Minera Mexicana Rosario S.A. de C.V., a wholly owned subsidiary of Guanajuato Silver, 100% holds and operates the Topia mine.
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Summary:
Deposit Type
The mineral deposits at Topia are adularia-sericite-type, silver-rich, polymetallic epithermal veins. Silver-gold-lead-zinc mineralization is found in fissure-filling veins along sub-parallel faults cutting andesitic flows, breccias, and pyroclastic rocks. Deposits are usually characterized by multiple veins in areas measuring 10 to 15 km2 with individual veins generally less than 2 m in thickness but up to 3 to 4 km in length.
Mineralization at Topia is reported to occur within a zone spanning 100 m to 200 m in depth, which is consistent with the epithermal model. The silver to gold ratio in these deposits is generally more than 300:1, and production from these deposits averages about 400 g/t silver.
Mineralization
Mineralization at Topia is hosted in a series of dominantly northeast-trending parallel veins that have historically been mined for gold, silver, lead, and zinc. Mineralization within the veins consists mainly of massive galena, sphalerite, with lesser pyrite, arsenopyrite, and tetrahedrite in a gangue of quartz, barite, and calcite. The vein constituents often include minor adularia and sericite, and the wider fault zones contain significant proportions of clay as both gouge and alteration products.
Metallic minerals occur as cavity-filling masses, comprising millimetre-scale crystals of galena and sphalerite. Some observations on metal zoning include: i) the lower parts of the mines are reported to contain slightly higher copper and gold contents than at higher elevations; ii) high silver grades are associated with higher proportions of base metals; and iii) the mines are located over an extreme range of elevations (1,000 to 1,800 masl) on the Property and that each has vertical limits of mineralization from 100 to 200 m with the deeper seated and more southern mines tending to contain more arsenopyrite and the western mines containing more pyrite.
The veins range in thickness from a few centimetres to two metres. They are very continuous along strike, with the main veins extending more than 4 km. The Madre vein has been mined for 3.5 km and the Cantarranas vein for 2.4 km. Many of the other veins have been mined intermittently over similar strike lengths. Vertically, the veins grade downward to barren coarse-grained quartz-rich filling and upwards to barren cherty quartz-calcite-barite vein filling. The main host rock is andesite of the lower volcanic series, which is usually competent, making for generally good ground conditions within the various mines. In wider sections, with greater clay content and/or zones of structural complexity, ground conditions are less favourable.