Alluvial* gold potential in buried palaeochannels in the Wyalong district, Lachlan Fold Belt, New South Wales

Kenneth C. Lawrie1, Roslyn A. Chan2, David L. Gibson2, & Nadir de Souza Kovacs3 Recent advances in understanding discovered palaeochannels may be pro- and likely climate control related to palaeodrainage in regolith terrains have spective for alluvial gold sourced by ero- eustatic sea-level changes (Gibson & led to the development of new conceptual sion of the vein deposits. Chan 1999: ‘Proceedings of Regolith 98 models for landscape evolution in the Conference, Kalgoorlie, May 1998’, CRC Lachlan Fold Belt. At the same time, new Geomorphic and LEME, Perth, 23–37). high-resolution airborne geophysical palaeogeographic setting Drilling and seismic refraction profil- datasets (magnetic, g-ray spectrometric, The Wyalong Goldfield is adjacent to the ing show that the Bland Creek and electromagnetic, AEM) have helped western margin of the north–south- palaeovalley has a crudely asymmetric delineate many regolith features with no trending Bland Creek palaeovalley cross-section owing to more pronounced surface expression — notably buried, (130 × 60 km; Fig. 1), which controlled the incision on its eastern side (Anderson et alluviated palaeoriver channels. Such northward flow of Tertiary palaeorivers al. 1993: NSW Department of Water Re- palaeochannels, mainly in areas adjacent discharging into the main westward-flow- sources, Technical Services Report to high ground, were identified in the ing palaeo-Lachlan River system. The 93.045). North-northwest-trending ridges 19th century in several of the goldfields palaeovalley drainage first incised (prob- in the palaeovalley apparently owe their in the Lachlan River catchment, where ably in the Paleocene) an already weath- expression to bedrock composition, in- some were mined for alluvial gold and ered terrain in which saprolite profiles in cluding alteration/mineralisation over- tin until the early 20th century. We have bedrock locally extended 50 m or more prints. On the western palaeovalley delineated previously unrecognised deep. Time-transgressive incision pro- margin, numerous magnetically deline- palaeochannels on high-resolution mag- ceeded by nickpoint retreat in response ated ENE- to NNE-trending palaeo- netic images near the Wyalong Goldfield, to the combined effects of differential channels appear to cut through these an historic source of primary gold in uplift associated with continental break- ridges. Steep-sided palaeochannels quartz veins. Some of these newly up, downwarping of the Murray Basin, (gorges?) underlying the Temora Gold- field are as deep as 140 m (Lishmund 1972: Records of the Geological Survey of NSW, 14(2), 133–157). Alluviation of the palaeo-Lachlan River system during the late Tertiary (Williamson 1986: Water Resources Com- mission, NSW, Hydrogeological Report 1986/12) buried the Bland Creek palaeovalley, whose alluvial fill comprises two formations. The Late Miocene to Pliocene Lachlan Formation — quartz- dominant, poorly sorted sand and gravel to cobble size — was deposited in a swampy, moderately reducing environ- ment. After a brief erosional hiatus, the Pleistocene Cowra Formation, brown gravel and clay, accumulated in a more oxidising environment in which gravel distribution across the palaeovalley indi- cates constant reworking by braided stream channels (Williamson 1986: op. cit.).

* We use the term ‘alluvial’ synonymously with the terms ‘placer’, ‘palaeoplacer’, ‘lead’ and ‘deep-lead’.

Fig. 1. The Bland Creek palaeovalley in relation to the Lachlan River and Wyalong Goldfield. AGSO Research Newsletter 30 May 1999

Alluvial gold associated with the rain in which colluvium and alluvium earliest Silurian Ungarie Granite (M. Lachlan River and Bland Creek associated with modern drainage form a Duggan, AGSO, personal comunication palaeovalleys thin veneer (<3 m) over much of the 1999). Epidote–quartz–chlorite wall–rock saprolitic bedrock. Relief in the Wyalong alteration coincides with the fault zones. Gold from alluvial palaeochannels contrib- area is subtle, and it is difficult to distin- Shear-parallel veins in these fault uted greatly to total gold production from guish between the mostly erosional ter- zones are much narrower (typically <1 m) several goldfields in the Lachlan River rain and the surrounding alluvial areas. and lenticular. The veins and host fault and tributary Bland Creek palaeovalleys. Between 1894 and 1920, the Wyalong zones display a variable dip — subvertical For example, 3.8 out of 4.2 t Au was goldfield produced ~14 t (450 000 oz) Au or steeply east-dipping near the surface, mined from the alluvial workings in the (Bowman 1977: ‘Forbes 1:250 000 mine and some veins flatten out at depth (<17° Temora Goldfield (Lishmund 1972: op. data sheets and metallogenic study’, to the east; Watt 1899: op. cit.). Ore shoots cit.), ~60km south of Wyalong. NSW Geological Survey, Report) — all of plunge to the north in most fault zones, Basal units in the Lachlan Formation it from several narrow north-northeast- but to the south in the east (Markham are the principal source of alluvial gold in trending subparallel veins (Watt 1899: 1987: ‘Gold deposits of the Lachlan Fold the palaeochannels. The auriferous NSW Geological Survey, Report, Mineral Belt’, NSW Geological Survey, unpub- sediments are typically 0.5–2 m thick and Resources 5; Bowman 1977: op. cit.). lished). Steeply plunging ore shoots up to 100 m wide (Andrews 1910: NSW Prospectors initially discovered the along strike were selectively mined. Min- Geological Survey Report, Mineral Re- location of individual auriferous veins in ing extended to depths generally less than sources 13; Wilson & McNally 1996: this area by tracing gold-bearing quartz 50 m in many veins, but to 100 m in high- ‘Symposium on the geological evolution float up very low-gradient slopes in dense grade zones, and below 250 m in the of eastern Australia’, Sydney University mallee scrub. The lack of alluvial gold was Neelds and True Blue mines, where aver- Consortium of Geology and Geophysics, attributed to the difficulty of prospecting age grades were 35 and 62 g/t respectively 71–73), but appear to be less continuous for such deposits in flat country (Pittman (Aliano & Schwebel 1981: NSW Geologi- laterally and downstream than those in 1900: Department of Mines, NSW, Annual cal Survey, Report 1981/544). palaeochannel deposits in Victoria Report for 1899, 164). Pittman surmised Primary gold is intimately associated (Nielsen 1998: Australian Institute of that the area was denuded in Tertiary with pyrite. Minor sulphides include Geoscientists [AIG] Bulletin 24, 121–125). times, and that higher rainfall in previous arsenopyrite, sphalerite, galena, and Gold within leads was mined up to 6 km times had conspired with the district’s chalcopyrite (Watt 1899: op. cit.). Coarse downstream of the channel heads elevation to form drainage channels. Oth- gold was recovered from white quartz in (Mullholland 1935: NSW Geological Sur- ers discounted the formation of alluvial veins in the east of the goldfield. Free vey, Report 1935/002; Lishmund 1972: op. prospects owing to the fine grainsize of gold is rare, but good grades were recov- cit.). Only in a few examples was it recov- the gold, which they asserted would have ered by cyanidation and/or chlorination. ered from horizons higher up than 2 m in been dispersed by aeolian processes in a the palaeochannel fill (e.g., at Golden moderately arid environment (Watt 1899: Oxide-zone enrichment Gate, Temora; Lishmund 1972: op. cit.). op. cit.). Lack of relief, low rainfall, and Gold grades of individual veins beneath Lateral gold distribution in the scarcity of groundwater for supporting the base of weathering are not uniform, palaeochannels is complex. It apparently sluicing operations (Lishmund 1972: op. but commonly <35 g/t. Despite their vari- was controlled by a combination of flu- cit.) also dampened enthusiasm for allu- ability, analysis of gold production fig- vial processes and local channel geom- vial-gold prospecting in the Wyalong ures for individual quartz reefs shows that etry. Gold was mined from tributary area. gold is enriched in the oxide zone (Aliano channels, and at the junction of tributar- Early mining, nevertheless, revealed & Schwebel 1981: op. cit.), where they are ies and the main channel in the Temora the presence of near-surface lumps of typically >60 g/t. Many shafts were not Goldfield (Lishmund 1972: op. cit.). Other gold-bearing quartz, locally known as deepened below the base of weathering examples of local controls on gold grades ‘spuds’, in the topmost regolith layers, owing to the increased difficulty of dig- include ‘depressions’ or scour holes in which were clearly the result of denuda- ging and diminishing grades (Watt 1899: the courses of channels, and bends in tion of outcropping auriferous quartz op. cit.). creeks within the main palaeochannel ‘gut- reefs. This, and analogies with other bed- In the oxide zone, fine gold is inti- ters’ (Andrews 1910: op. cit.; Lishmund rock-alluvial gold relationships within the mately associated with iron oxides; coarse 1972: op. cit.). same drainage system would suggest that gold, with colourless, white, and reddish Chemical dissolution and repre- gold may have accumulated in brown ‘opaline’ silica (Watt 1899: op. cit.). cipitation of gold in alluvial deposits may palaeodrainage channels adjacent to the The base of oxidation of the host rock have been important for redistributing Wyalong (and Hiawatha) Goldfield. Allu- and quartz veins is around 50 m. Above gold within some deposits in the Bland vial gold was mined from small this level, the ore and host rock could be Creek palaeovalley. For example, high palaeochannels farther north at Billys mined with pick and shovel. gold grades were associated with Lookout (Bowman 1977: op. cit.; Fig. 2). limonite-cemented gravels in palaeochannels at Temora (Lishmund Significance of geophysical 1972: op. cit.). Wyalong Goldfield — primary datasets for palaeochannel gold characteristics identification The Wyalong Goldfield — The auriferous quartz veins in the As a contribution to the National potential source for alluvial Au Wyalong Goldfield are structurally con- Geoscience Mapping Accord (NGMA) The Wyalong (and Hiawatha) Goldfield trolled, and localised in brittle–ductile last year, AGSO and the New South Wales (Fig. 2) occupies a mainly erosional ter- fault zones 2.5 m wide cross-cutting the Department of Mineral Resources (DMR)

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512000E 516000E 520000E 524000E 528000E 147°10' 147°15' C A3 G2 Billys Lookout NORTHERN 6268000N T ERR ITORY QUEENSLAND WESTERN I A A3 AUST R A L SOUT H AUSTRA LI A G2 NEW SOUTH WALES

G2 33°45' Forbes VICTORIA G1 1:250 000 sheet area TASMA NIA

6264000N G2 Magnetic palaeodrainage deposits G1 G2 Gold bearing quartz veins A1 SD2 G1 Boundary between depositional G1 A2 & erosional areas Hard rock gold workings (shafts) 6260000N G1 G1 SD3 0 5 km G1 G2 Hiawatha A2 Goldfield

G2 33°50' 6256000N

A1 G1

G2 G1 SD1 A2 6252000N G2 D1

G2 G1 A1 G1 Fig. 2. Explanatory diagram showing A3 regolith landform units and location of G1 major Au-bearing reefs. The O1 G1 photointerpreted boundary between thick 6248000N transported alluvial sediments of the G1 D2 O1 Bland Creek palaeovalley to the east and Wyalong Goldfield 33°55' the Humbug Creek alluvial plains to the north (A3), and the erosional landforms is shown. This sketch regolith-landform map is mainly derived from imaging high- O2 resolution g-ray spectrometric (Fig. 3A) 6244000N and magnetic (Fig. 3B) data recently O1 acquired by AGSO, field observations, and bedrock mapping. The erosional terrain A3 largely comprises in-situ highly A1 O1 weathered granitic plains and rises. G1 A2 has a low-potassium and moderate- to low- 6241000N 16/I55/44 thorium signature, and is characterised A1 Alluvium/colluvium in modern valley floors; sediment derived from low-response weathered granite. by ferruginous lag derived from mottled granite. G2 has a high-potassium A2 Alluvium/colluvium in modern valley floors; sediment derived from high-response Ordovician meta- sediments, and high-response weathered granite. signature, and appears to be less A3 Thick alluvium in Bland Creek palaeochannel; variable response depending on provenance. weathered. Magnetically delineated C Granite-derived colluvium forming distal low-angle colluvial fan; high response. palaeochannels (grey screen) containing highly magnetic detrital ferruginous G1 Granite, highly weathered to 60 m-100m lag of magnetic pisoliths, veneer of residual/colluvial sediments; low-response. Erosional plains and rises. pisoliths, sand, and clay cross this area, G2 Granite, mostly highly weathered; high-response; erosional plains and rises. and exit to the northeast. Colluvial and alluvial deposits associated with modern D1 Diorite, fresh; high response; steep rise. drainage form a veneer across these units D2 Diorite, weathered, masked by residual/colluvial deposits; low-response; erosional plains and rises. with two distinct provenances (A1 and A2). SD1 Palaeozoic strata, fresh; high total response; steep ridge. Variably weathered and covered diorite SD2 Palaeozoic strata, weathered outcrops on rises; high-response. (D1 and D2) and sedimentary rocks (SD1– 3 and O) surround the low-relief granitic SD3 Palaeozoic strata, weathered, veneer of residual/colluvial sediments; low-response; plains and rises. landscape. In the northeast, granitic O1 Ordovician metasediments, slightly weathered; high-response; low ridges. colluvium (C) is associated with steep O2 Ordovician volcanics, highly weathered; low-response; erosional plains and rises. granite hills in the northeast.

See AGSO Research Newsletter on the WWW at URL: http://www.agso.gov.au/information/publications/resnews/ AGSO Research Newsletter 30 May 1999 acquired high-resolution magnetic and g-ray spectrometric datasets along lines 50 m apart 60 m above the ground in the Wyalong area. The resolution of these datasets will be compared with that of others acquired over the area by AGSO and exploration companies at different heights and line-spacings. The new geo- physical data trace the outlines of palaeodrainage channels not previously identified (Figs. 2 and 3). The palaeochannels in the Wyalong Goldfield are magnetically delineated, and therefore apparent in magnetic images. They are also visible on AEM images (T. Munday, Cooperative Research Centre for Airborne Mineral Exploration Technolo- gies, personal comunication 1999), but not from the g-ray data. Other magneti- cally delineated palaeochannels occur to the south and east. The palaeochannels contain detrital ferruginous (maghemitic) pisoliths concentrated in lenses in sand and clay (Fig. 3A). These pisoliths have high magnetic susceptibilities, which may at least in part explain the magnetic char- acter of the palaeochannels. Palaeoflow was directed to the northeast, as indicated by the dendritic pattern in the magnetics image (Fig. 3A). The palaeochannels are not evident at the present-day land sur- face, but have been exposed in the West Wyalong rubbish dump. They are incised to unknown depths into weathered bed- rock. The proximity of some of the palaeochannels to the Wyalong (and Hiawatha) Goldfield — combined with the evidence for substantial surficial or near- surface mineralisation, evidence for ero- sion of these bedrock resources, and the justaposition of bedrock and alluvial de- posits elsewhere in the same valley catch- ment — implies that they may be prospective for alluvial gold. Further palaeogeographic reconstructive re- Th search will help identify the most pro- spective ones. Preliminary comparison of data from the different geophysical datasets sug- gests that airborne geophysical survey lines flown 50 m apart at £60-m elevation can reveal important information about regolith materials, including channel-fill deposits in the Bland Creek palaeovalley (T. Mackey, AGSO, personal communica- tion 1999). However, palaeodrainage can obviously be expressed and mapped at a range of scales, and it is important to match the specifications of each geo- physical survey to the scientific objec- Fig. 3. High-resolution magnetic (top) and g-ray spectrometric (bottom) images tives of a study. for the Wyalong district.

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For our study, survey specifications flown low (60 m) and closely spaced (50 m and for discussion; and Colin Pain, facilitated the mapping of palaeochannels apart) help map the distribution of buried Morrie Duggan, and Patrick Lyons for with a potential economic significance. In channels in the Bland Creek palaeovalley. manuscript reviews. addition to their potential for hosting al- Owing to their proximity to bedrock gold luvial gold deposits, palaeochannels in deposits, these palaeochannels and oth- 1 Minerals Division, Australian Geological Sur- vey Organisation, GPO Box 378, Canberra, the Lachlan River system are commonly ers to the south and east of Wyalong may ACT 2601; tel. +61 2 6249 9847; fax +61 2 reservoirs of saline groundwater (An- be prospective for alluvial gold. Similar 6249 9983; email [email protected]. drews 1910: op. cit.; Wilson & McNally buried palaeochannels adjacent to bed- 2 CRC LEME (Cooperative Research Centre for 1996: op. cit.). Therefore, mapping their rock gold deposits in analogous settings Landscape Evolution and Mineral Explora- distribution may also contribute to devel- elsewhere in the Lachlan River catchment tion), GPO Box 378, Canberra, ACT 2601; tel. +61 2 6249 9371 (RAC), +61 2 6249 9748 oping hydrogeological models for also may be potential sources of alluvial (DLG); fax +61 2 6249 9983; email roslyn. dryland salinity hazard assessment (W.R. gold. [email protected], [email protected]. Evans, Bureau of Rural Sciences, personal 3 University of Canberra, Canberra, ACT 2601; communication 1999). Three-dimensional Acknowledgments tel. +61 2 6249 4097; fax +61 2 6249 9983; mapping of regolith materials, including We thank David Denham, Ian Hone, and email [email protected]. palaeochannels, should also lead to a bet- Ken Horsfall for expediting the NGMA ter understanding of the hydromorphic geophysical data acquisition, and dispersion of metals. Murray Richardson for processing the datasets; Heike Apps and Lana Murray Conclusions for image preparation and figure drafting Modern airborne geophysical survey respectively; Golden Cross Resources for lines (magnetic and g-ray spectrometric) providing access to proprietary datasets

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