四川旺苍大河坝浅变质岩型石墨矿床地球化学特征与成因分析

Geochemical Characteristics and Genesis Analysis of Daheba Epimetamorphic Graphite Deposit in Wangcang,Sichuan Province

大河坝石墨矿是川北米仓山南缘石墨矿带新发现的大型显晶质石墨矿床。研究了该矿床的地质特征和矿石主量、微量及稀土元素地球化学特征,结果表明:含矿岩层为副变质岩,矿区石墨矿体原岩沉积于缺氧的还原环境,原岩为一套含碳质黏土质细—粉砂岩及含碳质泥灰岩组合。微量元素特征指示矿体原岩由近海陆源碎屑物沉积形成,沉积水体主要为盐度较低的、混合不均匀的淡水—半咸水。矿石ΣREE平均值为150×10^(-6),与泥灰岩相近;δCe值平均为0.91,呈弱负异常,δEu值平均为0.67,呈负异常,具滨海潮坪相沉积特征。含矿岩石δ^(13)C值为-21.4‰~-19.0‰,平均为-19.86‰,表明成矿碳质来源主要为有机碳,并混合了部分无机碳。矿床成因类型为沉积变质型,其变质作用可能包括多次区域变质作用并叠加了混合岩化作用。

Daheba graphite deposit is a newly discovered large phanerocrystalline graphite deposit in the southern margin of Micangshan,northern Sichuan.The geological characteristics of the deposit and the geochemical characteristics of major,trace elements and rare earth elements of minerals are studied in the paper.The results show that the ore bearing strata are parametamorphic rocks.The original rock deposition of graphite ore body in Daheba mining area is under anoxic reduction environment,and the protolith is composition of carbonaceous clayey fine to silt stone and carbonaceous marlite.The characteristics of trace elements indicate that the ore body is mainly composed of terrigenous clastic sediments.The sedimentary water is mainly fresh to brackish water with low salinity and uneven mixing.The average value of∑REE of the ore is 150×10^(-6),which is similar to marlite;The average value ofδCe is 0.91,Ce indicating weak negative anomaly;The average value ofδEu is 0.67,Eu is negative anomaly and has the characteristics of coastal tidal flat facies deposition;Theδ^(13)C values of the ore bearing rocks ranged from-21.4‰to-19.0‰,with an average of-19.86‰,indicating that the main source of metallogenic carbon is organic carbon,which may be mixed with some inorganic carbon.The genetic type of the deposit is sedimentary metamorphic.The metamorphism may be multiple regional metamorphism and superimposed migmatization.