四川南江柏林坪石墨矿床成因:岩石及C-O同位素地球化学约束

Genesis of the Bolinping graphite deposit in Nanjiang of Sichuan by constraints of lithological and C-O isotopic geochemistry

柏林坪石墨矿床是川北米仓山南缘石墨矿带新发现的晶质石墨矿床,该矿床主要赋存在晋宁澄江期岩浆岩体接触外带中的中-上元古界火地垭群麻窝子组中,矿体呈层状、似层状和透镜状产出于硅质板岩、大理岩中;矿石平均固定碳含量5.81%,石墨多呈鳞片状,多数片径大于1μm。对该矿床矿石和围岩的主量、微量元素及C-O同位素研究表明:石墨矿石和大理岩的稀土总量较低,在(45.03~79.28)×10^(-6),其与太古宙后澳大利亚平均页岩(PAAS)具有一致的球粒陨石标准化稀土配分模式;大理岩的δ^(13)C_(PDB)值为+3.1‰~+3.9‰,δ^(18)O_(V-SMOW)在17.17‰~19.53‰,指示成矿原岩为近海的含陆源碎屑的碳酸盐岩;矿石的有机碳δ^(13)C_(PDB)值为-5.2‰~-5.6‰,显示碳质来源主要为海相碳酸盐岩,可能混染了其他碳源;石墨方解石同位素温度计显示其形成温度在420~456℃。该矿床成因类型为区域变质型,海相碳酸盐环境是该类型矿床形成的物质基础,区域变质作用是其形成的必要条件。

The Bolinping graphite deposit is a newly discovered flake graphite deposit in the Micangshan graphite ore belt,northern Sichuan.The deposit is mainly hosted in the Middle Proterozoic Huodiya Group in the outer contact zone of Jinning-Chengjiang magmatic rock.The ore bodies are layered,stratoid and lenticular,and occur in siliceous slate and marble.The average fixed carbon content of the ore is 5.81%,and the graphite minerals are mostly scaly,larger than 1μm.The major elements,trace elements and C-O isotopes of the ore and wall rock of the deposit are studied.The results show that the total amount of rare earth element in the ore and wall-rock marble is low(45.03-79.28)×10^(-6).The ore and wall-rock marble have the same chondrite-normalized REE distribution pattern as the post Archean Australian average shale(PAAS).Theδ^(13)C_(PDB) are+3.1‰to+3.9‰,δ^(18)O_(V-SMOW),ranging from 17.17‰to 19.53‰,indicating that the ore-forming protoliths are carbonate rocks with terrigenous clasts.Theδ^(13)C_(PDB) of graphite ore range from-5.2‰to-5.6‰,indicating that the source of carbon is mainly marine carbonate.The graphite-calcite isotopic thermometer shows that the formation temperature is 420-456℃.It is considered that the genetic type of the deposit is regional metamorphic type.The marine carbonate environment is the material basis for the formation of this type of deposit,and regional metamorphism is the necessary condition for its formation.