Sediment-Hosted Disseminated Gold Deposits in Southwest Guizhou, PRC: Their Geological Setting and Origin in Relation to Mineralogical, Fluid Inclusion, and Stable-Isotope Characteristics

摘要

The sediment-hosted disseminated gold deposits in Southwest Guizhou, People's Republic of China (PRC) are located in faults on the flanks of anticlines or domes in clastic sedimentary rocks of Late Permian to Middle Triassic age on the southwestern edge of the Yangtze paraplatform. Lamprophyres crop out in the vicinity of the gold deposits. Mineralization in the area coincides with belts of weak Bouguer gravity and magnetic anomalies. The Lannigou and Yata deposits, described in detail in the present study, together with the Baidi deposit, are situated in the southeastern domain where mineralization was emplaced in fine turbidites of basinal facies of Middle Triassic age. The structures guiding this mineralization are high-angle reverse faults on domes or anticlines. To the northwest, the Getang deposit is one of a group of deposits including Zimudang, Sanchahe, Dayakou, and Xiongwu, which were emplaced in silicified breccias in impure carbonates or marls of Permian to Lower Triassic age. They are controlled by low-angle and bedding-parallel faults on anticlines. The clastic sedimentary host rocks are rich in illite and organic matter. Mineralization takes the forms of pervasive silicification. veinlets of quartz and disseminated auriferous arsenic-bearing pyrite and arsenopyrite, veins of quartz and calcite, and veinlets of realgar, cinnabar, and stibnite. Gold is mainly associated with arsenic-rich pyrite. The main-stage gold mineralization in pyrite is accompanied by pervasive silicification of host rocks. The Permian Emeishan basalts, widely distributed in the northwestern area, contain high average gold contents and may have been the primary source of the gold in the sediment-hosted deposits in Southwest Guizhou. The distribution of arsenic. antimony. and mercury in the host rocks and country rocks shows a pattern similar to that of gold. Gold is found mainly in pyrite and partly in illite. High-resolution electron-probe microanalysis (EPMA) of samples from the Lannigou deposit revealed that gold is located in pyrite rims in zones of intermediate arsenic content (3-5 wt percent). It is deduced that gold probably occurs as discrete submicron-sized particles rather than as a charged Au species in a coupled diadochic substitution with arsenic in the pyrite structure. The auriferous fluids at the Lannigou and Yata deposits are shown to be CO_2-rich (Xco_2 > 0.05) and of low salinity (<5 wt percent equiv. NaCl), with relatively high homogenization temperatures (mainly 240 deg to 300 deg C) and were probably trapped under high confining pressures (1.5 to 2.3 kbar). They are not typical epithermal fluids. At Lannigou, the delta~(34)S_(VCDT) values of sulfides range from +8.4 to +12.5 per thousand, the delta~(13)C_(VPDB) of carbon in calcite ranges from -0.1 to -3.6 per thousand, and the delta~(18)O_(VSMOW) of quartz and calcite are mainly around +17.6 and around +25.8 per thousand, respectively. At Getang, the isotopic compositions of hydrothermal minerals are in the range delta~(34)S_(VCDT) of -14.3 to +4.4 per thousand for sulfides, delta~(13)C_(VPDB) of -3.2 to -0.6 per thousand for calcite and delta~(18)O_(VSMOW) of +14.0 to +l5.3 per thousand for calcite and quartz. These isotope analyses show that sulfur was probably derived mostly from the sedimentary country rocks, and therefore inherited from the marine reservoir in which they were deposited, although part of the sulfur in the Getang deposit could be from altered or weathered basalt. Most of the carbon in the hydrothermal fluids was probably derived from the dissolution of carbonates in sedimentary rocks, although decarbonation reactions caused by low-grade metamorphism at deeper levels could have contributed some of the CO_2. The original hydrothermal fluids responsible for the gold mineralization are deduced to have formed by burial metamorphism at depths of 6-8 km with addition of meteoric water through deep fractures. Mineralization probably took place as a result o