International Geology Review, Vol. 3i. 1995. p. 945-958. Cop~' righl Cl 1995 h ~' V. H. Winslon & Son. Inc. All righl s reserved.

Evolution of an Active Ductile to Brittle Shear System Controlling Mineralization at the Chuquicamata

Porphyry Copper Deposit, Northern Chile

DARRYL D. LINDSAY, MARCOS ZENTILLI,

Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, B3H 3J5, Canada

AND JOSE ROJAS DE LA RIVERA

Superintendencia de Geologia Codelco·Chile, Division Chuquicamata, Chuquicamata, Chile

Abstract

The giant Eocene-Oligocene Chuquicamata porphyry copper deposit in northern Chile illustrates a remarkable degree of structural control from the time of porphyry emplacement through mineralization-alteration and supergene enrichment. The porphyry system developed within a transtensional fault-bend related to the> 2000-km-long, trench-parallel shear system known as the Precordilleran Fault System, which has been active in the Precordillera since at least the Eocene.

Detailed structural sampling and mapping at scales from 1: 10 to 1: 1000 within representative mineralized structural domains defined within the Chuquicamata open pit reveals that the bulk of mineralization is contained in fractures with defined preferred orientations that impart an anisotropy to ore grades at certain scales. Cross-cutting relationships in one domain, Estanques Blancos, establish a relative chronology of alteration-mineralization pulses related to an older (34 Ma), deeper potassic and a younger, superimposed (31 Ma), shallower quartz-sericitic event. The timing of these mineralizing events is supported by argon geochronology. Although there is evidence of shear-related ductile deformation from pre-porphyry times, fracture permeability was readily accessible to intrusions, hydrothermal fluids, and supergene waters at this localized extensional site, which may explain the remarkable copper concentration in Chuquicamata.

Kinematic indicators are compatible with a local N-S dextral shear system during the bulk of mineralization. Fabrics range from ductile through ductile-brittle and brittle, followed by regional sinistral brittle events within the Domeyko Fault system.

Introduction

loCATED BETWEEN THE Central Longitudinal Valley and the Western Cordillera, the Chilean Precordillera was the site of the narrow Andean magmatic front from the late Cretaceous to early Oligocene (Maksaev and Zentilli, 1988; Maksaev, 1990; Scheuber and Reutter, 1992). A dominant feature of the Precordilleran region is the Precordilleran fault system (Reutter et aI., 1991, 1993). This system, traceable for over 2000 km, is composed of various regional fault segments, each having undergone a distinct series of deformation events (Tomlinson et aI., 1994; C. Mpodozis, pers. commun., 1994). The segment in the Antofagasta region is known as the Domeyko Fault System (Maksaev, 1990), but often is referred to as the West Fissure, West Fault, or Falla Oeste after a dominant branch that traverses the Chuquicamata dis-

0020·6814/95/l52/945-14 $10.00 945

trict. Despite variations in tectonic history of the independent segments, many of the por­phyry copper deposits in northern Chile lie within the domain of this regionally extensive fault system.

Chuquicamata, one of the world's largest producing porphyry copper deposits, is inti­mately associated with, and truncated by, the Falla Oeste, or West Fissure, a dominant seg­ment of the Domeyko fault system. Related to the Falla Oeste fault zone, within the Chu­quicamata open pit, are several other fault sys­tems that influence district geology. The angular relation of each fault system with the Falla Oeste has provided an influence on struc­tural interpretations throughout the mine's production history. These models include a mineralized horse-tail fault termination (Lopez, 1939), a transtensional fault zone

946 LINDSAY ET AL.

241<1....1--1---::::

9 59 'qO km

')

~ BOLIVIA

~" -'"'" rif----+----'t-23°

I/"~ 68° 1,,\ ARGENTINA

2S'o--+--+-----'!..-j--"'::· -2S" ._.{ \ 26,o....:+r::::::::::t---j"'-+--T- 26°

EI Salvadore, (" \ ,.pot~eriIlOS\ 700 69°

e PORPHYRY COPPER DEPOSIT

FIG. 1. Location of the Chuquicamata porphyry copper deposit within the Antofagasta region of northern Chile in relation to the Domeyko fault system and other porphyry copper deposits (modified after Maksaev, 1990).

(Maksaev and Zentilli, 1988; Maksaev, 1990), and a Riedel strike-slip system (Martin et aI., 1993).

Recently, a variety of surficial mapping proj­ects have been undertaken, including struc­tural mapping and alteration and sulfide distribution, to provide a real base for the reno­vation and affirmation of the Chuquicamata geological model. We report here some findings from recent mapping and characterization of the fault systems, associated alteration and sul­fide mineralization, and detailed structural sampling that provide new insights into the complex structural evolution of this deposit and that may explain its great size and richness. In Chuquicamata, where most of the hypogene mineralization is contained within fractures, detailed observations within structural domains show that mineralized veinlet and stockwork systems are not chaotic vein net-

works, but have consistent and measurable ori­entations that impart strong anisotropies to ore grades.

Regional Geology

A control on stock emplacement

The localization of Andean porphyry deposits appears to be related in some manner to the Precordilleran fault system. Specifically, the porphyry copper deposits of northern Chile have a notable linear distribution (Fig. 1) that coincides with the domain of the Domeyko fault system (Maksaev and Zentilli, 1988). This fault system has been interpreted as an internal failure of the South American plate generated by the oblique convergence of the Nazca plate into the continental plate margin (Hollister, 1978). The fault-convergence relation is inter­preted as a trench-linked strike-slip fault sys-

946 LiNDSAY ET AL.

241"--f--t---::::

9 59 'qO km

')

~ BOLIVIA

~"-'"". rif----+----'j-23°

; / - 68"

1,"\ ARGENTINA 25,o-4--+--..!-+---':;-· 25" -_of

\ \

26,O"":+r:::.:::::::t-----j~+--T- 26" EI Salvadore' (

\ ,.pot~eriIlOS\ 700 69°

e PORPHYRY COPPER DEPOSIT

FIG. L Location of the Chuquicamata porphyry copper deposit within the Antofagasta region of northern Chile in relation to the Domeyko fault system and other porphyry copper deposits (modified after Maksaev, 1990).

(Maksaev and Zentilli, 1988; Maksaev, 1990), and a Riedel strike-slip system (Martin et aI., 1993).

Recently, a variety of surficial mapping proj­ects have been undertaken, including struc­tural mapping and alteration and sulfide distribution, to provide a real base for the reno­vation and affirmation of the Chuquicamata geological model. We report here some findings from recent mapping and characterization of the fault systems, associated alteration and sul­fide mineralization, and detailed structural sampling that provide new insights into the complex structural evolution of this deposit and that may explain its great size and richness. In Chuquicamata, where most of the hypogene mineralization is contained within fractures, detailed observations within structural domains show that mineralized veinlet and stockwork systems are not chaotic vein net-

works, but have consistent and measurable ori­entations that impart strong anisotropies to ore grades.

Regional Geology

A control on stock emplacement

The localization of Andean porphyry deposits appears to be related in some manner to the Precordilleran fault system. Specifically, the porphyry copper deposits of northern Chile have a notable linear distribution (Fig. 1) that coincides with the domain of the Domeyko fault system (Maksaev and Zentilli, 1988). This fault system has been interpreted as an internal failure of the South American plate generated by the oblique convergence of the Nazca plate into the continental plate margin (Hollister, 1978). The fault-convergence relation is inter­preted as a trench-linked strike-slip fault sys-

MINERALIZATION AT THE CHUQUICAMATA COPPER DEPOSIT 947

tern, as defined by Sylvester (1988). Wollard (1973) infers that extensional conditions pre­dominate above the subduction zone within the overriding continental plate, thus allowing for the emplacement of magmas. Continued exten­sion at higher structural (epizonal) levels, in the presence of fluids, leads to the formation of vein and stockwork systems whose orientations are controlled by local fault systems that comprise structural -domains. Williams (1994) suggests that changes in plate convergence, movement along trench-linked strike-slip faults, and the relatively short duration of fault movement are the principal focusing mecha­nisms of mineralizing fluids and, therefore, are primary controls on the size of northern Chilean porphyry copper deposits.

The two main branches of the Domeyko fault system (Fig. 2) in the Chuquicamata district are the Falla Oeste (West Fissure) and the Falla Mesabi (Mesabi Fault) . These faults border the Chuquicamata porphyry deposit to the west and east, respectively. Each fault zone reveals a partial deformation history for the system as a whole.

The Mesabi fault displaces volcano-sedimen­tary sequences consisting of Triassic siliciclas­tics interlayered with acid to intermediate volcanics (Chong and Pardo, 1993) and Jurassic marine sedimentary to Cretaceous continental volcaniclastics. This rock sequence was folded into N-S-trending anticlines probably during the Eocene Incaic tectonic phase (Reutter et aI., 1991, 1993) . Shearing parallel (or sub­parallel) to a limb of a volcano-sedimentary antiform entrained elongate slivers and blocks of the sequence into the Mesabi fault system. Dextral deformation within these units has been determined by mesoscopic ductile fabrics found within zones of strongly deformed marine sediments adjacent to the Chu­quicamata intrusive complex. Activity of the Mesabi fault system occurred before, was asso­ciated with, and probably assisted the emplace­ment of the host rocks of the Chuquicamata deposit. Evidence for this includes: (1) ductile deformation of Chuquicamata intrusive rocks ; (2) the intruding and cross-cutting apophyses of the Chuquicamata complex into the Mesabi deformation zone; (3) large xenoliths of quartz­vein fragments and deformed metasedimentary and metavolcanoclastic rocks within the Chu-

FIG. 2. Schematic representation of the major fault systems-Falla Oeste (FO). Estanques Blancos (EB) . and Mesabi (Ms)-within the Domeyko fault system at Chuquicamata.

quicamata complex; and (4) fault veins related to late hydrothermal mineralization that dis­place the principal trace of the Mesabi system.

The regionally prominent Falla Oeste places a non-mineralized Eocene granodioritic (For­tuna) complex in contact with the highly altered and mineralized Late Eocene­Oligocene Chuquicamata porphyry complex. Displacement-sense indicators within the black, clay-sized gouge of the Falla Oeste indi­cate sinistral strike-slip (Reutter et aI., 1993). This fault zone, characterized by various fault breccias (crush, attrition, implosion), includ­ing fine, black gouges, was active after emplace­ment, mineralization, and even supergene enrichment. It is possible that the fault gouge material, although proposed as an open sys­tem(s) by O'Neil (1984), may have acted, at some stage of mineralization, as an imperme­able barrier limiting the extent of alteration­mineralization (Anderson et aI., 1983). The Falla Oeste fault zone also may have been active during porphyry emplacement and mineraliza­tion, but this remains uncertain.

The Mesabi and Falla Oeste fault systems probably localized intrusive and hydrothermal activity during Eocene-Oligocene time. These faults were plausibly active as a subvolcanic arc­fault pair during the Late Cretaceous, focusing volcanic activity and plutonism within the Domeyko Cordillera at transtensive fault jogs (Maksaev and Zentilli, 1988) .

948 LINDSAY ET AL.

Fortuna Intrusive Complex

I San Lorenzo porphyries Tetera porphyry aplites Fiesta Fa Granodiorite

N5000

o 8 "<t Z

8 r'1 Z

N

N6000

N5000

N4000

FIG. 3. Simplified distribution of units in the Fortuna and Chuquicamata intrusive complexes.

Local Geology

The Chuquicamata deposit (Figs. 1 and 3) is located in the southern portion of the elongate (14 x 2 km) N100E-trending Chuquicamata intrusive complex. The Eocene-Oligocene (ca. 35 Ma) (Maksaev, 1990; Zentilli et aI., 1994) complex consists of three main lithological units-the Porfido Este, a matrix-poor monzo­granitic porphyry with an interstitial ground­mass; the Porfido Oeste, a monzogranitic porphyry with an aplitic groundmass; and the

Porfido Banco, a matrix-rich monzodioritic porphyry with an aphanitic groundmass (Ara­cena, 1981; Maksaev, 1990; Martin et aI., 1993). Eastward, the complex has an obscure relation­ship with the Cretaceous (?) (122 Ma, K/ Ar) Elena Granodiorite (Munizaga, 1976) that has intrusive contacts with Triassic through Cre­taceous metasedimentary and metavolcanic rocks. To the west, a regional cataclastic/ gouge­fault zone, the Falla Oeste, separates the por­phyry complex and the Chuquicamata deposit

MINERALIZATION AT tHE CHUQUICAMATA COPPER DEPOSIT 949

f (

I f>.Q I

~ as I

I

~ Sf< f1, I<K

Alteration

~~~~:~c~~~~;~~tive Sericitic

Sericitic overprinting potassic Chloritic over potassic

Potassic

Z 0-o 8

z VI o 8

z ~ 0 0 0

FIG. 4. Simplified distribution of alteration zones at Chuquicamata showing structural control in the northern end by Estanques Blancos and in the southern end by the Falla Oeste system.

from the non-mineralized Fortuna intrusive complex (35 to 36 Ma) (Maksaev, 1990).

The Chuquicamata deposit is concentrated in a zone of pervasive quartz-sericite alteration (ca. 31 Ma) (Zentilli et aI. , 1994) immediately east of the Falla Oeste (Fig. 4). It is controlled in the northern part of the deposit by NE-trending fault zones and vein arrays and is limited to the east in the southern portion of the mine by N-S-trending gouge-bearing faults. This altera­tion zone grades eastward into and variably

overprints a K-feldspar-biotite alteration zone (ca. 34 Ma) (Zentilli et aI., 1994). Potassic alteration gives way to a zone of chlorite-magne­tite-specularite(-epidote) alteration in the east­ernmost part of the deposit (Alvarez and Aracena, 1985; Maksaev, 1990; Martin et aI., 1993). As exposed, the Chuquicamata altera­tion zonation does not show the "typical" pattern of Lowell and Guilbert (1970), where a potassic core is concentrically surrounded by phyllitic-argillic and propylitic alteration

950 LINDSAY ET AL.

A

o o o on z

o o o .. z

o o

'" .., z

FIG. 5. A. Structural systems observed within Chuquicamata at 1995 level of exposure. B. Structural domains based on l:l00-scale mapping below the 2697-m level.

zones. Rather, it is hypothesized that Chu­quicamata represents at least two distinct mineralization-alteration events separated tem­porally by approximately 3 m.y. (Zentilli et aI. , 1994)_

Structural Domains

Structural domains in this study are based on 1: l00-scale mapping and are characterized by a series of faults and/ or veins, observable at the scale of 1 :2000, that are homogeneous with respect to orientation and displacement sense. Fault and vein systems divide the deposit into distinct structural domains (Maksaev, 1990; Martin et aI., 1993; Lindsay et aI., 1994) _ Within the deposit at the current level of exposure, the following structural domains can be defined (Fig. 5 and Table 1) : (a) Zaragoza, an early NNE-SSW hydrothermal vein system, possibly related to a NE-trending shear system; (b) Estanques Blancos, a NE shear system with

related early and late hydrothermal veins; (c) the Nor-Oeste fault system, a post-hydrother­mal mineralization NW-SE system; (d) Bal­maceda, a composite domain consisting of a NE-SW fault system (Estanques Blancos) related to late hydrothermal activity and a superimposed component of NW-SE faults (Nor-Oeste); (e) a weakly defined post-mineral­ization E-W system (El Negro, HI); (f) Mesabi, an early pre-mineralization ductile shear zone localized in Mesozoic rocks; and (g) Falla Oeste and Americana, a related pair of approximately N-S systems with regionally important post­mineralization movements.

Displacement senses for the majority of these fault systems have been determined from fabrics found within fault gouges representing most recent movements. NNE-trending shear zones, particularly the C-2 of the Zaragoza domain , are observed to have (at least) two periods of displacement , an earlier dextral duc­tile fabric overprinted by sinistral displace-

MINERALIZATION AT THE CHUQUICAMATA COPPER DEPOSIT 951

TABLE 1. General Characteristics of Structural Domains at the Chuquicamata Open Pit

Stuctural system Last displacemen t Orientation Mineral relation Dilation

Zaragoza Sinistral/normal NNE-SSW Syn-mineral High

Estanques Blancos Dextral NE-SW Syn-/post-mineral Moderately high

Balmaceda Dextral/sinistral NE-SW Late/ post-mineral Moderately high

Nor-Oeste Sinistral NW-SE Post-primary Low

Mesabi Dextral N-S Pre-mineral Low (?)

Banco HI E-W Post-mineral Low

Americana Sinistral

Falla Oeste Sinistral

men!. The C-2 fault also places oxide mineral­ization in contact with sulfide mineralization, indicating a normal west-side-down displace­ment (1. Rojas and G. Leiva, pers. commun., 1994) , oblique sinistral strike-slip, or altera­tion-mineralization controlled by fault-gouge permeability. The Estanques Blancos system has a dextral sense of displacement based on regional observations and recent fault gouges. The late, apparently non-mineralized, north­west (Nor-Oeste) fault system is associated with late sinistral motion of the Falla Oeste (Chong et aI., 1992; Martin et aI., 1993), having small «50 m) displacements. The Falla Oeste and Americana faults show similar fabrics within their gouges, indicating sinistral displacement. The Balmaceda is a domain where individual faults may show either dextral or sinistral dis­placement and the Mesabi is a distinct dextral, ductile shear system. Stockworks, or vein sets-interpreted as paleo-fracture systems formed within these domains as a result of active shearing of the system during cooling and consolidation of the plutonic complex-are guides to trends in mineralization and are expected to reflect the orientation of stresses during the evolution of the deposit.

Structural Sampling

In an attempt to begin to understand the complex history of the deposit and assess the extent of structural control and degree of anisotropy of the mineralized systems, several sites within the Estanques Blancos, Balmaceda, and Zaragoza domains were selected for struc-

N-S Post-mineral Moderately high

N-S Post-mineral Moderatel y high

tural sampling. These sites were chosen because they represent an area of the mine in which multiple generations of vein formation can be observed and for reasons of slope stability (safety). Domains farther south are strongly controlled by late movements associated with the Falla Oeste fault system and are described elsewhere. The methodology of observation and measurement, referred to here as "structural sampling," uses small (1 m x 2 m) sites within pre-determined structural domains to measure the preferred orientations of mineralized stock­work and veinlet systems and to observe a relative sequence of veinlet formation. This method, modified after Haynes and Titley (1980) , Rehrig and Heidrick (1972), and Titley et ai. (1986), emphasizes the temporal relation between distinct vein sets and thus specific trends in mineralization and evolution of inde­pendent domains. As a byproduct, a non-quan­titative fracture density is observable through the quantity of measurements made. However, that is not the purpose of this exercise (see Haynes, 1984). Uncertainties associated with structural sampling of this nature are discussed in Haynes (1984). In an attempt to minimize sampling bias, all sites are located along an east wall trending N-S with the exception of sites in the northern end of the pit, which are located on north walls trending E-W The sample sites are located in domains in such a way as to intercept the greatest number of features possi­ble at large angles.

Structural sampling was undertaken within the deposit to determine dominant orientations of vein sets. The Estanques Blancos and Bal­maceda faults (Fig. 6) have been viewed as

952 LINDSAY ET AL.

-500 m

~ Fortuna Granodiorite

Chuqui P~ry Alteration Zones L-.J Quartz-Sericite L...J Potassic

~ Propylitic

FIG. 6. Structural systems found at Chuquicamata as interpreted by Martin et aL (1993). Estanques Blancos (R) and Balmaceda (R') faults are emphasized. These faults had been interpreted as a Riedel-type strike-slip system, with the Falla Oeste being the master fault (cf. Fig. 5) . Structural sampling sites discussed in the text are shown.

Riedel shears (Martin et ai. , 1993) active during the formation of the deposit. Recent work has shown that these fault systems share the same sense of displacement and should not be consid­ered a conjugate Riedel (R, R') pair. The Estan­ques Blancos domain in the northeastern portion of the deposit is dominated by sub­parallel fault and fault-vein systems. Found extensively throughout this domain are miner­alized and non-mineralized veinlet (or stock­work) systems. Within this domain, fracture permeability, now observed as mineralized veins and stockwork systems, can be interpreted to have resulted from NW-SE extension (transten­sion) imposed by dextral shearing of the system as a whole. This is best illustrated by the pre-

ferred orientations of early, extensional, hydro­thermal , quartz-molybdenite veinlets (Fig. 7 A, N43 0 E, 52 0 NW). The Balmaceda domain, dominated by mineralized structural features similar to those found within the Estanques Blancos domain, also contains a series of non­to weakly mineralized NW-trending features. Sites located close to the Zaragoza-Estanques Blancos boundary within the Zaragoza domain exhibit fault-system characteristics similar to the Estanques Blancos domain. However, struc­tural sampling indicates that preferred orienta­tions of mineralized veinlet systems for the Zaragoza domain are distinct (Fig. 7B, N3 0 W, 47 0 W). A comparison of sampled sites within the above domains illustrates that each miner­alized vein let set has a definite trend in each domain (Fig. 7). Future domainal studies will provide information concerning the degree of control of the local modified tectonic stress field versus the intrusion-related stress field in terms of controls on fracture orientation.

Results and Discussion

The preliminary structural sampling study of veinlet anisotropy, of several sites in distinct structural domains and differing alteration zones (Fig. 6), has revealed a regular sequence of veining events (Fig. 8). The veinlet relation­ships (based on cross-cutting relations) at the structural sampling scale are summarized from older to younger as: K-feldspar ± quartz, quartz, quartz,molybdenite, quartz-Cu-Fe-sul­fide, Cu-Fe-sulfide, and (Cu-) Fe-sulfide-bearing veinlets with quartz-sericitic halos. Not all vein­let types are observed at each site.

Classification of the veinlet systems in terms of A-, B-, and D-type veins follows the estab­lished scheme of Gustafson and Hunt (1975), with sinuous K-feldspar to subplanar quartz and chalcopyrite-bornite comprising an A-vein group, quartz-molybdenite veins (<2-cm widths) as B veins, and quartz + sulfides (pyrite ± chalcocite ± covellite) with and without quartz-sericite halos and sulfide-bearing vein­lets comprising the D-vein group. At the scale of observation within the Chuquicamata open pit, quartz-molybdenite veins characteristic of the transitional B veins occur within the A-vein sequence. There is a scale-dependent factor that is discussed below.

MINERALIZATION AT THE CHUQUICAMATA COPPER DEPOSIT 953

Structural Sampling Structural Sampling

A s 53 Poles Rotte 14 Poles Plotted

DL93-25 Estanques Blancos DL94C Zaragoza

FIG. 7. Lower.hemisphere Schmidt stereographic projections of the preferred orientations of the quartz· molybdenite veinlet sets measured at structural sampling sites within the (A) Estanques Blancos and (B) Zaragoza

domains.

Sites 94A, 94B, and 94C-located within the potassic alteration zone of the Estanques Blancos structural domain, defined by NE­trending faults and fault-veins-show con­sistent trends in mineralized veinlet types (summarized in Fig. 8). These trends in miner­alization are comparable to sites 93-24 and 93-25, which are located in the quartz-sericite alteration zone of the same structural domain . It is believed that this structural domain has been active for the greatest length of time during deposit evolution and records the ear­liest formation of structurally controlled veins. Continued control of mineralization in the Estanques Blancos is observed in the unchang­ing preferred orientations of the majority of veinlet types. The dominance of the northeast trend is observed in the earliest-formed quartz veinlets through mineralizing sulfide deposi­tion events. Since the dominant orientation is observed to vary little within the potassic and quartz-sericite alteration zones, it is probable that there had been little change in stress regime throughout mineralization within the Estanques Blancos domain. A series of latest­formed vuggy quartz + pyrite ± covellite and/ or chalcocite veinlets is observed to maintain a dominantly northwest orientation, and may reflect the change in structural control imposed by the Falla Oeste and regional tectonics.

Site 94D, located near the Zaragoza-Estan­ques Blancos boundary, shows similar trends to

the aforementioned sites with respect to late hydrothermal activity. However, early quartz­molybdenite veinlets are oriented predomi­nantly N-S. This difference in orientation is most likely the result of a magmatic formation mechanism. Whereas the earliest quartz-molyb­denite mineralization is contained in an overall elliptical fracture pattern formed on emplace­ment and cooling of the Chuquicamata com­plex, the later stockwork fractures are controlled by local fluid pressures and domain­al stress systems related to the regional Domeyko shear system.

Similarly, sites 94E, 93-22, and 93-23 have mineralized veinlet types of different preferred orientations. It is proposed that this difference is associated with block rotation within the Balmaceda structural domain-a composite, or boundary, domain between the NE-SW Estan­ques Blancos and the superimposed NW-SE Nor-Oeste domains.

A change in scale

Distinct veinlet types are observed within the structural sampling sites. However, a portion of the copper mineralization and a majority of the molybdenite mineralization occurs within larger, discrete vein systems (scales of -1 :2000). These veins are classified as: "A" veins of quartz, generally pink to grey-white with widths of up to 30 cm; "B" veins of quartz­molybdenite, with widths of several meters; and

954 LINDSAY ET AL.

Site # / / I

94D i 94A I 948 94C 94E 93-22 93-23 93-24 93-25 93-26 Vein Type I , K-Feldspar I i '* '* Quartz / 1/ / I X - / X

Quartz + il' \ )' : / Molybdenite

Quartz :;. / 1/ / 1/ , , ~ -r " Pyrite i Sulfides / 1- ......... , / ~ " ""-(Cu) -Sulfide + / !- 1 ......... / QS halo --Chlorite + l I '/.. Magnetite i

I

Limonite + I .......... Pyrite

Ductile ......... -1-' / Shear

Alteration K K K SK SK K K QS QS Chi Type !

FIG. 8. Dominant orientations of veinlet sets observed and measured at structural sampling sites. Line orientations in this figure are approximate and dips are not shown.

late "D" veins of sphalerite ± galena and enar­gite ± pyrite, with widths ranging from 10 to 20 cm. The development of the finer stockwork system may have been continuous through the quartz stage and, in certain areas, in the quartz­molybdenite vein stage, but ended prior to the evolution of enargite ± pyrite and sphalerite ± galena veins, as demonstrated by the superim­position of the latest vuggy quartz-pyrite (-chal­cocite-covellite) veinlets on the large quartz­molybdenite veins, but not on the enargite- or sphalerite-bearing veins. These latest veins are observed to follow all fracture openings avail­able at the time of mineralization, regardless of orientation, which is indicative of a non-con­trolled extensional environment.

Scale and anisotropy of grades

The distinct preferred orientations of veinlet sets can be compared with larger, more contin­uous veins and contoured plots of metal grades. In the case of copper, grade-contour plots of excavated material adjacent to structurally sampled sites show a correlation of peaks that parallel the preferred orientation of Cu-sul-

fide-bearing veinlets. However, a second trend in copper distribution is notable that is not predicted by structural sampling. This second trend is the result of late hydrothermal, large­scale (1 :2000) copper-bearing features. Scale of observation and mapping thus plays an impor­tant role in mineral distribution. Plots of molybdenum distribution (Fig. 9) correlate with the preferred orientation of molybdenum veinlets and with the large quartz-molybdenite veins. This correlation is more convincing because the molybdenum distribution probably is confined to a single mineralizing event, whereas the copper distribution is the result of a series of hydrothermal and supergene miner­alizing events.

At the initiation of this project, the available structural model (Fig. 6) of Chuquicamata was used as a basis for establishing domains and thus the selection of sample sites was within domain boundaries. Analysis of data gathered from the various sample sites indicated the probable presence of subdomains and demon­strated that the structural model based on 1:2000-scale mapping was inaccurate for the

MINERALIZATION AT THE CHUQUlCAMATA COPPER DEPOSIT 955

scale at which mineralization occurs. The struc­tural domains defined by the work presented here were established on the basis of 1: 100-scale mapping.

The methodology of structural sampling pre­sented above allows for a relatively rapid anal­ysis of mineralized veinlet systems. This analysis includes the definition of a relative veinlet formation sequence and, if preferred orientations are found for the distinct veinlet sets, an anisotropy of mineralization can be described. The importance of scale of observa­tion arises when metal correlations are attempted in terms of mineralizing events. The primary mineralization of finer veinlets may have a preferred orientation that is overprinted by later mineralizing events. Later mineralizing events may be scale variant, as a result of changes in environment of formation.

Tectonic Summary

Host-rock emplacement is associated with an extensional (transtensional) system, locally controlled by the Mesabi fault system (concen­trated within Jurassic limestones, calcareous metasedimentary rocks, and adjacent gra­nitoids) and possibly, but not necessarily, an early pre-mineral Falla Oeste. Kinematic indica­tors consistently show dextral transcurrent dis­placements along shear zones of this system. The structural observations are consistent with plate tectonic reconstructions (see Pardo-Casas and Molnar, 1987) that indicate an acute oblique convergence between the South Ameri­can and Nazca plates (50 to 30 Ma) _

Ductile shear zones within the host rocks may have nucleated on early quartz veinlets, indicating a post-intrusion compressive event during early or pre-mineralization. These mylonitic shears are accompanied by variably distributed foliation within the host porphyry, and are interpreted to have formed contempo­raneously with emplacement during a time of oblique convergence. The convergence vector of the subducting plate migrated toward a posi­tion of normal convergence by the late Oligocene (Pardo-Casas and Molnar, 1987)_

At higher structural levels, this almost nor­mal convergence initiated extensional faults with a shear component (Estanques Blancos)

J:: t: o Z

Molybdenum Distribution

~N 4400

4300

FIG. 9. Contoured molybdenum distribution plot for an

area adjacent to structural sampling sites 93-24and 93-25 of

the Estanques Blancos domain. The shaded area encom·

passes the greater than 0.13% Mo contour; contour inter· vals are 0.02% . Note the strong northeast trend (cf. Fig. 5A).

after early hydrothermal activIty. This fault system continued to be active throughout the late hydrothermal and supergene history_ As normal convergence was approached, other fault systems formed (Americana, Falla Oeste, Banco HI) during the latest hydrothermal stage. Ultimately the system inverted (?) to sinistral, at some time after mineralization, and shear was concentrated along the Falla Oeste-Americana system with the formation of a fracture system (Nor-Oeste) containing latest hydrothermal mineralization (enargite, sphalerite, galena) centered over the quartz­sericite alteration zone.

The Falla Oeste-Americana system holds fault-bound blocks of low-grade, intensely silic­ified, or argillically altered rocks_ The bound­aries of the main faults of this system coincide with the limits of supergene alteration. How­ever, control of primary mineralization by these faults also is probable, at least in that initial

956 LINDSAY ET AI.

mineralization is displaced and contained between this fault pair.

Conclusions

The evolution of the Chuquicamata porphyry copper deposit from host-rock emplacement through supergene mineralization has been structurally controlled by fault systems within the regional Domeyko fault system. The devel­opment of local fault systems within the host porphyritic intrusion at epizonallevels allowed for recurring fracture events within individual domains, which provided the necessary per­meability for multiple mineralization events. It is these multiple fault systems and their related fracture systems and networks that are respon­sible for the great size and richness of the Chuquicamata deposit.

Primary and supergene mineralization is observed to be concentrated in stockwork-like structures. These structures are interpreted as the accessible permeability available at the time of mineralization. The veinlets constituting the mineralized stockwork systems show cross-cut­ting relationships that reveal a relative mineral­ization history associated with the evolution of permeability of the host rock. Structural domains can be described using the preferred orientations of vein sets measured and observed during structural sampling. Anisotropies of mineralization are associated with preferred orientations of distinct veinlet sets. These anisotropies are observable in contoured plots of metal distributions and thus indicate that this detailed scale of observation is necessary to reveal this important control on mineral anisotropy. Metal anisotropies and structural sampling trends are scale dependent, at least within the Chuquicamata open pit, where struc­tural control plays an important role in the deposit's evolution. Systematic structural map­ping and sampling at different scales are required to characterize this one control adequately.

The mineralization history is scale depen­dent, in that structural sampling indicates a finer sequence of definable mineralizing stages. In considering only the larger structural fea­tures (veins and faults), classification of min­eral stages into A, B, and D vein types is possible, but reveals little about the deposit's evolution.

The structural evolution of the deposit can be summarized as host-rock emplacement at depth within a dextral transtensional zone, uplift, and exhumation to higher structural levels followed by initiation of a series of hydrothermal events. This control is related to the convergence rate and angle of the Nazca plate with respect to the continental South American plate. Continued tectonic influence during hydrothermal stages with a tectonic inversion prior to the latest hydrothermal events is indicated by distinct preferred orientations of the sequential veinlet sets and by domain-defining fault and vein systems. A sinistral displacement along the Falla Oeste-Americana system not only is post­hydrothermal mineralization but probably post­dates supergene (?) enrichment as well.

The main results of this study can be summa­rized as follows. First, porphyry copper deposits that contain a high percentage of mineraliza­tion in veinlet and vein systems can be analyzed in a fairly rapid manner to determine relative veining sequences, but care must be taken to understand the distribution of structures throughout the deposit, because these sequences may be domain specific. Second, veinlets within a veining sequence may define distinct preferred orientations specific to domains or subdomains and thus describe an anisotropy of mineralization. Third, in deposits that contain vein, veinlet, and stockwork sys­tems, scale of mapping or sampling plays an important role in definition of an anisotropy of mineralization (cf. Heidrick and Titley, 1982). Finally, especially at Chuquicamata, one of the world's largest copper porphyry deposits, struc­tural control of mineralization in terms of frac­ture permeability at a variety of scales plays a dominant role in making this deposit a giant in both size and ore-grade richness.

Acknowledgments

Many fruitful discussions with Milton Graves (Cuesta Research) and the staff of the Superin­tendencia de Geologia at Chuquicamata have assisted in this initial understanding of this monstrous deposit. Special thanks are expressed to Juan Pallauta and Walter Martin for their initial apprehension, and Guillermo Ossandon C. for his continuing enthusiasm and interest in the multifaceted structural control

MINERALIZATION AT THE CHUQUICAMATA COPPER DEPOSIT % 7

at Chuquicamata. Financial assistance for this project has been provided by Codelco·Chile, Cuesta Research Ltd. (Canada), and a Dalhousie University Fellowship with support from NSERC through research grants to M. Zentilli.

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