Characteristics And Genesis Of The Chaoshan Skarn Gold Deposit, Tongling, Anhui Province

The ore-forming processes of gold skarn deposits have been one of the hottest topics in thefield of economic geology. The middle and lower Yangtze River Valley is the most importantmetallogenic belt of gold (copper) skarn deposits in China. The Chaoshan deposit is a typicalgold skarn deposit of this belt. The orebodies of this deposit have been buried in shallow depthand the ores generally have high grades. Because the ore-forming process was relativelycomplex, this deposit is of great value for genetic research. On the basis of the formerresearchers, the author has carried out petrographic, mineralogical and geochemical research onthis deposit. The following achievements have been obtained in this dissertation.The Baimangshan pyroxene diorite, which is the mother rock of the Chaoshan deposit,belongs to high-K and metaluminous igneous rocks and was formed in a postcollisional tectonicenviroment. The mother magma of the Baimangshan pyroxene diorite was alkalic basalticmagma which came from enriched lithospheric mantle. This magma underplated into the lowercrust, which resulted in the material exchange between the magma and the lower crust as well asthe formation of a magma chamber in that depth. Amphibole and clinopyroxene phenocrystswere formed just in this magma chamber. There is a lot of magnetite in the Baimangshanproxene diorite, which means that it is a oxidized pluton.There are seven main mineralization stages, which are contact thermal metamorphism stage,dry skarn stage, wet skarn stage, quartz-sulfide stage, carbonate-sulfide stage, sulfate stage andsupergene oxidization stage, in the formation of the Chaoshan deposit. Reaction skarns wereformed in the contact thermal metamorphism stage. The main mineral formed in the dry skarnstage is garnet, which is accompanied by minor diopside, scapolite and wollastonite. The mainminerals formed in the retrograde alteration stages (from wet skarn stage to sulfate stage) areepidote, actinolite, vesuvianite, chlorite, quartz, calcite, sericite and phlogopite. Goldmineralization had a close relationship with retrograde alteration, especially quartz and calcite.There are more garnet than clinopyroxene in dry skarns. The garnet cores generally have lowiron contents but the garnet rims generally have high iron contents. Clinopyroxene generally hasvery low iron contents. These characteristics all indicate that the Chaoshan deposit is an oxidizedgold skarn deposit.The characteristics and variations of mineral associations, mineral chemistry and whole rock chemistry in the alteration zone indicate that Si, Al, Fe, Mg, Na and K were mainly from themagma and Ca was mainly from the the carbonate in the strata. Stable isotope contents of theores of the Chaoshan deposit indicate that carbon and water of the ore-forming fluid was mainlyfrom the magma and sulfur was from the magma and anhydrock in the strata.The ore-forming fluid of the Chaoshan deposit changed from relatively acidic and reducedto relatively alkalic and oxidized. This change, which was caused partly by the boiling of thefluid, benefited the deposition of gold. Gold was carried in the form of sulfur complex in thefluid and was deposited at relatively low temperatures.