Multiple Orogenic Processes And Mineralization Of Orogenic Gold Deposits In The East Kunlun Orogen, Qinghai Province

This paper did some research on tectonic evolution in the East Kunlun orogen, Qinghaiprovince and characteristics of geology and geochemistry of gold deposits in this region. Someimportant conclusions have been reached. The East Kunlun area is a multiple orogen whichunderwent a very complicated evolutionary process and is characterized by archipelagic ocean/aulacogen, soft collision and multicyclicity. Many gold deposits in this area have similarcharacteristics of both geology and geochemistry, and are correlated to orogenic processes of thisorogen in genesis. As a result, they should be typical orogenic gold deposits. Each deposit iscontrolled strictly by three-order structural systems which are profound faults and/or collisionalbelts, large-scale ductile shear belts, and folds and faults, respectively.Although the wallrocks ofthese orogenic gold deposits vary in age and petrology, all of them were deformed andmetamorphosed into greenschist facies. According to ore types, the orogenic gold deposits can bedivided into two subtypes, namely altered rock type and quartz vein type. The pyritic-phyllicalteration is the most extensive and typical alteration. With evolution of the structures in oredistricts from ductile, ductile-brittle to brittle, the gold deposits also underwent differentmineralization phases and ore-forming stages.Based on a great deal of studies on fluid inclusion petrography and microthermometry of theWulonggou, Kaihuangbei, Dachang and Dongdatan orogenic gold deposits, the quartzrepresenting different ore-forming stages contain three main types of fluid inclusions: (1)primary CO₂-rich inclusions, (2) CO₂-H₂O inclusions with variable CO₂ content, (3) aqueousinclusion. The CO₂-H₂O inclusions are very plenty and content of CO₂ is comparatively high (≥5mol%) in this kind of inclusions. The ore-forming fluid is a type of middle-low temperature(118～378℃), low-salinity (0.35～9.54wt%NaCl) H₂O-CO₂-NaCl-CH₄±N₂ system. Thedeterminations of hydrogen and oxygen isotopes and studies on water-wallrock exchange suggestthat the distribution of hydrogen and oxygen isotopes of different stages of ore-forming fluid ineach deposit is very consistent with corresponding meteoric water evolutionary line, withtemperature reducing gradually and W/R ratio increasing correspondingly from early to late. Allabove inform us that the pre-ore fluids mainly originated from metamorphic and formationwater, but since the mineralization episode meteoric water joined and predominated gradually. Inaddition, carbon isotopic measurements of fluid inclusions also show us the meteoric water is veryimportant for mineralization. The unmixing of a homogeneous CO₂-H₂O parent fluid is the mostimportant mechanism that caused some ore-forming substance such as gold, silver and antimony, metallic minerals and quartz to be enriched and deposited. The major evidences forfluid unmixing include the CO₂-H₂O inclusions with a wide range of CO₂ content coexistingintimately in the same quartz, their similar homogenization temperatures and salinities.Isotopic geochemical studies show thatδ³⁴S values of different sulfides aren't big and can begrouped into 1.1～6.9 per mil for Wulonggou and Kaihuangbei gold deposits and -6.3～-0.9 permil for Dachang and Dongdatan gold deposits. The pyramidal distribution is very apparent. Thelead isotopic compositions vary in a small range, but the values are very high, therefore they areradiogenic lead. In the Zartman's (1981)tectonic diagram about lead isotope, all lead isotopic dataare put together near evolutionary line of orogen. It indicate that the ore-forming material wasderived from mixed little mantle and wall rocks. Leaching radiogenic lead from highly deformedwallrocks by predominant meteoric water is a major reason to make lead isotopic compositionbecome very high. According to field observation and isotopic dating, the main age of goldmineralizations is late Variscan-Indosinian. In addition, the mineralization of late Caledonianexisted in some deposits as well.In summary, intensive subduction and collision during late Caledonian and late Variscan-Indosinian not only formed deep faults, large-scale shear belt, and low-order folds and faults butalso make ore-forming fluid activated and mineralizing elements enriched initially. Subduction-related thermal events and episodically rising geothermal gradients drove formation water andmeteoric water migrate for long distance along collisional belts and large-scale shear belt andleached ore-forming substance from wallrocks during migration, and gold-bearing fluid wereformed. When ore-forming fluid flowed into ore-controlled structures, gold and antimony arelargely deposited as temperatures and pressures fall accompanying fluid immiscibility during themain mineralization stages.