白云鄂博地区相关地质单元的铁同位素特征及其对白云鄂博矿床成因的制约

摘要

白云鄂博Fe- REE- Nb矿床是世界著名的巨型多金属矿床,它的成因一直是个激烈争论的问题,观点主要集中在沉积成因和岩浆成因上,而铁的物质来源问题是争论的焦点之一.近年来Fe同化素的快速发展为解决白云鄂博铁矿的成因提供了新思路.对白云鄂博地区发育的白云鄂博群尖山组铁质板岩、宽沟北沉积型铁矿、腮林忽洞微晶丘、灰绿岩墙这些相关地质单元的Fe同位素组成特征进行了研究,为白云鄂博矿床成因研究提供了最直接的参考.结果表明,尖山组铁质板岩的δ56Fe值为-0.49‰～0.48‰,平均值为-0.03‰±0.84‰,2SD,n=5；宽沟北沉积型铁矿的δ56 Fe值为-0.68‰～0.23‰,平均值为-0.10‰士0.78‰,2SD,n=5；腮林忽洞微晶丘δ56 Fe值为-0.64‰～0.12‰,平均值为-0.28‰士0.57％,2SD,n=6；辉绿岩的Fe同位素组成δ56Fe值集中在0.11‰～0.16‰.腮林忽洞微晶丘总体上比白云鄂博赋矿白云岩寓集Fe的轻同位素,Fe同位素组成变化也相对更大,表明两者可能有不同的成因.白云鄂博地区尖山组铁质板岩、宽沟北沉积型铁矿与世界其他地区含铁沉积建造的Fe同位素组成类似,其共同特征是,Fe同位素变化较大,总体上δ56 Fe大于0‰.这一特征与白云鄂博铁矿的Fe同位素组成差别较大.白云鄂博矿床的δ56 Fe集中在0％附近,与白云鄂博地区灰绿岩、世界不同地区火成岩和岩浆型铁矿的Fe同位素组成特征一致.表明白云鄂博铁矿可能不是沉积成因的,更有可能与岩浆作用有关.%Bayan Obo Ore Deposit is a giant Fe-REE-Nb ore deposit, the largest light REE deposit in the world. However, its genesis has been a subject of longstanding debate, the main hypotheses including sedimentary origin and magmatic origin. Recently, the rapid development of high-precision measurement of Fe isotope ratios enables it to be used as a new proxy to trace the source of Fe for Bayan Obo ore deposit. To obtain direct Fe isotope baseline for Bayan Obo ore deposit of possible origins, Fe isotopes of geological formation related to the Bayan Obo ore deposit in Bayan Obo area were analyzed, including iron slate of Jianshan Formation (Bayan Obo Group), sedimentary iron ore deposit in north of Kuangou, micrite mound of Sailinhudong Group, and diabase dykes. The results show that iron slate of Jianshan Formation (Bayan Obo Group) yields δ55Fe values between -0. 49‰ to 0. 48‰, with the average — 0. 03‰±0. 84‰, 2SD, n = 5( that sedimentary iron ore deposit in north of Kuangou yields 856 Fe values between —0. 68‰ to 0. 23‰, with the average —0. 10‰±0. 78‰, 2SD, n = 5; that micrite mound of Sailinhudong Group yields 856Fe values between —0. 64‰ to 0. 1296m with the average —0. 28‰±0. 57‰, 2SD, n = 6; and that diabase dykes yields 85SFe values between 0. ll‰ to 0. 16‰. Compared to ore hosting dolomitic marble, the Fe isotope composition of micrite mound of Sailinhudong Group is systematically lighter, and is more variable, indicating the different origins between them. The iron slate of Jianshan Formation and sedimentary iron ore deposit in north of Kuangou display a similar Fe isotope composition to that of Banded Iron Formation (BIF) all over the world. All of them show heavy Fe isotope enriched and large variation in Fe isotopes. Their Fe isotope composition is different from that of Bayan Obo ore deposit, which yields δ56 Fe values of ca. 0‰. The Fe isotope composition of Bayan Obo ore deposit is identical with that of diabase dykes in Bayan Obo, magmatic rocks, and magmatic iron ore deposit, supporting the magmatic origin of Bayan Obo ore deposit.