Late Paleozoic mafic-ultramafic intrusions in southern Central Asian Orogenic Belt (NW China): Insight into magmatic Ni-Cu sulfide mineralization in orogenic setting

摘要

The mafic-ultramafic intrusions hosting magmatic Ni-Cu sulfide ore deposits, usually present within the Precambrian cratons and on their margin, have been extensively studied, whereas those occurring within the Phanerozoic orogenic belts have received little attention. The southern margin of the Central Asian Orogenic Belt is a region of late Paleozoic mafic-ultramafic intrusions emplaced during post-orogenic stage. In order to recognize the mineralization features, this study compares barren or weakly mineralized mafic-ultramafic intrusions in the Beishan with those hosting Ni-Cu sulfide ore deposit in the Altay and Eastern Tianshan in petrology, mineralogy and geochemistry aspects. Several mineralization features can be recognized with respects to petrology, mineralogy and geochemistry from comprehensive comparisons. Among the mineralization indicators, we highlight strong alteration accompanying sulfide emplacement, post-erosion exposure of magma conduit, abundant primary hydrous minerals and potential transportation mechanism of ore-forming elements. Other indicators are the presence of orthopyroxene and pyroxenite for potential sulfide formation through crystal fractionation, restricted Fo and Ni ranges in olivines for sulfide segregation, and some other special geochemical characteristics related to crustal contamination. The variation in mineralization could be ascribed to different degrees of partial melting, crustal assimilation and subduction-related modification of a mantle source. We suggest that the Ni-Cu sulfide mineralization of mafic-ultramafic intrusions within orogenic belts is remarkably different from that of cratonic and continental marginal settings in respects of petrogenesis and mineralizing mechanism. Magmatic conduit-type Ni-Cu sulfide ore deposit is a target for future prospecting and exploration for mafic-ultramafic intrusions in the orogenic belts.