Genetic Mineralogy Of The Zhesang Gold Deposit In Funing,Yunnan Province

The Zhesang gold deposit located in the structural belt of Yunnan-Guzhou-Guangxi gold triangle area, is a typical Carlin-type gold deposit.According to detailedly discussing mineralogy,rock structure and texture, finding out the Zhesang gold deposit formed though sedimentary diagenesis,hydrothermal metallogenesis and supergene oxidation period. Pyrites and arsenopyrites are the major gold-bearing minerals in the deposit. Gangue minerals like quartz are closely related to gold mineralization.The main gold-bearing minerals of the hydrothermal stage I of hydrothermal mineralization are arsenopyrites, which are better than pyrites. Pyrites are mainly pyritohedron, and arsenopyrites are mainly clintheriform or columnar, they occur both in coarse crumbs. The Ⅱ stage of hydrothermal mineralization is the most essential stage of gold mineralization. The main gold-bearing minerals are pyrites which contain obviously more gold than the arsenopyrites do. Pyrites of this stage are mainly cubic, meanwhile arsenopyrites are fine-gained needle columnar. All of these point out that the Zesang gold deposite is formed in middle-low temperature. The gold-bearing pyrites are colloidal, fine-gained or fragmentized, and the gold-bearing arsenopyrites are coarse columnar. Lower reflectance, darker reflection color, and lower Vickers hardness indicate the gold-bearing minerals have a stronger capacity of gold enrichment.The loss of S and Fe in pyrites and the enrichment of S and lack of As in arsenopyrites of the hydrothermal mineralized stages indicate that the Zesang gold deposit springs from low temperature. The gold-bearing pyrites are rich i n As, but the content of As and Au don’t present a positive proportion, Whic h suggest that the occurrences of Au are distinguish at different stages. A few pyrites are characteristiced by enrichment-As and poor-Au suggesting that As doesn’t only derive from the mineralized thermal fluid but also comes from As dispersed in the wall-rock, the soil and the underground water which are ext racted by the mineralized thermal fluid. The depth level (D) of the pyrites lies in the middle of the D interval(1.58～2.90), showing that the Zesang gold de posit formed in middle-shallow layer. The content of Co and Ni are0.06wt percent and0.02wt percent on average in pyrite respectively. The Co/Ni ratio is0.54-3.5, with average is1.54. The enrichment of Co(300-1200ppm) in arsenopyrites of each mineralized stage and the small ratio(1～1.67) of Co/Ni indicate that the pyrite formed in moderate temperature with fierce hydrothermal activity.REE’s data shows that pyrites and arsenopyrites are enrichment in LREE, with Eu negative anomaly obviously, but no Ce anomaly. The SREE in quartz from Stage I to Stage III is low, showing a transformation from enrichment in LREE to MREE. There’s no Ce anomaly in quartz of each stage, but Eu anomaly in Stage Ⅰ is negative without obviously anomaly in stage, positive anomaly in stageⅢ. All these evidences display the Zhesang gold deposit formed in reducing environment. Metallogenic matters do not derive from mafic magma but mainly originate from wall-rock. Moreover, magma activity probably serves as the heat source leading the hydrothermal circulation in the process of gold mineralization.The results of the cell parameter testing indicate that the ao of pyrite is5.43～5.4325, the ao has become large gradually from stage I to stage, suggesting As is enrich in metallogenetic hydrotherm. Gold-bearing arsenopyrites are characterized by enrichment in S, Co and poor in As leading b0,c0,β are all lower than theoretical value. The cell parameter of quartz in stage I is close to the theoretical value, and in stage Ⅱ the cell parameter of quartz is a little higher than the theoretical value, which indicate mineralizing fluid is middle-low temperature phrase reducing with the precipitation and enrichment of gold.Gold mineralization from stage Ⅰ to stage Ⅱ significantly shows different enrichment characteristics in pyrite and arsenopyrite. Hydrothermal stage I gold mainly occurs as Au complex (Au(HS)0) on pyrite and arsenopyrite, whereas gold in stage Ⅱ presents occurs as nanometer-sized particles of native Au (Au0). There is no Au in colloidal pyrites formed in sendimentary period, therefore the Enrichment-Au perhaps results from the adsorption of the large specific area of the native Au.Organic carbon content of the ores reaches0.12%～0.45%and the Au grade is growing with the increase of organic carbon, which is in broad agreement with the features of gold-antimony-mercury ore deposits in southwestern Guizhou Province, implying Zhesang gold deposit maybe relate to the palaeo-oil reservoir. High organic content suggests the reducing nature of the ore-forming fluid. Furthermore, the organic substance can adsorb and enrich the metallogenetic elements in clastic rocks and impure carbonate rocks. The δ34SV-CDT values of pyrite and arsenopyrite range from9.2%o to10.2%o, exhibiting enrichment of heavy sulfur. This proves the reaction between organic substance and wall-rocks and the metallogenic material comes from wall-rocks as well.Typomorphism of the fluid inclusions reveal that the ore-forming fluid is a middle-low temperature(80～245℃), hyposaline(0.18%～6.45%) and middle-low density system.δ18OH2O of the fluid inclusions ranges from-5.6%o to5.4%o and the δDV-SMOW ranges from-40.3%o to-74.8%o, showing that the ore-forming fluid comes from atmosphere precipitation. When the ore-forming fluid flowed through the organic-rich sedimentary rocks, primitive mefeoric water would be contaminated by the organic substance, resulting in the drift of hydrogen isotope. Preliminary analyses believe that during the oozing process, the ore-forming fluid mixed with the construction water and be heated to recycle upward by geothermal gradient and magma. So gold mineralization occurs in interaction process between the thermal recycle and wall-rocks, and enriches in proper space.