The San Jorge porphyry copper deposit, Mendoza, Argentina: a combination of orthomagmatic and hydrothermal mineralization

Ore Geology Reviews 14 Ž1999. 185–201

The San Jorge porphyry copper deposit, Mendoza, Argentina: a combination of orthomagmatic and hydrothermal mineralization W.C. Williams a

a,)

, E. Meissl b, J. Madrid

a,1

, B.C. de Machuca

b

Northern Orion Explorationsr Grupo Minero Aconcagua, Besares 1151, Chacras de Coria, 5505 Lujan de Cuyo, Argentina b Facultad de Ciencias Exactas, Fısicas, y Naturales, UNSJ, 5400 San Juan, Argentina ´ Received 5 October 1998; accepted 1 March 1999

Abstract The San Jorge porphyry copper deposit ŽSJPCD. is hosted by Carboniferous clastic sedimentary rocks and Permian intrusions located within the Permo-Triassic belt of Chile and Argentina. Its hypogene mineralization and alteration are products of superposed orthomagmatic and hydrothermal events that were strongly fault controlled. Copper related to orthomagmatic processes includes disseminated chalcopyrite in the matrix of porphyritic granodiorite and andesite, and chalcopyrite with tourmaline and quartz in breccias, both of which have accompanying potassic alteration. Soon thereafter, disseminated chalcopyrite is associated with a structurally controlled silicification of the sedimentary sequence. Finally, multiple episodes of hydrofracturing, probably driven by a deep-seated intrusion, deposited sulfide minerals in veinlets throughout the sedimentary sequence; the centers of these systems are characterized by potassic alteration. Total sulfides, which include chalcopyrite, pyrite, arsenopyrite, and pyrrhotite, and pyrite:chalcopyrite form a linear NNE trend, parallel to the main faults. Quartz–sericite is the dominant alteration and is ubiquitous. Zones of potassic alteration can be delineated even though phyllic alteration can be superposed. Much of the system evolved under reducing conditions. Despite uplift along a reverse fault during the Tertiary, and subsequent erosion, the system is preserved at high levels. Supergene processes redistributed copper in secondary oxides and sulfides. These processes were more effective where the deposit is covered by unconsolidated alluvial sediments. The unique history of the San Jorge deposit renders it an important variation of porphyry copper-style mineralization. q 1999 Elsevier Science B.V. All rights reserved. Keywords: San Jorge porphyry copper deposit; orthomagmatic; Permo-Triassic belt

1. Introduction The San Jorge porphyry copper deposit ŽSJPCD. is located in the east foothills of the Andean )

Corresponding author. Casilla de Correo 468, Correo Central, 5500 Mendoza, Argentina. E-mail: [email protected] 1 Current address: Pueyrredon ´ 138, 8183 Darregueira, Argentina.

Cordillera in the Uspallata Valley approximately 100 km WNW of the city of Mendoza, Argentina ŽFigs. 1 and 2.. Outcropping copper-oxide minerals prompted Cıa. ´ Minera Aguilar to option the property in 1964 and since then various companies have explored the deposit. To date, trench sampling, over 20,000 m of drilling, tens of kilometers of I.P. and magnetic surveys, and metallurgical testing have been used to evaluate and determine economic viability. Nonethe-

0169-1368r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 6 8 Ž 9 9 . 0 0 0 0 6 - 2

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less, the geologic characteristics of this rather unusual porphyry copper deposit ŽPCD. have not previously been described in detail. This communication provides a description and current interpretation of its petrogenesis. A Permian porphyritic granodiorite with disseminated sulfides, breccias with tourmaline and sulfides in the matrix, and a Carboniferous clastic sequence with disseminated and fracture-hosted sulfides comprise the SJPCD. The global resource is over two million tons of copper with byproduct silver and gold. Secondary oxide and sulfide species are the focus of future economic exploitation. An SXEW operation has been planned to produce up to 20,000 tons of cathode copper per year for as long as 10 years.

2. Regional geology The SJPCD is one of several porphyry copper occurrences located within the south extension of the Permian magmatic arc, which extends for over 2000 km from northern Chile to southern Argentina ŽFig. 1. ŽMpodozis and Ramos, 1989; Camus, 1998.. The SJPCD is on the west edge of the Uspallata–Calingasta–Iglesia Valley along the boundary between the Cordillera Frontal and Precordillera geologic provinces, which is interpreted as the site of an early Paleozoic suture zone ŽRamos, in press. ŽFig. 2.. Phyllites of the Devonian Cienaga del Medio Forma´ tion form the basement in this region ŽAmos and Rolleri, 1965.. The sequence was faulted, metamorphosed, and isoclinally folded at the end of the Devonian during the Famatinian Orogeny ŽMpodozis and Ramos, 1989.. It is unconformably overlain by the Carboniferous Yalguaraz Formation, a sequence of intercalated conglomerates, sandstones, siltstones, and shales ŽAmos and Rolleri, 1965.; N- to NNEstriking strike-slip faults, regional tilting, and lowgrade metamorphism characterize the late Paleozoic deformation of this unit ŽMpodozis and Ramos, 1989.. Permo-Triassic ignimbrites and andesites of the Choiyoi Formation unconformably overlie the above formations to the east and west of the Uspallata valley; coeval granitic stocks and batholiths are hosted by the Paleozoic sedimentary rocks. Mafic

Fig. 1. Location of the SJPCD within the Permo-Triassic belt of Chile–Argentina. Known Permo-Triassic PCDs and calderas are indicated with their approximate ages in parentheses. Modified after Camus Ž1998..

intrusions characterize Tertiary magmatism in the south part of the valley whereas felsic affinities prevail to the north and east. During mid-late Tertiary, the Andean Cordillera was uplifted and various structural blocks were transported along north-strik-

W.C. Williams et al.r Ore Geology ReÕiews 14 (1999) 185–201

ing, eastward-verging thrust faults ŽMpodozis and Ramos, 1989.. NW-trending lineaments, with apparent sinistral movement, are the youngest structures recognized. Alluvium and colluvium filled the basin thereafter and only small outcrops of the aforementioned rock types are exposed in the valley today.

187

3. Deposit geology 3.1. Host rocks The rock types that host SJPCD mineralization are: Ž1. Yalguaraz Formation clastic sedimentary rocks, Ž2. silicified sandstone, Ž3. porphyritic gran-

Fig. 2. SJPCD general geology and drillhole locations.

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odiorite, Ž4. igneous breccia, Ž5. tourmaline breccia, and Ž6. porphyritic andesite ŽFig. 2.. The Yalguaraz Formation consists of four rock types: Ž1. conglomerate, Ž2. sandstone, Ž3. siltstone, and Ž4. shale. The Yalguaraz Formation is homoclinal, striking 0 to N35W and dipping 10 to 458W, with locally developed non-plunging folds that trend north. Most of the sequence is red–gray subarkosic sandstone; the other rock types generally form fining-upward sequences from 20 cm to 1.5 m thick. The conglomerates are matrix supported with both intrusive and sedimentary rock clasts between 0.2 and 3 cm in diameter. Whereas the siltstone is a gray–red color, the shale is typically greenish; both rock types form beds less than 50 cm thick. Overall, the conglomerates, siltstones, and shales are a minor host rock to mineralization. The gray silicified sandstone is not a sedimentary rock type sensu stricto. It is an intensely and pervasively altered clastic rock, i.e., quartz recrystallization has totally destroyed sedimentary features. In most cases, the protolith is Yalguaraz Formation sandstone, but it was mapped and logged separately to distinguish it from less intensely silicified sandstone. This rock type commonly crosscuts sedimentary bedding. The silicified sandstone and sandstone are the principal host rocks to primary sulfide minerals. Hereinafter, unless specified, the Yalguaraz Formation and the silicified sandstone will be referred to as the sedimentary sequence. The porphyritic granodiorite is light gray–salmon, comprised of plagioclase, potassium feldspar, quartz, biotite, and minor hornblende. Phenocrysts of plagioclase, quartz, and biotite constitute 35 to 55% of the rock and are 1.5 to 6 mm in diameter. Quartz and potassium feldspar are the principal matrix minerals; up to 1.5% disseminated chalcopyrite and minor pyrite also occur in the matrix. The breccias are classified according to their tourmaline content. The igneous breccia crops out along the northwest contact of the porphyritic granodiorite and contains negligible tourmaline. Clasts are subrounded to subangular fragments of the sedimentary sequence and porphyritic granodiorite that are devoid of hydrothermal veinlets. The matrix is primarily silicic aphanitic granodiorite. The tourmaline breccia has subangular to angular clasts of the sedimentary sequence and porphyritic

granodiorite in a matrix of tourmaline, quartz, and sulfide minerals. This breccia is most common along the western contact of the porphyritic granodiorite ŽFig. 2., and mosaicrcrackle breccias are more voluminous than magmatic hydrothermal breccias, the latter being characterized by rotated clasts. The porphyritic andesite consists of plagioclase, quartz, and pseudohexagonal biotite phenocrysts Ž15%. in a very fine matrix of plagioclase, biotite, minor quartz and potassium feldspar, and up to 1.5% sulfide minerals, especially chalcopyrite. It occurs exclusively as dikes, of which one strikes northwesterly through the porphyritic granodiorite in the south part of the main San Jorge outcrop ŽFig. 2.. More numerous compositional zoning bands of the plagioclase primocrysts suggest a genetic relationship between this rock and the older porphyritic granodiorite. 3.2. Structural geology The main structural trend in the SJPCD is N to NNE; NW- and ENE-striking faults are subsidiary features ŽFig. 1.. The Gorda fault, which strikes NNE and dips 70 to 758W, is the most important structure because it controlled sulfide mineral distribution, along with parallel faults to the west, under a postulated strike-slip stress regime. In addition, it appears to have been the planar feature, at least in part, along which the deposit was brought to or near surface. Based on equivocal stratigraphic correlations, net movement along all other faults is normal. Finally, joints typically formed in unidirectional sheeted veinlets, comprised of tourmaline in the porphyritic granodiorite and quartz " sulfide in the sedimentary units, of various attitudes. Another important structural feature is a veinlet stockwork. The nature and distribution of these veinlets are discussed below. 3.3. Mineral assemblages 3.3.1. Hypogene sulfides Sulfide minerals are both disseminated sensu stricto and veinlet hosted in an area of approximately 1000 m by 600 m ŽFig. 3.. Chalcopyrite and pyrite with or without marcasite, cubanite, andror mackinawite are the principal sulfide minerals. Minor

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189

Fig. 3. Distribution of mineralization and location of potassic-alteration centers related to the hydrofracturing event.

pyrrhotite, arsenopyrite, native gold, and sphalerite occur unevenly throughout the deposit; very minor molybdenite, generally - 100 ppm, is hosted by the

porphyritic granodiorite. Bornite and galena occur in trace amounts. Silver tellurides, native bismuth, bismuthinite, and tellurobismuthinite are spatially asso-

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ciated with chalcopyrite; gold has been identified in chalcopyrite but this is not an important occurrence. Pyrite and some pyrrhotite and arsenopyrite are euhedral whereas the other minerals are anhedral. Sulfide, telluride, and bismuthide grain sizes range from 0.5 to 1.5 mm and native gold grains range from 0.004 to 0.2 mm. Sulfide mineral assemblages are characterized by the dominance of either chalcopyrite or pyrite both in individual veinlets as well as in areas of disseminated mineralization. Arsenopyrite is typically associated with chalcopyrite and tourmaline in veinlets. Pyrrhotite is dominantly with chalcopyrite; the bird’s eye textures of marcasite suggest replacement of pyrrhotite. Sphalerite occurs as individual grains and as exsolution features in chalcopyrite. The bismuth minerals and silver tellurides are commonly attached to the edges of chalcopyrite grains; silver and copper are the only elements that correlate in a ratio of approximately 1:1250. Silver is notably more abundant in the tourmaline breccia. Gold is generally evenly distributed throughout the deposit at 200 ppb, but tends to be more abundant in the silicified sandstone. The pyrite:chalcopyrite ratio ŽPy:Cp. and the greatest sulfide mineral abundances are distributed in a NNE-trending linear pattern, parallel to the main faults ŽFig. 3.. Just west of the Gorda fault, total sulfides constitute 1% of the rock volume with Py:Cp< 1 both in the porphyritic granodiorite Ždisseminated. and sedimentary sequence Ždisseminated and veinlet.. Total sulfides of 1 to 3% are coincident with higher grades of copper and the potassic-alteration centers Žsee below.; Py:Cp is - 1. Greater percentages of total sulfides with Py:Cp4 1 correspond to the ‘pyrite halo’, which consists of variable proportions of pyrite, pyrrhotite, and arsenopyrite with minor chalcopyrite. Within these three zones, however, local variations are the rule rather than the exception. For instance, arsenopyrite-rich pods are encountered proximal to subsidiary faults, high Py:Cp bodies occur in the high-grade copper belt, and about 100 m of high Py:Cp, high total sulfide overlies a low Py:Cp with 1 to 2% chalcopyrite in drillhole SJD-16 Žpyrite cap.. The aforementioned sulfide species distribution includes disseminations as well as veinlet hosted occurrences. Sulfide mineral dissemination occurs in

all rock types; their diameters are generally less than 1.5 mm in the porphyritic granodiorite and andesite, and 0.5 mm in the sedimentary sequence. Sulfides in veinlets occur with or without associated quartz and tourmaline and, in lesser amounts, with or without biotite, potassium feldpsar, and calcite. Veinlet widths are commonly 0.5 to 2 mm. Many of the sulfide-only veinlets are discontinuous, irregular, and less than 0.25 mm wide, especially the pyrite-only type, which are a variety of ‘D’ veinlets ŽGustafson and Hunt, 1975.. Veinlet frequency, as measured over a linear meter within every 2 m of core from SJD-drilling or over a square meter at the surface, is rock-type dependent. However, primary copper grades are not necessarily sympathetic to veinlet density. In the first instance, veinlet frequency clusters decrease from 60 to 70 in the tourmaline breccia, 50 to 60 in the quartzite, sandstone, siltstone, and shale, 40 to 50 in the conglomerate, and 30 to 40 in the porphyritic granodiorite ŽFig. 4.. With respect to copper distribution, hypogene average grades decrease in the rock types as follows: tourmaline breccia Ž1.10% ., conglomerate Ž0.49% ., quartzite Ž0.45%., sandstone Ž0.39%., siltstone Ž0.35%., shale Ž0.36%., porphyritic granodiorite Ž0.31%.. The tourmaline-bearing hydrothermal breccias, and their related mosaicrcrackle breccias, are spatially associated with faults and are crosscut by a high frequency of veinlets. Nonetheless, chalcopyrite is principally hosted by the tourmaline–quartz matrix and, in places, is indeed massive Žup to 10%.. On the other hand, the veinlets in the porphyritic granodiorite are composed mainly of quartz " potassium feldspar " apatite " sulfides and tourmaline " sulfides; yet, nearly all of the chalcopyrite occurs as disseminations in the matrix. Furthermore, not all veinlets are sulfide-bearing. Thus, a description of veinlets in the sedimentary units is more pertinent to understanding the hydrothermal system. By volume, silicified sandstone and sandstone host the majority of hypogene copper minerals, especially the more continuous higher-grade zones. Veinlet frequencies are similar, but the average hypogene copper grade of the silicified sandstone is, on average, 10% greater than that in the sandstone. On the other hand, average hypogene copper grade in the less permeable siltstones and shales is relatively lower despite similar veinlet frequencies. The con-

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191

Fig. 4. Veinlet frequency histograms sorted by rock type. Data collected from SJD-series core. Abbreviations are as follows: Cgl.s conglomerate, Sil. SS s silicified sandstone, Porph. GD s porphyritic granodiorite, and Tourm. Bx s tourmaline breccia.

glomerate notoriously hosts greater hypogene copper grades, but veinlet frequencies are relatively low. In sum, intensity of silicification and permeability of the sedimentary unit are more related to average hypogene copper grade than veinlet density, i.e., they are sympathetic to a greater quantity of disseminated mineralization. The data base for trace elements of the host rocks is limited Ž105 samples., but manifests fundamental

differences between the porphyritic granodiorite and the sedimentary sequence with hypogene sulfide minerals ŽTable 1.. Most notably, arsenic, bismuth, nickel, and cobalt abundances are, on average, 2.5 to 4 times greater in the sedimentary sequence Ž441, 27, 44, and 29 ppm, respectively. than the intrusion Ž190, 7, 14, and 13 ppm, respectively.. Although zinc and manganese abundances are slightly greater in the sedimentary sequence than the porphyritic

Table 1 Average concentrations of selected trace elements in the hypogene alteration zone determined by MS-ICP Rock type

n

Cu Mo Pb Zn Ag Mn As Sb Hg Bi Ni Cr Co Fe Ti P Žppm. Žppm. Žppm. Žppm. Žppm. Žppm. Žppm. Žppm. Žppm. Žppm. Žppm. Žppm. Žppm. Ž%. Ž%. Ž%.

Porphyritic granodiorite Sedimentary sequence Silicified sandstone Ss, Cgl, Sltst, Sh

19 3250

17

15

105

2.2

179

190

2

9

7

14

67

13

2.50 0.02 0.079

86 4470

6

15

128

2.8

233

441

4

13

27

44

86

29

3.44 0.05 0.059

33 5558

6

17

167

3.5

248

513

5

15

31

44

92

37

3.57 0.04 0.059

53 3792

6

15

104

2.4

224

396

4

12

24

43

83

24

3.35 0.06 0.060

Sampling was random from SJD-series core holes. Sedimentary sequence results are averages from the silicified sandstone, sandstone ŽSs., conglomerate ŽCgl., siltstone ŽSltst., and shale ŽSh..

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granodiorite, their values average approximately 124 and 223 ppm, respectively. Pyrite and chalcopyrite composite samples from individual drillholes were analyzed for lead and sulfur isotopes ŽTable 2.. Fig. 7 shows the lead-isotope values as compared to other regions in southern South America. Sulfur isotopes are near 0‰. Their significance are discussed below. 3.3.2. Supergene sulfides and oxides Supergene minerals include chalcocite, digenite, covellite, and minor cuprite and native copper in the enriched zone, and chrysocolla, malachite, pitch limonite, delafossite, pseudomalachite, and minor tenorite, brochantite, and neotocite in the oxide zone. The area covered by this type of mineralization is about 800 m by 400 m ŽFig. 3.. Leached cap minerals include goethite with minor hematite, jarosite, and black oxides. Where the deposit crops out, delafossite rims chalcopyrite in the porphyritic granodiorite and covellite–digenite rims chalcopyrite in the sedimentary sequence in the north sector. On the other hand, where the deposit is overlain by unconsolidated gravels that have been dissected by post-gravel drainages, copper oxides with relatively higher grades overlie hypogene chalcopyrite, certain sectors with oxide minerals are thoroughly stained with transported limonite, and cuprite and native copper occur with chalcocite. Secondary sulfide minerals comprise the enriched zone. Two types of enriched mineralization are recognized: Ž1. a higher grade zone in the southwest sector where sooty chalcocite is the principal mineral with associated minor cuprite and native copper, and Ž2. a mixed or transitional zone, principally located

below outcrop, where digenite andror covellite rim chalcopyrite cores. Chalcocite coatings on pyrite are rare in both zones. Three zones of copper oxide mineralization are defined: Ž1. in the southwest sector, overlying the sooty chalcocite zone, where malachite and minor brochantite and azurite are encountered, Ž2. on the main outcrop where chrysocolla, malachite, pseudomalachite, pitch limonite, and delafossite are the principal copper species, and Ž3. in the north sector where malachite is the main copper mineral. In general, high copper grades are encountered in the southwest sector, along the prophyritic granodiorite–sedimentary rock contact, and at the north end of the outcrop area ŽFig. 2.. Copper oxide minerals also occur on detrital grains in unconsolidated gravels within 100 m west and east of the outcrop but are not economically significant. 3.4. Alteration Alteration minerals include quartz, tourmaline, biotite, potassium feldspar, calcite, and minor chlorite, kaolinite, and gypsum of variable intensity and pervasiveness. Whereas silicic and phyllic alteration is ubiquitously pervasive, biotite is locally pervasive. All alteration types, however, are veinlet related. Pervasive silicic alteration of varying intensities has affected all rock types. In the case of the silicified sandstone, which crosscuts bedding boundaries, the original sedimentary textures are not discernible. The other rock types are less intensely silicified and their sedimentary textures are preserved. Pervasive silicification of the porphyritic granodiorite and andesite is most notable in their matrices.

Table 2 Summary of radiogenic and stable isotope analyses Sample SJD-02, 106–124 m, sericite SJD-06, 175 m, biotite SJD-11, 228–270 m, Cp SJD-16, 89–138 m, Py SJD-06, 168 m, calcite SJD-07, 234 m, calcite

d 34 S

d 18 O

d 14 C

q9.1 q2.9

d 13 D

206

Pbr 204 Pb

Pbr 204 Pb

208

Pbr 204 Pb

y93 y125 18.207 19.716

y1.0 q1.5 q25.9 q7.8

207

q0.4 y6.5

15.629 15.721

38.225 39.906

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Tourmaline occurs in the matrix of magmatic hydrothermal and mosaicrcrackle breccias, disseminated radiating clusters, and in hairline, discontinuous veinlets with or without fine quartz and sulfide minerals. Chlorite after biotite, rutile, and apatite may also occur with tourmaline in veinlets. Discontinuous potassium-feldspar halos occur on the margins of these veinlets. Tourmaline is ubiquitous but is especially abundant west of and along the intrusive-sedimentary rock contact in the southwest sector of the SJPCD, where massive chalcopyrite accompanies the tourmaline in the matrix of the breccias

193

ŽSJD-09.. Proximal to the tourmaline breccias, a milky quartz occurs as irregular veinlets and cavity fillings in the sedimentary section. This type of silicic alteration is found peripheral to the contact breccia as well. Potassic alteration is manifested by biotite after biotite andror hornblende as well as potassium feldspar after plagioclase in the intrusions, quartz " sulfide" tourmaline" biotite veinlets with potassium feldspar halos, disseminated biotite in the sedimentary sequence, and quartz y potassium feldspar " sulfide and biotite" quartz " sulfide veinlets. In

X Fig. 5. Distribution of mineralization types and alteration along sections A–A Ž8650E. Žsee Fig. 2 for location.. BiotiterKsp is potassic alteration associated with hydrofracturing event. Note superimposition of calcite alteration on the potassic alteration in southern part of section.

194

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the intrusions, secondary biotite pseudomorphically replaced primary biotite forming biotite aggregates within the original crystal. Potassium-feldspar replacement of plagioclase in the intrusive rocks is often incipient. Rutile and euhedral apatite are spatially associated with potassium feldspar and quartz in the veinlets hosted by the porphyritic granodiorite. Based on its patchy occurrence and habit, the disseminated biotite in the sedimentary sequence is considered to be potassic alteration and not the result of contact metamorphism Žhornfels.. The potassiumfeldspar veinlet assemblages crosscut and are crosscut by the biotite-bearing veinlets. Whereas the former occurs as irregular, sinuous veinlets characteristic of ‘A’ veinlets ŽGustafson and Hunt, 1975., the latter occurs as continuous semi-rectilinear veinlets. In sum, four distinct potassic-alteration events related to mineralization are recognized: Ž1. replacement biotite and potassium feldspar in the intrusions, Ž2. potassium feldspar halos and veinlet biotite associated with the tourmalinization, Ž3. finely disseminated biotite in the sedimentary sequence, and Ž4. potassium feldspar halos and veinlet biotite. Three centers of the lattermost, spatially related to highergrade copper mineralization, have been delineated in the sedimentary section ŽFigs. 3, 5 and 6.. An altered biotite from a porphyritic andesite dike in SJD-06 has a K–Ar age of 263 " 6 Ma ŽTable 3.. Its oxygen and deuterium isotope values are found in Table 2 and plotted in Fig. 8. Continuous, rectilinear veinlets with sharp contacts, and commonly with grain sizes increasing from their contacts inward, are characteristic of ‘B’ veinlets ŽGustafson and Hunt, 1975.. These veinlets are typically sheeted over a width of as much as 1.5 m; many of these veinlets are the site of repeated fluid injection. Quartz with variable quantities of chalcopyrite, pyrite, pyrrhotite, andror arsenopyrite comprises these veinlets. Silicic alteration, which often bleaches the reddish gray host rock to a creamy white proximal to the contacts, is typically associated with these veinlets; sericite accompanies the silica. Thus, a phyllic alteration formed along the veinlet margins. Sericite is ubiquitous. It occurs as fine disseminations in the sedimentary sequence on the deposit scale and locally occupies the sites of biotite, potassium feldspar, and plagioclase. Sericite replaces bi-

Fig. 6. Distribution of mineralization types and alteration along X sections B–B Ž2775N. Žsee Fig. 2 for location.. BiotiterKsp is potassic alteration associated with hydrofracturing event. Note superposition of calcite alteration on the potassic alteration in western part of section.

otite in veinlets within certain zones of the deposit and in addition, it envelopes veinlets. In fact, the discontinuous, irregular pyrite veinlets have sericite halos. No zoning pattern is recognizable. Finegrained sericite in a quartzite from drillhole SJD-02 yielded a K–Ar age of 257 " 5 Ma ŽTable 3.. Its oxygen and deuterium isotopes are found in Table 2 and plotted in Fig. 8. Propylitic alteration is minor. Local zones of chlorite, often incipient, after biotite do occur but no pattern is discernible. Epidote is more common in fault zones. Calcite veinlets, with or without minor

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195

Table 3 Summary of KrAr ages Sample

K Ž%.

40

SJD-02, 106–124 m, sericite SJD-06, 175 m, biotite

4.900 7.503

5.845 8.951

K Žppm.

sulfide minerals, occur throughout the deposit but are particularly more dense in the southwestern and northeastern potassic centers. These veinlets crosscut all veinlet types. Calcite was sampled from two distinct veinlets from drillholes about 100 m apart ŽTable 2, Fig. 2.. The sample from drillhole SJD-07, extracted from a calcite–quartz veinlet, has a d13 C of y6.5‰. On the other hand, the sample from drillhole SJD-06, extracted from a 2-cm wide calcite veinlet, is almost q0.4‰. Clay alteration is unevenly distributed. Kaolinite and minor montmorillonite are common in the zones of sulfide leaching and as constituents of fault gouge. In the northeast part of the deposit, gypsum is spatially associated with kaolinite. Incipient replacement of plagioclase in the porphyritic granodiorite and of sericite is also common.

4. Discussion Superposed orthomagmatic and hydrothermal events resulted in the unique SJPCD occurrence. Chalcopyrite and minor pyrite crystallized in the matrix of the porphyritic granodiorite because of liquidus saturation. Clasts in the contact breccia are crosscut by but a few veinlets thus establishing the porphyritic granodiorite as the earliest event with copper minerals. Quartz" sulfide" apatite veinlets, with or without potassium feldspar halos do not crosscut the breccias nor the sedimentary sequence. Tourmaline-bearing veinlets are nearly always crosscut by all other types of veinlets. Therefore, brecciation, and coeval veinlets, with or without matrix tourmaline, quartz, and sulfide minerals followed the initial intrusion. The milky quartz alteration associated with the breccias may be considered a late orthomagmatic phase of tourmalinization. The copper minerals hosted by the porphyritic granodiorite and tourmaline breccia, potassium alteration related

40

40

Age " error ŽMa.

0.09378 0.1473

0.01604 0.01645

257 " 5 263 " 6

Ar Žppm.

Arr 40 K

to veinlets in the porphyritic granodiorite and to tourmaline-bearing veinlets, and the milky-quartz alteration peripheral to both types of breccias are considered orthomagmatic processes. As defined, the silicified sandstone is a recrystallized sandstone formed by silica flooding. This intense and pervasive silicic alteration crosscuts sedimentary bedding and, in plan, trends north ŽFig. 2.. The other sedimentary rock types are also pervasively silicified, but less intensely. These rock types host finely disseminated sulfide minerals, and disseminated secondary biotite, but the silicified sandstone contains more chalcopyrite and native gold. These attributes suggest that the silica flooding, the disseminated sulfide minerals, and the disseminated secondary biotite in the sedimentary sequence were facilitated by structural control, principally along the Raya Roja fault ŽFig. 2., and resulted in the linear zoning of total sulfides and Py:Cp, both of which increase from east to west ŽFig. 3.. In the silicified sandstone, chalcopyrite comprises most of the total sulfides. The fine veinlets hosted by the sedimentary sequence and tourmaline breccia formed during hydrofracturing. Higher-grade copper and potassic alteration characterize the centers of hydrothermal activity; the centers were also structurally controlled, most notably by the Portazuela and Raya Roja faults ŽFig. 3.. These veinlets generally did not penetrate the porphyritic granodiorite rendering it non-receptive to hydrofracturing due to its physical properties. Many ‘A’ veinlets crosscut and are crosscut by ‘B’ veinlets ŽGustafson and Hunt, 1975. thus confirming a multi-stage process. Sulfide-only veinlets are characterized by the dominance of one species. Potassic alteration is manifested by potassium feldspar halos and veinlets with biotite" sulfide minerals. In areas, phyllic alteration overprints the potassic alteration, but a concentric alteration pattern is not discernible. In the waning stages of this hydrothermal activity,

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calcite " quartz veinlets were emplaced preferentially in the potassic zones ŽFigs. 5 and 6.. This system superposed itself along the west and north perimeters of the focus of silica flooding and forms the zone with 1 to 3% total sulfides and a Py:Cpf 1. It is surmized that the parent magma of the porphyritic intrusions was the progenitor of this hydrothermal event. The porphyritic andesite dikes, which also host disseminated chalcopryite" pyrite, crosscut all veinlets except those with calcite. The deposit evolved, in part, under relatively low sulfur and oxygen fugacities as indicated by the occurrence of pyrrhotite and arsenopyrite ŽHemley et al., 1992.. Pyrrhotite and arsenopyrite appear to be temporally related to the late orthomagmatic and silica flooding events, which are also characterized by elevated bismuth, silver, nickel, and cobalt, whereas pyrite is more common to the hydrothermal event. Despite the superposition of the aforementioned systems, a strong structural control maintained the linear pattern of mineralization. The concentric sulfide and alteration zoning characteristic of many PCDs Žcf. Lowell and Guilbert, 1970. is precluded here. This lack of concentricity directly affects the distribution of secondary copper oxides and sulfides. The supergene history of the SJPCD is intimately related to its postulated uplift along the reactivated Gorda fault Žreverse faulting.. Nonetheless, the SJPCD is preserved at high levels based on the evidence that the potassic centers are at least 150 m below the surface and capped by pyrite as well as the characteristic thin veinlets, - 2 mm but usually F 1 mm. Even though a zonation of total sulfides and Py:Cp has been delineated, local variations of Py:Cp and horizontal transport of copper in solution have created a distribution of oxide and sulfide minerals other than would be predicted from simple vertically-directed leaching. In the porphyritic granodiorite, where chalcopyrite is the dominant sulfide, black oxides, e.g., delafossite, pitch limonite, tenorite, are more common than chrysocolla, i.e., oxidation of sulfides prevails over leaching of sulfides. Where the sedimentary sequence crops out, the enriched zone is characterized by covellite andror digenite rimming chalcopyrite attesting to the immaturity of the leachingrenrichment process. On the other hand, in the gravel-covered parts of the deposit, post-gravel drainages have apparently recharged areas so as to

redistribute copper both vertically and horizontally. In fact, outlying drillholes west of the main sulfide mineral concentration did not encounter a leached section under gravels, implying erosion and leaching of any pre-existing leached zone and the subsequent transport of acidic fluids ‘downstream’ ŽFig. 3.. It is in these areas where oxide copper grades typically average ) 0.60% and the enriched zone consists of sooty chalcocite with grades ) 1.00%, although the underlying primary minerals may contain - 0.25% copper. Horizontal transport of fluids under ambient conditions was an important process throughout the deposit because in areas where the Py:Cp was not generally sufficient to generate sulfuric acid to leach and mobilize all copper in solution vertically, an enriched zone developed as opposed to the predicted oxide mineralization. Thus, the distribution of secondary copper oxides and sulfides is nearly coincident. Finally, erosion and oxidation outpaced supergene processes locally, and copper oxide occurs on detrital grains in colluvium proximal to the deposit. The reconnaissance sampling of isotopes provides more insight into the petrogenesis of the SJPCD ŽTable 2.. The sampled pyrite contains radiogenic Pb and d 34 S consistent with a magmatic source ŽBarnes, 1979. ŽFig. 7.. The sampled chalcopyrite contains less radiogenic, but crustal Pb and a d 34 S consistent with a magmatic source ŽBarnes, 1979. ŽFig. 7.. These lead isotopes show that the lead source, and by implication the metal source, was not homogenized and confirm the complex petrogenetic history deciphered from field and petrogaphic observations. The lead in the pyrite was derived from a high-m, crustal source, typical of late Paleozoic–early Mesozoic rocks of northern Chile ŽWilliams and BouseSchaeneman, 1992; Tosdal, 1994; Williams, 1995., which mark the western edge of Gondwana ŽRamos, in press.. Thus, based on regional considerations, the source for the pyrite was derived from the same source as the San Jorge intrusion; this interpretation requires confirmation of lead isotopes from the intrusion. The lead in the chalcopyrite was derived from a source with a lower m not atypical of Andean Mesozoic and Cenozoic rocks ŽTosdal, 1994., but with a 207 Pbr206 Pb value above the average crustal growth curve ŽFig. 7.. These two data points suggest that the metals were not derived from an homogeneous source, a trait of the world class Chilean PCDs and

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Fig. 7. Uranogenic and thoranogenic diagrams illustrating Pb isotopes from San Jorge compared to fields defined in proximal areas in South America. Fields defined based on data from Zentilli et al. Ž1988., Williams and Bouse-Schaeneman Ž1992., Tosdal Ž1994., Aitcheson et al. Ž1995., and Williams Ž1995.. Data for Aguilar and Capillitas, deposits located in northern Argentina, after Zentilli et al. Ž1988..

the prolific auriferous Maricunga Belt ŽFig. 7.. These preliminary data not only have important implications for the tectonic history, but for the magmatic and metallogenic history characteristic of this part of Argentina. Stable isotopes also confirm the dynamic, multistage history of the SJPCD. Whereas the sulfur isotopes indicate a magmatic source ŽBarnes, 1979., the carbon isotopes suggest bimodal sources for the late fluids and the oxygen–deuterium isotopes implicate magmatic sources with later fractionation ŽFig.

8.. The lighter carbon isotope could have resulted, in part, from a carbonaceous source, such as the Devonian phyllites that comprise the basement in this region, whereas the near-neutral carbon isotope was probably sourced from the known host rocks. The oxygen–deuterium isotopes from the altered biotite plot near the field for potassic alteration at Butte, Montana implicating magma degassing ŽFig. 8. ŽSheppard and Taylor, 1974., a plausible fractionation mechanism for the SJPCD since a part of the Choyoi Formation volcanics are essentially time-

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Fig. 8. Oxygen and deuterium isotopes for San Jorge compared to the isotopes for K-silicate alteration in other PCDs. Data for El Salvador from Sheppard and Taylor Ž1974., for Butte from Sheppard et al. Ž1971., for magmatic waters from Taylor Ž1974., and for Yerington from Dilles et al. Ž1992..

equivalent to the SJPCD. In this instance, isotopic exchange with heated meteoric waters can be precluded, but not necessarily discarded. On the other hand, the isotopes for the sericite are consistent with isotopic exchange during the passing of heated meteoric waters ŽTaylor, 1974.. Additional data are required to better constrain the affect that magma degassing andror a late hydrothermal event had on the evolution of the deposit.

5. Conclusions Magmatism and hypogene alteration and mineralization in the SJPCD are unequivocally Permian. Field observations, petrography, and isotopes have constrained a multi-stage evolution of this unique deposit. Based on the data collected to date, the following paragenetic sequence for hypogene mineralization is proposed ŽFig. 9.:

Ž1. Porphyritic granodiorite with up to 1% disseminated chalcopyrite Žliquidus saturation. was intruded. Potassic alteration related to this event may be considered late orthomagmatic. Ž2. Tourmaline " quartz " sulfide—rich fluids were emplaced forming breccias; tourmaline and sulfides are most abundant along the western porphyritic granodiorite–sedimentary rock contact. Potassic alteration is manifested as potassium feldspar envelopes on tourmaline veinlets and minor biotite in veinlets. The milky-quartz alteration, which also occurs peripheral to the tourmaline-deficient igneous breccia, is a late-stage alteration related to this event. Tourmalinization andror brecciation is considered latest orthomagmatic–earliest hydrothermal. Ž3. Contemporaneous with andror immediately after tourmalinization, silica-rich fluids were transported vertically along faults and distributed disseminated chalcopyrite throughout the sedimentary sequence. All rock types were silicified and mineralized, but the more permeable sedimentary rocks were intensely and pervasively silicified Žsilicified sandstone., i.e., sedimentary textures were completely obliterated, albeit across bedding boundaries. The ubiquitous, but patchy, disseminated biotite in the sedimentary sequence represents the potassic alteration associated with this event. Disseminated tourmaline in the sedimentary sequence may implicate synchroneity with Ž2.. Maximum disseminated chalcopyrite is estimated at 1%. Ž4. Hydrofracturing and sulfide mineralization, with strong structural control by inferred strike-slip faults, was focused above a deep-seated magma chamber, probably the progenitor of the porphyritic granodiorite. The multiple pulses and superposition of individual systems resulted in complex crosscutting relationships of veinlets, e.g., ‘A’ veinlets crosscutting ‘B’ veinlets. The higher-grade copper zones occur where this event is superposed on event Ž3., i.e., chalcopyrite related to this event is approximately 1% in the core of the mineralization, thus forming a zone with up to 3% total sulfides and 2% chalcopyrite. Ž5. Porphyritic andesite dikes intruded, with approximately 1% disseminated chalcopyrite due to liquidus saturation, throughout the area. These dikes are genetically related to the porphyritic granodiorite and therefore the progenitor of the entire system.

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Fig. 9. Schematic model of the evolution of the SJPCD. ŽI. Granodiorite intrusion, emplacement of tourmaline-rich fluids and brecciation, and silica flooding via faults that infiltrated permeable units of sedimentary sequence. Disseminated chalcopyrite, with minor pyrite, in porphyritic granodiorite Žliquidus saturation. and sedimentary sequence, and massive chalcopyrite in matrix of tourmaline-bearing breccias. Pyrite" pyrrhotite" arsenopyrite ‘halo’ formed west of main copper mineralization during silica flooding event. ŽII. Repeated hydrofracturing, strongly controlled by NNE-striking structures, focused in sedimentary sequence. Deep magmatic source drove this sytem. ŽIII. Mineralized area uplifted via reverse movement along Gorda fault; movement deviates from previous fault plane and flattens out with depth. Commencement of erosion and supergene processes.

Ž6. Widespread and pervasive phyllic alteration and introduction of discontinuous pyrite veinlets Ž‘D’ veinlets. followed. Ž7. Calcite" quartz veinlets were emplaced, especially within the centers of potassium alteration.

The distribution of higher-grade copper, therefore, is spatially associated with the potassic alteration related to the hydrofracturing events where they superposed the earlier, silicification event. The belt with 1% total sulfides and a Py:Cp- 1 not only

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includes the disseminated copper minerals hosted by the porphyritic granodiorite, but the disseminated copper minerals related to the silica-flooding event in the sedimentary sequence where veinlet copper minerals are less abundant, i.e., at the periphery of the these systems’ centers ŽFig. 3.. It is therefore concluded that the linear arrangement of the total sulfides was established during the silicification event, with the pyrite halo focused westward, and the later hydrothermal events primarily added copper minerals to this system decreasing the Py:Cp, i.e., their contribution of pyrite was negligible. Supergene processes were generated after uplift of the deposit via the Gorda fault. They were more efficient where the deposit is covered by unconsolidated gravels, which were later dissected by drainages that emanated from the Andean Cordillera to the west; it is here that repeated water recharge allowed for more thorough leaching of pyrite, generation of sulfuric acid, and leaching of copper minerals to form an enriched blanket of secondary copper minerals with or without cuprite and native copper. In addition, acidic waters also leached and transported copper horizontally, eventually depositing additional copper oxides in already established zones of copper oxide minerals. Black copper oxides formed by oxidation of chalcopyrite occur in the outcrop and, below the outcrop in the sedimentary sequence, mixed zones of chalcocite, digenite, andror covellite with chalcopyrite are encountered, i.e., a transitional zone, demonstrating the incipient nature of the supergene processes here. The SJPCD can be classified as a ‘wall-rock porphyry’ ŽTitley, 1972.. Even though the outcropping porphyritic granodiorite is an important contributor of copper in oxides, the engine that drove the mineralized hydrothermal system is its postulated deep-seated progenitor. The orthomagmatic and early hydrothermal events that disseminated copper, the later cupriferous hydrofracturing events that formed under reducing conditions, the strong structural control of the system, and the complex supergene history render the SJPCD an important variation of porphyry copper mineralization. In fact, the interplay of the various mineralizing events and geomorphologic processes make the SJPCD ideal for further study. Whereas most of the interpreted history is based on field observations and thin and polished

sections, additional radiometric and isotope studies will better constrain the paragenetic sequence.

Acknowledgements The authors are grateful to Northern Orion Explorations and its Argentine subsidiary, Grupo Minero Aconcagua, for granting permission to publish, and one author ŽEM. ackowledges the logistical support provided in pursuit of a PhD at the University of San Juan, Argentina. Critical reviews by R. Hodder and by OGR reviewers H. Bonham and J. Price improved the manuscript and their efforts are greatly appreciated. Discussions with various Chilean colleagues helped refine the ideas presented herein. J. Galdames and M. Carrizo prepared all figures. Radiometric and stable isotope data provided by Geochron Laboratories.

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