Textures of gold-bearing sulfides and gold precipitation mechanism, Wangu gold deposit, Jiangnan Orogen

The Wangu gold deposit, with a proven gold resource of similar to 85t, is located in the middle section of the Jiangnan Orogen and occurs in metamorphic rock series of the Neoproterozoic Lengjiaxi Group. It formed under the control of the NNE-NE-trending Changsha-Pingjiang fault zone and the EW-trending Jiuling-Qingshui ductile shear zone. The main ore types of the deposit include arsenopyrite-, pyrite-, sericite-, and quartz-altered slate and quartz-sulfide veins, followed by slate breccia within hydrothermal quartz. Arsenopyrite and pyrite are the main gold bearing minerals in the deposit, which are widely distributed in the deposit. Gold mineralization can be divided into four stages : I, the milky quartz-muscovite-scheelite ; II, the smoky gray quartz-muscovitearsenopyrite-pyrite-gold; III, the smoky gray quartz-muscovite-pyrite-arsenopyrite-polymetallic sulfide-gold; and IV, the milky quartzcalcite. Among them, the II and III are the main mineralization stages. Based on the results of Electron Probe Micro Analysis ( EPMA) of arsenopyrite from the main mineralization stages, the contents of As in arsenopyrite at stage II range from 42. 19% to 44. 84% , with an average of 43. 42% ( n = 56 ). The contents of As in arsenopyrite at stage III range from 40. 08% to 43. 36% , with an average of 42. 08% ( n =19). According to the phase diagram of arsenopyrite thermometer, the formation temperature of Apy-1 in the stage II is estimated to be 364 +/- 21 degrees C with a sulfur fugacity varying within 10(-9.7) similar to 10(-7). The formation temperature of Apy-2 in the stage III is 319 +/- 22 degrees C , and its sulfur fugacity is 10(-11)(.5) similar to 10(-8.6). The contents of invisible gold in gold-bearing arsenopyrite and pyrite were 0. 01% similar to 0. 66% and 0. 01% similar to 0. 11% , respectively, revealed by EPMA data. The Au-As data of pyrite are plotted on both sides of the gold solubility curve, indicating that gold in pyrite mainly exists in the form of nano-scale particles and solid solution or lattice gold. The proportion of nanometer gold particles in Py-1 pyrite is 73. 33% , more than that in Py-2 (67. 80% ). The above data indicate that in the process of water-rock reaction, the iron-bearing minerals in surrounding rocks react with H2S in ore-forming fluid to form arsenopyrite and pyrite. Accompanied by a strong water-rock reaction, the main gold-bearing sulfides changed from arsenopyrite to pyrite during mineralization from stage II to III with the decrease of mineralization temperature and sulfur fugacity, i. e. , strong sulfurization results in instability of the gold-sulfur complex and the release of gold. As a result, gold is deposited into the sulfide lattice by substitution or in the form of microscopic-ultra microscopic gold particles to form gold-bearing sulfide. Therefore, sulfidation is the main mechanism of the precipitation of invisible gold in sulfides at the Wangu gold deposit.