东秦岭地区一种富稀土热液型钡解石的发现及其意义

对东秦岭地区河南嵩县一带进行地质调查,发现了一系列具有一定规模的含稀土碱性碳酸岩矿脉并在其中发现一种特殊的钡解石矿物。依据该钡解石主量元素组成,计算分子式为Ba_1._(04) Ca_0._(81)Sr_(0.14)(CO_3)_2,为锶钡解石,LA-ICP-MS分析表明其富Na、K、Mn、Pb、REE、Y等元素,稀土元素总量最高为4 080×10-6,总体表现为轻稀土元素富集、重稀土元素亏损。该矿物与常见于沉积岩中的钡解石存在显著差别。钡解石呈现出与霓辉石共生(钡解石正晶型,霓辉石围绕钡解石生长;霓辉石正晶型,钡解石围绕霓辉石生长),或与石英、方解石、磷灰石共生(它形)两种状态。早期方解石与钡解石共生,形成于碱性岩演化早期的碳酸盐与硅酸盐不混溶阶段;晚期方解石则以布丁状分布于钡解石和霓辉石中,为碳酸盐交代阶段产物。霓辉石、钾长石、钠长石、辉石、磷灰石、方解石、石英和钡解石共生的组合与已知火成碳酸岩的矿物组合相似。该区碳酸岩富集REE、Ba和Sr,与已知大型富稀土碳酸岩矿床(如牦牛坪稀土矿)特征一致。结合已发现矿脉地质特征,认为该区有较大的成矿潜力,为东秦岭地区寻找火成岩型稀土矿提供了依据。

The discovery of a rare earth-rich barytocalcite in the East Qinling area and its significance

This paper reports a new type of high temperature REE-rich barytocalcite, which occurs in REE-enriched alkaline carbonate veins in Songxian County, Henan Province, East Qinling. The molecular formula of the studied barytocalcites is Ba_1.04Ca_0.81Sr_0.14(CO₃)₂ (Sr-barytocalcite) based on Stoichiometric calculation using the major element compositions. LA-ICP-MS analyses show high concentrations of Na, K, Fe, Mn, Pb, Y and REEs in the barytocalcite. The total REE concentrations of the barytocalcites are up to 4 080×10^-6, which generally show LREE-enriched and HREE-depleted patterns. Contrasting to sedimentary barytocalcite formed in low temperature, backscattered electron images suggest that the studied barytocalcites have two types of high-temperature textures intergrowing with aegirine augite or with quartz + calcite + apatite. The intergrowth of orthomorphic barytocalcite with primary calcite suggests that they were formed in the early stage of alkaline melt evolution, likely crystallized in a carbonate and silicate immiscible environment. The later stage calcites were found in barytocalcites and aegirine augites in patch shape, implying a carbonate metasomatic stage. The assemblage of aegirine augite + K-feldspar + albite + pyroxene + apatite +calcite + quartz + barytocalcite is consistent with that in carbonatites. The features of high Ba, Sr and REE content in the studied carbonatites are also of high similarity to well-known large-scale REE-rich carbonatite ore deposits (e.g., the Maoniuping REE deposit). Together with detailed geological features of the alkaline carbonate veins, the results obtained by the authors could provide a general guideline for exploration of carbonatite REE deposits in the East Qinling region.