Paragenesis and mineral chemistry of alabandite (MnS) from the Ag-rich Santo Toribio epithermal deposit, Northern Peru

ISSN： 0026-461X

Source： Mineralogical Magazine, Vol.67, Iss.1, 2003-02, pp. : 95-102

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Abstract

The Miocene, epithermal, Ag-rich polymetallic Santo Toribio deposit is hosted by the volcanics of the Quiruvilva-Pierina subbelt, Northern Peru, which also comprises the world-class, high sulphidation Pierina deposit. The Ag-rich, alabandite-bearing veins of the Santo Toribio deposit formed during two major stages. The early stage is characterized by deposition of arsenopyrite, pyrite, quartz, Mn-sphalerite, stannite, alabandite and minor miargyrite. Sphalerite associated with this stage is exceptionally enriched in Mn (up to 14.5 wt.%) and alabandite is optically and mineralogically zoned. Its brown zones have greater Fe+Sb and smaller Mn contents than the green zones and Fe+Sb replaces Mn in its structure. During this early stage, fs₂ must have been high to allow the stabilization of alabandite relative to rhodochrosite. In the second stage, the physicochemical conditions changed and the CO₂/S ratio increased, causing dissolution of alabandite and the deposition of abundant rhodochrosite and a second generation of arsenopyrite, pyrite and quartz, sphalerite with chalcopyrite inclusions, mirargyrite, ramdohrite, and finally stibnite. This polymetallic ore probably formed due to an abrupt decrease in H⁺ and/or Cl^- concentration caused by boiling or dilution of the high-salinity hydrothermal fluids and constitutes an example of "intermediate sulfidation-state" epithermal deposits.