Tomingley - Wyoming, Caloma
New South Wales, NSW, Australia
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The Tomingley gold operation includes the Wyoming 1 and Wyoming 3, and the Caloma and Caloma 2 deposits, and is located ~300 km NW of Sydney, ~40 km NNE of the Northparkes porphyry Cu-Au deposit, 52 km SW of Dubbo and ~15 km north of Peak Hill in the Lachlan Fold Belt of New South Wales, Australia (#Location: 32° 35' 22"S, 148° 13' 0"E).
Wyoming 3 is ~750 m north of Wyoming 1, while Caloma is ~500 m east of Wyoming 3, and Caloma 2 is ~500 m south of Caloma 1, and 500 m east of Wyoming 1, forming the four quadrants of a square. Mining commenced in 2014.
These deposits are located near the eastern margin of the Ordovician Junee-Narromine volcanic belt of the early Palaeozoic Macquarie Arc, in the Lachlan Orogen of eastern Australia, and is close to the interpreted Parkes Thrust. The Junee-Narromine volcanic belt extends north-south for several hundred kilometres and includes the 450.9±4.2 Goonumbla Volcanic Complex which hosts the nearby Northparkes porphyry Cu-Au system. The north-south arc parallel Parkes Thrust separates the Goonumbla Volcanic Complex from the Mingelo Volcanics, a thin slice of north-south trending andesitic volcanics identified by regional aeromagnetic data and interpreted to be equivalents of the Goonumbla Volcanics. This thin volcanic belt, which hosts the Tomingley and Peak Hil gold deposits, has a strike length of ~35 km varies from 2 km in width in the north, to ~500 m in the south.
Ordovician rocks west of the Parkes Thrust are weakly deformed, with broad open folds and sub-greenschist metamorphic assemblages, while the Ordovician - Silurian sequences east of the fault, including the host Mingelo Volcanics, exhibit tight to isoclinal folding, strong axial planar cleavage with greenschist metamorphic assemblages.
The immediate Tomingley area is almost entirely covered by alluvial sequences of clays, sand and gravel of Quaternary to Cretaceous age that are up to 50 m in thickness.
The gold deposits at Tomingley are hosted within volcaniclastic sediments, rare lavas and shallow intrusive porphyritic rocks. The volcanic units are trachy-andesites to basaltic trachy-andesites, with very rare detrital quartz in the volcaniclastic rocks. The volcaniclastic rocks are dominated by well bedded sandstones and siltstones with minor breccias, lithic conglomerates and black mudstones. The sandstones and siltstones are dominant and have a primary composition of plagioclase and augite, which has been largely altered to sericite, carbonate, chlorite and albite with the rare primary quartz. These volcaniclastic rocks are intruded by numerous sill-like coarse feldspar±augite porphyritic bodies of trachy-andesitic to mafic trachy-andesite affinity which are weakly concordant to the bounding sediments. Rare peperitic textures suggest these intrusions were emplaced at a relatively shallow level.
To the west, the andesitic volcaniclastic sequence has a sharp, apparently not faulted, contact with an overlying succession of well-bedded fine quartzose sandstone and laminated siltstone with a diagnostic basal quartz rich conglomerate that may be either equivalents of the Cotton formation, part of the Goonumbla Volcanics sequence (Sherwin, 1996) or the separate Silurian Forbes Group (Squire et al., 2007). The eastern margin of the volcaniclastic sequence is uncertain.
A narrow, marginally discordant, chlorite-talc schist has also been encountered in drill core just to the east of the porphyry sills at Wyoming One, and may represent a mafic-ultramafic precursor, similar to olivine rich lavas (picrites) which are known from the Molong Belt (A Crawford, pers. comm. 2004).
The volcaniclastic sequence strikes NNW and dips steeply east, with the feldspar porphyry that hosts the Wyoming 1 deposit located near the axis of a tight, easterly vergent, antiform. Within this massive feldspar porphyry, brittle fracture is dominant and a number of vein directions are evident. Major structures are orientated WNW, as shown by the near vertical faults that appear to dislocate the porphyry and several sub-parallel vein sets within the porphyry. A pervasive set of shallow north dipping veinlets also have a WNW to ENE strike. A sinistral transpressional event is indicated by structural analysis, with a rotation of the stress field to develop the mineralised vein array seen at Wyoming 1, the structures at Wyoming 3 and the regional foliation.
Gold mineralisation at Wyoming One is distributed both around and within a sub-vertical, south plunging, feldspar±augite phyric sill. The deposit can be separated into distinct mineralised zones (Alkane Resources ASX Release, September, 2015):
i). the porphyry zone - where mineralisation is hosted by a quartz stockwork within the carapace of a sub-volcanic sill with dimensions roughly 60 x 150 m;
ii). contact zone - high grade mineralisation developed along the eastern contact of the porphyry zone;
iii). hangingwall zone - a linear zone of mineralisation situated ~30 m into the hangingwall of the 'porphyry' mineralisation and hosted within silicified and brecciated carbonaceous mudstone, and is interpreted to fold around the northern end of the porphyry;
iv). the '376' zone - and east west zone of high grade mineralisation developed at the northern contact of the porphyry, interpreted to be a bounding structure and primary fluid conduit;
v). the '831' zone - one of a number of high grade east-west structures that appear to be developed in an en echelon pattern to the south of the '376' zone; and
vi). the footwall zone - a low grade zone located in a similar stratigraphic position to the hangingwall zone but in the footwall of the porphyry.
Gold mineralisation at Wyoming One is characterised by strong quartz±carbonate (ankerite)±albite±pyrite±arsenopyrite veins within intense sericite-carbonate (ankerite)-albite-quartz-(±chlorite±pyrite±arsenopyrite) alteration of the feldspar±augite-phyric intrusion and the volcaniclastic sediments. The hangingwall zone appears stratigraphically controlled by a fine-grained carbonaceous mudstone and the '376' and '831' are high grade east west zones truncating and transecting the porphyry.
The gold mineralisation at Wyoming Three also shows a strong spatial relationship with feldspar porphyritic rocks, although pervasive alteration is limited or absent, with mineralisation hosted within structurally controlled quartz±carbonate±chlorite±pyrite±arsenopyrite veining striking about 105°.
The Wyoming deposits appear to be the result of a rheological contrast between the brittle fractured porphyry host and the ductile deformation of the surrounding volcaniclastic and metasedimentary rocks. Although the timing of alteration and mineralisation in the Wyoming deposits is uncertain, a relationship with possible early to middle Devonian deformation is considered likely (Chalmers et al., 2007).
As at 31 December 2006, published mineral resources were (Alkane Resources web site, 2007):
Measured resource - 4.835 Mt @ 2.24 g/t Au
Indicated resource - 1.025 Mt @ 2.76 g/t Au
Inferred resource - 1.270 Mt @ 4.09 g/t Au
Total resource - 7.130 Mt @ 2.70 g/t Au for 18.86 tonnes of Au.
Prior to mining in 2014, JORC compliant ore reserves and mineral resources were (Alkane Resources web site, 2016):
Measured + indicated + inferred resources
Wyoming 1 - 4.742 Mt @ 2.1 g/t Au,
Wyoming 3 - 0.649 Mt @ 1.7 g/t Au,
Caloma - 5.909 Mt @ 1.8 g/t Au,
Caloma 2 - 2.169 Mt @ 2.1 g/t Au,
TOTAL - 13.469 Mt @ 1.9 g/t Au.
Proved + probable reserves (included within the resources)
Wyoming 1 - 1.864 Mt @ 1.6 g/t Au,
Wyoming 3 - 0.389 Mt @ 1.7 g/t Au,
Caloma - 1.928 Mt @ 2.2 g/t Au,
Caloma 2 - 0.239 Mt @ 3.6 g/t Au,
Stockpiles - 0.186 Mt @ 1.9 g/t Au,
TOTAL - 4.606 Mt @ 2.0 g/t Au.
The most recent source geological information used to prepare this summary was dated: 2007.
Record last updated: 5/8/2016
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.
Tomingley - Wyoming 1
References to this deposit in the PGC Literature Collection:
Chalmers D I, Ransted T W, Kairaitis R A and Meates D G, 2007 - The Wyoming gold deposits: volcanic-hosted lode-type gold mineralisation in the eastern Lachlan Orogen, Australia : in Mineralium Deposita v42 pp 505-513|
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