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Woods Point - Morning Star

Victoria, Vic, Australia

Main commodities: Au
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The Woods Point dyke swarm and associated orogenic gold deposit is located in the eastern Lachlan fold belt, Victoria, Australia, near the town of Wood's Point, ~100 km ENE of Melbourne and 85 km NW of Sale (#Location: 37° 34' 8"S, 146° 15' 4"E).

The dyke swarm is located in the eastern Lachlan fold belt of south-eastern Australia, within the Melbourne zone, a sequence of deformed and metamorphosed Siluro-Devonian turbiditic sandstones and siltstones unconformably overlying Cambrian volcanic rocks and metasedimentary rocks to the east of the Tasman Line (Crawfordand Keays, 1978; Willman et al., 2010; Cayley et al., 2011).

The suite of metasedimentary rocks at Woods Point can be divided into three distinct units (Phillips and Hughes, 1996): i). the strongly deformed Early Devonian Wilson Creek Shale, ii). the Walhalla synclinorium (host to the dyke swarm), containing multiple thin interbedded shales, and iii). Ordovician shales along the east and west of the Melbourne zone margin.

The boundaries of the Melbourne zone are fault defined, including the Mount Wellington fault zone to the east an the Heathcote fault zone to the west (Gray, 1988). In comparison to the Ballarat zone of the Lachlan fold belt (1950 t of Au from 143 deposits), the Melbourne zone has a lower overall tonnage of discovered Au and fewer known Au deposits (Phillips and Hughes, 1996). However, the areally relatively limited Walhalla-Woods Point-Jamieson belt, which includes the Morning Star dyke, dominates Au production from the Melbourne zone, having contributed ~120 t of Au to date (Phillips and Hughes, 1996).

The Woods Point dyke swarm was intruded into Late Devonian sedimentary rocks (Bierlein et al., 2001a) and host both Au deposits and minor magmatic Cu-Ni mineralisation (e.g., Thomson River Copper Mine, Keays and Kirkland, 1972). The gold mineralisation is interpreted to have followed peak metamorphic and regional deformation events in the Melbourne zone and closely postdates dyke intrusion (Cox et al., 1987).

The dyke swarm is located on the western limb of the Walhalla synclinorium, within and to the east of the Mount Easton Fault Belt, and extends north-south for over 150 km, with some dykes traceable along strike for >20 km (VandenBerg and Gray, 1988; Phillips and Hughes, 1996), truncated by Late Devonian granites to the north (Green, 1974). Individual dykes vary from hornblende peridotites through gabbros and gabbro-diorites to granophyres, with gabbro-diorites dominating (Junner, 1920; Hills, 1952; Green et al., 1982). The dykes intrude steeply dipping, subvertical Middle to Early Devonian slates, shales, sandstones and siltstones (Singleton, 1965; Garratt, 1983), emplaced immediately after the Tabberabberan Orogeny, and may have been a precursor to the widespread post-Tabberabberan igneous activity preserved across Victoria (Birch and Gleadow, 1974; Clemens, 1988).

Minor magmatic Cu-Ni-PGE sulphides occur in many of the gabbroic rocks of the dyke swarm but are rare in the gabbro-diorites. Most of the dykes in the swarm underwent pervasive deuteric alteration during cooling immediately after emplacement (Green et al., 1982; Carmichael, 1994). In addition, some sections of individual dykes have also experienced intensive postmagmatic hydrothermal alteration from fluid flow related to Au mineralization (Green et al., 1982).

Gold occurs in both its native form and as inclusions within sulphides, and is hosted by quartz-carbonate veins with halos of intense hydrothermal alteration (Green, 1974). The average content of the pyrite-bearing alteration envelopes adjacent to the quartz veins at the Morning Star and Loch Fyne Mines is 0.74±0.36 g/t Au (80 samples, Keays, 1987, recorded in Jowitt et al., 2012). The hydrothermal alteration associated with these veins is gradational, and the hosting veins do not crosscut each other. The quartz reefs/veins, were developed either in reverse faults that crosscut the dykes or in veins sub-parallel to dyke strike. The quartz veins frequently cross-cut entire dykes, but only persist for short distances, if at all, into the enclosing country rock. Gold mineralisation is found as high-grade zones within the vein, with the highest grades of >1500 g/t Au occurring in 20 to 30 cm wide laminated veins (Green et al., 1982). Gold is also associated with hydrothermal sulphides, particularly arsenopyrite, while dykes with poorly developed hydrothermal sulphides are usually unmineralised (Junner, 1920; Carmichael, 1994).

Gold mineralisation appears to slightly post-date dyke intrusion by a maximum of 2 to 4 m.y.(Jowitt et al., 2012. 40Ar/39Ar of hydrothermal sericite associated with Au mineralisation within the Morning Star dyke), with ages of 374±2 Ma at Morning Star and 372±2 Ma at Walhalla, which are within error or postdates dating of hornblende from dykes of the Woods Point dyke swarm (40Ar/39Ar; Bierlein et al., 2001). Dyke intrusive ages were obtained from hornblende from the Jericho (376±4 Ma), Mountain Home (377±4 Ma), and A1 (378±6 Ma) dykes (Foster et al., 1998; Bierlein et al., 2001). Less precise K-Ar dating of hornblende from the Morning Star dyke (378±14 Ma, Richards and Singleton, 1981) is also within error of the Au associated sericite age.

The Morning Star dyke, which is ~1 km to the NW of the settlement of Woods Point, contains high-grade Au mineralisation hosted by quartz-ankerite veins that are preferentially located within the dyke, but close to the dyke-country rock contact. Veining becomes more frequent and higher grade with depth (McAndrew, 1965). The dyke has a maximum width of 120 m, can be traced for ~400 m along strike, and persists for at least 800 m deep. It has an asymmetric, variable composition, with the western side of the dyke being generally more mafic than the east, and a gradational boundary between the gabbro and gabbro-diorite portions of the dyke. Felsic patches, and more rarely, felsic magmatic veins occur within the dykes, with a zonation within the dyke that is not parallel to the dyke margins. No layering has been observed in the core of the dyke, although some flow banding and layering is visible toward the dyke margins.

The pre-alteration mineralogy of the Morning Star dyke is dominantly amphibole, plagioclase and quartz, all of which are generally between 2 and 5 mm in size, with quartz as a late interstitial phase, with minor clinopyroxene and accessory minerals. The more sections of the dyke have the highest amphibole content, while more felsic zones are dominated by plagioclase and quartz, with the latter usually present as myrmekitic intergrowths with plagioclase. Amphibole occurs as euhedral phenocrysts up to 1 cm in size and as interstitial anhedral crystals. Plagioclase is commonly euhedral, and comprises 10 and 20% of of the rock where unaltered. Quartz occurs both within granophyric intergrowths with rare K feldspar and as distinct anhedral phenocrysts, both of which show undulose extinction. Accessory magnetite and rarer ilmenite are also common within the more evolved gabbro-diorite but are rarer in the gabbro. Rare euhedral primary apatite is also present as are small amounts of magmatic chalcopyrite, pyrrhotite and pentlandite in the more primitive gabbroic sections of the dyke.

The Walhalla-Woods Point-Jamieson belt, (including the Morning Star dyke) has historically produced ~120 t of Au

The most recent source geological information used to prepare this summary was dated: 2012.    
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:
Jowitt S M, Keays R R, Jackson P G, Hoggart C R and Green A H,  2012 - Mineralogical and Geochemical Controls on the Formation of the Woods Point Dike Swarm, Victoria, Australia: Evidence from the Morning Star Dike and Implications for Sourcing of Au Within Orogenic Gold Systems: in    Econ. Geol.   v.107 pp. 251-273


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