Anatomy of a large Ag-Pb-Zn deposit in the Great Xing5an Range, northeast China: metallogeny associated with Early Cretaceous magmatism

摘要

The Bairendaba vein-type Ag-Pb-Zn deposit, hosted in a Carboniferous quartz diorite, is one of the largest polymetallic deposits in the southern Great Xing'an Range. Reserves exceeding 8000 tonnes of Ag and 3 million tonnes of Pb + Zn with grades of 30 g/t and 4.5% have been estimated. We identify three distinct mineralization stages in this deposit: a barren pre-ore stage (stage 1), a main-ore stage with economic Ag-Pb-Zn mineralization (stage 2), and a post-ore stage with barren mineralization (stage 3). Stage 1 is characterized by abundant arsenopyrite + quartz and minor pyrite. Stage 2 is represented by abundant Fe-Zn-Pb-Ag sulphides and is further subdivided into three substages comprising the calcite-polymetallic sulphide stage (substage 1), the fluorite-polymetallic sulphide stage (substage 2), and the quartz-polymetallic sulphide stage (substage 3). Stage 3 involves an assemblage dominated by calcite with variable pyrite, galena, quartz, fluorite, illite, and chlorite. Fluid inclusion analysis and mineral thermometry indicate that the three stages of mineralization were formed at temperatures of 320-350°C, 200-340°C, and 180-240°C, respectively. Stage 1 early mineralization is characterized by low-salinity fluids (5.86-8.81 wt.% NaCl equiv.) with an isotopic signature of magmatic origin (δ~(18)O_(fluid)) = 10.45-10.65%o). The main ore minerals of stage 2 precipitated from aqueous-carbonic fluids (4.34-8.81 wt.% NaCl equiv). The calculated and measured oxygen and hydrogen isotopic compositions of the ore-forming aqueous fluids (δ~(18)O_(fluid) = 3.31-8.59%o, (δD_(fluid) = — 132.00%o to — 104.00%o) indicate that they were derived from a magmatic source and mixed with meteoric water. Measured and calculated sulphur isotope compositions of hydrothermal fluids (δ~(34)S_(Ξs) = —1.2—3.896o) indicate that the ore sulphur was derived mainly from a magmatic source. The calculated carbon isotope compositions of hydrothermal fluids (δ~(18)O_(fluid) = — 26.52%o to —25.82%o) suggest a possible contribution of carbon sourced from the basement gneisses. The stage 3 late mineralization is dominated (1.40-8.81 wt.% NaCl equiv.) by aqueous fluids. The fluids show lowerδ~(18)O_(fluid) (—16.06%o to —0.70%o) and higher δD_(fluid) (—90.10%o to — 74.50%o) values, indicating a heated meteoric water signature. The calculated carbon isotope compositions (δ~(18)O_(fluid) = — 12.82%o to — 6.62%o) of the hydrothermal fluids in stage 3 also suggest a possible contribution of gneiss-sourced carbon. The isotopic compositions and fluid chemistry indicate that the ore mineralization in the Bairendaba deposit was related to Early Cretaceous magmatism.