Genesis of the Bangbu Orogenic Gold Deposit, Tibet： Evidence from Fluid Inclusion, Stable Isotopes, and Ar-Ar Geochronology

The Bangbu gold deposit is a large orogenic gold deposit in Tibet formed during the AlpineHimalayan collision. Ore bodies（auriferous quartz veins） are controlled by the E-W-trending Qusong-Cuogu-Zhemulang brittle-ductile shear zone. Quartz veins at the deposit can be divided into three types： pre-metallogenic hook-like quartz veins, metallogenic auriferous quartz veins, and postmetallogenic N-S quartz veins. Four stages of mineralization in the auriferous quartz veins have been identified：（1） Stage S1 quartz＋coarse-grained sulfides,（2） Stage S2 gold＋fine-grained sulfides,（3） Stage S3 quartz＋carbonates, and（4） Stage S4 quartz＋ greigite. Fluid inclusions indicate the oreforming fluid was CO_2-N_2-CH_4 rich with homogenization temperatures of 170–261°C, salinities 4.34–7.45 wt% Na Cl equivalent. δ^（18）Ofluid（3.98‰–7.18‰） and low δDV-SMOW（-90‰ to-44‰） for auriferous quartz veins suggest ore-forming fluids were mainly metamorphic in origin, with some addition of organic matter. Quartz vein pyrite has δ^（34）SV-CDT values of 1.2‰–3.6‰（an average of 2.2‰）, whereas pyrite from phyllite has δ^（34）SV-CDT 5.7‰–9.9‰（an average of 7.4‰）. Quartz vein pyrites yield 206Pb/204 Pb ratios of 18.662–18.764, 207Pb/204 Pb 15.650–15.683, and ^（208）Pb/204 Pb 38.901–39.079. These isotopic data indicate Bangbu ore-forming materials were probably derived from the Langjiexue accretionary wedge. 40Ar/39 Ar ages for sericite from auriferous sulfide-quartz veins yield a plateau age of 49.52 ± 0.52 Ma, an isochron age of 50.3 ± 0.31 Ma, suggesting that auriferous veins were formed during the main collisional period of the Tibet-Himalayan orogen（-65–41 Ma）.

Multiple and prolonged porphyry Cue Au mineralization and alteration events in the Halasu deposit, Chinese Altai, Xinjiang,northwestern China

The Halasu area is located in the southeastern margin of the Chinese Altai in Xinjiang, China. It is part of the Altaid orogenic collage where a number of porphyry-type Cue Moe Au deposits have been discovered in recent years. Geological mapping and drilling indicate the presence of various mineralized porphyritic intrusions in the Halasu Cue Au deposit, which is currently under exploration. Ue Pb dating of zircon crystals from four different mineralized porphyries reveals three significantly different ages of magmatic intrusion, i.e., ca. 372e382 Ma granodioritic porphyry and porphyritic granite, ca. 266 Ma quartz monzonitic porphyry, and ca. 216 Ma quartz dioritic porphyry. Ree Os dating of molybdenite from veinletdissemination ores in the granodioritic porphyry yields an age of mineralization of ca. 377 Ma, and Ar e Ar dating of K-feldspar from K-feldsparequartzechalcopyrite veins produces ages of ca. 269 and ca.198 Ma. The mineralization（and alteration） ages correspond broadly to the three episodes of magmatic intrusion, suggesting three overprinting porphyry mineralization events that are significantly separated in time. The first episode of porphyry intrusion and mineralization may be related to the magmatic arc being above a plate subduction zone, and the second was formed in a late-collisional environment during the closing of the Junggar Ocean, whereas the third episode of mineralization took place in the postcollisional stage. This case study suggests that in orogens where major porphyry deposits have been found in magmatic arc environments, the potential of discovering late- to post-collisional porphyry deposits cannot be neglected; conversely, in orogens where most porphyry deposits have late- to postcollisional ages, more attention should be paid to porphyries that were formed earlier in magmatic arc environments.

Petrogenesis of Cenozoic Basaltic Rocks from Jiangsu Province,China:Evidence from Geochemical Constraints

Cenozoic （Miocene to Pleistocene） basaltic rocks in Jiangsu province of eastern China include olivine tholeiite and alkali basalt. We present major, trace element and Sr-Nd isotopic data as well as Ar-Ar dating of these basalts to discuss the petrogenesis of the basalts and identify the geological processes beneath the study area. On the basis of chemical compisitions and Ar-Ar dating of Cenonoic basaltic rocks from Jiangsu province, we suggest that these basalts may belong to the same magmatic system. The alkali basalts found in Jiangsu province have higher ~FeO, MgO, CaO, Na20, TiO2 and P205 and incompatible elements, but lower A1203 and compatible elements contents than olivine tholeiite which may be caused by fractional crystallization of olivine, pyroxene and minor plagioclase. In Jiangsu basaltic rocks the incompatible elements increase with decreasing MgO/~FeO ratios. The primitive mantle-normalized incompatible elements and chondrite-normalized REE patterns of basaltic rocks found in Jiangsu province are similar to those of OIB. Partial loss of the mantle lithosphere accompanied by rising of asthenospheric mantle may accelerate the generation of the basaltic magma. The 143Nd/144Nd vs. S7Sr/S6Sr plot indicates a mixing of a depleted asthenospheric mantle source and an EM1 component in the study area. According to Shaw＇s equation, the basalts from Jiangsu province may be formed by 1%-5% partial melting of a depleted asthenospheric mantle source. On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen （1982）, we suggest that basalts from Jiangsu province may belong to a magmatic system with JF-2 as the primitive magma which has undergone fractional crystallization and evolved progressively to produce other types of basalts.

Age of Yingfeng rapakivi granite pluton on the north flank of Qaidam and its geological significance

The Yingfeng rapakivi granite on the north flank of Qaidam is a newly discovered Proterozoic rapakivi pluton in China, which was found after the discovery of Shachang rapakivi in Miyun County, Beijing and Kuandian rapakivi in Jilin Province. Yingfeng rapakivi pluton is ex-posed on the north side of the suture belt between Qinling-Kunlun orogenic belt and North China plate. U-Pb zircon isotopic dating and Ar-Ar isotopic dating of both hornblende and K-feldspar from the Yingfeng rapakivi granite have been conducted. The results show that the age of (1776±33)Ma at the upper intercept in Concorde diagram represents the age of formation of the plu-ton, whereas the age of (526 281) Ma at the lower intercept and Ar-Ar mineral dating of horn-blende and K-feldspar correspond to the age of a later event affecting the pluton, suggesting that Yingfeng pluton has ever been affected by strong regional Caledonian-Hercynian tectonic movement after its formation. The discovery of middle Proterozoic Yingfeng rapakivi granite pro-vides a petrologic evidence for the timing of cratonization of both the continental crust basement in western China and the basement of the North China plate and for a rifting event taking place in the mid-Proterozoic, suggesting that an amalgamation of ancient China continents had ever happened during the L liang Movement?between the early Proterozoic and the mid-Proterozoic.