| The Chuxiong Meso-Cenozoic basin is a typical terrestrial red-bed basin in Yunnan province,southwestern China,which is also an important base of coal mines(natural gas as well),copper mines and salt mines.When evolved into Cenozoic era,under the intense tectonic extrusion of Ailaoshan orogenic belt,fault-fold tectonic activities and hypabyssal magma intrusion occurred in the Chuxiong Basin intensely.Fault-fold tectonic activities resulted in the tectonic-reworked mineralization and Cu polymetallic enrichment to the sandstone-type copper deposit.While,the alkali-rich porphyries leaded to porphyry-type Cu-polymetallic deposits,such as the Zhiju Cu-Mo deposit and the Yao'an Pb-Ag-Au deposit.They were both the metallogenic effects to the Chuxiong basin impacted by Himalayan orogeny.Aiming at the sedimentary-diagenesis,tectonic-reworked effect and magma intrusing activities during the evolution of the Chuxiong basin,also based on the metallogenetic timing,space,and substance relationship between the sandstone-type and porphyry-type copper deposits,this article achieved at conclusions as follows:(1)Two typical types of mineralization occurred in sandstone-type copper deposits:sedimentary-diagenesis type and tectonic-reworked type.The former was represented by the Liuju deposit,mainly forming disseminated and laminated ores in stratiform orebodies.This type of mineralization was controlled by the structural plane of sedimentary tectonics:slope belt of basin margin,unconformity,permeable sandstone layer.The Mouding Copper Belt,orebody groups and orebodies was controlled respectively by these three structural plane of sedimentary tectonics.When the reducing and oxidizing brime migrated and convected in these three spatial structures above,reducing-oxidizing,physico-chemical transitional planes formed respectively in the transition zone from purple to grey sandstone,resulting in the mineralization during the diagenesis period.The tectonic-reworked mineralization was represented by the Haojiahe deposit,mainly forming banded and veined ores in/nearby the fractures and joints.This type of mineralization was controlled by the fault-fold tectonic system,involving three types of ore-forming structural plane:contemporaneous faults,folds and faults.Liuju and Haojiahe deposits has experienced four periods of tectonic process,the principal compressive stress directions were successively:NE-SW?EW?NW-SE?SN.(2)The geochemical research of sandstone-type copper deposits suggested that the precipitation of sulfophile elements,such as Cu,Ag,Pb,Zn,Mo etc.,were controlled by reducing barrier.These mineralized elements enriched in grey sandstone strongly,especially in the transitional zone from purple to grey sandstone,or in the tectonic hydrothermal grey sandstone.The ore-forming fluid in diagenesis period was characterized by low temperature(90?160?),medium low salinity(515 wt%NaCl),riched in SO42-,Ca2+ and CH4,etc..The source of ore-forming fluid was dominated by organic degraded water;while the sulfur derived mainly from the sulphate thermal reduction,containing a small amount of bacteria reductive sulfur;Isotope tracers of Pb and Sr show that the source of metallic elements of diagenetic mineralization was dominated by crust-source,mainly came from the sandstone wallrock.The ore-forming fluid in reworked period was characterized by relatively high temperature(140?240?),relatively low salinity(2?13 wt%NaCl),riched in Cl-?Na+ and a small mount of CO2,and was the mixed fluid of magmatic thermal fluid,organic degraded water and meteoric water.The sulfur was characterized mainly by the deep-source,partly from the sulphate thermal reduction.The metallic elements of reworked mineralization came from the basin basement,partly from the sandstone wallrock.The geochemical features above show that the ore-forming fluid,sulfur and metallic elements in reworked mineralization possessed the characteristics of magmatic source.The formation of sandstone-type copper deposits experienced sedimentary-diagenesis process and tectonic-reworked process.The diagenetic mineralization occured during the middle stage of diagenetic period.Acidic and reduced hydrocarbon fluid,which derived from the grey sandstone,expanded and migrated along the permeable sandstone layer.Cement minerals,such as carbonate,iron and gypsum etc.,were dissolved and reduced by the hydrocarbon fluid,which could form the secondary intergranular pore and provide the reduced sulfur.Ore mineral precipitation was resulted from the convection and interaction between the alkaline oxidized copper-bearing fluid and acidic reduced hydrocarbon fluid.The ore-forming fluid in reworked period,driven by tectonic compression and overthrust effect,could leach metallic elements from basement and sandstone wallrock,when moving along the contemporaneous faults.Therefore,reworked metallogenesis was the result of the deep-source,copper-bearing and hydrocarbon fluid trapped by favorable structural space(such as core of anticline,secondary fractures of contemporaneous fault,joints etc.),which leading to precipitation and mineralization when mutation happened to the metallogenic condition.(3)Not only did fault-fold tectonism and tectonic-reworked mineralization of sandstone copper deposits occur in the Chuxiong Basin after Cenozoic,but also the alkali-rich magma intrusion and porphyry Cu-polymetallic metallogenesis happens,such as the Zhiju porphyry Cu-Mo deposit.Zhiju deposit occurred in a set of porphyritic monzonitic syenogranite.From the inside intrusion out,four types of alteration occurred successively:K-silicate alteration,sericite-quartz-pyrite alteration,sericite-chlorite propylitic alteration and epidotic hornfelsic alteration.Corresponding to the alteration zone,four types of mineralization occurred:K-feldspar-quartz-molybdenite type,quartz-pyrite-molybdenite-bornite-chalcopyrite type,chlorite-quartz-bomite-chal cocite type and epidote-quartz-molybdenum-chalcopyrite-chalcocite type.The Daxueshan anticline and the northwestward transtension-extensional faults were the controlling structure of granite intrusion;while the northwestward faults and joints were ore-control structure as well.Syenogranites with porphyroid texture(porphyritic texture partly)which formed in 30.7±2.5 Ma,was the main metallogenic porphyry body.The metallogenic age of the Zhiju deposit was 31.28 ±0.39 Ma.The metallogenic granitic body is the typical alkali-rich porphyry.It belongs to weakly peraluminous shoshonictic A-type granitic series(SiO2 = 64.08%?68.61%;K2O+Na2O=8.27?10.78;K2O/Na2O=1?2.29;A/CNK=1?1.21).The granitic porphyries in the Zhiju orefield are generally enriched in LILEs,such as Rb,Ba,Th,U,K,Sr and so on;but depleted in HFSEs,such as Nb,Ta,Zr,Hf,P and so on.Meanwhile,the REE distribution patterns belong to LREE enrichment type.All the geochemical traits imply that the Zhiju porphyry intrusions were adakite-like granites.The original ore-forming fluid was magmatic hydrothermal fluid with high temperature(to the maximum of 531.7?)and high salinity(to the maximum of 64.15 wt%NaCl).It was also riched in metallic elements and mineralizer(such as CO2,Cl-,etc.).Copper and molybdenum sulfides precipitated when ore-forming fluid phase separation happened under the conditions of fluid boiling,mixing.Isotope tracer of He-Ar shows that the fluid was dominated by crust-source,containing a small amount of mantle-source.And so was the metallogenic material.(4)As the Chuxiong Basin evolved into Cenozoic era,under the intense geological process of tectonic compression,magma intrusion and hydrothermal process,two types of copper metallogenesis occurred in the favorable geological environment:?As the granitic magma intruded into favourable depth and structural environment,the temperature and pressure conditions change could lead to the hydrothermal differentiation,boiling and ore mineral precipitate to form the porphyry copper deposits;? As the position of magmatic emplacement was too deep to form porphyry deposit,the magmatic hydrothermal fluid would migrate upward along the faults.The upward magmatic hydrothermal fluid,bearing metallogenetic elements and mixing with the basin brime,migrated to the tectonic place which was favourable for tectonic-reworked mineralization of sandstone copper deposits.The timing of magma-hydrothermal mineralization of porphyry deposit(occurred in 35?30 Ma)was approximately close to the reworked mineralization age of the sandstone deposits(occurred in 50?30 Ma).Both of two types of copper matallogenesis were controlled by the NW-SE compressive tectonic system in the Oligocene epoch.Evidences from trace elements,ore-forming fluid and isotope tracers showed that two types of copper matallogenesis had intimate genetic connections between sandstone and porphyry copper deposit.In a word,both of the tectonic-reworked mineralization of sandstone Cu deposit and the hydrothermal porphyry Cu-Mo deposit formed under the same tectonic-magmatic-hydrothermal matallogenesis system,that is the metallogenesis in late-collisional transformation setting of Tibetan collisional orogenic belt. |
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