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Peng H.-J.,Chengdu University of Technology | Zhang C.-Q.,Chinese Academy of Geological Sciences | Mao J.-W.,Chinese Academy of Geological Sciences | Santosh M.,China University of Geosciences | And 2 more authors.
Journal of Asian Earth Sciences | Year: 2015

The Late Cretaceous Hongniu-Hongshan porphyry-skarn copper deposit is located in the Zhongdian area of northwestern Yunnan Province, China. Garnets from the deposit have compositions that range from Adr14Grs86 to almost pure andradite (Adr98Grs2) and display two different styles of zoning. The garnets are predominantly of magmatic-hydrothermal origin, as is evidenced by their 18Ofluid (5.4-6.9‰) and low Dfluid (-142‰ to -100‰) values, both of which likely result from late-stage magmatic open-system degassing. Three generations of garnet have been identified in this deposit: (1) Al-rich garnets (Grt I; Adr22-57Grs78-43) are anisotropic, have sector dodecahedral twinning, are slightly enriched in light rare earth elements (LREEs) compared with the heavy rare earth elements (HREEs), have negative or negligible Eu anomalies, and contain high concentrations of F. Fluid inclusions within these Al-rich garnets generally have salinities of 12-39wt.% NaCl eq. and have liquid-vapor homogenization temperatures (Th) of 272-331°C. The Grt I are most likely associated with low- to medium-salinity fluids that were generated by the contraction of an ascending vapor phase and that formed during diffusive metasomatism caused by pore fluids equilibrating with the host rocks at low W/R (water/rock) ratios. These garnets formed as a result of the high F activity of the system, which increased the solubility of Al within the magmato-hydrothermal fluids in the system. (2) Fe-rich garnets (Adr75-98Grs25-2) have trapezohedral faces, and are both anisotropic with oscillatory zoning and isotropic. These second-generation Fe-rich garnets (Grt II) have high σREE concentrations, are LREE-enriched and HREE-depleted, and generally have positive but variable Eu anomalies. All of the Fe-rich garnets contain high-salinity fluid inclusions with multiple daughter minerals with salinities of 33-80wt.% NaCl eq. Some of them show higher temperatures of halite dissolution (465-591°C) than liquid-vapor homogenization temperatures (319-473°C), and several Fe-rich garnets contain inclusion groups indicative of boiling. The Grt II are associated with high-temperature, hypersaline fluids that were segregated from magma at a depth of at least 5.6km and reacted with carbonates at depths shallower than 2.0km. (3) Al-rich garnet veins (Adr14-60Grs86-40) contain allotriomorphic crystals, have lower HFSE (high field strength element) and REE concentrations than the other garnets, and have HREE-enriched and LREE-depleted patterns with small Eu anomalies that are typical of the majority of garnets. The Grt III most likely formed from residual metasomatic fluids. © 2014 Elsevier Ltd. Source

Peng H.,Chinese Academy of Geological Sciences | Li H.Y.,Chinese Academy of Geological Sciences | Pei R.F.,Chinese Academy of Geological Sciences | Zhang C.Q.,Chinese Academy of Geological Sciences | And 4 more authors.
Acta Petrologica Sinica | Year: 2014

The Hongniu-Hongshan deposit is a newly discovered porphyry-skarn copper deposit in Zhongdian volcanic arc, which belongs to Southwest Sanjiang metallogenic belt. The deposit occurs within the hydrothermal alteration zone of the Yanshanian felsic intrusion which emplaced into Triassic carbonate and marble. The biotite and amphibole phenocryst in the quartz monzonite porphyry related to the mineralization contain abundant fluorine, respectively reached 1.49% and 2.62%, indicating that the magma are rich in water and fluorine. The quartz phenocryst show a skeletal texture, which indicate the magma undergone rapid rising, and the growth embayments, resorption surfaces, and high-K edge indicate the quartz phenocryst undergone autometasomatic process. In addition, the magmatic hydrothermal breccias in a drill core suggest that magmatic hydrothermal breccias were generated by explosive, fluid released from crystallizing porphyry, and there probably be concealed porphyry in the deep. The prograde stage coule be divided into three main substage: aluminum-riched garnets (And22-57) and diopside (Hd7-27) in the first substage formed by diffusive metasomatism, and the fluid formed from high temperature magmatic degassing: Iron-riched garnets (And75-98) and pyroxenes (Hd10 99) in the second substage formed by advective metasomatism, and the large scale fluid formed by magmatic fluid exsolution: The skarn mineral veins in the last substage probably formed from residual fluid of the metasomatism. The formation of skarn lead to the CO 2 lost, mineral volume decreased, and open space created, permeability and porosity increased, which well-produced for mineralization. In the retrograde stage, the mineral assemblage mainly is epidote, amphibole, chlorite and sulfides. In the early retrograde stage, porosity decreased owning to hydrous minerals, fluid pressure increased leaded to fluid boiling and cryptoexplosion, as the result, the fluid temperature and pressure decrease, and copper precipitation. In the last stage, owning to the repeated boiling of fluid, formed open system, acidic magmatic fluid mixed with atmospheric water, copper precipitation. Source

Peng H.-J.,Chinese Academy of Geological Sciences | Zhang C.-Q.,Chinese Academy of Geological Sciences | Zhou Y.-M.,Yunnan Gold Mining Industry Group | Tian G.,China University of Geosciences | And 4 more authors.
Geology in China | Year: 2012

The Hongniu deposit is located in the Zhongdian island arc belt of northwest Yunnan and lies in the middle part of southwest Sanjiang region, belonging to the southern segment of the Triassic Yidun island arc belt. This ore deposit is one of the promising copper-polymetallic deposits with vast potential, and its Cu reserves have reached the middle size. Geological features of the ore bodies and element geochemical characteristics indicate that Hongniu is a typical skarn - hornfels type copper polymetallic ore deposit. It has typical alteration zoning and metallogenic element zoning of the skarn type deposit in both vertical and horizontal directions. The?element geochemical characteristics of the skarn are obviously affected by the elements of marble and quartz monzonite porphyry. According to geological features of the ore bodies, the authors also infer that the deep porphyry bodies have great metallogenic potential. Source

Peng H.-J.,Chengdu University of Technology | Peng H.-J.,Chinese Academy of Geological Sciences | Peng H.-J.,James Cook University | Mao J.-W.,Chinese Academy of Geological Sciences | And 7 more authors.
Economic Geology | Year: 2016

The Hongniu-Hongshan Cu skarn deposit (77.8 Mt at 1.8% Cu) is located in the central part of the Zhongdian porphyry and skarn Cu belt in southwestern China. Skarn and orebodies occur mainly between the different units of the Upper Triassic Qugasi Formation or within altered limestone adjacent to Late Cretaceous intrusions (78-76 Ma). Three main paragenetic stages of skarn formation and ore deposition have been recognized on the basis of petrographic observations: (1) pre-ore-stage hornfels with diopside (Di87-72Hd12-7), small-scale endoskarn with reddish grossular (Adr22-57Gr78-43), diopside (Di83-92Hd7-15), vesuvianite, and abundant exoskarn with red-brown andradite (Adr75-98Gr2-22), sahlite (Di28-41Hd58-71), and wollastonite; (2) syn-ore-stage retrograde minerals, sulfides (pyrite, chalcopyrite, pyrrhotite, molybdenite, galena, and sphalerite), quartz, and calcite; and (3) post-ore-stage calcite veins. Sulfur isotope values of sulfides are relatively high, with an average δ34S = 4.9% (n = 40), suggesting that the ore-forming fluid was magmatic and that the sulfides precipitated from a relatively reducing ore fluid. The coexistence of silicate melt and primary fluid inclusions in quartz phenocrysts of the mineralizationrelated quartz monzonite porphyry indicates the simultaneous entrapment of fluid and melt, and records the process of the aqueous fluid exsolving from the crystallizing melt. The initial single-phase fluid has a salinity of 8.8 to 12.7 wt % NaCl equiv and homogenization temperatures of 566° to 650°C, corresponding to pressures of 680 to 940 bar and lithostatic depth of 2.5 to 3.5 km. The primary fluid inclusions in the pre-ore-stage garnet and pyroxene composed of coeval vapor-rich (V type) and halite-bearing (S-I and S-II types containing sylvite) inclusions (32->79 total wt % salts) share similar homogenization temperatures (450°-550°C), indicative of the occurrence of fluid unmixing under lithostatic pressures of ∼550 to 780 bar (>2.0-km depth). Primary fluid inclusions trapped in syn-ore quartz, calcite, and epidote show the common development of S-type inclusions (∼37.3 wt % NaCl equiv) with coexisting V-type, liquid-rich (L type), and CO2-bearing (C-I type) inclusions, all of which have homogenization temperatures of 300° to 400°C and trapping pressures of 100 to 400 bar (∼1.5-km depth). Brine inclusions homogenized by halite dissolution after vapor disappearance in both the pre- and syn-ore stages are interpreted to have been trapped under overpressured conditions (>1,520 bar). Oxygen isotope analyses were conducted on garnet, wollastonite, epidote, quartz, and calcite. The pre-orestage garnet and wollastonite have δ18Ofluid values of 5.6 to 8.1%, whereas the syn-ore-stage epidote, quartz, and calcite have more variable δ18Ofluid values in the range of 3.9 to 17.5%. The δ18Ofluid values of the postore-stage vein calcite (15.2-21.3%) are much higher than both the pre- and syn-ore stages. The vapor phase of inclusions contains H2S, CH4, and C2H6 in the syn-ore stages. All these observations reveal that (1) the formation of the Cu skarn deposit was dominated by a magmatic hydrothermal system, (2) multiple fluid pulses contributed to the formation of the pre- and syn-ore-stage skarn minerals and sulfides, and (3) the increase in pH due to the neutralization of the acidic fluid could be the main factor controlling the large-scale ore deposition in Hongniu-Hongshan. © 2016 Society of Economic Geologists, Inc. Source

Peng H.-J.,Chinese Academy of Geological Sciences | Mao J.-W.,Chinese Academy of Geological Sciences | Pei R.-F.,Chinese Academy of Geological Sciences | Zhang C.-Q.,Chinese Academy of Geological Sciences | And 4 more authors.
Journal of Asian Earth Sciences | Year: 2014

The Hongniu-Hongshan porphyry and skarn copper deposit is located in the Triassic Zhongdian island arc, northwestern Yunnan province, China. Single-zircon laser ablation inductively coupled plasma mass spectrometry U-Pb dating suggests that the diorite porphyry and the quartz monzonite porphyry in the deposit area formed at 200Ma and 77Ma, respectively. A Re-Os isotopic date of molybdenite from the ore is 78.9Ma, which indicates that in addition to the known Triassic Cu-(Au) porphyry systems, a Late Cretaceous porphyry Cu-Mo mineralization event also exists in the Zhongdian arc. The quartz monzonite porphyry shows characteristics of a magnetite series intrusion, with a high concentration of Al, K, Rb, Ba, and Pb, low amount of Ta, Ti, Y, and Yb, and a high ratio of Sr/Y (average 26.42). The Cretaceous porphyry also shows a strong fractionation between light and heavy rare earth elements (average (La/Yb)N 37.9), which is similar to those of the Triassic subduction-related diorite porphyry in the Hongniu-Hongshan deposit and the porphyry hosting the Pulang copper deposit. However, in contrast to the older intrusions, the quartz monzonite porphyry contains higher concentrations of large ion lithophile elements and Co, and lesser Sr and Zr. Therefore, whereas the Triassic porphyry Cu-(Au) mineralization is related to slab subduction slab in an arc setting, the quartz monzonite porphyry in the Hongniu-Hongshan deposit formed by the remelting of the residual oceanic slab combined with contributions from subduction-modified arc lithosphere and continental crust, which provided the metals for the Late Cretaceous mineralization. © 2013. Source

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