Summary:
The San Gabriel deposit is hosted in the breccias of a complex of intrusive volcanic diatremes that cross through the sedimentary rocks deformed in the Yura Group. The volcanic rocks are found deep in the Canahuire zone and consist of aphanitic rhyolitic volcanic sequences.
The diatreme complex is related to the complex of rhyolitic domes of Chucapaca and associated dikes, and includes various breccias with distinct characteristics of facies. The domes present an aphanitic texture and flow bands. The complex of diatreme breccias comprises phreatomagmatic and phreatic breccias. The latter have been fragmented by steam pressures while the former includes a magmatic component. The breccias show a wide range of composition and size of clasts, matrix, and cement and variations in the internal organization of the breccia.
The diatreme complex presents as an elongated body, which is deeper to the west, is approximately 1,300 m long, 250 m wide and possesses an average thickness of 170 m. It is important to note that the root of the diatreme remains open at depth. Additionally, the breccias appear to have a genetic relation with the mineralization at San Gabriel.
Mineralization
Two stages of mineralization are interpreted. Both, although closely related in space and time, occurred under different conditions.
Early Stage Mineralization Cu-Ag-As
Early mineralization of copper and silver is characterized by the presence of pyrrhotite, pyrite 1, chalcopyrite, arsenopyrite 1 and sphalerite; these minerals are present in all the rock types. The pyrite is typically cubic, measures up to 8 mm, and contains small inclusions of pyrrhotite. The chalcopyrite and the arsenopyrite all contain small inclusions of pyrrhotite, which is a distinctive feature of this stage. This early mineralization manifests as cement that fills open spaces in the breccia, acting as a replacement in the limestone of the Gramadal Formation and in the veins and veinlets of the sediments in the Gramadal and Labra formations.
The most common gangue minerals are siderite and carbonate; these are broadly distributed in both rock types, mainly as cement in breccia or as a replacement for veins/veinlets. The magnetite is associated with early mineralization and is found predominantly in sandstone at greater depths, in distal zones of mineralization with pyrrhotite. The early stage represents no economic content.
Main Stage Mineralization Au-Cu-(Ag)
The main stage of mineralization of gold, copper and silver represents the most important economic phase of the deposit and partially replaces the early stage. The mineralogy includes gold, electrum, maldonite, pyrite II, arsenopyrite II, marcasite, chalcopyrite, tetrahedrite, tetradymite, sulphosalts of Bi-Sb and Ag, sphalerite and galena. The gangue minerals include carbonates (siderite, ankerite, mixed carbonates and impure rhodochrosite), quartz, chalcedony, opal, clay and adularia.
Gold mineralization is found mainly in the limestone in the middle member of the Gramadal formation, which presents the most continuous replacement alternation and mineralization at San Gabriel. The mineralized areas show strong vertical zoning controlled by lithology, while the lateral limits are defined by brecciated faults or processes related to oblique extension. This zoning is evidenced by the absence of mineralization in the Labra and Hualhuani formations as well as in some portions of the phreatomagmatic breccia.
Gold is concentrated mainly in the limestone of the Gramadal Formation, which explains the soft dip of the mineralization to the west-northeast. It is also concentrated in polymictic breccia, which contain variable proportions of clasts and carbonated matrix. The high reactivity of the hydrothermal fluids and limestones controls this mineralization. In the case of phreatomagmatic breccia, the muddy matrix reduces permeability, which limits mineralization to a zone of contact with polymictic breccia.
The higher-grade areas of mineralization, dominated by the hydrothermal replacement, are found in the intersections of the fossiliferous limestones with thicker grains.
Deposit Types
Epithermal deposits are important sources of silver and gold, but they may also contain subproducts such as copper, lead, zinc and mercury. The characteristics of the individual epithermal deposits can vary widely, depending on their mode of formation, and can result in mineralization of only a few thousand tons to >100 Mt, with grades of 0.1 g/t to 30 g/t of Au and between 1 g/t and 1,000 g/t of Ag.
These commonly represent the most superficial parts of the largest and deepest hydrothermal systems, driven mainly by magmatic heat and, to a lesser extent, by circulation of deep underground waters that run along the faults that border basins.
The epithermal high sulphidation (HS) deposits typically contain mineral associations rich in sulphur and in the state of high sulphidation such as pyrite-bornite, pyrite-enargite, pyriteluzonite, pyrite-famantinite and pyrite-covellite.
The epithermal deposits with intermediate sulphidation (IS) are of great interest in mining given their economic and scientific importance, whether due to their high content of metals such as Ag-Au-Pb-Zn or as inputs to study epithermal-porphyry systems (Wang et al. 2019). This group of epithermal deposits tends to be closely associated with volcanic and sub-volcanic rocks with an andesitic-dacitic composition and are formed at depths that oscillate between approximately 0.3 km and more than 1 km, and generally at temperatures between 150 °C and 300 °C. The IS deposits are typically related to oxidated magmatism that is calcic or calc-alkaline in nature (Wang et al. 2019).
These deposits tend to occur in tectonic zones that have experienced a prolonged history of subduction and/or collision of plaques and can be found in magmatic arcs, backarc basins, and post-collisional orogenic belts. The porphyry-type intrusions tend to provide the heat source for these hydrothermal systems; as such, the epithermal mineralization can be spatial and genetically related to the mineral deposits of copper and molybdenum associated with porphyry. Epithermal mineralization occurs when focalized ascending and rapidly circulating fluids present a brusque change in their composition a few hundred feet from the surface.
The location and form of the ore bodies in most gold-silver epithermal deposits are intimately related with the structural characteristics associated with igneous (diatreme), sedimentary, tectonic, topographical and hydrothermal fluids. These structures provide routes and barriers for the flow of hydrothermal fluids and sites for the deposit of gangue and ore minerals. Many epithermal deposits contain various types of mineralization due to the interaction of multiple sets of structural characteristics. For example, epithermal deposits of low and intermediate sulphidation commonly present high-grade veins as well as low-grade mineralization that is often disseminated in stockwork and/or breccia; the aforementioned can represent different parts of a single hydrothermal system or mineralization formed at different times.
In the case of the San Gabriel deposit, this sample shows many characteristics of an epithermal deposit with IS, formed within a backarc environment, as described by Wang et al. (2019) as well as Sillitoe and Hedenquist (2003).
Most of the epithermal deposits with IS are found in andesitic-dacitic arcs with a calc-alkaline composition; locally, however, these deposits can be present in more felsic lithologies. Some IS deposits can be located close to advanced argillaceous lithocaps, with or without associated high-sulphidation deposits (HS) (Sillitoe and Hedenquist 2003). The San Gabriel deposit is located next to the Chucapaca Dome Complex and an associated lithocap.