The relation between iron-formation and low temperature hydrothermal alteration in an Archean volcanic environment

Abstract

The uppermost section of the Hunter Mine group (HMG) (2728 Ma), a bimodal volcanic complex in the Abitibi greenstone belt, contains both oxide and carbonate facies banded iron-formation (BIF). This paper explores the relationship between volcanic activity and the development of the two types of iron-formation. The oxide facies, represented by chert-jasper-magnetite iron-formation is widespread and recurs at several stratigraphic levels. Field evidence suggests it deposited directly on the sea floor during periods of volcanic quiescence, and is most probably derived from fluids seeping from volcaniclastic sediments and the compaction of felsic shard-rich tuffs. Seeping occurred along synsedimentary and synvolcanic faults and then along bedding planes. Carbonate (siderite) facies iron-formation, in contrast, formed locally below a silica cap rock by in situ low-temperature hydrothermal replacement of chert-tuff beds and even of the oxide formation. Thus the two types of iron-formation, although spatially and stratigraphically juxtaposed, are not the result of the same process, and only the carbonate facies appears to be directly related to a low temperature hydrothermal volcanic process. The contrasting genesis of oxide and carbonate iron-formations indicates that current simplistic models relating banded iron-formation to volcanic massive sulphide deposits need to be re-evaluated. The notion that oxide iron-formation represents a distal manifestation of volcano-generated hydrothermal activity may be the exception and not the rule, because, as indicated by this study, BIF occurs in the central part of a volcanic edifice, rather than on the flanks.