Hydrothermal evolution and origin of the Suixian molybdenum deposit in the Tongbai orogenic belt, China: Constraints from geology, fluid inclusions and multiple isotopes (H[sbnd]O[sbnd]C[sbnd]S[sbnd]Pb)

Abstract

The Suixian Mo deposit is a recently discovered deposit in the northern Hubei Province, Tongbai orogen. The Mo orebodies of this deposit are all hosted in the metamorphic rocks (leuco-gneiss, amphibolite and marble) of the Neoproterozoic Tianwangzhai Group, and these orebodies have no direct contact with the granite surrounding the host rocks. The mineralization types are dominated by disseminated ore and fine Mo-bearing quartz-sulfide veins. Hydrothermal alteration consists of abundant feldspathization, epidotization and chloritization, together with less garnetization, tremolitization, and muscovitization. A detailed study on fluid inclusions and multiple isotope (CHOSPb) analyses, together with geologic features, are carried out to investigate the fluid composition, fluid origin and evolution, the source of ore-forming materials, and the ore genetic type. In line with microthermometric measurements and laser Raman analysis on fluid inclusions from three hydrothermal stages, the ore-forming fluid in this deposit is characterized by medium–high temperatures and salinities, as well as richness in CO2 and CH4. Three hydrothermal stages are identified in this deposit, including (I) feldspar-quartz-amphibole-muscovite stage, (II) quartz-sulfides stage, and (III) quartz-calcite-scheelite-fluorite stage. From stages I, II to III, the temperatures change from 290 to 354 °C, through 229–312 °C, to 173–269 °C, with decreasing salinities of 12.6 to 16.3 wt% NaCl equiv., 4.5 to 13.4 wt% NaCl equiv., and 0.2 to 6.9 wt% NaCl equiv., respectively. Most of sulfur isotope data of sulfides (chalcopyrite, pyrite, pyrrhotite and molybdenite) show a narrow δ34S range from −2.0 to 3.0 ‰, suggesting a dominant magmatic source, but some data show either positive or negative values (δ34S: −13.3 to −10.6 ‰ and 9.8 to 11.2 ‰) which may indicate small contribution from the restricted wall rocks. The Pb isotope data (molybdenite, pyrite and pyrrhotite: 206Pb/204Pb: 17.239 to 18.483; 207Pb/204Pb: 15.378 to 15.717; 208Pb/204Pb: 37.889 to 38.850) imply that the lead of sulfides pertains to a mixture of magma (derived from the lower crust) and upper crustal materials (host metamorphic rocks). Based on evident CO2 and CH4 in fluid inclusions, the gradually depleted carbon isotopic compositions (δ13C: −5.4 to −9.9 ‰ at stage I, −14.8 to −19.7 ‰ at stage II, and −17.0 to −24.2 ‰ at stage III) and the HO isotope data, it is suggested that the ore-forming fluids were mainly magmatic fluids with mixture of some organic-containing water, and the thermochemical reaction between the fluids and organic matters (e.g., CH4) from host metamorphic rocks and fluid immiscibility are two important mechanisms for Mo mineralization. Compared with typical Mo deposits in the same orogen, the Suixian Mo deposit belongs to the magmatic hydrothermal vein-type deposit associated with intrusions rather than porphyry or skarn type deposit.