Gejiu Tin Concentrate Geological And Geochemical Research

The Gejiu tin-polymetallic district consisting of five ore fields (Malage, Songshujiao, Gaosong, Laochang and Kafang) in Yunnan province, is one of the most important tin-producing region in the world. The district mainly contains skarn, vein-type and stratabound carbonate-hosted ore. Now, the mine resource in Gejiu tin-polymetallic district has become serious lack for long-time mining. The aim of this study is to provide some new information for the establishing of gechemical model in Gejiu tin-polymetallic district and to research some new theory and new thinking in the second round exploitation for the old mines in our country. In this paper, the petrology, mineralogy and geochemistry of basic-ultrabasic volcanic rock, granite and skarn related to the Sn-Cu skarn-type ore and the vein-type ore in Laochang ore field have been researched systematically, some conclusions are summarized as follows:In Gejiu tin-polymetallic district, the middle Triassic volcanic rocks belonging to calc-alkaline basalt-tholeiite serie, mainly consist of pyroxene, hornblende, feldspar, chlorite and phlogopite; All the granites are peraluminous granites related to the Yanshanian collision-orogeny. Laoka granite closely related to the tin-polymetallic deposits emplaced in the post- orogenic.The metal elements show obviously vertical zones with the variety of skarn types in dimensional extension; The occurrence of garnet skarn and diopside skarn alternately developed in the horizontal zones of the contact between granite and carbonatite are mainly related to two factors: one factor is the infiltration metasomatism of post-magmatic hydrothermal solution, the another is the metrical strata of marble and calcic-dolomite developed in country rocks. The infiltration metasomatism of post-magmatic hydrothermal solution makes the marbles to form garnet skarn zone and the calcic-dolomites to form diopside skarn zone.Schorl in the granite and dravite in carbonatite are respectively related to the magmatic fluid and basin fluid, while the tourmaline with optical and chemical zones in vein ore is mainly due to the mix of post-magmatic hydrothermal solution and basin fluid; The chemical composition variety of garnet in the horizontal zones are mainly related to the post-magmatic hydrothermal solution and the accessorial fluid from sedimentary basin.In different type skarns including altered granite and massive skarn, there are positive linear correlations between Sn and ∑LREE, majority LILEs (such as Cs, Nb, Rb and Sr) or negative linear correlations between Sn and radioactive elements (such as U, Th). In garnet skarn, Sn mainly substituted Fe into andradites and its concentration always increased with the enlargement of Fe/Fe+Al ratio, minority of Sn existed in andradites as cassiterite (SnO₂) inclusions.