The Los Gatos Joint Venture holds mining concessions through its 100%-owned Mexican subsidiary company, Minera Plata Real S. de R.L. de C.V. (MPR). MPR is 70% owned by Gatos Silver, Inc. (formerly Sunshine Silver Mining & Refining Corporation) and 30% owned by Dowa Metals & Mining Co., Ltd.
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Summary:
Los Gatos is interpreted as an intermediate sulfidation epithermal vein.
Veins in the Cerro Los Gatos deposit show textures and gangue mineralogy (local chalcedony and calcite, and quartz-replaced lattice texture calcite) that indicate a relatively high-level hydrothermal system in the boiling environment. Breccia with clasts of vein quartz indicates a protracted hydrothermal system during multiple faulting events.
Mineralization at Cerro Los Gatos is characterized by silver, lead, zinc, and copper sulfides and their corresponding oxides, along with fluorite, manganese, barite, and traces of gold associated with quartz and calcite veins.
The veins vary in orientation from West-Northwest to Northwest to North-Northwest and vary in thickness from 20 cm to 30 m in the mine operation. Study of the veins in hand specimens and thin sections suggest they are epithermal in origin and are likely of intermediate sulfidation composition.
The Los Gatos District hosts a series of quartz, quartz-calcite, and calcite veins in at least fifteen separate vein systems that are exposed along a strike length of approximately 30 km and an outcrop belt width of approximately 5 km.
The veins containing silver, lead and zinc at Los Gatos are hosted primarily by the Andesite rocks immediately above the contact with the dacitic lithic tuff. Vein thickness is variable. Figure 7.7 shows an isometric view of the dacite lithic tuff unit (pink) which marks the position of a NE facing monoclinic fold that is disrupted by brittle deformation along the Lower Gatos and the antithetic Anti-Gatos faults (In red: Ag, Pb, Zn Mineralization).
Economic mineralized grades are not present at surface; however, epithermal alteration textures are present. The general Northwest trending, Northeast dipping Cerro Los Gatos vein system is persistent with a mapped extension in the order of 10 km, with true widths of as much as 25 m at depth as demonstrated by diamond drilling. Banded quartz veins and breccias are cemented by quartz, calcite, and abundant manganese oxides.
The Cerro Los Gatos deposit is a listric-shaped mineralized horizon hosting steeply to shallowly dipping mineralized-shoots at depth. Mineralization of interest occurs for approximately 1,800 m in length, between an elevation varying roughly between 800 masl and 1,400 masl. The top of the mineralized horizon at Cerro Los Gatos is generally located at an elevation of 1,400 masl. The natural topographic surface is in the order of 1,570 masl ± 50 masl.
The veins at Cerro Los Gatos contain silver, zinc and lead. Lower concentrations of gold and copper are also associated with the veining.
Silver mineralization occurs as acanthite (argentite) and native silver and has been detected in thin sections as proustite as small inclusions within galena grains.
Zinc mineralization occurs as sphalerite, zinc silicates and zinc carbonates of variable grain sizes disseminated in quartz vein material, as open-space filling in cavities, and as replacements in the andesitic and dacitic flow units. Sphalerite ranges from yellow to brown in color and is deposited in a similar style but is not always associated with the galena mineralization. Zinc oxides after sphalerite also exist down along fault structures through the deposit.
Lead mineralization occurs primarily as galena and lead oxide minerals of varying grain sizes that are disseminated in quartz vein material, as open-space filling in cavities, and as replacements in the andesitic and dacitic flow units.
Copper mineralization occurs as chalcopyrite and occasional native copper disseminated within quartz veins.
Gold mineral species have not been identified visually but are present in small quantities in assay results.
The veins themselves display variable gangue mineralization, depending on the depth of exposure within the epithermal environment. It is common to observe calcite or manganese oxide mineralization at high levels within the epithermal system. which transitions to barite, fluorite, and quartz at lower levels.
Within the mineralized portions of the veins, it is common to see quartz and fluorite and occasional minor calcite associated with lead, zinc, silver, copper, and gold mineralization. Fluorite is a significant component of the mineralized zones. The veins are typically rhythmically banded on a scale of 1 mm to 10 mm per band, with repeated pulses of quartz carrying the metals and other gangue minerals.
It is common to see multiple pulses of mineralization where small veins crosscut each other. It is also common to see various coloration of quartz in the multiple pulses, ranging from milky white to vitreous gray to amethystine purple.
It is apparent that most of the economic mineral values are associated with sulfide mineralization. Oxide mineralization is limited but present at depth, and is commonly related to fracture, breccia zones, and open spaces within the veins.
The Los Gatos vein mineralization has a halo of hydrothermal alteration. The distribution of alteration is complex because the halo was subsequently offset by the major listric fault movement.
The Los Gatos footwall rocks are mostly fresh and tough. Fresh mafics and feldspar are common, even as close as 20 m to the fault zone. Approaching the Los Gatos vein(s), magnetite is destroyed and the andesites become pale green. The illite alteration is typical argillic alteration found around epithermal veins. The dacitic tuffs are less affected; they remain siliceous and contain only a minor percentage of illite + pyrite. Illite alteration is seen at depth and at surface. At deeper levels, epidote + chlorite + pyrite alteration is widespread (propylitic).
High elevations within the hanging wall, particularly within rhyolite, are affected by massive chalcedony replacement and veinlets of chalcedony. Kaolinite + silica alteration occurs preferentially within the acidic volcanic rocks.