A mineralogical study of the Guanajuato, Mexico, silver ores

Abstract

The silver-gold ores now being worked in the Guanajuato, Mexico, mining district consist chiefly of argentite and native gold-silver, with minor amounts of polybasite, pyrargyrite, and some argyrodite, a silver germanium sulfide. With these are always associated galena, sphalerite, chalcopyrite, pyrite, and minor amounts of marcasite. The gangue is generally a breccia of various rock types, mostly volcanics, cemented by calcite that has been largely replaced by quartz. The quarts is often amethystine, and is the carrier of the metallic minerals. The breccia fragments are pyritized, but not markedly mineralized otherwise; most of the ore minerals are in the quarzitic cementing materials; however, relatively large crystals of the complex sulphosalts appear to be younger than, i.e., deposited on, the quarts crystals in vugs. The mineralization is not of the banded-vein type, but consists of discrete ore particles in patches and streaks in the quarts. In general there is no,clear indication that any of the base-metal sulfides are in a definite age sequence; there is evidence of argentite replacing galena, as well as the other sulfides. There are local variations in the gangue - a soapstone ("jabon") is worked locally, and also a porous leached (?) siliceous rock ("charasco"). The character of the mineralisation, however, is essentially constant throughout the area studied, and in particular no significant variation was noted in the several vein systems. Investigation of gas-liquid inclusions in the quartz suggests that the ores were deposited at about 254°C and, therefore, that the mineralization is hydrothermal. Possible recovery of the base-metal content of the ore is considered; it is computed that several hundred thousand dollars worth of copper, zinc, and lead is mined and milled annually, and discarded as tailings and/or slag. This with similar waste from not-too remote mining districts might be processed at some central point for profitable recovery. Hitherto unrecognized germanium is also present, but in the absence of a cheap and reliable method of estimating small percentages of this metal, it is not likely that germanium can be produced economically.