Supergene ore modifications of epithermal silver mineralization at Martha Mine, Deseado Massif, Argentina

PAEZ, G.; RUIZ, R.; PERMUY VIDAL, C.; LOPEZ, L.; GUIDO, D.; JOVIC, S.

Upsala

Encuentro; 12th SGA Biennial Meeting; 2013

SGA

The Martha underground silver mine is located in the southwestern portion of the Deseado Massif, Santa Cruz province, Argentina. Martha is an intermediate sulphidation epithermal deposit with a complex mineralogy dominated by Ag-As-Sb sulphosalts and Cu-Pb-Zn sulphides. The deposit is characterized by a structurally controlled vein system characterized by a complex history of fracturing and cementation, arranged in nine paragenetic stages that can be grouped into four episodes. Grade vs. depth profile show a shallow 20 m thick oxidation zone dominated by mean Ag/Au ratio of about 800:1. This upper zone is composed an association of kaolinite, plumbogummite, limonites, jarosite, cerussite, chlorargyrite/embolite, native silver, malachite and azurite. Below the oxidized zone, a 30 m thick enrichment zone can be recognized by a sharp increase in the Ag/Au ratio (up to 20,000:1). This zone is composed by a mineral assemblage dominated mainly by argentite/acanthite, pyrargirite, native silver, chalcocite, digenite, coveline, and minor bornite and spionkopite. Below the enrichment zone, the primary hypogene zone is characterized by a progressive reduction in the Ag/Au ratio to values of about 1000:1. These changes are the first mention of a supergene enrichment blanket in the Deseado Massif; and should be considered during exploration, as high grade ore-shoots may be concealed below a lower grade oxidized horizon at surface.