| The Zhongdian arc, characterized by extensive arc magmatic activities and a large number of porphyry-skarn deposits related to subduction, was formed in Triassic during the subduction of Ganzi-Litang ocean plate. Pulang copper deposit is the largest porphyry copper deposit within the Zhongdian arc so far, with the copper reserves of 4.18 Mt, associated gold reserves of 145 t, associated molybdenum reserves of 84800 t. The Pulang ore-bearing porphyry complex is composed of early-stage quartz-diorite porphyry, the second-stage quartz-monzonite porphyry, and the third-stage granodiorite porphyry. Mineralization is closely related to the second-stage quartz-monzonite porphyry. So far, the published geochronological data of the three mineralized porphyries do not conform to their contact relationship; the oxidative or reductive nature of the ore-forming magma remains controversy; the characteristics of the volatile phase of the ore-forming magma and the controlling factors of the large-scale accumulation of the ore-forming elements is remain lack of research; the tectonic setting has not formed a unified understanding.In view of the major scientific problems, this research focuses on Wengshui volcanic rocks in northern Zhongdian arc and Pulang giant porphyry copper deposit. Based on systematic field investigation and microscopic observation, we clarify the emplacement order, mineral composition and mineralization characteristics of Pulang mineralized porphyries, and the mineral composition of the Wengshui volcanic rocks. We ascertain the formation age of the three-stage Pulang porphyries and Wengshui volcanic rock by zircon LA-ICPMS U-Pb dating. Then systematically study on apatites and zircons of the three-stage mineralized porphyries from Pulang include:(1) detailed observation of the characteristics of the inclusions in apatites and zircons;(2) analysis of the major and trace elements composition, SO3, F and Cl content, and Nd isotopic composition of apatite in the three-stage mineralized porphyries from Pulang;(3) analysis of the composition of the melt inclusions in zircons and the elements composition of the apatite inclusions. According to our new data, the character, evolution and deep process of metallogenic magma from Pulang giant porphyry copper deposit are discussed. Together with previous works on geochemistry of Pulang mineralized porphyries and contemporary volcanic rocks in Zhongdian arc, the tectonic setting is analyzed.The following aspects are the innovative understandings of this research:(1) The formation age of Wengshui volcanic rock is 211.1±1.5Ma, and we clarify the spatial and temporal distribution of Indosinian volcanic rock in northern Zhongdian arc. Precise determination the formation age from early to late of Pulang three-stage mineralized porphyries are 218.6±2.1Ma, 218.0±1.9Ma and 217.7±2.1 Ma, respectively; which is consistent with the geological observation. The formation ages of porphyries are consistent with volcanic rock, implying that all of them are formed during the subduction of Ganzi-Litang ocean plate.Pulang porphyry intrusions, northern Wengshui and southern LannitangQiansui-Langdu andesites all belong to high K calc-alkaline or shoshonite and metaluminous rock. They are all characterized by arc-like rare earth and trace elements patterns, such as listric-shaped LREE enriched patterns without significant negative Eu anomalies(Eu N/Eu* = 0.70 ~ 1.05), enrichment in LILE(such as Rb、Ba、Sr、Th、U), depletion in HFSE(such as Nb, Ta and Ti). It’s likely that in Late Triassic, during the westward subduction of Ganzi-Litang ocean plate, the H2O-rich magma of partial melting of the mantle wedge metsomatized by slab fluid and sediment melt, undergoing late differentiation evolution, and then partially erupt to the surface and form Wengshui and Lannitang-Qiansui-Langdu andesites, partially emplace in the crust and form Pulang porphyry intrusions.(2) Zircons of the three-stage mineralized porphyries from Pulang all have high Ce4+/Ce3+ and Eu/Eu* ratios, rich in magnetite mineral inclusion. Apatites of them have anhydrite mineral inclusions and high SO3 content. These characteristics suggest that Pulang ore-forming magma is a highly oxidized magma.Pulang mineralized porphyries have average zircon Ce4+/Ce3+ ratios range of 334 ~ 515, zircon Eu/Eu* ratios range of 0.47 ~ 0.68, and with magnetite mineral inclusions widespread in zircons. In addition, the apatities trapped in zircon have relative high SO3 contents range of 0.20% ~ 1.03%, suggesting that the primary ore-forming magma is a high sulfur and highly oxidized magma. Apatites which contain less fluid inclusion in Pulang quartz-diorite porphyry and granodiorite porphyry have relative high Eu/Eu* ratios(most range of 0.35 ~ 0.74), low Ce/Ce* ratios(0.94 ~ 1.05), high SO3 contents(most range of 0.11% ~ 0.40%), and anhydrite mineral inclusions, also showing highly oxidized magma feature.(3) Apatites in Pulang mineralized porphyries can be classified as apatites rich in fluid inclusions(fluid-rich apatites) and apatites lack of fluid inclusion(fluid-poor apatites), which have obviously different geochemical characteristics. The fluid-rich apatites are probably transformed by late-stage ore-forming fluid, and cannot reflect the nature of the primary magma, while the fluid-poor apatites can record the characteristics of the primary magma. The fluid-rich apatites probably can be used as an indicator for the rock which has a close relationship with porphyry deposit.Apatites in Pulang mineralized porphyries can be classified as fluid-rich apatites and fluid-poor apatites. Quartz-diorite porphyry has two kinds of apatites, apatites in strongly mineralized quartz-monzonite porphyry are all fluid-rich apatites, and apatites in granodiorite porphyry are all fluid-poor apatites. The fluid-poor apatites develop anhydrite mineral inclusions, have relative high SO3 contents(0.11% ~ 0.40%), high Eu/Eu* ratios(0.35 ~ 0.74) and low Ce/Ce* ratios(0.94 ~ 1.05), showing highly oxidized magma feature, which is consistent with the analysis results of zircon. The fluid-rich apatites have no anhydrite mineral inclusions, have relative low SO3 contents(0.01% ~ 0.08%), low Eu/Eu* ratios(0.12 ~ 0.52) and high Ce/Ce* ratios(1.03 ~ 1.15), showing low oxygen fugacity feature of magma, which is contrary to the analysis results of zircon. The elements composition analysis of the early crystalline apatites trapped in zircons show that all of them rich in SO3 contents(0.20% ~ 1.03%), which is consistent with the fluid-poor apatites. Combined with the fluid-rich apatites are mainly developed in strongly mineralized rock, we considered that the fluid-rich apatites are probably transformed by late-stage ore-forming fluid, and cannot reflect the nature of the primary magma. Therefore, the fluid-poor apatites and the early crystalline apatites trapped in zircons can record the characteristics of the magma; the fluid-rich apatites record the metallogenesis of the ore-forming fluid, which can be used as an important mineralogical indicator in search of ore-forming bodies.(4) Pulang ore-bearing magma has generally experienced widespread magmatic immiscibility, the mafic melt and the felsic melt may be mixed in the source region or shallow magma chamber.Si-rich and Fe-rich melt inclusion are widespread exist in zircons from the three-stage ore-bearing porphyry of Pulang. Si-rich melt inclusions have Si O2 content range of 69.5% ~ 82.0%, Al2O3 content range of 8.25% ~ 14.1%, K2 O content range of 0.77% ~ 4.69%, Na2 O content range of 0.02% ~ 2.73% and Fe O content range of 0.24% ~ 2.16%. Fe-rich melt inclusions have Si O2 content range of 2.29% ~ 5.31%, Al2O3 content range of 0.05% ~ 1.77%, K2 O content range of 0.03% ~ 0.40%, Na2 O content range of 0.16% ~ 0.55%, Fe O content range of 83.1% ~ 91.5%, Mg O content range of 0.00% ~ 0.57%.Besides, the evidences of mafic microgranular enclaves(MMEs) developed in Pulang quartz-diorite porphyry, and the distinguishing flat REE pattern and relative high 147Sm/144 Nd ratios(0.1650 ~ 0.2123) of one kind of apatite, also suggesting that two melts from different sources may be mixed in Pulang ore-bearing magma source region or shallow magma chamber. The mixing between mafic melt and felsic melt is considered to be the way for felsic melt to achieve a large amount of sulfur and metal and have a great metallogenic potential. |
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