Characteristics Of The Pay Rocks In Laowangzhai Gold Ore Deposit And Its Metallogenetic Mechanism

Ailaoshan gold belt is one important metallogenetic belts in China. The Laowangzhai gold ore deposit lies in folded zone between Yangtz plate and Indian which is controlled by three deep faults: Jiujia-Mojiang fault on the west, Ailaoshan fault in the middle, and Honghe fault on the east. Geological setting studies suggest that, after deformation and metamorphism of Luliang movement, Ailaoshan gold belt experienced intracontinental subduction, main collision orogenesis and then was overprinted by the Himalayan orogenic movement. Active crust-mantle process, complicate tectonics and special geologic features seem to tell us that mantle fluid and structures play a critical role in the ore-forming process. We choose the Laowangzhai gold ore deposit as the subject investigated, based on the studies of microscopic characteristics of the rocks (ores), as well as major elements , trace element and REE of rocks (ores), and ore-controlling structure in addition, the crust-mantle overprinting metallogenetic process is acknowledged.There are ultrabasic rocks in linear distribution along the Jiujia-Mojiang fault which is the first order structure controlling rocks occurrence and channelizing metallogenetic materials. All the secondary structures are reversed faults on the upper blocks, striking NE-SW with steep top and subdued bottom. The third structures are ore-bearing faults which control the shapes, attitudes and sizes of the ore bodies. The special structural frameworks provide excellent geologic conditions for ore-forming: most ore-bodies occurrence in EW reversed faults and NW shear zones; furthermore, the intersections of faults are the best ore-forming space.The wall-rocks of Laowangzhai gold ore deposit are altered ultrabasic rocks, altered basic volcanic, altered lamprophyres, carbonaceous slates (phyllites), metamorphic quartzwackes, and altered porphyries and so forth. Microscopic researches suggest that carbonatation, sulfidation, silicification and pyritization have a close relationship with the gold-mineralization. The stronger the alterations and the more structures, the stronger the gold-mineralization, especially polystage altered overprinting. Along the crannies and cleavages in pyroxenes, black opaque submicroscopic material develop, which is confirmed through the electron microprobe and scanning electron microscope as well as energy spectrum analsys. The black opaque material consists of submicroscopic quartz, silicate and carbonate, as well as sulphide, rutile and specularite in liquation-intergrowth state. The fact that black opaque submicroscopic material almost syngenesis with minerling alterations (slication, carbonation and sulphidiation), implicates that black opaque submicroscopic material may be the vestiges of mantle fluid which initiates minerling alterations.Through geochemical studies of rocks (ores) and veins, it is concluded that:(1) SiO2 content of the altered quartz sandstone, (stibium) quartz porphyry, carbonaceous slates and silicificated lamprophyres are higher than other types of wall-rocks. Alkali content of most wall-rocks increase after alteration.(2) LILE of rocks (ores) are enriched with the shape of"hump", and Th are conspicuously enriched while Ta relatively depleted. ILE and HSFE contents of wall-rocks→mineralized rocks→ores change from weak enrichment to conspicuous enrichment, while Ni and Cr change from weak discrepancy to relatively strong discrepancy.(3) REE distribution pattern seems to be right deviation with LREE enriched, and the assemblage of Eu and Ce anomalies are different as to the wall-rocks and ores, which suggest that wall-rocks are not the source of metallogenic fluid come from, as least not the main source.?We propose that conspicuously enrichment of REE is a important sign of mantle fluid participating in the metallogenic process, which implicates that the ore-forming process is probably under the control of mantle fluid rich in REE. Combined with the structure studies, it is believed that during the alteration process of mantle fluid, the crust-mantle contamination and overprint take place.