Jump to Search Jump to Navigation Jump to Content

Geological history and genesis of the high-grade iron ore deposits of the Joyner's Find greenstone belt

Joyner's Find, Western Australia

The project consisted of 1:2000 scale field mapping of the iron formations of the Joyner's Find Greenstone belt (JFG) and associated petrographic studies of iron ore and BIF. Numerous ore types including microplaty hematite and specular hematite and a detailed geological history involving folding, thermal metamorphism and crustal scale shearing followed by two marine transgressions are described.  

Numerous generally small bodies of high-grade hematite are associated with banded iron formations (BIF) at two horizons; a lower sequence (B Band) of up to seven thin typically less than 20m thick BIF bands interbedded with mainly ultramafic schists and an upper sequence (C Band) of five thicker BIF units of 10 to 200m thickness separated by up to 300m of ultramafic to felsic volcanic rocks. Most of the larger ore bodies consist of finely banded porous microplaty hematite hosted by BIF units ranging from 5 to 200m in thickness. Numerous lenses of massive non-porous hematite occur both within the bedded ore deposits and rarely as separate entities within BIF and mafic rocks.

Mineralization in the JFG belt typically consists of porous bedded microplaty hematite and lenses of massive hematite with grades typically in excess of 68wt% Fe. Hydrothermal specular hematite and quartz-hematite veins are common but no evidence of these upgrading BIF to high-grade ore was observed. However, the veins commonly caused recrystallization of bedded hematite and deposition of specular hematite in pore spaces transformed some porous bedded ore to massive ore. Less common are veins and lenses of hematite that appear to have formed by replacement of BIF by magnetite in which the bedding is at least partially preserved. The microplaty hematite ore is commonly underlain by martite goethite ore (60-65wt% Fe).

The high-grade deposits are interpreted as weathered synsedimentary/diagenetic ore bodies that have been locally invaded and replaced by hydrothermal specular hematite or magnetite. BIF and ore in the northern part of the belt was exposed to subaerial weathering in the early Paleoproterozoic resulting in the formation of microplaty hematite, whereas BIF and ore that was only weathered during later erosion cycles formed anhedral hematite-goethite with no microplaty hematite.

The juxtaposition of two separate unmetamorphosed epeirogenic sedimentary sequences with only minor deformation has enabled both of the erosion surfaces produced by the marine transgressions to be preserved on BIF in the northern part of the area. The alternation of supergene weathering and diagenetic processes associated with the unconformities and the absence of post Archean deformation and metamorphism has greatly assisted the interpretation of the ore deposit's genesis and the geological history of the JFG belt.