EPMA analysis of gold bearing minerals and its indications in Gaolong gold deposit

• 1、State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023
• 2、 East China Mineral Exploration and Development Bureau for Non-ferrous, Nanjing 210007
• 3、School of Geosciences, China University of Petroleum, Qingdao 266580

Abstract：Gaolong gold deposit is a typical large and medium size Carlin-type gold deposit in Northwestern Guangxi, China. In this paper, studied the main gold bearing minerals by microscopic observation and electron probe microanalysis (EMPA). There are different types and genius of the gold bearing pyrite and arsenopyrite, and the metallogenic process with multi stages evolution. Hydrothermal arsenic pyrite is the main gold-bearing minerals and Au-rich, which is greater than the early sedimentary pyrite and the middle-late hydrothermal arsenopyrite, and the zonal pyrite has the typical "three zone" structure. The content of Au and As in ore-forming fluid has a process of increase and decrease from early to late, Au and As show a certain correlation, indicating that the ore-forming fluid is constantly changing. Au mainly occurs as "invisible gold" in the gold bearing sulfide minerals, and the mechanism for "invisible gold" formation is controlled by fluid evolution. In the sedimentary pyrite or the early hydrothermal activities transformed pyrite with As-poor, the major modes of occurrence for gold are the microscopic inclusion gold grains, the free Au0 is mixed into minerals by mechanical means. In the hydrothermal pyrite with As-rich and arsenopyrite, solid solution of Au+ is dominant in arsenic pyrite and arsenopyrite ,Au+ may be occupying the lattice defect positions of gold-bearing minerals due to the displacement of As and S. The Co/Ni ratio is closelyrelated todeep magmatic activity. Combined with the previous research results, it is considered that the ore-forming fluid has undergone many evolutionary transformations: Rock formation water is a major component of basin fluid dominated the early metallogenic stage, deep-source magmatic hydrothermal fluid and mixed with basin fluid were dominated the main metallogenic stage, the late metallogenic stage mainly mixed with meteoric water. It is speculated that the ore-forming materials were mainly derived from deep mantle or magmatic activity, secondarily has inherited and transformation Au of the early pyrite, and some Au has been extracted in stratum by hydrothermal fluid. Therefore, this paper thinks that the genesis of Gaolong gold deposit belong to the magmatic hydrothermal gold deposit of the middle-low temperature. the mineralization process can be divided into four stages: Au pre-enrich during the diagenetic period→ Intrusion of deep concealed rock mass (thermal energy, metallogenic minerals)→ On the "Si-Ca interface", the metallogenic fluid is migrated, precipitated and enriched (passable structure, host structure)→ In the late stage, it entered the supergene weathering stage because of the uplift of the surface.

Keywords： Ore deposit geochemistry Gold bearing minerals EPMA Magmatic hydrothermal-type Gaolong