扬子北缘新元古代基性岩体铂族元素特征及找矿意义

Characteristics of Platinum Group Element in Neoproterozoic Mafic Intrusions in the Northern Margin of the Yangtze and Exploration Implications

扬子地块北缘的汉南地区是中国最重要的基性–超基性岩体分布区之一。新元古代毕机沟和望江山岩体是汉南地区出露最好、研究程度最高的两个层状基性岩体。毕机沟和望江山岩体被认为是由亏损地幔经历10%~20%部分熔融形成的。远高于原始地幔的Cu/Pd值(Cu/Pd毕机沟值为3.52×10^(4)~3.97×10^(5),Cu/Pd望江山值为1.78×10^(4)~1.61×10^(6)),表明岩体母岩浆在侵入浅部地壳之前就经历了早期硫化物熔离。毕机沟岩体中铱族元素(IPGE)与全岩Ni呈正相关,Cu/Ir与Ni/Pd呈负相关,说明在浅部岩浆房硫化物未饱和时,铂族元素的分配主要受橄榄石控制。望江山岩体中铂族元素与全岩Ni、V、TiO_(2)无相关性,Cu/Ir与Ni/Pd呈正相关,说明望江山岩体中铂族元素受二次熔离硫化物的控制。毕机沟和望江山岩体中矿物不具有定向性,加上望江山岩体中二次熔离出的硫化物中铂族元素依然亏损,说明这些岩体更可能是岩浆单次贯入冷却形成的,而非岩浆通道。因此,浅部可能不具备赋存大型矿床的条件,今后的找矿工作应该聚焦于更深部。

Multiple layered mafic intrusions occur along the northern margin of the Yangtze Block,SW China.The Neoproterozoic Bijigou and Wangjiangshan mafic intrusions are two of the best exposed intrusions in the region.The Bijigou and Wangjiangshan mafic intrusions are thought to be generated by 10%to 20%of partial melting of a depleted mantle source.Uniformly high Cu/Pd(3.52×10^(4)~3.97×10^(5)for the Bijigou samples and 1.78×10^(4)~1.61×10^(6)for the Wangjiangshan samples)indicate that the parental magma of these intrusions experi-enced prior sulfide segregation before their intrusions into the shallow crust.Positive correlation between IPGE with whole-rock Ni,and negative correlation between Cu/Ir and Ni/Pd illustrate that the distribution of PGE is mainly controlled by the accumulation of olivine under an S-unsaturated condition.In comparison no linearly correlation between PGE and whole-rock Ni,V and TiO_(2),and positive correlation between Cu/Ir and Ni/Pd il-lustrate that the PGE in the Wangjiangshan intrusion is controlled by the second-stage sulfide saturation.The general lack of parallel alignment of tabular minerals in the Bijigou and Wangjiangshan intrusions,combined the PGE depletions in the Wangjiangshan second segregated sulfides,indicates that these intrusions probably intrud-ed and cooled under a single episode of magma replenishment,rather than a dynamic magma conduit system.Therefore the shallow part may not have the conditions to host large deposits and future prospecting work should focus on the deeper part.