Fluid inclusion,rare earth element geochemistry,and isotopic characteristics of the eastern ore zone of the Baiyangping polymetallic Ore district,northwestern Yunnan Province,China

The Baiyangping Cu–Ag polymetallic ore district is located in the northern part of the Lanping–Simao foreland fold belt, which lies between the Jinshajiang–Ailaoshan and Lancangjiang faults in western Yunnan Province, China. The source of ore-forming fluids and materials within the eastern ore zone were investigated using fluid inclusion, rare earth element (REE), and isotopic (C, O, and S) analyses undertaken on sulfides, gangue minerals, wall rocks, and ores formed during the hydrothermal stage of mineralization. These analyses indicate: (1) The presence of five types of fluid inclusion, which contain various combinations of liquid (l) and vapor (v) phases at room temperature: (a) H₂O (l), (b) H₂O (l) + H₂O (v), (c) H₂O (v), (d) C_mH_n (v), and (e) H₂O (l) + CO₂ (l), sometimes with CO₂ (v). These inclusions have salinities of 1.4–19.9 wt.% NaCl equivalents, with two modes at approximately 5–10 and 16–21 wt.% NaCl equivalent, and homogenization temperatures between 101 °C and 295 °C. Five components were identified in fluid inclusions using Raman microspectrometry: H₂O, dolomite, calcite, CH₄, and N₂. (2) Calcite, dolomitized limestone, and dolomite contain total REE concentrations of 3.10–38.93 ppm, whereas wall rocks and ores contain REE concentrations of 1.21–196 ppm. Dolomitized limestone, dolomite, wall rock, and ore samples have similar chondrite-normalized REE patterns, with ores in the Huachangshan, Xiaquwu, and Dongzhiyan ore blocks having large negative δCe and δEu anomalies, which may be indicative of a change in redox conditions during fluid ascent, migration, and/or cooling. (3) δ³⁴S values for sphalerite, galena, pyrite, and tetrahedrite sulfide samples range from ?7.3‰ to 2.1‰, a wide range that indicates multiple sulfur sources. The basin contains numerous sources of S, and deriving S from a mixture of these sources could have yielded these near-zero values, either by mixing of S from different sources, or by changes in the geological conditions of seawater sulfate reduction to sulfur. (4) The C–O isotopic analyses yield δ¹³C values from ca. zero to ?10‰, and a wider range of δ¹⁸O values from ca. +6 to +24‰, suggestive of mixing between mantle-derived magma and marine carbonate sources during the evolution of ore-forming fluids, although potential contributions from organic carbon and basinal brine sources should also be considered. These data indicate that ore-forming fluids were derived from a mixture of organism, basinal brine, and mantle-derived magma sources, and as such, the eastern ore zone of the Baiyangping polymetallic ore deposit should be classified as a “Lanping-type” ore deposit.