Noble Gas and Stable Isotopic Constraints on the Origin of the Ag-Cu Polymetallic Ore Deposits in the Baiyangping Area, Yunnan Province, SW China

摘要

The Lanping basin, Yunnan province, SW China, is located at the juncture of the Eurasian and Indian Plates in the eastern part of the Tibetan Plateau. The Lanping basin, in the Sanjiang Tethyan metallogenic province, is a significant Cu-Ag-Zn-Pb mineralized belt in China that includes the largest sandstone-hosted Zn-Pb deposit in the world, the Jinding deposit, as well as several Ag-Cu deposits (the Baiyangping and Jinman deposits). These deposits, with total reserves of over 16.0 Mt Pb+Zn, 0.6 Mt Cu, and 7,000t Ag, are mainly hosted in Meso-Cenozoic clastic rocks and are dominantly controlled by two Cenozoic thrust systems developed in the western and eastern segments of the basin. The Baiyangping, Babaoshan, and Hetaoqing ore deposits are representative of the epithermal base metal deposits in the Lanping basin. The microthermometric data show that the ore-forming fluids for these deposits were low temperature (110-180 degrees C) and had bimodal distribution of salinity at moderate and mid to high salinities (approximately 2-8wt.% and 18-26wt.% NaCl equivalent). The C and O isotope data indicate that the ore-forming fluids were related to hot basin brines. We present new He and Ar isotope data on volatiles released from fluid inclusions contained in sulfides and in barite in these three deposits. He-3/He-4 ratios of the ore-forming fluids are 0.01 to 0.14 R/Ra with a mean of 0.07 Ra (where R is the He-3/He-4 ratio and Ra is the ratio for atmospheric helium). This mean value is intermediate to typical He-3/He-4 ratios for the crust (R/Ra=0.01 to 0.05) and the ratio for air-saturated water (R/Ra=1). The mean ratio is also significantly lower than the ratios found for mantle-derived fluids (R/Ra=6 to 9). The Ar-40/Ar-36 ratios of the ore-forming fluids range from 298 to 382 with a mean of 323. This value is slightly higher than that for the air-saturated water (295.5). The He-3/He-4 ratios of fluids from the fluid inclusions imply that the ore-forming fluid for the Baiyangping, Babaoshan, and Hetaoqing deposits was derived from the crust and that any mantle-derived He was negligible. The content of the radiogenic Ar ranges between 0.2 to 20.4%, and the proportion of air-derived Ar-40 averages 94.1%. This indicates that atmospheric Ar was important in the formation of these deposits but that some radiogenic Ar-40 was derived from crustal rocks. Based on these observations coupled with other geochemical evidence, we suggest that the ore-forming fluids responsible for the formation of the Ag-Cu-Pb-Zn polymetallic ore deposits in the Baiyangping area of the Lanping basin were mainly derived from crustal fluids. The fluids may have mixed with some amount of air-saturated water, but there was no significant involvement of mantle-derived fluids.