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Liu B.,Chinese Academy of Geological Sciences | Liu B.,Beijing Research Institute of Uranium Geology | Liu H.,Chinese Academy of Geological Sciences | Zhang C.,Chinese Academy of Geological Sciences | And 5 more authors.
Ore Geology Reviews | Year: 2015

The Beiya gold-polymetallic orefield, with gold reserves of 305t, is one of the most representative porphyry-skarn orefields in the Jinshajiang-Ailaoshan Cu-Au ore belt within the Sanjiang region of southwest China. The orefield contains seven deposits: the Wandongshan, Hongnitang, Dashadi, Bijiashan, Weiganpo, Matouwan, and Bailiancun deposits. In this paper we report on the geochemistry and geochronology of porphyries associated with mineralization from the seven deposits. The results show that all the porphyries have similar geochemistry, with high alkalinity, high contents of SiO2, Al2O3, K2O, and Sr, high K2O/Na2O ratios, low MgO, Y, and Yb contents, enrichments in Ba, K, and Pb, depletions in P, Ti, Nb, and Ta, and non-evident to weak Eu depletions (δEu=0.42-0.99). In the SiO2 vs. Th/Ce diagram, the porphyry samples are distributed in the area of thickened lower crust, and in the Sr/Y vs. Y and La/Yb vs. Yb diagrams, the porphyries broadly followed the batch-melting trend of amphibolite containing up to 10% garnet. LA-MC-ICP-MS zircon U-Pb dating analysis suggests that the porphyries were emplaced between 34.62±0.25 and 36.72±0.25Ma. They were coeval with lamprophyres (34 to 36Ma) in the Beiya area and with potassic-ultrapotassic intrusive rocks (40 to 35Ma) within the Jinshajiang-Ailaoshan magmatic belt, indicating possible genetic relation between these rock types. We suggest that the porphyries in the Beiya gold-polymetallic orefield were derived from the partial melting of a K-rich mafic source in the thickened lower crust, with the melting triggered by asthenospheric upwelling following the removal of lower lithospheric mantle. © 2015 Elsevier B.V.

Liu H.,Chinese Academy of Geological Sciences | Wang Q.,China University of Geosciences | Zhang C.,Chinese Academy of Geological Sciences | Lou D.,Chinese Academy of Geological Sciences | And 2 more authors.
Journal of Geochemical Exploration | Year: 2016

The Pulang ore deposit, one of the largest porphyry copper deposits in China, is located in the Yidun continental arc, SW China. The alteration zones in the deposit transit upward and outward from early potassium-silicate, through quartz-sericite, to later propylitization. The wallrock near the porphyry stock was mostly changed into hornfels. The former two alterations host the main orebodies, constituting the core of mineralized zone; the later two alterations only develop weak mineralization surrounding the core. In this paper, various fractal indices, including the exponent of lacunarity, multifractal spectrum, correlation dimension and Hurst exponent, are applied to characterize the Cu spatial distribution in 114 drillcores in the Pulang ore deposit, with the aim to correlate the element spatial pattern with its dynamic drive. Compared to fractal indices in the propylitic zone and hornfels, the exponents of lacunarity in the potassium-silicate and quartz-sericite zones exhibit lower and more stable values, the correlation dimensions are higher and more consistent; yet the values of height difference of multifractal spectrum are lower and largely varied, and the Hurst exponents show little difference. Variations of the former three indices suggest that the core of mineralized zone has more homogeneity, stronger compactness of high concentrations, and greater proportion of high concentrations in the Cu distribution compared to the other parts of the deposit. More importantly, the correlation dimension, indicating the complexity of controls underpinning the system, is closely correlated to the exponent of lacunarity, which represents the homogeneity of spatial pattern. This correlation between those two indices implies a genetic link, that is to say the greater complexity of controls results in a more homogeneous spatial distribution of Cu in the porphyry deposit. The stability of the two indices is considered to reflect the development of thick orebody, providing a new perspective to understand the genesis of porphyry ore deposit. This interpretation from the fractal perspective is consistent with the geological understanding for the formation of porphyry deposits, which is considered to be subject to the complexity of ore fluid evolution with multifaceted physicochemical conditions. For a pragmatic use, these two fractal indices are successfully applied in the delineation of the core of mineralized zone in the plane view. © 2015 Elsevier B.V.

Liu H.,Chinese Academy of Geological Sciences | Zhang C.Q.,Chinese Academy of Geological Sciences | Jia F.D.,Chinese Academy of Geological Sciences | Zhou Y.M.,Yunnan Gold and Mineral Group Co. | Lou D.B.,Chinese Academy of Geological Sciences
Acta Petrologica Sinica | Year: 2015

Magma mixing is the topic issue of petrology, although it is stiil under discussion, and it is instructive to unravel the interaction of crust-mantle, and discuss the geodynamic setting of magma and ore-forming process. Pulang ore deposit, located in the south end of Yidun continental arc of SW Sanjiang, is one of the largest porphyry copper deposits in China The orebodies were developed in Pulang porphyries, in which many mafic microgranular enclaves (MMEs) developed. The MMEs are fine-grained in texture and generally ellipse in shape, and characterized by resorbed quartz, needle-like apatites, and plagioclase phenocrysts with compositional and textural disequilibrium. Compared to the host rock, the MMEs are rich in biotite and amphibole. Geochemically, the MMEs have lower SiO2 (53.67% ∼ 61.50%) content and higher MgO (3.12% ∼ 5.40%) and Fe2O3 T(3.38% ∼ 9. 00%) contents than the host rock. Furthermore, the MMEs have similar trace element signatures and REE pattern with the host rock, characterized by enrichment in LILEs (e. g., K, Rb, Ba, Sr) and LREEs, depletion in HFSE (e. g., Nb, Hf, P, Ti) and HREEs, and non-evident Ce depletions. However, the MMEs are different from the host rock in having relatively high Σ REE (119.0 × 10-6 ∼ 308.9 × 10-6) and obvious Eu depletions (δEu =0.56 ∼ 0.95). These results reveal that the MMEs are derived from magma mixing. Moreover, we applied the multifractal model to describe the distribution of elements Mg, Fe, Ca, Al, K, Ti, P and Ba in MMEs, with the aim to quantify the intensity of magma mixing. It shows that the wider multifractal spectrum and lower correlation dimension suggesting a higher degree of magma mixing.

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