电子探针技术研究大别造山带富硫独居石地球化学特征及稀土矿化成因

Study on Geochemical Characteristics and REE Mineralization of S-enriched Monazite in the Dabie Orogenic Belt by Electron Probe Microanalysis

独居石是常见的赋稀土矿物,也是许多稀土矿床中重要的矿石矿物,而含硫独居石却相对罕见,只在为数不多的一些碳酸岩、金伯利岩、变质岩中被发现。本文在大别造山带蕲春县白羊沟地区发现的富稀土矿样品中,利用偏光显微镜镜下鉴定、电子探针定性和定量分析相结合的技术手段测定富硫独居石中的硫、稀土元素等主要化学成分的含量,研究其地球化学特征以及成因。结果表明:研究区稀土矿化角闪片岩中的富硫独居石大多呈胶状、团块状,部分呈现板状、柱状自形晶体,且呈粒状集合体状,和磷灰石之间存在相互交生、包含、半包含的空间关系,或呈网脉状交代切割磷灰石,岩相学表明富硫独居石与磷灰石之间存在着类似热液蚀变的交代关系。富稀土矿样品中富硫独居石SO_(3)含量最高达14.57%,平均含量为10.54%,是目前国内外已知的硫含量最高的独居石,同时测得富硫独居石CaO含量较高,NdO含量偏低,与花岗岩成因的独居石和热液交代的不含硫独居石成分差异较大,地球化学显示S^(6+)与P^(5+)呈负相关性以及(Sr,Ca)^(2+)、S^(6+)与REE^(3+)、P^(5+)呈负相关性,并可以用“硬石膏耦合”置换反应来解释其独居石含S的原因,即(Sr,Ca)^(2+)+S^(6+)REE^(3+)+P^(5+),也就意味着白羊沟地区存在着与稀土矿化相关的热液活动,结合白羊沟地区的地质背景推断其热液来源可能与白垩世以来该地区经历了强烈的岩石圈伸展运动和岩浆活动晚期热液有关。研究结果为白羊沟地区的稀土多金属矿化成因研究提供了新线索。

BACKGROUND:Monazite is a common rare earth mineral and an important ore mineral in many rare earth deposits,while sulfur-containing monazite is relatively rare and is only found in a few carbonate rocks,kimberlites,and metamorphic rocks.OBJECTIVES:To accurately analyze the chemical composition of S-enriched monazite,and to infer its genesis.METHODS:The main chemical components of S-enriched monazite from the REE ores in the Baiyanggou area of Puchun County,Dabie orogenic belt were determined by polarized light microscopy and electron probe microanalysis.RESULTS:The sulfur-enriched monazite in the rare-earth mineralized amphibole schist in the study area was mostly colloidal and agglomerate,some were plate-like and columnar euhedral crystals,which were granular aggregates.There was mutual interaction,inclusion,and a semi-inclusion spatial relationship between sulfur-enriched monazite and apatite.Crosscut of apatite by network veins was also present.Petrography showed that there was a metasomatism similar to hydrothermal alteration between sulfur-enriched monazite and apatite.The SO 3 content of S-enriched monazite in the rare earth-enriched mineral samples was as high as 14.57%,with an average content of 10.54%,which was the monazite with the highest S content.The S-enriched monazite has a higher CaO content and a low NdO content,which was quite different from the composition of the granite genesis monazite and hydrothermal metasomatic sulfur-deficient monazite.According to the negative correlation between S^(6+) and P^(5+),as well as the negative correlation between(Sr,Ca)^(2+),S^(6+)and REE^(3+),P^(5+),the S-containing monazatite can be explained by the“anhydrite coupling”displacement reaction,that is,(Sr,Ca)^(2+)+S^(6+)REE^(3+)+P^(5+).This indicated that hydrothermal activity related to REE mineralization in the Baiyanggou area was present.CONCLUSIONS:Combined with the geological background of the Baiyanggou area,it is inferred that the source of the hydrothermal fluid may be related to the intense lithospheric extensional movement and the late magmatic activity in the study area since the Cretaceous period.This provides new clues for research on the origin of rare earth polymetallic mineralization in this area.