Architecture and emplacement of the Nebo–Babel gabbronorite-hosted magmatic Ni–Cu–PGE sulphide deposit,West Musgrave,Western Australia |
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| Authors: | Zoran Seat Stephen W Beresford Benjamin A Grguric Rob S Waugh Jon M A Hronsky M A Mary Gee David I Groves Charter I Mathison |
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| Institution: | (1) Centre for Exploration Targeting, School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009, Australia;(2) BHP Billiton—Minerals Exploration, Level 34 Central Park , 152–158 St Georges Terrace, Perth, WA, 6000, Australia |
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| Abstract: | The Nebo–Babel Ni–Cu–platinum-group element (PGE) sulphide deposit in the West Musgrave Block, Western Australia, is the largest
nickel sulphide discovery in the last 10 years. The deposit is hosted within a concentrically zoned, olivine-free, tube-like
(chonolithic), gabbronorite intrusion associated with the, approximately, 1,078-Ma Giles Complex-layered intrusions in the
Warakurna large igneous province. Emplaced into sulphide-free amphibolite facies orthogneiss, the fault-offset Nebo–Babel
chonolith extends for 5 km and has a cross-section of 1 × 0.5 km. Igneous mineralogy, fabrics, and textures are well preserved.
The lithostratigraphy includes variably textured leucogabbronorites (VLGN) that form an outer shell around mineralised gabbronorite
(MGN), with barren gabbronorite (BGN) and oxide–apatite gabbronorite (OAGN) in the middle and lower parts of the chonolith.
Mineral and whole-rock geochemistry indicate that the units become progressively evolved in the order: VLGN, MGN, BGN, and
OAGN, and that incompatible trace-element concentrations increase downwards within the MGN and BGN. The mineralisation, which
is confined to the early, more primitive units (VLGN and MGN), occurs as massive sulphide breccias and stringers and as disseminated
gabbronorite-hosted sulphides. The massive sulphides were emplaced late in the intrusive sequence, have different PGE chemistry
and Cu tenor to the disseminated sulphides, and have undergone sulphide fractionation. The distribution of disseminated sulphides,
which are primary magmatic in origin, is related to chonolith geometry and magma flow regimes, rather than to gravitational
settling. Sulfur-bearing country rocks are absent in the Nebo–Babel deposit area, and thus, local crustal S addition was unlikely
to have been the major mechanism in achieving sulphide immiscibility. The Nebo–Babel intrusion is part of an originally continuous
magma chonolith with multiple and related magma pulses. The parental magma was medium- to low-K tholeiite with 8–9 wt% MgO.
The initial magma pulse (VLGN), the most primitive and sulphide saturated, was probably emplaced along a linear weakness in
the country rock. After crystallisation of VLGN, marginally more fractionated, sulphide-saturated magma was injected through
the thermally insulated core of the conduit, forming the MGN. Successive pulse(s) of more fractionated magma (BGN) were emplaced
in the core of the intrusion. After magma flow ceased, closed system crystal fractionation produced consistent mineral and
chemical fractionation trends within BGN and OAGN. After crystallisation, the intrusion was overturned and then offset by
the Jameson Fault resulting in the apparent ‘reverse’ chemical and mineral trends in Nebo–Babel. |
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| Keywords: | Nebo– Babel Musgrave block Australia Ni– Cu– PGE Magmatic sulphide deposit |
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