EMPLACEMENT OF SMALL, PGE-DEPLETED GABBROIC INTRUSIVE ROCKS IN THE APPALACHIANS AND CALEDONIDES. IS THERE A LINK TO LARGER MAGMA RESERVOIRS AT DEEPER LEVELS IN THE CRUST?

The Appalachians of North America and the Caledonides of Europe contain a host of small gabbroic intrusives generally less than 10 km in diameter. The Skjækerdalen igneous complex is a lenticular intrusion approximately 4 km long and 1.5 km wide situated in Gula Group meta-pelites of the Trondheim Nappe complex in the central Norwegian Caledonides. The intrusion is a polymagmatic mafic breccia consisting of quartz gabbro with a quartz diorite rim and a central brecciated core consisting of gabbro fragments in a leucocratic dioritic matrix. Interstitial magmatic sulphides are present in some of the gabbro fragments in the brecciated core. Recalculated to 100% sulphide, average concentrations for base metals in the gabbro are Ni; 7.6 wt%, Cu; 4.6 wt%, & Au; 634 ppb, while PGE average 167 ppb Ru, 68 ppb Ir, 90 ppb Rh, 99 ppb Os, 171 ppb Pd, & 120 ppb Pt. PGE concentrations in magmatic sulphides at Skjækerdalen are low in comparison with deposits of comparable Ni and Cu contents (e.g. Sudbury, Noril'sk). A mechanism to explain these lower concentrations is the formation of an early olivine-chromite-PGM phase during differentiation in a magma chamber at deeper crustal levels in which precipitation of Pt-rich phases scavenged much of the PGE from the silicate magma. Subsequent droplets of sulphide liquid segregated at higher levels settled through a PGE-depleted magma. Resistance of the solidified olivine-chromite-PGM phase to remobilisation during final emplacement of intrusive rocks at Skjækerdalen resulted in sulphide-rich gabbro with a low PGE tenor. A similar mechanism can be used to explain low PGE values in other small sulphide-rich gabbroic intrusions in orogenic settings.