D.O.I. : https://doi.org/10.30550/j.agl/2018.30.1/1

Petrography, geochemistry and geochronology of San Jorge porphyry Cu-Au deposit, Mendoza, Argentina. Constraints for the timing of magmatism and associated mineralization

 

Mirta M. GARRIDO¹, Laura E. GRECCO², María V GONZÁLEZ¹, Cecilia M. PAVÓN PIVETTA¹-²

¹    Departamento de Geología. Universidad Nacional del Sur (UNS). Bahía Blanca. Argentina Correo electrónico: mgarrido@criba.edu.ar, violeta.gonzalez@uns.edu.ar, Cecilia.pavon@uns.edu.ar
²   Instituto Geológico del Sur (INGEOSUR). Departamento de Geología (UNS). Bahía Blanca. Argentina Correo electrónico: mlgrecco@criba.edu.ar

Abstract — The San Jorge porphyry Cu-Au deposit, Argentina, integrates the Paleozoic to Early Jurassic metallogenic belt recognized in the Southern Andes. San Jorge is currently the only deposit considered economically viable due to its supergene enrichment. Previous studies have reported Middle to Upper Permian ages (257-270 Ma) for the intrusion-mineralization processes. Granite porphyry, two granodiorite porphyries (G1 and G2) and an alkali-feldspar granite dike were recognized. These granites intrude sedimentary rocks (Yalguaráz Formation) that hosts the main mineralization. Two tourmaline alteration events occurred prior to the potassic episode. They are overprinted by phyllic alteration that has an elongated shape with potassic mineralized cores. The mineralization is linked to the potassic alteration in the sedi-mentary sequence and in the granite porphyry that has the highest Cu contents (1% to 3%). The phyllic alteration has pyrrhotite, arsenopyrite and minor chalcopyrite. Digenite, chalcocite and covellite appear in the supergene; malachite and brochantite in the oxide; and goethite, hematite and jarosite in gossan zones.

The granitic porphyry, granodiorite porphyries G1 and two potassic alteration biotites are of Early Permian age. This event produces the potassic alteration and mineralization and is 15 Ma older than the ages determined by other authors. The granodiorite porphyry G2 and the alkali feldspar granite dike are Upper Permian in age. The results suggest that the evolu-tion of the western margin of Gondwana began in late Carboniferous and continued during the Permian period. In this Paleozoic to early Jurassic metallogenic belt, La Voluntad Cu-Mo porphyry is the oldest deposit (early Pennsylvanian, Garrido et al. 2008). Mineralized and no mineralized San Jorge granites (Early Permian-Late Permian) integrate the intrusive event of the Choiyoi Group.

Keywords: San Jorge copper-gold deposit-petrography-geochemistry-⁴⁰Ar/³⁹Ar and U-Pb geochronology.

Resumen — “Petrografía, geoquímica y geocronología del pórfido Cu-Au San Jorge, Mendoza, Argentina. Límites para el magmatismo y la mineralización asociada”. El depósito de pórfido Cu-Au de San Jorge, Argentina, integra el cinturón metalogénico del Paleozoico

Algunos derechos reservados. Esta obra está bajo una Licencia Creative Commons Atribución - No Comercial - Sin Obra Derivada 4.0 Internacional al Jurásico Temprano, reconocido en los Andes del Sur. San Jorge es actualmente el único depósito considerado económicamente viable debido a su enriquecimiento supergénico. Estudios previos dieron edades del Pérmico Medio a Superior (257-270 Ma) para los procesos de intrusión-mineralización. Se reconocieron pórfidos de granito, dos pórfidos de granodiorita (G1 y G2) y un dique de granito alcali-feldespático. Estos granitos intruyen rocas sedimentarias (Formación Yalguaráz) que alberga la principal mineralización. Dos eventos de turmalinización ocurrieron previos al episodio potásico. Están sobreimpuestos por una alteración fílica que tiene una forma elongada con núcleos mineralizados potásicos. La mineralización está ligada a la alteración potásica en la secuencia sedimentaria y en el pórfido de granito que tiene los mayores contenidos de Cu (1% a 3%). La alteración fílica contiene pirrotina, arsenopirita y menor calcopirita. Digenita, calcosina y covelina aparecen en la zona supergénica; malaquita y brochantita en la de óxidos; y goethita, hematita y jarosita en las zonas gossan. El pórfido granítico, el pórfido de granodiorita G1 y dos biotitas de alteración potásica son de edad Pérmico inferior. Este evento produce alteración potásica y mineralización y tiene 15 Ma más que las edades determinadas por otros autores. El pórfido de granodiorita G2 y el dique de granito álcali-feldespático son de edad Pérmico superior. Se propone un mínimo de 2 Ma de edad para la duración del sistema hidrotermal.
Los resultados sugieren que la evolución del margen occidental de Gondwana comenzó a finales del Carbonífero y continuó durante el período Pérmico. En este cinturón metalogénico del Paleozoico al Jurásico Temprano, el pórfido La Voluntad Cu-Mo es el depósito más antiguo (Pensilvaniano temprano, Garrido et al., 2008). Los granitos mineralizados y no mineralizados de San Jorge (Pérmico Temprano-Pérmico Superior) integran el evento intrusivo del Grupo Choiyoi.

Palabras clave: Depósito de Cu-Au San Jorge, petrografía, geoquímica, geocronología de ⁴⁰Ar/³⁹Ar y U-Pb.

INTRODUCTION

Porphyry copper deposits of similar age from the Andes in western South América have been grouped into a series of metal-logenic belts. These NS belts are a few km wide and several km long and occur parallel to the Pacific margin. The San Jorge porphyry Cu-Au deposit, Argentina, is one of 13 porphyry Cu systems in the Paleozoic to Early Jurassic metallogenic belt (Fig. 1) rec-ognized so far in the Southern Andes (Camus 2003; Sillitoe and Perelló 2005). San Jorge is currently the only porphyry Cu-Au deposit in the Paleozoic Belt considered economically viable. Resources are estimated at 194.5 Mt with 0.48% Cu, and 0.21 g/t Au with a cut-off grade of 0.3% Cu (Compañía Minera San Jorge-Coro Mining Corporation 2008). The supergene enrichment gives an economic possibility horizon to this deposit.

The purpose of this paper is to provide new geological information on the deposit and to better constrain the age of the San Jorge porphyry system using more reliable and robust isotopic systems.

The new geological information enables a detailed petrologic-geochemical study on the magmatism linked to the mineralization in an attempt to understand its tectonic for-mation environment. The granite porphyry and alkali feldspar dike rocks analysed in this work are comparable to those described by Williams et al. (1999) as granodiorite porphyry and the andesitic porphyry dike respectively Here we use U-Pb zircon dat-ing to determine the age of crystallization of intrusive bodies and a precise 4oAr/39Ar technique on magmatic and hydrothermal biotite to better estímate the duration of the magmatic-hydrothermal process. Previ-ous geochronological studies on the deposit have reported Middle to Upper Permian ages. A magmatic biotite from a granodiorite stock yielded a K-Ar age of 270±4 Ma (Sillitoe 1977), whereas a hydrothermal biotite from a porphyritic andesite dike was dated using the K-Ar method at 263 ±6 Ma. Ad-ditionally sericite from a quartzite sample yielded a K-Ar age of 257±5 Ma (Williams et al. 1999).

Figure 1. A) Map showing the location of San Jorge porphyry Cu-Au deposit along igneous rocks of the Choiyoi magmatic province from Sato et al (2015). B) Location of Late Paleozoic to Early Jurassic porphyry copper deposits of the Andes in Chile and Argentina. Numbers in pa-rentheses are K-Ar ages from Sillitoe (1977), Camus (2003), and Sillitoe and Perelló (2005). Age for La Voluntad is on Re/Os molybdenite [Garrido et al, 2008). Age for San Jorge is on U/Pb zircon (286 Ma) and on Ar/Ar biotite (288 Ma) ages from this study.

 

Figure 2. Geological map showing the location of the San Jorge porphyry copper deposit. Modified from Williams (1999) and Compañía Minera San Jorge SA (2008).

The results contribute to understanding the Andean Paleozoic porphyry systems and provide insights into the evolution of the western margin of Gondwana during the Permian.

GEOLOGICAL SETTING

The San Jorge deposit (32°10’S- 69°27’W) is located in the Uspallata valley between the eastern foothills of Cordillera Frontal and Precordillera geologic provinces, at approximately 100 km WNW of Mendoza city, Argentina (Fig. 2).

The copper deposit is hosted by Carboniferous clastic rocks and Permian intrusions within the Permo-Triassic belt that stretches along the border between Chile and Argentina (Camus, 1998; Mpodozis and Ramos, 1989; Fig. 1).

The geology in the deposit área is inte-grated by Carboniferous sediments of the Yal-guaráz Formation (Amos and Rolleri 1965). The sequence has mainly red and grey sub-arkosic sandstones interbedded with several layers, from 20 cm to 1.5 m, of conglomer-ates and shales. Structurally it is homoclinal with a strike from 0^o to N35°W and a dip-ping of 10°-45°W. They lie unconformably over Ciénaga del Medio Formation basement formed by Devonian marine greywackes , siltstones and shales (Amos and Márchese 1965). A Permian granodiorite body (K-Arbiotite age of 270 ± 4 Ma, Sillitoe 1977) in-truded the Yalguaráz sedimentary sequence and outcrops in the southeastern edge of this área. An andesitic dike crosscuts this body in the southern part of the deposit and is con-sidered post-Permian in age. Several breccia types have been recognized in the deposit, i.e. igneous, silicified and tourmaline brec-cias. The main fault structures such as the Gorda, Flaca, Raya Roja, and Portezuela have NS and N-NE strikes (Fig. 2).

Ignimbrites and andesites from the Per-mo-Triassic Choiyoi Group overlie the Yalguaráz Formation (Llambías 1999). A sig-nificant amount of alluvial deposits were produced during the Tertiary due to the rapid Andean uplift of the Cordillera Frontal and the Precordillera which filled the Us-pallata-Calingasta-Iglesia Graben.

The main structural system has an N-NNE orientation and is parallel to the Uspallata graben, whereas secondary structures show NNW, NW, NE, and EW directions (Compañía Minera San Jorge S.A., internal report 2008).

ANALYTICAL METhODS

Thirty nine samples were collected from the less altered granitic outcrops and twelve samples along the drill cores SJT4 (Fig.4a) and SJT15 (Fig.4b) also shown in Fig. 2. All samples were affected by a pervasive mete-oric alteration that obscures and oblitérate the primary features. Outcrop samples 1, 4, 25, 26, 27, and 33 correspond to gran-ite porphyry Samples 30 and 32 are from granodiorite porphyry. Sample 34 comes from an alkali feldspar granite dike.

The geochemical investigation was car-ried out by analysing 12 samples for major and trace elements of granitic rocks. The analyses were realized by ICP-MS at ACMÉ, LF202 total whole rock characterization (Canadá). International geostandards were used for calibration. Major elements were not used for igneous rock classification schemes because the rocks present supergene and hydrothermal alteration that modify their chemical compositions; only immobile trace elements were used for tectonic setting discrimination (Table 1).

The geochronological studies were done on three samples using 4oAr/39Ar dating and three samples with U-Pb methods. Drill core samples from SJT15 at 112.4 m and SJT4 at 57.10 m depths were dated with 4oAr/39Ar measurements. For the first drill core, a magmatic biotite from granodiorite porphyry and a hydrothermal biotite from a veinlet that crosscuts this rock were dated. For the second drill core a pervasive hydrothermal biotite from subarkosic sandstone was selected. Sample locations were plotted on the geo-logic map (Fig. 2), and their description is summarized in Table 2.

The rock samples were crushed in a ring mili, washed in distilled water and ethanol, and sieved when dry to -40 +60 mesh. Ap-propriate mineral grains were picked out of the bulk fraction. Mineral separates were wrapped in aluminium foil and stacked in an irradiation capsule with similar-aged samples and neutrón flux monitors (Fish Can-yon Tuff sanidine (FCs), 28.03 Ma (Renne et al. 1998). The samples were irradiated at the McMaster Nuclear Reactor in Hamilton, Ontario, for 45 MWH, with a neutrón flux of approximately 6 x 1013 neutrons/cm2/s. Analyses (n=30) of 10 neutrón flux monitor positions produced errors of <0.5% in the J valué. The samples were analysed at the Noble Gas Laboratory Pacific Centre for Isotopic and Geochemical Research, Uni-versity of British Columbia, Vancouver, BC, Canadá. The mineral separates were step-heated at incrementally higher powers in the defocused beam of a 10W C02 láser (New Wave Research MIR10) until fused. The gas evolved from each step was analysed by a VG5400 mass spectrometer equipped with an ion-counting electrón multiplier. All measurements were corrected for total system blank, mass spectrometer sensitivity mass discrimi-nation, radioactive decay during and subse-quent to irradiation, as well as interfering Ar from atmospheric contamination and the irradiation of Ca, Cl and K (isotope produc-tion ratios: (4oAr/39Ar)K=0.0302±0.00006, (3 7Ar/3 9Ar)Ca=1416.4±0.5,(3 6Ar/

 

Table 1. Chemical analyses of samples from San Jorge porphyry copper deposit (SJPD). Samples 1,4, 25, 26 and 27: granite porphyry; samples 30, 32 and 33: granodiorite porphyry ( G2); granodiorite porphyry samples (G1) from drill cores SJT15-101m and SJT15 -112m; sample 34: alkali feldspar granite dike.

Sample	1	4	25	26	27	30	32	33	34	SJT101	SJT112
S¡02	68.8	67	66.04	71	68.41	72	71.9	67.9	65.3	67.7	65.96
T¡02	0.6	0.6	0.58	0.5	0.6	0.53	0.5	0.6	0.61	0.61	0.65
AI203	14.6	14.8	15.02	13.7	15.1	14.4	14.3	14.2	15.8	15.7	16.03
Cr203	0.01	0.07	0.002	0.01	0.01	0	0.03	0.02	0	0	0.003
Fe203	1.42	2.25	2.66	2.02	1.22	1.69	2.05	2	2.56	3.4	3.79
MnO	0.01	0	0.01	0.1	0	0.02	0	0.01	0.01	0.01	0.01
MgO	0.7	1.1	0.91	1.6	0.8	0.94	1.3	1	1.4	1.54	2.37
CaO	0.3	0.2	0.15	0.3	0.3	0.85	2.8	0.2	0.29	0.4	1.35
Na20	0.58	0.56	0.47	0.33	0.25	3.07	3.84	0.46	0.77	3.34	4.39
K20	11.1	10.8	11.53	8.51	11	4.51	1.65	10.7	10.5	4.15	2.5
P205	0.2	0.2	0.01	0.2	0.2	0.16	0.2	0.2	0.18	0.21	0.19
LOI	1.3	1.8	1.7	1.2	1.5	1.6	1.1	1.6	1.8	2.6	2.5
Total	99.62	99.38	99.22	99.5	99.39	99.8	99.67	98.89	99.2	99.6	99.74
(Nb/Zr)N	1.14	1.02	1.01	1.01	1.04	1.08	1.03	0.89	0.97	0.97	0.86
Ba	1987	1840	1626	1261	1273	189	126	2016	1280	306	431
Rb	258	251	276.9	202	272	165	56.5	246	297	154	119
Sr	124	125	71	72	97	218	360	108	102	77.1	174
Y	7	13.3	11.7	15.1	17.8	9.7	14.6	5.8	12.2	7.3	7.2
Zr	157	176	175.9	167	177	157	149	183	170	184	183.7
Nb	11.4	11.4	11.3	10.7	11.7	10.8	9.8	10.4	10.3	11.2	10
Th	10.2	11.9	7.9	8.3	9.1	5.3	6.1	10.4	7.4	6.6	4.2
Pb	9.7	7.8	8.2	8.4	9.2	12.1	9.2	6.1	6.9	51.2	7.7
Ga	14	18	13	18	14	18.6	19	15	17.3	23	24.5
Zn	29	22	23	11	6	141	255	20	31	81	63
Cu	1557	3371	4336	2883	4324	715	1945	6971	4044	2050	733.9
V	48	66	63	52	57	53	61	52	65	59	72
Hf	4.2	5.1	4.7	4.3	5	4.1	4	5.9	5	5.2	5
Cs	3.4	3.6	4.3	3.5	4.6	5.5	3	3.2	6.2	9.3	9
Se	4	5	5	6	5	5	6	5	6	5	7
Ta	1.1	1	0.9	0.9	1	1.1	1	1	1.1	0.9	1.3
Co	3.4	4.4	4.2	1.9	1	3.7	7.5	3.5	5	13.3	7.9
Be	2	1	4	1	1	1	1	1	1	2	4
U	5.2	4	2.9	2.3	3.5	1.4	1.5	4.2	3.7	3.9	2.4
W	38	42	32.1	30	51	25.4	47	39	56.2	32.7	3.6
Sn	12	15	19	23	18	5	8	30	17	9	2
Mo	4.6	13	9.7	1.6	2.4	2.5	6.2	4.6	1.5	4.4	6.3
Au	431	57.1	11.8	58.4	89.4	21.5	270	171	64.2	236	44.1
Ni	47	230	9.8	32	37	10.4	116	92	7.7	9.5	12.4
As	43.9	36	273.2	12.4	148	123	78.1	62.3	75.6	384	40.2
Cd	0.1	0.1	0.1	0.1	0.1	1	5.3	0.1	0.3	0.7	0.2
Sb	0.6	0.8	3.5	0.4	2	2.1	1.6	0.4	0.9	1	0.3
B¡	7.2	7	10.3	25	22	1.9	19	15	8.3	11.6	1.6
Ag	0.3	0.5	0.6	1	1.1	0.5	1.1	1.1	0.8	0.8	0.4
TI	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.4
Se	0.9	0.8	7.2	0.5	1.4	0.5	1.2	2.8	0.9	4.8	0.9
La	3.1	52.3	27.7	32.2	36.4	5.5	10	6.7	21.1	21	3.8
Ce	8.3	108	55.7	67	73.4	14.7	29.9	14.4	44.8	42.1	8.6
Pr	1.05	11.84	6.42	7.19	8.1	1.92	4.04	1.74	4.94	4.54	1.15
Nd	5.3	42	24.9	27	28	9.8	18	7.4	19.3	17.4	5.1
Sm	1.5	7.5	4.6	5.4	5.5	2.17	4.2	1.5	3.73	2.99	1.27
Eu	0.4	0.67	0.59	0.74	1.08	0.59	0.94	0.38	0.73	0.45	0.61
Gd	1.8	4.99	4.07	4.35	4.72	2.12	3.56	1.54	3.18	2.48	1.49
Tb	0.3	0.6	0.57	0.7	0.7	0.32	0.5	0.2	0.49	0.31	0.23
Dy	1.7	3.2	2.63	3.4	3.6	1.52	2.7	1.2	2.62	1.43	1.28
Ho	0.3	0.5	0.47	0.6	0.7	0.36	0.5	0.2	0.44	0.3	0.21
Er	0.75	1.18	1.27	1.55	1.79	0.86	1.44	0.5	1.2	0.82	0.71
Tm	0.1	0.2	0.2	0.2	0.3	0.12	0.2	0.1	0.18	0.12	0.11
Yb	0.7	1.3	1.22	1.5	1.9	0.73	1.4	0.6	1.08	0.66	0.63
Lu	0.1	0.2	0.18	0.2	0.2	0.13	0.2	0.1	0.16	0.13	0.14

Table 2. Descnption of dated samples from SJPD.

Sample	Location	Sample Description plus hydrothermal alteration	Dated material	Method	Ages	Other Ages
25	S32°14'46 " WÓ9°26'17"	Granite porphyry. K feldspar, quartz and biotite phenocrysts in quartz- K feldspar matrix. Accesory minerals as apatite and zircon. Tourmaline occurs as radiating clusters and in veinlets. Low potassic alteration	zircon	U-Pb	285.7± 3.0	nd
30	S32°14'57" W69°26'12"	Granodioritic porphyry. Mainly plagioclase phenocrysts, minor K feldespar, quartz and biotite in felsic matrix. Tourmaline occurs as disseminated radiating clusters and in hairlines in the magmatic rocks. Modérate potassic alteration: biotite in the matrix and replacing K feldespar phenocrysts. Vein of K feldespar + quartz.	zircon	U-Pb	258.8 ±3.5	biotite. K/Ar 270±4 Ma (Sillitoe 1977)
34	S32°14'50" WÓ9t>26'19"	Alkali feldspar granite porphyry dyke. Mainly K feldspar, quartz and biotite phenocrysts in felsic matrix. High pervasive phillic alteration occurs in phenocrysts and matrix.	zircon	U-Pb	257.1 ±2.9	hydrotermal biotite. K/Ar 263 ± 6 Ma (Williams 1999)
SJT15 112.4 m	S32°14'48" W69°26'18"	Granodioritic porphyry. Mainly plagioclase phenocrysts with minor K feldespar and quartz in K feldspar, quartz and biotite matrix . Potassic alteration	magmatic biotite	40Ar/MAr	288.0 ±2.0	nd
SJT15 112.4m	S32°14'48" W69°26'18"	FK+Bi+Qtz replacement veinlet that cut granodioritic porphyry	hydrothermal	40Ar/39Ar	285.7 ±6.1	nd
SJT4 57.10 m	S32°14'53" W69°26'22"	Subarkosic sandstone. Pervasive biotite alteration that destroyed sedimentary features	hydrothermal biotite	40Ar/39Ar	282.3 ±2.0	Sericite. K/Ar 257 ±5Ma (Williams 1999)

 

39Ar)Ca=0.3952±0.0004, Ca/K= 1.83±0.01 (37ArCa/39ArK)). Details of the analyses, in-cluding plateau 7 (spectrum) and inverse correlation plots, are presented in Excel spreadsheets. Initial data entry and calcu-lations were carried out using the software ArArCalc (Koppers 2002). The plateau and correlation ages were calculated using Iso-plot ver.3.09 (Ludwig 2003). Errors are quoted at the 2-sigma (95% confidence) level and are propagated from all sources except for mass spectrometer sensitivity and age of the flux monitor.

For the U/Pb dating three samples (25, 30 and 34) were dated using the U-Pb single zircon láser ablation multicollector ICP-MS technique, sample 25 from a granite porphyry, sample 30 from a granodiorite por-phyry and sample 34 from an alkali-feldspar granitic dike. The samples were crushed and milled, and zircons were handpicked under a binocular microscope. They were mount-ed in epoxy and polished for láser ablation analysis. Textural studies were conducted using a microscope or scanning electrón microscope; prior to analysis the zircons were selected by size and crystallographic integrity That is, the crystal should not have cracks or altered outer surfaces, and the sys-tem behaviour should have remained isoto-pically closed and their U, Th and Pb dates be concordant (Richards and Noble 1998). Zircons were analysed by LA-MC-ICP-MS (Láser Ablation Multi Collector Inductively Coupled Plasma Mass Spectrometry) at the Centro de Excelencia en Geotermia de los Andes (CEGA), at the University of Chile. The ablation system used is an ArF excimer láser with a wavelength of 193 nm (Photon Machine Analyte G2) coupled to the Nep-tune Plus MC-ICP-MS instrument (Thermo Scientific). The láser was operated at a fre-quency of 7 Hz and its ablation diameter was 30 ¡im. The calibration of the MC-ICP-MS was performed by ablation of standard Plesovice zircons (Slama et al., 2008). The data reduction is performed by the operator with the Iolite software (Patón et al., 2010) and the results are plotted with the Excel Isoplot supplement (Ludwig, 2012).

RESULTS Petrography

In the deposit the porphyry rocks, based on phenocrystal composition and propor-tions, were identified as granite and two granodiorites (Gl and G2) and an alkali feldspar dike. The granite porphyry analysed in this work is equivalent to the granodiorite porphyry described by Williams et al. (1999). All these rocks intrude subarkosic grey sandstone, with quartz clasts up to 172 /im in size, from the Yalguaráz sequence (SJT4- 57.10 m, Fig. 2 and Fig. 4a) that ac-cording to Williams et al. (1999) host the main mineralization.

The granite porphyry is light grey with porphyritic texture and outcrops in the central part of the deposit (Fig. 2). The phenocrystal to groundmass relations are 40:60, and K-feldspar, quartz, plagioclase and bio-tite are phenocrystals (Fig. 3A). Euhedral-subhedral cloudy K-feldspar and quartz are the most abundant and range from 2 to 8 mm in size (Fig. 3B). Plagioclase crystals are scarce and are altered to sericite. Primary biotite crystals range from 0.5 to 3.5 mm. The groundmass is composed of fine-grained quartz, feldspar and biotite with zircon and apatite as accessory minerals.

Two granodiorite porphyries were deter-mined. The first granodiorite porphyry (Gl) is located at drill core SJT15 where a litho-logical change from granite porphyry at the surface to granodiorite porphyry at a depth of 101-112.4 m has been observed (Fig. 3C and Fig. 4b). The contact between both rocks seems to be transitional. It has phenocryst to groundmass relations of 60%-40% and is dominated by plagioclase with quartz, K-feldspar and biotite. Plagioclase phenocrysts (oligoclase-andesine) are coarse ranging in size from 3 to 13 mm. Quartz crystals are anhedral, 1.2 to 4 mm in size. K-Feldspar (orthoclase) phenocrysts are scarce like biotite. The groundmass feldspars and quartz are generally <0.1 mm in size (Fig. 3C). The second granodiorite porphyry (G2) is located at the eastern margin of the Gorda fault (Fig. 2, samples 30 and 32). It clearly crosscuts the granite porphyry and has a phenocryst     crysts are smaller and less abundant (Fig. to groundmass ratio of 60:40. Phenocrysts     3E). The groundmass is felsic, and zircon are mostly plagioclase and quartz with sizes     and apatite are present as accessory miner-up to 2.5 mm. Biotite and K-feldspar phenoals.

Figure 3. A) Granite porphyry with quartz, feldspar and biotite phenocrystals in felsitic ground-mass. B) Detailed view of K feldspar phenocrystal with cloudy appearance. C) Granodiorite porphyry G1. Mainly plagioclase phenocrysts with minor K-feldspar and quartz in felsitic ground-mass (drillhole, SJT15-112.4 m). D) K feldspar, biotite and quartz replacement veinlet that crosscuts granodiorite porphyry G1 (in drillhole SJT15-112.4 m). E) Granodiorite porphyry G2 (sample 30) showing plagioclase, quartz and biotite phenocryst in a felsitic groundmass. F) Alkali-feldspar granite porphyry dike (sample 34). Qtz:quartz; Kfs: K-feldspar; Pl: plagioclase; Bt: biotite.

 

2.458.600                                   2.458.800                                   2.459.000
Figure 4a. Section 2775N. Taken and modified by Compañía Minera San Jorge SA [2008].

An alkali-feldspar granite porphyry dike crosscuts the granite porphyry in the central-SW área and is equivalent to the andesitic dike described by Williams et al. (1999). The dike (sample 34) has a porphyritic texture with abundant and coarse K-feldspar phe-nocrysts (<8 mm) and minor plagioclase, biotite and quartz. The groundmass has a felsic composition (Fig. 3F).

GEOCHEMÍSTRY

Major, trace and rare earth element chemical analyses of granite, granodiorite porphyries and the alkali-feldspar dike are shown in Table 1.

The spider diagram of trace elements and the REE diagram have been normalized to primordial mantle (Taylor and McLennan 1985) and chondrite (Boynton 1984) respectively (Fig. 5A and Fig. 5B). The granite, granodiorite porphyries and alkali feldspar dike show an enrichment in Rb, U, Hf and Zr, and a depletion in Th, Nb, Ta and Y. The spi-der diagram of these types of rocks (Fig.5A) indicates that LILEs are enriched relative to HFSEs with the Rb concentration being the highest related to others trace elements. Ba is enriched in granites and alkali feldspar dike and depleted in the granodiorites. Sr is higher in granodiorites than granites and dike. Nb and Ta negative anomaly is indica-tive of a continental margin zone magmatism according to Pearce (1984). A minor negative anomaly in La and Ce in granodiorites in Gl and G2 is observed. La and Ce anomaly is not observed in the REE diagram.

 

Figure 4b. Section 2875N. Taken and modified by Compañía Minera San Jorge SA (2008).

The total rare earth element content (LREEs) is 25 to 166 ppm for granites and 25 to 95 ppm for granodiorites. The gran-ite is enriched in light rare earth elements (LRREs) related to granodiorite, with (La/ Yb)_N ratios of 12.9 to 15.4 and 4.1 to 5.1 respectively The granite shows a significant negative Eu anomaly suggesting plagioclase fractionation compatible with Sr negative anomaly. The granodiorites (Gl and G 2) show a slightly negative Eu anomaly. They present a constant level of HREEs and do not show any trend consistent with the fractionation of any particular phase represented by garnet or hornblende (Fig. 5B). The Ta vs Nb tectonic diagram enables the discrimi-nation between are, within-plate, collisional and orogenic granites (Pearce et al. 1984). The studied samples plot between volcanic are and syn-collisional granites (Fig. 5C). In the Zr vs (Nb/Zr) diagram, the samples plot mainly in the field of subduction-zone magmatism and post-orogenic zone rocks re-ported by Thiéblemont and Tegyey (1994) (Fig. 5D).

ALTERATION AND MINERALIZATION

Detailed hydrothermal alteration stud-ies were done over surface samples, and on SJT15, SJT4; in SJT5 drill hole some results were taken from Garrido et al. (2010). The earlier type of alteration has been identi-fied as tourmalinization. Tourmaline oceurs as radial fibrous crystals replacing feldspar and quartz phenocrystals of granite porphyry (Fig. 6A and B); there is a second tourmaline event as breccias cement in the granite porphyry and sedimentary rocks (Fig. 6C). Following the tourmalinization, potassic and phyllic events occurred affecting the granite porphyry, the sandstone wall rock (Yalguaráz Formation) and the tourmaline breccia body (Garrido et al. 2010).

Figure 5. A) Normalized to primordial mantle (Sun and McDonough 1989) diagram of SJPD. B) Chondrite normalized REE diagram (Boynton 1984). C) Ta vs Yb diagram (Pearce et al. 1984). D) zr vs (Nb/zr)_N diagram of Thieblemont and Tegyey (1994).

 

 

Potassic alteration is represented by bio-tite, K-feldspar, quartz, associated with pyrite and chalcopyrite. Biotite occurs as pervasive-ly in sandstones (SJT4-57.10m, Fig.6D and 6E), Gl granodiorite porphyry groundmass (Fig. 3C), and in veinlets (SJT15-112m, Fig. 3D). These veinlets also have K-feldspar, quartz, chalcopyrite and pyrite (Fig. 6F and G); quartz and biotite; quartz with minor K-feldspar or biotite that crosscut the grano-diorite porphyry (Fig. 3D) .

Figure 6. A) Tourmaline soles as replacement of potassic feldspar in granite porphyry. B) Same as A with plane polarized light. C) handspecimen of tourmaline breccia. D) Sandstone with potassic alteration. Disseminated biotite in the groundmass (drillhole SJT4- 57.10m). Cross polarized light. E) Same as D with plane polarized light. F) Potassic alteration veinlet with quartz, K-feldspar and sulphides.

 

Figure 6, continuation. G) Same as F showing pyrite (py) and chalcopyrite (cpy). H)- Chalcocite (Cc) and covellite (Cv) halos as replacement of chalcopyrite (Ccp). I) Pyrite and chalcopyrite surrounded by chalcocite. J) Brochantite (Bro) and malachite (Ma) vein that crosscut tht sandstone rock.

 

The potassic alteration is overprinted by a phyllic alteration that consists of veinlets of quartz and sericite; sericite; and quartz, pyrite, chalcopyrite and arsenopyrite. Wil-liams et al. (1999) found that the phyllic alteration zone has an elongated shape with a dominant NNE-SSW direction with potassic mineralized cores. The mineralization occurs in the granite porphyry and in the sedimen-tary sequence that has the highest Cu con-tents (1% to 3%). At the Western margin of the Gorda fault, the total sulphide content is 1% with Py:Cpy <<1. The phyllic alteration has an external pyrite halo with pyrrhotite, arsenopyrite and minor chalcopyrite, dis-

seminated in the granite porphyry and dis-seminated and in veinlets in the sandstone. Supergene minerals consist of digenite, chalcocite and covellite in the enriched zone (Fig.6H and I); malachite and brochantite (Fig. 6J) in the oxide zone; and goethite, hematite and jarosite in the gossan zone.

GEOCHRONOLOGY

With 4oAr/39Ar three incremental heat-ing age spectra were obtained: one spectra is on magmatic biotite and two spectra are on hydrothermal biotites. One hydrothermal biotite is related to a replacement veinlet and the other to a pervasive alteration in sandstone (Table 2). The best estímate of the age from the igneous biotite phenocrysts in the granodiorite porphyry Gl (drill core, SJT15-2.4m) givesaplateauageof288.0 +/-2.0 Ma (Table 3, Fig. 7A). The reverse isochron forced through the air composition of argon is within error, confirming the age (Fig. 7A’).

The age determination of hydrothermal biotite from replacement veinlet (drill core SJT5-2.4m) shows the step heating pro-file without a clear plateau (Table 3, Fig. 7B). The argón systematics is definitely dis-turbed, probably due to the hydrothermal vein. The reverse isochron age is 285.7 +/-6.1 Ma (Fig.7B’).

Figure 7. ⁴⁰Ar/ ³⁹Ar age spectrum for biotites in drilling samples and their inverse iso-chron diagrams respectively of: A ) and A’) magmatic biotite (mag bi) from granodiorite G1, SJT15-112.4m; B) and B’) hydrothermal biotite (hy bi) vein from SJT15-112.4m; C) and C’) hydrothermal biotite from sandstone, SJT4-57.10m.

 

The best estímate of the age from hydrothermal pervasive biotite (drill core SJT4-57.10m) is given by the plateau age of 282.3 +/- 2.0 Ma. The reverse isochron forced through the air composition of argón is within error, confirming the age (Table 3, Fig. 7C and C’)-

Figure 8. U/Pb weighted average plot for A) granite porphyry, B) granodiorite porphyry G2, and C) alkali-feldspar granite dike.

 

For the U/Pb dating twenty-seven zircon grains were measured from the granite porphyry (sample 25, Fig. 2). The doubly-terminated prismatic zircons analysed are light greenish to colourless and range from 70 to 125 ¡im in size. Zircons from this sample have U concentrations that vary from 423 to 1374 ppm. Cathodoluminescence images show that the zircons present the well-devel-oped growth zoning characteristic of evolved magmas (Corfú et al. 2003).The results are shown in Fig. 8A, 8B, and 8C; each bar rep-resents a spot analysis. These zircons yielded a weighted average 206pb/238U age of 285.7 ± 3 Ma (n=27, MSWD=1.4; Table 4, Fig. 8A). No inherited zircon was recognized.

For the granodiorite porphyry G2, sixteen zircon grains were analysed (sample 30). Zircons from this sample have U concentrations that vary from 595 to 1357 ppm and yielded a weighted average 206pb/238U age of 258.8 ± 3.5 Ma (MSWD= 1.8; Table 4, Fig. 8B). Five grains yielded a Permian inherited component (—273 and 284 Ma).

For the alkali feldspar granite porphyry dike (sample SJ34) only sixteen from twenty-seven zircon analysed grains were chosen; eleven analysed grains were disregarded because they represent inherited zircons (in bold) and Pb loss (in italics). The sixteen analysis selected yielded 206pb/238U ages with a weighted mean of 257.1 ± 2.9 Ma, MSWD= 1.4, probability 0.14. (Table 4; Fig. 8C). However, age variation range is higher than 50 Ma ( 289Ma and 231 Ma). There-fore, the best results are obtained consid-ering only five samples yielding 206pb/238U ages with a weighted mean of 258.4 ± 1.3 Ma. (Fig. 8C).

DISCUSSIONS

The rocks found in San Jorge deposit were a granite porphyry, two granodiorite porphyries; one in SJT15 at 112 m (Gl), other (G2) on the eastern margin of the Gorda fault, and an alkali-feldspar granite dike. The alkali-feldspar granite dike crosscuts the granitic porphyry and is contemporaneous with the granodiorite porphyry G2 (Fig. 2).

The geochemical characteristics from spi-der diagram show Nb and Ta negative anom-aly indicative of a continental margin zone magmatism according to Pearce (1984). The Ta vs Nb tectonic diagram enables the dis-crimination between are, within-plate, col-lisional and orogenic granites. The studied samples plot mainly between volcanic are and syn-collisional granites.

Based on the new biotites 4oAr/39Ar ages and zircon U-Pb ages in granites show that there are two periods of time for hydrother-mal magmatic system. The first group is ear-ly Permian and was obtained on the granitic porphyry (sample 25), on magmatic biotite in the granodiorite porphyry Gl (SJT15 at 112.4 m), and on two biotites from the po-tassic alteration (SJT15 at 112.4 m; SJT4 at 57.10 m).

The two ages from the granodiorite Gl and the granite 25 suggest that the magmatism related to the formation of porphyry copper appears to be within period of time, between 290 Ma and 283 Ma, (early Permian, Fig. 9). The evidences show transitional contact between these rocks and similar geochemical patterns suggest they are probably genetically linked. The 4oAr/39Ar ages on hydrothermal biotites (in vein and pervasive) indicate the associated mineralization overlaps with the granite age (25, U/Pb) and continuous by 2 million years after intrusive emplacement.

In the second group is late Permian, appears to be within period of time between 263 Ma and 254 Ma, (Fig.9) and are rep-resented by the granodiorite porphyry G2 (sample 30) and the alkali feldspar granite dike (sample 34). G2 outerops in the south-ern part of the deposit and it is separated from granite porphyry by an inferred fault (Gorda fault). Evidences of mineralization are not observed in this group.

 

Tablas 3 y 4

 

The copper mineralization and alterations oceur in the sandstones from the Yal-guaráz Formation and in lesser amount in the granite porphyry. A hydrothermal biotite from a vein that crosscuts the granodiorite porphyry Gl (SJT15-112.4 m, 288 Ma 4oAr/ 39Ar) yielded 285.7 ±6.1 Ma (4(Ar/39Ar) age, and a pervasive hydrothermal biotite in sandstone (SJT4-57.10 m) gave an age of 282.3 ± 2.0 Ma (4oAr/39Ar ). These two ages overlap with the granite porphyry age (285.7± 3 U/Pb) suggesting that the potas-sic alteration is related to the early Permian granite porphyry (Fig. 9) and the hydrother-mal system remains active during 2 Ma post emplacement of granite.

Figure 9. Summary of the timing of igneous and hydrothermal episodes at San Jorge porphyry copper deposit. 25- Granite porphyry; 30- Granodiorite porphyry G2; 34- Alkali feldspar granite dike; SJT15, SJT4 drillholes; MBi: magmatic biotite; hBi: hydrothermal biotite.

 

Mpodozis and Ramos (1990) defined an Early Permian (286-272 Ma) metallogenic province in the less eroded parts of a volcanoplutonic are developed along the Gond-wana Pacific margin in Chile and western Argentina. In this province there are several porphyry copper deposits, being San Jorge deposit one of them. On the other hand, porphyry copper oceurrences in Chile and western Argentina yielded Late Permian to Early Jurassic ages and are part of a large gran-ite-rhyolite igneous province named Choiyoi (Kay et al 1989). LLambias and Sato (2011) and Sato et al (2015), define Choiyoi Mag-matic Province between an Early Permian San Rafael orogenic phase and the Triassic extensional Huarpica phase in the región of Argentine Frontal Cordillera, Precordillera and San Rafael Block (U/Pb 286-247 Ma, Early Permian-Early Triassic).

In this paper, according to new U/Pb and 4oAr/39Ar geocronological results, mineral-ized and no mineralized San Jorge granites intégrate the intrusive event of the Choiyoi Group. The obtained ages differ from K-Ar age of 270±4 Ma (Sillitoe 1977) on mag-matic biotite from a granodiorite stock (de-nominated as granite porphyry in this paper) and from K-Ar age of 263 ±6 Ma (Williams et al. 1999) on hydrothermal biotite from por-phyritic andesite dike (denominated alkali feldspar granite porphyry in this paper).

CONCLUSIONS

The detailed petrography indicates that in the deposit the igneous activity has under-gone two episodes. The first is represented by granite and granodiorite porphyry Gl and the second by another granodiorite porphyry G2 and a dike. The granite porphyry and the alkali-feldspar granite dike correspond to the granodiorite porphyry and andesite dike described by Williams et al. (1999).

Spider diagram show Nb and Ta negative anomaly indicative of a continental margin zone magmatism according to Pearce (1984). The Ta vs Nb tectonic diagram enables the discrimination between are, within-plate, collisional and orogenic granites (Pearce et al. 1984). The studied samples plot mainly between volcanic are and syn-collisional granites.

An Early Permian magmatic event is re-sponsible for the granite and granodiorite porphyry Gl intrusions and the potassic alteration-mineralization. A second event of Late Permian age is represented by the granodiorite porphyry G2 and the alkali feldspar granite dike. The results indicate the hydrothermal system in Early Permian, continued for 2 Ma after the emplacement of the intrusive body

The results suggest that the evolution of the western margin of Gondwana began in late Carboniferous and continued during the Permian period. In this Paleozoic to early Jurassic metallogenic belt, La Voluntad Cu-Mo porphyry is the oldest deposit (early Pennsylvanian, Garrido et al. 2008). Mineralized and no mineralized San Jorge granites (Early Permian-Late Permian) intégrate the intrusive event of the Choiyoi Group.

ACKNOWLEDGMENTS

We are grateful to the Universidad Nacional del Sur (Bahia Blanca, Argentina) for the financial support through research grants (SECYT) and PICT 2013-2713 of ANPCYT. We also thank Dr Angelo Peri (Vice Presi-dent of Exploration of the Mining Company Coro Mining Corporation) for allowing us access to the deposit and especially to Geolo-gist Alejandro Palma (Compañía Minera San Jorge, Argentina) for his logistical support and for facilitating the access to samples of the drilling holes needed to carry out this study We wish to thank Dr. Fernando Barra for performing U/Pb analysis. We also ap-preciate the support given by Dr Eduardo A. Domínguez for very helpful discussions on the manuscript.

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