Woodcutters is a polymetallic vein-style deposit hosted by sediments of
the Palacoproterozoic Whites Formation which forms part of the
sedimentary sequence in the central Pine Creek Geosyncline. Past
production and identified resources total over eight million tonnes at 5.5
per cent lead, 12.1 per cent zinc and 110 gA silver. The local geology is influenced by Archaean basement highs; the Rum
Jungle and Waterhouse Complexes. Unconformably overlying basement
are interbedded clastic and crystalline carbonate units (Batchelor Group)
representing cyclic changes in the depositional environment. These
exhibit onlapping features with the underlying rocks. Sediments of the
overlying Frances Creek Group suggest a quiet period during subsidence
and subsequent increase in water depth (the carbonaceous dolomitic
shales and the interbedded dololutites of the Whites Formation) followed
by proximal uplift resulting in the inundation of turbiditic flows (Acacia
Gap Quartzite). All of the Palaeoproterozoic rocks show evidence of
being affected by penecontemporaneous, basement circling faults with
steep dips. The stress regime during the deposition of the Frances Creek
Group was characterised by alternating episodes of thrusting and
transpression. Periods of relaxation also occurred. The compression
resulted in major NS strike-slip (transpressional) faulting, en-echelon
NNE trending folds and NW-SE tensional cross faults. Thrusting
produced N-S striking, steep dipping reverse faults and a possible
decollement immediately above the Batchelor Group. Relaxation is
evidenced by the intrusion of lamprophyre dykes and dolerite sills (Zamu
Dolerite). Mineralisation at Woodcutters occurred in faults within the
Whites Formation, probably synchronous with deformation and the
deposition of the upper parts of the Frances Creek Group. Greenschist
facies metamorphism occurred as the rocks were further buried. Several massive sulphide bodies occur at Woodcutters. A number of
features influence the location and shape of these bodies. They are: the N-S transpressional faults and in particular associated dilation
zones; intersection of the N-S transpressional faults with the axial plane of
the NNE trending Woodcutters Anticline; intersection of the N-S transpressional faults with the NW-SE
cross-faults; intersection of dololutite rich units with the NS transpressional faults
where local replacement takes place; thickening of the lithological units in the vicinity of the anticlinal
axis; and the anticline doubly plunging to both the north and south. The mineralisation exhibits vein-like features as well as epigenetic
textures. Its boundaries are sharp, its thickness varies rapidly down dip
and along strike, and no alteration halo exists though stringer veins of
quartz-carbonate-pyrite sphalerite/galena occur in close proximity. A
number of sulphide mineral assemblages have been recognised, each
separated by periods of deformation and partially replacing the earlier
assemblages. Vertically continuous zonation patterns highlight the
presence of rich galena-sphalerite-silver in zones of maximum dilation
and pyrite-arsenopyrite-gold concentrated at the along strike peripheries. The genetic model proposed has the mineralisation occurring whilst the
transpressional regime was active. It was deposited from the same fluid
that elsewhere in the Geosyncline formed the stratiform deposits higher in
the sedimentary pile. Hydrocarbons sourced from the organic-rich Whites