Arsenic as an indicator element for gold exploration in the region of the Xiangxi Au–Sb–W deposit, NW Hunan, PR China |
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| Authors: | S X Yang N Blum |
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| Institution: | a Freie Universität Berlin, FR Rohstoff- und Umweltgeologie, Malteserstrasse 74–100, 12249 Berlin, Germany;b Forschungszentrum Jülich, Projektträger Biologie, Energie, Umwelt (BEO 52), Bundesministerium für Bildung und Forschung (BMBF), Seestrasse 15, 18112 Rostock-Warnemünde, Germany |
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| Abstract: | The Xiangxi Au–Sb–W deposit, the largest of its type in northwestern Hunan, China, is a sulfide-dominated ore body hosted by low grade metamorphic red slates of the Neoproterozoic Madiyi Formation. Three stages of mineralization, quartz–scheelite, quartz–gold–pyrite, quartz–gold–stibnite, and one metal-barren stage of veining, quartz–calcite, are recognized. Arsenopyrite occurs only as a minor mineral phase in the second stage. Analyses for 21 trace elements show that the enrichment factors of As in the metal deposit (EC =element concentration of sample/average content of an element in the upper crust]: 190; 43 samples) in ore veins and in the Guanzhuang and Yuershan reference sections (3.7 km and 2.7 km away from the Xiangxi mine, EC: 3.5; 96 samples) are much smaller than those of Sb (52855 in ore veins], 117 in the sections]), W (5665, 7.5) and Au (2727, 5.3). The background concentrations of Au and As in the two sections were 1.4 ppb and 1.4 ppm, respectively. Arsenic (with an anomaly coefficient AC = number of anomalous samples/total number of samples] of 76%) forms a larger geochemical halo than W (AC: 8%) and Au (AC: 32%). Gold and As in the deposit were transported mainly as metal complexes such as Au(HS)−2, HnAs3S−(3−n)6 (n=1, 2 or 3) and HAsS02. Au(HS)−2 is rapidly precipitated by a geochemical oxidation barrier — the red slates of the Madiyi Formation. As–S complexes in the stratigraphic horizon can be transformed into As–O complexes (e.g., H3AsO03) under oxidizing conditions, and are continuously transported. Therefore, they can be widely distributed in the red slate units, thus forming extensive geochemical haloes, so that As can be used as an indicator element for Au exploration in the Xiangxi region. |
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| Keywords: | Au As Sb W indicator elements exploration transport Xiangxi China |
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