Ore-forming mechanism of Caledonian W-Sn deposit in the Nanling Range: A case study from the Luojiachong deposit, sourthwestern Hunan Province.

Recently, many Caledonian W-Sn deposits have been discovered in the Nanling Range, whereas their ore -forming mechanism are still unclear. The Luojiachong deposit in the northwestern Miao'ershan batholith, southwestern Hunan Province, is a newly -discovered Caledonian W-Sn deposit. It is considered to be a promising tungsten -tin reserve, whilst the fluids evolution processes and ore genesis of the deposit still remain enigmatic. Based on detailed geological observation, the mineralization process can be divided into four ore -forming stages, namely altered granite-scheelite stage ( I), greisen-scheelite-cassiterite stage (II), quartzscheelite stage (III) and quartz -sulfide stage (IV). We present new cassiterite and zircon U-Pb ages and zircon Hf isotopes of stage I, as well as detailed fluid inclusion, H-0 isotopic studies of quartz in all stages to constrain the ore genesis of the Luojiachong deposit. The cassiterite Tera-Wasserburg U-Pb age (433.0 +/- 11Ma, MSWD =0. 75) is consistent with the zircon Pb-206/U-238 weighted mean age (430.7 +/- 2.3Ma, MSWD = 0. 05) of biotite monzogranite, suggesting that the W-Sn mineralization is spatial -temporally associated with the Late Caledonian biotite monzogranite. Fluid inclusion microthermometry shows that the homogenization temperatures of the fluid inclusions decrease from stage I to stage IV (260 similar to 380 degrees C for stage I, 260 similar to 320 degrees C for stage II, 200 similar to 320 degrees C for stage III and 180 similar to 220 degrees C for stage IV, respectively), whereas the salinities of the fluid inclusions drastically decrease from stages I to II and within a narrow range from stages II to IV ( ave. 16.9%, 3. 9%, 4. 8% and 3.7% NaCleqv, respectively). The ages of granite emplacement and mineralization, the characterics of ore -forming fluids, combined with zircon epsilon(Hf)(t) values ( -7. 6 similar to -5. 0) and two stage model ages t(Dm2) (1565 similar to 1711 Ma), the delta(OH2O)-O-18, value of quartz decreses from stage I (3. 97 similar to 5. 34 parts per thousand) to stage IV ( - 4. 99 similar to 5. 10 parts per thousand), as well as the reported geochemical analysis of Miao'ershan batholith, we suggest that the ore -forming material in