Abstract:The Baiyinchang mining area, an important industrial base of base metals in China, consists of five pyrite type deposits of different sizes, four of which have already been worked. These five deposits might be grouped into two types: Cu-bearing (predominatingly Cu) pyrite type deposits, such as Zheyaoshan, Huoyanshan and Tongchanggou, and polymetallic pyrite deposits containing principally Cu, Pb and Zn, such as Xiaotieshan and Sigejuan. These two types of deposits occur close to each other in the same formation-Baiyinchang spilite-keratophyre sequence and have similar structures and petrofacies, suggesting the possibility of forming two different types of deposits in the same tectonic unit and volcanic rock series. The deposit lies in the eastern part of the volcanic complex within the Qilian eugeosyncline, its genesis remains a problem of much controversy. Ln recent years, the author has made an integrated study and analysis of the data accumulated over the past thirty years and also performed some field investigation to check up these references. The results acquired have led him to postulate the hypothesis that the deposit was formed by the concentration of volcanic hydrothermal fluids in depression and the activity of subvolcano-magmatic fluids. The formation of the massive pyrite in the main ore body of Zheyaoshan is believed to be the result of the lateral effusion from the Qingshanbao vent of the third subcycle appearing presently as silicic rock formation, i.e., the result of the accumulation in the second subcycle crater depression of the ore material effused from the pyrrhotite ore chimney. This might be borne out by the existence of abundant massive copper-bearing pynhotite ore with well-developed textures of corrosion and replacement in the chimney as well as by the ring-like distribution of the metallic minerals around the chimney caused by the spread of the fluids from this center. Geological mapping and analyses of the data of drill core and petrofacies support the claim that the ore body is located at the crater depression. Minerals in massive pyrite ore body are coarse in the west and fine in the east, suggesting also the migration of ore fluids from east to west into the center of depression. The massive ore body cutting obliquely the strata, the relatively weak alteration of country rocks and the remarkable variation in copper content of the ore body all seem to have resulted from the ascending movement of the copper-bearing ore fluids brought by subvolcanic activity along the ore chimney and its fracture zone formed during the crustal reversion. The Xiaotieshan polymetallic pyrite deposit is of subvolcano-magmatic fluids type. Its ore-forming processes were strictly controlled by late subvolcanic activity which, in turn, was governed by fracturing. The ore body overlies the hanging wall of quartz albitophyre and shows close relationships with subvolcanic rocks in dimension and grade while the texture and structure of ore and the mineral constituents vary in accordance with the diverse country rocks. The examination of the structure of volcanos, fractures, variation in petrofacies, regularity of subvolcanic activity, the extent of wall rock alteration and characteristics of mineralization, therefore, makes up the prerequisite for the search of such concealed ore deposits. The Jinfangou ore belt expends along a EW striking fracture zone characterized by complex lithological characters and petrofacies as well as by frequent subvolcanic activities. Mineralization and ore bodies are exclusively confined to the fracture zone. The repeated crustal reversion during: the late stage of the development of rock formation resulted in the folding and faulting of the strata, thus facilitating the differentiation and migration of the magma. The intrusion of the subvolcanic magma not only brought sufficient ore-bearing fluids for mineralization, but supplied heat for their migration. This is due to the fact that heat: accelerated the circulation of the ore fluids, which dissolved ore elements dispersed in the volcanic rock and unceasingly infiltrated into and replaced the country rocks as well. As a result ore fluids concentrated to form ore deposits at places with favorable structure and country rock conditions.
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严济南.1983.祁连山白银厂黄铁矿型矿床成因探讨[J].矿床地质,2(3):24~32.1983.A discussion on the genesis of the Baiyinchang pyrite type deposit, Gansu Province[J].Mineral Deposits2(3):24~32
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