Geology and genesis of the Hehuaping magnesian skarn-type cassiterite-sulfide deposit, Hunan Province, Southern China
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Cassiterite
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The Bainiuchang polymetallic deposit in Yunnan Province,China,is located geographically between the Gejiu deposit and the Dulong deposit.The cause on the genesis of the deposit is controversial due to the lack of reliable chronology data.This article reports the first precise in situ LA-MC-ICP-MS U-Pb age of cassiterite from the Baiyang ore block,which performs the most obvious characteristic of sedimentary.The results show that the 238U/207Pb-206Pb/207Pb isochron ages of two cassiterite samples are(87.5±3.6) Ma(MSWD=7.8) and(87.9±3.1) Ma(MSWD=8.1),respectively.A comparison of the U-Pb cassiterite ages with zircon U-Pb ages of(84.1±2.687.83±0.39) Ma from Bozhushan granite demonstrates that there is a close relationship between tin mineralization and magmatism.Combined with the results of TIMS U-Pb chronology from other tin-polymetallic deposits in Nanling area,there is a great advantage on direct dating of tin-polymetallic deposit with cassiterite,and it provides a new way for dating the same type deposit with the method of in situ LA-MC-ICP-MS U-Pb.With higher common lead in cassiterite,this method entails further study to meet accurate age dating.
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Cassiterite is the main ore mineral of Sn deposits, and may be used to date tin mineralization processes/processes. Both isotope dilution thermal ionization mass spectrometer (ID-TIMS) and laser ablation multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS) have been applied to cassiterite U–Pb dating. Here we show that cassiterite U–Pb dating results using LA-ICP-MS are comparable to those using LA-MC-ICP-MS. Our results also show that Tera–Wasserburg U–Pb intercept age is far better than the previously used 206Pb/207Pb versus 238U/207Pb “isochron” age because the latter weighted data points with low proportions of common Pb more heavily, such that introducing larger errors. Moreover, 206Pbi/207Pb changes with age and 238U/207Pb ratios, such that yields results younger than the real ages with larger errors. Samples with multiple common Pb sources are difficult to date. Nevertheless, multiple common Pb in cassiterite are often associated with fluid inclusions that may be identified and avoided. The precisions of cassiterite U–Pb ages are not as good as zircon ages, but they provide direct dating on the mineralization processes/processes. Therefore, coupled studies on zircon and cassiterite dating may provide constraints on the relation between magmatism ± hydrothermal activities and mineralization.
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Abstract This contribution presents new insights into the origin and age relationships of the Geyer tin deposit in the Erzgebirge, Germany. Tin mineralization occurs in skarns, greisen, and in cassiterite-bearing fluorite-quartz veins. Skarn alteration replaces marble layers of the Cambrian Jáchymov Group and occurs in two clearly distinct stages. The first skarn stage forms skarnoid textured assemblages of clinopyroxene, garnet, and wollastonite with no tin phases recognized. Garnet U-Pb ages of this skarn stage (~322 Ma) relate the earlier skarn stage to the emplacement of the Ehrenfriedersdorf granite (~324 to 317 Ma). The second stage of skarn alteration is marked by the occurrence of malayaite and cassiterite associated with garnet recording ages of 307 to 301 Ma. Greisen- and skarn-hosted cassiterite-bearing veins provide U-Pb ages in the range of 308 to 305 Ma, relating greisenization and vein formation to the same magmatic-hydrothermal event as the second skarn stage. This suggests that tin mineralization at Geyer is related to a distinctly younger magmatic-hydrothermal event, clearly postdating the Ehrenfriedersdorf granite, which was previously assumed as the source of the tin-rich fluids. Fluid inclusions show salinities in the range of 1.0 to 31.5 % eq. w(NaCl±CaCl 2 ) and homogenization temperatures between 255 and 340 °C. Cassiterite-associated fluid inclusions show indications for heterogeneous entrapment and dilution of hydrothermal with meteoric fluids. Dilution of high-salinity fluids with low-salinity fluids and cooling of the system was probably a decisive process in the precipitation of cassiterite in the Geyer Sn system.
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Abstract Skarn ores have recently been identified beneath the historically mined placer Sn deposit at Kanbauk of the Dawei region, southern Myanmar. A large-tonnage skarn ore reserve at Kanbauk is estimated to be over 100 million tonnes, with reported ore grades of 0.17% WO3, 0.26% Sn, and 15.4% CaF2, potentially making it one of the largest W-Sn skarn deposits in the Southeast Asian tin belt. The mineralized skarns lie between marbles to the east and metasediments of the Mergui Group to the west. The timing of the mineralization is unclear, and thus the genetic relationship with regional magmatic events is not known. We report laser ablation-inductively coupled plasma-mass spectrometry U-Pb ages of garnet and cassiterite from the mineralized skarns. Garnet grains from the massive prograde skarns are typically subhedral to euhedral and show both sector and oscillatory zoning. They have 15 to 23% andradite (Ad), 55 to 67% grossularite (Gr), and 16 to 30% pyralspite (Py) (Ad15-23Gr55-67Py16-30) and contain 0.08 to 306 ppm U with a lower intercept 206Pb/238U age of 56.0 ± 1.5 Ma. Cassiterite grains from retrograde veinlets are subhedral to anhedral and have U contents from 110 to 12,000 ppm with a lower intercept 206Pb/238U age of 54.2 ± 1.7 Ma. Garnet and cassiterite have ages consistent within error and can be taken to indicate the formation of the Kanbauk W-Sn(-F) skarn deposit at around 55 Ma. Together with published ages of primary Sn-W deposits in the Dawei region, our study confirms a westwardly younging trend of mineralization toward the coast and provides support for rollback of the Neo-Tethyan subducting slab since the Late Cretaceous, which is considered as the main mechanism for the regional, extensive Sn-W mineralization.
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Isochron dating
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