Chromite, platinum group elements and nickel mineralisation in relation to the tectonic evolution of the Dun Mountain Ophiolite Belt, east Nelson, New Zealand

摘要

In the Dun Mountain Ophiolite Belt of east Nelson, New Zealand, chromite segregations (chromitites) occur in a lower zone of serpentinised harzburgite and dunite from Wooded Peak (Dun Mountain) to Little Ben, and in dunite-rich zones in the Red Hills massif and Baldy Ridge outlier. Electron probe microanalyses of chromite grains from the chromitites and also the Red Hills peridotite reveal a range in chromite compositions that are consistent with depletion of peridotite and reaction with island arc tholeiite and boninitic melts in a forearc setting of a supra-subduction zone. Podiform chromitite samples in the harzburgite zone show elevated Ir and Ru values, similar to the levels of enrichment of these platinum group elements (PGE) found in podiform chromite deposits in ophiolites worldwide. The PGE values in dunite-hosted chromitite from Baldy Ridge (Matakitaki) have elevated Pt, Pd and Rh, as is found in cumulate dunite zones in some other ophiolites. Disseminated Ni mineralisation as awaruite +/- magnetite +/- ferritchromit +/- heazlewoodite +/- pentlandite +/- native Cu occurs in serpentinised peridotites, and was likely produced as a by-product of hydration as a result of detachment faulting allowing penetration of seawater into underlying mantle peridotite.