Geology and genesis of the San Sebastian epithermal vein system, Durango, Mexico

The San Sebastian intermediate sulfidation, Ag-Au-Cu-Pb-Zn, epithermal deposit is in the Saladillo district, 97 km northeast of Durango City, in Durango State, Mexico. The district contains two known Ag-Au +/- Pb-Zn deposits (San Sebastian and Don Sergio), a historic mercury mine (La Roca), and a historic antimony mine (El Caballo). Since 2001, the San Sebastian mine has produced 23.7 Moz of Ag and 0.25 Moz of Au and sits within the Mexican Silver Belt. San Sebastian is rare within epithermal deposits worldwide as it is hosted within mudstones and graywackes, as opposed to more common volcanic rocks. The San Sebastian deposit consists of parallel veins (Professor, Francine and Middle) offset by post mineral movement along the San Ricardo fault. A coherent, deposit scale, geologic model, based upon 3D interpretations of major structures, lithology, and veins, was developed. It allows for a broad understanding of how the main geological components of the deposit interact and influence one another, which in turn reveals key observations into the genesis of the deposit.
Marine sedimentary rocks are the oldest exposed in the district. These include the mid-Cretaceous, Aurora and the Cuesta del Cura limestone formations. Overlying the limestone is the late-Cretaceous, Caracol Formation mudstone (with intercalated graywacke), which is host to the San Sebastian vein system. Nonmarine sedimentary rocks, in the form of the late-Cretaceous to early-Paleogene Ahuichila conglomerate, unconformably overlie the marine sedimentary rock package which are further overlain by volcanic rocks. Extrusive coherent and volcaniclastic rocks have two compositions, andesite, and rhyolite. Intrusive rocks in the district include the Andrea diorite, andesite sills, and rhyolite.
Dating of district volcanic rocks (U/Pb in zircons and Ar/Ar in K bearing minerals) and hydrothermal alteration (Ar/Ar in adularia) indicates six populations, from oldest to youngest: 1) Andrea diorite (average 82.48 Ma.), 2) intensely altered coherent volcanic rocks, (average 45.58 Ma.), 3) intensely altered clastic volcanic rocks, (average 37.61 Ma), 4) a series of volcanic rocks from La India and Pedernalillo areas (32.09 to 30.40 Ma), 5) early potassium feldspar alteration (29.21 to 28.18 Ma), and 6) Pedernalillo and La India volcanic units (28.49 to 27.78 Ma).
Four phases of deformation are recognized to have occurred within the district. D1 records ENE compression and is responsible for the NNW trend of bedding, as well as NW-NNW trending thrusts (San Ricardo fault) and folds seen within the district. D2 records N-S to NNE-SSW compression resulting in reactivation of crustal scale structures as transpressional shear zones and folds. D3 records a N-S extensional environment allowing for reactivation of D2 thrusts as normal to trans-tensional faults. The veins of San Sebastian formed within this stress regime. D4 records ENE extension and is responsible for listric normal faults which have tilted local stratigraphy up to 20 degrees to the NE. A compressional stress regime (D1 and D2) persisted in the region until mid-Oligocene when extension began in a NS orientation and rotated to the NNE (D3 and D4). This extensional regime likely reactivated the San Ricardo fault in a normal to trans-tensional sense.
Within this extensional environment, it is inferred that an intrusion exploited the San Ricardo fault and was emplaced beneath the location of the current vein system. Exsolving fluids from the intrusion, mixing with those circulating within the hydrothermal system, reacted with the host rock mudstones resulting in intense potassium metasomatism. This alteration significantly hardened the rock and made it more amenable to brittle failure. A triggering event destabilized this system and resulted in fracturing and vein formation.
The veins are multi-generational, indicating numerous opening and mineral precipitation events. Veins have been defined over 3 km in strike length, 500-700 m in dip length, with true widths averaging 1.5- 2.5 m. Vein textures include crustiform, colloform and cockade banding, as well as massive infill. Gangue mineralogy is dominated by quartz, calcite, chlorite, and sericite, with lesser adularia, rutile, and trace titanite. Vein hypogene ore mineralogy consists of acanthite, tetrahedrite, polybasite, freibergite, pyrargyrite, sphalerite, chalcopyrite, pyrite, and galena, with trace gersdorffite and cobaltite. The upper portions of both veins have experienced varying degrees of supergene weathering, resulting in secondary stromeyerite, naumannite, electrum, bromargyrite, digenite, and cerussite. Adularia, muscovite, and chlorite distribution, the location of colloform banding, and the location of recrystallization and replacement textures, all indicate that the base of the boiling horizon was between the elevations of 90 and 245 m below surface in the Francine vein, and greater than 350 m below the surface in the Middle vein. The significant volume of calcite, in the upper portion of Middle vein is indicative of the presence of carbonate-rich water that forms on the margins of these deposits, and which can be drawn down into veins as upwelling wanes.
Metal distribution within both veins indicates two distinct mineralized zones and two distinct depositional processes. A lower zone is dominated by base metals and an upper zone is dominated by precious metals. The metal tenor and vein textures of these zones suggests the dominant means of metal transport and deposition was chloride complexing and deposition along a decreasing temperature gradient within the lower zone and bisulfide complexing and deposition due to episodic boiling within the upper zone. In both veins, intermittent, but repeated, fracturing events allowed fluids along the lower pathways to rise near vertically, with little additional metal deposition, until boiling triggered precious metal deposition within the upper zone.
Veining within the Saladillo district formed between 32.03 and 28.24 Ma. The Andrea, Francine, and Don Sergio veins span 32.03 Ma to 30.69 Ma. The Middle vein is slightly younger (29.85 Mato 28.24 Ma). The ages of the veins overlap, though each vein appears to have had a duration of 1 to 1.5 million years.
At San Sebastian, the hydrothermal system and post mineral weathering have resulted in three alteration horizons within the sedimentary host rocks: an upper weathered horizon, a middle least altered horizon, and a lower early potassium feldspar horizon. Vein-related hydrothermal alteration affects each horizon, further dividing the San Sebastian stratigraphy into three alteration horizons, and six alteration domains. Least altered mudstones have a modal mineralogy of muscovite, albite, quartz, calcite, chlorite, and potassium feldspar, with trace apatite, rutile, and pyrite. Relative to this mineralogy: the early potassium feldspar domain is enriched in potassium feldspar, and depleted in muscovite, albite, calcite, and quartz. The vein over early potassium feldspar domain is enriched in potassium feldspar and titanite and is depleted in muscovite, albite, chlorite, and pyrite. The vein over least altered domain is enriched in quartz and potassium feldspar and depleted in calcite. The weathered over least altered domain is enriched in quartz and depleted in calcite, muscovite, albite, and chlorite. The weathered over vein domain is enriched in potassium feldspar and depleted in albite. The early potassium feldspar horizon occurs at depths greater than 300 m while the weathered horizon is restricted to the upper 120 m of stratigraphy. Vein-related alteration is restricted to within 10 cm of the vein within the early potassium feldspar horizon and to within several meters of the vein in the least altered and weathered horizons.
Elemental mobility has been quantified for each protolith-altered equivalent pair and elemental additions and depletions greater than 2x are summarized here in order of occurrence, and in decreasing magnitude of addition/depletion. 1) Early potassium feldspar alteration resulted in addition of As, Au, S, Na2O, Sb and depletion of Hg, Be, and C. 2) Vein over early potassium feldspar alteration resulted in addition of Mo, Be, Ag, Au, Cu and depletion of Na2O,As, and B. 3) Vein over least altered alteration resulted in addition of Hg, Sb, Au, Ag, Zn, S, Pb, Se and depletion of Na2O, CaO, C, and Sr. 4) Weathering over least altered alteration resulted in addition of Au and depletion of S. 5) Weathering over vein alteration resulted in the addition of Au and depletion of S, C, Sr, and Se.
The alteration domains have distinct SWIR characteristics. Least altered samples are characterized by white mica of phengitic composition and chlorite of intermediate composition. Within the early potassium feldspar domain white mica is distinctly muscovite and chlorite is intermediate in composition. Within the vein over early potassium feldspar domain, white mica is muscovite and chlorite is Mg-rich. Within the vein over least altered domain white mica is muscovite and chlorite is Mg-rich. Within the weathered horizon white mica is phengitic-illite. Hydrothermal white mica is muscovite and is compositionally zoned and hydrothermal chlorite is Mg-rich and is also compositionally zoned. Hydrothermal white mica and chlorite are limited to the vein over least altered domain and the early potassium feldspar horizon.
Within the Francine vein, fluid temperatures (from fluid inclusion measurements) range from 223 °C near surface (2,065 m elevation) to 323 °C. at depth (1,536 m elevation) and span over 500 meters of dip length. Within the Middle vein, fluid temperatures range from 169 °C near surface (2,068 m elevation) to 312 °C. at depth (1,744 m elevation) and span over 300 m of dip length. Vein fluid inclusion temperatures do not correlate with carbon geothermometry estimates from nearby .......