Post-subduction magmatism and gold mineralisation in the Colorado Mineral Belt: insights from accessory minerals

摘要

Magmatism that occurs following subduction (post-subduction) is commonly associated with gold (Au) and tellurium (Te) mineralisation (Richards 2009). These post-subduction magmas tend to be low volume, hydrous melts that are variably alkali-enriched, and often associated with porphyry-epithermal deposits. The processes controlling post-subduction enrichments in gold and tellurium are thought to begin during active subduction. Arc magmas have a 'cryptic' amphibole signature, whereby La/Yb and Dy/Yb ratios suggest the presence of amphibole during magma differentiation, but arc magmas lack a phenocrystic amphibole phase (Davidson et al. 2007). Therefore, it is plausible that amphibole remains as a residue in the lower crust. These cumulates act as a 'sponge' and can filter water throughout the subduction to mineralisation type in carbonatites appears to be at least partially controlled by confining pressure, with almost all examples of economic interest at palaeo-depths <5 km. The potential exception is phoscorites (representing immiscible Fe-P-rich melts) where some evidence suggests deeper, even >10 km depths of crystallisation. Much like silicate systems, orthomagmatic REE-Nb-P mineralisation occurs in igneous cumulates (of apatite, pyrochlore) and roof-type REE-F mineralisation in complex polyphase assemblages (of fluorite, REE-F carbonates) near the upper margins of magmatic bodies. Furthermore, weathering has played an important role in some carbonatites, upgrading and redistributing the commodities of interest.