ABSTRACT
The Juramento Cu-(Ag) stratiform copper deposit of northwestern Argentina consists of disseminated sulfides hosted by Late Cretaceous shallow marine or lacustrine carbonates and siltstones overlying rift-filling continental redbeds. The host greybeds were prepared for mineralization by the deposition of finegrained syndiagenetic pyrite and by an increase in porosity accompanying early diagenetic dolomitization. The ore solution, arriving from the oxidized footwall redbeds, presumably consisted of a low-temperature chloride brine capable of leaching metals (principally Cu, Pb, Zn and Ag). The resulting mineralization consists mainly of disseminated cupriferous sulfides, galena and sphalerite which replaced pyrite and, to a lesser extent, gypsum. Sulfur isotopes of ore minerals are heterogenous in values and heavier than those of pre-ore pyrite, indicating a possible contribution of sulfur from gypsum. The host sediments were subsequently silicified and compacted during deep burial, and then disturbed by the Andean tectonic event and recent supergene alteration.
