Geological and mineralogical characteristics, and extraction history of the Guanajuato Mining District, Mexico.Possible geochemical sceneries for the tailings

Abstract

We describe the geological features and the extraction methods historically applied in the Guanajuato Mining District, in order to propose hypothesis about the geochemical conditions that prevail in the tailings. Information about geographical conditions of the district, ore type, production history, processing methods, and the recovered amount of gold and silver was compiled. We estimate the approximate volume of tailings and the concentration of calcite and pyrite in relation with the location on the source deposit.

According with the production history, approximately 95 million tons of mineralized rock have been extracted and processed by different methods: smelting and amalgamation (from 1548 to 1905), cyanidation (from 1905 to present), and flotation (from 1946 to present). The elements present in the tailings that may be hazardous because of their high toxicity are: As, Cd, Cu, Pb, Se, and Zn, in addition to Hg and Cyanide, which have been added during processing.

Due to the heterogeneity of the deposits, the sulfide content in the tailings is highly variable. The sulfide concentrations are higher when the ore occurs as veins and breccias than when it is contained in stockworks. In the areas with argillic alteration, pyrite is present in high concentration. When the ore is oxidized, its concentration of sulfide is very low. Therefore, the sulfide concentration in the tailings varies in accordance with the source mineralogy in the following relation: Veta Madre center, deep (Rayas) > La Sierra » La Luz >Veta Madre south (stockwork) > Veta Madre center, shallow (oxidation).

The amount of calcite in the tailings, and therefore the capacity for acidity neutralization, depends on the calcite concentration in the gangue, which is controlled by the host-rock mineralogy and the predominant alteration type in each section of the deposit. The only type of alteration that favors the neutralization capacity is the propylitic alteration. This alteration type is widely spread in the deposit (farther than that 10 km from the veins). The content of calcite in the tailings varies with the location of the source deposit as follows: La Luz > La Sierra » Veta Madre south > Veta Madre center. Some tailings have a high acidity generation capacity, namely those that come from the deeper section of the deposit.