A Fluid Inclusion Study of Vein-type Copper Mineralization in the East Wulasigou Pb-Zn-Cu Deposit,Altay,Northwestern China

The Wulasigou Cu-Pb-Zn deposit,located 15 km northwest of Altay city in Xinjiang,is one of many Cu-Pb-Zn polymetallic deposits in the Devonian Kelan volcanic-sedimentary basin in southern Altaids.Two mineralizing periods can be distinguished：the marine volcanic sedimentary PbZn mineralization period,and the metamorphic hydrothermal Cu mineralization period,which is further divided into an early bedded foliated quartz vein stage（Q1） and a late sulfide-quartz vein stage（Q2） crosscutting the foliation.Four types of fluid inclusions were recognized in the Q1 and Q2 quartz from the east orebodies of the Wulasigou deposit：H2O-CO2 inclusions,carbonic fluid inclusions,aqueous fluid inclusions,and daughter mineral-bearing fluid inclusions.Microthermometric studies show that solid CO2 melting temperatures（T（m,CO2）） of H2O-CO2 inclusions in Ql are from-62.3℃ to-58.5C,clathrate melting temperatures（T（m,clath）l） are from 0.5 C to 7.5 C,partial homogenization temperatures（T（h,CO2）） vary from 3.3℃ to 25.9℃（to liquid）,and the total homogenization temperatures（T（h,tot）） vary from 285℃ to 378℃,with the salinities being 4.9%-15.1%NaCl eqv.and the CO2-phase densities being 0.50-0.86 g/cm-3.H2O-CO2 inclusions in Q2 have T（m,CO2） from-61.9℃ to-56.9℃,T（m,clath）from 1.3℃ to 9.5℃,T（h,CO2） from 3.4℃ to 28.7℃（to liquid）,and T（h,tot） from 242℃ to 388℃,with the salinities being 1.0%-15.5%NaCl eqv.and the CO2-phase densities being 0.48-0.89 g/cm-3.The minimum trapping pressures of fluid inclusions in Q1 and Q2 are estimated to be 260-360 MPa and180-370 MPa,respectively.The δ-（34）S values of pyrite from the volcanic sedimentary period vary from2.3‰ to 2.8‰（CDT）,and those from the sulfide-quartz veins fall in a narrow range of-1.9‰ to 2.6‰（CDT）.The δD values of fluid inclusions in Q2 range from-121.0‰ to-100.8‰（SMOW）,and theδ-（18）O（H2O） values calculated from δ-（18）O of quartz range from-0.2‰ to 8.3‰（SMOW）.The δD-δ-（18）O（H2O）data are close to the magmatic and metamorphic fields.The fluid inclusion and stable isotope data documented in this study indicate that the vein-type copper mineralization in the Wulasigou Pb-Zn-Cu deposit took place in an orogenic-metamorphic enviroment.

Characteristics of Hydrocarbon Fluid Inclusions and Their Significance for Evolution of Petroleum Systems in the Dabashan Foreland,Central China

Field investigation combined with detailed petrographic observation indicate that abundant oil,gas,and solid bitumen inclusions were entrapped in veins and cements of sedimentary rocks in the Dabashan foreland,which were used to reconstruct the oil and gas migration history in the context of tectonic evolution.Three stages of veins were recognized and related to the collision between the North China block and the Yangtze block during the Indosinian orogeny from Late Triassic to Early Jurassic（Dl）,the southwest thrusting of the Qinling orogenic belt towards the Sichuan basin during the Yanshanian orogeny from Late Jurassic to Early Cretaceous（D2）,and extensional tectonics during Late Cretaceous to Paleogene（D3）,respectively.The occurrences of hydrocarbon inclusions in these veins and their homogenization temperatures suggest that oil was generated in the early stage of tectonic evolution,and gas was generated later,whereas solid bitumen was the result of pyrolysis of previously accumulated hydrocarbons.Three stages of hydrocarbon fluid inclusions were also identified in cements of carbonates and sandstones of gas beds in the Dabashan foreland belt and the Dabashan foreland depression（northeastern Sichuan basin）,which recorded oil/gas formation,migration,accumulation and destruction of paleo-reservoirs during the D2.Isotopic analysis of hydrocarbon fluid inclusions contained in vein minerals shows that δ^（13）C_1 of gas in fluid inclusions ranges from-17.0‰ to-30.4‰（PDB） and δD from-107.7‰ to-156.7‰（SMOW）,which indicates that the gas captured in the veins was migrated natural gas which may be correlated with gas from the gas-fields in northern Sichuan basin.Organic geochemical comparison between bitumen and potential source rocks indicates that the Lower Cambrian black shale and the Lower Permian black limestone were the most possible source rocks of the bitumen.Combined with tectonic evolution history of the Dabashan foreland,the results of this study suggest that oil was generated from the Paleozoic source rocks in the Dabashan area under normal burial thermal conditions before Indosinian tectonics and accumulated to form paleo-reservoirs during Indosinian collision between the North China block and the Yangtz block.The paleo-reservoirs were destroyed during the Yanshanian tectonic movement when the Dabashan foreland was formed.At the same time,oil in the paleo-reservoirs in the Dabashan foreland depression was pyrolyzed to transform to dry gas and the residues became solid bitumen.

Transmission electron microscopic study of the fine-grained vein matrix in the Suizhou L6 meteorite

The mineralogy of shock vein matrix in the Suizhou meteorite has been investigated by optical and transmission electron microscopy. It was revealed that the vein matrix is composed of majorite-pyrope garnet, mag- aesiowtistite, and ringwoodite, with FeNi-FeS inter- growths. The observation and character of ring-like selected electron diffraction （SAED） patterns indicate that Lhe idiomorphic garnet crystals in the vein matrix have different orientations. The polycrystalline nature of magnesiowtistite is also confirmed by a ring-like SAED pattern. Both garnet and magnesiowtistite crystals showed sharp Jiffraction spots, signifying the good crystallinity of these ：wo minerals. The SAED pattern of cryptocrystalline 5ngwoodite shows only diffuse concentric diffraction tings. FeNi metal and troilite （FeS）, which were molten during the shock event, occur in the matrix as fine eutectic FeNi-FeS intergrowths filling the interstices between garaet and magnesiowiistite grains. Based on the phase dia- gram of the Allende chondrite and the results of this TEM study, it is inferred that majorite-pyrope garnet first crystallized from the Suizhou chondritic melt at 22-26 GPa, Followed by crystallization of magnesiowtistite at 20-24 GPa, and then ringwoodite at 18-20 GPa. The eutectic intergrowths of FeNi-metal and troilite are proposed to have crystallized during meteorite cooling and solidified at the last stage of vein formation.

NaAlSi_3O_8-hollandite and other high-pressure minerals in the shock melt veins of the Suizhou meteorite

The Suizhou L6 chondrite contains a few very thin shock melt veins of 0.02-0.09 mm in width. In spite of small width of the veins, shock-induced high-pressure phases, such as coarse-grained NaAlSi3O8-hollandite, ringwoodite, majorite and fine-grained matrix majorite-pyropess have been discovered in these veins. NaAlSi3O8-hollandite, the high-pressure phase of plagioclase, in Suizhou shock veins occurs as a single phase mineral, no silicate glassy phase, such as albitic glass, was incorporated with it. The presence of above-mentioned high-pressure phases constrains the high pressure (up to 23-24 GPa) and high temperature (up to 1900-2000℃) regime in Suizhou shock veins, and indicates that the duration of high-pressure regime in the veins should be long enough (a few seconds) for phase transformation and crystallization of minerals under pressure. The discovery of the first natural single-phase crystalline NaAlSi3O8- holland-ite in Suizhou meteorite is of important significance in understanding the Earth’