Geochemistry and petrogenesis of unaltered and altered volcanic sequences in the southern Abitibi greenstone belt

Abstract

The Abitibi greenstone belt is a volcanic-plutonic dominated subprovince of the Archean Superior structural province. In the southern volcanic zone (SVZ) of the Abitibi greenstone belt, there are numerous, well preserved, komatiite-tholeiite sequences in fault-bounded tectonostratigraphic terranes, or "assemblages". Volcanism commenced at 2750 Ma and terminated at $\sim$2700 Ma. New analyses of relatively fresh komatiites by ICP-MS from the classical Pyke Hill locality for spinifex textured komatiites, confirmed the Nb, LREE-depleted, Al-undepleted characteristics of the komatiites. The new analyses show positive Zr and Hf anomalies such that the Zr/Hf ($\sim$30) ratios are less than the chondritic ratio of 36. The anomalies may be explained by a two stage melting model. Komatiite-tholeiite sequences in Tisdale Township, $\sim$80 km west of Pyke Hill include domains of carbonate-dominated alteration. Two compositional varieties of komatiite are present. Type 1 has flat REE patterns, and is Al-undepleted. Type 2 is LREE and HREE depleted, yielding concave up REE patterns, and is Al-depleted. Type 2 is more magnesian than type 1. Type 1 was derived from an undepleted, but type 2 from a depleted mantle source. Two different mantle sources are required for the komatiites, both different from the mantle source for Pyke Hill komatiites. Proximal to faults, komatiite-tholeiite sequences are intensely deformed and altered, and locally contain large gold deposits. Komatiites are of the Nb, LREE-depleted, Al-undepleted type, as at Pyke Hill, and also have positive Zr and Hf anomalies. Some komatiites show evidence of crustal contamination in the conjunction of La/Nb $>$ 1, LREE enrichment, and negative P and Ti anomalies. Hence in the western Abitibi SVZ, distinct volcanic sequences were derived from separate mantle sources, and erupted variously through oceanic or continental crust, and were subsequently thrust together. Least altered komatiites in Whitney Township have smooth mantle normalized diagrams for Au and PGE. Komatiites have primary gold contents of 2-4 ppb--not gold rich relative to average crust. Altered komatiites have smooth PGE patterns but positive gold anomalies. Intensely altered komatiites in Whitney Township show unusual REE patterns, with relatively stable LREE and MREE, but HREE patterns that are smoothly depleted or concave down. The megaplume model for greenstone belts is not supported. Komatiites must have come from multiple plumes sampling different mantle sources, and coexisting basalts either from the same source as komatiites in some volcanic sequences, or from different mantle sources in other sequences. (Abstract shortened by UMI.)