Geology, mineralogy and pyrite trace elements constraints on gold mineralization mechanism at the giant Dayingezhuang gold deposit, Jiaodong Peninsula, China

Abstract

Identifying the mechanism of gold deposition is important to reveal ore-forming processes and genesis. The giant Dayingezhuang gold deposit, with 283 tons of proven gold resources, is located in the northwestern of the Jiaodong gold province, China. Pyrite is the dominant sulfide minerals and visible gold dominates Au resources. Five types of pyrite were identified at the Dayingezhuang gold deposit: (1) Py-1a is euhedral grain with Co– and Ni-rich content; (2) Py-1b is subhedral to anhedral grain with intermediate rich in Co, Ni, As and minor Te; (3) Py-2a is rich in As and crystalizes with a mass of mineral inclusions; (4) Py-2b is also rich in As but glossy; and (5) Py-3 is mainly euhedral grain and crystalizes with much more galena.Laser ablation spectra commonly shows Au and Ag-Bi-Pb spikes in Py-1b, Py-2a, Py-2b and Py-3, reflecting the occurrence of nanoparticles in pyrite. LA-ICP-MS mapping analysis shows elemental zoning of Co, Ni and As, which is considered as the element depletion (Co and Ni) of pre-existing Py-1a and enrichment (As) from ore-forming fluid via coupled dissolution-reprecipitation (CDR) mechanism. Au in pyrite is totally below 1 ppm (average 0.154 ppm and median 0.264 ppm) and without arresting variation in different pyrites, and mass balance indecates that invisible gold mobilized from pyrite may not interpret the formation of world-class gold deposit in the Jiaodong gold province. Most of the gold grains distribute in the microfractures of pyrites which is caused by brittle deformation. Fracture of rocks and minerals significantly improve the efficiency of water–rock interaction and result in sudden drop of fluid pressure. A probable model is that a fluid with extremely different physicochemical conditions and high As-Au-Cu-Pb-Bi-Te content give rise to Au mineralization, and visible gold precipitate in a gold-supersaturated hydrothermal fluid which is attributed to sulfidation and fluid phase separation via hydrofracturing. In addition, similar occurrences of gold and silver (individual minerals in microfractures of pyrites and spikes on laser ablation spectra of pyrite) and paragenetic sequence indicate that gold and silver mineralization belong to single geological event.