摘要
辽宁东部古元古界底部地层(南辽河群)中赋存着大型的硼酸盐矿床,含矿层位中广泛分布含电气石的变粒岩和电英岩。空间上这些含电气石的岩石与硼酸盐有着密切的联系,电气石可以作为区域硼矿找矿的标志。已有研究结果表明,该地区的硼酸盐矿床是变质蒸发岩成因。本研究对该区不同产状的电气石和硼酸盐的地质特征,全岩和矿物成分、硼同位素组成进行了分析。本区的电气石包括层状和脉状两大类,而电气石的富集与硼酸盐关系密切,电英岩往往分布在硼酸盐矿体的上盘。而矿体的下盘一般不产出富电气石的岩石。当长英质脉体穿过硼酸盐矿体时,脉体中往往会富集电气石。含电气石岩石的全岩地球化学分析表明,它们的REE及其他微量元素特征以及相关性关系与周围不含电气石的同类岩石十分相似,反映出一种成因上的联系。本区电气石主要属于镁电气石—铁电气石系列,靠近硼矿体的电气石比远离硼矿体的电气石更加富镁,有着更高的Mg/Fe比值。电气石和硼酸盐的硼同位素成分分析显示出二者在同位素组成上的相似性,前者比后者的δ1lB稍低,这可能是由于热液活动过程中同位素分馏的结果。电气石的硼同位素组成在空间上显示出变化规律:远离硼酸盐矿体的电气石的δ1lB值(-5.2‰～+3.6‰)比矿体附近的电气石低(平均+10.5‰)。
We have analysed the major element, trace element and boron isotope composition of borates, tourmaline-rich rocks and their non-tourmalinised equivalents from the lower portion of the Paleoproterozoic South Liaohe Group (N.E. China). These rocks host economically important borate deposits that formed in an evaporitic environment. The sequence contains abundant tourmaline in leptynites, stratiform tourmalinites, quartz-tourmaline veins and pegmatites that have spatial and temporal relationships to the borates.Tourmalines distal to the borates have lower δ1 lB values (-5.2 ‰ to +3.6 ‰) than those proximal to the borates (+1.8 ‰ to +9.4 ‰)that are slightly lower than those of the borates (av. +10.5 ‰). Mg/Fe ratios of the distal tourmalines are low (<1.0) relative to proximal tourmaline-rich rocks (>to 2.6). These differences reflect the fact that the evaporites are a major source of B and Mg. The geochemistry of immobile elements in the stratiform tourmalinites is similar to that of their unmineralised equivalents (interpreted as meta-tuffites), suggesting that the tourmalinites formed via alteration of Al-rich tuff layers by boron-rich fluids derived from leaching of the underlying borates. Field evidence and geochemical data support a three-stage model of tourmaline formation. The first stage was coeval with deposition of the borates, during which boron from hot springs was adsorbed by clay minerals in the tuffs that were later metamorphosed to tourmaline-bearing leptynites. In the second stage, hot fluids leached boron from the evaporites and passed through overlying tuffs to form the stratiform tourmalinites. During later emplacement of granites at the base of the sequence, felsic veins and pegmatites intruded the borates and tourmalinites. Boron from the borates diffused into the felsic veins, forming coarse-grained tourmaline at their margins. This association of tourmaline with the borate deposits emphasizes the significance of tourmaline-rich rocks as a prospecting guide for borates in this area and has implications for the origin of tourmalinites in other metamorphic terrenes.
出处
《中国地质》
CAS
CSCD
2004年第3期240-253,共14页
Geology in China
基金
国家自然科学基金(49573175,40373023)
英国皇家学会中英合作项目联合资助
关键词
电气石
硼酸盐
古元古代
蒸发岩
辽宁
tourmaline
borates
Paleoproterozoic
evaporite
Liaoning
