Advances in NiCu-PGE Sulphide Deposit Models and Implications for Exploration Technologies

It is commonly possible to identify groups of orebodies within one cluster or camp of temporally and spatially constrained deposits that have common empirical traits, and to then develop successful exploration technologies based on these similarities. Models developed for exploration for contact, footwall, and offset styles of mineralization at Sudbury are examples at the camp scale, whereas the models for massive sulphide and disseminated sulphide deposit types associated with the ultramafic rocks of the Yilgarn represent examples where the models have been applied on the scale of greenstone belts. Exploration at the edges of economically mineralized systems is often rewarding as the detailed quantitative data can be used to inform robust predictive models, and the exploration risk is commensurately lower. Inherent in this approach is the bias towards exploring with known technologies and models that do not consider previously unrecognized geologic settings. The risk of missing a new style of mineralization in a mature camp or mine environment or in a new geological environment is thus heightened. In greenfields exploration, data sets are often unavailable or incomplete, and successful exploration requires holistic geological models that embrace the common characteristics of nickel systems and maximize value from empirical geoscience observations that form the basis for project selection and then targeting of exploration using advanced geochemical and geophysical technologies. The petrologic characteristics of the host rocks can effectively be used to rank exploration approaches in prospective environments; for example, the petrological associations and geochemical signatures of the associated mafic and ultramafic rocks provide an indication of whether the magma is from a productive source and has equilibrated with sulphide. At the deposit scale, linkages between the abundance of metal in the sulphides within the rock and the geological context offer ways to inform geological models, and they can focus efforts to define drill targets and to better delineate orebodies. This paper summarizes the state of present understanding of the key requirements for the formation of large nickel sulphide systems. It also reviews recent advances in our understanding of these systems and illustrates how geological models are increasingly informed by an array of geochemical and geophysical data. INTRODUCTION AND OBJECTIVES Ni-Cu-PGE sulphide ores typically form by the equilibration of immiscible magmatic sulphide and silicate magma (Naldrett, 2004). The extent to which the sulphides are enriched in Ni, Cu, and platinum group elements (PGE) is then a measure of not only the composition of the parental magma, but it is a function of the inherent efficiency of the chemical and physical processes of equilibration between the two melts (Naldrett, 2004). Without a common set of empirical characteristics together with technologies based on geochemistry and geophysics, we are challenged to recognize metallotects with the potential to host economic concentrations of Ni sulphide mineralisation; exploration expenditures on drilling and delineation depend on the right choices. Several features are common to igneous systems that contain Ni sulphide mineralisation; these have been referred to as ‘key features’ (e.g., Keays and Lightfoot, 1994), and they are commonly used in evaluation of exploration opportunities. Some of the principal observations that relate to many, but importantly not all, major Ni sulphide deposits include such features as available source of metals (mafic and ultramafic magmas), a source of S to saturate the magma (e.g., sulphidic black shales), gravitational segregation of dense immiscible sulphide liquid, and concentration of the sulphides into physical traps at the base of intrusions, within conduits, or in rock bodies emplaced in transpressional shears. Geochemical characteristics attendant with these traits include a within-plate magma signature (with the obvious exception of Sudbury, which is a crustal melt sheet; see below), depletion of the silicate melt in Ni, Cu, and PGE due to the removal of sulphide melts (into which these elements strongly partition), and contamination of the magma by assimilation of continental crust. The geophysical signatures of Ni systems can (but don’t always) include magnetic high signatures, gravity highs, and/or either conductivity (due to the Ore Deposits and Exploration Technology _________________________________________________________________________________________