Paragenesis and origin of staurolite from a palladium-rich gabbronorite: an unusual occurrence from the Lukkulaisvaara layered intrusion, Russian Karelia

Microcrystalline staurolite and other highly aluminous secondary minerals ( Cl-rich pargasitic amphiboles, almandine, epidote, chlorite, Zn-rich hercyn ite, phlogopite, corundum, Al2SiO5 and AlO(OH) phases) occur in altered coa rse-grained gabbronorite (ACGG) that is associated with a sill-like body of microgabbronorite within the layered series of the Early Proterozoic Lukku laisvaara intrusion, Russian Karelia. The ACGG and a related plagioclase-be aring pyroxenite contain various base-metal sulphides (BMS) and platinum-gr oup minerals (PGM). The magmatic texture and relies of primary minerals are preserved in the staurolite-bearing rock. The composition of primary miner als in the ACGG is indistinguishable from that in the pristine host microga bbronorite, and indicates the existence of a close relationship between the se rocks, Textural data imply that the Al-rich assemblage largely formed at the expense of the plagioclase. The content of chlorine of the Al-rich amphiboles in the ACGG is unusually high (up to 4.5 wt.% Cl), including the amphibole closely associated with t he PGM. Inclusions of phlogopite in chalcopyrite also are Cl-rich (up to 1. 1 wt.% Cl). Thus, Cl was prominent in the fluid causing the alteration. The ACGG formed by crystallisation of isolated volumes of H2O-saturated mel t, in situ, during solidification of the microgabbronorite. The Al-rich amp hibole-garnet-staurolite assemblage appears to have formed in the range 560 to 670 degrees C at a deuteric stage. Ultimate loss of the fluid phase, wh ich had dissolved the original minerals incongruently, shifted the bulk com position of the gabbronorite to a corundum-bearing assemblage on the scale of tens of centimetres. Textural evidence shows that both the EMS and PGM w ere mobile during the alteration event; their mobility was rather restricte d, however, in spite of an elevated Cl concentration in the fluid phase.