-tered phyllic argillic; b. argillic alunite and argillic silicic.


granitoid rocks described as porphyritic quartz monzonite and granodiorite of the Andean Tin Belt (Kontak and Clark, 2002). These deposits are of Late Oligocene age. The main cassiterite lodes contain abundant gangue chlorite, which also occurs as relatively narrow (


The trends displayed in Figure 7a,b correspond closely to the mineralogical changes reported by Williams-Jones and Kontak (1998).

Discussion

Alkali/alumina binary molar ratio plots, unlike separate ternary chemical and ternary mineralogical diagrams, por-tray both chemical and mineralogical trends on the same diagram, thus facilitating interpretation of alteration data. K2O/Al2O3 versus Na2O/Al2O3 diagrams are particularly useful for rocks consisting dominantly of quartz, feldspars, and micas. Examples are altered rhyolites hosting VHMS deposits (Davies and Whitehead, 2006) and granites, quartz monzonites, and granodiorites hosting porphyry deposits.

In most of the cases considered in this paper, the chem-icalmineralogical trends portrayed by the molar ratio dia-grams agree well with the observed mineralogical changes accompanying alteration.

However, in some examples, the plots do not support the mineralogical changes reported: for example Bingham (Fig. 2) and Sibert (Fig. 5). Many samples from the actino-lite-chlorite alteration zone at Bingham have considerably higher K2O/Al2O3 values than the least-altered quartz mon-zonite, and some samples coincide with the quartz-ortho-clase-phlogopite alteration (Fig. 2). However, authors of papers on Bingham (e.g., Lanier et al., 1975, 1978) made

downward from greisen at the surface to weakly sericitized leucogranite at depth. The alkali/alumina data are plotted in Figure 7, where the sequence from weakly and per-vasively sericitized leucogranite through the three main stages of alteration are shown by arrows as follows: (1) K-feldspar to albite, (2) albite to quartz-sericite, and (3) sericite (muscovite) to quartz-topaz. Figure 7b shows the changes in K2O/Al2O3 relative to depth within the drillhole. The cassiterite deposit occurs between 0 m and 100 m from the collar of the hole.

The least altered leucogranite at about 850 m depth (Fig. 7b) was described by Williams-Jones and Kontak (1998) as a homogeneous assemblage of quartz, muscovite, albite, and K-feldspar. The leucogranite becomes increasingly sericitized upward as a result of replacement of K-feldspar by sericite. The decrease in K2O/Al2O3 in the leucogranite (Lg) as predicted by reaction 1 is shown in Figure 7a,b (more clearly on 7b). This decrease K2O/Al2O3 in the leuc-ogranite does not involve a significant increase in Na2O/Al2O3.

The initial alteration of sericitized leucogranite involved replacement of K-feldspar by albite (reaction 5, going to the left), resulting in a decrease in K2O/Al2O3 and a small increase in Na2O/Al2O3 (trend 1 in Fig. 7). The samples of rocks showing albitization, which contain quartz, musco-vite, and albite, lie on or near the albitemuscovite join of Figure 7a. Replacement of albite by muscovite and the for-mation of quartz-muscovite greisen is represented by trend 2, where K2O/Al2O3 increases and Na2O/Al2O3 decreases as predicted by reaction 6. Subsequent leaching of K+ ions from muscovite to form topaz (reaction 4) is manifested by trend 3.

Fig. 6. K2O/Al2O3 vs. Na2O/Al2O3 molar ratio plot of degrees of alteration in the upper 750 m of the San Rafael tin deposit, Peru.

Fig. 7. a. K2O/Al2O3 vs. Na2O/Al2O3 molar ratio diagram showing altera-tion trends (arrows) in leucogranite (Lg) hosting the greisen tin deposit, Kemptville, Nova Scotia, Canada. b. K2O/Al2O3 trends relative to dis-tance from collar of drillhole from which samples were taken.


no mention of a hydrothermal K-bearing mineral in the actinolite-chlorite zone. This suggests the possible failure to recognize introduced hydrothermal orthoclase.

The difference between the molar ratio and field-petro-graphic interpretations of alteration of the Sibert deposit (Fig. 5) are striking. If the petrographic interpretation mis-identified the feldspars, that is K-feldspar versus plagio-clase, the reason might have been the result of a sericite alteration that obscured the optical properties of the feld-spars. In any case, the molar ratio interpretation cannot be reconciled with the petrographic interpretation of the ori-ginal authors, Beaufort and Meunier (1983).

Kontak and Clark (2002) employed staining tech-niques in an attempt to alleviate the problem of identify-ing feldspars in a pervasively altered granitoid rock at the San Rafael lode tin deposit. That this approach appears to have been only partially successful is suggested by an al-kali/alumina molar ratio plot of fresh to strongly altered samples (Fig. 6). Pervasive alteration was mainly potas-sic with some isolated areas of albitization. Most samples plot where expected on the alkali/alumina diagram, but two samples of moderately altered rocks and one identified as strongly altered plot in the same region as fresh granite.

This study has shown that alkali/alumina molar ratio plots are a convenient way of correlating chemical and mineralogical characteristics in altered granitoid rocks hosting porphyry ore deposits and in identifying possible discrepancies between chemistry and mineralogical iden-tification.

Alkali/alumina molar ratio plots make a significant con-tribution to the study of hydrothermal alteration whether or not trace element data are available.

References

Barrett, T.J., and MacLean, W.H., 1991, Chemical, mass, and oxygen isotope changes during extreme hydrother-mal alteration of an Archean rhyolite, Noranda, Quebec: Economic Geology, v. 86, p. 406414.

Barrett, T.J., MacLean, W.H., Cattalani, S., Hoy, L., and Riverin, G., 1991, Massive sulfide deposits of the Noranda area, Quebec, III: The Ansil mine: Canadian Journal of Earth Sciences, v. 28, p. 16991730.

Beaufort, D., and Meunier, A., 1983, A petrographic study of phyllic alteration superimposed on potassic alteration, the Sibert porphyry deposit, Rhone, France: Economic Geology, v. 78, p. 15141527.

Bray, E.L., Lanier, G., and John, E.C., 1975, General geol-ogy of the open pit mine: Guide Book, Bingham Mining District, Society of Economic Geologists, p. 4859.

Chesley, J.T., and Ruiz, J., 1997, preliminary Re-Os dat-ing on molybdenite mineralization from Bingham can-yon porphyry copper deposit, Utah, in John, D.A., and Ballantyne, G.H., eds., Geology and ore deposits of the Oquirrh and Wasatch mountains, Utah: Society of Eco-nomic Geologists, Guidebook Series, v. 29, p. 165169.

Creasey, S.C., 1966, Hydrothermal alteration, in Titley, S.R., and Hicks, C.L., eds., Geology of the porphyry copper deposits-southwestern North America: Tucson, University of Arizona Press, p. 5174.

Davies, J.F., and Whitehead, R.E., 1994, Molar ratios in the study of unaltered and hydrothermally altered grey-wackes and shales: Chemical Geology, v. 111, p. 85100.

Davies, J.F., and Whitehead, R.E., 2006, Alkali-alumina and MgO-alumina molar ratios of altered and unaltered rhyolites: Exploration and Mining Geology, v. 15, p. 7790.

Deino, A., and Keith, J.D., 1997, Ages of volcanic and in-trusive rocks in Bingham mining district, Utah, in John, D.A., and Ballantyne, G.H., eds., Geology and ore de-posits of the Oquirrh and Wasatch mountains, Utah: So-ciety of Economic Geologists, Guidebook Series, v. 29, p. 9195.

Gresens, R.L., 1967, Composition-volume relationships in metasomatism: Chemical Geology, v. 2, p. 4755.

Hemley, J.J., and Jones, W.R., 1964, Chemical aspects of hydrothermal alteration with special emphasis on hy-drogen metasomatism: Economic Geology, v. 59, p. 538569.

Keith, J.D., Whitney, J.A., Hattori, K., Ballantyne, G.H., Christiansen, E.H., Barr, D.L., Cannan, T.M., and Hook, C.J., 1997, The role of magmatic sulfides and mafic al-kaline magmas in the Bingham and Tintic mining dis-tricts, Utah: Journal of Petrology, v. 38, p. 16791690.

Kontak, D.J., and Clark, A.H., 2002, Genesis of the Bon-anza San Rafael lode tin deposit, Peru: Origin and sig-nificance of pervasive alteration: Economic Geology, v. 97, p. 17411777.

Lanier, G., Folsom, R.B., and Cone, S., 1975, Alteration of equigranular quartz monzonite, Bingham District: Guide Book, Bingham Mining District, Society of Eco-nomic Geologists, p. 7397.

Lanier, G., Raab, W.J., Folsom, R.B., and Cone, S., 1978, Alteration of equigranular quartz monzonite, Bingham District, Utah: Economic Geology, v. 73, p. 12701286.

Lowell, J.D., and Guilbert, J.M., 1970, Lateral and verti-cal alteration-mineralization zoning in porphyry ore de-posits: Economic Geology, v. 65, p. 373408.

MacLean, W.H., 1990, Mass change calculations in altered rock series: Mineralium Deposita, v. 25, p. 4449.

MacLean, W.H., and Hoy, L., 1991, Geochemistry of alter-ation at the Horne mine, Noranda, Quebec: Economic Geology, v. 86, p. 506528.

Moore, W.J., 1978, Chemical characteristics of hydrother-mal alteration at Bingham, Utah: Economic Geology, v. 73, p. 12601286.

Moore, W.J., and Nash, J.F., 1974, Alteration and fluid in-clusion studies of the porphyry copper orebody at Bing-ham, Utah: Economic Geology, v. 69, p. 631645.

Mutschler, E.G., Wright, E.G., Ludington, S., and Abbott, J.T., 1981, Granite molybdenite systems: Economic Geology, v. 76, p. 874897.

Pearce, T.H., 1968, A contribution to the theory of varia-tion diagrams: Contributions to Mineralogy and Petrol-ogy, v. 19, p. 142157.

So, C.-S., Zang, D., Yun, S.-T., and Li, D., 1998, Alter-ation-mineralization zoning and fluid inclusions of the high sulfidation epithermal Cu-Au mineralization at Zijinshan, Fujian province, China: Economic Geology,


v. 93 p. 961980.Stanley, C.R., and Madeisky, H.E., 1994, Lithogeochem-

ical exploration for hydrothermal ore deposits using Pearce element ratio analysis, in Lentz, D.R., ed., Alter-ation and alteration processes associated with ore-form-

ing systems: Geological Association of Canada, Short Course Notes, v. 11, p. 193211.

Williams-Jones, A.E., and Kontak, D.J., 1998, Origin and evolution of the greisenizing fluids at the East Kempt-ville tin deposit, Nova Scotia, Canada: Economic Geol-ogy, v. 93, p. 10261062.

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