Mineralogical Characteristics and Geological Significance of Skarn in the Saishitang Copper Deposit, Xinghai County, Qinghai Province

The Saishitang copper deposit is an important skarn deposit located in the Southeast of Ngola Mountain Area, Eastern margin of East Kunlun orogenic belt. The skarn occurs along the contact between Indosinian quartz diorite and middle-lower Triassic Tb2 1-2 lithologic section. The ore bodies which are stratoid or lenticular mainly occur in exoskarn. The Tb2 1-2 lithologic section consists of intermediate volcanic rocks, marble and metamorphic siltstone. Cu mineralization is adjacent to the metamorphic andesitic tuff and andesite spatially. Petrography studies of Cu-bearing skarn show four stages of skarn formation and ore development, including skarn stage, retrograde alteration stage, quartz-sulfide stage and quartz-carbonate stage. Skarn stage consists of garnet, pyroxene and wollastonite and retrograde alteration stage dominates by epidote, amphibole and magnetite. In quartz-sulfide stage a large amount of sulfides were precipitated. Electron microprobe analysis shows that components of garnet and pyroxene are Gro0.00~91.00And7.02~100.00(Pyr+Alm+Spe)0.00~4.27 and Di12.80~91.75Hd2.41~79.80(Jo+Jd+Opx)0.00~13.47, respectively. The result indicates that the skarn in Saishitang deposit belongs to typical calcic skarn. High concentrations of grossular and epidote are found in skarns located closer to the contact between quartz diorite and andesite. Andradite is richer near the marble with wollastonite and Mn-bearing hedenbergite. Mineralogical characteristics and compositional variations suggest: from skarn stage to quartz-sulfide stage, the property of fluid changed episodically with two redox fluctuations at least, probably resulting from the influx of meteoric waters in ore-forming fluid. Saishitang copper deposit is a skarn deposit. The quartz diorite-dominated intrusions carrying plenty of heat and fluids intrude into middle-lower Triassic strata. The materials exchange between intrusions and wall rocks may be the mechanism of the development of skarn and ore bodies in Saishitang deposit, accompanying the bimetasomatism between marble and metamorphic siltstone and intermediate volcanic rocks.