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Ore-Controlling Structure of Orefield and Ore-forming Chemical Kinetics of Mineral Assemblage Area in Tongling

Author: LiJinWen
Tutor: PeiRongFu;ZhangDeQuan;ZhangRongHua
School: Chinese Academy of Geological Sciences
Course: Mineral prospecting and exploration
Keywords: Tongling mineral assemblage area Tectonomagmatic activity Structure of orefield Ore-forming chemical kinetics Ore-forming fluid Formation and evolution of the mineral assemblage area Concentration regularity of copper deposits
CLC: P617
Type: PhD thesis
Year: 2004
Downloads: 445
Quote: 5
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Abstract

The Tongling mineral assemblage area, situated in Anhui province, is a large one in theMiddle-lower Yangtze River metallogenic belt. The recent researches showed that there are closetemporal, special and genetic relations between the copper（gold, sulfur and iron） deposits and theintermediate-acid intrusive rocks. The rocks intruded from 148.20±2.13 to 134.35±3.03Ma and ingranodiorite→quartz monzodiorite→pyroxene monzodiorite order, and controlled by the structuresof basement and covers mainly distribute along the Tongling-Shatanjiao tectonomagmatic belt.They consist of alkaline series and sub-alkaline series, but both the series have no evolutionalrelation. The original magma of the high-K talc-alkaline series was mainly partial melting materialof old metamorphic basement rocks, and mixed by a little basaltic magma of mantle. The primalmagma of the alkaline series was alkaline basalt which originated from the enriched mantle andunderwent assimilation, fractionation and crystallization. Delamination of lower crust orlithospheric mantle followed by thermal mantle moving up brought directly on formation of theintrusive rocks. The magma activities are the indication of tectonic setting changing fromcompression to extension, and the large-scale copper（gold, sulfur and iron） ore-forming processoccurred in the extensional tectonic setting. The intrusive rocks develop structures of "intrusivebodies in body". The multiple intrusive activities have co-magmatic complementary differentiationwhich may be beneficial to mineralization. The cooling rates of the intrusive bodies are commonlycounter-correlation to mineralization.The metallic deposits in Tongling could be classified into three main types: hydrothermalvein-type, skarn-type and stratabound skarn-type, and these deposits are controlled by differentstructures. According to the ore-controlling characteristics, the structures of orefield in the Tonglingmineral assemblage area were classified into "row-line-cluster" pattern, contact structure system ofmagma intrusion and "depression in swell". They occur respectively epizone（0-1km）,mesozone（1-3.0km） and hypozone（＞3.0km） in the space so as to form a vertical zoning of thestructures of orefield. The three zones of ore-controlling structures control respectivelyhydrothermal vein-type deposits, skarn-type deposits and stratabound skarn-type deposits. May besingle or both, or maybe all of them that form a integrate vertical zoning of the structures of orefield,the epizone, mesozone and hypozone may combine different formats. The vertical zoning of thestructures of orefield is the main cause controlling the metallogenic assemblage and multistorymineralization. The structures of orefield mainly went through three stages: formation anddevelopment of"depression in swell" in the Hercynian; formation of the folds and interlayer glidingstructures in the Indosinian; development of interlayer gliding structures, formation anddevelopment of contact structure system of magma intrusion and "row-line-cluster" pattern in theYanshanian.Three types of fluid inclusions, namely gas-rich, liquid-rich and daughter mineral-bearingpolyphase brine inclusions were distinguished in quartz, calcite and garnet. The temperature ofhomogenization of fluid inclusions ranged from 128℃to＞570℃. The salinities of three types ofinclusions varied remarkable from 1.07 to 60.72 wt％NaCI equivalent and the halite-bearinginclusions with high salinities from 30.27 to 66.75 wt％NaCl equivalent. Ore-forming fluid of the skam mineralization might behave in a supercritical state in the early stage, and occurredimmiscible separation or boiling of fluid in the major mineralization stage. The changingcharacteristics of the temperature of homogenization and salinities of the fluid inclusions showedthat the mineralization of hydrothermal vein-type appreciably lagged the skarn mineralization. Thecomponents of fluid inclusions are mainly H₂O, CO₂, N₂, Na⁺, K⁺, Ca²⁺, Mg²⁺, SO₄^2-, Cl^-, and alittle CH₄, He, Ar, O₂, C₂H₆ and H₂S. Only a few samples contain F^-. It was suggested that contentsof CO₂, O₂, Ca²⁺, K⁺, SO₄^2-, Cl^- and F^- in the ore-forming fluid might control the metallogenic size.Changing from reduction increasing to dropping, ore-forming fluid displayed a dynamicevolvement from early stage to late stage. Reduction increasing is beneficial to deposit of thesulfides. Ore-forming fluid mainly originated from magma water, and it contained someore-forming elements like copper and so on in the primitive magma-derived fluid. Ore-formingelements in the fluid were gradually increasing in the process of the interaction between magma andfluid.The experimental studies on kinetics of water-rock reaction showed that slowly flowing fluidbenefited ore-forming elements like Cu, Mo, Zn and so on dissolving from rocks, and thewater-rock reaction rate was commonly counter-correlation to flow rate of the fluid and correlationto holdup time of the fluid. The water-rock reaction rate varied in a non-linearity state. Copper inore-forming fluid might migrate in the supercritical state, and deposited under critical state. As agreat deal of zinc in rocks still released into the fluid at 350℃so that it always make up spatialzone for ore-forming elements like Cu, Zn and so on.Based on isotopic ages of the deposits, it’s considered that metallogeny in Tongling mineralassemblage area had undergone the Hercynian （from 313.2±32.7 Ma to 290±10Ma） and theYanshanian （from 140.3±1.7Ma to 134.2±3.9Ma）. The major metallogenic period belong to theYanshanian, and ore-forming material mainly derived the intermediate-acid intrusive rocks. It wasestimated that about 10 percent ore-forming material were provided for the stratabound skam-typedeposits by the Hercynian primal sedimentary source bed. The hydrothermal mineralization relatedto intrusive rocks was predominated by magma-derived fluid in the early stage and meteoric waterin the major and late ore-forming stage. It’s vary important that the ore-forming fluid derived fromdeep magma chamber complemented the large-scale copper-gold mineralization.Based on the evolvement of regional tectonics, applying the concept of evolutionarymetallogeny, the ore-forming evolutionary history of Tongling mineral assemblage area wassummarized. The relation between interior structure of Tongling mineral assemblage area andgeochemical blocks had been studied, and the metallogenic intensity of copper in the Yanshanianhad been evaluated based on the research on the relative abundance of the ore reserve and the timelimit of metallogenic processes. In the final, the prospecting direction for the copper deposits hadbeen pointed out.

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