Geology, Geochemistry And Genesis Of Huanxiangwa Gold Deposit In Dongqinling, Henan Province

Altered cataclastic rock-type gold deposits are the main types of gold deposits in Xiong'ershan region of East Qinling. Although many publications have been done on the geological features, geochemistry and metallogenic regularity, there are still controversy on the sources of ore-forming fluid and material, the age and the geodynamic mechanism of gold mineralization. Based on detailed field study of medium-sized Huanxaingwa gold deposit, using some feasible methods such as EMPA, fluid inclusion homotemperature measuring, stable isotope measuring and radioactive isotope measuring, this thesis recognizes its characteristics and source of ore-forming fluid, source of ore-forming materials, the age of the Huanxiangwa gold deposit, and then gives a reasonable explanation about its genesis. The main conclusions we got are as follow:Based on cross-cutting relationships of the ore veins, mineral assemblages and ore fabrics, the ore-forming process can be divided into two mineralization periods: hydrothermal and supergene. The hydrothermal period can be further divided into four mineralization stages: (â… ) K-feldspar stage, (â…¡) pyrite-quartz stage, which is the main ore-forming stage, (â…¢) quartz-polymetallic sulphides stage, (â…£) quartz-carbonate stage. Wall rock alteration in the Huanxiangwa gold deposit includes potassic feldspar alteration, silicification, biotitization, pyritization, calcitization. Of these, pyritization are most closely associated with gold mineralization, biotitization is a kind of Characteristic alteration.In the main ore-forming stage, gas phase of fluid inclusions is mainly composed of H₂O (83.8-91.5%), CO₂ (7.45-15.6%); liquid phase of fluid inclusions mainly contains Na⁺ and SO₄^2-. Some fluid inclusions in the main ore-forming stage contain small amounts of C₂H₆, H₂S, N₂ and CH₄, indicating a reductive ore-forming fluid. In the late stage, various ions decreased significantly.The homogenization temperatures of the fluid inclusions in the K-feldspar of early stage are higher than 500℃. The main ore-forming stage contains two(NaCl-H₂O) and three phase(CO₂-H₂O) fluid inclusions; two types of primary fluid inclusions mainly homogenize at temperature between 220～300℃, with salinities ranging from 5 wt% to 11wt%, indicating a moderate temperature and low-salinity ore-forming fluid. The late stages has only one type of fluid inclusions-the NaCl-H₂O type, which have a peak homogenization temperature at 200～260℃and a peak salinity at 1～4wt%.Theδ¹⁸O_H2O value of main ore-forming stage fluid are 3.5‰～6.2‰, similar to magmatic water and mantle fluid. Theδ³⁴S of pyrite in vein-type ore are in the range of -3.4‰～0.6‰and represent the initial sulfur characteristics, showing that the ore-forming materials are probably derived from the upper mantle.Pyrite-quartz vein of the main ore-forming stage cut a granitic pegmatite vein in F8 fault. Re-Os isotopic dating of two molybdenite from the granite pegmatite gives a mean model age of 155.0±2.2Ma. This age is in agreement with Wuzhangshan granite zircon SHRIMP U-Pb age(156.8±1.2). Combined with other geological features, we proposed that Wuzhangshan granite had a genesis relationship with the granite pegmatite, but no relationship with Huanxiangwa gold deposit.The Huanxiangwa gold deposit formed at Early-cretaceous, consistent with that of the Miaoling and Qiyugou gold deposits in the Xiongershan region. On the basis of geochronological data and tectonic evolution of East Qinling-Dabie area, we propose that these deposits were driven by the Late Mesozoic lithospheric extension in East Qinling-Dabie area.