1 CRC LEME, c/- CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 1670
2 Perseverance Exploration Pty. Ltd., McCormicks Road, Fosterville, VIC, 3557
Key Words: gold exploration, weathering, transported cover, partial leaching, Central Victoria
The Goornong South Gold Deposit is blind, generally occurring below more than 5 m of transported overburden. It is located 4 km north of the Fosterville mineralisation and 25 km east of the major historical gold-producing district of Bendigo. Mineralisation is structurally controlled along a north-south fault and is hosted by Ordovician turbidites of the Ballarat Trough of Central Victoria. Mineralisation occurs over a strike length of 1.3 km within the 40 m deep oxidised zone.
Gold mineralisation at Goornong South is characterised by the absence of potential pathfinder elements except for As. During complex weathering events in the Tertiary and Quaternary, As and Au have been dispersed within the residual regolith to produce haloes defined by cut offs of 100 ppm and 50 ppb respectively. Dispersion also occurs into the overlying transported material where slightly lower levels of these elements are present in the soil developed in overburden above mineralisation. Detailed study of the soils shows that the fine (63 mm) fraction generally contains higher Au contents than the coarse (+2 mm) fraction. Arsenic >100 ppm in the coarse fraction of the soil defines an anomaly twice the size of that in bedrock whereas Au in this fraction gives a similarly-sized anomaly to that in bedrock. However the Au anomaly (>10 ppb) defined using the fine fraction of the soil is nearly four times as large as that derived by bedrock geochemistry.
Soils developed over gravels in a palaeochannel in the middle of the mineralisation also reveal an extensive area with Au >10 ppb in the fine fraction whereas the coarse fraction only returns an anomalous value at the western edge of the mineralisation. However partial leaching studies suggest that this Au is not easily leached and so it is suspected that the Au may be introduced by mechanical means. Although Au in the fine fraction of the soil gives the largest anomaly, that fraction is potentially contaminated by aeolian or recent colluvial/alluvial processes. Thus the easily collected coarse ferruginous fraction of the soils may actually be the most cost-effective sampling medium for areas of significant cover in Central Victoria.
Gold mineralisation in Central Victoria occurs in structurally-controlled locations within the sandstones of Ordovician turbiditic sequences, as exemplified by the world class deposits at Ballarat and Bendigo (e.g. Taylor, 1998; Turnbull and McDermott, 1998). At Fosterville, 25 km east of Bendigo (Figure 1), oxidised Au mineralisation occurs in sandstones in two parallel shear zones (Fosterville and O’Dwyers Faults) along a strike length of up to 10 km (Zurkic, 1998). Such mineralisation was worked from 1894 until 1952. Recent production commenced in 1991 with gold being produced from several pits and treated by heap leaching by Perseverance Corporation Ltd. since 1992. Away from the Mining Lease, outcrop is poor but, since late 1995, regional exploration has outlined a 1.3 km long anomalous Au zone below at least 5 m of transported cover at Goornong South, 4 km to the north of the Fosterville operations. Systematic drilling (73 RC holes) has delineated 783,000 tonnes of oxide ore @ 1.4 g/t Au (Gold Mining Journal, 1998) in a body which is still open-ended in all directions.
The Goornong South mineralisation occurs at the southern end of what was originally delineated as three separate exploration prospects Goornong South, Howards Prospect and the Cemetery Prospect (from south to north; Figure 2). Outcrop is absent over the whole length of the deposit, although mineralised float occurs about a low rise at the southern end and shallow prospecting pits occur at both the southern and northern ends. Ferruginous float from the southern end may contain up to 1.8 ppm Au and 1700 ppm As, whilst subcrop from the Cemetery area in the north may contain up to 5.2 ppm Au and 2700 ppm As. (Other chalcophile element contents are low in such materials: Scott and van Riel, 1999). This Au-As association in regolith materials, similar to that at the Fosterville deposits to the south, encouraged Perseverance Exploration Pty. Ltd. to commence a geochemical exploration program based on sampling weathered bedrock. During late 1995, three lines of augering, 50 m apart and a sampling interval of 10 m were conducted over the rise at the southern end of the deposit. These outlined a broad (>150 x 150 m) anomaly in As (>100 ppm) and Au (>50 ppb) in this area. However, the drilling of 9 RC holes in 1996 yielded disappointing results with the best intersection being 4 m @ 1.02 g/t Au. Nevertheless, the intersection of porphyry dykes, reflecting the presence of a favourable structure, during this drilling and the alignment of the anomaly parallel to the 340º magnetic trend of the Fosterville mineralisation, encouraged persistence with the bedrock augering program. As the depth to bedrock increased in the middle of the whole prospect, RAB drilling replaced augering and an almost continuous anomalous line 1.3 km long was outlined (Figure 2). In 1997, a further 36 holes were drilled (2200 m of RC drilling) over the whole anomalous line but with a focus in the area just to the north of the original drilling. This program was designed to test oxide resources suitable for heap leaching and produced results such as 10 m @ 4.6 g/t Au, 6 m @ 7.5 g/t Au and 18 m @ 1.8 g/t Au. Subsequent infill drilling during 1998 (30 holes) has defined a deposit of 783,000 tonnes of oxide ore @ 1.4 g/t Au in the southern portion of the Goornong South anomaly (Gold Mining Journal, 1998).
Subsequent to drilling, the assumption that the oxide ore at Goornong South is similar to that at Fosterville has been confirmed (e.g. Scott and Van Riel, 1999). In both areas, the Au was originally hosted in pyrite and arsenopyrite which break down during weathering but still leave the fine-grained Au intimately associated with the resultant Fe oxides. Weathering of the Ordovician sandstones and siltstones extends to 30-40 m depth, but at Goornong South, this regolith material is overlain by a variable thickness of Quaternary cover. This cover is alluvium, derived from the recent streams (Campaspe River system) and possibly, aeolian material (e.g. Cherry and Wilkinson, 1994). Kotsonis (1998) has also drawn attention to the possibility of sludge (derived from 19th century mining operations at Bendigo) being present in the soils of the area. Pedogenic calcrete is developed in the top metre of the soil over much of the deposit. At the extreme ends of the deposit the cover may be shallow (<1 m) but over the economic mineralisation it is generally 5-10 m thick. Over the central part of the anomaly, RAB drilling intersected thick sequences of gravel (up to 30 m) which probably represent an infilled Tertiary palaeochannel.
In an effort to develop an easy, cost-effective exploration method, the surficial soils above the mineralisation at Line GS5, a highly mineralised section of the deposit, were studied. The top 6 cm of soil was sampled at 25 m intervals along a 350 m traverse. Results of sieving indicated that the coarse (+2 mm) fraction of the soils is Fe-rich, often containing pisoliths as well as lithic fragments and sub-rounded quartz. The amount of lithic material decreases as the depth to bedrock increases. The fine -63 mm material represents 50-70% of soil and is clay-rich.
Au is generally enriched in the fine fraction of the soils relative to the coarse (Figure 3). The lateral extent of the anomaly in the fine material is similar to that of the anomaly defined by bedrock sampling using a 10 ppb cut-off in both cases. The coarse soil fraction also contains anomalous Au in the central portion of the Au anomaly, centred on the GSRC28 area.
As is enriched in the coarse, Fe-rich fraction of the soils with a broad anomaly, almost twice the size of that defined by bedrock geochemistry using a 100 ppm cut-off (Figure 4). The As content of the fine fraction is generally at least an order of magnitude lower than that in the coarse fraction but a low level anomaly defined by As >20 ppm may occur centred on the region of the dyke within the eastern portion of the bedrock anomaly area (Scott and van Riel, 1999).
Surficial soils were collected along a 250 m transverse (GH6) from the middle of the extended anomaly across an inferred palaeochannel 500 m north of the GS5 transverse (Figure 2). The soils were separated into fine (-63 mm) and coarse (+1 mm) fractions, as well as being analysed as whole soils after a total digestion and by the partial leaching techniques, Regoleach and MMI.
Au contents above background occur in the western-most sample (sample 1) and sample 8 (or 7) in both the total digest and Regoleach and MMI digests (Figure 5). The absolute concentrations of Regoleach Au are <2/3 those obtained from the total digest, whereas the MMI Au is only 1/20 of the value determined by total digest. Although Regoleach As contents are significantly lower than total values, both methods show values which are slightly elevated above the background in samples 1 and 8. The fine fraction of the soil is anomalous (>10 ppb Au) over the whole transverse but the coarse faction shows anomalous Au only in the western-most sample. Although As occurs at 15 ppm in the coarse fraction of the adjacent sample (Figure 6), this level is well below the anomalous level (>100 ppm) used for along Line GS5.
All five sample types/analytical methods indicate anomalous Au (generally with low but detectable As) at the western end of the traverse. (Bedrock sampling also shows Au = 40 ppb at this site.) However, the presence of an Au anomaly (> 10 ppb) in the fine soil fraction over the whole traverse with no correlation with bedrock chemistry or the expected position of the ore horizon contrasts with the close association seen along GS5 (Section 4.1) and suggests that either there is extensive hydromorphic dispersion through the gravels or there is mechanical dispersion of fine-grained gold-bearing material into this palaeochannel area. The presence of such anomalous Au in the fine material may also suggest that the influence of aeolian material in the -63 mm fraction may not be as great as originally feared (Scott and van Riel, 1999) and the potential of the fine fraction of the soils as a sample medium should be further evaluated.
The initial evaluation of surficial soils developed in transported material over the mineralisation recommended that the coarse fraction of the soils be used to avoid the possibility of dilution by aeolian material (Section 4.2; Scott and van Riel, 1999). However the study of the soils developed over gravels in the central portion of the Goornong South anomaly suggests that the influence of aeolian material in fine fractions of the soil may not be as severe as originally feared.
Furthermore, the study of different sampling/analytical procedures over the gravels reveals significant features.
· Au values determined by the Regoleach, MMI and total digestion methods show anomalies in similar positions along the GH6 traverse. The magnitude of the anomalies is reduced by at least 1/3 in the case of Regoleach, but by a factor of 20 with MMI. Because MMI involves only a weak leach which liberates ions adhering to the surface of soil particles (Birrell, 1996), it would appear that most of the Au along the GH6 traverse is not attached to soil particles and perhaps is present as discrete Au grains. Thus use of MMI and total dissolution techniques may give information about the form of the Au in soil samples.
· The high Au contents in the fine fraction of the soils over the whole traverse rather than just above the mineralisation, suggest that the Au is either hydromorphically dispersed or is mechanically transported to this area. If it were hydromorphically dispersed, it might be expected to adhere to soil particles but the MMI analyses suggests that this is not the case. Topographic variations are subdued but the area above the palaeochannel may be still receiving fine silt from the west where mineralised float has also been found. This scenario suggests that detailed landscape mapping using digital elevation models may be necessary to fully interpret the geochemical survey results.
Regoleach, MMI and total digest methods applied to surficial soils over gravels at Goornong South show similar results although the magnitude of the Au anomaly is different. The very difference in MMI and total Au abundances for the same samples suggests that most the Au in the soil along the traverse is not loosely bound and hence is more likely to be incorporated into the soil by mechanical means rather than by hydromorphic processes. This suggests that very careful evaluation of regolith processes within a prospect area is necessary to gain the confidence to interpret results from a geochemical survey once the presence of transported material is suspected.
The assistance of Perseverance Corporation Ltd geologists (especially Chris Roberts, Trevor Jackson and Phil Kinghorn) in encouraging and facilitating this study is gratefully acknowledged. We also had useful discussions about the geomorphology of the area with Andrew Kotsonis (University of Ballarat) and Don Cherry (New Holland Mining NL). Soil samples were sieved by Jeff Davis (CSIRO Exploration and Mining, North Ryde).
Birrell, R., 1996. MMI geochemistry: mapping the depths. Mining Magazine 174, 306-307.
Cherry, D.P. and Wilkinson, H.E., 1994. Bendigo and part of Mitiamo, 1:100 000 map geological report. Geological Survey of Victoria Report 99.
Gold Mining Journal, 1998. Perseverance Corporation Ltd. Furthering oxide mine life is key to short term success. Gold Mining Journal 1 (50), 41.
Kotsonis, A., 1998. Fosterville Goldfield: Landscape history and regolith development, near Bendigo, southeastern Australia (unpublished report).
Scott, K.M. and van Riel, B., 1999. The Goornong South Gold Deposit and its implications for exploration beneath cover in Central Victoria, Australia. J. Geochem. Explor. 67 (in press).
Taylor, D.H., 1998. Ballarat gold deposits. In “Geology of Australian and Papua New Guinean Mineral Deposits”. Eds. D.A. Berkman and D.H. Mackenzie, 543-548, Australas. Inst. Min. Metall., Melbourne.
Turnbull, D.G. and McDermott, G.J., 1998. Deborah line of reef gold deposits, Bendigo. In “Geology of Australian and Papua New Guinean Mineral Deposits”. Eds. D.A. Berkman and D.H. Mackenzie, 521-526, Australas. Inst. Min. Metall., Melbourne.
Zurkic, N., 1998. Fosterville gold deposits. In “Geology of Australian and Papua New Guinean Mineral Deposits”. Eds. D.A. Berkman and D.H. Mackenzie, 507-510, Australas. Inst. Min. Metall., Melbourne.