Regional Geology
The Maggie Hays and Emily Ann orebodies are hosted within a komatiitic belt of rocks within the Archaean c. 2.85 Ga Lake Johnston Greenstone Belt (LJGB). There are three ultramafic horizons recognised within the LJGB stratigraphy; the Eastern, Central, and Western ultramafic. The entirety of the belt's nickel endowment is hosted within the Central Ultramafic Unit, known as the CUU. However, disseminated and low tenor nickel mineralisation is known from the other ultramafic units, especially the Western Ultramafic Unit (WUU).
The general stratigraphy of the belt is, from base upwards, a thick sequence of felsic orthogneiss composed of fragmental to glomerocrystic feldspar gneiss, known as the footwall felsic sequence; the ultramafic units of komatiite affinity, 'overlain' by grunerite-magnetite-quartz-amphibole banded iron formation of the Honman Formation, tholeiitic basalt and metasedimentary rocks.
Regionally, several subvolcanic lopolithic layered intrusions have been identified from mapping and drilling. These are interpreted to represent the feeder conduits to extrusive ultramafic and mafic igneous rocks stratigraphically higher in the belt. Examples include the Medcalf Ultramafic Intrusion, a 3.5 km long, ~1 km thick pile of gabbroic to pyroxenitic cumulates which contain subeconomic stratiform vanadiferous magnetite deposits.
The extrusive and intrusive igneous rocks are underlain and intruded by a series of I-type granite intrusions and granite domes of c. 2.65 Ga age. A late dyke swarm of Proterozoic age intrudes the belt, most notably the Jimberlana Dyke which attains a thickness of some 600m and transects the Emily Ann and Maggie Hays orebodies.
The Lake Johnston Greenstone Belt is metamorphosed to amphibolite facies, and in parts is extremely highly strained. Structurally the belt is composed of overturned west-vergent isoclinal fold trains, separated by thrusts. Deformation intensity is heterogeneous, ranging from weak foliation overprints to extreme 10:1 or 20:1 l-tectonite mylonite development.
Read more about this topic: Emily Ann And Maggie Hays Nickel Mines