四川南江坪河石墨矿炭质来源及成矿机制

Carbon Source and Metallogenic Mechanism of Pinghe Graphite Deposit at Nanjiang,Sichuan Province

南江坪河石墨矿赋存于中元古界火地垭群麻窝子组下段,岩性为白云质大理岩与石墨绢云母片岩组成的互层,原岩为一套灰岩-炭质泥岩韵律沉积,属于潮坪-泻湖沉积环境。沉积地球化学结果表明围岩白云质大理岩形成于氧化环境,而赋矿的石墨绢云母片岩形成于缺氧环境。南江坪河石墨矿的δ13C值波动在-24.5‰^-22‰之间,与有机质的δ13 C分布范围重叠,表明其中的炭质来源于生物有机质,而大理岩中的炭质则是原始沉积碳酸盐岩的原始组分。结合地质、地球化学特征,初步提出了南江坪河石墨矿成矿机制:在闭塞缺氧的泻湖环境中,有机碳在还原成岩环境中容易保存,形成富含有机质的沉积物;后期成岩过程中经过区域变质作用,有机质重新结晶形成石墨矿源层;随后,经过岩体侵位接触变质,使得含矿岩系中的石墨片径增大,富集成石墨矿床。

Nanjiang Pinghe graphite ore hosts in Midlle Proterozoic Huodiya Group Mawozi Formation.Its lithology mainly contains dolomitic marble and graphite silk mica schist,originally in limestone-carbonaceous mudstone rhythm,formed in the tidal flat-lagoon depositional environment.Sedimentary geochemical results show that surrounding rock dolomitic marble is formed in oxic environment,and the graphite silk mica schist is formed in anoic environment.Theδ13 C of graphite ore,-24.5‰to-22‰,in accordance with theδ13 C of the organic matter,shows that the carbon derived from biological organic matter,and the carbon in the marble is derived from the sedimentary carbonate.The metallogenic mechanism is in the lagoon occlusion anoxic environment,and so organic carbon is not easily oxidized to CO2 escape,which is preserved in the formation,rich in organic matter of sediment rock.Through regional metamorphism,the sources of organic matter are recrystallized to form graphite.The later emplacement contact metamorphism,leads to the incease of the flake graphite in size and thus the formation of graphite deposit.