安徽冬瓜山矽卡岩铜矿石榴石成分特征及其成因探讨

对安徽铜陵冬瓜山铜矿石榴石的地球化学特征进行了研究,并进行了成因探讨。野外调查及镜下观察发现,冬瓜山铜矿石榴石分为两期形成,第Ⅰ期石榴石环带发育,颜色较深,呈褐-棕黄色;第Ⅱ期石榴石呈他形-半自形穿切Ⅰ期,颜色较浅,呈浅黄-蜡白色,具非均质性。石榴石主量、稀土元素的等离子光谱(LA-ICP-MS)分析结果显示这两期石榴石有较大差别:第Ⅰ期石榴石钙铁榴石组分含量较高,可达94.14%,稀土元素配分模式为轻稀土元素富集、Eu正异常的右倾曲线;第Ⅱ期石榴石则相对富铝,钙铝榴石组分含量达44.06%,稀土元素配分模式为重稀土元素略微富集、Eu负异常的平缓曲线。这些特征表明,第Ⅰ期石榴石为岩浆成因,形成于较氧化环境;第Ⅱ期石榴石为热液交代成因,形成于较还原环境。

The composition characteristics and origin of garnets in the Dongguashan skarn copper deposit, Anhui Province

Located in northern Shizishan orefield of Tongling area,Anhui Province,the Dongguashan skarn copper deposit is a typical stratabound skarn copper deposit in China.Garnet is the main mineral in the skarn system,whose content is up to 95%.According to the garnet geochemical features of the study area,this paper intends to provide evidence of its origin.Based on field survey and microscope observation,the authors have found that garnet in the Dongguashan copper deposit can be divided into two stages: stageⅠ garnet has well-developed girdle,and its color is dark brown;stageⅡgarnet cuts the first,is anhedral-subhedral in shape,and has heterogeneity,with its color being light pale yellow-wax white.The major elements and REE compositions of the two phases of garnets were analyzed by plasma spectrometry(LA-ICP-MS),and the results show remarkable differences between them: the andradite component of stageⅠgarnet is higher,which is up to 94.14%,its REE distribution is characterized by rightly-oblique curves of enriched light rare earth elements(LREE) with positive Eu anomaly,while stage Ⅱ garnet is Al-rich,its grossular component is up to 44.06% and its REE patterns display a tendency of slight heavy rare earth elements(HREE) enrichment and strong negative Eu anomaly.These features indicate that the first stage garnet is of magmatic genesis and was formed in oxidizing conditions,whereas the second stage garnet is of hydrothermal metasomatic genesis and was formed in a reductive environment.