Lithogeochemistry And Mineralogical Mapping Of The Dongyaozhuang Gold Deposits In Wutai County, Shanxi Province

Dongyaozhuang gold deposit located in Wutai County of Shanxi Province is a new type of greenstone gold deposits. Its ore-forming age is 2451 Ma±. It occurs in Hongmenyan formation of Wutai Group. Sericite albite (quartz) schists and albite chlorite schists are the ore-hosting rocks. Gold mineralizations are controlled by occurring of sericite albite (quartz) schists on ore-hosting rocks and strictly by Taihuai-Lijiazhuang homoclinal reversing compound syncline and Lijiazhuang-Dacaoping ductile shear zone on strcture. Therefore it is also a typical of shear zone-type gold deposit. Study on mineralogy, petrology, mineral deposit geology and determining prospecting criteria of such type gold deposit have great significance to prospecting they in Wutaishan greenstone belt.This paper comes down to three aspects, including the lithogeochemistry characteristics of ore-hosting rocks and fluid inclusions in quartz of ores, and mineralogical mapping by which the relationship between altered rock mineral and gold deposits is studied. Primary understandings achieved as follows:1. Trace element analysis shows that the metamorphic rocks are characterized by LREE enrichment, HREE loss and weak positive Eu anomaly. Restoring the original rocks indicates that both of chlorite schists and sericite albite (quartz) schists are volcanogenic, and thay are belong to calc-alkaline and high-K calc-alkaline series. The original rocks of chlorite schists and albite (quartz) shoud be andesite-basalt and rhyolite-dacite respectively.2. The fluid inclusion in quartz of ores can be classified into V-L inclusion, V inclusion, CO2 inclusion and silicate melt inclusion. V-L inclusion is the major inclusion type. They are large in 2¹2μm mostly. The histogram of homogenization temperature is nearly normal muti-peak distributed. There are four peak value intervals of homogenization temperature which are 360⁴00℃, 280³40℃, 240²60℃, 100²20℃respectively. The fourth intervals is the major mineralization stage. The major liquid composition of quartz fluid inclusion is Mg²⁺, Na⁺, SO₄^2-, Cl^-, small K⁺, Ca²⁺, F⁺. The gas composition of quartz fluid inclusion is H₂, N₂ and CO mostly. The ore-forming fluid comes from tectonic-metamorphic hydrothermal fluid.3. Altered minerals are idetified by the Portable Infrared Mineral Analyzer (PIMA). The result suggests that muscovite (or sericite) and chlorite are two speciese of the most widely distributed altered minerals. The spatial distribution and the content of altered minerals have regularity and largely are controled by lithological characters. The alteration zone is obvious., muscovite (in particular muscovite of high crystallinity degree and low intensity of interlayer water), illite and tourmaline have close relationship with gold mineralization. The magnetic susceptibility of altered rock indicates that, the susceptibility values of various kinds of typical altered rocks are in decreasing order as follows: magnetited rocks, chloritization rocks, phyllic rocks, silicification -tourlinization rocks, silicification -carbonatization rocks. The values of susceptibility tend to be lower where near the gold ore body. The magnetic susceptibility has negative correlation with gold grade.By comprehensive analysis of the results of the study, the author propose several prospecting criterial as fllows: a. the vicinity of the ductile shear zone; b. belt of sericite albite (quartz) schists which occur among belt of epidiabase; c. tourlinization+ pyritization+silicification development zone; d. the emergence of sericite and chalcopyrite; e. aera of low magnetic anomaly, in particular the low-value anomaly area among the high value abnormal area.