Magmatic Nickel Deposit Model

Nickel deposits fall into three main types – Archaean komatiite hosted deposits, laterite nickel deposits and magmatic nickel sulphide deposits.  A fourth type (hydrothermal)  is emerging in importance Australia’s eastern states with discoveries in Tasmania and Queensland.  The Thomson fold belt is prospective for the magmatic and hydrothermal types, which are usually associated with mafic to ultramafic igneous intrusions.  

Nickel occurs as sulphides such as the mineral pentlandite and is thought to accumulate by gravitational settling and similar processes related to magma flow.  Copper, chromite and Platinum group element (PGEs) deposits share the same association.  

The magmas ascend through the crust and cool as they encounter cooler crustal rocks. If the original sulphur content of the magma is sufficient, or if sulphur is added from crustal wall rocks, a separate sulphide liquid forms as droplets dispersed throughout the magma. Because the partition coefficients of nickel, copper, and PGE as well as iron favour sulphide liquid over silicate liquid, these elements preferentially transfer into the sulphide droplets from the surrounding magma. The sulphide droplets tend to sink toward the base of the magma because of their greater density, and form sulphide concentrations. On further cooling, the sulphide liquid crystallizes to form the ore deposits that contain these metals.