Mineralogical Characteristics And Genetic Significance Of Skarn In The Cishan Gold Deposit, Tongling

The Cishan Au deposit is located in Shizishan ore-field, Tongling city, Anhui province. The ore bodies mainly occur in skarn which lie between pyroxene diorite rock mass and limestone of Fenshuiling Formation. A comprehensive study has been carried out on the orefield and ore deposit geology, petrography and mineralogy, especially the detailed study of petrography and mineralogy. The difference of mineralogical characteristics between skarn gold deposit and skarn copper deposit in Tongling area have been discussed, with a focus on the analysis of skarn formation and its genetic significance based on the study. with a focus on the analysis of skarn formation and its genetic significance based on the study.The skarns in Cishan ore districts are typical calcareous skarn of contact metasomatic skarn based on field investigation and indoor study found. Skarn to the surrounding rock and rock mass present as gradient zoning characteristics which is the typical zoning phenomenon of contact metasomatic type. The skarn minerals include mainly garnet, pyroxene, epidote, chlorite and actinolite and so on. Electron microprobe analysis suggests that garnet comprises mainly andradite and grossular and pyroxenes are mainly composed of diopside. Components of garnet and proxene are And0~97.42Gro11.40~84.21Other0~12.46 and Di51.53~88.56Hd8.40~45.00Other0.02~0.07, respectively, and their wide compositional variation range suggests that skarns were not formed under a totally enclosed equilibrium condition.On the basis of occurrence characteristics of ore-body and observed mineral assemblages, two periods of ore deposition were divided, i.e., gas-high temperature hydrothermal period and hydrothermal period, which could be further subdivided into five metallogenic stages, namely gas-high temperature hydrothermal includes early skarn stage, late skarn stage, magnetite stage while hydrothermal period comprises quartz-sulfide stage, carbonate stage. In quartz-sulfide stage, on the condition of low oxygen fugacity, low concentration of sulfur, low temperature and alkaline environment, sulfur complex of gold disintegrates and a large amount of gold element precipitates, forming the Cishan skarn gold deposit. So quartz sulfide stage is the main metallogenic stage.Study of skarn mineral assemblage for skarn gold deposits and skarn copper deposit suggests both skarn minerals are mainly garnet, pyroxene, and other secondary minerals are epidote, actinolite, chlorite, quartz, calcite, etc. Garnet is andradite-grossular series, with the difference of their content. Pyroxene is the diopside- hedenbergite series which is mainly diopside. Secondary mineral species is roughly same, but each mineral content is not the same. The main elements of the garnet between skarn gold deposit and skarn copper doposit have similar characteristics and also differences. The pyroxene Mn/Fe ratio of skarn gold deposit and skarn copper deposit are relatively small, mostly less than 0.2. The pyroxene Mg/Fe ratio of skarn copper deposit ranges largely whereas the pyroxene Mg/Fe ratio of skarn gold deposit is relatively stable with small range.