| 其他摘要 | Several Cu-Ni-PGE sulfide deposits, such as Jinbaoshan, Yangliuping, Limahe, Baimazhai, and Dacao-Abulangdang mineralized rocks, are hosted in mafic-ultramafic sills in the Emeishan Large Igneous Province (ELIP). Various mineralized types occur in these ore deposits, some of which are enriched in PGE but poor in Ni and Cu (e.g. the Jinbaoshan Pt-Pd deposit); some of which are enriched in PGE, Ni and Cu (e.g., the Yangliuping PGE-Ni-Cu deposit); and some of which are poor in PGE but rich in Ni and Cu (e.g., the Limahe and Baimazhai Ni-Cu deposits). In addition, one type of mineralized rock, which is composed of olivine and chromite cumulates, for instance, the Dacao-Abulangdang, is also very important. What are the causes for the variation of mineralized type of Ni-Cu-PGE sulfide ore deposits in the ELIP? What is the character of the parental magma? How is the degree of partial melting for the mantle? What are the reasons for sulfide segregation in the magma, and which factor has played a key role for the sulfide saturation? What are the difference and its cause among the metallogenic magmas of these ore deposits? Considering these questions, we summarize the studies on the Cu-Ni-PGE ore deposits worldwide and firstly reported systematic geochemical data on the Dacao-Abulangdang mineralized rocks, especially the data of trace elements, PGE, and Sr-Nd isotopic elements. On the basis of geochemical study on the Jinbaoshan Pt-Pd deposit, Limahe Ni deposit, and Dacao-Abulangdang intrusion, in combine with other studies on Ni-Cu-PGE ore deposits, such as the Yangliuping, Baimazhai, we aim to elucidate these above mentioned questions. Some important conclusions are obtained in this dissertation as follows:
1）According to the characteristics of metallogenic elements, the magmatic Ni-Cu-PGE sulfide deposits can be divided into several types, including PGE deposits (e.g. Jinbaoshan Pt-Pd ore deposit), Ni-Cu-PGE deposits (e.g. Yangliuping ore deposit), Ni-Cu deposits (e.g. Limahe and Baimazhai), and Ni-Cu-PGE mineralized rocks which are composed of early-crystallized olivine and chromite (e.g. Dacao-Abulangdang intrusion) in the ELIP. The study on metallogenic parental magma suggests that they have a composition similar to that of the Emeishan picritic magma, which is enriched in Ni and PGE and was produced by relatively large degree with mantle partial melting.
2）The multi-stage of sulfide segregation may be an important factor for the variation of mineralized types of Ni-Cu-PGE deposits in the ELIP. Take the Jinbaoshan deposit as an example; the sulfide saturation occurred at depth due to olivine and chromite crystallization. Immiscible sulfide droplets were transported to the Jinbaoshan conduit where they accumulated and reacted with mamga successively passing through the conduit to achieve high PGE concentrations. The sulfides of Yangliuping were produced from magmas that had experienced minor sulfide removal. In contrast, the sulfides in the Limahe and Baimazhai intrusion were segregated from strongly PGE-depleted parental magmas that experienced earlier sulfide removal. The Dacao-Abulangdang rocks consisting of early-crystallized olivine and chromite cumulates, are crystallized from PGE-undepleted parental magma.
3）R factor (ratio of silicate to sulfide liquid) may have played a main role in controlling the variation of mineralized types of Ni-Cu-PGE deposits in the ELIP. The sulfides in Jinbaoshan deposit have extremely high R factors (＞10000), the sulfides in Yangliuping and Zhubu have moderate R factors (2000~5000), the sulfides in Limahe deposit were formed from residual magma (with R factor for 200) after the first magma with R factor for 2000.
4) The difference of crustal contamination may have played a key role for the variation of mineralized types of Ni-Cu-PGE deposits in the ELIP. The degree of crustal contamination of the Jinbaoshan deposit is relatively low. Fractional crystallization of early-staged olivine and chromite may have caused the sulfide saturation in the parental magma. As far as the Dacao-Abulangdang intrusion is concerned, sulfide saturation occurs around the intrusion of the margin, which may be caused by assimilation of the surrounding rock. In the case of the Yangliuping Ni-Cu-PGE deposit, Limahe and Baimazhai Ni-Cu deposits, the evidence of significant crustal contamination giving rise to large amount of sulfide saturation, could be deduced from many aspects, such as the strong Nb-Ta negative anomaly in primitive mantle normalized trace element spidergrams, the high initial radiogenic Os isotope compositions , γOs(t)=100~120）, and the sulfur isotopic characteristics. |
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