Ore-forming fluids and isotopic (H-O-C-S-Pb) characteristics of the Fujiashan-Longjiaoshan skarn W-Cu-(Mo) deposit in the Edong District of Hubei Province, China

Abstract

The Fujiashan-Longjiaoshan deposit is located in the southern area of the Edong District of the Middle-Lower Yangtze River metallogenic belt. W-Cu-(Mo) orebodies are found in the contact zone between the granodiorite porphyry and Carboniferous to Triassic carbonate strata. Four stages of ore formation are identified: (1) the prograde skarn stage, (2) retrograde skarn stage, (3) quartz-sulfide stage (further subdivided into the early 3a and later 3b), and (4) carbonate stage. In this study, we applied fluid inclusion microthermometry, laser Raman spectroscopy, and H-O-C-S-Pb isotopic compositions to constrain hydrothermal processes and ore precipitation mechanisms of the Fujiashan-Longjiaoshan deposit. Early stage (1 and 2) ore-forming fluids belong to a high-temperature (above 550 °C), high-salinity (60–70 wt% NaCl equiv. and 15–20 wt% NaCl equiv, respectively), high-pressure (above 700 bars) and high-oxygen-fugacity NaCl-KCl-H2O system, and in the quartz-sulfide stage 3, the system changes into a NaCl-KCl-CO2-H2O system. Homogenization temperatures and salinities of ore-forming fluids gradually decrease throughout the evolutionary process. Fluid temperatures vary at >550 °C in prograde skarn stage 1, from 350 to 400 °C in retrograde skarn stage 2, from 250 to 300 °C in quartz-sulfide stage 3, and finally from 100 to 150 °C in carbonate stage 4. Salinity levels vary from 15 to 20 wt% NaCl equiv. in prograde skarn stage 1, from 5 to 20 wt% NaCl equiv. in retrograde skarn stage 2, from 10 to 15 wt% NaCl equiv. in quartz-sulfide stage 3a, from 5 to 10 wt% NaCl equiv. in quartz-sulfide stage 3b, and from 0 to 5 wt% NaCl equiv. in carbonate stage 4. The H-O-C isotopic compositions indicate that the ore-forming fluids were derived from magmatic water (δD of −123‰ to −83‰, δ18OV-SMOW of 6.1‰ to 7.4‰) with the input of meteoric water and organic matter in the quartz-sulfide stage (δD of −91‰ to −53‰, δ18OV-SMOW of 2.4‰ to 4.6‰, and δCV-PDB of −14.7‰ to −6.4‰). Sulfur and Pb isotopic compositions show that the ore-forming materials were mainly derived from granitic magma (δ34SV-CDT values of granodiorite-porphyry sulfides and skarns of −5.7‰ to +6.6‰) and in part from host sedimentary strata (δ34SV-CDT values of sulfides of country rocks clustering at −20‰ and at −10‰). Fluid boiling in the retrograde skarn stage gave rise to the first precipitation of scheelite. The precipitation of scheelite, chalcopyrite and molybdenite in the quartz-sulfide stage was probably triggered by the input of meteoric water and organic carbon. According to mineral assemblages and all available geological and geochemical data, we conclude that the Fujiashan-Longjiaoshan deposit is an oxidized skarn-type W-Cu-(Mo) deposit.