层控夕卡岩及有关矿床形成过程的稀土元素行为——以安徽冬瓜山矿床为例

REE behavior during formation of strata-bound skarn and related deposit: a case study of Dongguashan skarn deposit in Anhui province, China.

对安徽省冬瓜山层控夕卡岩型铜(金)矿床中穿层方向上不同蚀变程度的大理岩、顺层方向上距岩体远近不同但垂向深度相近的块状石榴子石夕卡岩及其主矿物石榴子石、不同演化阶段的石英及矿石等到的REE特征进行了系统研究。结果显示:在穿层方向上,大理岩被交代的程度愈深其稀土总量(∑REE)愈高,Eu负异常愈显著;尽管,夕卡岩全岩的REE分布模式与原岩(大理岩)相似,但前者的稀土总量(∑REE)远高于后者,且Eu异常更为显著,显然不是继承原岩的REE特征所致,而是受控于其主矿物石榴子石的REE特征,后者又由参与交代作用的岩浆热液REE所决定;热液中的REE具有缓和右倾型分布模式、LREE富集、较显著的Eu负异常等基本特征;石榴子石晶体基本承袭了热液的REE特征。REE的空间变化特征结合地层的构造特征可以揭示夕卡岩及相关矿体形成过程中流体的输运路径及输运方式。REE的研究有助于深化对层控夕卡岩及其相关矿床形成过程的认识。

Systematic studies have been carried out on REE composition, of hydrothermal alterated marbles, massive garnet skarns and ores, as well as garnet and quartz formed in mufti-stage skarn and deposit formation. The result show that stratabound skarn and its related Cu deposits were formed in a coraplex hydrothermal system during mutistage infiltration metasomatism. The REE characteristics of skarns, however, are similar to those of the marbles. They were not simply inherited from the marble protolith, but were controlled by garnets, and the latters were determined by the hydrothermal solutions participating infiltration metasomatism. The related hydrothermal fluids characterized with slightly right-hand dip REE patterns, are enriched in LREE and have strong negative Eu anomalies. The uptake of REE from hydrothermal fluid during garnet crystallization resulted in that garnet shared the similar REE patterns. REE in skarns were provided mainly by hydrothermal fluids associated with the emplacement of granodiorite plutons in the mining district. Based on the dimensional variation of REE in skarns combining with the structures of hosting strata, the transporting route and pattern of fluids involved in the formation of skarns and related deposits can be better understood. The present study demonstrates that extensive REE metasomatism can occur in hydrothermal system at shallow to intermediate depths and REE geochemistry are useful in discerning the origin of skarns and skarn-related deposits.