Magmatic sulfide ore deposits

Magmatic sulfide ore deposits are products of natural smelting: concentration of elements from silicate magmas (slags) by immiscible sulfide liquid (matte). Deposits occupy a spectrum from accumulated pools of matte within small igneous intrusions or lava flows, forming orebodies mined primarily for Ni and Cu, to stratiform layers of weakly disseminated sulfides, mined for platinum group elements, within large mafic-ultramafic intrusions. One of the world’s most valuable deposits, the Platreef in the Bushveld Complex in South Africa, has aspects of both of these end members. Natural matte compositions vary widely between and within deposits, controlled largely by the relative volumes of matte and slag that interact with one another

Use and calibration of portable X-Ray fluorescence analysers: application to lithogeochemical exploration for komatiite-hosted nickel sulphide deposits

<p>Portable X-Ray Fluorescence (pXRF) analysers allow on-site geochemical analysis of rock powders and drill core. The main advantages of pXRF analysis over conventional laboratory analysis are the speed of data collection and the low cost of the analyses, permitting the collection of extensive, spatially representative datasets. However, these factors only become useful if the quality of the data meets the requirements needed for the purposes of the study. Here, we evaluate the possible use of portable XRF to determine element concentrations and ratios used in exploration for komatiite-hosted nickel sulphides. </p> <p>A portable XRF analyser was used to measure a series of chalcophile and lithophile element concentrations (Si, S, K, Ca, Ti, Cr, Fe, Ni, Cu, Zn, As, Sr, and Zr) of 75 samples from three komatiite units associated with nickel sulphide ores in the Yilgarn Craton, Western Australia. Crucial steps in the study were the development of a strict calibration process as well as numerous data quality checks. The 670 analyses collected in this study were compared with conventional laboratory XRF data on discriminant diagrams commonly utilized in exploration for komatiite-hosted nickel sulphides (Cr vs Ni and Ni/Ti vs Ni/Cr). After comparing the results obtained with pXRF during this study with the laboratory values, we can conclude that portable XRF analyses can be used for rapid assessment of the nickel sulphide prospectivity of komatiites provided that strict control protocols are followed. </p

3D textural evidence for the formation of ultra-high tenor precious metal bearing sulfide microdroplets in offset reefs: an extreme example from the Platinova Reef, Skaergaard Intrusion, Greenland

The Platinova Reef in the Skaergaard Intrusion, east Greenland, is an example of a type of layered-intrusion-hosted, precious metal-enriched, stratiform, disseminated sulphide deposit referred to as “offset reefs”. These typically show platinum-group element (PGE) enrichment immediately below a major increase in the abundance of Cu-rich sulphides, with a prominent peak in Au enrichment exactly at that transition between the PGE-rich and the Cu-sulphide-rich zones. The reasons for the relative sequence of offsets in metal peaks, and the occasionally very high metal tenors have been subject to great debate. Here we use an integrated approach of high-resolution X-ray computed tomography (HRXCT), SEM, synchrotron and desktop microbeam XRF mapping, and thin section petrography to comprehensively classify the textural relations of the precious metal-bearing sulphides of the Platinova Reef as an extreme end member example of an exceedingly high tenor offset deposit. Our results show that in the zones of PGE enrichment, precious metal minerals (PMMs) are intimately associated with Cu sulphide globules, mostly located at, or close to, silicate and oxide boundaries. The textures are identical in zones enriched in Pd and Au, and thus we do not see any evidence for different processes forming the different zones. The PMM:Cu sulphide ratio in each globule varies significantly but overall the size of the globules increases from the Pd-rich, through the Au-rich, and into the Cu zone, with a significant corresponding decrease in PM tenor. As such, this records a progression of exceedingly high tenor, microdroplets of sulphide, which progressively get larger up through the section, and decrease in tenor proportionally to their size. Cumulus droplets of Cu sulphide became enriched in metals, and were trapped in situ without significant transport from their point of nucleation. The transition to larger sulphides represents a change from sulphides nucleated and trapped in situ, to larger ones that liquated from magma devoid of crystals, and that were able to grow and sink. This feature is common in all offset reef deposits, and is marked by the major enrichment in Au. Although the metal ratios of PGE to Au in the Pd- and Au-rich offset zones differ, the identical textures and comparable mineralogy show the physical mechanisms of concentration are the same, indicating a similar physical method of concentration. The relative position of the Pd, Au and Cu peaks in the Platinova Reef is essentially the same as that in numerous other offset reefs, suggesting that common overarching processes are responsible for the enrichment in metals, and relative offsets in peak metal concentrations in all such deposits. The most important of these processes are their relative Dsul/sil values and the diffusivities of the metals, which determine the order of offsets and the high tenors of the smallest sulphide droplets. The Platinova Reef therefore records the extreme enrichment via equilibrium and diffusive partitioning into sulphide liquid microdroplets very close to their point of nucleation