Geological, fluid inclusion, and H-O-S-Pb isotopic studies of the Xiaban epithermal gold deposit, Fujian Province, southeast China: Implications for ore genesis and mineral exploration

摘要

The Xiaban gold deposit is located in the Dehua goldfield of central-eastern Fujian Province, southeast China. Detailed field observations combined with mineral exploration in the study area indicate that the majority of mineralization within the Xiaban gold deposit is hosted by porphyritic rhyolite units in areas proximal to E-W trending faults with lesser amounts located within alteration zones along lithological contacts. The deposit contains silicified banded, disseminated, brecciated, fine-grained stockwork, and quartz-sulfide vein-hosted mineralization, all of which is associated with a quartz-sericite-illite-chalcedony alteration assemblage. The main ore minerals are electrum and pyrite with lesser amounts of galena, Fe-rich sphalerite, and marcasite, all of which are associated with sericitic and argillic alteration as well as silicification. Fluid inclusion microthermometric analysis of quartz-hosted fluid inclusions associated with the main stage of mineralization yielded homogenization temperatures (T-h) of 157-207 degrees C (mean of 180 degrees C) and salinities of 0.7-2.9 wt% NaCl equivalent, which are indicative of an epithermal-type system. The precipitation of gold was most likely caused by fluid mixing between dominant meteoric and minor magmatic fluid components. Fluids within quartz-hosted fluid inclusions have H and O isotopic compositions (delta D-H2O = - 106.3 parts per thousand to - 81.8 parts per thousand; delta(OH2O)-O-18 = -5.8 parts per thousand to -3.0 parts per thousand) that are indicative of ore-forming fluids derived from a meteoric-water-dominated source. Pyrite in the study area has S isotopic compositions that vary between - 2.8 parts per thousand and - 1.6 parts per thousand, indicative of S derived from a homogeneous magmatic source. Lead isotopic compositions of the pyrite within the deposit (Pb-206/Pb-204 = 18.2953-18.3203, Pb-207/Pb-204 = 15.6458-15.6502, and Pb-208/Pb-204 = 38.6974-38.7271) plot between the orogenic-belt and upper-crustal evolution lines on tectonic discrimination diagrams as well as in the upper-crustal zone of a Delta gamma-Delta beta genetic classification diagram, indicating that Pb within the Xiaban deposit was derived from a homogeneous upper-crustal source. Taken together, these mineralization and alteration, gangue and ore mineral assemblage, fluid inclusion, and H-O-S-Pb isotopic characteristics suggests that the Xiaban gold deposit is a lowsulfidation epithermal deposit This result, together with the inferred minor amount of denudation that has affected the study area, means that the surrounding region should be considered prospective for this style of mineralization, with further exploration for epithermal mineralization within the Dehua goldfield ideally focusing on areas that show similar alteration characteristics to those found in the Xiaban deposit.