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The geology and evolution of the Manantial Espejo epithermal silver(-gold) deposit, Deseado Massif, Argentina Wallier, Stefan


Quartz-adularia epithermal veins record the interplay between magmatism, tectonism, and derivative geothermal systems. At Manantial Espejo, southern Argentina, such veins form a Ag(-Au) deposit with a proven and probable reserve of 38.5 Moz Ag and 0.55 Moz Au. The district located in the Deseado Massif is part of the Middle to Late Jurassic Chon Aike Province, a large bimodal igneous province dominated by silicic volcanic rocks that was formed in association with the Gondwana break-up. The district stratigraphy consists of a more then 400 m thick andesitic to rhyolitic volcanic succession formed between 162 and 157 Ma. The precious metal veins post-date the volcanic host rocks and formed contemporaneously with normal faults at 154 ± 1 Ma. Mineralized veins are Ag-rich examples of a low- to intermediate-sulfidation epithermal deposit. In the Maria Vein, the major orebody of the district, six hydrothermal stages are identified, but only stages 1 and 4 contributed significantly metal. The distribution of the hydrothermal stages along the vein is primarily controlled by fault formation and records the growth and linkage of vein segments to form an integrated curviplanar vein. Alteration mineral zoning about the Maria Vein with proximal adularia and distal illite and illite-smectite assemblages shows higher temperature mineral assemblages extending over a broader halo and to shallower levels along a high-grade vein segment where compared to a low-grade segment. The dominant changes in whole-rock alteration geochemistry toward the Maria Vein are the gain of K and the loss of Na, Ca, and Mg. Precious metals, base metals, and pathfinder elements have typically increasing concentrations toward the vein. Boiling was the dominant depositional process and metal ratios in veins reflect the abundance of dissolved metals in solution. Barren quartz with abundant boiling textures was formed by low-salinity fluids of temperatures between 230° and 270°C. Mineralizing fluids had lower temperatures, but salinities between 4 and 10 wt % NaC1 equiv. Lead isotopic constraints indicate a mostly common source for Pb in volcanic rocks of the Chon Aike Formation and Pb in hydrothermal sulfides, or that the Pb in the hydrothermal veins was derived from the Chon Aike Formation.

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