LA-ICP-MS in situ trace element analysis of pyrite from Dongtongyu gold deposit and its metallogenic significance, Xiaoqinling gold district

Situated in the southern margin of the North China craton, the Xiaoqinling district is one of the most important gold metallogenic belts and has been the second largest gold producer in China. Although most gold deposits have been extensively studied, issues related to the source of the ore materials and fluids remain debated. This paper presents a study of the distribution characteristics of trace elements in gold-bearing pyrite to explore the source of ore-forming materials, the precipitation mechanism and ore genesis. The Dongtongyu lode gold deposit is the important deposit in Xiaoqinling and pyrite is the predominant sulfide mineral. Three generations of pyrite were identified, broadly corresponding to the three mineralization stages, and termed as the first generation, second generation and third generation (PyI, PyII and PyIII). The first generation (PyI) is mainly present as isolations or as aggregate masses dispersed in milky quartz veins, consisting of coarse-grained, euhedral to subhedral crystals. The second generation (PyII) pyrite occurs as veins or veinlets generally associated with light gray quartz or cutting milky quartz. Many grains are porous and contain abundant micro-fractures that are usually filled with later-stage quartz, other sulfide minerals and free gold. The third generation (PyIII) pyrite is commonly intergrown with other sulfide minerals including chalcopyrite, sphalerite, and galena. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) results show that the average content of As in PyI is 16.63×10-6, and the contents of Au, Ag and Te are low and often below detection limit. In contrast to PyI, the content of As in PyII is relatively lower, and that of Au, Ag and Te are slightly higher. Particularly, the stage PyIII exhibits distinguished compositions and significantly enriched in Au (up to 35.58×10-6), Ag and Te (up to 79.79×10-6), but with most of the content of As blow the detection limits. Moreover, The Co/Ni ratios of pyrite in different generations are basically >1, and the contents of Co, Ni and Co/Ni in PyIII are much lower than those in PyII and PyI. The results show that As is always below or only marginally higher than the detection limits, and plays an insignificant role in gold mineralization. There is prominently positive correlation relationship between Au, Ag, and Te in the third generation of pyrite (PyIII), indicating that tellurium is closely related with gold and silver. Moreover, significant Au, Ag and Te are enriched in the PyIII, highlighting the role of Te as important scavengers in transfer, transport, enrichment and precipitation of gold and silver. An intimate Te-Au-Ag association has been widely noticed in widespread gold mineralization in Xiaoqinling gold district, and low-As, high-Te in pyrite, suggesting that the ore-forming materials and ore-forming fluids of the gold deposits may have come from the deep magma devolatilization or mantle degassing, rather than being involved with the regional metamorphism. © 2016, Editorial Department of Earth Science. All right reserved.