Application of platinum-group element (PGE) geochemistry to select geological problems

Abstract

Porphyry deposits produce nearly 75% of the world's Cu, 95% of its Mo and 35% of its Au. These deposits are related to subduction zones, and their tectonic setting is a critical factor in determining their occurrence. However, not all porphyries give rise to economic mineralisation. Over 90% are barren, even though they share many features with the economic porphyries, so the critical unanswered question is "what controls magma fertility?". The present study used the concentrations of platinum-group elements (PGEs), analysed by the fire-assay isotope dilution method, to determine the timing of sulfide saturation in an evolving magma system, which in relation to volatile saturation is one of the most important factors controlling magma fertility. The time gap between these two processes can determine the type of ore deposit formed. PGEs were used because they partition strongly into sulfide phases (KD ca. 1000 higher than Cu and ca. 100 higher than Au). When sulfides precipitate, the PGEs are stripped from the melt first, making them sensitive indicators of sulfide saturation. Additionally, their solubility in hydrothermal fluids is low, so they are less mobile than Cu and Au. Mount Hagen, a mid-Pleistocene stratovolcano located in Western Highlands of Papua New Guinea, has a very complex tectonic setting that makes it challenging to unravel the history of magma evolution. Trends in the PGE, however, record evidence of two sulfide saturation events. The first occurred in the mantle or at a lower crustal level, as evidenced by the presence of sulfide inclusions in megacrysts, even in the most mafic samples. The second occurred once the evolving magma reached 8.5 wt.% bulk MgO. At about 5.5 wt.%, however, the trends from all of the PGEs reverse sharply before commencing a second decline. This is due to a new magma pulse entering the system, which coincided with the appearance of plagioclase, hornblende and olivine megacrysts, and reverse zoning in plagioclase. Based on modelling using Petrolog3, it is estimated that the magma had undergone ca. 10% of fractional crystallisation by the time it became sulfide saturated at ca. 8.5 wt.% MgO. Fractionation modelling of Pd, Au and Cu showed that the enrichment factor of these metals was close to 1. Early sulfide saturation makes it unlikely that the first Mount Hagen magma pulse have produced economic porphyry-style mineralisation. Further petrological studies on samples from Mount Hagen confirmed that the system underwent various magma mixing events, possibly remelting and assimilation. Multiple lines of evidence demonstrate that Mount Hagen is a good example of a mush zone - disequilibrium between the mineral phases and the melt and each other, varied chromium concentrations in the clinopyroxenes, plagioclase with the highest An# occurring in the most felsic samples, no interstitial sulfides and a continuous Cu-Fe-S evolutionary trend in magmatic sulfides, with no distinction between bulk-MgO or host mineral. The Permian-Triassic mass extinction (EPME) at 251.9 Ma was one of the most significant events in Earth's history. It resulted in the disappearance of 90% of marine species and 70% of vertebrates. The two leading theories regarding the cause of the extinction are: (i) a chain of events triggered by a bolide impacting the Earth, and (ii) the products of Siberian Traps flood volcanism. The Ir concentrations and Re/Ir in sediments are a sensitive indicators of the presence or absence of an extra-terrestrial component. Analyses of multiple end-Permian sediment samples from four locations in China and Japan for major, trace elements, PGEs, and calculated enrichment factors of trace elements sensitive to redox conditions point, without a doubt, to Siberian volcanism, not bolide impact, being the primary cause of the EPME. Show more