Geochemical Characteristics And Genesis Of The Lunshan Gold Deposit, Jiangsu Province

Lunshan gold deposit is a small gold deposit, which is newly discovered in the Ningzhen district. Based on the systematic field investigation of the Lunshan gold deposit, sample collection and a variety of testing methods, we discussed the geological characteristics, occurrence, properties of ore-forming fluid, source of ore-forming materials, metallogenic age and the genesis of Lunshan gold deposit, etc.The main known results are as follows:(1) Metallogenic stages can be divided into two stages: sedimentary diagenetic stage and hydrothermal stage, the later could be divided into hydrothermal sub-stage I and hydrothermal sub-stage II. The sedimentary diagenetic stage and hydrothermal sub-stage I are the main metallogenic stages.(2) An integration of optical microscopy, scanning electron microscopy equipped with energy dispersive system and electron microprobe indicated that the occurrence of gold could be divided into microscopic invisible and native species. The water-rock reaction between wall-rock and gold sulfur complex of the ore-forming fluid finally led to the enrichment of gold, also the organic matter had a promoting effect for the forming of Lunshan gold deposit.(3) The analysis of fluid inclusions indicated that the homogenization temperatures of two phases(gas+liquid) fluid inclusions in quartz and calcite of the hydrothermal sub-stage I were respectively vary from 330 to 366℃ and 150 to 240℃,with the corresponding salinities being 4.96%~6.74%, 0.71%~9.80%, belonging to high-medium temperature and low salinity fluid. The homogenization temperatures of two phases(gas+liquid) fluid inclusions in quartz, calcite and fluorite of hydrothermal sub-stage II vary from 124 to 260℃, with the corresponding salinities being 0.71%~10.98%, belonging to medium-low temperature and low salinity fluid.(4) The studies of Hydrogen and Oxygen isotope suggested that the ore-forming fluid of hydrothermal sub-stage I was magmatic fluid, and the hydrothermal sub-stage II was meteoric water with minor magmatic fluid. The analysis of sulfur isotope using pyrites of sedimentary diagenetic stage indicated that sulfur came from evaporate bedof the Triassic Qinglong Group. The sulfur of hydrothermal sub-stage I was partly from the activation, migration and enrichment of pyrites of the sedimentary diagenetic stage. The magmatic sulfur also supplied ore-forming materials of hydrothermal sub-stage I.(5) The Sm-Nd isotopic dating was carried out on the fluorite of hydrothermal sub-stage II of the Lunshan gold deposit, yielding an isochron age of 93.7 ± 3.1Ma.The upper limit age of the Lunshan gold deposit was 93.7 ± 3.1Ma, and belonged to the early Upper Cretaceous, which represented the latest mineralization of the Middle-Lower Yangtze River Metallogenic Belt.(6) The mineralization of the Lunshan gold deposit may be as follows. In the late early Mesozoic(109~100Ma), after affected by the subduction of Pacific plate to Eurasian plate, Ningzhen district break out a large number of intrusion of granitic intrusive rock, and the mineralized area was mainly characterized by the intrusion of quartz diorite porphyrite and diorite porphyrite. The intrusion of granitic rock provided a large number of ore-forming materials. At the same time, the ore-forming fluid of the magma activation migrated the pyrites of sedimentary diagenetic stage,and enriched to the weak tectonic zone(the southwest turning side of the Lunshan anticline axis), the minerals of the ore-forming fluid unloaded under an appropriate metallogenic physical and chemical condition, which finally led to the formation of Lunshan gold deposit.