Porfidocu El Abra

Economic Geology Vol. 72, 1977, pp. 1062-1085

Geology of the E1 Abra Porphyry Copper Deposit, Chile JOZSEF A•IBRUS

Abstract

The E1 Abra porphyry COl)per deposit, located 42 km north of Chuquicamata, was mined on a small scale in the past, and several studies of the deposit were carried out prior to 1972. From 1972 to 1975, the Corporaci6n Nacional del Cobre de Chile completed an evaluation program known as the E1 Abra Project.

In common with several other major porphyry copper deposits in the region, E1 Abra is related to a regional structure known as the West Fissure and is eraplaced in Tertiary intrusive rocks of intermediate composition, which in turn form part of an intrusive and volcanic complex, the oldest members of which are Jurassic in age. The orebody is controlled by a west-northwest structural trend •vhich continues to the east where it is related to other minor hydrothermal occurrences.

The host rocks for the mineralization range from diorite to syenite. Late in the magmatic cycle, these rocks were altered, first by a period of K-feldspar-quartz addition and later by a period of strong biotization. The intrusive complex, called the E1 Abra diorite, is intruded by several dacitic necks which produced a remobilization of potassic alteration, developing biotite breccias on their tops. Age relations are obscured by early alteration stages, but it can be concluded that E1 Abra primary mineralization was almost synchronic with the Chuquicamata orebody. Primary mineralization related to potassic alterations exhibit high copper/iron ratios, with increasing iron in time. The dacitic porphyries have their own zoning pattern.

Hydrothermal alteration and mineralization is found inside the deposit as bands controlled by structures and as discontinuous occurrences in fringe zones. There, quartz veins with sericitic alteration in host rocks show a pyrite-rich assemblage, but only the roots of the phyllic zone are now exposed. Argillic and supergene alterations coexist with phyllic alteration in the same structures, being hydrothermal events obscured by this superimposition. The lack of pyrite and the basicity of host rocks had an unfavorable effect on supergene leaching and enrichment. However, an important oxidation blanket which has an area of one square kilometer covers primary sulfides. The dominant oxide mineral is chrysocolla.

Alteration and mineralization at E1 Abra has been interpreted as a deep exposure of a porphyry copper due to erosion of upper zones. A multiple intrusion-alteration-mineralization model is proposed, which is essentially the classical model repeated under slightly different conditions during time. The deep exposure of the orebody in a porphyry copper column in this deposit approaches a magmatic environment with the geo- metric patterns deformed by premineral structural control and by the complexity of the intrusion-alteration history.

Introduction

Er• ABe^ is a major porphyry copper deposit, located in E1 Loa Province, Chile, 42 kilometers north of the Chuquicamata open-pit mine (Fig. 1). It is emplaced in the same mountain range as Chuquica- mata at 4,000 meters above sea level. Two third- class roads connect E1 Abra with Chuquicamata--the first, 86 kilometers long passes through Conchi vil- lage and the second, 50 kilometers long, was recently built for direct access from Chuquicamata. The climate is a marginal desert climate which is affected by the high altitude, with cold nights and windy days, minor snow during July and August, and some rain during January and February.

The discovery of E1 Abra predates the Spanish

conquest as proved by some Indian workings, probably mined for turquoise and chrysocolla for orna- mental purposes. At the beginning of this century, several vein deposits were mined in the area, especially the Maria Vein by the British Compafiia Minera de Calama. Direct smelting ores were shipped to Conchi by a now dismantled railroad. Most mining in the area stopped around 1925, and at that time the Chile Exploration Company, a subsidiary of the Anaconda Company, bought the mining rights to the deposit. Several surface exploration programs were carried out by the Chile Exploration Company between 1945 and 1965 (Mulchay and Shephens, 1945; Brewer and Beltrfin, 1958; Hunt, 1965). This work led to the drilling of seven holes (Applegate,

1062

GEOLOGY OF THE EL •tBR.4 PORPHYRY COPPER DEPOSIT 1063

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1965-68). That work proved the existence of an important disseminated primary orebody below the outcropping oxide ore. :Between 1957 and 1970 more than 100,000 tons of high-grade siliceous copper ore were shipped to the Chuquicamata smelter for flux, and several vein structures were mined for this purpose in small surface mines. In 1971 the Chilean Government took over the Chile Exploration Company properties, including the E1 Abra prospect,

and in 1972 Chuquicamata's Geological Division started a new drilling program which included underground exploration and detailed surface mapping. About 33,000 meters of diamond drilling had been completed by mid-1975 together with 1,500 meters of tunneling and shaft sinking, plus numerous geological studies and metallurgical tests. More than 1.3 billion tons of ore has been proved xvith an average of 0.6 percent Cu grade on the basis of a 0.3

1064 JOZSEF AMBRUS

GEOLOGY OF THE EL ABRA PORPHYRY COPPER DEPOSIT 1065

percent Cu cutoff. This reserve includes a core of plus 1 percent Cu ore considerably in excess of 100 million tons.

The present paper is based on the final geological report completed in 1975 on the part of Chuqui- camata's geological staff headed by the author and including Luis Pdrez O. and •Ram6n Araneda G. (Ambrus et al., 1975). Mining rights covering E1 Abra and other Chilean porphyry copper prospects owned by Codelco-Chile (Corporaci6n Nacional del Cobre, a state agency) are at present under inter- national negotiation and the deposit might become a major copper producer within the next 10 years.

Geological Setting

The El Abra porphyry copper deposit lies on the southern slope of Cerro Pajonal (4,542 m) and is characterized by a copper oxide outcrop one kilometer square (Fig. 2) located in an intrusive complex of Tertiary age.

Most of the principal copper mines and prospects in the region, such as Chuquicamata, E1 Abra, and (•uebrada Blanca, are located along a narrow north- south trending band and are controlled by an apparently premineral structure known as the West Fis- sure. The West Fissure is a north-south regional fault whose relative movement is not yet well defined. It is known to be about 200 km long and is part of a major structural system that can be observed for several thousand kilometers along the western slope of the Andean range. A second alignment of porphyry-type deposits, represented by Mantos Blancos, Sierra Gorda, Lomas Bayas, and others (Fig. 1), could be defined in the Atacama desert, but it has not been determined whether it is controlled by a north-east faulting system as stated by Frutos and Oyarzfin (1974) or by structures older than, but parallel to, the West Fissure as suggested by Llau- met et al. (1975) at Andacollo, about 1,000 km south of E1 Abra.

At Chuquicamata the ore deposit is eraplaced in the eastern block in direct contact with the fault.

At E1 Abra, the orebody lies 2 km east of the trace of the West Fissure and is connected to it by a west- northwest-trending fault system which controls part of the alteration pattern in El Abra and extends beyond El Abra to the east-southeast, where a number of other minor copper deposits have been described as being controlled by the same system.

Sedimentary and volcanic rocks

The oldest stratified rocks cropping out in the area (Fig. 2) are a Jurassic sedimentary and volcanic series which is widely exposed around the El Abra intrusive complex, especially west of the \Vest Fis-

sure. Andesite lava flows and tuffs are interbedded with several sandstone and limestone horizons where

some fossiliferous beds are present, especially in the lower part of the series. The sedimentary beds disappear in the upper part of the series, which consists of andesitic breccias together with andesitic flows. The upper part of the Jurassic section has been correlated with the extensive Formaci6n La Negra (Thomas, 1967) of the Antofagasta region.

The Jurassic rocks are strongly folded and faulted in the E1 Abra area with fold axes trending north- south to N20øE. Contact and dynamic metamorphism have affected the rocks in the vicinity of intrusive bodies and faults. During the upper Terti- ary, after the main Andean uplift, the Jurassic volcanics and the middle Tertiary intrusives were exposed to erosion under semi-arid conditions and nearly one hundred meters of talus debris with interbedded mud flows was deposited in the lower parts of the paleotopography. In a few areas these sediments overlie mid-Tertiary limestone and conglom- erates, suggesting a wetter dimate in that period. Cenozoic volcanic activity began in the upper Mio- cene or lower Pliocene with the deposition of some irregular beds of volcanic ash, followed by a thick cover of rhyolitic ignimbrite (Formaci6n Altos de Pica).

In the (•uaternary most of the ignimbrites were eroded from the eastern block of the West Fissure, a thin talus cover was deposited on the flanks of the mountain ranges, and the bottoms of deep valleys were covered by the alluvium deposited by mud flows during the intermittent rainy seasons.

Intrusive rocks

The principal intrusive events in the E1 Abra area occurred during the upper Paleozoic, the upper Cretaceous, and the Tertiary. In general, basicity of the intrusives increased with time, as is also true in the Chuquicamata area.

Mesa granite: The Mesa granite, the oldest intrusive rock cropping out in the area, is found in the western edge of mapped area (Fig. 2). It under- lies the Jurassic volcanic and sedimentary series west of the West Fissure and tectonic events affecting the Mesa granite also affect the Jurassic series. The Mesa granite was described by Renzetti (1957) as the most sialic member of the Chuquicamata intrusive series. Unaltered Mesa granite has a distinc- tive reddish-orange color with a very low total mafic content usually less than 2 percent. The texture is a medium- to fine-grained allotriomorphic aggregate of orthoclase, microcline, low-An plagioclase, and rounded quartz blebs. A single Pb-alpha age dating, showing 292---30 m.y., was lnad½ i.n a

1066 JOZSEF AMBR US

quartz-eye porphyry cutting Mesa granite east of Chuquicamata (Munizaga, 1964). The Mesa granite described west of E1 Abra was correlated with its

Chuquicamata equivalent by petrographic and emplacement similarities (Thomas, 1967).

Light-colored granodiorite: The so-called light- colored granodiorite of Hunt (1965) crops out in the southern part of the E1 Abra intrusive complex (Fig. 2). It is characterized by a rough topography and intense fracturing. It is a medium- to coarse-grained granite to granodiorite with sparse porphyritic phases. The principal minerals are anhedral quartz, perthitic orthoclase, plagioclase approximately An 25, and less than 10 percent biotite and hornblende. Due to petrographic similarity and regional continuity, it can be correlated with the Paleozoic East Side granite of Chuquicamata.

E1 Abra intrusive complex: The E1 Abra complex is a regional unit composed of a large number of hypabyssal intermediate intrusive rocks which contain most of the mineralization and related hydrothermal alteration in the area. Inside the margins of the E1 Abra mineralized zone and in surrounding minor deposits, the rock ranges from diorite to syenite in an almost synchronic series, cut by porphyry stocks. Outside of the altered zones the rock is a granodiorite porphyry (called Southern granodiorite on E1 Abra detailed mapping), similar in many respects to the Fortuna granodiorite at Chuqui- camata.

The outcrops of this unit begin in a bleached zone dose to the West Fissure and continue east-southeast

in an irregular band about 4 km wide to disappear under Jurassic volcanics intruded by the rocks of the complex near the Conchi Viejo mine (Fig. 2). The rock appears in the field as a medium-gray, medium- to coarse-grained granodiorite with abundant porphyritic phases containing hornblende and plagioclase phenocrysts. The mafic minerals are usually weakly altered to chlorite. Under the microscope a weak potash metasomatism can be generally observed, with replacement rims of potash feldspar around plagioclases. Quartz is anhedral, usually as irregular phenocrysts but also as interstitial groundmass. No corrosion of phenocrysts by the groundmass has been observed.

Pajonal diorite: The Pajonal diorite occupies the top and upper slopes of Cerro Pajonal (Fig. 2), with sharp contacts with E1 Abra intrusives. This intrusive body is regular in shape and the sedimentary rocks on its northern boundary are affected by contact metamorphism.

The Pajonal diorite is completely devoid of the west-northwest-east-southeast structural system that dominates the area, and it appears in outcrop as

dark-colored, coarse-grained, weathered rounded boulders. Abundant zoned plagioclase, scarce interstitial quartz and orthoclase, plus a fairly uniform amount of hornblende and biotite are present in an allotriomorphic equigranular aggregate. This rock is believed to represent the last major intrusive event in the area.

Other intrusive rocks: Some minor rhyolite necks have been described near the West Fissure, but no detailed studies have been made on them. Andesitic

dikes are not uncommon in the light-colored granodiorite, and they also cut the Jurassic sediments and volcanics. However, they do not appear in Tertiary intrusives.

About 5 km east of E1 Abra a granodioritic intrusive stock cuts Jurassic volcanics. The granodiorite is a gray, equigranular rock, similar to the usual Andean Cretaceous granodiorites, but no detailed field work has been done to correlate it xvith

the rocks described in the area of the deposit.

Structure

As stated above, the West Fissure is the dominant structure in the area. Easily traced on aerial photo- graphs, it cuts a straight path N10øE through most of the rocks in the area, about 3 km from the western edge of the E1 Abra ore deposit. An eastern branch of the West Fissure has been observed southwest of E1 Abra. In the field it is sometimes difficult to

locate the fault precisely due to the intense shattering in the fault zone and the presence of Quaternary cover, but generally the fault zone can be identified by its topographic expression. The West Fissure, interpreted at Chuquicamata as a premineral structure, was subject to very recent movement. The upper Tertiary ignimbrite. s are almost completely eroded from the eastern block, while they form a fairly continuous layer in the western one. It is believed that the last movement on the West Fissure

was an uplift of the eastern side, but very little can be said about older movements.

The trend of the major folding axes affecting Jurassic rocks in the area ranges from N15øE to N25øE. Probably the intense faulting of the pre- Tertiary intrusives (light-colored granodiorite) was produced by the same tectonic stresses. The pre- vailing fracture system in the area is N40øW with a subordinate N40øE trend. These fracture systems and associated andesitic dikes affect both the light- colored granodiorite and the Jurassic volcanics.

The major ore-controlling fracture system trends N65øW, beginning in the vicinity of the West Fis- sure and extending along most of the Tertiary granodiorite to the Conchi Viejo area. The porphyry- type deposits occur along this trend, as do all the


copper-bearing vein systems between E1 Abra and Conchi Viejo. These structures are considered to be premineral weakness lines, which controlled first the emplacement of the Tertiary intermediate intrusives and later provided the openings for copper mineralization.

Ore deposits

The Abra porphyry copper deposit is the major ore deposit in the area, but other minor copper deposits are present and are probably genetically related to the principal center of mineralization.

Northwest of E1 Abra, in the vicinity of the West Fissure, there is an altered zone elongated north- south parallel to the fault that could be considered a potential orebody (Fig. 2). Several studies have been made covering an area about 6 km long and 2 km wide, but no clear evidence of mineralization has been found to date (Moreno and Vivallo, 1974; Love, 1974). Replacement by clay minerals and destruction of original textures affect both intrusive and volcanic sedimentary rocks. Neither quartz veining nor an alteration zoning pattern has been observed, and geochemical exploration has demonstrated background values throughout the area. Supergene leaching can be noted, probably affecting a fair amount of pyrite. On the northern edge of the altered zone, there are two small gold-bearing quartz veins which have been mined on a small scale in the past.

On the southern boundary of this altered zone a small deposit of exotic copper oxide is exposed; its origin is not yet clear. The deposit crops out over an area of 100 x 300 meters in the bottom of Ichuno

Valley. The main copper minerals are green and black chrysocolla, copper wad, and minor atacamite. The mineralization occurs mostly as cement in granodiorite debris and, to a minor extent, in fractures of the granodioritic bedrock. Since the deposit is about 2 km southwest of the E1 Abra mineralized area, in the past it was assumed to be copper mobilized from E1 Abra. However, no con- nection can be observed with the E1 Abra deposit, neither through paleodrainage nor through geochemical continuity. Recently, a strongly eroded, medium-size tourmaline breccia pipe has been described in the vicinity of the exotic body, and erosion of the copper mineralization peripheral to the pipe could be the source of the solutions for deposition of the exotic copper (Zamora, J. C., 1976, pers. commun. ).

With the exception of the E1 Abra, Maria Vein is the most important ore deposit of the area. It is located 1 km east of the southern end of the porphyry copper deposit and is a vein 1.8 km long, 2 to 4 m

wide (N65øW), emplaced in granodioritic porphyry. Weak chlorite-kaolinite •vall-rock alteration with

weak disseminated copper mineralization is present up to 10 meters from the vein. Two diamond drill holes demonstrated high-grade primary copper mineralization (chalcopyrite-bornite) at depths of 300 meters. An upper oxide zone and a deep secondary sulfide enrichment were intensely mined in the past.

To the east-southeast of the Maria Vein there are

a number of nilnor copper and copper/gold-bearing veins with the same strike (N60øW) all of which are much less important than the Maria Vein. The best known of these are in the Baquedano, Arturo Prat, and Huantajoyita groups.

Along the eastern edge of the E1 Abra intrusive complex where it disappears under Jurassic rocks (Fig. 2), there is a small porphyry copper-type deposit called the Conchi Viejo mine, described by Langerfeldt (1963) and Sillitoe and Neumann (1972). It is the easternmost expression of copper mineralization related to the E1 Abra system. The Conchi Viejo altered zone lies in granodiorite intruded by a quartz-eye porphyry and covers an area of 1,200 x $00 meters. The dominant hydrothermal alteration is argillic, with quartz-sericite spots. This zone is surrounded by a propylitic ring. Most of the alteration zoning is controlled by N60øW structures. Pyrite is the principal sulfide, and minor chalcopyrite is present. Spotty copper oxide as brochantite-atacamite is developed in veins close to the surface, and a high-grade supergene enrichment blanket, essentially chalcocite, was mined out in the past.

It can be concluded that an alignment of copper deposits oblique to the West Fissure represents the most obvious structural control of the majority of copper occurrences in the area, all of which are the products of a similar hydrothermal environment. So, while the West Fissure seems not to be directly responsible for the occurrence of the orebodies, a genetic relation cannot be excluded, and it may represent the primary structural ore control of the district. I

Geology of the E1 Abra Orebody

The E1 Abra orebody lies on a natural step on the southern slope of Cerro Pajonal, cut by four steep valleys, separated by 40- to 70-meter-high ridges. Mineralized outcrops extend from an elevation of 3,910 meters to an elevation of 4,100 meters. Many small surface workings have changed the original topography, the most important of which is the Ojo de Gallo mine (Fig. 3), where about a million tons of material has been removed.

Present knowledge of the ore deposit has been

1068 JOZSEF AMBRUS

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obtained through detailed surface mapping (about of 200 to 300 meters, four inclined diamond drill 90% of the area is rock outcrop), a grid (100 m x sections with 28 holes o• 350 to 500 meters depth 100 m) of vertical diamond drill holes with depths (one hole reached 1,000 m), and a 1,200-meter-long


adit connected to shafts of 56 meters and 97 meters, respectively, driven in the oxide zone of the deposit. Lateral boundaries of the ore deposit are well defined, but no indication of interruption of geologic patterns at depth was found.

Petro#raphy

Six major intrusive units, further subdivided into ten varieties, can be defined in the mapped area (Fig. 3). All of these units are very closely related in time, but intrusive relations can be observed in most instances.

S,o•tthern granodiorite: The Southern granodiorite occupies the southern and southeastern portion of the mapped area (Fig. 3) and is the equivalent of the regional E1 Abra pluton (Fig. 2). As previously described, porphyritic texture is usually present with evidence of weak potassic metasomatism even in propylitic alteration zones (Fig. 4). Con- tacts with other units are generally sharp and vertical, and the unit is locally intruded by most of the other rock types in the mine area. Farther away from the ore deposit, the Southern granodiorite becomes a uniform granodiorite porphyry, and other intrusives are lacking.

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FIG. 4. Photomicrograph of Southern gr•odiorlte showing anhedral quartz grains (Q) and a w•k replacem•t of orthoclase (Kf) on pla•oclase (P).

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E1 Abra diorite: This unit is the main host rock for mineralization at E1 Abra. Three textural varieties can be distinguished--a coarse-grained diorite (3 mm average grain size), especially abundant in central parts of the deposit, grading into a fine-grained diorite (0.5 mm average grain size) that tends to occur near the periphery of the deposit, and a biotite breccia related to dacitic porphyry intrusions. The term diorite does not reflect the wide

compositional variation of these rocks. Although the bulk of the E1 Abra diorite is a biotite-rich diorite, quartz diorites, monzonites, monzodiorites, granodiorites, and even granites and syenites are common (Fig. 5). Contacts between these petrographic varieties are usually gradual changes reflect- ing an increase or decrease of quartz, orthoclase, or marlcs. In some instances relative abundances of different rock-forming minerals define parallel west- northwest-east-southeast structures. Grain-size dif-

ferences and porphyritic phases have been observed defining the same lineaments.

The only unaltered E1 Abra diorite in the area occurs at a distance from the orebody, on the edges of the propylitic alteration zone. The model composition of the unaltered diorite is: plagioclase (oligo- dase-andesine) 60 percent, biotite 2 percent, orthoclase 5 percent, quartz 7 percent, chlorite-epidote 3 percent (as alteration products). Inside the altered zones, late magmatic alteration probably took place before many of the rock-forming minerals had crystallized, producing significant changes in the final textures (Figs. 6 and 7). The main mineralogical changes were the addition of biotite and/or orthoclase and quartz, partly as different types of veinlets,

1070 JOZSEF AMBRUS

FIG. 6. Photomicrograph of the El Abra diorite with advanced biotization (dots) of K-feldspar (Kf) and plagioclase (P). Quartz (Q) remains stable.

but mostly as interstitial grains or as replacement of former feldspars. The late magmatic potassium metasomatism also influenced the grain size of the diorite.

The strongest expression of biotite alteration at E1 Abra is found in hydrothermal breccias (Fig. 8), one variety of which occurs as a biotite-orthoclase pegmatite. Both occurrences of this rock are closely related to dacitic porphyry intrusions in the El Abra diorite. Breccia fragments are in all cases altered diorite, and the matrix is fine-grained biotite. All mapped breccias lie on the digitated tops of dacitic porphyry bodies. On the flanks of eroded porphyry bodies, diorite is transformed to pegmatite near the contact. These pegmatites are composed of coarse crystals of hydrobiotite intergrown with fine-grained rock fragments completely replaced by tiny orthoclase crystals. Biotite breccias and pegma- tire are both later than the period of potassium metasomatism that accompanied the intrusion and cooling of dacitic porphyries.

Quartz monzonite: Quartz monzonite at E1 Abra is represented by a single east-west elongate intrusion in the southwestern part of the mapped area and by a smaller outcrop 500 meters east (Fig. 3), where it is intruded by dacitic porphyry. In all instances, the quartz monzonite occupies topographic highs and is intrusive into the Southern granodiorite. Contacts are sharp, in many instances defined by steeply dipping fault segments with small displace- ments. Quartz monzonite differs from the E1 Abra diorite in its lack of mafic minerals, but diamond drilling reveals an increase of biotite in depth where the rock approaches the composition of some of the El Abra varieties. The texture of the quartz monzonite is hypidiomorphic equigranular with a 2-mm average grain size. Orthoclase and minor micro-

cline, usually microperthitic in texture, are the mait alkali feldspars. Plagioclase is subhedral with nor. mal zoning and quartz is anhedral interstitial. Som, brownish biotite flakes are locally present. An ap proximate modal composition of this rock is: alkal feldspar 30 to 35 percent, plagioclase 30 to 40 per cent, quartz 15 to 25 percent, biotite 0 to 5 percent Most of the outcrops of quartz monzonite are af. fected by a weak to medium pervasive phyllic altera tion, but propylitic alteration is also represented This alteration obscures the original texture in th, rock.

•lplitic dikes: Aplitic dikes are scattered aroun½ the edges of the main altered area and are intrusive only into the E1 Abra diorite and to a lesser exten' into the Southern granodiorite. The dikes lorn irregular to tabular bodies which are usually vertica without a preferred orientation. Many of these dike: have been displaced by west-northwest-trendin• structures, and some of them are cut by daciti• porphyry bodies. Contacts are sharp, alteratim rims in the intruded diorite are absent, and the dike•, invariably weather as a positive feature above th• intruded rocks.

Mineralogical -composition varies from alkal granite (Fig. 5), with variable amounts of perthiti• orthoclase and subhedral quartz grains in apliti• aggregates. Some dikes contain a small amount o: biotite (55½). In the northeastern part of the mappet area, some of the aplitic dikes contain a fair amoun• of plagioclase and grade into alkali-rich fine-grainec El Abra diorite. Aplitic dikes are not spatially re. lated to the main quartz-monzonite stock, althougt they have certain similarity to it. The principal dif. ferences are the finer grain size in the aplitic dike•,

FIG. 7. Photomicrograph of a syenitlc extreme of • diorite at El Abra. All gradations between this rock an( the one shown in Figure 6 •/re found.


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and the presence of plagioclase in the lnodal composition of the quartz monzonites. There is no good evidence regarding their relative abundance in depth, but some dikes were cut by deep drilling and are similar in composition to those observed on the surface.

Dacitic porphyry: Dacitic porphyry is distributed throughout the orebody and is second only to the E1 Abra diorite as a host rock for mineralization.

Dacitic porphyry intrusions form a major N10øE dike and many irregular minor bodies. Diamond

drilling has demonstrated that most of these porphyry intrusions are vertical with an upper digitated shape and a deeper dikelike form. The regular dike in the central part of the mapped area (Fig. 3) can be interpreted as a deep exposure of the same type of porphyry as the elongated bodies which crop out in the Ojo de Gallo area, and it is also the same as those irregular intrusives associated with hydrothermal breccias on the northern and southwestern edges of the orebody.

Textural and compositional differences due to

Fro. 9. Three varieties of dacitic porphyry. Fresh rock (left) has euhedral hornblende (black) and plagioclase (white) phenocrysts, in an aplitic fine-grained groundmass. Potassic altered (center) shows replacement of all marlcs by biotite and K-feldspar addition through veinlets. Propylitlc (right) has altered feldspars and chloritized marlcs.

1072 JOZoeEF AMBRUS'

Fro. 10. Photomicrograph of dacitic porphyry showing plagioclase phenocrysts (P) and quartz eyes (Q) with corrosion (arrow) by aplitic groundmass.

different late magmatic activity and different depth of exposures define three types of dacitic porphyry (see Fig. 9). The main porphyritic dike is a granodiorite porphyry with plagioclase, quartz, and fer- romagnesian phenocrysts. Plagioclase is anhedral, often zoned, and the dominant mafic mineral is hornblende, generally completely replaced by fine- grained biotite. Quartz eyes, consisting of rounded, unbroken single crystals, are moderately abundant. In general, these crystals are corroded by the groundmass. The groundmass is an aplitic, fine-grained (0.03-0.1 mm) aggregate of anhedral grains of quartz and plagioclass, fine-grained biotite, and interstitial alkali feldspar (Fig. 10).

The modal composition of the dacitic porphyry varies within the following ranges:

Phenocrysts Plagioclase 40-60% (50-85%) Quartz 5-20%

Biotite-hornblende 5 %

Groundmass Quartz 20% (30-65 %) Plagioclase .5-20%

' Alkali feldspar 0-20% Biotite 5%

A dark porphyry dike of limited extent, but of obviously different composition, cuts through the central portion of the main prophyritic dike. Petro- graphically, the dark porphyry is similar to the main dike, but the plagioclase is more calcic (An 30-40) and is invariably zoned, and the groundmass con- tains abundant fine-grained biotite. No alkali feldspar has been observed in this rock, and quartz eyes are rare.

The irregular porphyry bodies along the northern and southwestern edge of the ore deposit are essentially the same as the main porphyry dike in min-

eralogical composition and texture, although lighter in color due to clay and sericite replacement of feldspar and to diffuse and rounded masses of chloritized biotite in place of anhedral marlcs. At-depth many of these bodies change to a normal texture with anhedral biotitized hornblende Crystals, but pervasive alteration of the groundmass persists to deeper zones.

It is possible to conclude that each of the dacitic porphyry exposures at E1 Abra represents a separate porphyry copper system, with a deep core of. fresh- looking rock where potassic alteration is present and an upper apophysis of phyllic and propylitic alteration (Fig. 12). In every instance, the intruded diorite shows the effects of strong biotitization (biotite breccias on top and pegrnatites on the flanks).

The pattern of primary copper-iron mineralization coincides with the alteration zoning pattern in the porphyry (Fig. 20). Only pyrite is present in the uppermost zones. Chalcopyrite appears where biotite is stable, and in deep drilling, bornite-chalcopyrite mineralization appears and the ore grade increases considerably.

Pajonal diorite: The Pajonal diorite crops out in the northeastern corner of the area (Fig. 3) where it is characterized by the lack of alteration and structural patterns observed in the other rock types. The contact between the unaltered Pajonal diorite and the altered E1 Abra diorite is sharp. Propylitic alteration is absent, but a number of tourmaline-rich dikes are related to the contact zones.

Age dating relationships

A few K-Ar radiometric age dates have been made on E1 Abra biotite. Samples were prepared at the Universidad de Chile's Geological Department's laboratories. Age determinations were made at S5o Paulo University, Brazil, by Mr. Francisco Muni- zaga. Four of five biotite ages of different rock types range between 33.5 and 35.4 million years, with a considerable overlap of expected measurement error, so it is difficult to conclude specific age differences. Probably all of these determinations were made on "hydrothermal" biotites which could have been formed more or less. simultaneously in most of the rocks of the intrusive complex.

Biotite from the Pajonal diorite was slightly older (35.9---+ 0.7 m.y.) than biotite from rocks related to mineralization, and' in this instance, there is a rea- sonable certainty that determinations were made on magmatic-stage biotite. However, the expected error of the determination overlaps the apparent age of the altered rocks, and the determinations do not fit the relative ages of the different rock units and hydrothermal events established from the field relation-

ships (Fig. 11). Both radiometric age dating and


relative age relations suggest a close time relationship between magmatism and potassic alteration as well as between the different intrusives in the area.

Based on the age of the alteration biotite in mineralized rocks and pure magmatic biotites in later intrusives (Pajonal diorite), it is possible that the latest intrusion took place as the earlier magmas were still cooling and producing their first hydrothermal solutions.

At Chuquicamata, the two principal host rocks for mineralization, the east and west porphyries, are 33.6- 0.9 m. y. and 31.8- 0.9 m. y. old, respectively. These ages were determined in 1968 by K-At method in biotites from the potassic alteration zone. In both the E1 Abra and Chuquicamata deposits, late magmatic processes and related primary mineralization were probably simultaneous. The Fortuna granodiorite, found west of the West Fissure, has a K-At age of 35 ñ 1.0 m.y. and is the same rock type as the Southern granodiorite at E1 Abra (34.5 ñ 05 m.y., as determined on biotite that is probably an alteration product).

The Pajonal diorite at E1 Abra is virtually identi- cal to the Atahualpa diorite, located northwest of Chuquicamata. K-At dating on fresh biotite from the Atahualpa diorite indicated an age of 38.1 ñ 1.0 m.y. The Pajonal diorite is apparently slightly younger with a K-Ar age of 35.9ñ0.7 m.y.

Apart from the work on the probable alteration biotite, there has been no age dating on the hydrothermal minerals at E1 Abra. A K-Ar determination

on serfcite from the envelope bordering the Chuqui- camata quartz vein showed 30.3 ñ 1.2 m.y., which indicates an age difference between late magmatic processes and pure hydrothermal activity of about one or two million years. The same range of time has also been determined in other porphyry copper deposits such as Bingham (Moore and Lanphere, 1971). Assuming a similar age difference between intrusion and phyllic alteration at E1 Abra, hydrothermal activity should have affected most of the intrusive rocks in the area susceptible to alteration.

I/Fall-rock alteration

Potassic, phyllic, and propylitic alteration stages can all be recognized at E1 Abra, with potassic alteration the best developed of the three.

Potassic alteration: Most intrusive rocks in the

mapped area (Fig. 12) underwent late magmatic changes, the characteristics of which vary from one rock unit to the next. In this early alteration, K- feldspar and biotite were the principal alteration products. Quartz as an alteration product is less abundant in this assemblage than it is in phyllic alteration zones but is more abundant than in chlori-

Southern Granocl',orif e"

"KI Abrcz Diorite"

Quartz Monzahire

Aplitic Dikes

Dc•cit ic Porphyry

K-f- libor o TIME

Blot.

Blot.

K- felcl,

I

K-feld -Siot.

"Pojonol Diorl te"

In trusIon

•! K - Meto$omotJsm K- feld$1)or 8ioti te

I• Alteration on host rock

Fro. 11. Relative ages of intrusions and K + metasomatism at E1 Abra.

tized or fresh rocks. There is no mobilization of

iron at E1 Abra in potassic assemblages, and magnetite is a common mineral in biotitized diorites.

Ilmenite is also present in this stage and is usually subordinate to magnetite. Near the surface, hematite replaces magnetite, but the overall iron content remains on the order of 10 percent total Fe. Pyrite is absent in this alteration stage.

In the E1 Abra diorite, early K-feldspar alteration occurs mainly by replacement of plagioclase by an orthoclase or microcline perthitic rim (Fig. 13), generally mixed with minor clay minerals. Early quartz veinlets cut through diorites and although they have no visible alteration halo, petragraphically it is possible to observe that plagioclase has been altered to K-feldspar in the vicinity of the veinlets. In some areas orthoclase veinlets crisscross the rock, producing a rock of syenitic composition, with quartz and K-feldspar comprising most of the rock, with some altered plagioclase present as relict grains. In E1 Abra diorite, biotitization represents the late phase of potassic alteration and is generally superimposed on the early K-feldspar alteration. Two biotite generations are present, an early generation of euhedral brownish crystals which is cut by a late, greenish, fine-grained variety (Fig. 14), which occurs in hairline veinlets and as fine crystals disseminated through the rock. The biotite breccias and pegmatites are interpreted as extreme examples of this late-stage potassic alteration.

The weakest expression of biotitization in the E1

1074 J OZSEF .•IMBR US

.•oo

,

s c ,• L œ

o .•oo M, i

Fro. 12. Alteration zoning at E1 Abra porphyry copper deposit.


Abra diorite is the replacement of other ferromag- nesians, especially hornblende, by a fine-grained, brownish biotite aggregate.

The great variation in composition of the E1 Abra diorite (Fig. 5) is due to the relative importance of K-feldspar alteration and biotitization. Four different zones of potassic alteration can be defined in the E1 Abra diorite (Fig. 12). The first is a central core corresponding to the location of most high- grade copper mineralization, where potassic alteration is evidenced by coexistence of pervasive early K-feldspar alteration with pervasive later biotitization.

The second zone is characterized by an increase in biotitization. Sometimes an earlier K-feldspar alteration can be recognized, but the rock is now composed of fine-grained biotite, with quartz grains and little K-feldspar flakes. This type of potassic alteration occurs as an irregular ring around the central zone and is best developed on the northern and western sides of the deposit.

The third zone corresponds to the outer fringe of potassic alteration. Biotite content decreases and finally disappears and only K-feldspar alteration is present. The rock units affected by this alteration phase could be confused with acid intrusions (syenites or quartz monzonites) if it were not for the grada- tion from biotitized rocks at the inner contacts to normal diorite on the outer side. The abundance of

K-feldspar veins and the replacement of plagioclase also indicates that this rock has undergone appreci- able potassic alteration.

The previously described biotite breccias constitute the fourth potassic alteration zone but are related to the dacitic porphyry infusions raher than to the other potassic alteration zones in the E1 Abra diorite. At depth, the major potassic alteration zones tend

....... .;, • ,• ß - •. .- • . .:. . . ..%.• ,

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... , : . . . ß - ' • c; ' • ' •: .t • . ' ' ..•. • '. . •'

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Fro. 13. Photomicrograph of plagioclase (P) being replaced by orthoclase (Kf).

ß

1076 JOZSEF AMBRU• e

hornblende. Megascopically, such altered rocks have a fresh appearance and the original texttire has not been obliterated. Minor alteration products in this rock are rutile (pseudomorph of sphene), apatite, zircon, and anhydrite. In contrast to the potassic alteration suite in diorite, iron oxides such as magnetite and hematite are rare.

K-silicate alteration is generally restricted to the deeper portion of the dacitic porphyry and crops out only in the principal dike. Important primary copper mineralization appears about 400 meters deeper in this dike, accompanied by an increase of potassic alteration products, but even in this zone alteration pervasiveness is much less than in weakly altered diorites. Higher level dacitic porphyry exposures show a mixed replacement of amphiboles by biotite and chlorite, grading into propylitic assemblages. Intermediate plagioclase (oligoclase) in deep levels grades up•vard into more sodic compositions by developments of albitic rims.

_Phyllic alteration: Feldspar-destructive hydrothermal alteration at E1 Abra is represented by sericitic and advanced argillic alteration suites. The two types were not differentiated in the present study, since they generally occur in the same structures and argillic assemblages can be confused with the effects of supergene alteration, the importance of which is not yet well understood.

The structures controlling sericitic alteration are subparallel west-northwest-east-southeast lenses that dip north in the southern zone, are nearly vertical in the center, and dip south at very flat angles in the northern area (Fig. 20). The zones of sericitic alteration become narrower in depth and tend to disappear altogether more than 300 meters below the surface. For this reason it was initially sup- posed that all sericitic alteration might be supergene, but the increase of quartz and pyrite in these zones does not support arguments for a supergene origin.

Sericitic alteration affects all the rock units in and

around the ore deposit, including quartz monzonites and, locally, the Southern granodiorite. This ob- servation suggests that the main phyllic-stage hydrol- isis occurred as a single event late in the development of the intrusive complex rather than as a series of separate events following each intrusion. Local sericitic alteration of the dacitic porphyry may be an exception to this generalization.

Within the ore deposit sericitic alteration appears only along the west-northwest-east-southeast structures mentioned above, but at the periphery of the orebody (especially in the northwestern and southwestern fringes), a zone of sericitic alteration forms a spotty and discontinuous ring around the potassic zones (Fig. 12).

The structures controlling sericitic alteration at El Abra are essentially the same as the features known as "I)" veins at E1 Salvador (Gustafson and Hunt, 1975) and at Chuquicamata. "D" veins are large, steep quartz veins are braided veinlet zones, where pyrite is the dominant sulfide, sometimes oc- curring with subordinate chalcopyrite and enargite. The most characteristic feature of "D" veins is a

wide quartz-sericite envelope, many times wider than the quartz vein itself. In most of the rock units in the mineralized area at E1 Abra, these "D"-type veins occur as braided quartz veins a few meters wide separating silicified and brecciated country rocks. The sericitic halo is generally many meters wide, with quartz sericite and pyrophyllite near the vein and an outer zone of sericite and kaolinite that

grades outward into potassically altered rocks. The coexistence of sericite, kaolinite, and pyrophyllite indicates simultaneous "argillic" and "phyllic" alteration, perhaps with superimposed supergene effects. Probably this alteration developed at a higher tem- perature than would be typical of a pure quartz-sericite assemblage (Meyer and Hemley, 1967). The presence of this assemblage at E1 Abra may reflect the relatively deep level of exposure (Fig. 21).

Different rock types show different responses to sericitic alteration at E1 Abra. In biotitized diorites, alteration did not completely remove iron from the biotite and the resultant rock is a chalky mass with altered but recognizable mafics. Quartz is a major product of the sericitic alteration of K-feldspar, and feldspars are replaced by sericite without completely losing their original form. Alteration of quartz monzonite produces the same effect.

The dacitic porphyry seems to be more resistant to pervasive alteration than other rock units, and sericitic halos are poorly developed around quartz veins. However, dacitic porphyry is the only rock in the area in which a separate stage of phyllic alteration was developed. Immediately north of the Ojo de Gallo cut (Fig. 12), the porphyry groundmass is replaced by a uniform gray quartz-sericite aggregate in which the mafic phenocrysts are completely altered to a coarse aggregate of mica and chlorite. This occurrence seeins to be independent of the general zoning pattern displayed by the phyllic alteration elsewhere in the area.

Propylitic alteration: A poorly developed propylitic ring occurs around the E1 Abra ore deposit outside of the previously described alteration zones. The outer boundaries of this ring grade over a short distance into fresh rocks. The narrowness of the

propylitic alteration zone at E1 Abra is additional support for the argument that the deposit is deeply eroded (Fig. 21).


The time relationship between the propylitic, potassic, and sericitic phases of alteration is not clear. On a large scale, both sericitic and potassic alteration zones seem to be related to chloritization because

of their proximity and the continuity between them. Biotitized marlcs have undergone chloritization within the potassic zone, and chlorite and epidote replace biotite along veinlets in biotite breccias located southwest of the orebody. However, the coexistence of biotite and chlorite can be considered part of the potassic alteration suite in basic-intermediate rocks (Guilbert and Lowell, 1974).

Propylitic alteration of the E1 Abra diorite, the quartz monzonite, and the Southern granodiorite consists of albitization of plagioclase, variable chloritization of all marlcs, and chlorite, epidote, and specu- lar hematite replacement along veinlets. Quartz and K-feldspar appear unaffected in this zone, except in contact with chlorite-filled veinlets where K- feldspar is altered to kaolinire and chlorite. Chlorites of the propylitic zone have been identified as clinochlore and pennine, and they usually coexist with minor epidote and zeolites. Carbonates are uncommon but may occur as fracture filling in the Southern granodiorite.

The change from potassic to propylitic assemblages is in all instances accompanied by a decrease of total sulfides, and no increase in pyrite content has been noted. Chalcopyrite and bornite are the common sulfides in propylitic zones. Overall iron content does not increase over the normal levels of

the potassic alteration zone. Magnetite drops to background values only when propylitic alteration grades into fresh rocks.

The dacitic porphyry is bordered by a propylitic alteration zone that is unrelated to the alteration zon-

ing of the rock types mentioned above, and which is found above small intrusive fingers of porphyry. The rock has a characteristic population of rounded phencrysts (Fig. 9), consisting of fine-grained marlcs altered to chlorite (clinochlore). Crystal boundaries are diffuse and grade into the aplitic groundmass. The groundmass is also slightly altered to chlorite but consists predominantly of clay and sericite. Where this alteration zone in the porphyry is en- countered at depth, pyrite is the only sulfide present.

Tourmalinization: Small, irregular tourmaline breccias with silicified fragments are erratically distributed around the E1 Abra diorite and also cut the

Pajonal diorite outside the mapped area. Some of these tourmaline breccias are spatially related to irregular siliceous outcrops consisting of rounded, silicified rock fragments cemented by fine-grained quartz. No significant tourmaline has been found in the mineralized zones.

/

/ /

\ \

\ /

I •> 0.8 ø/o Cu in rock

• >0.$% Cu inrock

g

IL EI•EHID

7'• •> •.2•- I•inso,I [>ZOl•m)

• < •ol]' Pb insoil (•- I• ppm)

N •'000

NIOO0

Fro. 15. Surface mineralization and geochemical pattern at E1 Abra porphyry copper deposit (modified from Page and Corm, 1973).

Hypo!lene mineralization

The general distribution of copper, molybdenum, lead, and arsenic in the E1 Abra area was studied by Page and Corm (1973) utilizing soil and rock geochemistry (Fig. 15). The resulting pattern shows a copper-rich core with a peripheral increase of molyb- denurn from medium-high values in the core to very high values in the fringe zone of copper outcrop. The highest molybdenum anomaly does not completely surround the copper core and is centered southwest of the principal copper anomaly.

Leads show background values throughout the deposit, with some negative anomalies restricted to fringe zones and to the Pajonal diorite.

Arsenic is present in background amounts over the area of the copper zone, and a few positive anomalies occur along the southern boundary of the copper anomaly.

The study was extended over the Maria Vein where a major difference in geochemical pattern was observed: no molybdenum anomaly is present over this vein, but copper, lead, zinc, arsenic, and silver showed high values.

The above-described zoning pattern is characteristic of deep exposures in porphyry copper systems (Lowell and Guilbert, 1970). E1 Abra has a high- copper, low-sulfur central core, which correlates with the K-feldspar-biotite alteration zone and corresponds to early stages of mineralization. Surrounding this core is a later stage of mineralization with a higher sulfur/copper ratio. Most of this later ring is now eroded and this zone is discontinuous and narrow.

The earliest phase of this later mineralization is rich

1078 IOZSEF ,4MBRU$

ß •'*-" Cp

Bn

0.1 ram. I

Fro. 16. Photomicrographs of early sulfide assemblages in the E1 Abra porphyry copper deposit.

A. Chalcopyrite-bornite intergrowth; this is the most abundant sulfide assemblage throughout the ore deposit and is related to several early alteration stages, beginning with the K-feldspar stage. Generally it occurs disseminated.

B. Bornite-chalcocite-chalcopyrite, where chalcopyrite replaces chalcocite, which replaces bornire. It is a common assemblage in the high-grade core zone, but chalcopyrite or chalcocite might be absent.

C. Pyrite-bornite-chalcopyrite. Bornire appears at the edges of pyrite grains and chalcopyrite fills up the remaining space. It occurs in the northern part of the deposit, in advanced biotitization zones.

D. Pyrite-chalcopyrite, where chalcopyrite reacts with pyrite. It occurs in deep zones below bornite-chalcopyrite and also in the margins of the deposit, with molybdenite.

in molybdenum and the late hydrothermal stage is represented by an increase of lead and arsenic values.

The Maria Vein can be interpreted as a very late hydrothermal stage formed by crystallization of residual solutions in an open fracture.

The depth to hypogene mineralization at E1 Abra varies from 45 to 330 meters. The surface of the

top of sulfides is very irregular and is controlled by west-northwest-trending structures which roughly define channels of oxidation and supergene alteration. I-Iypogene mineralization at E1 Abra is largely related to wall-rock alteration.

Early ,nineralixation: The late magmatic K-feld-

spar and biotite alteration was accompanied by low- sulfur, high-copper disseminated sulfide mineralization. The earliest sulfide assemblage known at E1 Abra consists of an intergrowth of chalcopyrite and bornite (Fig. 16), in which chalcopyrite is generally dominant and is finely disseminated in the K-feldspar alteration phase of the E1 Abra diorite. This is the most abundant assemblage in the deposit and con- stitutes the host for subsequent stages of mineralization. Where chalcopyrite and bornite occur without subsequent introduction of other sulfides, the copper grade does not rise above 0.4 percent. This mineral assemblage does not appear to change appreciably


with increasing depth, although a pyrite-chalcopyrite assemblage appears near the bottom of DD-58, about one kilometer below the surface (Fig. 20). The marked uniformity of chalcopyrite-bornite mineralization throughout the E1 Abra diorite permits the assumption that this mineralization, together with late magmatic alteration, is a characteristic of this intrusive body.

Several sulfide assemblages are related to the biotitization of the E1 Abra diorite. Bornite-chalco-

cite-chalcopyrite (Fig. 16B) and bornite-chalcocite assemblages occur in the high-grade copper core zone. The grain size is coarser than in the first disseminated mineralization but this mineralization is also disseminated. Bornite seems to be the first

mineral to have crystallized, and it is surrounded by discontinuous rims of blue anisotropic chalcocite which are in turn surrounded by an outer rim of chalcopyrite. Bornite is generally more abundant than the other two sulfides in this assemblage. In those instances where chalcocite and/or chalcopyrite are absent, or present only in trace amounts, it can be difficult to distinguish the assemblage from the earlier K-feldspar-related ore. How- ever, the coarser grain size and the presence of chalcopyrite rims around bornite provided the main cri- teria for recognizing this type of mineralization.

The assemblage pyrite-bornite-chalcopyrite (Fig. 16C) is common in the northern and eastern edges of the deposit. Where superimposed on earlier "background" mineralization, good copper grades may result. Typically, bornite replaces pyrite, and the remaining mineralization consists of chalcopyrite.

The assemblage pyrite-chalcopyrite (Fig. 16D), with chalcopyrite replacing the margins of the pyrite, is common at the fringes of the biotitized zone and it seems to be a bit later in time than the previously described assemblage. This phase of mineralization is often accompanied by molybdenite and is spatially related to an assemblage of bornite, molybdenite, and chalcopyrite (Fig. 17) that represents the earliest appearance of molybdenite in the sequence of mineralization. The pyrite-chalcopyrite assemblage, with or without molybdenite, is considered to represent a stage of primary mineralization between potassium and hydrogen metasomatism.

The sulfide mineralization is fine to medium

grained and is usually disseminated through the rock, but it may also be present in hairline veinlets. In many instances, magnetite and ilmenite occur inside pyrite grains, a future which has not been observed in the previously described pyrite-bornite-chalcopyrite assemblage. Quartz veinlets without an ob- servable alteration halo may be present, generally in areas where molybdenite is present.

Fro. 17. Bornite-molybdenite-chalcopyrite is considered to belong to an intermediate stage between early and hydrothermal mineralization.

Pyrite-chalcopyrite-molybdenite is the dominant sulfide assemblage in deep zones of the quartz-monzonite intrusion in the southern part of the deposit. In this area, the mineralization is disseminated and in veinlets, and is not directly related to quartz veining. Minor bornite may be present.

Inside the boundaries of the K-feldspar and biotite alteration zones, molybdenum grades average only about one-third of the grades normal for other porphyry coppers. However, where this intermediate- stage primary mineralization is well developed, around the southern edge of the deposit, the molybdenum grade increased by a factor of five to ten.

Hydrothermal rnineralization: About 90 percent of the primary sulfide mineralization at E1 Abra be- longs to the early and intermediate stage, so that late-stage hydrothermal mineralization does not have a major influence on ore distribution. The sulfide mineralization related to this stage accompanies phyllic alteration in subparallel bands inside the potassic alteration zones and in discontinuous patches around the edges of the potassic zone.

The most important sulfide assemblages in this zone are:

mPyrite-bornite mPyrite-bornite-tennantite (sometimes galena) •Chal•opyrite-pyrite-sphalerite --Pyrite-chalcopyrite --Pyrite (alone or accompanied by ilmenite)

No enargite has been found in this zone, but significant arsenic assays and the presence of some olivenite in the oxide zone suggest that it might be present. Pyrite and enargite form a common assemblage in the Maria Vein about one kilometer east of E1 Abra.

1080 JOZSEiV •IMBRUS

Fro. 18. Pyrite-bornite texture, bornire replacing pyrite along cleavages. Chalcocite might be with bornire in some cases. It is usually found in veinlets located in dacitic porphyry, in the intermediate zone.

This mineralization is clearly superimposed on all the former stages and generally occurs as veinlets associated with hydrothermal quartz veins that cut the low-sulfur phases. The sericitic halos bordering these veins also contain disseminated sulfides typical of this stage. In some instances where pervasive alteration has affected major volumes of host rock, euhedral pyrite crystals fill cavities and rugs in the rock. Usually this pyrite is not associated with other sulfides. This hydrothermal-stage mineralization does not display an obvious zoning pattern, and it is interpreted as having formed in the root of a cupola of late hydrothermal activity where sulfides crystallized from solutions that became supersatu- rated through loss of water brought about by reac- tions with the wall rock.

Primary mineralization in the dacitic porphyry: As stated above, the dacitic porphyry at E1 Abra shows a separate pattern of alteration-mineralization zoning that cuts the pattern of intermediate primary mineralization developed in the E1 Abra diorite. Late hydrothermal mineralization along structural features affects the dacitic porphyry. The rock is not pervasively altered, and the mineralization is restricted to the controlling structures.

The uppermost mineralization assemblage typical of the dacitic porphyry is pyrite-chalcopyrite. Chal- copyrite rims and replaces pyrite along fractures and the iron/copper ratio increases upward without an increase in the total amount of sulfides in the rock.

Disseminated pyrite with trace amounts of chalcopyrite are generally the only sulfides present where fingers of dacitic porphyry appear with chloritic alteration.

The assemblage pyrite-bornite-chalcopyrite appears below the pyrite-chalcopyrite zone. The copper

grade increases in this zone. Individual sulfide a• gregates may consist of bornite and chalcopyri! (very common) or pyrite and bornite (Fig. 18), i which bornite replaces pyrite along cleavage plane.

The assemblage chalcocite-bornite-chalcopyrite a t pears deep in this rock unit. The sulfides are get erally disseminated and fine grained. Both blu digenite and gray anisotropic chalcocite are preset and are replaced successively by bornite and chalcc pyrite. Other assemblages present at the same deptlq are: bornite-chalcocite, chalcocite-ilmenite-bornit• and bornite-chalcopyrite. The latter is the mo•, abundant assemblage in deep levels of the daciti porphyry, as it is in the early stages of mineralizatio in the E1 Abra diorite.

The different sulfide assemblages in the daciti porphyry grade into one another and the coppe grade increases steadily with depth. Biotite breccia along the upper contacts of the dacitic porphyry an the biotite-orthoclase pegmatites generally contai coarse-grained bornite and chalcopyrite and dis semihated chalcocite-bornite mineralization, with close relationship between the sulfides and second ary biotite. No pyrite has been observed in thes breccias, although it is the most abundant sulfide i' the unbrecciated country rock adjacent to the brecci bodies.

Super#ene alteration

Supergene wall-rock alteration cannot be clearl distinguished from some phyllic-argillic assemblages, However, a considerable amount of supergene hy drolysis is superimposed on earlier hydrotherm• processes. Although supergene effects decreas abruptly below the top of sulfides, some obviou supergene obliteration of primary wall-rock texture have been observed at depths of 300 meters.

The depth of oxidation and penetration of super gene alteration at E1 Abra is indicated by the partia or total oxidation of magnetite to hematite. Ma.g netite at the surface is completely oxidized but ap pears as relicts with hematite (marrite) in deep pot tions of the copper oxide zone. Immediately belo• the top of sulfides, the rock becomes slightly magneti due to a rapid increase in the magnetite/hematit ratio, and deeper in the sulfide zone the martite tex ture disappears and magnetite is the dominant iro• oxide. Paralleling this change in oxidation of iro• is a strong kaolinization, often accompanied by mont morillonite. This clay alteration results in complet replacement of feldspars by kaolin, amorphous clays and by micas. The original marlcs are less altere, than feldspars. Copper oxides have been leache• from this zone, but some montmorillonite-coppe mixtures are usually present. Minor zeolites (leon


hardite-laumontite) are also common in this zone. The rocks affected by this alteration usually consists of a friable mass of day minerals in which the original boundaries between crystals are preserved. Super- gene alteration affects all rock types in, and surrounding, the ore deposit, but is deeper and better developed in coarse-grained E1 Abra diorite and in the Southern granodiorite. In the fresh porphyry the penetration of clay alteration is negligible. The southwestern portion of the orebody near the contacts of dacitic porphyry and quartz monzonite shows the deepest supergene alteration. The contacts between these rock units and E1 Abra diorites probably acted as natural dams for underground water (Fig. 3).

Super•7ene rnineralization

The lack of pyrite in the inner alteration zones of the deposit and the relative basicfry of the host rocks produced unfavorable conditions for secondary sulfide enrichment. Supergene activity was restricted to oxidation of primary sulfides, with minor trans- port of copper in supergene solutions and essentially in situ development of a copper-oxide blanket. A minor amount of copper reached a water table that fluctuated up and down with the rainy season and produced a mixed ore zone of partially oxidized primary sulfides and secondary sulfides. Only in those areas of quartz-sericite alteration, accompanied by fairly large amounts of pyrite, was some of the solution precipitated as secondary sulfides that are generally restricted to veins (Fig. 19).

Oxide zone: The most outstanding feature of the E1 Abra deposit is an area of about one square kilometer where copper oxides crop out at the surface; in the central zone of this orebody, rainable grades

of oxide mineralization are present over an area of about 800 X 400 meters to a depth of 100 to 150 meters. The lack of overburden and the fact that it

overlies the highest sulfide grades make this zone an obvious area for initial mining.

Oxide mineralization in the central zone is predominantly chrysocolla, with traces of atacamite, pseudomalachite, antlerite, and brochantite. The chrysocolla is the greenish-blue, copper-rich, iron- poor variety. Manganese-bearing black chrysocolla is abent. Mineralization occurs along fractures, as replacement of feldspars, and disseminated through the rock as replacement of primary sulfides. Major structures generally play an important role in ore distribution, and produce high-grade oxide veins. The bottom part of this oxide zone is under the present water table. The water is neutral and is in

equilibritm• with the oxide ore. The presence of chrysocolla in the oxidized zone

at E1 Abra may be due to the neutralization of a weakly acid supergene solution by reaction with the feldspars of the host rock.

A different type of oxide ore is present outside the central zone and is especially well developed in the southern part of the ore deposit. In this area, chrysocolla has in turn been 'altered. Subordinate manganese-rich chrysocolla (copper pitch) appears, and a copper-rich clay, in which copper is generally adsorbed in montmorillonite, becomes the second most abundant copper mineral. Minor amounts of cuprite, amorphous oxides, copper wad, and non- tronite are also present. Snpergene clay is the principal alteration product in this zone, and copper oxides appear to have been affected by a recent remobilization. This change occurred in a more acidic environment than existed during formation of chryso-

Fit;. 19. Typical sulfide assemblages related to quartz-sericite alteration. A. Quartz veinlet containing pyrite and chalcopyrite generally in individual grains. It is the primary assemblage. B. Near the top of sulfides, secondary enrichment of chalcocite occurs in veinlets, where all primary sniffdes except pyrite are replaced.

1082 JOZoeEF dtMBRUo e

+• / I

• K-FELDSPaR iND 810TITE ILTERiT•ON

LOCALLY SUPERGENE CLAYS) • WEAK PROPYLITIZ&TIO•

:1 3000 • OACITIC PORPHYRY • El •000

PRINCIPAL SULFiO[ ASSEMBLAGES • DO-71 SRILL HOLE NUMeER AND LOCATION OHALCOOl

(DRILL HOLES OF erie A•E NOT PLOTTED)

FIG. 20. Northeast-southwest section (A-A')

colla in the central zone, probably because of the higher pyrite content in the outer zones. Some leaching of copper took place, but present underground water has also been neutralized by wall rocks.

Mixed zone: A zone of mixed oxides and sulfides

is found between oxide and sulfide ore throughout the ore deposit. The host rocks in this zone are more altered to clay than the underlying rocks, and the copper mineralogy grades from an upper zone of altered chrysocolla, cuprite, tenorire, native copper, and amorphous oxides to a lower zone of weak chalcocite films on primary sulfides. Cuprite in the mixed zone is usually present as euhedral crystals filling cavities in the rock but also replaces secondary chalcocite. Rarely, it replaces chalcopyrite or bornite.

through ]El Abra porphyry copper deposit.

Native copper occurs generally along fractures and disseminated, together with cuprite or with secondary hematite.

The total thickness of the mixed zone is variable; in certain areas there are only a few centimeters of transition between oxides and sulfides, and at the eastern side of the deposit there is a maximum thickness of about 60 meters. Biotitized diorite is the host rock for most of this mixed oxide-sulfide mineralization. It is rare in dacitic porphyry and in K- feldspar-rich diorite. It has to be noted that most of oxide ore occurs in the same rock type. The mixed zone cannot be considered to be a secondary enrichment blanket. Copper grades in this layer are never higher than those found in the underlying

GEOLOGY OF THE EL .4BR/t PORPHYRY COPPER DEPOSIT 1083

PHASE I

Intrusion of Diorites [•1 in Gro•o(liorite • • development of' K- foldspot olterotion zone .[•:TI week propylitlzotionl=:::•

PHASE

Intrusion of DM:itic Porphyry ]• ond develol•ment of biotite breccias •

PHASE 2

+

+

+

-\ -

Biotltizotion of Diorites m preserving K-feldspar eltera-

tion in central zone •-1 Intrusion of Cluartz-monzonite :•

PHASE 4

a) Phyllic alteration I on top of previous alterorions, development of oropylitic alteration • structural control in the bottom.

b) IntruZlOn Of "Polohal Diorite" •

1 Km.

IDEALIZED NE-SW SECTION ON THE DEPOSIT

SHOWING PRESENT ROCK TYPE AND ALTERATION ZONES.

(• IE IN ID

SOUTHERN GRANODIORITE

D IORI TE(PAJONAL) K- FELDSPAR ALTI[ RATION

K- FELDSPAR AND BIOTITE ALTERATION

ADVANCEO BIDTITIZATION

QUARTZ MONZONITE

DACITIC PORPHYRY

BIOTITE BRECC•A

pHYII IC ALTERATION

PROPYLITIC ALTERATION.

FzG. 21. Proposed model for E1 Abra alteration and primary mineralization.

1084 JOZSEF ./tMBRU$

sulfides and in most cases loxver than the overlying oxides.

Conclusions

The relationship between wall-rock alteration and hypogene mineralization at E1 Abra and the zonal pattern of alteration phases show the usual porphyry copper patterns (Lowell and Guilbert, 1970; Rose, 1970).

The genetic model applied to E1 Abra is generally similar but somewhat more complicated than that envisioned for most porphyry copper deposits (Fig. 21). The variations are the result of the development of different degrees of "early" and "late" types of mineralization and alteration (Gustarson and Hunt, 1975) and variations in structural and litho- logical control (Guilbert and Lowell, 1974).

Two main features obscure or distort the usual

pattern of alteration and mineralization at E1 Abra. A premineral structural controI, that resulted in the emplacement of mineralized bodies along narrow belts such as the West Fissure and its subsidiary west-northwest-east-southeast system, distorted the normally circular or oval pattern of K-silicate and quartz-sericite alteration. The second feature is a sequence of multiple intrusions, each accompanied by an independent sequence of alteration and mineralization. At least two different intrusions, probably very closely related in time, proved to be individual porphyry copper systems.

An early, extended period of K-feldspar metasomatism took place, accompanied by a low-sulfur disseminated mineralization in the El Abra diorite.

Relations between this early alteration stage and pure magmatic processes are not clear enough to warrant separation into independent stages. This early stage was more extensive than the present ore deposit, and probably a weak chloritic ring was developed beyond it. Soon afterwards, residual solutions spreading beyond the center of the present high- grade deposit resulted in an iron-magnesium metasomatism that increased in intensity outward from the central core. The resulting pattern of early alteration zoning in the E1 Abra diorite consists of a K-feldspar and biotite-rich core zone, accompanied by higher copper grades, an increasing biotitization outward from the center where K-feldspar has been replaced, and an outer ring beyond the limits of biotitization where the former K-feldspar is the only alteration product (Fig. 20). The late-stage extreme biotitization at the top of dacitic porphyry intrusions in diorite can be interpreted as remobilization of high potassium-iron-magnesium solutions and their invasion into the cooling diorites. The primary mineralization accompanying these alteration stages is of the type generally attributed to the early stages

of the development of the core zone of porphyry copper deposits. A slight increase with time in the copper/iron ratio has been suggested.

The zoning pattern of the dacitic porphyry is clearly superimposed on all the diorite zoning patterns and mineral assemblages, with each individual dacite porphyry intrusion constituting a separate system, with a chloritic roof, chloritic-sericitic-pyritic neck, and a fresh to potassic root.

Not enough data are available to establish a definite relationship between the southwestern quartz-monzonite intrusion and the El Abra diorite, but the two units could be closely related. The well-defined increase in molybdenum in, and close to, this unit might be considered as an early- to intermediate- stage zoning with respect to the diorite.

All rock types involved in the early stages of alteration and mineralization were affected by the main period of hydrothermal alteration and mineralization, although the different rock units show different effects. The presence of phyllic alteration assemblages inside earlier alteration zones, and the relatively narrow and discontinuous phyllic ring around the earlier zones, can be interpreted either as deep emplacement of the porphyry copper or as a deep erosion level in the system. In either event, the scarcity of phyllic exposures at E1 Abra demon- strate a relatively low ratio of meteoric to magmatic hydrothermal solutions, at least at the presently exposed depth. Regional geological data support a deep erosion of the whole range, since middle Tertiary time. In addition, the vertical changes in the dacitic porphyry are suggestive of rather shallow emplacement. Therefore a hypabyssal eraplacement can be accepted, with deep erosion of the original porphyry copper system to produce the patterns now exposed at the surface.

A deep, low-grade central assemblage has not been observed at E1 Abra, but magnetite throughout the biotitized zone might be suggestive of such a core zone. A classical deep assemblage might exist at depth. In the deepest drilling in the deposit the chalcopyrite-bornite assemblage of the main ore zone is replaced by a pyrite-chalcopyrite assemblage about 900 meters below the present surface.

The very late hydrothermal stages at E1 Abra are now largely missing except for a poorly developed area restricted to the southern part of the deposit. It is possible that such late-stage zones were once present at E1 Abra, but have now been removed by erosion.

Acknowledgments

The author wishes to thank the Directors of the

Corporaci6n Nacional del Cobre de Chile for per-

GEOLOGY OF THE EL ./tBR./t PORPHYRY COPPER DEPOSIT 1085

mission to publish this paper. The work on which this paper is based would not have been possible without the support of that Corporation, the en- thusiastic interest of Mr. Andrds Zauschquevich, Production Vice President, and Mr. Nicol/ts Tschis- chow, General Manager of the Chuquicamata Divi- sion.

The E1 Abra Project was carried out with the permanent and active participation of project geologists Luis Pdrez and Ram6n Araneda, but most of the geological staff at Chuquicamata also cooperated in it, as did several other Chilean institutions. The author is indebted to all of them.

Special thanks are due to Mr. J. David Lowell and John G. Stone for critical review of the manuscript and Mr. Alvio Lagos for aid in preparation of the paper. The drawings were prepared by Mr. Carlos Muffoz.

SUPERINTENDENCIA DE GEOLOG•A CODELCO-CHILE

D•WSI6N CYIU•UICAMATA C•u•uic•, C•i•.

January 12, March 15, 1977

REFERENCES

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