The Yangcun talc deposit in Guangfeng County of Jiangxi Province is one of the superlarge talc deposits in the world. The ore reserves are over a hundred million tons. The talc ore consists mainly of oolite granular talc with a small amount of quartz, carbonates and disseminated organic matter, assuming typical sedimentary oolite granular structure. The wall rock dolomite is strongly silicified, and consists mainly of dolomite and quartz. The talc ore and dolomite show similar REE patterns, with low REE content, weakly negative Ce anomaly, and weak enrichment of HREE in relation to LREE. They also show similar Sr isotopic compositions, with 87Sr/86Sr rnratios of 0.7092～0.7101 for both talc and dolomite. The dolomite has δ 13CV-PDB values of -4.9‰ to 0.7‰, similar to the range of the contemporaneous seawater. These data indicate that the magnesium in the talc ore and dolomite probably shared the same source and might have originated from the seawater. However, some of the samples also show positive Eu anomalies, lower δ 18Ov-SMow(14.5%o～19.7%o), and slightly higher 87Sr/86Sr values than the contemporaneous seawater values, and these characteristics may suggest an additional source besides seawater that contributed to the formation of talc ore. The authors hold that the required silicon in talc was probably provided by silica-rich hydrothermal fluid with slightly higher 87Sr/86Srvalues than the seawater. It is considered that the Yangcun talc deposit may have a syngenetic hydrothermal sedimentary origin. Magnesium was mainly derived from magnesium-rich seawater and silicon was mainly derived from silicon-rich and high 87Sr/86Sr hydrothermal fluid which was formed by seawater infiltrating and leaching of the old crustal strata. After synsedimentary deposition, talc ores also underwent strong metamorphism and deformation, which formed talc schist with typical schistosity structure.%江西广丰杨村滑石矿储量达上亿吨,是世界上超大型滑石矿床之一.矿石主要由鲕粒状滑石组成,含少量石英和碳酸盐矿物及浸染状有机质,具有典型的沉积鲕粒状结构；赋矿围岩白云岩硅化较强烈.研究结果表明,白云岩与滑石矿页岩标准化稀土元素分配模式基本一致,重稀土元素相对轻稀土元素弱富集,稀土元素总量低,弱负铈异常,并且它们具有相同的87 Sr/86 Sr比值(0.7092～0.7101),白云岩的δ 13 CV-PDB值变化范围在-4.9‰～+0.7‰之间,与同期海水大体一致.这些地球化学特征暗示了滑石矿和白云岩具有相同的物质来源,滑石成矿所需的镁可能来源于海水.部分样品显示正铕异常,且白云岩中δ 18OV-SMOW值(+14.5‰～+19.7‰)较同期海水偏低,87Sr/86Sr比值比同期海水稍高,暗示成矿物质并非由海水单一提供,可能还有高87Sr/86Sr比值、富硅的热液提供了成矿所需的硅.综合滑石和白云岩岩相学和地球化学特征,认为江西广丰杨村滑石矿为同生热水沉积成因,滑石中镁来源于富镁海水,硅来源于海水下渗淋滤古老硅铝质地层形成的富硅、高87Sr/86Sr比值的热液,海水中镁与热液中硅结合形成滑石.滑石矿在经历了同生沉积-热液期之后,又受到了后期强烈的变质变形作用的叠加,形成了具片理构造的滑石片岩.
展开▼
