Summary:
The deposits within the San Sebastián del Oeste district are considered to be examples of low sulphidation epithermal deposits.
The San Sebastián del Oeste area, including the Terronera Project, is underlain by an intermediate to felsic volcanic and volcaniclastic sequence correlated with the middle to lower Cretaceous Lower Volcanic Group of the Sierra Madre Occidental geological province. This volcano-sedimentary sequence consists of shale, sandstone, and narrow calcareous-clayey interbeds overlain by tuffs, volcanic breccias, and lava flows of mainly andesitic composition. The volcano-sedimentary units crop out in the north-central part of the district. Further to the north, granitic to granodioritic intrusive rocks are present.
In the San Sebastián del Oeste district, silver and gold mineralization represents the upper portion of an epithermal vein system. Illite, sericite, and adularia are characteristic alteration assemblages that typically occur in the veins and in the vein wall rocks. In higher elevation areas, where limited mining (colonial-era artisanal mining), has occurred, such as the El Hundido and Real de Oxtotipan mines, the quartz is amorphous and milky white, indicative of a low-temperature environment.
The epithermal veins' silver-gold ± base metal mineralization is hosted in structurally controlled quartz and quartz breccia veins. The principal Terronera Vein has been traced by drilling for 1.5 km on strike and from the surface to the maximum depth of drilling at 546 m. The Terronera Vein strikes at approximately 145° and dips 80° east. The actual width of the principal Terronera Vein ranges from 1.5 to 15 m and averages 3.9 m. In addition to the main Terronera Vein, there are additional hanging wall and footwall veins. The veins are primarily hosted in volcanic flows, pyroclastic, and epiclastic rocks and associated shales and their metamorphic counterparts (Lewis and Mulahwi (2012); Munroe (2013)).
Metallic minerals include galena, argentite, and sphalerite associated with quartz, calcite, and pyrite gangue constituents. Munroe (2013) reported that high silver and gold values from 2011 sampling of underground workings in the Terronera Vein were primarily obtained from crystalline quartz veins, drusy in places, with limonite and manganese oxides lining box works after sulphides and fine-grained disseminated pyrite and traces of dark grey sulphides, probably silver sulphides.
Regionally, known deposits contain polymetallic sulphide mineralization in wide vein structures. The veins at higher elevations may represent the tops of ore shoots containing significant silver and gold mineralization at depth.
High-level hydrothermal systems form low-sulphidation epithermal deposits from depths of ~1 km to surficial hotspring settings. Deposition is related to regional-scale fracture systems related to grabens, (resurgent) calderas, flow-dome complexes, and rarely, maar diatremes. Extensional structures in volcanic fields (normal faults, fault splays, ladder veins, and cymoid loops) are common; locally graben or caldera-fill clastic rocks are present. High-level (subvolcanic) stocks and dikes and pebble breccia diatremes occur in some areas. Locally resurgent or domal structures are related to underlying intrusive bodies.
Most types of volcanic rocks can host the deposit type; however, calc-alkaline andesitic compositions predominate. Some deposits occur in areas with bimodal volcanism and extensive subaerial ash-flow deposits. A less common association is with alkalic intrusive and shoshonitic volcanic rocks. Clastic and epiclastic sediments can be associated with mineralization that develops in intra-volcanic basins and structural depressions.
Ore zones are typically localized in structures but may occur in permeable lithologies. Upwardflaring ore zones centred on structurally controlled hydrothermal conduits are typical. Large (> 1 m wide and hundreds of metres in strike length) to small veins and stockworks are familiar with lesser disseminations and replacements. Vein systems can be laterally extensive, but ore shoots have a relatively limited vertical extent. High-grade mineralization is commonly found in dilational zones in faults at flexures, splays, and cymoid loops. Textures typical of low-sulphidation deposits include open-space filling, symmetrical and other layering, crustification, comb structure, colloform banding, and multiple brecciation.
Deposits can be strongly zoned along strike and vertically. Deposits are commonly zoned vertically over 250 to 350 m from a base metal-poor, gold–silver-rich top to a relatively silver-rich base metal zone, and an underlying base metal-rich zone grading at depth into sparse base metal, pyritic zone. From surface to depth, metal zones can contain gold–silver–arsenic– antimony–mercury, gold–silver–lead–zinc–copper, or silver–lead–zinc. In alkalic host rocks, tellurides, vanadium-mica (roscoelite), and fluorite may be abundant, with lesser molybdenite.
The Terronera orebody is tabular, with a strike length of approximately 1,200 m, a thickness between 3 and 15 m, extending from surface to 570 m deep. The orebody is oriented southeast-northwest and dips on average 75° to 80° towards 050°. The footwall rock is mainly rhyolite (uniaxial compressive strength (UCS) approximately 92 MPa), with the hanging wall rock predominantly andesite (UCS approximately 175 MPa); the ore zone is termed the Terronera Vein (UCS approximately 62 MP based on limited samples).
The La Luz orebody is also tabular and steeply dipping, with a strike length of 500 m, thickness of 1 to 1.5 m, extending from 60 m below surface to a depth of 250 m. The orebody is oriented east-west and dips 80° towards 005°. The La Luz Vein is very narrow and is wholly contained in an andesite (UCS = 115 MPa) rock unit that forms both the footwall and hanging wall.