Maggie Hays

Western Australia, WA, Australia

Main commodities: Ni
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The Maggie Hays nickel deposit is located some 540 km east of Perth and 125 km west of Norseman in Western Australia (#Location: 32° 13' 21"S, 120° 31' 01"E).

It lies within the Archaean Lake Johnston greenstone belt between the Southern Cross-Forrestania belt to the west and the Norseman-Wiluna belt to the east within the Archaean Yilgarn Craton, on the central-southern margin of the Southern Cross Province (and Youanmi Terrane) of the Yilgarn craton.

The Lake Johnston greenstone belt trends NNW-SSE and is ~100 km in length. It varies in width from 20 to <6 km, and is bounded to the east and west by Archaean granitic batholiths and migmatitic gneisses. Upper greenschist- to amphibolites-facies assemblages are found in the central portion of the belt with peak pressure of 5 to 7 kbars and temperatures of 596 to 678°C (Joly et al., 2008). Four phases of deformation are identified within the greenstone belt, comprising three early shortening, and one late brittle event (Joly et al., 2010). These deformation are not evenly distributed, and are partitioned into discrete zones and ductile rock types (Heggie et al., 2012).

The felsic volcanic unit that underlies the Maggie Hays deposit indicates the greenstone is 2921±4 Ma, with emplacement of ultramafic units occurring after 2903±5 Ma (Wang et al., 1996).

The Lake Johnston greenstone belt comprises a folded west facing homoclinal sequence of felsic to mafic volcanics and sedimentary rocks, including banded iron formation (BIF) and ultramafic units of possible intrusive origin. The belt occurs as a series of cuspate to linear shaped blocks separated by granitoid intrusive bodies, the most continuous of which hosts the Maggie Hays deposit. Within this interval there are three stratigraphic units, each of which contains an ultramafic unit.

The greenstone belt is composed of three stratigrapic units, the Maggie Hays, Honman and Glasse Formations (Gower and Bunting 1972, 1976). Sulphide Ni mineralisation is exclusively associated with ultramafic rocks of the middle Honman Formation (Barnes, 2006). The Honman Formation spans a strike length of 12 km and has been overturned, comprising five variably deformed, distinct lithostratigraphic units that stratigraphically dip at 60 to 80°E and young to the W. From east to west these comprise:
i). Felsic volcanic unit, a thick sequence of fragmental felsic volcanic rocks dated at 2921±4 and 2903±5 Ma (zircon U-Pb SHRIMP; Wang et al., 1996);
ii). Transition zone unit, a 50 to 75 m thick, laterally extensive sequence of sulphidic volcanic and sedimentary rocks, dominated by Fe-rich silicates, abundant garnet, thin chert units and both disseminated and stringer sulphides, representing the stratigraphic transition from felsic volcanic to banded iron formation (Heggie et al., 2012). The upper sections of the Transition zone unit show an increase in Fe (magnetite) and Si (chert) content over a 5 to 10 m transition into the overlying BIF unit;
iii). BIF unit, which comprises alternating bands of magnetite and chert forming a 120 m thick (on average) regional stratigraphic unit;
iv). Sedimentary unit, which caps the BIF unit, occurring as a thin, <15 m, averaging ~3.2 m thick sequence of quartz arenite, with minor narrow (<10 cm) bands of garnet-grunerite and sulphide-rich intercalations, overlain by a ~2 m thick sulphide rich sub-unit, dominated by massive nodular pyrite, narrow stringers of pyrrhotite, within a fine matrix of quartz. The transition from quartz arenite to sulphide ubit is gradational, with minor narrow stringers of sulphide in the underlying quartz-arenite;
v). Differentiated komatiitic flows, known as the Western ultramafic unit, emplaced on top of the volcanosedimentary sequence, representing the stratigraphically youngest unit of the Honman Formation. The massive sulphide mineralisation occurs at the base of this unit. The Western ultramafic unit is characterised by thin, <20 m flows with well developed upper spinifex-textured A zones and lower cumulate textured B zones.

The stratigraphic succession that accumulated prior to komatiite volcanism, specifically, the contact between the Felsic volcanic and BIF units, is interpreted to have acted as a magma trap that controlled the emplacement of intrusive subvolcanic feeders (the Central ultramafic unit) to the komatiite flows (the Western ultramafic unit; Heggie et al., 2012). The orthomagmatic mineralisation that defines the Maggie Hays Ni deposit occurs along the basal contact of the intrusive Central ultramafic unit.

The Central ultramafic unit occurs as an irregular, subhorizontal tube-like mass that crosscuts the Honman Formation stratigraphy and trends southward for ~3.5 km from the northern terminus, where the Maggie Hays Ni sulphide mineralisation is located (Heggie et al., 2012). The maximum cross sectional developement (300 to 400 m thick and >400 m in width) occurs in the vicinity of the Ni mineralisation, decreasing in size to the south. It comprises peridotite and dunite (olivine mesocumulate to adcumulate) with volumetrically minor pyroxenite and gabbroic to feldspathic-pyroxenite lithologies. Pyroxenite lithologies (retrograded to amphibolite) occur as a thin (<10 m wide) margin along the preserved sharp contact with the wall rock (Heggie et al., 2012). Where this intrusive contact is not preserved, the boundary of the intrusion is sheared and deformed. The pyroxenite locally contains small (<5 cm) xenoliths and irregular-shaped patches of crystallised felsic melts. The pyroxenite to peridotite-dunite transition is gradational, over a narrow 5 to 10 m interval. The peridotite-dunite within the pyroxenite occurs as a homogeneous sequence of olivine cumulates with varying trapped liquid abundances, that are replaced by metamorphic assemblages formed through hydration and dehydration during prograde metamorphism, to produce in mineral assemblages of metamorphic olivine, talc and anthophyllite. In the core of the Central ultramafic unit, all primary igneous textures are obscured by metamorphic olivine and zones of random anthophyllite that formed during static prograde metamorphism (Joly et al., 2008, 2010). Within the upper sections of the intrusion, there is a transition from olivine-dominant cumulates, through increasing pyroxene content, to gabbroic rocks with increasing feldspathic mineralogies (Heggie et al., 2012).

The Maggie Hays orebody is a 1400 m long, with a gradational up-dip margin at 100 to 180 m below surface. It has distinctly different southern and northern zones.   The Maggie Hays Main Zone (in the south) is 850 m long, and extends for 400 m down the 60° E dip, and comprises a zone of dominantly disseminated ore (<40% sulphide) up to 40 m thick, underlain by a narrow 7 m thick basal massive sulphide (the Main Zone Lower Massive Sulphide).

The Maggie Hays North zone is entirely hosted by felsic volcanics which form the footwall to the ultramafic unit that hosts the Main Zone. Intense deformation is partitioned within the felsic volcanic unit with pervasive shearing, localised mylonites and boudinage. The Main and North zones are contiguous, the boundary being the fault termination of the ultramafic unit. The North zone is a tabular body of complex stringer and massive sulphide mineralisation in a felsic breccia matrix. It has a strike length of 500 m and is typically between 3 and 9 m thick (maximum thickness of 9.5 m averaging just over 3 m). It is controlled by a dextral shear zone which dips at 60° to the east and terminates the northern continuation of the ultramafic and its associated Main Zone mineralisation. The North zone represents mobilisation associated with shearing of Main zone massive Ni sulphide along its basal contact (Joly et al., 2008, 2010).

In addition to these major mineralised zones, localised structurally controlled massive/stringer sulphide mineralisation, termed the Upper Massive Sulphide zone, occurs close at the northern pinch-out of the ultramafic, up-plunge of the Main Zone mineralisation.

The sulphides in both zones comprise polycrystalline aggregates of pyrrhotite-pentlandite ±pyrite and chalcopyrite.   Supergene alteration involved the replacement of pentlandite and to a lesser extent pyrrhotite by violarite and pyrrhotite by pyrite, locally to depths of 400 m.

A third ultramafic mass, the Eastern ultramafic unit occurs on the eastern side of the Honman Formation and has only been encountered by diamond drilling. Although a significant igneous body, no mineralisation has been identified within this unit (Heggie et al., 2012).

The indicated resource at Maggie Hays in 2003 at a 0.7% Ni cut-off was 10.8 Mt @ 1.5% Ni, plus an inferred resource of 0.97 Mt @ 1.10% Ni as disseminated ore. In addition the Main Zone Lower Massive Sulphides amounted to 0.475 Mt @ 3.55% Ni.

Heggie et al., 2012 report the Maggie Hays deposit comprises - 12.3 Mt at 1.5 wt % Ni.

Maggie Hays was purchased by Norilsk Nickel as a result of the LionOre Mining International Limited acquisition on 28 June 2007 and is part of the Lake Johnston operation.

The most recent source geological information used to prepare this summary was dated: 2012.     Record last updated: 24/6/2013
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Buck P S, Vallance S A, Perring C S, Hill R E, Barnes S J  1998 - Maggie Hays nickel deposit: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne   Mono 22 pp 357-364
Heggie G J, Fiorentini M L, Barnes S J and Barley M E,  2012 - Maggie Hays Ni Deposit: Part 2. Nickel Mineralization and the Spatial Distribution of PGE Ore-Forming Signatures in the Maggie Hays Ni System, Lake Johnston Greenstone Belt, Western Australia : in    Econ. Geol.   v.107 pp. 817-833
Heggie G J, Fiorentini M L, Barnes S J and Barley M E,  2012 - Maggie Hays Ni Deposit: Part 1. Stratigraphic Controls on the Style of Komatiite Emplacement in the 2.9 Ga Lake Johnston Greenstone Belt, Yilgarn Craton, Western Australia : in    Econ. Geol.   v.107 pp. 797-816

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