To constrain the mixing properties of (Ca,Fe,Mg)2SiO4 olivine, Fe-Mg exchange experiments have been carried out on the assemblages of low-Ca olivine+clinopyroxene, low-Ca olivine+high-Ca olivine+clinopyroxene and high-Ca olivine+clinopyroxene with bulk Fe/(Fe+Mg) in the range of 0.4 to 0.6, mainly about 0.5, at pressures from 1 atm to 2.0 GPa and temperatures from 1100°C to 1300°C, mainly at 1 atm and 1150°C. The phase relations of (Ca,Fe,Mg)2SiO4 olivine have been also determined at 1 atm and 1150°C. Experiments under 1 atm were performed in an electric furnace equipped with CO2/H2 control device. The oxygen fugacity around the sample space in the furnace was kept to that realized with the iron-wüstite buffer. High-pressure experiments were carried out using a piston-cylinder apparatus. The Fe-Mg distribution coefficient between olivine and clinopyroxene, KD[=(XMgCpxXFeOl)/(XFeCpxXMgOl)], increases with bulk Ca/(Ca+Fe+Mg) content, but insensitive to pressure. The solvus between low-Ca and high-Ca olivines becomes narrow with pressure and Fe/(Fe+Mg) of the system, but the pressure effect on the solvus is very small. Thermodynamic analyses were made on the present and previous data to refine the compositional dependence on KDof olivine-clinopyroxene pair and the solvus of quadrilateral (Ca,Fe,Mg)2SiO4 olivine using the Fe-Mg mixing data, and the free energy changes of the Fe-Mg exchange reactions between these phases and in olivine. The symmetric and asymmetric two-site regular solution models were applied to (Ca,Fe,Mg)2SiO4 olivines in terms of the Fe-Mg intracrystalline exchange between M1 and M2 sites and the excess energies of Ca-Fe-Mg mixing. In the present model, it is assumed that substitution of Ca is limited to M2 sites alone whereas Fe and Mg substitute on both M1 and M2 sites. The mixing parameters of the asymmetric Ca-Mg and symmetric Fe-Mg and Ca-Fe models are evaluated as WCaMg=36.31±0.19, WMgCa=32.73±0.18, WCaFe=21.34±0.06, WCaFeMg=6.12±0.98, δ1=2.24±1.30 and δ2=−8.45±0.94 (unit in kJ), whereas the symmetric Ca-Fe-Mg mixing model (WCaFeMg=0) resulted as WCaMg=WMgCa=34.20±0.18, WCaFe=23.75±0.58, δ1=−1.50±2.35 and δ2=−10.20±1.37. The present results are consistent with the previous analyses of miscibility gaps in Ca-Fe-Mg olivines. The Ca-Mg mixing is slightly less non-ideal than the previous estimates on the forsterite-monticellite solvus. Considerations about the Fe-Mg intracrystalline exchange show a slight preference of Fe in M1 site in Ca-free olivines, and indicate preferential order of Fe to M2 site with increase of Ca contents of olivine.
In the Youjiang basin, South China, numerous sediment-hosted micro-disseminated gold (SMG) deposits were found in Upper-Paleozoic to Triassic calcareous siltstone, mudstone, sandstone and chert breccias, and usually regarded as Carlin-type gold. Ore bodies show remarkable stratabound features, and occur often in transitional zone between shallow water facies and deep-water facies of the basin, particularly on marginal slope of submarine carbonate highs constrained by syndepositional faults. In ores and in hostrocks there are abundant synsedimentary-syndiagenetic fabrics of sulfides such as lamination, convolute bedding, slumping texture, diagenetic crack, soft deformation etc, indicating syndepositional faulting and diagenetic compaction. Abundant fluid-escape and liquefaction fabrics in ores and hostrocks imply strong fluid migration during sediment basin evolution. High content of organic matters and various kinds of biogenic-organogenic fabrics in ores imply a probable genetic connection between ores and sedimentary organic matters. Mineral associations (pyrite, arsenopyrite, realgar, cinnabar, stibnite, chalcedony) as well as alteration assemblages are typically low-temperature. Ore and its sedimentary hostrock shows similar trace element, REE and S isotope features implying that both of them are probably product of similar processes, with no evidence for a strong later (epigenetic) material-introduction after sediments were lithified. C-O isotope study reveals that CO2 in ore-forming fluids might be mainly produced by dissolution of carbonates in surrounding sedimentary rocks. All these imply a close relationship between SMG ore-formation and basin evolution, and are inconsistent with the popular genetic models of Carlin-type gold, because Carlin-type gold is regarded as a typical epigenetic hydrothermal deposit with little genetic connection to basin evolution.
The Ladakh batholith is exposed along the 600 km long and 20 to 80 km wide NW-SE trending Ladakh range north of the Indus-Tsangpo Suture Zone. It was emplaced with in an unmetamorphosed thick pile of mafic and felsic volcanics, ultramafics and sediments of Upper Cretaceous-Eocene age (Dras Volcanics, Khardung Volcanics). The granites from the Ladakh batholith (60 Ma phase) within the Leh-Khardung La and Sakti-Chang La sections (samples collected between altitude of 3600 M and 5440 M above mean sea level) have been estimated for pressure and temperature of crystallization employing the hornblende geobarometer of Schmidt (1992) and hornblende-plagioclase geothermometer of Blundy and Holland (1990), with the results of pressure of 254±67 MPa and temperature of 695±20°C. Therefore, these granites were solidified at a depth of 8.6±2.3 km suggesting an unroofing of this thickness in this region.