Summary:
The Cinco Minas Vein system at Los Ricos is a classic epithermal precious metal deposit, which exhibits at least three phases of quartz and sulphide mineralization and deformation. The vein, which is up to 30 m in width and has been traced over 3.5 km on the concessions, occurs along a northwesterly trending structure that roughly marks the boundary between two calc-alkaline magmatic arcs: 1) the older Sierra Madre Occidental volcanic province to the north; and 2) the younger Trans-Mexican volcanic arc to the south. The Sierra Madre Occidental province is the largest accumulation of pyroclastic flows and ignimbrites in the world (Nebocat, 2002). The age of these volcanics ranges from Cretaceous (100 Ma) to Neogene (18 Ma) (previously Tertiary).
Los Ricos (Cinco Minas) is a classic Neogene (previously Tertiary) age, volcanic-hosted and low-sulphidation epithermal precious metal deposit.
The geology of the Hostotipaquillo District is characterized by late Oligocene to Pliocene volcanic and sub-volcanic intrusive rocks deformed by a set of northwest and east-west trending, graben-forming normal faults. Oligocene and Miocene volcanics are primarily andesite flows, rhyolite ash flow and air fall tuffs, and rhyolite and dacite flow-domes that are partially covered by Pliocene to Recent basalt flows. The northwest-trending graben that extends across most of the district is one of several late Miocene to Quaternary tectonic depressions formed in the area of the intersection of the south SMO and the TMVA, and is part of the larger regional west-northwest trending Zacoalco Graben system.
The Rio Santiago River flows northwest through the district along the northeast margin of the Hostotipaquillo district graben structure, including, from northwest to southeast, the La Trini-Mololoa-Monte del Favor group of mines, Gran Cabrera group of mines, Santo Domingo-La Española Mine group, and Cinco Minas-El Aguila Mines vein systems. These faults form prominent scarps that are the canyon walls on the southwest and south side of Rio Santiago River. The mineralized vein systems in these faults form dip slopes in the river canyon walls at several locations, such as Cabrera and Santo Domingo-La Española.
Andesite occurs in various colors and textures. Northwest of the El Aguila Mine, near the bottom of the vein and in San Miguel Creek and the mouth of La Calera Creek, the andesite is greenish grey in color and very fine-grained. It contains abundant quartz phenocrysts and previous investigators classified the rock as quartz andesite.
At the Village of Cinco Minas, outcrops of the andesite form the hanging wall to the Cinco Minas Vein. They are of reddish-purple, porphyritic andesite and overlie andesite tuffs (Rivera and Vazquez). These andesites have been observed in an open pit exposure at the El Abra workings (P&E, 2019). These volcanics have been down-dropped along the dip-slope of the Cinco Minas Vein by a large, post-mineralization fault, such that they appear to conformably overlie the vein/fault surface.
In the El Abra Mine, rhyolites occur overlying the andesite in the lower parts of the vein. On surface, the rhyolites are found principally in outcrops above the vein and underlie most of the higher hills found to the northeast of it. The rhyolites are pink and light green in colour and contain quartz phenocrysts. Quartz phenocrystic rhyolite was observed along crosscuts connected to the La Famosa level haulage, located in the footwall of Cinco Minas Vein (P&E, 2019).
Andesite and rhyolite tuffs are present. The andesite tuffs outcrop in Cinco Minas Creek are light green in colour, fine-grained, and locally have purplish inter-bands and show indications of internal folding. The latter type outcrops in the higher parts of the hillside in the extreme northwest part of Cinco Minas Vein near the San Juan historical workings. Here they are pale pink in colour and contain abundant quartz and biotite phenocrysts and feldspars phenocrysts that are kaolinized.
Mineralisation
The mineralized vein at Los Ricos South, which is as much as approximately 30 m wide, has had at least three quartz vein formation and metal precipitation events (Nebocat, 2004). The vein strikes across the Property for 3.5 km, following a northwest-southeast trending fault, and dips approximately 65° to 70° to the west.
The chief gangue mineral is quartz, whereas crystalline quartz is plentiful and colourless, cream colored, milky or occasionally even amethyst, the bulk of the quartz is a dense fine-grained grey, yellowish of greenish porcelain quartz intermixed with chalcedonic quartz of a slightly later origin. This quartz is associated with less grey, brown and white calcite. Some of the calcite was deposited contemporaneously with the quartz, however, most of it post-dates the latter. The contemporaneous calcite is more abundant in the vein near surface than at depth. Comb quartz and open vugs in the quartz vein were more abundant in the upper workings near surface. Siderite, rhodochrosite and adularia were also noted as rare occurrences, whereas kaolinite was also present near secondary enriched oxidized mineralized materials. Chlorite and possibly epidote were also in evident in association with the quartz, or in partially replaced rock fragments.
The chief sulphide minerals are galena, chalcopyrite, sphalerite, pyrite, covellite, bornite and silver sulphides. The higher-grade sulphide zones are generally associated with finegrained galena and minute specks of chalcopyrite. Coarse-grained sulphides generally carry low metal values. Recognizable silver sulphides are rare. However, specks of pyrargyrite were observed in the high-grade sulphides that might have been tetrahedrite, argentite or other silver sulphide minerals. Native silver flakes and specks were observed coating fractures in high-grade sulphide mineralized zones.
There appear to have been either several periods of vein deposition or a long period of mineralization interrupted periodically by faulting and brecciation of the quartz. The mineralization appears to have formed relatively early. The location of the mineralized shoots, which appear to rake at 65° to 70° northwestwards, seems to have been controlled by:
• segregation of the minerals during deposition from the original primary solutions;
• periods of movement and brecciation that followed the first period of mineralization; and
• formation of the diagonal slips or cross-faults that confined or diverted the later primary mineralizing solutions into these breccia-filled channels.
The portions of the quartz vein between the various mineralized shoots show little evidence of brecciation of the original quartz. The richest mineralized shoots appear to have been associated with the greatest width of quartz and the portions of the veins showing the greatest number of periods of brecciation.
The Magdalena and the San Pedro-San Juan mineralized shoots, although of minor importance, appear to have a true shoot-like character and plunge approximately 65° to the northwest. On the Destajos Level and above, there was originally almost continuous mineralization from the southeast edge of the San Nicolas Stope to the northwest edge of the Main Destajos Stopes at approximately 320 m northwest of the main shaft.
Garrey (1923) reports that the San Nicolas, San Diego, Destajos, Cinco Minas and North Cinco Minas deposits were probably part of what appears to have been one large, mineralized shoot, subsequently broken-up along faults into smaller shoots. Drill hole LRGG-19-079 intersected the Los Ricos Vein from 128.2 m to 144.7 m, which displays typical gold and silverbearing quartz veins in the area of the historical Cinco Minas underground mine.