Multistage evolution of the Calera epithermal Ag-Au vein system, Orcopampa District, southern Peru; first results

Abstract

In the Orcopampa district, southern Peru, the Calera vein and its splits comprise a complex multistage epithermal fissure vein system of early Miocene age hosted by slightly older silicic and intermediate volcanic rocks. The several veins of the district are hosted by normal faults that strike N 45 degrees E to east-west. The Calera vein system is the largest and economically most important yet discovered in the district. Produced ore and reserves contain over 40 million ounces of silver and 0.4 million ounces of gold.Five paragenetic stages as well as multiple episodes of fracturing, faulting, and hydrothermal brecciation are recognized. The early stage is defined by quartz + or - adularia + or - sericite + or - pyrite altered wall rock and by mineralogically similar veinlets. Altered wall rock rich in adularia borders the vein at depth and yields laterally to sericite-rich rock within a few meters of the vein, whereas altered wall rock with abundant sericite borders the vein in upper levels. Early-stage material is cut and brecciated by several substages of the Mn stage, which are characterized by abundant rhodonite and rhodochrosite containing bands composed largely of sphalerite and other sulfide minerals or tetrahedrite. Rich ores contain 10 2 to 10 3 ppm Te, mainly in hessite (Ag 2 Te). The Mn stage is followed by two quartz-rich substages; the second substage contains tetrahedrite. This quartz stage is in turn succeeded by the bonanza stage, which contains native gold, miargyrite, pyrargyrite, and several other silver sulfosalts and precious and base metal tellurides in quartz gangue; this stage contains about 50 percent of the precious metal values of the Calera system. Late quartz and sphalerite + galena veins with barite, marcasite, and stibnite and hydrothermal breccia dikes of the late stage are volumetrically minor.Ore shoots typically occur in zones of intense fracturing indicated by abundant stockwork veins and vein splits. Based on the form and zoning of the west-plunging main ore shoot, ore-forming fluids appear to have moved diagonally upward. Several superimposed stages have different distributions within the main ore shoot; the Mn stage is better developed at depth and the quartz stage is dominant in upper levels. The Au-rich ores of the bonanza stage are found preferentially in a zone of intersection of vein splits that appears to have controlled fluid flow. Mineralization throughout the district is very similar. The bonanza stage, however, has been identified only in the Calera vein.The paragenetic sequence has resulted from a series of progressively evolving hydrothermal fluids. Initial abundant rhodonite is replaced by rhodochrosite and eventually by quartz as the dominant gangue mineral. The base metal sulfide and tetrahedrite bands must have formed from compositionally very different hydrothermal fluids. The bonanza stage also represents a distinct fluid with much lower Ag/Au, relatively high a Te /a S , and low manganese content. The close spatial and temporal association with dacite dikes and flows, the relatively high Te content of the ores, and the high a Te /a S ratios implied by some of the phase assemblages suggest the involvement of deep-seated hydrothermal fluids.