川东南脉状萤石－重晶石矿床同位素地球化学

Isotope Geochemistry of Vein Fluorite and Barite Deposits in Southeast Sichuan

川东南脉状萤石－重晶石矿床主要产于下奥陶统碳酸盐岩中。其成矿作用与岩浆活动无明显联系，大致发生在燕山运动晚期或稍后。武隆、酉阳及彭水二河水矿区的萤石、方解石和重晶石中包裹体水的Ｄ值为－８．９‰～－４２．９‰（ＳＭＯＷ），其中彭水二河水矿床方解石中包裹体水的Ｄ（－２９．２‰）和从方解石计算得到的１８ＯＨ２Ｐ（＋０．３６５‰０，ＳＭＯＷ）接近海水的同位素组成；仅彭水郁山矿床石英中包裹体水的Ｄ值（－８９．２‰）落在我国东部侏罗纪～白垩纪时期大气水的Ｄ值范围（－７０‰～９０‰。）内，从石英计算得到的１８Ｈ２Ｏ为－１９．７７‰～－５．２９‰。以上有限的Ｈ和Ｏ同位素数据，加之包裹体气相成分中Ｎ２含量甚微，表明上述地区成矿流体主要来源于地层水（如封存的海水），部分来源于大气降水。重晶石Ｓ为十１６．１３‰～＋４２．１‰（ＣＤＴ），主要为十２３‰～＋３６‰，与该区寒武系石膏的Ｓ（＋２３．１‰～＋２９．７‰）相似，表明硫与该区寒武系的蒸发岩有关。重晶石及萤石的锶同位素组成（８７Ｓｒ／８６Ｓｒ为０．７０８８０～０．７１３００，主要为０．７０８８０～０．７１１１３）与该区下奥陶统碳酸盐岩（８７Ｓｒ／８６Ｓｒ为０．７０９２１～０．７?

Vein fluorite and barite deposits in southeast Sichuan occur mainly in the Lower Ordovician carbonate rocks. Geological evidence indicates no obvious relationship between the mineralization and igneous activityl the mineralization formed during the Yanshanian movement (middle Jurassic to late Cretaceous)or slightly later. The δDH2ovalues of inclusions in fluorite, calcite, and barite in Wulong, Youyang, and Erheshui ore district of Pengshui range from - 8. 9‰ to -- 42.9‰, (SMOW), and δH2o(-29.27‰) of inclusions in calcite and δ18OH2o (+0.365‰, SMOW) computed from calcite of the Erheshui ore district are close to those of SMOW; only δDH2o of inclusions in quartz of the Yushan ore district is - 89.5‰ which is within the range of δD values (-70‰ to - 90‰ ) of meteoric water during Cretaceous time in eastern China; δ18OH2o values computed from quartz are -19. 77‰ to - 5. 29‰, The above cited limited H and O isotope above and negligible N2in the gaseous component of inclusions suggest that the ore-forming fluid originated mainly from stratum waters (e. g. trapped seawater), plus some meteoric water. δ34S value of barite ranges from + 16. 13‰ to +42. 1‰(CDT), mostly varies in a range of +23‰, to +36‰, which is similar to that reported in Cambrian evaporitic gypsum of the region (δ24S= + 22.1￣+29.7‰), suggesting that the source of sulfur was related to the Cambrian evaporites in this district. The sr isotopic compositions in fluorite and barite (87Sr/86Sr= 0. 70880￣0.71300, mostly range from 0. 70880 to 0. 71113), are close to those in lower Ordovician carbonate rocks (87Sr/86Sr= 0. 70921 ￣ 0. 71010) in this district, showing that Sr in the ore-forming solutions was derived mainly from the Lower Ordovician carbonate rocks, and therefore, the Ca and Ba were presumably derived from the carbonate rocks because of the close geochemical similarity of the two elements to Sr. However, these carbonate rocks can supply only a small volumes of barium for the large-scale mineralization of fluorite and barite in southeast Sichuan because of low barium (generally<500μg/g) in the rocks. It is possible that the main source for barium is the black-shale series of the Upper Sinian to the early Cambrian Niutitang Formation, the only significant Ba-bearing rock (containing 1135￣13220μg/g of barium) in southeast Sichuan and vicinity.