Geology, Geochemistry Of Malage Tin-copper Deposit In Gejiu, Yunnan Province

Gejiu is famous for the largest tin polymetallic orefield in the world. There arefive ore deposits from north to south, which are Malage, Songshujiao, Gaosong,Laochang and Kafang. Based on the pervious researches, we selected the Malagedeposit to carry out our study. In this paper, a systematic study of geologicalcharacteristics, fluid inclusions, stable isotope, geochronology of the deposit and theevolutionary and petrogenesis of granites in the deposit have been carried out. Themain achievements of this study are shown as followings.1. Malage is a Sn-, Cu-dominant deposit, and two types of ore styles are figuredout, which are skarn ores and oxidized stratiform ores. Alteration in Malage deposit isextensively developed, and skarnization is the major alteration type. Moreover, otheralterations such as potash feldspathization, sericite-muscovieization, albitization,greisenization, chloritization, silicification, marbleization also commonly observed.The formation of the deposit experienced a period composed of skarn stage,retrograde alteration stage, quartz-sulfide stage and carbonate stage.2. Late Cretaceous granites, comprising Baishachong and Beipaotai granites arethe major igneous rocks in the Malage deposit. The Baishachong and Beipaotaigranites are equigranular biotite granite and porphyritic biotite granite, respectively.The Baishachong granite has high contents of SiO₂, Al₂O₃, CaO and LOI, andhydrothermal alteration has a strong effect on its major elements. The Beipaotai stockbelongs to shoshonitic metaluminous granite. Both granites are enriched in Rb, Pb andU, and depleted in Ba, Sr, Nb, P and Ti. The Baishachong granite shows a lowerΣREE, but stronger negative δEu anomaly than the Beipaotai granite. What’s more,the Baishachong granite reflects a tetrad-like effect with a shape of―M‖. The sourceof both the Baishachong and the Beipaotai granites are mainly derived in the crustalmaterials and experienced crystal fractionation process. Compared with the Beipaotaigranite, the Baishachong granite experienced a much stronger fractionated evolutionprocess.3. There are two dominant types of fluid inclusions in the Malage deposit:liquid-rich and gas-rich inclusions. Homogenization temperatures of skarn stage,quartz-sulfide stage, carbonate stage are362.3⁵95.0℃,165.6⁴56.1℃,137.8³02.4℃, respectively; and salinities （NaCleq） are16.6%²2.7%,4.2%²0.2%,2.2%¹2.5%, respectively. Densities range from0.72to0.97g/cm3. 4. Hydrogen and oxygen isotopic data show that the ore-forming fluids in theMalage deposit are dominated by magmatic water in the early stage, but in the latestage, it could be produced by mixing magmatic and meteoric water. Thecompositions of sulfur isotope show that the most sulfur are derived from granite.5.40Ar/39Ar dating of muscovites in skarn and oxidized ore yielded ages of95.7±1.2Ma and89.68±0.65Ma, respectively. Our new data are consistent with theages of other ore deposits in the Gejiu ore district, suggesting they were formedalmost at the same time, during the Late Yanshanian.