Studies On Material Composition And Genesis Of The Shangwang Gold Deposit,Shaanxi Province

The Shuangwang gold deposit is located in the Feng-Tai ore-centralization area in the Q inling Orogen and is hosted by the albitite breccias in the Upper Devonian Xinghongpu Formation. This study focuses on the geologic characteristics, mineral components, lithogeochemistry, isotopic geochemistry and geochronology of the Shuangwang gold deposit and discusses the gold metallogenetic age and the sources of ore-forming fluids and materials.The minerals related to the hydrothermal mineralization in gold ores can be divided into five categories:(1) native element and intermetallic minerals, including native gold, native bismuth and maldonite;(2) sulfide and sulfosalt minerals, including pyrite, chalcopyrite, arsenopyrite, pyrrhotite, pentlandite, tennantite and gersdorffite;(3) silicate minerals, including albite and dravite;(4) carbonate minerals, including ankerite and calcite;(5) oxide minerals, including magnetite, hematite and chromite. Pyrite is the most important gold-bearing mineral which can be divided into four generations.The magmatic rocks in the deposit are mainly the Xiba granodiorite, monzonitic granite, granite porphyry, spessartine and imandrite. The granitoids belong to high potassic calc-alkaline series(total alkali contents = 6.95~9.11%) and are of quasi-aluminous to peraluminous intermediated-acidic magmatic rocks(A/NK = 1.19~1.66, A/CNK = 0.93~1.11). The spessartine belongs to calc-alkaline basalt series(CaO content = 5.84~6.28%, total alkali content = 4.71~5.28%), the imandrite belongs to calc-alkaline quasi-aluminous series(total alkali content = 6.94%~8.44%,A/CNK = 0.93~1.09).The zircon U-Pb dating results indicate that the emplacement ages of Xiba granodiorite and monzonitic granite are 221.8±1.2Ma and 214.8±1.2Ma, respectively; the ages of granite porphyry outcropping in No. I breccia and Jiupinggou are 219.9±1.5Ma and 213.1±0.89 Ma, respectively; the ages of spessartine and imandrite veins are 214.4±2.7Ma and 213.0±1.5Ma, respectively. Two periods of magmation in the late Indosinian were identified: 219.9~221.1Ma and 213~214.8Ma. The zircon Lu-Hf isotopic values show that the calculated two-stage model ages(tDM2) are 1373~1634Ma, yielding a gap of 1100~1300 Ma between tDM2 and emplacement ages. This suggests that the source was the Mesoproterozoic basement crustal.During the late Indosinian, the tectonic setting transformed into extension stage of the Q inling intercontinental collision, resulting in the formation of crust-sourced granitoids. The magmation provided part of the ore- forming fluids and materials for the formation of the Shuangwang gold deposit, which contributed to the existing of Au in fluids. The ore-forming fluids were of the mixture of magmatic water and formation water, and the ore- forming materials had the characteristics of both crust and mantle sources. The ages of spessartine and imandrite veins(214.4±2.7Ma and 213.0±1.5Ma, respectively) cutting into breccias and orebody limit the ceiling age of gold mineralization.