Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Many magmatic ore deposits are zoned with an Fe-rich portion and a Cu-rich portion. Nickel is generally evenly distributed. This distribution is attributed to fractional crystallization of a sulfide liquid, with Fe-rich monosulfide solid solution (Fe-mss) crystallizing first and forming an Ferich cumulate. The fractionated liquid eventually crystallizes as Ni-rich mss (Ni-mss) and a Cu-rich intermediate solid solution (iss). Rhenium, Os, Ir, Ru and Rh concentrate in the Fe-rich rocks. Platinum, Pd and most other chalcophile elements concentrate in the Cu-rich rocks. From 600 ¡ C pyrrhotite (Po) and pentlandite (Pn) exsolve from mss and chalcopyrite (Ccp) exsolves from iss. Much of the Pd is hosted in Pn. This is conterintuitive because Pd is incompatible with mss (DMss/Liq = 0.2). Thus one would expect the Pd to concentrate in the Cu-rich liquid and partition either into the iss or, if it is incompatible with iss, then it should crystallize as a Pd mineral. In order to explain this conundrum we have previously proposed that Pd diffuses into the Pn during the exsolution process.

To investigate the affects of diffusion we have determined the Pd content of Pn both from Fe-rich and Cu-rich ore, looked at the variation of Pd content in Pn with texture and at the zonation of Pd within Pn. During cooling and exsolution the texture of Pn changes. The Pn formed at high temperature is coarse grained and granular, the Pn formed at lower temperature forms narrow chains around the Po and the Pn formed at the lowest temperatures occurs as small flames within Po. The granular Pn is richest in Pd, the flames poorest in Pd. Furthermore the granular Pn is zoned, with highest concentrations in the center. We interpret these observations to suggest that the Pn that formed at high temperature (the center of the granular Pn) absorbed most of the Pd in its viscinity leaving a depleted halo around it. Thus the next layer of the granular Pn that formed was depleted in Pd. The chain Pn and flame Pn that form at lower temperatures contains less Pd for three reasons: the granular Pn had depleted the sulfides in Pd; the temperature was lower and hence diffusion is slower; there was less time for diffusion to occur before diffusion ceases altogether.