EXAMINING ACCESSORY MINERALS ASSOCIATED WITH THE PICKET PIN PLATINUM-RICH ZONE
Though the sulfide deposits have been studied extensively, little work has been carried out on their accessory minerals outside the ore zones. This study focuses on the minerals found in Anorthosite Subzone II (ANII), which hosts the Picket Pin deposit. Thin section analysis of both pyroxene-rich and pyroxene-poor ANII samples has shown distinct mineral assemblage variations between samples that are high in interstitial pyroxene and samples containing no visible pyroxene.
Pyroxene-rich samples (with visible interstitial pyroxenes) show overall lower sulfide concentrations relative to pyroxene-poor samples, but have a much higher ratio of intracrystalline to interstitial sulfides. Intracrystalline sulfides seem to be more associated with products of plagioclase alteration, like sericite, while interstitial sulfides seem to be accompanied by high temperature, hydrous minerals like amphibole and mica.
The lack of interstitial sulfide crystals in pyroxene-rich samples, coupled with the variation of accessory minerals, suggests the possibility of two sulfur saturation events during the formation of the complex. The first would have deposited the intracrystalline sulfides found in the plagioclase. Interstitial sulfides in these rocks may have escaped, possibly during outgassing of magmatic volatiles, allowing pyroxene oikocrysts to form in the gaps. In samples which did not contain interstitial pyroxene, a second sulfide deposition event, likely hydrothermal, may have deposited interstitial sulfides. The next stage in this study will further investigate the sulfides and accessory minerals using an electron microprobe, providing a more complete record of the mineral assemblages associated with both the intracrystalline and interstitial sulfides. Further electron microprobe work on the accessory minerals will serve to narrow down the mechanism of deposition.