Flood basalts and metallogeny: The lithospheric mantle connection

摘要

Continental flood basalts, derived from mantle plumes that rise from the convecting mantle and possibly as deep as the core-mantle boundary, are major hosts for world-class Ni-Cu-PGE ore deposits. Each plume may have a complex history and heterogeneous composition. Therefore, some plumes may be predisposed to be favourable for large-scale Ni-PGE mineralisation ("fertile"). Geochemical data from 10 large igneous provinces (LIPS) have been collected from the literature to search for chemical signatures favourable for Ni-PGE mineralisation. The provinces include Deccan, Kerguelen, Ontong Java, Parana, Ferrar, Karoo, Emeishan, Siberia, Midcontinent and Bushveld. Among these LIPs, Bushveld, Siberia, Midcontinent, Emei Mt and Karoo are "fertile", hosting magmatic ore deposits or mineralisation of various type, size and grade. They most commonly intruded through, or on the edges of, Arcbaean-Paleoproterozoic cratonic blocks. In contrast, the "barren" LIPs have erupted through both continental and oceanic crustal terranes of various ages. Radiogenic isotopic signatures indicate that almost all parental LIP magmas are generated from deep-seated mantle plumes, and not from the more widespread depleted asthenospheric mantle source: this confirms generally accepted plume models. However, several important geochemical signatures of LIPS have been identified in this study that can discriminate between those that are "fertile" or "barren" in terms of their Ni-PGE potential. The fertile LIPS generally contain a relatively high proportion of primitive melts that are high in MgO and Ni, low in Al2O3 and Na2O, and are highly enriched in most of the strongly incompatible elements such as K, P, Ba, Sr, Pb, Th, Nb, and LREE. They have relatively high Os contents (>= 0.03 to 10 ppb) and low Re/Os (< 10). The fertile LIP basalts display trends of Sr-Nd-Pb isotopic variation intermediate between the depleted plume and an EM1-type mantle composition (and thus could represent a mixing of these two source types), and have elevated Ba/Th, Ba/Nb and K/Ti ratios. These elemental and isotopic signatures suggest that interaction between plume-related magmas and ancient cratonic lithospheric mantle with pre-existing Ni- and PGE-rich sulfide phases may have contributed significantly to the PGE and Ni budget of the fertile flood basalts and eventually to the mineralisation. This observation is consistent with the location of fertile LIPS adjacent to deep old lithospheric roots (as inferred from tectonic environment and also seen in global tomographic images) and has predictive implications for exploration models. Barren LIPs contain fewer high-MgO lavas. The barren LIP lavas in general have low Os contents (mostly <= 0.02 ppb) with high Re/Os (10- >= 200). They show isotopic variations between plume and EM2 geochemical signatures and have high Rb/Ba ratios. These signatures may indicate involvement of deep recycled material in the mantle sources or crustal contamination for barren LIPs, but low degrees of interaction with old lithospheric-type roots.