| Shimensi tungsten polymetallic deposit, located in the west of the Jiulling metallogenic belt in Wuning county, Jiangxi province, formed the Dahutang tungsten-copper polymetallic ore area with Shiweidong, Dalingshang, Kunshan and other series of large-sized tungsten deposits. In this article, Shimensi alterated wall rock is the main object of the study. Through the study of the wall rock’s petrology, mineralogy, feature of the alteration and elements migration, the altered wall rocks can be divided into different zones, and the origin, mineralization enrichment and migration patterns can be discussed.Jinning biotite granodiorite and yanshanian granite are the main rock mass in Shimensi ore area. Yanshanian granite can be divided into three periods, and from early to late,they are porphyritoid biotite granite, fine grained biotite granite and granite porphyry. The study of mineralogy and geochemical features shows that Jinning biotite granodiorite and yanshanian granite are peraluminous S-type granitoids.The ore bodies in the Shimensi deposit mainly occur in the upper part of the Yanshanian acidic granitic body and the external contact zone of the biotite granodiorites of late Jinning period. Various kind of wall rock alteration is extensive. Except the biotitization in external contact zone, greisenization, sericitization, choritization and weak epidotization, fluoritization and carbonation can be found in internal and external contact zone. The intensity of alteration in external contact zone is stronger than in internal one. Different types of alteration with different intensity overlap with each other, and greisenization is the strongest. From the contact interface to the extended place, according the intensity of greisenization, internal contact zone can be devided into secondary and weak alteration parts and external contact zone can be devided into secondary, strong and secondary parts.Major elements, Ree and trace elements are taken in or out of the wall rock because of the alteration. Na2 O,Ca O and Fe O are taken out and Al2O3,K2 O,Fe2O3 are taken in of different aprts in external contact zone; K2 O is taken out, and except Na2 O, other elements are taken in of different parts in internal contact zone. Migratory amount increases with alteration intensity strengthening. Ree remains steady. In fresh and altered rocks, there are similar chondrite-normalized REE patterns which shows obviously light and heavy rare earth fractionation and right- incline pattern. Due to plagioclase in Jinning biotite granodiorites resolved in the process of alteration, Eu is further inadequate. Migration amount of W, Cu,Mo and intensity are positively correlated relationship. The migration amount of W is obvious because W is mostly affected, and the migration amount in strong alteration zone is several or dozen times greater than in secondary alteration zone. The feature of the trace elements status shows that a great quantity of ore-forming elements are taken into wall rock during the altering, so W, Cu, Zn and Mo mineralization are closely related to greisenization, and hydrotherm replacing with wall rock and ore forming fluid may be the same fluid system.Jiuling Shuangqiaoshan group argillaceous rock which is the source rock of Yanshannian granite is rich of W, and supplys W in ore forming fluid. Under the influence of tectonic movement, ore forming fluid keeps rising. A large number of joints formed in biotite granodiorites having good sealing because of the great pressure during the rise of fluid, and supply a good ore-contained space. In the ore forming fluid, W usually takes the form of [WO4]2-.When greisenization alteration occurs,Ca separates from the wall rock, and combines with [WO4]2-.As a reault, they forn the scheelite.So greisenization is one of the most important ore-prospecting criteria in the filed. |
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