Arsenic as an indicator element for gold exploration in the region of the Xiangxi Au-Sb-W deposit, NW Hunan, PR China

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 mineralizat ion, quartz-scheelite, quartz-gold-pyrite, quartz-gold-stibnite, and one me tal-barren stage of veining, quartz-calcite, are recognized. Arsenopyrite o ccurs only as a minor mineral phase in the second stage. Analyses for 21 tr ace elements show that the enrichment factors of As in the metal deposit (E C [=element concentration of sample/average content of an element in the up per crust]: 190; 43 samples) in ore veins and in the Guanzhuang and Yuersha n 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 concen trations of Au and As in the two sections were 1.4 ppb and 1.4 ppm, respect ively. Arsenic (with an anomaly coefficient [AC = number of anomalous sampl es/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 main ly as metal complexes such as Au(HS)(2)(-), HnAs3S6-(3-n) (n = 1, 2 or 3) a nd HAsS20. Au(HS)(2)(-) is rapidly precipitated by a geochemical oxidation barrier - the red slates of the Madiyi Formation. As-S complexes in the str atigraphic horizon can be transformed into As-O complexes (e.g., H3AsO30) u nder oxidizing conditions, and are continuously transported. Therefore, the y 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 e xploration in the Xiangxi region. (C) 1999 Elsevier Science B.V. All rights reserved.