Mineralogical study on pyrites from the micro-disseminated gold deposits in the Southwest Guizhou and its significance:A case study of the Shuiyindong gold deposit

The micro-disseminated gold mineralization in Southwest Guizhou is a typical medium-low temperature hydrothermal deposit, which is generally controlled by different levels of structures. However, the lens and lamellar orebodies, characterized by the strata-bounded nature of Au mineralization, are quite different from the spatial distribution of most hydrothermal deposits, whose genetic mechanism is not clearly understood. The morphology, composition and distribution of pyrite and the occurrence of Au in the Shuiyindong gold deposit have been analyzed through EPMA, ESEM and other high-precision in-situ test methods. The results indicated that there are at least three types of pyrites, which are all closely associated with organic matter. (1)The coarse pyrite:It is densely disseminated or banded in the calcareous sandstone of the Member I of the Longtan Formation. Condensate and hydrothermal minerals such as apatite, rutile and calcite are often found in the pores of coarse pyrite. Some coarse pyrite has a regular core with 2 ~ 3 outer layers of secondary enlarged rings containing As, which are the major gold-bearing minerals. Invisible Au mainly exists in the outer ring, and the content of Au is 0. 06% on average and up to 0. 24% . The elements such as S, As and Au show an annular distribution, indicating that the inner and outer zones were formed in different metallogenic stages. (2)The fine pyrite:It is distributed in a scattered or sparse fashion in the calcareous tuff, rich in bioclasts in the Member Ⅱ of the Longtan Formation, and constitutes the economic orebodies. It is mostly symbiotic with arsenopyrite, and rich in Au and As, with Au constituting up to 0. 14% . The organic matter contains natural gold particles with sizes of 5 ~ 10μm and is found in the microcracks of the surrounding rock. (3)The framboidal pyrite:It is distributed in the Member I and II of the Longtan Formation in the microfossils (Ty1)and microfractures (Ty2) as aggregates. Ty1 has a metasomatic bioclastic formation and organic matter cladding, whereas Ty2 is found in microfractures rich in organic matter, and metasomatizes some coarse and fine crystals of pyrite, indicating that it is formed slightly later than the main metallogenic stage. The above research on the structure, composition, and spatial occurrence of pyrite in the Shuiyindong deposits indicates that this type of gold deposit has the characteristics of multiple overprinting of hydrothermal activities. Under the metasomatism of an ore-forming hydrothermal solution, numerous bioclasts in the Longtan Formation and its underlying strata were decomposed into organic matter and other elements such as C, P, N, Ca, Na, K, etc. The ore-forming fluid carries organic matter upward and the dense Member Ⅱ of the Longtan Formation and its upper strata provide lithofacies traps for the Au mineralization like a cap. During the mineralization process, the organic matter which formed by fluid metasomatized bioclasts may help to reduce the solubility of Au, promoting its precipitation and enrichment, and forming the orebodies with fracture-controlled and strata-bounded characteristics. © 2022 Science Press. All rights reserved.