Mineralogical constraints on the genesis of the Dahu quartz vein-style Au-Mo deposit, Xiaoqinling gold district, China: Implications for phase relationships and physicochemical conditions

摘要

The quartz vein-style Dahu Au-Mo deposit, with an average grade of 6.8 g/t for 38 tons Au, and an average grade of 0.24% for 100,000 tons Mo, is situated in the Xiaoqinling region in the northernmost part of Qinling orogen in China. It is characterized by enrichment of native gold, molybdenite, telluride and Bi-sulfosalt. The current understanding of telluride and Bi-sulfosalt mineralization in the Dahu Au-Mo deposit is very poor, and this paper is the first systematic study of these minerals in this deposit. The Au-Mo ores occur mainly as veins and stockworks as well as disseminated in quartz veins, and they are hosted by biotite-plagioclase gneiss, amphiboleplagioclase gneiss, and amphibolite of the Archaeozoic Taihua Group. Four paragenetic stages of mineralization are recognized that reflect early formation of sulfide minerals and subsequent precipitation of telluride minerals: (I) quartz-K-feldspar-pyrite-molybdenite, (II) quartz-pyrite-molybdenite, (III) sulfide-telluride-sulfosalt-gold, and (IV) carbonate-barite. The telluride minerals are mainly altaite, tellurobismuthite, and tetradymite, and to a minor extent calaverite, petzite, hessite, and stiitzite. Some Bi-sulfosalt minerals also exist in the deposit, including aikinite, lindstromite, saddlebackite, krupkaite, and hammarite. Of these minerals, saddlebackite, krupkaite and hammarite are reported in this deposit for the first time herein. The Au-Ag-Te-Bi-S minerals substantially formed in the Dahu Au-Mo deposit under conditions of T = 260 degrees C, P approximate to 100 MPa, pH = 19 to 7.9, logfO(2) = -40.9 to -33.8, logfTe(2) = -12.5 to -7.5, logfS(2) = -13.5 to -8.2, logfH(2)S = -2.4 to -0.3 and log alpha Aut((a)(q))(+)/alpha Age((ag))(+) = -6.7 to -6.2. The deposit formed from an ore-forming fluid dominated by HTe center dot or/and Te-2(2)-, in which Au was transported in the forms of Au(HS)(2)(-) and/or an unknown aqueous Te species (e.g., AuTe22-). Furthermore, during the mineralization stages of the Dahu deposit, the ore-forming fluid evolved from an initial oxidation type to a later relative reduction type, and the telluride minerals, Bi-sulfosalt minerals and native gold were formed in a relatively reduced condition. The Au-Ag-Te-Mo-Bi metal associations in the Dahu Au-Mo deposit were derived from a magmatic-hydrothermal system during the Late Triassic.