1. Introduction
Sediment-hosted stratiform copper deposits are tabular, bedding-parallel bodies of zonally-distributed, disseminated and veinlet-controlled copper and copper-iron sulfides located near and at oxidation-reduction boundaries in the host sedimentary sequence (Kirkham, 1989; Brown, 1992; Hitzman et al., 2005). The deposits are important sources of copper, cobalt, and silver and, in the last decade, have accounted for 15 to ~25% of global copper production, second only to porphyry copper and related deposits (Sillitoe, 2012). The host sedimentary sequences are typically characterized by thick packages of siliciclastic red beds, with additional carbonates and evaporites, deposited in intra and/ or pericratonic basins (Hayes et al., 2015). The largest deposits of the class were formed in basins where large-scale flow of metal-bearing fluid was focused into relatively small rock volumes containing abundant reductants (Hitzman et al., 2010). Less commonly, mineralization can also be hosted by synorogenic terrigenous strata deposited in continental foreland basins (Barton et al., 2018; Perelló et al., 2019).
The timing of mineralization in sediment-hosted stratiform copper deposits relative to host-rock sedimentation has historically remained elusive (e.g., Sillitoe et al., 2017a, 2017b, and references therein), a factor that has complicated the definition of a genetic model for the deposit type and its assignment to specific metallogenic epochs. Multiple syngenetic, diagenetic, and epigenetic models have been outlined over the years (Garlick, 1961; Sales, 1962; Bartholomé et al., 1972; Brown, 1981; Jowett, 1991; Hitzman et al., 2010), including proposals of late diagenetic timing of mineralization, up to 100 m.yr. after active sedimentation of the host strata (Aleinikoff et al., 2012; Milton et al., 2017). Models involving numerous diagenetic and epigenetic mineralization stages spanning protracted basin evolution are currently accepted by some investigators (Selley et al., 2005; Hitzman et al., 2010, 2012; Muchez et al., 2010), whereas others support an overall synorogenic timing of copper introduction (Sillitoe et al., 2017a and references therein; Saintilan et al., 2018; Perelló and Sillitoe, 2018; Perelló et al., 2019, 2021; Twite et al., 2020).
Independent of deposit scale and metallogenic province, copper sulfide-bearing veinlets are common in sediment-hosted copper deposits and are an ubiquitous accompaniment to the stratiform sulfide mineralization (Sillitoe et al., 2010, 2017a, 2017b). Traditionally, however, the veinlets have been considered as products of local remobilization of the disseminated stratiform mineralization and assigned to one or more overprinting late-diagenetic to metamorphic events, with little bearing on primary ore genesis (Brummer, 1955; Garlick, 1961, 1967; Mendelsohn, 1961; Whyte, 1971; Ghandi and Brown, 1975; Hayes and Einaudi, 1986; Price et al., 1988; Richards et al., 1988; Annels, 1989; Sweeney et al., 1991; Cailteux et al., 2005; Selley et al., 2005; Dewaele et al., 2006; Brems et al., 2009; El Desouky et al., 2009; Huang et al., 2013; Brown, 2017; Oummouch et al., 2017).
This paper briefly describes the geologic setting and salient features of the sediment-hosted copper mineralization in the San Marcos area within Sabinas basin strata exposed in Coahuila, northeastern Mexico (Figure 1). Field observations of sulfide-bearing veinlets accompanying the sediment-hosted stratiform mineralization are described and, in conjunction with available geologic descriptions of the mineralization in the study area, are used as evidence to show that copper introduction was contemporaneous with veinlet emplacement during basin inversion resulting from regional Laramide orogenic processes.
2. Regional geologic setting and mineralization
2.1. The Sabinas basin
As summarized elsewhere (Padilla y Sánchez, 1986, 2016; Salvador, 1987, 1991; Wilson, 1986; Goldhammer, 1999; Eguiluz de Antuñano, 2001), the Sabinas basin formed part of peninsular Mexico during the Mesozoic opening of the Gulf of Mexico consequent upon dispersion of equatorial western Pangea (e.g., Salvador and Green, 1980; Pindell et al., 2020). Sabinas basin strata, presently exposed in the Sierra Madre Oriental ranges of northeastern Mexico (Figure 1), were deposited on Precambrian and Paleozoic-Triassic basement of predominantly Gondwanan affinities (Dickinson and Lawton, 2001; López et al., 2001; Rubio-Cisneros and Lawton, 2011; Ocampo-Díaz et al., 2014; Centeno-García, 2017; Martini and Ortega-Gutiérrez, 2018). Approximately 6 km of marine, evaporitic, and fluviatile sedimentary rocks accumulated from the Middle Jurassic to the Paleogene (Eocene), between ~170 and 40 Ma (e.g., Padilla y Sánchez, 1986; Goldhammer, 1999; Eguiluz de Antuñano, 2001; Pindell et al., 2020), in three principal stages: an initial, rapidly subsiding rift-fill stage (Middle-Late Jurassic), with siliciclastic sedimentary rocks and evaporites; an intermediate stage of platform-dominated facies with carbonate, evaporite, and coastal siliciclastic deposits (Early Cretaceous); and a terminal, regressive stage with terrigenous sedimentation in a foreland setting (Late Cretaceous-Paleogene).
Red-bed strata are present in all three stages, although the more voluminous and continuous sequences accumulated during the middle stage in the Late Jurassic (e.g., McKee et al., 1990). Early-stage sedimentation was transgressive over Early Jurassic terrigenous and volcanogenic sequences deposited during initial opening of the Gulf of Mexico (e.g., Barboza-Gudiño et al., 1999; Fastovsky et al., 2005; Rubio-Cisneros and Lawton, 2011; Pindell et al., 2020, and references therein), whereas late-stage, synorogenic sedimentation in a foreland basin setting accompanied uplift of the Mexican orogen (Fitz-Díaz et al., 2018; Juárez-Arriaga et al., 2019a, 2019b; Davison et al., 2020).
The Sabinas basin has been traditionally assigned to an equatorial, intra- or pericratonic setting as a passive margin embayment accompanying the opening of the Gulf of Mexico, resulting from drifting of North and South America during western Pangea break-up (Salvador, 1987, 1991; Goldhammer, 1999; Pindell and Keenan, 2009; Bird et al., 2005; Marton and Buffler, 2010; Martini and Ortega-Gutiérrez, 2018; Pindell et al., 2020; Figure 2a). An alternative interpretation proposes that the Gulf of Mexico originated as a back-arc basin during the Jurassic, and assigns the Sabinas and Chihuahua basins to a series of borderland rift basins along a 2000-km-long magmatic arc associated with subduction of Farallon plate oceanic crust (Stern and Dickinson, 2010; Figure 2b). Irrespective of the geotectonic setting of the basin, Pangea break-up promoted the shuffling of continental blocks, generating an irregular topography in which numerous fault-bounded horsts and grabens controlled the sedimentation patterns within the Sabinas basin as proto-Atlantic sea waters inundated first the Gulf of Mexico and then the adjacent peninsular lands (Padilla y Sánchez, 1986, 2016; Salvador, 1987, 1991; Goldhammer, 1999; Eguiluz de Antuñano, 2001; González-Sánchez et al., 2009; Davison et al., 2020; Pindell et al., 2020). Under these conditions, the northwest-trending Sabinas basin evolved between two large basement highs, the Burro-Peyotes peninsula to the northeast and extensions into the Tamaulipas archipelago and the Coahuila block to the southwest, with additional internal basement blocks at La Mula and Monclova islands (Wilson, 1990; Eguiluz de Antuñano, 2001; Figure 1). Principal bounding structures of the basin included the regional, high-angle San Marcos and La Babia faults, both inferred to have been active during basin sedimentation and later reactivated at the time of basin inversion (Chávez-Cabello, 2005; Chávez-Cabello et al., 2005, 2007; Bolaños-Rodríguez, 2006; Figures 1 and 3a).
Deformation of the Sabinas basin as well as other basins in the northeast Mexican fold and thrust belt in the Sierra Madre Oriental took place during various stages of continuous and progressive transition from thin- to thick-skinned structural styles, between the Late Cretaceous (~90 Ma; Turonian) and the middle Eocene (~40 Ma; Bartonian), in response to a combination of Sevier and Laramide orogenic pulses (Eguiluz de Antuñano et al., 2000; Eguiluz de Antuñano, 2001; Gray and Lawton, 2011; Ramírez-Peña and Chávez-Cabello, 2017; Fitz-Díaz et al., 2018; Williams et al., 2020). Final basin inversion occurred in the middle Eocene (~50-40 Ma), with folding, thrusting, and high-angle reverse faulting during the regional, relatively low-temperature (100-300°C; Fitz-Díaz et al., 2011; Fitz-Díaz and van der Pluijm, 2013) shortening of the Laramide orogeny (Eguiluz de Antuñano, 2001; Chávez-Cabello, 2005; Chávez-Cabello et al., 2005, 2007, 2009; Molina-Garza et al., 2008). Recent literature refers to this wholesale deformation event as the Mexican orogeny (Juárez-Arriaga et al., 2019a; Davison et al., 2020; Gray et al., 2020).
2.2. Sediment-hosted stratiform mineralization
Strata within the Sabinas basin and bounding blocks host a variety of mineralization styles genetically related to basin evolution and deformation, principally Mississippi Valley-type (MVT) lead-zinc-celestine-barite-fluorite deposits and a currently subeconomic, although metallogenically important group of red bed-type sediment-hosted stratiform copper-silver deposits and prospects (e.g., Tritlla et al., 2006, 2007; González-Sánchez et al., 2009; Camprubí, 2013, 2017; Figure 3b). The copper-silver group defines a ~1000-km-long northwest trend from central Chihuahua to Tamaulipas (Clark and de la Fuente, 1978; Price et al., 1988; Camprubí, 2013, 2017; García-Alonso et al., 2011; González-Partida et al., 2013, 2017; Figure 3a). The deposits and prospects are hosted by thick (up to 1 km) red-bed sequences of Jurassic to Early Cretaceous (Neocomian) age (Figure 3b), with many of them located in the vicinity of regional basement-bounding faults of which the San Marcos area prospects are good examples (Figure 3a).
3. Sediment-hosted stratiform copper- silver mineralization in the San Marcos area
3.1. Stratigraphy and mineralization
The sediment-hosted stratiform mineralization in the San Marcos area (also called Cuatro Ciénegas or Cuatrociénegas; González-Sánchez et al., 2009; García-Alonso et al., 2011), is located approximately 45 km south of the town of Cuatro Ciénegas in central Coahuila (Figures 1 and 4a) along and near the San Marcos fault, is part of a 250-km-long segment of the main north-west-trending copper-silver belt (Figure 4a). The San Marcos fault constitutes the regional structural contact between the Coahuila block and the Sabinas basin and controlled basin subsidence and sedimentation in the area (McKee et al., 1990; Chávez-Cabello 2005; Chávez-Cabello et al., 2005, 2007; Figures 1 and 4).
Over 4000 m of Late Jurassic and Early Cretaceous stratigraphy are exposed in the San Marcos area comprising, from bottom to top, the following principal sequences (McKee et al., 1990; Bolaños-Rodríguez, 2006; Figure 4b): ~2000 m distributed in three Late Jurassic units of upward-fining siliciclastic rocks, including red beds (Las Palomas, Sierra El Granizo, and Tanque Cuatro Palmas beds, all correlated with the La Casita Formation; Mckee et al., 1990; Eguiluz de Antuñano 2001); ~1000 m of Early Cretaceous (Neocomian), coarse-grained siliciclastic red beds (San Marcos Formation); ~350-600 m of lime-stone and shale of late Early Cretaceous (Barremian-Aptian) age (Cupido and La Peña Formations); and >500 m of Late Cretaceous (Albian-Cenomanian and younger) carbonate rocks (Aurora and Acatita Formations and correlative strata; Figure 4b). Basement units comprise low-grade metasiliciclastic rocks of Late Paleozoic age intruded by Permian-Triassic granitoids (McKee et al., 1990; Bolaños-Rodríguez, 2006).
The stratiform disseminated copper-silver mineralization occurs predominantly at or close to an Early Cretaceous redox boundary between foot-wall siliciclastic red-bed strata of the San Marcos Formation and hanging-wall dolomitic limestone of the Cupido Formation. Scattered mineralized showings within San Marcos sandstone and in Late Jurassic red beds (Sierra El Granizo beds) are also present (e.g., Barbosa-Luna et al., 1998; Ramírez-Gutiérrez and Ramos-Anleu, 2008; Figure 4b), and copper-silver-bearing veins and fractures occur in isolated exposures of basement granite in the San Marcos valley (Barbosa-Luna, 1998; Barbosa-Luna et al., 1998; Bolaños-Rodríguez, 2006; Figure 4a), thereby spanning a stratigraphic interval of ~3000 m.
Exposures of the sediment-hosted mineralization occur along the foothill slopes of Sierra El Granizo and Sierra San Marcos y Pinos, which together are inferred to form the limbs of a large, composite, northwest-trending antiformal structure (e.g., González-Ramos et al., 1997; Chávez-Cabello, 2005; Figures 4 and 5). The southwestern limb of this antiform at Sierra El Granizo runs along the San Marcos fault and is intensely disrupted by east-vergent thrust faults and associated folds (McKee et al., 1990; Chávez-Cabello, 2005; Chávez-Cabello et al., 2005, 2007; Bolaños-Rodríguez, 2006; Alonso-Manuel et al., 2020), whereas the exposures along the Sierra San Marcos y Pinos are relatively continuous over several kilometers along bedding (Figures 4 and 5). The principal single mineral occurrence is Manto Negro (also known as Manto Rojo; Barbosa-Luna, 1998) at Sierra El Granizo, with an estimated artisanal production of 100 t of concentrate containing 70-75% Cu from ore with head grades of ~2 to 3% Cu (Rivera-Martínez, 1993). Silver contents of ~40-50 ppm reported for Manto Negro and neighboring showings (Rivera-Martínez, 1993) are within the upper range of silver values for numerous prospects from the larger Monclova region (10-70 ppm; González-Ramos et al., 1997; Barbosa-Luna, 1998; Barbosa-Luna et al., 1998; Santiago-Carrasco, 1998; Ramírez-Gutiérrez and Ramos-Anleu, 2008), and underscore the copper- silver signature of the sediment-hosted stratiform mineralization regionally (García-Alonso et al., 2011; Camprubí, 2013, 2017; González-Partida et al., 2017). The copper mineralization predominantly occurs as supergene carbonates, sulfates, silicates, and oxides (malachite, azurite, chalcanthite, chrysocolla, cuprite, and manganese wads), but hypogene copper-bearing sulfides, including chalcocite-group minerals (chalcocite, digenite, djurleite), bornite, and chalcopyrite, have also been reported (Rivera-Martínez, 1993; González-Ramos et al., 1997; Barbosa-Luna, 1998; Santiago-Carrasco, 1998; Ramírez-Gutiérrez and Ramos-Anleu, 2008). The host for silver in the copper mineralization is unknown, but it is likely to be in solid solution with the copper-bearing sulfides, mainly chalcocite and bornite, as documented in sediment-hosted copper-silver deposits elsewhere (e.g., Kelepile et al., 2020).
3.2. Mineralization styles
On both limbs of the large San Marcos anticline, the studied stratiform copper-silver mineralization occurs within a stratigraphic interval comprising carbonaceous siltstone, micritic limestone, and arkosic sandstone, which marks the transitional contact between coarse-grained, poorly-sorted, red-bed siliciclastic footwall strata of the San Marcos Formation and hanging-wall carbonate rocks of the Cupido Formation (Figures 4b and 6). In the Sierra San Marcos y Pinos, the copper-bearing interval averages ~1.5 m thick, with mineralization occurring as disseminations and fractures along gently northeast-dipping, thinly bedded, illite-stable, dark-gray to black, carbonaceous silt-stone, silty sandstone, and calcareous shale inter-bedded with thickly- and locally cross-bedded, greenish sandstone and fine-grained conglomerate (Figures 6a, 6b, 6e, 6f and 6g). In the foothills of Sierra El Granizo, the ~4- to 6-m-thick, southwest-dipping mineralized section is contained in an identical, although highly distorted package of alternating greenish sandstone and dark gray to black pyritic siltstone, within a structural wedge of San Marcos Formation strata located between undifferentiated carbonate rocks of the Cupido and Aurora Formations in the hanging wall and reddish siliciclastic rocks of the Sierra El Granizo beds in the footwall (e.g., McKee et al., 1990; García-Alonso et al., 2011; Figures 5d and 6c). Although the majority of the sediment-hosted stratiform copper-silver mineralization in the San Marcos area is confined to the carbonaceous strata, arkosic sandstone and conglomerate are also mineralized at Sierra San Marcos y Pinos (e.g., García-Alonso et al., 2011; Figures 6e and 6f). Locally, along the San Marcos valley, discrete stratigraphic horizons of the San Marcos red-bed sequence contain copper mineralization in decimeter- to meter-thick bleached conglomerate and sandstone lenses in which fossilized wood fragments have been partially replaced by copper-bearing minerals (Figures 6d and 6h), identical to sediment-hosted stratiform copper-silver mineralization elsewhere in the region (e.g., Las Vigas, Chihuahua; González-Partida et al., 2017). Also, at San Marcos, the inherent primary porosity of certain conglomerate lenses next to localized redox boundaries in otherwise fine-grained strata, appears to have aided fluid flow and sulfide precipitation (Figure 6f).
The bulk of the exposed copper mineralization in the San Marcos area comprises various supergene copper-bearing oxidation products (malachite, azurite, chrysocolla, copper pitch, neotocite, pitch limonite, and undifferentiated copper wads; Figures 6 and 7), although hypogene copper-bearing sulfides can be found in shallow workings and as relict grains in polished slabs. Chalcocite, as a general field term for chalcocite-group minerals, is the principal sulfide species observed (Figures 6i and 7c, 7d, 7e, 7k), and the presence of localized cores of bornite assists in definition of a typical paragenetic sequence from early bornite to later chalcocite (Figure 6i). Djurleite, digenite, and chalcopyrite have also been described from exposures in the area (Barbosa-Luna, 1998; Santiago-Carrasco, 1998). Fine-grained chalcocite disseminations occur as arborescent replacements of minute, framboidal, diagenetic pyrite grains in the stratiform disseminated mineralization (e.g., García-Alonso et al., 2011), whereas coarse-grained chalcocite is characteristic of massive, centimeter-wide, chalcocite-only veins or together with bornite, as well as veins and veinlets with various proportions of quartz, calcite, and barite (Figures 6i and 7c, 7d, 7e). Laterally distanced from the chalcocite-dominant zones along certain exposures in the Sierra de San Marcos y Pinos, veinlets and disseminations of pitch limonite, neotocite, and chrysocolla are present: an association considered to represent the in-situ oxidative weathering products of low-pyrite, chalcopyrite-bearing hypogene mineralization (e.g., Buckley and Woods, 1984; Sillitoe, 2005; Figures 7a and 7b). These assemblages suggest that a lateral zonation, from central chalcocite-bornite to peripheral chalcopyrite, is present at San Marcos, although detailed mapping would be required to confirm this.
3.3. Veins, veinlets, structure, and stratiform copper-silver mineralization
Various types of mineralized veins and veinlets are associated with the sediment-hosted stratiform copper-silver mineralization in the San Marcos area. The majority comprises vertical to sub-vertical (80-90°), planar veins with well-defined margins and a characteristic syntaxial, blocky fill of one or more of quartz, calcite, barite, and copper-bearing sulfides and/or their derivative oxidation products. They can occur singly and approximate 1 cm wide or multiply along subparallel arrays, which locally coalesce to form zones from 1 to several meters wide (Figures 7f and 7h). Sigmoidal and bedding-parallel veinlets also occur locally, where deformation of the host rock is more intense (Figure 7j). Although the paucity of exposure inhibited measurement of statistically meaningful populations during the present study, single and composite vein/veinlet swarms typically strike from N35°E to N60°E (Figure 5a), which are parallel to the dominant extensional joint orientations, and typically run nearly perpendicular to the strike of the rotated host stratigraphy (Figure 7). The veins and veinlets characteristically display sharp walls and lack brecciation and gouge zones, although slickensides along vein margins are very locally developed and indicative of strike-slip motion, but with minimal vertical offset of the host rocks. Dilation is apparent in most veins and veinlets, hence the characteristic syntaxial, blocky, and massive fill. These features, as well as the overall lack of fault plane indicators in the wall rocks, suggest that they originated as predominantly opening mode fractures, with opening concordant with northwest-southeast extension (σ3). Locally, tree-like arrays occur (Figure 8h), whereby discrete thin veins, veinlets, and fractures propagate vertically and merge into a single, wider, and bigger vein, like the roots of a tree, reflecting the upward ascent of overpressured fluid along planes of weakness (e.g., Bons et al., 2012). Elsewhere in the larger San Marcos area, larger copper-bearing vein systems, typically tens to hundreds of meters in strike length, display similar northeast-trending attitudes and opening modes (e.g., Ramírez-Gutiérrez and Ramos-Anleu, 2008; Figure 5a). It is noteworthy that everywhere in the San Marcos and nearby areas, veins, veinlets, and disseminated stratiform mineralization possess the same copper-bearing species and assemblages, a feature that in sediment-hosted stratiform copper deposits elsewhere has been considered to demonstrate the contemporaneity of both the vein-controlled and disseminated stratiform mineralization (e.g., Sillitoe et al., 2010, 2017a, 2017b, and references therein).
In general, veining intensity at San Marcos is proportional to the intensity of deformation affecting the host sequence: an empirical observation that attests to a close relationship between geologic structure and sediment-hosted stratiform mineralization. For example, at Manto Negro (Figures 4 and 5), stratiform copper mineralization is hosted by zones of low-angle, bedding-parallel thrust faults predominantly accommodated by fine-grained silty and carbonaceous horizons interbedded with more rigid sandstone and conglomerate of the San Marcos Formation (Figure 8). The thrust faults are characterized by scaly fabrics involving polished anastomosing surfaces, phacoidal forms, sheared competent blocks and concretions, and injected and foliated shale bands, typical of thrusts elsewhere (e.g., Vannucchi et al., 2003; Morley et al., 2017; Vannucchi, 2019; Figures 8a, 8b, 8c, 8e, 8g). Tectonic thickening of the host mineralized sequence is apparent: 4 to 6 m in comparison to 1-2 m in less deformed sections in the San Marcos area. Associated chevron folds in structurally overlying strata, as well as sigmoidal (S-type), copper-bearing quartz-calcite veinlets within deformed strata define an overall northeast-directed shortening and subhorizontal direction of transport (Figure 8d), congruent with the structural observations and interpretations of Chávez-Cabello (2005) for areas nearby. More-over, the copper-bearing quartz-calcite veins and veinlets display mutually crosscutting relationships with the thrust faults: while some single veinlets and swarms are truncated and fragmented by the faults other sets of similar mineralogy transect almost perpendicularly both host stratigraphy, thrust slip planes, and fault zones (Figures 7h and 7i), thereby implying contemporaneity of deformation and veining events (e.g., Engelder, 1985; Laubach and Ward, 2006; Fitz-Díaz et al., 2011; Williams et al., 2020). Similarly, along Sierra San Marcos y Pinos, stratiform copper mineralization is also hosted locally by shallow-angle, bedding-parallel, northeast-vergent slip surfaces characterized by concretional structures and centimetric zones of cleaved shale tectonically injected into more competent host sandstone (Figure 8g).
3.4. Stratiform lead-zinc-silver mineralization
Numerous sediment-hosted stratiform prospects in the larger Monclova region display the conventional zoning from centrally located copper-silver to peripheral lead-zinc-silver mineralization. The prospects and old workings at Mineral Refoma in Sierra La Purísima and those along Sierra La Gavia (Figure 4a) are classic examples of this metal zonation (González-Ramos et al., 1997; Santiago-Carrasco, 1998), but many others are also present at San Marcos, where the disseminated stratiform mineralization is spatially associated with veins containing various proportions of lead, zinc, copper, and silver (Barbosa-Luna, 1998; Ramírez-Gutiérrez and Ramos-Anleu, 2008).
The majority of the stratiform lead-zinc-silver prospects are hosted by dolomitic limestone of the Cupido Formation, typically located tens to hundreds of meters stratigraphically above and laterally to the copper-bearing siliciclastic strata of the San Marcos Formation (Figures 3b and 4c). Sphalerite and galena, accompanied by various proportions of quartz, barite, and calcite, occur as disseminations, fracture fillings, veins, chimney-like breccias, and karst cavity fillings, and a comparison with the MVT mineralization style has long been established (Barbosa-Luna, 1998; Tritlla et al., 2006, 2007; González-Partida et al., 2007, 2009, 2013; Camprubí, 2017). However, in other cases, the lead-zinc mineralization is disseminated and stratiform, and the transition from copper to lead- zinc takes place within single horizons or across thin stratigraphic intervals in the uppermost part of the San Marcos Formation, or immediately at the contact with hanging-wall strata of the Cupido Formation (González-Ramos et al., 1997).
4. Discussion
4.1. Timing of sediment-hosted stratiform copper mineralization in the San Marcos area
The predominant northeast-southwest trend of the copper-bearing veins and veinlets indicative of northwest-southeast extension, the northeast-vergence of the mineralized thrust faults and associated folds, and the mutually crosscutting relationships between mineralized veins/veinlets and thrusts are all consistent with a northeast-trending horizontal shortening direction and in agreement with structural interpretations of both in the San Marcos area and more regionally (Chávez-Cabello, 2005; Chávez-Cabello et al., 2005, 2007; Molina-Garza et al., 2008; Bolaños-Rodríguez, 2006; Figure 5). In the broader Monclova region, principal vein trends in numerous sediment-hosted stratiform prospects containing one or more of copper, silver, lead, zinc, fluorite, and barite are also oriented northeast-southwest (González-Ramos et al., 1997; Barbosa-Luna et al., 1998; Ramírez-Gutiérrez and Ramos-Anleu, 2008), a preferred direction that is taken to indicate that the prevalent stress conditions were everywhere the same for all mineralization styles. Additionally, in the vicinity of the northwest-trending San Marcos fault, copper-, lead-, and zinc-bearing veins associated with sediment-hosted mineralization are oriented predominantly subparallel to the fault (e.g., Ramírez-Gutiérrez and Ramos-Anleu, 2008), providing examples of the effective control on fluid up-flow and discharge along this regional structure.
Hence, the principal structural and mineralization features of the San Marcos area confirm an intimate relationship between sediment-hosted stratiform copper-silver mineralization and structural deformation. Deformation in the region was previously interpreted to have taken place during two consecutive and progressive periods of Laramide orogenic activity (Chávez-Cabello, 2005; Chávez-Cabello et al., 2005, 2007; Bolaños-Rodríguez, 2006; Molina-Garza et al., 2008; Fitz-Díaz et al., 2018; Alonso-Manuel et al., 2020; Figure 9): an early thin-skinned structural event, characterized by shallow, bedding-parallel thrusting and associated fault-bend folding along the frontal portion of regional, northeast-vergent detachment structures of the Coahuila block; and, a second event, with kinematic inversion of deep-seated, basin-bounding faults and generation of basement-rooted, high-angle brittle faults under a thick-skinned tectonic regime. Paleogene-age Laramide activity along the San Marcos fault system took place predominantly during the second event and was responsible for the uplift of Paleozoic basement blocks, involving from 1300 to 3000 m of vertical displacement and ~500 m of horizontal shortening, under a horizontal, northeast- to north-northeast-directed principal (σ1) stress field (McKee et al., 1990; Chávez-Cabello, 2005; Bolaños-Rodríguez, 2006; Alonso-Manuel et al., 2020). This activity occurred contemporaneously with tectonism along the Mexican fold-thrust belt and orogen-wide uplift, which triggered massive detrital sedimentation in paired foreland basins (Chávez-Cabello et al., 2006; Gray and Lawton, 2011; Fitz-Díaz et al., 2011, 2014, 2016, 2018; Ramírez-Peña and Chávez-Cabello, 2017; Juárez-Arriaga et al., 2019a, 2019b; Davison et al., 2020; Gray et al., 2020; Williams et al., 2020).
The presence of mineralized veins and veinlets accompanying the disseminated stratiform copper mineralization at San Marcos is taken as evidence that copper-bearing fluids operated at various scales during the deformation history of the host strata, with vein/veinlet emplacement induced by tectonic stress imposed by the Laramide orogeny. Hence, these veins and veinlets represent the episodic passage of far-travelled copper-bearing fluids and their propagation and growth as over-pressured mobile hydrofractures (e.g., Oliver and Bons, 2001; Bons et al., 2012), which in the study area occurred at and near an important upper crustal discontinuity: the regional San Marcos fault system at the boundary between the Coahuila block and Sabinas basin (e.g., García-Alonso et al., 2011; Figure 1). Indeed, the San Marcos fault system originated in the Jurassic as a basin-bounding structure during the opening of the Gulf of Mexico, continued as a growth fault during Sabinas basin sedimentation, and was finally inverted as a series of reverse faults and fault panels during Laramide contraction in the Paleogene (McKee et al., 1990; Chávez-Cabello, 2005; Chávez-Cabello et al., 2005, 2007; Bolaños-Rodríguez, 2006; Fitz-Díaz et al., 2018; Alonso-Manuel et al., 2020).
By analogy with sediment-hosted stratiform copper deposits elsewhere, the mineralized veins and veinlets present in the San Marcos area imply cogenesis between the two styles of mineralization (e.g., Sillitoe et al., 2010, 2017a, 2017b). Hence, the described crosscutting relationships between stratiform mineralization, mineralized veins and veinlets, and the tectonic structures affecting the host sequence suggest that emplacement of the copper-silver mineralization at San Marcos occurred during the transition from shallow-angle slip and thrusting to high-angle, basement-involved faulting (Figure 9). Both deformation and mineralization were promoted by the prevalent northeast-southwest-oriented, horizontal shortening during inversion of the Sabinas basin in the Paleogene (e.g., Chávez-Cabello, 2005; Chávez-Cabello et al., 2005, 2006, 2007; Molina-Garza et al., 2008) which, elsewhere in the Mexican fold-thrust belt, propagated progressively to the east between the Late Cretaceous (~90 Ma, Turonian) and the middle Eocene (40 Ma, Lutetian; Fitz-Díaz et al., 2011, 2014, 2018; Juárez-Arriaga et al., 2019a; Gray et al., 2020).
Hence, contrary to previous syngenetic (Rivera-Martínez, 1993) or syndiagenetic (García-Alonso et al., 2011) proposals for the timing of the sediment-hosted copper-silver mineralization in the area, the geologic evidence indicates that mineralization was predominantly epigenetic and took place ~70 to 40 m.yr. after sedimentation of the host strata of the San Carlos Formation and the establishment of the redoxcline with the carbonate rocks of the Cupido Formation in the Early Cretaceous. Such timing would be congruent with a gravity-driven fluid-flow model for the origin of the mineralization associated with tectonic loading and crustal thickening resulting from the eastward migration of the Laramide orogenic front in northeastern Mexico (e.g., Fitz-Díaz et al., 2011) and possibly elsewhere in contiguous parts of the southwest US. For example, the emplacement of the sediment-hosted stratiform copper mineralization in strata of the Paradox basin exposed in the Colorado Plateau of Utah and Colorado (e.g., Barton et al., 2008) has also been assigned a Laramide timing (early Eocene; ~51 to 48 Ma), as suggested by the isotopic dates obtained from fault-controlled and disseminated copper-bearing products (Kirk et al., 2020).
Although multiple driving forces can be responsible for fluid movement during basin evolution (e.g., Kyser, 2007; Boiron et al., 2010; Barton et al., 2018), fluid expulsion, convective circulation, and upward brine migration along regional structural channel ways such as the San Marcos fault system (e.g., Yang et al., 2004), were likely best accomplished by tectonic squeezing during Laramide contraction and fault inversion in the Mexican orogen (e.g., Oliver, 1992; Sibson, 1995; Zhang et al., 2006; Fitz-Díaz et al., 2011; Gibson et al., 2017; Sillitoe et al., 2017a, 2017b). In the case of the San Marcos area, one additional source of heat to sustain the upward convection and transport of the dense, deeply circulating, copper-bearing brines could have been provided by the arc magmatism of the Candela-Monclova belt, which migrated eastward across the region contemporaneously with deformation (Chávez-Cabello, 2005; Molina-Garza et al., 2008; Chávez-Cabello et al., 2009; Figure 4a). Although such a relationship is possible, because many of these intrusions are mineralized with copper, silver, lead, and zinc (e.g., González-Ramos et al., 1997), it should be noted that elsewhere in the Mesozoic basins hosting stratiform base-metal mineralization in northeastern Mexico, a genetic relationship with magmatism is absent (e.g., Trittla et al., 2006, 2007; González-Sánchez et al., 2007, 2009; González-Partida et al., 2017).
4.2. Implications for the timing of sediment-hosted stratiform base metal mineralization in the region
In common with many sediment-hosted copper deposits elsewhere, the fluids responsible for mineralization at San Marcos were dense, near neutral, saline, Na-Ca-bearing, moderately oxidized, chloride-dominant basinal brines that likely infiltrated deeply in the basin and its basement (e.g., Rose, 1976; Jowett, 1991; Brown, 2014). Fluid temperatures, salinities, and compositions (García-Alonso et al., 2011) were similar to those of other sediment-hosted stratiform copper-silver deposits in the region (e.g., Las Vigas, Chihuahua; González-Partida et al., 2017). The processes that gave rise to the copper mineralization were also similar to those in other deposits of the class, whereby sulfate was reduced by hydrocarbons either bacterially or inorganically, in the latter case via thermochemical reduction (García-Alonso et al., 2011; González-Partida et al., 2013, 2017).
Given that the fluid characteristics and processes responsible for regional base-metal-bearing MVT mineralization in the Sabinas basin were strikingly similar, a common origin and a predominantly syntectonic timing have been proposed for both sediment-hosted stratiform copper and MVT mineralization (González-Sánchez et al., 2007, 2009; González-Partida et al., 2013; Camprubí, 2017), with a postulated permissive interval of formation between ~110 Ma (Albian) and 30 Ma (Oligocene) resulting from a combination of Sevier and Laramide orogenic contraction (e.g., Camprubí, 2017). Indeed, the uplift of the Cretaceous carbonate platform sequences is likely to have resulted in important karstification that would have contributed to permeability and ground preparation for the incoming MVT fluids. If such a common origin for both deposit styles is correct, it follows that the bulk of the MVT mineralization in the Sabinas basin was not only the product of widespread Laramide shortening and deformation (González-Sánchez et al., 2009), but also that its timing can be more precisely constrained by the wholesale basin inversion event in the Paleocene-Eocene from ~60 to 40 Ma. This is coincident with large-scale fluid circulation in basins of central and northeastern Mexico (Laubach and Ward, 2006; Fitz-Díaz et al., 2011, 2018; Williams et al., 2020).
4.3. Implications for the tectonic setting of sediment-hosted stratiform copper- silver mineralization in the region
Current reconstructions of the tectonic setting of central and eastern Mexico during the early phases of the Laramide orogeny in the Late Cretaceous (Turonian-Maastrichtian) postulate that a large, compartmentalized, predominantly marine foreland basin (collectively named Mexican foreland basin; Juárez-Arriaga et al., 2019a, 2019b) was established along the eastern front of the Mexican fold-thrust belt (Centeno-García, 2017; Fitz-Díaz et al., 2018; Juárez-Arriaga et al., 2019a, 2019b; Davison et al., 2020; Pindell et al., 2020; Gray et al., 2020). Between ~90 Ma (Turonian) and 40 Ma (Lutetian), this basin received a massive input of siliciclastic detritus, in excess of 6 km in thickness, principally from the erosion of the Mexican orogen that was undergoing contraction and uplift (Lawton et al. 2009; Juárez-Arriaga 2019b; Davidson et al., 2020; Gray et al., 2020). Hence, the Paleogene sediment-hosted stratiform copper-silver mineralization and the lead-zinc-fluorine-barium-strontium MVT deposits of the Sabinas and adjacent basins formed contemporaneously with regional foreland sedimentation, at a time when their original intracratonic rift setting consequent upon opening of the Gulf of Mexico was undergoing important tectonic reorganization. Such a marine environment would help to explain the predominant seawater affinity of the mineralizing fluids responsible for the sediment-hosted copper mineralization in the San Marcos area and regionally (García-Alonso et al., 2011; González-Partida et al., 2017), whereas the meteoric component would result from the ingress of meteoric water in the highlands of the Mexican orogen (Fitz-Díaz et al., 2011).
5. Conclusions
The sediment-hosted stratiform copper-silver mineralization in the San Marcos area of Coahuila is characterized by disseminations, veins, and veinlets that are comprised everywhere by the same copper-bearing sulfide and oxide mineral assemblages. The mineralization is principally controlled by an Early Cretaceous redox boundary located at the transition between footwall siliciclastic red beds of the San Marcos Formation and hanging-wall carbonate strata of the Cupido Formation. Additional manifestations of the same mineralization style in lower horizons of the San Marcos Formation and in Late Jurassic strata of Sierra El Granizo beds, along with copper-bearing veins in basement blocks of Permian-Triassic age show that mineralization spans a stratigraphic thickness of ~3,000 m of basin fill.
The geologic and structural features of the sediment-hosted stratiform copper-silver mineralization in the San Marcos area corroborate a synorogenic timing of emplacement during inversion of the Sabinas basin triggered by Laramide contractional tectonism. In accord with the orogen-wide deformation chronology, the preferred timing for the mineralization at San Marcos, and elsewhere in the larger Monclova region, is Paleocene-Eocene (~60-40 Ma) coincident with voluminous fluid circulation in the basins of northeastern Mexico. This timing is equally applicable to the MVT mineralization of various styles, but mainly of lead-zinc-silver, if the previously established genetic and spatial relationships between the two mineralization types are correct.
Hence, although the Early Jurassic to earliest Late Cretaceous (~190-90 Ma) sedimentary fill of the Sabinas basin took place in a pericratonic setting initially associated with opening of the Gulf of Mexico, sediment-hosted stratiform copper-silver (and MVT) mineralization occurred during orogenic uplift and conversion of the original basin into a paired orogen-foreland setting, a situation remarkably similar to the tectonic environment of the Cambrian-aged sediment-hosted stratiform copper mineralization in the Zambian Copperbelt (Sillitoe et al., 2017a, 2017b).