| Abstract:The Longmenshan copper deposit is of skarn type, its ore-forming wall rocks being Middle and Early Triassic limestone, dolomite, lime dolomite and calcic siltstcliif, and metallogenic parent rocks being early YanSliianian hypabyssal calc-alkaline-subakaline granodiorite-diorite, with the orebodies present both in xenolith-like skarns at the depth of the intrusive body and along the endo- and exocontact zone at small depth. The main orebody is a middle-sized one occurring at depth, which is lenticular in shape with localized expansion and ramification, being extremely unstable and having a thickness variation coefficient of Vm=123. Of all copper ore types, garnet skarn ore, scapolite skarn ore and magnetite ore are predominant. Metallic minerals comprise abundant chalcopyrite and bornite, moderate amounts of magnetite and hematite, and less molybdenite and skutterudite. As for gangue minerals, the commonly-seen ones include garnet: diopside, scapolite, albite, anhydrite, epidote, actinolite, quartz, calcite, K-feldspar and hydromica. The texture and structure are rioted for metasomatic and intercrystalline filling types. Copper is pretty high in content, but shows an uneven distribution, being frequently concentrated at convex or concave position and in the middle of ore bed. Orebodies at shallow depth are of small size, occurring in groups. According to their position of occurrence, they might be grouped into two types: (1) veinlike ones in intrusive body frequently arranged in feather shape; (2) lenticular orebodies occurring conformably in wall rocks, conditioned by structural styles. Ores are largely diorite type and brecciated marble type. In addition to the above mentioned copper minerals, considerable amounts of pyrite, molybdenite and copper oxides are observed. The gangue minerals are mostly calcite, dolomite and quartz. Ores contain relatively low copper, belonging to copper-poor type. Ore-controlling factors may be summarized as follows: (1) Intrusive body played its part: in metallization in such aspects as metallogenic time, space, exchange, differentiation and concentration of rock-forming and ore-forming materials. (2) Tongue-like structure, syncline at small depth and strutural fissures acted as ore-governing structures. (3): The role played by wall rocks finds expression in such, phenomena as variation in chemical composition of skarn, structure of ore, selected metasomatism and space occupied by orebodies. In addition, sulfur isotope determinations might be of referential value. Ore-controlling wall rocks are part of Middle and lower Triassic carbonate-evaporite formation. Metallization of the ore deposit might be divided into skarn period, quartz-sulfides period and hypergene period, in association with four groups of mineral assemblage, i.e., magnetite-hematite-pyrite, pyrite-chalcopyrite-bornite, pyrite -chalcopyrite-molybdenite and covellite-malachite-bitumen-like limonite. This deposit is genetically of contact metasomatic skarn type, with the ore materials supplied largely by the intrusive body and Middle Triassic clastic layer. |
