High 3He/4He ratios in the Wangu gold deposit, Hunan province, China: Implications for mantle fluids along the Tanlu deep fault zone.

Abstract

The Wangu shear zone hosting gold deposit is located on splay faults of the regional scale Tanlu deep fault zone that extends to the MOHO and likely through the lithosphere.Fluid inclusions in pyrite from gold bearing veins have 3 He/ 4 He (R/Ra) values of 3.5 to 9.8.Fluid inclusions in pyrite from barren vein have R/Ra values of 0.8, and 0.9.The projections of the analyses in 3 He/ 4 He vs. 40 Ar/ 36 Ar diagram can be separated obviously as two groups and display the variable degrees of mixing between mantle and crustal fluids.Fluid inclusion in quartz from auriferous veins are characterized by a higher temperature (Th = 207 to 310°C), dilute, aqueous carbonic fluids, with δD and δ 18 O values overlapping the magmatic fluids, whereas barren veins have low temperature (Th = 138, 145°C), more saline, aqueous fluid with relatively low, light δD and δ 18 O values, consistent with evolved meteoric water.Lithosphere extension accompanied with displacement of the Tanlu sinistral fault, with development of gas and oil-bearing sedimentary basins proximal to the fault.The CO 2 -bearing fluids have high He contents of up to 0.1% He, 3 He/ 4 He = (3~7.2)R/Ra,40 Ar/ 36 Ar = 573~7744, and δ 13 C CO 2 = -3.4 to -16.9‰ (Xu et al., 1995;Tao et al., 1996).During the lithosphere extension and decompressional melting of asthenosphere, noble gases and CO 2 exsolved from basalt melts, advected up to the Tanlu fault zone into splay faults where gold mineralization developed, and through sedimentary basin.There was mixing with isotopically and compositionally evolved meteoric water.mineralization, based on helium isotope studies (Simmons et al., 1987;Stuart et al., 1995;Hu et al., 1998;Burnard et al., 1999).All of the 3 He/ 4 He ratios in these deposits are between two end members, implying variable degrees of mixing between mantle and crustal fluids.To further address possible contributions of mantle fluids to gold metallogenesis, we have conducted a study of He and Ar isotopes and fluid inclusions in the Wangu gold deposit, Hunan province, China.This is one of several gold deposits distributed along the Tanlu deep fault system.High concentrations of mantle-derived helium and CO 2 have been reported in natural gases from several basins along the Tanlu fault zone (Xu et al., 1995;

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