Henty, Mount Julia

Tasmania, Tas, Australia

Main commodities: Au
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The Henty (-Julia) gold deposit comprises a string of overlapping gold lenses that are located ~10 km south of Rosebery and ~30 km north of Queenstown on the west coast of Tasmania, Australia (#Location: 41° 52' 27"S, 145° 33' 5"E).

Mineralisation is hosted by the Cambrian Mount Read Volcanics, which within the Henty area, comprise an early sequence of feldspar porphyry dacite lavas and volcaniclastics, the Central Volcanic Complex, and the overlying Tyndall Group composed of rhyolitic lavas, crystal rich volcanic sandstones and volcanic conglomerate. The transition between the Central Volcanic Complex and the Tyndall Group marks a change from feldspar-phyric to predominantly quartz-phyric volcanic lithologies. These volcanic rocks are, in turn, unconformably overlain by the Late Cambrian to Early Ordovician molassic Owen Conglomerate.   Two major faults, the NNW-SSE Great Lyell Fault and the NNE trending Henty Fault intersect near the deposit which occurs between the two, south of their confluence. The Great Lyell Fault continues for >30 km south to the Mount Lyell deposit and beyond, dipping at 30 to 60°W. The Henty Fault extends to the NNE for >50 km, through the Tullah district to the vicinity of the Que River-Hellyer VHMS deposits. At Henty it is composed of two planar structures separated by a 20 to 200 m melange and dips at ~70°W. In the deposit area, the two structures of the Henty Fault splits into the separate North and South Henty faults south of Mount Julia. The two faults are separated by the Cambrian Henty Fault Wedge, a sequence of quartz-phyric, mafic and ultramafic volcanic rocks and black shale.

Gold and sulphides are found immediately below the west dipping Henty Fault straddling the Central Volcanic Complex to Tyndall Group boundary in rocks that are principally submarine volcaniclastic and hyaloclastic dacites with interbedded carbonates and calcareous volcaniclastic sandstones (Callaghan, 2001). The bulk of the mineralisation occurs within a zone of intensely silicified massive quartz alteration (Lorrigan et al., 2017). This alteration zone occurs as a subvertical tabular sheet >3 km long and from 10 to 100 m wide. Whilst this zone is basically stratabound in the north, it transgresses stratigraphy and is located 50 to 300 m deeper to the south, well below the base of the Tyndall Group. The alteration zone, which is strongly deformed by the South Henty Fault, is limited up-dip by that fault and downdip by an abrupt decrease in alteration intensity (Callaghan, 2001).

Alteration is asymmetric and has been subdivided into three:
Footwall alteration, which comprises intensely foliated to mylonitic sericite-quartz ±pyrite ±carbonate altered schistose rhyolitic and dacitic volcaniclastic rocks, lavas and sills of the Central Volcanic Complex;
The main mineralisation (A zone), is zoned from intensely leached, massive quartz, to quartz-sericite, to an outer quartz-sericite-pyrite-chlorite alteration. Minor stratabound lenses of massive pyrite (and rarely massive polymetallic base metals sulphides) occur at the top of the A zone. These are up to 2 m thick, and are discontinuously distributed over a length of 600 m and 150 m down dip. Along strike, away from the centre of mineralisation the pyritic band becomes more carbonate rich and thickens to 20 m. Bedded carbonates and calcareous volcaniclastic rocks are also predominantly associated with the upper A zone alteration interval, but also occur toward the footwall and well up into the overlying sequences. The massive quartz alteration makes the rocks brittle and they have been repeatedly fractured and annealed with multiple generations of quartz veinlets containing sulphide and calcite. It is distinguished by quartz replacing all feldspars and sheet silicates, and the depletion of K, Na and Al. The massive quartz is cut by irregular fractures that contain free gold along with pyrite, chalcopyrite and galena with minor tellurides and bismuth sulphosalts (Lorrigan et al., 2017). Gold, copper and bismuth are mainly confined to the massive quartz and quartz-sericite alteration zones, whereas the outer quartz-sericite-chlorite alteration halo is dominated by pyrite. As such, metal zonation extends from a gold-silver-rich core associated with copper, lead and bismuth, to a proximal halo of copper, lead and bismuth, and then to a distal halo of zinc (Callaghan, 2001). Within the main A zone alteration there are a number of facies variations, including quartz-sericite-pyrite alteration with disseminated base metal sulphides, ~5% sulphides, 0.1 to 1 ppm Au and an apparent fragmental texture. A sulphide poor variation is distinguished by pale green mica that is concentrated in cleavages planes that envelope intensely silicified domains. This latter facies contains minor chalcopyrite and galena in coarse grained concentrations in silicified domains accompanied by sparse sphalerite, pyrite and occasional fluorite (Lorrigan et al., 2017). The rocks adjacent to the high silica zone are far less brittle, and ductile deformation is evident both adjacent to and distal from the Henty Fault (Lorrigan et al., 2017; Callaghan, 2001). Unlike the Henty orebodies, not all massive quartz alteration at Mount Julia contains significant gold mineralisation, with areas of low-grade (<1 g/t Au) massive quartz being common (Callaghan, 2001).
Hanging-wall alteration comprises chlorite-albite-quartz alteration in andesitic volcaniclastic rocks and albite-quartz alteration of rhyolitic volcaniclastic rocks and lavas (Lorrigan et al., 2017; Callaghan, 2001).

There is intense Na2O depletion and K2O enrichment in the A Zone and Footwall zones due to sericite alteration and feldspar destruction. In contrast the hanging wall is strongly Na2O enriched due to intense albitisation (Lorrigan et al., 2017; Callaghan, 2001).

A strong deformational fabric, in places mylonitic, is evident throughout the deposit area. Faulting everywhere affects the distribution of massive quartz and gold mineralisation and is the result of Devonian NE-SW compression. This faulting was responsible for the dismemberment of the deposit into separate ore lenses along strike and down dip. As well as this dismemberment, there is apparently evidence that shallow dipping splays from the Great Lyell Fault control the distribution of gold (Lorrigan et al., 2017). The Great Lyell Fault is a composite of Middle and Late Cambrian and Middle Devonian aged faults. It includes a collage of Middle Cambrian faults that were active during emplacement of the Mt Read Volcanics and Late Cambrian extensional growth structures that controlled deposition of the Owen Conglomerate. These faults were reverse reactivated during initial compressive Middle Devonian orogenesis (Noll and Hall, 2005). The earliest movement on the Henty Fault is brittle-ductile and is east-directed thrusting, pre-dating Devonian folding, and may coincide with the early Ordovician displacement on the Great Lyell Fault. Devonian reactivation includes high-angle reverse faulting, post-dating Devonian folding, but synchronous with the Granite Tor intrusion. Sinistral wrenching during the later Devonian involved displacements of <5 km, generally involving brittle deformation overprinting the earlier brittle-ductile fabric. There are also late phases of reactivation of the Henty Fault, including possibly Jurassic sinistral wrench movement, and Tertiary normal faulting (Taheri and Green, 1991; Berry, 1989).

Geochemical pathfinders include ratios of elements, i.e., averages of Ag:Au of 11:1; As:Au of 200:1; and Cu:Au of 1000:1. The Ag tenor does not vary significantly through the deposit, although As is higher to the south and at depth, while Cu tends to be elevated in the central deeper sections of the deposit with many exceptions (Lorrigan et al., 2017).

The mining operation has exploited a series of small tonnage (<0.5 Mt), high grade (10 to 30 g/t Au) lenses within an extensive (>20 Mt) mass of altered volcanic rock. Individual lenses are 250 to 600 m long, 3 to 20 m wide and 60 to 120 m down dip (Lorrigan et al., 2017). They are relatively planar and are sub-parallel to parallel to the Henty Fault and generally 10 to 20 m below the massive pyrite band at the top of the A Zone. The contact between the silicified and quartz-sericite zones is knife sharp, as is the gold cut-off (Lorrigan et al., 2017; Callaghan, 2001). In longitudinal projection, these lenses form a gently south plunging string of laterally overlapping planar bodies that flattens from ~30° to horizontal at depth over a length of 3 km (Lorrigan et al., 2017). From north to south they comprise the Sill, Intermediate, 96, 15, Tyndall, Newton, Mount Julia, Darwin, Read and South Darwin zones. Most are parallel to the Henty Fault, although the Darwin zones change to a SE strike, while Read continues parallel to the Henty Fault. As such the deposit apparently splits into two mineralised trends to the south. The interval between the two trends represented by the Darwin and Read zones and their alteration halos, is in part occupied by the Read Dacite, a characteristic strongly oxidised, massive quartz-phyric lava overprinted by pervasive carbonate alteration (Lorrigan et al., 2017).

Two main clusters of deposits were initially identified, namely:
Henty, which comprised Zone 96 with 0.506 Mt @ 26.9 g/t Au, and the Sill Zone with 0.023 Mt @ 36 g/t Au, which were discovered prior to 1992 and were put into production in 1996 (Callaghan et al., 1998).
Mount Julia, ~1 km to the south, was discovered subsequently, and in 2001 had a resource of 0.731 Mt @ 7.6 g/t Au (Callaghan, 2001).

New zones were progressively discovered over the life of the mine connecting the Henty and Mount Julia lenses and then extending down plunge, with the most recent being the Read Zone in 2009.

A total endowment for Henty and Mount Julia in 2007 was calculated as 2.83 Mt @ 12.5 g/t Au for 35 t of contained gold (Seymour et al., 2007).

The remaining Measured + Indicated + Inferred Mineral Resources as at 30 June, 2015 were: 1.565 Mt @ 4.6 g/t Au, for 7.2 t of contained gold (Unity Mining, Annual Report, 2015).

For more detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2017.     Record last updated: 20/5/2019
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
   Selected References:
Callaghan, T.,  2001 - Geology and host-rock alteration of the Henty and Mount Julia Gold deposits, western Tasmania: in    Econ. Geol.   v.96, pp. 1073-1088.
Callaghan, T., Dunham, S. and Edgar, W.,  1998 - Henty gold deposit: in Berkman, D.A. and Mackenzie, D.H. (Eds.), 1998 Geology of Australian & Papua New Guinean Mineral Deposits, The AusIMM, Melbourne,   Mono 22, pp. 473-480.
Halley S W, Roberts R H  1997 - Henty: a shallow-water Gold-rich volcanogenic massive Sulfide deposit in western Tasmania: in    Econ. Geol.   v92 pp 438-447
Large R R, McPhie J, Gemmell J B, Herrmann W, Davidson G J  2001 - The spectrum of ore deposit types, volcanic environments, alteration halos, and related exploration vectors in submarine volcanic successions: some examples in Australia: in    Econ. Geol.   v96 pp 913-938
Lorrigan, A., Blake, M. and Purvis, G.,  2017 - Henty gold deposit: in Phillips, G.N., 2017 Australian Ore Deposits, The Australasian Institute of Mining and Metallurgy,   Mono 32,  pp. 473-478.
Taheri, J. and. Green, G.R.,  1991 - The origin of the gold mineralisation at the Henty Prospect: in    Tasmania Department of Resources and Energy, Division of Mines and Mineral Resources,    Report 1991/32,  88p.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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