Tectonic inversion in a segmented foreland basin from extensional to piggy back settings: The Tucumán basin in NW Argentina

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Journal of South American Earth Sciences 31 (2011) 432e443

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Journal of South American Earth Sciences

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The Cretaceous sediment-hosted copper deposits of San Marcos (Coahuila,Northeastern Mexico): An approach to ore-forming processes

Donají García-Alonso a,b, Carles Canet b,*, Eduardo González-Partida c, Ruth Esther Villanueva-Estrada b,Rosa María Prol-Ledesma b, Pura Alfonso d, Juan Antonio Caballero-Martínez e, Rufino Lozano-Santa Cruz f

a Facultad de Ciencias de la Tierra, Universidad Autónoma de Nuevo León, Carretera a Cerro Prieto Km. 8, Linares, Nuevo León, Mexicob Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, 04510 México D.F., MexicocCentro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Boulevard Juriquilla 3001, 76230 Santiago de Querétaro, Qro., MexicodDepartament d’Enginyeria Minera i Recursos Minerals, Universitat Politècnica de Catalunya, Avinguda Bases de Manresa 61-73, 08242 Manresa, Catalunya, Spaine Servicio Geológico Mexicano, Blvd. Felipe Ángeles Km. 93.5, Venta Prieta, 42080 Pachuca, Hidalgo, Mexicof Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, 04510 México D.F., Mexico

a r t i c l e i n f o

Article history:Received 3 August 2010Accepted 26 February 2011

Keywords:Red-bedsStratabound depositsSilver depositsSupergene alterationRiftingTransgressive

* Corresponding author. Tel.: þ52 55 56224133; faxE-mail address: [email protected] (C. Can

0895-9811/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jsames.2011.02.012

a b s t r a c t

In the San Marcos ranges of Cuatrociénegas, NE Mexico, several sediment-hosted copper deposits occurwithin the boundary between the Coahuila Block, a basement high mostly granitic in composition andLate Paleozoic to Triassic in age, and the Mesozoic Sabinas rift basin. This boundary is outlined by theregional-scale synsedimentary San Marcos Fault. At the basin scale, the copper mineralization occurs atthe top of a w1000 m thick red-bed succession (San Marcos Formation, Berrisian), a few meters belowa conformable, transitional contact with micritic limestones (Cupido Formation, Hauterivian to Aptian). Itconsists of successive decimeter-thick roughly stratiform copper-rich horizons placed just above the red-beds, in a transitional unit of carbonaceous grey-beds grading to micritic limestones. The host rocks arefine- to medium-grained arkoses, with poorly sorted and subangular to subrounded grains. The detritalgrains are cemented by quartz and minor calcite; besides, late iron oxide grain-coating cement occurs atthe footwall unmineralized red-beds. The source area of the sediments, indicated by their modalcomposition, is an uplifted basement. The contents of SiO2 (40.70e87.50 wt.%), Al2O3 (5.91e22.00 wt.%),K2O (3.68e12.50 wt.%), Na2O (0.03e2.03 wt.%) and CaO (0.09e3.78 wt.%) are within the ranges expectedfor arkoses. Major oxide ratios indicate that the sedimentary-tectonic setting was a passive margin.

The outcropping copper mineralization essentially consists in a supergene assemblage of chrysocolla,malachite and azurite. All that remains of the primary mineralization are micron-sized chalcocite grainsshielded by quartz cement. In addition, pyrite subhedral grains occur scattered throughout the copper-mineralized horizons. In these weathered orebodies copper contents range between 4.24 and 7.72 wt.%,silver between 5 and 92 ppm, and cobalt from 8 to 91 ppm. Microthermometric measurements of fluidinclusions in quartz and calcite crystals from footwall barren veinlets gave temperatures of homogeni-zation between 98 �C and 165 �C, and ice-melting temperatures between �42.5 �C and �26.1 �C.

The primary copper mineralization formed during the early diagenesis, contemporary with the activelife of the Sabinas Basin. The mineralizing fluids were dense, near neutral, moderately oxidized brinesthat originally formed from seawater that, driven by gravity, infiltrated to the deepest parts of the basinand dissolved evaporites. As a result, they became hydrothermal fluids of moderate temperature capableof leaching high amounts of copper. The source of this metal could be mafic detrital grains and ironoxides of the underlying Jurassic and Lower Cretaceous red-beds. Copper precipitation took place whenthe brines passed through the redox boundary marked by the transition from red- to grey-beds. Theupward movement of the brines was promoted by a high heat flow that allowed their convectivecirculation and their ascent along the synsedimentary San Marcos Fault.

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1. Introduction

Only surpassed by the porphyry copper-type, sediment-hostedcopper (SHC) deposits are the second source for this metal, prob-ably containing about 20e25% of the global copper reserves

D. García-Alonso et al. / Journal of South American Earth Sciences 31 (2011) 432e443 433

(Kirkham, 1989). They are also a significant source for silver andcobalt and, to a lesser extent, for zinc, lead and uranium. A few SHCdeposits, as well, have economic concentrations of vanadium, goldand platinum-group elements (Hitzman et al., 2005).

SHC deposits, which frequently have been labeled as “strati-form” or “stratabound” and/or “diagenetic”, have been divided intotwo broad categories (e.g., Kirkham, 1989; Hitzman et al., 2005;Cabral et al., 2009): (a) The Kupferschiefer type, given by copperdeposits of regional extent but stratigraphically restricted tocarbonaceous pelitic horizons deposited in shallow marine orlacustrine environments, and (b) the red-bed type, which occurs inthe contact between red and grey clastic horizons in red-beddominated sequences. With average reserves of about 40 Mt andgrades of 1.8% Cu, those of the Kupferschiefer type can be consid-ered as giant copper deposits. On the other hand, the poorly knownred-bed copper-type has a wider range of tonnage and grades,including many deposits of minor importance, with a few excep-tions as the Dzhezkazgan deposit, in Kazakhstan (Gablina, 1981),and Revett, in Montana, USA (Kirkham, 1995).

The San Marcos ranges of Cuatrociénegas, in central Coahuila,northeasternMexico, contain several SHC deposits and occurrencesof the red-bed type, scattered in an area of about 400 km2 (Figs. 1and 2). These deposits and occurrences are thought to be

Fig. 1. Major tectonic features and sedimentary basins of Coahuila (based on Chávez-CabelloSan Marcos ranges near Cuatrociénegas (this study), (B) Sierra Mojada, and (C) Pinos.

representative of a regional SEeNW-trending belt of red-bed SHCdeposits, with likely economic potential extending for almost onethousand km from the state border between Tamaulipas and NuevoLeón, at the southeast, to northern Chihuahua, at the northwest.This metallogenic belt was initially identified by Clark and De laFuente (1978) and named Cupriferous Sandstone Belt (CSB). Itincludes, among others, the deposits of El Huizachal, in Tamaulipas,San Marcos (this study) and Sierra Mojada, in Coahuila, and ElCoyote and Las Vigas, in Chihuahua. Only a few studies have beenpublished on the CSB and they have been carried out primarily inLas Vigas, which, so far, can be considered the biggest copperdistrict in the entire belt (Price et al., 1988). The SHC deposits andoccurrences of the CSB occur systematically (a) hosted within red-bed clastic sequences of one km or more in thickness, whose agesrange from Late Triassic to Early Cretaceous, becoming progres-sively younger from SE to NW, and (b) in the vicinity of regionalfaults that originally acted as normal synsedimentary faults in theedge of basement highs. These faults, which include the SanMarcosand La Babia faults in the Coahuila segment of the CSB (Fig. 1),controlled the evolution and structure of the basins along with thenature of the sediments, and, probably, the circulation of basinbrines, and were reactivated multiple times up to the Plioce-neeQuaternary (Chávez-Cabello et al., 2007).

et al., 2005). The location of the main sediment-hosted copper districts is shown: (A)

Fig. 2. LEFT: location and detailed geological map of the San Marcos sediment-hosted copper deposits and occurrences: (A) Rincón de la Presa, (B) La Rinconada, (C) El Rosillo, and(D) Manto Negro. RIGHT: schematic stratigraphic section (after Bolaños-Rodríguez, 2006 and Chávez-Cabello et al., 2007). (XeY) Location of the geological section shown in Fig. 3.

D. García-Alonso et al. / Journal of South American Earth Sciences 31 (2011) 432e443434

The only copper deposit hitherto mined in the entire CSB is LasVigas, between 1950 and 1973. Copper grades ranged between 2and 4%, and silver and gold were recovered as byproducts, with80e100 g/t and up to 1 g/t, respectively (Giles et al., 1973). On theother hand, the San Marcos SHC deposits were only sporadicallymined before the decade of 1990s by artisanal miners. Most of thecopper ores were extracted from the Manto Negro deposit, with anestimated production between 1968 and 1975 of 100 t at about 7%Cu (Rivera-Martínez, 1993). Preliminary exploration conducted bythe Consejo de Recursos Naturales (former Servicio Geológico Mex-icano) showed copper grades up to 7.1%, and 50 and 90 ppm of Agand Co, respectively (Vargas, 1993).

Further north of the CSB, a major porphyry copper provinceextends from the Trans-Pecos region, in Texas, to southern Arizonaand northern Sonora. However, unlike those of the CSB, thesecopper deposits are genetically connected to the Laramide(75e54 Ma) subduction-related magmatism (Gilmer et al., 2003).

Geological, geochemical, mineralogical and fluid inclusion dataare presented here with the objective of elucidating the genesis ofthe SanMarcos SHC deposits. We also provide clues for the regionalexploration of a metallogenic belt that has remained under-explored despite its economic potential.

2. Geological and tectonic setting of northeastern Mexico

The state of Coahuila, northeastern Mexico, contains, besidesSHC deposits, several fluorite, celestine, barite and leadezincdeposits akin to the Mississippi Valley Type (MVT). In fact, itencloses the most important Sr province elsewhere (González-

Sánchez et al., 2009). Geologically, Coahuila is shaped by alter-nating basement highs and sedimentary basins, a broad structurethat dates back to the opening of the Gulf of Mexico, in Triassictimes (e.g., Padilla y Sánchez, 1986; Salvador, 1987, 1991;Goldhammer, 1999) (Fig. 1). The basement highs are composed ofPaleozoic to Triassic granitic and metasedimentary rocks and,besides determining the nature and distribution of sedimentsduring Mesozoic to Tertiary times, controlled the patterns ofdeformation of the Laramide orogeny, in Late Creta-ceousePaleogene (Horbury et al., 2003; Molina-Garza et al., 2008).The Laramide orogeny resulted in the Coahuila Fold Belt and theSierra Madre Oriental, the two most conspicuous orographicfeatures of northeastern Mexico.

The main basement highs, the Burro-Peyotes Peninsula, the LaMula Island and the Coahuila Block, are bounded by the Sabinasand Parras basins (Fig. 1) (Eguiluz de Antuñano, 2001). Theirerosion led, during Jurassic and Lower Cretaceous, to clasticsedimentation of thick red-bed sequences, including those of theCSB, simultaneously with the development of a calcalkalinevolcanism of local extent (Padilla y Sánchez, 1982; Goldhammer,1999). Two NWeSE regional faults, La Babia and San Marcos,had a pronounced influence on the sedimentation, tectonics andorography since the Triassic (Padilla y Sánchez, 1986; McKee et al.,1984). They began as normal faults in the Triassic, and sufferedseveral reactivations during the active life of the Sabinas Basin andinversions during the Laramide orogenesis (Chávez-Cabello et al.,2007).

During MiddleeLate Jurassic (CallovianeOxfordian), theformation of oceanic crust in the center of the proto-Gulf of Mexico

Table 1Bulk-rock chemical composition (major, minor and trace elements) of selected unmineralized sandstone samples of the Lower Cretaceous San Marcos Formation.

# Sample SiO2

(wt.%)TiO2

(wt.%)Al2O3

(wt.%)Fe2O3t

(wt.%)MnO(wt.%)

MgO(wt.%)

CaO(wt.%)

Na2O(wt.%)

K2O(wt.%)

P2O5

(wt.%)LOI(wt.%)

Total(wt.%)

CIA

1 RC-1 69.60 0.40 12.70 1.26 0.06 1.78 3.33 0.96 6.23 0.06 3.41 99.79 472 RC-4 78.54 0.13 9.04 1.04 0.05 0.59 2.43 1.49 4.00 0.04 3.05 100.40 453 RC-8 71.90 0.30 13.18 1.97 0.02 2.22 0.57 2.02 5.09 0.05 2.51 99.83 574 RC-10 66.57 0.29 13.11 2.45 0.06 1.77 3.78 2.03 5.33 0.06 4.68 100.13 455 RP-7 61.00 0.29 20.40 0.85 0.00 2.45 0.24 0.09 11.00 0.00 3.21 99.54 626 RP-8 58.10 0.72 22.00 1.13 0.00 1.67 0.40 0.38 12.50 0.00 2.77 99.68 607 RP-13 59.70 1.02 20.00 1.13 0.02 3.45 0.41 0.73 10.20 0.03 2.99 99.68 618 MCG32 64.70 0.78 15.70 1.22 0.00 4.43 0.23 1.02 7.04 0.06 2.78 97.96 629 RIN3 87.50 0.00 5.91 0.01 0.10 0.35 0.44 0.54 3.92 0.00 0.64 99.42 5010 RIN4 60.80 1.00 16.30 3.53 0.14 3.95 0.85 0.68 8.46 0.06 3.94 99.71 58

# Rb(ppm)

Sr(ppm)

Ba(ppm)

Y(ppm)

Zr(ppm)

V(ppm)

Cr(ppm)

Co(ppm)

Cu(ppm)

Pb(ppm)

1 143 102 814 47 81 59 47 89 161 112 136 72 970 40 37 163 8 107 1760 83 141 91 888 33 92 47 20 80 495 124 129 121 811 42 82 74 15 78 51 115 220 205 3005 33 62 38 64 20 699 186 165 48 628 29 247 71 25 24 1532 107 176 104 541 30 251 117 42 22 225 318 75 86 2266 17 293 67 45 44 2400 119 136 64 679 28 35 12 15 61 10 1210 145 80 525 30 174 114 39 16 10 10

LOI¼ lost on ignition. CIA¼ chemical index of alteration (CIA¼ [Al2O3/(Al2O3þ CaOþNa2Oþ K2O)]� 100; CaO* represents calcium attributable to silicates). Method ofanalysis: X-ray fluorescence.

D. García-Alonso et al. / Journal of South American Earth Sciences 31 (2011) 432e443 435

took place, followed by a generalized event of thermal subsidence(Pindell, 1985; Salvador, 1991). As a result, what is today north-eastern Mexico underwent a broad marine transgression and thebasement highs were progressively flooded (Echánove, 1988). Onthe large platforms thus formed, the Minas Viejas and Olvidoevaporitic formations were deposited either conformably over red-bed units or unconformably overlying the basement (Goldhammer,1999). Later in the Kimmeridgian and Tithonian, this transgressioncontinued and induced the deposition of sandstones and biopelitesof the La Casita, La Caja and La Pimienta formations, overlying theevaporitic beds (Eguiluz de Antuñano, 2001).

In Early Cretaceous (Berrisian), the opening of the Gulf of Mexicoceased and the subsidence rate decreased, thus favoring theformation of thick marine platform carbonate deposits(Goldhammer et al., 1991). In the center of the Sabinas Basin high-energy platform carbonates and pelites were deposited (MenchacaFormation; Imlay, 1940). However, in the edge of the Coahuila Blockthe sedimentation of red-beds still continued giving place to theSan Marcos Formation. During HauterivianeAptian times, the coralreefs of the Cupido and Lower Tamaulipas formations restricted theconnection of the Sabinas Basin to the open sea. Consequently, thesedimentation of the La Virgen evaporitic formation on a sabkhaenvironment took place (Eguiluz de Antuñano, 2001).

The maximum marine transgression occurred at the end of theAptian, flooding the entire basin and the Coahuila Block. La PeñaFormation (Fig. 2, right), which consists of deep-marine pelites andmicritic limestones, was deposited during this event (Goldhammer,1999). Above it, at the northern edge of the Coahuila Block, plat-form carbonates accumulated, resulting in the Aurora Formation(Fig. 2, right), AlbianeCenomanian in age, which laterally gradesinto the Washita Group (Goldhammer, 1999). It was not until theConiacianeSantonian that the sea began to recede, as reflected bythe sedimentation of shallow-marine limestones and pelites(Austin and Indidura formations; Padilla y Sánchez, 1982;Goldhammer, 1999).

In the Upper Cretaceous (Santonian) the beginning of the Lar-amide orogenesis was accompanied by the accumulation in theSabinas Basin of thick synorogenic clastic series, rich in coal beds,

deposited in deltas and continental alluvial plains (La DifuntaGroup, Maastrichtian to Paleocene; Padilla y Sánchez, 1986). Theorogenic processes shaped the Coahuila Fold Belt, which involvesall the basins and basement highs and consists in NWeSE-trendingtight anticlines and synclines with minor N-vergent thrusts(Goldhammer, 1999). In addition, the Laramide orogenesis trig-gered themobilization of basinal brines that formed abundant MVTdeposits of celestine, fluorite, barite and zincelead by replacementof Mesozoic evaporites and carbonates (González-Sánchez et al.,2009).

3. Sampling and methods

The four most important copper deposits and occurrences of thestudyarea in termsof extent, thickness andoregradeswere sampledand named Rincón de la Presa (RP: 26�43037.4100N/102�4026.1100W),La Rinconada (RIN: 26�43025.4400N/102�6037.6800W), El Rosillo (RC:26�29058.8100N/101�51058.6400W) and Manto Negro (MCG:26�34040.0800N/102�2011.7600W) (Fig. 2). Thirty-seven rock sampleswere collected from these copper occurrences.Mineral assemblageswere studied in 13 polished thin sections using a standard petro-graphic microscope (with transmitted and reflected illumination).The GazzieDickinson point-counting method (Ingersoll et al., 1984)was followed to determine modal sandstone compositions. Addi-tionally, all the polished thin sections were analyzed with a HitachiTM-1000 “table-top” scanning electron microscope (SEM), whichallowed us to obtain back-scattered electron (BSE) images andenergy dispersive spectrometry (EDS) qualitative analyses.

Bulk mineralogy was confirmed in seven samples by X-raydiffraction (XRD), using a Philips 1400 diffractometer equippedwith a Cu anode tube as X-ray source and directing the collimatedCu Ka1,2 radiation (l¼ 0.15405 nm) towards a randomly orientedsample. X-ray radiation was generated at 40 kV and 20 mA. Scanswere recorded from 4� to 70� (2q) with a step-scan of 0.02� and 2 s/step. To support clay mineral characterization, all samples wereanalyzed by short-wave infrared (SWIR) reflectance spectroscopyusing a portable LabSpec Pro Spectrophotometer and following tothe methodology of Canet et al. (2010).

Table 2Bulk-rock chemical composition (major, minor and trace elements) of selected mineralized samples from the San Marcos sediment-hosted copper deposits.

# Sample SiO2

(wt.%)TiO2

(wt.%)Al2O3

(wt.%)Fe2O3

(wt.%)MnO(wt.%)

MgO(wt.%)

CaO(wt.%)

Na2O(wt.%)

K2O(wt.%)

P2O5

(wt.%)SO3

(wt.%)CuO(wt.%)

LOI(wt.%)

Total

1 RC-5B 69.00 0.25 8.82 0.06 0.02 0.88 0.14 0.32 4.19 0.03 0.73 9.14 4.91 98.492 MCG-36 40.7 1.48 19.7 1.98 n.d. 4.72 0.479 0.11 8.17 0.068 1.37 9.66 9.05 97.493 RC-6 72.60 0.30 10.60 0.06 0.33 1.17 0.12 0.40 5.60 0.03 0.06 5.31 3.21 99.794 RIN-7 55.3 0.778 15.4 1.57 0.362 1.20 0.442 0.818 9.92 0.095 0.723 6.17 4.18 96.965 RP-9 59.50 0.44 14.20 5.30 0.03 0.80 0.51 0.36 9.67 n.d. 0.05 6.62 3.57 101.05

# Cu(wt.%)

V(ppm)

Cr(ppm)

Co(ppm)

Ni(ppm)

Zn(ppm)

Ga(ppm)

As(ppm)

Se(ppm)

Sr(ppm)

Y(ppm)

Zr(ppm)

Mo(ppm)

Ag(ppm)

Cd (ppm)

1 7.30 19 3.9 58.3 2.5 2.0 1.1 1.7 2.6 8.6 2.56 1.0 1.5 14.2 <0.012 7.72 32 15.6 10.7 27.4 40.8 8.0 1.3 0.8 37.6 3.22 3.5 1.4 91.9 0.243 4.24 63 4.1 90.8 2.0 2.8 1.4 <0.1 1.3 7.4 4.29 0.6 0.2 5.5 0.034 4.93 12 8.0 8.1 7.1 19.4 3.0 34.9 1.0 10.7 7.76 4.9 38.5 43.2 0.035 5.29 38 5.0 40.1 16.1 70.4 3.6 206.0 1.4 21.4 10.3 8.3 30.7 69.7 <0.01

# In(ppm)

Sb(ppm)

Te(ppm)

Ba(ppm)

Au(ppb)

Pb(ppm)

Bi(ppm)

Th(ppm)

U(ppm)

1 <0.02 <0.02 0.04 121.0 <0.5 7.3 4.8 1.6 6.72 <0.02 <0.02 0.11 55.9 <0.5 97.2 2.1 4.3 5.13 <0.02 <0.02 <0.02 92.8 0.9 6.4 6.0 2.2 9.94 <0.02 <0.02 0.04 52.3 <0.5 42.5 0.3 6.3 28.85 0.04 <0.02 0.02 59.6 <0.5 110.0 0.1 9.2 16.0

LOI¼ lost on ignition; n.d.¼ not detected. Methods of analysis: SiO2 to SO3, X-ray fluorescence; Cu, inductively coupled plasma optical emission spectrometry; V to U,inductively coupled plasma mass spectrometry.

D. García-Alonso et al. / Journal of South American Earth Sciences 31 (2011) 432e443436

Bulk-rock chemical analyses were performed in thirty-threesamples (Tables 1 and 2). Major oxides (SiO2, TiO2, Al2O3, Fe2O3t,MnO, MgO, CaO, Na2O, K2O, SO3 and P2O5) and some minorelements (Rb, Sr, Ba, Y, Zr, V, Cr, Co and Pb) were measured by X-rayfluorescence (XRF) at the Instituto de Geología (UNAM) usinga Siemens SRS 3000 sequential spectrometer. The sample prepa-ration procedure and the reference materials used to build thecalibration curves are explained by Lozano and Bernal (2005). Cuconcentrations were measured in all the samples by inductivelycoupled plasma optical emission spectrometry (ICPeOES), whereastrace elements (V, Cr, Co, Ni, Zn, Ga, As, Se, Mo, Ag, Cd, In, Sb, Te, Au,Bi, Th and U) were analyzed in five mineralized samples byinductively coupled plasmamass spectrometry (ICPeMS) at Actlabs(Ancaster, Ontario, Canada). The software used for the statisticaltreatment of geochemical data was STATISTICS, version 6.0.

Suitable fluid inclusions for the microthermometric study werefound in quartz and calcite veinlets spatially related to the sedi-ment-hosted copper deposits (Table 3). Fluid inclusion data wereobtained from six doubly polished sections (100e150 mm thick),with a Linkam THMSG-600 heating-freezing stage. No data wereobtained from fluid inclusions where leakage was suspected. Cali-bration runs show that measurements are accurate to �0.2 �C forlow-temperature measurements and to �2 �C for high-tempera-ture measurements. Salinities were calculated assuminga H2OeCaCl system (Crawford, 1981).

Conditions of mineralizing fluids involved in primary oreformation were estimated by a geochemical modeling study,

Table 3Summary of fluid inclusion microthermometric data of quartz and calcite veinlets relate

Sample Mineral # Th (�C),max./avg./mi

RC-1 Q 43 146/136/128RC-2 Cal 43 115/108/98RC-3 Q 40 146/140/131RP-2(b) Q 42 159/153/147RP-3(b) Cal 33 130/124/118RP-4(a) Q 43 165/158/150

Key: Th¼ temperature of homogenization, Tmi¼ temperature of ice melting (freezinavg.¼ average; max.¼maximum value. Abbreviations: Cal e calcite; Q e quartz.

performed with the assistance of the software THE GEOCHEMIST’SWORKBENCH, version 6.0.

4. Geology of the San Marcos sediment-hosted copperdeposits

The studied SHC deposits are located about 30 km south from thecity of Cuatrociénegas. They occur within the boundary between theCoahuila Block and the Sabinas Basin, which is outlined by theregional-scale San Marcos Fault (Figs. 1e4). Four SHC occurrencesand deposits are found in both limbs of a faulted NWeSE recumbentanticline. The northeastern limb configures the San Marcos y Pinosrange (Fig. 4A), which encloses the copper occurrences of Rincón dela Presa, La Rinconada and El Rosillo. The southwestern limb shapesthe El Granizo range, where the Manto Negro deposit, the largest inthe study area, crops out (Figs. 3 and 4B).

4.1. Local stratigraphy of the San Marcos area

The basement rocks of the Coahuila Block are very poorlyexposed throughout the study area. A few outcrops adjacent to theSan Marcos Fault comprise metasandstones with minor interlay-ered marble, presumably Late Paleozoic in age (McKee et al., 1990).However, granitic to granodioritic rocks of PermianeTriassic(Wilson et al., 1984) or Late Triassic (McKee et al., 1990; Jones et al.1995; Molina-Garza, 2005) age constitute the predominant clastlithology in the overlying Jurassic conglomerates.

d to the San Marcos sediment-hosted copper deposits.

n.Tmi (�C),max./avg./min.

Salinity (wt.% CaCl eq.),max./avg./min.

�38.5/�38.5/�38.5 35/35/35�26.1/�26.6/�27.0 27/27/26�37.6/�37.6/�37.6 34/34/34�40.0/�41.1/�42.5 39/37/36�35.0/�36.1/�37.0 34/33/32�31.0/�31.0/�31.0 29/29/29

g point depression). #¼ number of analyzed inclusions; min.¼minimum value;

Fig. 3. Schematic geological section of the El Granizo and San Marcos y Pinos ranges, showing the Manto Negro sediment-hosted copper deposit (location and legend shown inFig. 2). Stratigraphic and lithodemic units (in chronologic order): Ps, Paleozoic metasedimentary rocks; Pgr, Late Paleozoic to Triassic granitoids; Jrb, Jurassic red conglomerates andvery coarse sandstones, possibly of La Casita Fm.; Ksm, Lower Cretaceous arkosic red-beds of the San Marcos Fm., which at the top hosts the copper mineralization; Kcu, LowerCretaceous limestones of the Cupido Fm.; Kpe, Lower Cretaceous lutite beds of the La Peña Fm.; Kau, Mid Cretaceous micritic limestones of the Aurora Fm.; Qaf, Quaternary alluvial,lacustrine and playa deposits.

D. García-Alonso et al. / Journal of South American Earth Sciences 31 (2011) 432e443 437

Jurassic rocks mostly crop out in the northern foothills of the ElGranizo range, in reverse fault contact with the overlying Creta-ceous sequence (Fig. 3). With an overall thickness possibly up to2000 m, they consist of red conglomeratic and very coarse-grainedsandstone units, generally decreasing in grain size upward in thesuccession. Their clasts are polymictic, poorly sorted and have a lowdegree of roundness. They could be correlated with the La CasitaFormation (KimmeridgianeTithonian; Eguiluz de Antuñano, 2001).The Jurassic sequence is separated from the overlying Cretaceousseries by a minor syn-rift stratigraphic discontinuity (Fig. 2).Resting on the discontinuity are the Lower Cretaceous red-beds ofSan Marcos Formation, which in the study area attain w1000 m in

Fig. 4. Photographs of the Mesozoic sedimentary sequence, the associated sediment-hosted(A) Landscape view of the southwestern slope of the San Marcos range, showing the sequenformations; (B) weathered stratiform copper orebodies at the top of the San Marcos Formatiohighly weathered copper-bearing sandstones. Abbreviations: Azu e azurite, Chr e chrysoco

thickness. The San Marcos Formation consists mostly of coarse-grained, poorly sorted, red arkoses, commonly showing crossbedding. The base and the top of the San Marcos Formation aremarked respectively by (a) minor conglomerate beds, and (b) greymicritic limestones and carbonaceous siltstones. The SHC miner-alization occurs at the top of the San Marcos Formation, a fewmeters below the transitional contact with the overlying lime-stones of the Cupido Formation (Hauterivian to Aptian; Imlay,1937;Eguiluz de Antuñano, 2001). The latter formation ranges from 300to 600 m in thickness in the study area, and consists of bedded,micritic limestones rich in chert nodules at the base, with reeffacies at the top. It is conformably overlain by the deep-marine

copper orebodies, and representative mineralized samples from the area of San Marcos.ce of Early (San Marcos) to Mid Cretaceous (Cupido, La Peña and Aurora) sedimentaryn, cropping out in the Manto Negro deposit; (C, D and E) slabbed hand specimens of thella, Mal e malachite.

Fig. 5. Episodes and general sequence of crystallization of the San Marcos sediment-hosted copper deposits.

Fig. 6. Transmitted-light photomicrographs (AeD) and scanning electron microscope (SEM-hosted copper deposits. (A) Chrysocolla veinlet cross-cutting coarse sandstones of the Sangrained arkosic sandstones with a matrix rich in illite and muscovite (the reddish tint inchrysocolla veinlets. (F) Microscopic aggregate of primary chalcocite (with an arborescemineralized sandstone; SEM-BSE image. Abbreviations: Brt e barite, Chc e chalcocite, Chr equartz.

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lutite beds of the La Peña Formation (Aptian; Goldhammer, 1999),which in the San Marcos y Pinos range attains only few meters inthickness (Fig. 4A). Conformably overlying them are the micriticlimestones of the Aurora Formation, which represents the top ofthe Mesozoic sequence that is exposed in the study area(AlbianeCenomanian; Padilla y Sánchez, 1982).

4.2. Structure and host rocks of the orebodies

The San Marcos SHC deposits occur as several successive deci-meter-thick, concordant, copper-mineralized, fine- to medium-grained clastic layers (Fig. 4B). The mineralized interval showsfining upward cycles that consist of alternating decimeter tometer-thick grey-beds of conglomerate, sandstone and siltstone. Thecopper mineralization essentially consists of supergene chrys-ocolla, malachite and azurite. These secondary copper mineralsgive to the mineralized horizons a striking blueegreen color

BSE) images (EeF) of textures and mineral assemblages from the San Marcos sediment-Marcos Formation. (B) Open-space filling malachite aggregate. (C and D) Very coarse-photomicrograph C is due to Fe-oxyhydroxides). (E) Barite crystals crosscut by late

nt overgrowth that possibly is also of chalcocite) included in the quartz cement inchrysocolla, Ill e illite, KF e potassium feldspar, Mal e malachite, Ms e muscovite, Q e

Fig. 7. Discrimination diagrams for sandstones from the Cretaceous San Marcos Formation, which are the host rock of the copper mineralization. (A) Provenance discriminationdiagram of Dickinson et al. (1983) based on modal compositions of sandstone (clastic grains: F e feldspars including plagioclase, Q e quartz, L e lithic). (B) Diagram of Roser andKorsch (1986) for the determination of the sedimentary-tectonic setting using major oxides. (C) Chemical Index of Alteration (C.I.A.) ternary diagram of Nesbitt and Young (1982),which allows estimating the degree of weathering of sandstones.

D. García-Alonso et al. / Journal of South American Earth Sciences 31 (2011) 432e443 439

(Fig. 4). All the deposits and occurrences are well exposed by shortexploration trenches and shafts that, nevertheless, do not penetratebeyond the weathered cover of the orebodies.

Two minor copper occurrences, Rincón de la Presa and Rinco-nada, situated 3.5 km from each other, are almost symmetricallylocated on either sides of the axis of the NWeSE recumbent anti-cline; hence they could be equivalent mineralized intervals (Fig. 2).The Rincón de la Presa occurrence lies in the inverted limb of theanticline, dipping 70� SE towards the fold axis. Four mineralizedlayers, 10e50 cm thick, are present and consist of medium-grainedsandstone beds consistently found at the top of conglomerate bedsand underlying grey siltstone strata. On the other hand, the Rin-conada occurrence is found in the normal limb of the anticline,dipping 45� NE. An overall mineralized interval of w14 m isexposed; however, in addition to some unimportant supergenecupriferous crusts, only a single mineralized layer occurs. It isa 15 cm-thick layer of greenish fine-grained sandstone that lies atthe top of a conglomerate bed and below a silty layer.

Located some 32 km further southeast in the same slope of theSan Marcos y Pinos range, the El Rosillo SHC deposit occurs more orless at an equivalent stratigraphic position to that of the Rinconada,but with almost horizontal strata. In the El Rossillo deposit, twomineralized layers of 30 and 60 cm in thickness extend laterally forat least 300 m. They are fine-grained cupriferous sandstones thatoccur on the top of grey microconglomerate beds.

Manto Negro is the most important copper deposit in the studyarea in terms of thickness and ore grades. It is located in the

northern slope of the El Granizo range, about 16.5 km south of theRinconada and Rincón de la Presa occurrences (Figs. 2 and 3). Thedeposit is hosted in a thin wedge of the San Marcos Formation,which is in tectonic contact with the Jurassic clastic sequence. Itsstrata dipw20� to the northeast. The overall mineralized interval isabout 4 m thick, contains three discrete copper-bearing layers, of30, 90 and 50 cm (from base to top), and has an exposed lateralextension of at least 200 m (Fig. 4B). These layers are fine-grainedsandstones, very enriched in secondary copper minerals, which areinterbedded with decimeter- to centimeter-thick carbonaceoussilty layers.

5. Mineral assemblages

The sequence of crystallization of the San Marcos SHC depositsand occurrences is provided in Fig. 5, and major textural featuresare shown in Fig. 6. Both mineralogy and textures are consistentwith a supergene mineralization product of weathering of coppersulfides (Figs. 4C, D, E, 6A and B). All that remains of the primarymineralization are chalcocite microcrystals shielded by quartzcement (Fig. 6F).

According to their compositional and textural analysis, thecopper-bearing rocks are fine- to medium-grained arkoses. Theirgrains are poorly sorted and subangular to subrounded in shape,and the matrix, always below 15% modal, is rich in illite, as indi-cated by XRD and SWIR analysis (Fig. 6C and D). Plotting modalcontents of quartz, feldspars and lithic grains in the provenance

Fig. 8. Histogram showing the distribution of homogenization temperatures (Th) offluid inclusions in quartz and calcite from the El Rosillo (shaded bars) and Rincón de laPresa (white bars) sediment-hosted copper occurrences (mineralized area of SanMarcos). The top-left inset shows a photomicrograph of representative primary, liq-uidevapor fluid inclusions in quartz from the footwall of the mineralized arkoses.

Fig. 9. Binary EhepH stability diagram for copper-bearing phases and species, con-structed under the conditions estimated for the San Marcos sediment-hosted copperdeposits (chemical activities: SO4

�2¼10�3, Cl� 0.3, Cu¼ 10�2.8; phase boundaries havebeen calculated for 60 �C and 140 �C). The Fe2þ-hematite boundary has been included,as a reference of sandstone reddening. The circle (A) and the dashed area indicate thepossible conditions of the recharge water of a deep circulating copper-mineralizingsystem (mw, meteoric water; sw, seawater). The circle (B) indicates the optimal brineconditions for copper leach and transport. High copper solubilities are due to theformation of chloride complexes. The circle (C) indicates the conditions that areattained for copper precipitation as sulfide (chalcocite) and, therefore, for the forma-tion of the sediment-hosted deposits.

D. García-Alonso et al. / Journal of South American Earth Sciences 31 (2011) 432e443440

diagram of Dickinson et al. (1983), it follows that the source area forthese sediments is an uplifted basement (Fig. 7A), which agreeswith the geological context. Feldspar grains, including K-feldsparfollowed in abundance by plagioclase, are partially altered tokaolinite and illite. Quartz grains are mostly monocrystalline and,in general, show higher degree of roundness and a smaller grainsize than those of feldspars. In accessory amounts (modal con-tents< 1%), biotite, muscovite, chlorite, zircon, apatite and rutilealso occur as detrital grains. Lithic grains are, for the most part,granite-derived and rarely higher than 1% modal. The detritalgrains are cemented by quartz and minor calcite. In addition, lateFe-oxyhydroxide grain-coating cement occurs at the footwallunmineralized series, reddening them (Fig. 6C).

Probably related to the formation of quartz cement, a few barrenmillimeter-thick quartz-calcite veins have been found at the foot-wall of the mineralized interval.

Chalcocite occurs as scarce, small inclusions, 2e40 mm indiameter, within the quartz cement (Fig. 6F). The nature of theseparticles was deduced from their optical properties and verified byEDS analysis. Pyrite, up to 25 mm in diameter, is found as subhedralgrains scattered throughout the copper-mineralized layers of fine-grained sandstone. In most cases, pyrite remains as relicts includedwithin goethite pseudomorphs.

The main copper mineralization is an assemblage of chrysocollaand malachite with minor azurite (Figs. 4C, D, E, 6A and B).Chrysocolla platy crystals, up to 300 mm wide, occur as sub-millimetric veinlets filling, cross-cutting the arkose host rock(Figs. 4D and 6A). Malachite occurs as botryoidal aggregates ofprismatic crystals, up to 500 mm in length, mostly forming crustsand open-space fillings. The latter compose a groundmass thatsurrounds and embeds detrital grains of quartz and feldspars.Azurite has the same mode of occurrence as malachite, but isconsiderably less abundant.

Centimeter-thick undeformed barite veins crosscut the copper-mineralized sandstones. Barite crystals are tabular, up to few mmwide, and locally occur embedded in a groundmass of secondarycopper minerals (Fig. 6E). Therefore, even though barite formationis prior to the supergene alteration, it clearly took place after theprimary process of copper sulfide mineralization (Fig. 5).

6. Major and trace element geochemistry

Whole-rock analysis for major and trace elements of the barrenhost-rock sandstones is presented in Table 1. By plotting the K2O/Na2O ratio vs. the SiO2 contents (diagram of Roser and Korsch,1986;Fig. 7B), it can be deduced that the sedimentary-tectonic settingwas a passive margin, in agreement with the regional geologicalcontext. The contents of SiO2 (40.70e87.50 wt.%), Al2O3(5.91e22.00 wt.%), K2O (3.68e12.50 wt.%), Na2O (0.03e2.03 wt.%)and CaO (0.09e3.78 wt.%) are within the ranges expected forarkoses and arkosic arenites. The positive correlation betweenAl2O3 and K2O (r¼þ0.86) is because both oxides are mostlycontrolled by feldspars and, to a lesser extent, by diagenetic illite.On the other hand, the negative correlation between Al2O3 and SiO2(r¼�0.82) indicates that feldspar increases as quartz decreases.

Whole-rock chemical analyses of the copper-mineralizedsandstones are shown in Table 2. In general, major elements varywithin the same ranges observed for the unmineralized rocks. Onthe other hand, the mineralized sandstones are characterized bycopper contents between 4.24 and 7.72 wt.% Cu, silver between 5and 92 ppm, and cobalt from 8 to 91 ppm. Contents of gold (belowthe detection limit of 0.5 ppb), vanadium (12e163 ppm), lead(6e110 ppm), zinc (2e70 ppm) and uranium (5e29 ppm) are lowerthan those reported in many SHC deposits.

7. Fluid inclusion microthermometry

Fluid inclusions were studied in quartz and calcite from barrenveinlets in the footwall of the Rincón de la Presa and El Rosillo SHCdeposits. Fluid inclusions mostly occur homogeneously distributed,

Fig. 10. Conceptual model for the basin circulation of copper-mineralizing brines and the formation of the sediment-hosted deposits of San Marcos. Top-right inset: schemaillustrating copper precipitation (after Brown, 1992).

D. García-Alonso et al. / Journal of South American Earth Sciences 31 (2011) 432e443 441

being of primary origin accordingly to criteria provided by Roedder(1984). Inclusions showing evidences of post-entrapment modifi-cations, such as necking down, were avoided for micro-thermometric measurements. Primary fluid inclusions have regularmorphology and small size, usually between 5 and 10 mm, and areaqueous two-phase liquidevapor (LþV), with bubbles represent-ing w5 vol.% of the inclusion at room temperature (Fig. 8).

Homogenization temperatures (Th) and ice-melting tempera-tures (Tmi) were obtained in 244 fluid inclusions (Table 3). Duringheating runs, homogenization to the liquid phase occurredbetween 98 �C and 165 �C (Fig. 8). Eutectic temperatures were closeto �52 �C, thus indicating that other salts different than NaCl andKCl occur in solution, probably CaCl2 (cf. Crawford, 1981; Daviset al., 1990). Tmi vary from �42.5 �C to �26.1 �C, correspondingto a salinity range of 26e39 wt.% CaCl equivalent.

8. Discussion

According to our data, the San Marcos copper deposits meet theprincipal tectono-sedimentary, morphological and geochemicalcharacteristics that are distinctive of the red-bed type SHC deposits(e.g., Gustafson and Williams, 1981; Kirkham, 1989; Brown, 1992;Hitzman et al., 2005): (a) They are hosted at the top of a morethan 1000 m thick red-bed sequence, (b) occur in the vicinity ofa large synsedimentary fault that outlines the edge of basementblocks and acted as a fluid pathway, (c) the orebodies are roughly

stratiform and placed very close to the redox barrier that is markedby the stratigraphic transition from red- to grey-beds, and (d) havehigh grades of copper and silver, with subordinate cobalt.

Although the studied outcrops of SHC mineralization are totallyweathered, the occurrence of relict copper sulfide grains identifiedas chalcocite, shielded by quartz cement, yields information abouttiming and conditions for the primary mineralization. Not only thecoppermineralization is affected by supergene alteration, so are thehost arkoses. The chemical index of alteration (CIA; Nesbitt andYoung, 1982), ranging from 25 to 67 (Table 1), indicatesa moderate degree of weathering of the arkoses outcroppingnearby the SHC deposits. In the Las Vigas SHC deposit, located some400 km northwest of San Marcos and hosted in Lower Cretaceousred-beds, mining operations exposed the non-weathered levels ofthe mineralization revealing a primary paragenesis dominated bychalcopyrite and chalcocite (Price et al., 1988).

Our textural observations suggest that the primary coppermineralization formed during the diagenesis, being at least in partcontemporary with the cementation of the arkoses (Fig. 5). Thus,copper mineralization processes took place during the active life ofthe Sabinas Basin, preceding at least w30 m.y. the formation of thesyn-Laramide MVT deposits of celestine, barite, fluorite and zinc-lead (González-Sánchez et al., 2009). The syn-rift tectono-sedi-mentary processes that induced the metallogenesis of the CSB ofnorthern Mexico are ultimately a consequence of the break-up ofPangea and the opening of the Atlantic Ocean basin. Because of the

D. García-Alonso et al. / Journal of South American Earth Sciences 31 (2011) 432e443442

much larger than regional scale of this event, SHC deposits verysimilar in age and in stratigraphy to those of the CSB can be found asfar away as the Sub-Andean Ranges of Argentina (Durieux andBrown, 2007).

8.1. Ore-forming fluids

An approach to the conditions of the mineralizing solutions andtheir evolution during recharge, leaching, transport and precipita-tion of copper was done in the EhepH space (Fig. 9). To constructthe diagram, the CueOeHeSeCl chemical system was taken intoaccount. The Cl� activity was of 0.3, which corresponds to a CaCl eq.concentration of w0.5 M. This last value is consistent with thesalinity range obtained from fluid inclusions (Table 3). High-chlo-ride concentrations are required by low-temperature hydrothermalsolutions to effectively leach and transport copper (Rose, 1976;Collings et al., 2000). The range of temperatures that was consid-ered for the geochemical modeling is 60e140 �C. The highest valueof this interval, although slightly above the maximum temperatureexpected for typical SHC mineralizations (120 �C; Brown, 2009), isapproximately the average Th measured in our fluid inclusions. Onthe other hand, the lowest temperature of the interval was con-strained by considering the geothermal gradient of rifts (25e30 �Ckm�1; e.g. �Cermák and Rybach, 1991) and a minimum depth forfluid circulation of 1300 m, which in turn was deduced from theoverall thickness of the Cupido and San Marcos formations. Sincepressure does not significantly affect the stability of the consideredcopper species, except when very high values are exceeded,a pressure of 1 atm was taken into account. The conceptual modelof the mineralizing fluid circulation is shown in Fig. 10.

Taking into account a syn-rift timing for the copper minerali-zation, the most likely source for fluid recharge is seawater,although a meteoric component, which has been invoked forvarious SHC deposits (e.g., Kirkham, 1989; Brown, 2005, 2009),cannot be discarded. During its deep circulation throughout thebasin sediments, the fluid may have interacted with evaporites (i.e.Olvido and Minas Viejas formations, Jurassic; La Virgen Formation,Lower Cretaceous) becoming a dense, near neutral, moderatelyoxidized brine capable of leaching and transporting high amountsof copper (Figs. 9 and 10). The hematite cement of the footwall red-beds confirm the oxidizing conditions the fluids prevalent duringdiagenesis; in addition, the interaction of these fluids with evapo-ritic sulfates may have contributed to maintain the oxidizingconditions of the fluids (Cathles et al., 1993), even if they loose partof the oxygen through altering the mafic detrital grains. Underthese conditions, Cu2þeCl complexes are dominant and allow highcopper solubilities, up to 0.1 M CuCl2 according to the experimentsof Collings et al. (2000). The source of copper could be the maficdetrital grains and the iron oxides of the underlying red-beds(Hitzman, 2000), which is especially reasonable in view of the largethickness of these sediments in the study area (up to 2000 m ofJurassic and 1000 m more of Lower Cretaceous; Eguiluz deAntuñano, 2001).

Copper precipitation probably took place when the upwardmoving brine was abruptly reduced when passing the redoxboundary marked by the transition from red- to grey-beds (Figs. 9and 10). Thus, the carbonaceous, pyrite-bearing silty horizons thatoccur in the latter may have acted as the geochemical trap forcopper, which precipitated as sulfide minerals, probably chalcocitefor the most part.

Three factors should be considered for their possible influenceon the circulation of the mineralizing brines throughout the basin:(a) the high density of the fluids due to the dissolution of evapo-rites, (b) a likely high heat flow, and (c) the synsedimentary SanMarcos Fault. Thus, on one hand, the density of the brines should

promote a gravity-driven infiltration to the deepest parts of thebasin, and, on the other hand, heat flow allowed a convectivecirculation of the brines and their up flow along the permeable SanMarcos Fault.

9. Concluding remarks

According to their tectono-sedimentary, morphological andgeochemical characteristics, the SanMarcos copper deposits belongto the red-bed SHC ore deposit type. Their occurrence, size andgrades reinforce the exploration potential of the regional-scale CSBmetallogenic unit.

Although the observable copper assemblages have originated bysupergene alteration, the primary copper mineralization formedduring the early diagenesis, when the Mesozoic Sabinas rift basinwas still active.

The mineralizing fluids were dense, near neutral, moderatelyoxidized brines that form from seawater and, driven by gravity,infiltrated to the deepest parts of the basin and dissolved evapo-rites. As a result, they became low temperature (�140 �C), highsalinity (w30 wt.% CaCl eq.) hydrothermal fluids capable of leach-ing and transporting high amounts of copper. The source of thismetal could be the mafic detrital grains and the iron oxides of thethick underlying Jurassic and Lower Cretaceous red-beds. Copperprecipitation took place when the upward moving brine passedthrough redox boundary marked by the transition from red- togrey-beds.

The upward movement of the mineralizing brines, from thedeep basin to the position of the SHC deposits was promoted bya high heat flow that allowed their convective circulation andascent along the synsedimentary San Marcos Fault.

Acknowledgements

Funding was provided by the UNAM project IN-117409 (PAPIIT).M. Antúnez Argüelles, A. Camprubí Cano, J.L. Farfán Panamá, E.G.García González, F. Orozco, J.A. Romero Guadarrama, L.I. SánchezVargas and V. Vázquez Figueroa are thanked for their assistanceand comments during fieldwork. A. R. Cabral and J. Kellogg arethanked for their comments and suggestions.

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