Zircon U - Pb age, fluid inclusion, and H - C - O - He - Ar - S isotopic compositions as an index to the VMS-type mineralization: A case study from the Wulasigou polymetallic deposit, Altay Orogenic Belt, Northwest China

摘要

The Wulasigou polymetallic (zinc-copper-iron-gold) deposit is located in the volcano-sedimentary Kelan Basin of the Chinese Altay. The orebodies of the Wulasigou polymetallic deposit are hosted in the second unit of the upper member of the Lower Devonian Kangbutiebao Formation and exhibit the characteristics of a stratabound deposit. Three mineralization periods are identified: an exhalative-sedimentary period, a regional metamorphism period, and a supergene period. The exhalative-sedimentary period is the main mineralization period and can be subdivided into stage 1 (the pyrite-magnetite stage), stage 2 (the pyrite-sphalerite-magnetite stage), and stage 3 (the pyrite-chalcopyrite-native gold stage). Zircon U-Pb dating of a metamorphic volcanic tuff from the second unit of the upper member of the Kangbutiebao Formation yields an age of 397.9 +/- 2.8 Ma, consistent with the mineralization age (391.3 +/- 2.7 Ma), indicating that the orebodies and the host rocks share close spatial and temporal relationships and are genetically related. Liquid-rich biphase (L type) fluid inclusions are the most common type in the quartz and calcite samples from every stage of the exhalative-sedimentary period. The ore forming fluids of the exhalative-sedimentary period homogenize at 564-120 degrees C and are of moderate to low salinity (17.61-0.71 wt% NaCl equiv). The quartz and magnetite samples from stage 1 of the exhalative-sedimentary period yield delta(OH2O)-O-18 concentrations ranging from 0.9 parts per thousand to 6.1 parts per thousand and from 8.3%o to 9.7%o, respectively, and 8D values that range from-110.1 parts per thousand to-104.5 parts per thousand and from-123.1 parts per thousand to-113.8%o, respectively. The delta(OH2O)-O-18 values for the quartz samples from stages 2 to 3 of the exhalative-sedimentary period are 2.1 parts per thousand and-1.5 to 4.0 parts per thousand, respectively, and the 8D values for stages 2 and 3 are-104.6 parts per thousand and-107.3 parts per thousand to-95.4 parts per thousand, respectively. The fluid inclusions in pyrite yield He-3/He-4 ratios clustering within a small range from 0.53 to 0.89 Ra, Ar-40/Ar-36 ratios between 444.6 and 656.5, and the percentage of radiogenic Ar-40 (Ar-40*) in the range of 33.54% to 54.99%. The combined fluid inclusion studies and their H-O-He-Ar isotopic compositions show that the ore-forming fluids belong to a simple H2O-NaCl system and have a dominantly magmatic signature mixing with an increasing seawater component vertically upward from the volcanic conduit towards the sea floor. The delta(CPDB)-C-13 values and delta(OSMOW)-O-18 values of four calcite samples from the marble vary between-1.6 parts per thousand and 0.1 parts per thousand and between 9.4 parts per thousand to 10.1 parts per thousand, respectively. Calcite samples from the sulfide-(quartz) -calcite veins have delta(CPDB)-C-13 values and corresponding delta(OSMOW)-O-18 values of-6.3 parts per thousand to-3.9 parts per thousand and 8.6 parts per thousand to 9.4 parts per thousand, respectively. The S-834 values of the sulfides exhibit a narrow range from-4.41 parts per thousand to 4.20 parts per thousand, similar to most VMS deposits. These isotopic data suggest that the ore-forming materials of the Wulasigou deposit may have been derived from both a direct deep magma source and leaching of volcanic rocks, with a larger contribution made by the magma source.