Micro-geochemical Analysis And Metallogenic Mechanism Of Tin-copper Associated Deposits

Most tin deposits in the world are directly related to highly differentiated S-type biotite granites,but there are also some tin deposits,which occur in volcanic or sedimentary strata in the form of massive or stratiform orebodies.The genetic mechanism of these deposits has long been controversial.And whether there exists sedimentary exhalative tin mineralization has always been a hot issue.The Neves Corvo tin-copper polymetallic deposit in Portugal has the typical characteristics of submarine sedimentary exhalative deposits in both geological and geochemical aspects.There are stratified tin mineralization occurring between marble beds,and copper-tin mineralization occurring within basalt and in contact zone between basalt and wall rock in Gejiu,Yunnan Province.Both types of mineralization are intuitively not directly related to granite.At present,there are different understandings about the metallogenic mechanism of these types of tin mineralization.At the same time,tin-copper coexistence exists in the Neves Corvo deposit and the Gejiu deposit.Traditionally,it is believed that magmatic hydrothermal tin deposit differs significantly from the copper deposit in point of the parent magma oxygen fugacity.The mechanism of high-grade tin-copper coexistence in the Neves Corvo deposit and the Gejiu deposit needs special metallogenic processes to explain.At present,geochemical analysis techniques of ore deposits are developing towards trace,in-situ and micro-area.Laser ablation plasma mass spectrometry?LA-ICPMS?and laser ablation multicollector-inductively coupled plasma mass spectrometry?LA-MC-ICPMS?are the fastest developing techniques.The geochemical analysis methods of micro-area,especially the in-situ analysis of elemental contents and isotope ratios of ore minerals,can provide direct evidence for precise determination of metallogenic age and fine tracing of metallogenic processes.In this paper,detailed micro-area geochemical studies of the Neves Corvo deposit in Portugal and the Gejiu deposit in Yunnan Province are carried out,and the following conclusions are obtained:1.Three main ore types were classified on a geological basis of the sulfide stratigraphy in the Neves Corvo deposit:fissural ores in the lower part,massive sulfide ores and rubanéores in the upper part.Cassiterite-rich ores exist in all types of ores.By LA-ICP-MS U-Pb isotope dating of cassiterite from the Neves Corvo deposit,the formation age of cassiterite is 363-366 Ma,which is identical to the age of the host volcanic rocks and represents the age of tin mineralization in the Neves Corvo deposit.The age provides direct evidence for the formation of the Neves Corvo deposit in Carboniferous submarine hydrothermal exhalative sedimentation.2.A detailed in-situ micro-geochemical analysis was carried out directly on the ore minerals in the Neves Corvo deposit,including trace element compositions of cassiterite and pyrite,S and Pb isotopic compositions of sulfides.The results show that the cassiterite has high Fe content,which is significantly different from the cassiterite in the VMS deposit,and is similar to that in the granitic magmatic-hydrothermal deposit.?³⁴S values of sulfides range from-32.3‰to+17.4‰among the different ore types and are largely consistent with microbiological reduction of seawater sulfate?BSR?,but with some positive values resulting from thermochemical sulfate reduction?TSR?.In contrast,the Sn-rich ores have a limited range in?³⁴S that clusters near 0‰,which suggests derivation from magmatic fluids.Pb isotope analysis of ore sulfides in the Neves Corvo deposit revealed that the Neves Corvo deposit has a lead isotopic composition different from other deposits in the Iberian pyrite belt?IPB?,and the content of radioactive lead in the Neves Corvo deposit is relatively higher.A considerable number of sample points of the Neves Corvo deposit do not fall within the range of lead isotope compositions in other deposits in the area,indicating that the lead of Neves Corvo ore has multiple sources.On the basis of the above analysis,it is believed that the addition of magmatic hydrothermal fluid contributes greatly to tin mineralization in the Neves Corvo deposit besides the cyclic submarine sedimentary exhalative hydrothermal system dominated by seawater.It is the addition of tin-bearing magmatic fluid that leads to the occurrence of high-grade Sn mineralization in the process of sedimentary exhalative deposition.3.Five types of mineralization have been found in the Gejiu deposit:?1?greisen-skarn type tin-copper polymetallic mineralization in the contact zone between granite and marble;?2?layered tin mineralization between marble layers,mostly in the form of oxide mineralization;?3?vein tin mineralization at the top of granite;?4?stratiform-like copper-tin mineralization within basalt body and near the contact zone with surrounding rock;?5?disseminated and vein tin mineralization within granite body.LA-ICP-MS U-Pb isotopic dating of zircons from granites in the Gejiu mining area shows that the ages of granites are 82.4-88.2 Ma.Titanite dating indicates that the Jiasha gabbro is emplaced at about80.8 Ma.The LA-ICP-MS U-Pb isotope dating of cassiterite in the Laochang deposit in Gejiu shows that the formation age of cassiterite is 84.1 Ma,which is consistent with the emplacement age of granite in the mining area,indicating that tin mineralization should be closely related to granitic magmatic hydrothermal activity in the area.4.The geological,mineralogical and geochemical studies of the deposits in Gejiu?trace element compositions of cassiterite,S and Pb isotopic compositions of sulfides?show that cassiterite has high Fe content,which is different from cassiterite in VMS deposit,but similar to that in granitic magmatic hydrothermal deposit.The data of?³⁴S of different ore types in the Gejiu deposit mostly distributes around 0,indicating that the sulfur in ore-forming fluid mainly comes from magmatic sulfur.The?³⁴S values of sulfides in skarn ore of the Malage deposit are higher,and distribute in the range of surrounding carbonate rocks,indicating the sulfur in ore-forming fluids mainly comes from carbonate strata.The?³⁴S values of sulfides of ores in the Kafang basalt strata are mostly distributed within the range of sulfur isotope compositions of basalt.It is thus indicated that the sulfur may come from the basalt.The Pb isotopic compositions of the altered granite ores,skarn ores and layered oxide ores are basically within the initial Pb isotopic range of the granite,indicating that the lead in these ores may come from the granite.However,the lead isotopic compositions of ores in the layered copper-tin orebody in the Kafang mining area are obviously different from those of the above ores.They show more characteristics of mantle lead.We speculate that the lead of the layered copper-tin orebody in Kafang is derived from the basalt.Combined with the above analysis,it is found that the ore-forming fluids have the properties of granite magmatic hydrothermal fluids,which are obviously different from those of sedimentary exhalative fluids.The different types of mineralization are the response to the tectonic location and precipitation mechanism.5.In the Neves Corvo deposit,the addition of tin-bearing magmatic hydrothermal fluid into copper-bearing exhalative hydrothermal fluid,formed the tin-copper co-genetic mineralization.In the Gejiu area,leaching copper from the basalt by tin-bearing magmatic-hydrothermal fluid,formed tin-copper co-genetic mineralization.