The Emily Ann and Maggie Hays nickel deposits are situated 117 km west of the town of Norseman, Western Australia, within the Lake Johnston Greenstone Belt.
The mines had been operational since 2001 when Nornickel suspended mining at Emily Ann and Maggie Hays in early 2009 because of drastically falling nickel prices. Poseidon Nickel acquired the Lake Johnston Nickel Project from Norlisk in 2014.
Discovery
The Maggie Hays deposit was developed in 1996 by LionOre. It was discovered by prospecting in the 1970s by Anaconda Co. and Union Miniere, who had first located a nickel geochemical anomaly and drilled the disseminated halo, but missed the lucrative high-grade massive sulfide mineralisation by as little as three metres.
The recognition of the Maggie Hays orebody by LionOre geologists in the late 1990s was based upon electromagnetic geophysical surveys and deep diamond drilling of conductive anomalies. LionOre geologists credited the discovery to recognition of the electromagnetic response and drilling of the anomaly, however it is widely recognised that the initial discovery was based upon literature research and the fact that in the late 1970s and 1980s Union Miniere/Anaconda relinquished the tenements in a period of unfavourably low nickel prices.
Reassessment of the geophysical signature at Maggie Hays indicates that the hangingwall banded iron formation is as conductive as the massive sulfides, and that on this basis, as well as the steep orientation of the massive nickel sulfides, Maggie Hays is essentially blind to discovery.
The Emily Ann orebody is situated approximately 1200 m north of the Maggie Hays orebody and was drilled first in 1998 after a prolonged and saturated effort to electromagnetically prospect the entire prospective belt. The Emily Ann orebody was unequivocally discovered by geophysical surveying, the result of a flatter orientation of the orebody, the fact it is hosted within conductively dead felsic gneiss, and the depth of the upper parts of the orebody reaching to within 200m of the surface.
The Emily Ann discovery was a technical triumph, because it is a mechanically displaced recumbent fold of sheared massive sulfide hosted several hundred metres off the original ultramafic-felsic contact in a position not generally expected to host nickel sulfides.
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.
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.
Mining
Production at Emily Ann begun in late 2001, with the mine being owned by LionOre. Maggie Hays, 3 km south of Emily Ann, at that time, was owned partly by LionOre and BHP. Mining at the deposit was started after the conclusion of the ramping up of Emily Ann.