Proximal to distal hydrothermal alteration patterns around epithermal low-intermediate sulfidation vein deposits and their implications for precious metal exploration

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Simmons, S.F. 2017 Proximal to distal hydrothermal alteration patterns around epithermal low-intermediate sulfidation vein deposits and their implications for precious metal exploration. Lower Hutt, N.Z.: GNS Science. GNS Science report 2017/28 39 p.; doi: 10.21420/G2FW6C

Abstract: Low-intermediate sulfidation epithermal vein deposits form at shallow levels in hydrothermal systems and contain important resources of gold and silver. Individual veins dip steeply, extend for strike lengths of 500 to 5000 m, whereas vein districts cover areas ranging from 2 Within epithermal veins, ore bodies are often localised, extending several hundred metres along strike, owing to the boiling conditions that favour gold and silver precipitation within upflow zones. The patterns of hydrothermal alteration zoning surrounding epithermal veins are best developed in volcanic host rocks. Deep regional propylitic alteration gives way upward to alteration dominated by clays, quartz, calcite and pyrite. At shallow levels, strong to intense clay-carbonate alteration develops across the tops and/or the sides of veins and vein districts. At the highest level in and around the boiling water table, advanced argillic alteration forms from steam-heated acid sulfate waters, but where the water-table intersects topography, near boiling hot springs deposit silica sinter. Clay-carbonate and advanced argillic alteration are generally barren of gold or silver, even where they directly overlie vein-hosted ore bodies. Boiling conditions that cause gold-silver deposition also produces quartz-adularia alteration, which is a strong proximal indicator of veining and mineralisation. As adularia is very fine grained, thin section petrography, x-ray diffraction, or whole rock geochemistry are used to define the extents of such alteration envelopes which extend