The komatiite nickel sulfide association at Scotia; a petrochemical investigation of the ore environment

Abstract

The Scotia nickel deposit is typical of the volcanic peridotite-associated type of Archean sulfide deposit. It occurs at the basal contact of a massive dunitic unit which is itself the basal unit of a thick extrusive komatiitic sequence. The ore zone is marked by a thickening of the overall ultramafic sequence and is located in a depression at the footwall contact. Sulfide textures in the massive ore indicate that it has been strongly deformed, which is in marked contrast to the overlying matrix ore and komatiitic sequence. Textural and mineralogical preservation of olivine in the ultramafic sequence shows that the sequence is made up of a series of thin flow units with gravity or flow-differentiated olivine-rich zones. Between these flows are thicker flows, characterized by massive olivine harrisites, commonly with ocelli structures. Throughout the sequence there are spinifex-textured olivine zones and well-preserved ultramafic volcanic breccias.Major element data indicate that the primary komatiitic magma had 30 wt percent MgO (anhydrous) and this gave rise to all the flow types. Microprobe data on relic brown olivines show that the spinilex-textured olivines are strongly zoned (Fo (sub 75-93) ) while the harrisitic olivines are remarkably constant at about Fo 92 . Olivine chemistry within the ore zone reflects the position of the olivine with respect to the massive sulfides and also subsolidus reequilibration. A particular feature of the ore zone is the occurrence of colorless olivine which appears to be degraded after brown igneous olivine. The colorless olivine has abnormally low Ni concentrations associated with Fo- and Mn-rich cores. Rims to these grains are Mn depleted, Ni enriched, and more fayalitic than the cores.Various possible models are discussed for the origin of the orebody and it is proposed that the sulfide was erupted as a dispersed liquid phase within an intratelluric mush of olivine and komatiitic liquid. The sulfides are thought to fill topographic lows or rifts on the sea floor in which the supernatant komatiite liquid is decanted off as the trough fills with later erupted olivine grains. Within the ore-bearing ultramafic unit there is little evidence for in situ olivine crystallization.