Abstract:The northern Qilian Mountain is tectonically a trench-island arc folded system where Middle Cambrian-Ordovician sodium-rich volcanic series-spilite-keratophyre formation is the main ore-localizing stratigraphic horizon for volcanogenic sulfide deposits. Tens of ore deposits and occurrences of this type have already been observed. Petrochemical calculations indicate that these volcanic rocks are of hypersthene series equivalent to basalt-andesite-rhyolite association of the orogenic belt except for some pre-Cambrian rocks(Changcheng Period)and some Cambrian-Ordovician rocks at Yusigou which belong to Kuno’s pigeonite series, equivalent to oceanic tholeiite in composition. The ore deposits can be grouped into two major types, i. e., island arc type and oceanic basin type. The former type occurs at uplifts or volcanic depressions of the ancient island arc belt and might be further divided into five subtypes on the basis of base metals: (1)Copper-zinc-pyrite deposits such as Zheyaoshan; (2)copper-pyrite deposits, such as Tongchanggou; (3)lead-copper-zinc-pyrite deposits, such as Xiaotieshan; (4) lead-zinc-copper-pyrite deposits, such as Shiqingdong and Jiaolongzhang; (5)lead-zinc deposits, such as Huangyakou. The latter type, equivalent to Condie’s ancient oceanic island belt, can also be further divided into two subtypes: (1)Copper-zinc-pyrite deposits, such as Yindonggou; (2)copper(zinc)-pyrite deposits, such as Honggou. The ore deposits of island arc type 3ie of volcanic composite origin. Copper in ore-forming fluids must have been derived mainly from basic volcanic rocks (spilite)which contain 466 ppm Cu as compared with 232 ppm for regional country rocks. Lead, antimony and mercury came chiefly from country rocks. The Convection caused by thermal difference between water on sea floor and water-bearing country rocks enabled the ore-bearing fluids to extract uninter-ruptedly such elements as copper, lead and zinc from the country rocks during the circulation to form ore-rich composite fluids which filled in or replaced quartz-keratophyre overlying spilite, thus producing base metal-bearing sulfide deposits. Of these deposits, the early stage ones are dominated by copper (zinc)ore occurring near volcanic conduit Or vent while the late stage ones by copper, zinc and copper ores. This type of deposits, therefore, are frequently characterized by obvious metallogenic evolutionary series. Ore deposits of oceanic basin type are of volcanic hydrothermal fractionation origin. They unexceptionally occur in spilite and are controlled by fissures inside this rock, exhibiting the characters of pneumato-high-temperature hyckrothermal ores. The near-ore spilite generally has copper content of 500 ppm, and the orebodies and mineralized country rocks all contain such elements as copper, zinc, silver, iron, cobalt, chromium and nickel, assuming mineralization zoning. Ore-forming process might be divided into three stages, i. e., pyrite-arsenopyrite stage, pyrite-magnetite stage, and chalcopyrite stage with the late stage superimposed Oil the early stage. Ore fluids were tormed as a result of differentiation and enrichment of the initial spilite magma. Both of these two types of deposits are related to central eruption. Especially at the central eruption terrain characterized by early stage fractures, such as Baiyin area, some important ore deposits have been found. On the contrary, geological terrain dominated by fissure eruption and overflow often has small ore deposits and occurrences. The Honggou ore deposit is a composite volcanic eruption and overflow deposit distributed along the fracture. The spilite-quartz keratophyre characterized by continuous differentiation or polar fractionation serves as an important indicator for ore prospecting. Sulfur and lead isotopic compositions suggest that the ore-forming materials were mainly derived from lower crust or upper mantle and the ore-forming temperatures were generally higher than 300℃.
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向鼎璞，戴天富.1985.北祁连山火山成因硫化物矿床区域成矿特征[J].矿床地质,4(1):64~74.1985.Regional metallogenic characteristics of the volcanogenic sulfide deposits in northern Qilian mountain[J].Mineral Deposits4(1):64~74
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