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The X-Y-Z of Geochemical Dispersion from Mineralisation in the Cobar Terrain Neil F Rutherford Exploration Field Workshop Cobar Region 2004, Proceedings. CRC LEME Report, 107pp Eds. McQueen, K. G. and Scott, K. M. Cobar, 24th -26th May 2004 Email: rminres@zip.com.au Web: www.geochem.zip.com.au
Abstract There are many instances in exploration using geochemistry in the Cobar region where failure to consider the implications of weathering, erosion, transport and deposition of material in the landscape have wasted significant exploration budgets. Analytical information might be spurious and misleading if these factors are not considered. Geochemical sampling methods and strategies need to be modified across the region to accommodate changes in the geochemical and physical landscape. Geochemical dispersion is a three dimensional process. Different media disperse or concentrate anomalism from the same source in different ways and to differing extents. We generally treat geochemical anomalism as a two dimensional (X-Y) entity in routine surface exploration, only adding the third (Z) dimension when we drill. Sometimes we only consider the “Z”-dimension by undertaking drill programs to avoid the near surface rocks. In many instances the practice of not considering all three together in their relation to the geochemical landscape can result in failure to delineate source targets and to only focus on widely dispersed secondary geochemical anomalism remote from source. The physical landscape (and in particular the regolith) is only one part of a geochemical landscape. A geochemical landscape also considers the influence of Eh and pH, groundwater hydrology both above and below the ground, time constraints, chemical speciation and a range of similar factors. Some primary anomalies may be subtle, short-lived and mobile while related secondary anomalies may be strong, persistent and fixed. Primary anomalies of this type require sensitive analytical techniques to define them, most techniques will find the secondary one perhaps hundreds or thousands of metres away from source. Three examples are presented to illustrate the dispersion of geochemistry in the Cobar terrain, Anomaly P4 a quartz-pyrite alteration system, Anomaly LP3, Pb-Zn mineralisation hosted in carbonate and Mafeesh, quartz-Au-As-pyrite mineralisation. Cobar Landscape Tertiary and Quaternary erosion has produced a variably incised landscape that shows markedly different outcrop expressions of the Late Cretaceous-Early Tertiary deeply weathered profile that dominates much of the Cobar region. We see remnants of a mature lateritic terrain away from the Cobar Trough boundaries and main drainage lines. In some areas, particularly along the Cobar Trough margins, the profile has been completely eroded. Elsewhere it is partially stripped, for example, about McKinnons Gold Mine. There are sites where an immature profile is re-developing over eroded elements of the old. Regionally red earths, locally calcareous, derived from sheet-wash erosion of the laterite profile and wind blown materials dominate at the surface. Redox fronts were established at different depths in the landscape in response to rising and falling water tables. Active redox fronts now lie between about 40-60 metres depth and occur close to the base of weathering. Much of the old terrain relief is infilled and covered with the eroded components of the weathered landscape. A significant depth (~0.5m) of sheetwash erosion has occurred in the last 100 years or so in some areas following land clearing and introduction of stock resulting in loss of original A-horizon soils (Leah, 1996; Cohen et al, 1996). This gives rise to a range of sample settings and media, both at the surface and at depth. Figure 1 models some of the laterite zone characteristics of mature (upper) and immature (lower) terrains observable in the Cobar region. This complexity needs to be taken into consideration in sampling and in interpretation of geochemical data, especially for surface materials such as soil or stream sediment and shallow drill samples. It requires good field records of sample site attributes in terms of their regolithic and geochemical setting with particular emphasis as to erosional level exposed to interpret the data. This is generally lacking for most sampling undertaken in the Cobar District.
Figure 1: Schematic illustration of characteristics of mature (upper) and immature (lower) lateritic terrains. Elements of both occur in the Cobar region. (Da Costa, M.L., 1993). Weathering Processes and Weathered Profile Geochemistry
Interpretation of analytical data is directly influenced by the geochemical
processes that produced the deeply weathered lateritic terrain and continue to
modify it. By understanding the way in which elements are redistributed through
a profile during its formation and subsequently it is possible to determine
where you are in a weathered terrain. The processes are summarised in Figure 2.
 
Figure 3 illustrates the “classic” form of a geochemical profile for both major silicate cations and trace metals through a weathered zone from surface to fresh bedrock. It shows the effect of strong leaching in the profile coincident with the upper saprolite. Element values increase to depth as weathering intensity declines (lower saprolite and saprock). It may not be possible therefore to recognise primary lithologies from major element geochemistry alone in weathered bedrock. This will be dependant upon other observations such as rock texture, mineralogy from logging or trace elements associated with resistate minerals in the primary rock. The figure also illustrates how geochemical profiles might appear in drill sections at sites near mineralisation but eroded to different depths. The changes in surface values are of particular note in regard to stream or soil programs. This is one of the dilemmas we face in the Cobar region - where are we in the profile?  
Figure 3 : Variation in the geochemistry of gold through a weathered profile. The relationship of profile (curve) form to erosion level is important to recognise and understand. In Cobar most profiles are eroded to 3, 4 or 5, rarely are they 2 or 1. The Cobar profiles are often capped by recent post-erosion fluvial gravel, sand and silt. These sections do not illustrate the sharp peak frequently seen corresponding to a redox front. (Da Costa, 1993). Sampling in Weathered Profiles The recognition of position in a weathered profile is relatively simple with clues seen in airborne magnetic and radiometric data, in outcrop and position within the Cobar Trough. The colour of drill cuttings or rock exposure in a trench or pit also provides a useful clue as to depth for sampling. This is normally supported by textural evidence (log cuttings dry). The most important colour changes to recognise in drilling from a geochemical perspective are those that occur near the base of the weathered bedrock, the transition zone corresponding to the saprock (partially weathered bedrock) and fresh bedrock. The colour changes from pale browns, buff, yellows and orange-yellows to green to green-grey to dark green-grey to grey. This is generally obvious and easy to log. This change frequently corresponds to the active redox position in the profile. About Cobar this is at about 40-60 metres depth but it can vary depending upon local hydrology and extent of erosion. Elsewhere, such as along deeply incised drainage lines saprock may be capped by more recent fluvial sediments. Sampling the active redox front should be one of the objectives in geochemical exploration in deeply weathered terrains. The reason for this is two-fold. Firstly, geochemical responses are likely to be significantly higher at the redox interface for many ore related elements if present and thus they will be easier to detect. Secondly, as this also generally closely corresponds to the groundwater interface and we are likely to get significant lateral dispersion by groundwater producing a broader anomaly halo. The dispersion of Fe-oxide colloidal precipitates from the redox front is of note in this regard. These have a high adsorption capacity for many ore pathfinder elements and dispersion of them accounts for much of a geochemical halo (along with species such as alunite-jarosite). We should also try to sample fresh bedrock and should drill until we get dark grey or green-grey material, normally within a metre or two of the change from light pallid colours to dark colours. From this we can reliably log bedrock geology, alteration and mineralisation. Where rocks are altered or of a favourable lithotype we might analyse the chips to assess whether they contain low order halo signatures to build up a picture of regional geochemical zonation for areas under cover. The relationship of colour variation and distribution of Fe-oxides through a weathered profile is often confusing. Iron, the fourth most abundant crustal element, produces coloured compounds. The colour changes seen in weathered profiles often only reflect changes in the chemical and mineralogical state of Fe-oxides (or other Fe-bearing minerals). The Fe-oxides change colour in response to changes in their hydration and crystallinity and variation in the local pH conditions through a weathered profile. The primary form of Fe-oxide that results from oxidation of Fe2+ under the low pH conditions that prevail at the redox front is goethite (or perhaps similar species such as feroxyhyte at slightly higher pH). It is usually a distinctive yellow to orange colour. Where kaolinite or other clays are also abundant its colour is subdued by them to give buff to light straw-yellow colours. It is this material that we should sample as our redox sample. It frequently occurs in the first metre or so of pallid coloured material above the transition from green and grey colours. Sample a composite of the first two metres of buff to light straw yellow-brown colours above the colour transition. Yellow orange zones may occur than once in a single hole. These frequently correspond to palaeo-redox positions. It is the lowermost that is generally of most interest as this will the current active redox front, although this depends on depth at which the water table is intersected. Anomalism, Thresholds and Backgrounds in Weathered Rocks What should be clear is that there is no single “anomalous” or “threshold” value, calculated or assumed, for a particular element that might indicate mineralisation or alteration is represented by a particular analysis value. This is particularly so with geochemical data from surface samples and those taken from the saprolite zone. The qualitative spatial distribution of major and trace “pathfinder” species characteristic of an ore, alteration or rock type are more important than their absolute values. High ore values are always important but subtle near background values, where these are co-anomalous with other “pathfinder” elements, are often overlooked by using thresholds to define anomalism or background. Co-anomalism may be indicative of blind mineralisation, reflect dispersion remote from source mineralisation, or be related to an anomalous halo about a mineralised vein in an alteration zone and be more important than a high analysis value. Depletion or enrichment halos are often completely missed by making assumptions about thresholds or backgrounds as these are often relative (proximal-distal, stronger-weaker source) and the scale at which they occur difficult to define. Correlation matrices and factor analysis can assist in definition of co-anomalism or depletion relationships and this should be applied to data from different geological and geochemical settings. Factor analysis can often highlight subtle relationships that can be lost in processing large suites of elements particularly when the volume of analytical data is large. Plotting of spatial relationship of factors, “multi-element enrichment” or “alteration depletion” ratios or stacked line profiles is generally more definitive than using single element plots as these take into consideration the inter-relationship of co-anomalism and relative dispersion as well as spatial distribution. Analytical Strategy for Geochemical Samples Levels of many useful pathfinder elements associated with mineralised systems are low. Their dispersed geochemical halo therefore normally falls below the limits of detection of routine “total” ICP-AES (ppm level) analytical methods. Consequently low detection ICP-MS (ppb level) analytical methods should be utilised in field geochemical studies in intensely weathered rocks (Cohen et al., 1998). ICP-MS analysis can measure values to the low ppb range in rocks and soils. In waters it can measure into the ppt range. It can only do this however, where ionic solution strengths resulting from sample digestion are low, otherwise the ICP-MS detectors are overwhelmed by signal and sensitivity declines. This makes ICP-MS an ideal analytical tool for partial leach extractions as they produce solutions with relatively low ionic solution strengths that are suitable for determination of trace analytes to the ppb level. This is the prime reason for using partial leach methods in exploration geochemistry. An example of an ICP-MS based partial leach method designed exploit this situation in weathered samples is Regoleach, (ALS Chemex method ME-MS08). It uses a combination of ICP-MS for trace species to achieve very low levels of detection of ore-related pathfinder elements and ICP-AES for major species to determine lithology and alteration. By limiting geochemical analyses to a limited number of discrete intervals in drill holes the cost of undertaking multi-element ICP-MS geochemistry is low. There is no need to analyse whole holes but rather a few selected holes at the start of a program to determine the local character of the geochemical profile then sample only the intervals of interest. The significant enhancement in detection limit of ICP-MS over ICP-AES permits a much greater range of pathfinder analytes to be reliably determined. Examples of Geochemical Dispersion – Anomalies P4, LP3 and Mafeesh Geochemical dispersal patterns differ between outcropping mineralisation or its weathered remnants and mineralisation that impinges into an eroded weathered zone, but is buried under more recent cover. Dispersion in the first case is primarily by normal surface processes; in the second case dispersion is strongly influenced by hydrology under and within the cover and local Eh-pH constraints. It is important to recognise the difference when exploring in covered terrains as surface techniques may not see mineralisation under even shallow depths of cover. This is highlighted in the following examples. Host lithologies in the local region are bedded siltstones, fine to medium-grained sandstones, mudstone, minor limestone (Lerida Limestone) and various calcareous, fossiliferous units of the Devonian Amphitheatre Group along the western margin of the Cobar Trough. Folding occurs along the NNW trending Nullawarra Anticline with its axis approximately 1 km E of Anomaly P4 and similarly to the west of LP3. Deep-seated shears parallel this anticline and the tectonic margin of the Cobar Trough and are likely the pathway for regional mineralization, such as at the McKinnons deposit and the anomalies discussed. Regionally geochemical anomalies are controlled by lithology, regolith and proximity to structures. Elements such as Cu, Tl, Sb and Zn, for example, concentrate in and along margins of the Quaternary drainage system in association with recent ferricretes. Quartz float broadly defines faults and shears in subcrop areas with sheet-wash cover. Anomaly P4 is a Au-Ag-As-Bi-Cu-Hg-Mo-Pb-Sb-Tl-Zn anomaly found by partial leaching of a composite of two 100m spaced soil samples (<75m fraction) collected from 250-300 mm depth along 400 m spaced traverse lines. Regional pisolith sampling shows a local low order anomaly from the vicinity of the Anomaly P4 site with low base metals but elevated As and Sb compared with pisoliths from surrounding catchments; this is consistent with patterns in the soil data. Low numbers of pisoliths indicate the upper parts of the laterite profile (mottled zone) have been removed by erosion. Grey vein quartz with abundant pyrite occurs below the redox front enclosed by a siliceous, pyritic envelope, in excess of 50 m in width. As with the McKinnons Au deposit nearby the mineralization is hosted within a shear zone. It probably also occupies a local, but now buried, topographic high within the pre-Quaternary landscape. The present redox and weathering front is at approximately 45-46 m depth over the anomaly and is sharp, with a 10-30 mm transition from completely weathered to fresh sulphide. The weathering front is deeper, away from mineralization, particularly to the east. Anomaly P4 does not appear to contain an economic resource. (Johnston, 1996; Salt et al., 1996). Maximum concentrations in RAB drilling are 0.3 g/t Au, 200 ppm Cu, 940 ppm Pb, 1000 ppm Zn, 2.4 ppm Ag, 2250 ppm As, 380 ppm Sb. There has been dispersion of soluble Cu, Zn, Tl and Sb for some 1000 m down catchment, detectable at the ppb level. This highlights the need to assess geochemistry on a catchment-by-catchment basis to define dispersion patterns for different elements. Where low pH results from oxidation of pyrite a target zone may have abnormally low abundances for many elements in the weathered profile, as soluble elements (eg Cu, Zn, As, Sb) may have been totally leached and redistributed down catchment (Figure 4). Elements that form insoluble salts (eg Pb as plumbo-jarosite) or Au (as the metal) precipitate and become enriched in the vicinity of the ore zone and at palaeo-redox fronts and show limited dispersion. This is readily apparent in drill cross sections and points to the folly of only taking shallow bottom-of-hole samples in RAB programs in such terrains and of not drilling to below the base of oxidation to assess the geochemistry of the whole weathered profile (Figure 5, Rutherford, 2000). Near mineralisation the saprolite units are intensely leached and generally grey, light grey, beige-pink or white in colour. Just above the redox front, goethite has stained the rocks orange to medium-brown. Elsewhere it is generally yellowish, mustard, light brown or medium brown, or near surface, pinkish red-brown, due to hematite (Figure 5). PIMA analysis indicates increasing kaolinite content in the saprolite away from the mineralization both vertically and laterally, in a similar manner to the nearby McKinnons gold mine (Marshall & Scott, 1999). Silicification occurs toward the top of the saprolite over the centre of the anomaly, beneath the transported overburden. This is attributed to precipitation of silica leached from bedrock and the alteration halo under a low pH generated by oxidation of pyrite at the redox front. Anomaly LP3 occurs within the same structural corridor as the McKinnons Gold Mine and along strike of low grade base metal mineralization at that mine site. RAB drilling suggests mineralization is fault-hosted within and at the margins of fault bound blocks of massive Lerida Limestone units of the Amphitheatre Group that extends from just north of McKinnons for at least 2 km north. Although speculative, the irregular depth of intersection of the upper surface of the limestone unit and presence of abundant Fe-Mn oxides at the contact could suggest development of a karst surface during the Tertiary, prior to burial by 10-40 m of gravels and silt during the Late Tertiary and Quaternary although faulting could also account for the variation. Erosion of the weathered profile left thick saprolite zones in some areas while, a short distance away, it is thin or missing. This indicates strongly dissected terrain prior to infill during the Late Tertiary and Quaternary. Boundaries between soil, transported cover and weathered bedrock are difficult to distinguish in RAB cuttings due to high clay contents, but the presence of quartz gravels defines base of alluvial cover. Calcrete is frequent at a depth of 5-8 m and is considered to reflect the presence of the limestone beneath (Salt & Donnelly, 1996; Johnston, 1996). Immediately above the limestone unit and extending away from the fault-bounded blocks is a dark brown-black Fe-Mn-rich horizon that is typically enriched in Zn. Lead is enriched immediately adjacent to the limestone contact and within the limestone unit. Lead, Zn and Au are also locally enriched at distinctive brown coloured redox boundaries in the saprock. RAB drilling did not reach primary mineralization. It is however, considered to be similar to that in the McKinnons pit where minor galena, sphalerite, with a trace of chalcopyrite, arsenopyrite and gold occur within a broad silicified shear. Shear-hosted disseminated pyrite causes a well-defined IP anomaly beneath the McKinnons gold mineralization and extends NNW toward LP3 where pyrite is the only sulphide observed in saprock. There are at least three, possibly four, mineralized fault or shear zones, two along the boundaries of the limestone blocks and one or more within faulted limestone within a 600 m wide interval over 1000 m strike length. Mineralization beneath the limestone unit is untested.
Figure 4: Geochemical expression of P4 and LP3 from partial leach soil results. Anomaly LP3 is best defined by soil partial leach Ag geochemistry. Silver forms a broad 2000 m long anomaly, trending NNW from the McKinnon’s deposit, with a maximum of 230 ppb. Mercury, Mn and to a lesser extent, Co, Mo, Cu and Zn also show anomalism (Figure 4). The strong Ag anomaly was originally thought by Burdekin Resources to be derived from the McKinnon’s deposit (Bywater et al., 1996). Although Ag itself is near continuous from McKinnons due to continuity of mineralized host structures into Anomaly LP3 the dispersion of base metals, e.g. Zn into the cover sequence has been strongly inhibited by the high pH alkaline environment developed about the limestones and calcareous sandstones in the sequence. Elements such as Ag, Hg, Mo, (and possibly also locally Co, Mn, Bi, Au) may by quite soluble in this environments as thio-complexes (after oxidation of pyrite, Mann, 1984, 1998, Webster, 1984) hydroxy-species or oxyanions and disperse through the profile (Thornber, 1992). Thus metals like Ag, Hg and Mo become the target species for exploration geochemistry in high pH environments rather than relying on base metals whose dispersion can be very low. Levels of these trace elements in the soils are very low hence ICP-MS analysis of partial leachates is the best tool for their detection (Cohen et al. 1998; Rutherford, 2000). Most concentrations are well below the detection limits for routine “total” analysis. Pisoliths are rare or absent over most of the anomaly area and none have been analysed.
Figure 5: Distribution of elements around ore zone at P4. Immediately above ore there is a “geochemical hole”. Mafeesh is a quartz-Au-As-Sb-pyrite shear sediment-hosted occurrence that provides a useful example of anomaly dispersion by sheetwash erosion of soil and pisoliths from an outcropping source (Schmidt, 1991). It highlights the need to treat dispersed geochemistry on a catchment basis. In this case outcropping mineralisation has provided a source for widespread anomalism extending for several kilometres down catchment in both media. Broad spaced, for example, 400 x 200m (100 x 100m composites) or even wider, partial leach soil data is far more effective at resolving anomaly targets in such weathered and sheetwash affected terrains where low order anomalism becomes widely dispersed. While this might take longer to sample and have a higher initial cost it is much easier to interpret and far more subtle in its outcomes.
Figure 6: Dispersion of As in soil and pisoliths from Mafeesh. References Bywater, A., Johnston, C., Hall, C.R., Wallace Bell, P. & Elliott, S.M. 1996. Geology of McKinnons Gold Mine, Cobar, New South Wales. In: W.G. Cook et al. (Eds) The Cobar Mineral Field - A 1996 Perspective. Aust. Instit. Mining and Metall., Melbourne, pp279-291. Cohen D.R., Rutherford N.F., Dunlop A.C. & Alipour. S. 1996. .Geochemical Exploration in the Cobar Region. In: Cook W.G., Ford A.J.H., McDermott J.J., Standish P.N., Stegman C.L. & Stegman T.M. (eds). The Cobar Mineral Field - A 1996 Perspective. Australian Institute of Mining and Metallurgy, Melbourne. pp 125-155. Cohen D.R., Shen X.C., Dunlop A.C. & Rutherford N.F. 1998. A comparison of selective extraction soil geochemistry and biogeochemistry in the Cobar Area, New South Wales. Journal of Geochemical Exploration, 61:173-190. Da Costa, M.L. 1993. Gold distribution in lateritic profiles in South America, Africa and Australia: applications to geochemical exploration in tropical regions. Journal of Geochemical Exploration, 47: 143-163. Johnston, C., 1996. EL 3232: Nullawarra. Annual report for the period ending 7th December 1996. Report to NSW Mines Department. Burdekin Resources NL. Leah P.A. 1996. Relict lateritic weathering profiles in the Cobar District, NSW. In: Cook W.G., Ford A.J.H., McDermott J.J., Standish P.N., Stegman C.L. & Stegman T.M. eds. The Cobar Mineral Field - A 1996 Perspective, pp. 157-177. Australian Institute of Mining and Metallurgy, Melbourne. Mann A.W. 1984. Mobility of gold and silver in lateritic weathering profiles: some observations from western Australia. Economic Geology 79: 38-49. Mann A.W. 1991. Geochemical processes in the secondary environment. Advanced Exploration Geochemistry, AMF Workshop Course 763/91. Mann A.W. 1998. Oxidised gold deposits: relationships between oxidation and relative position of the water table. Australian Journal of Earth Sciences 45: 97-108. Marshall S.M. & Scott K.M. 1999. Supergene derived McKinnons gold deposit and associated features in the surrounding weathered rock. In: Taylor G. & Pain C. eds New Approaches to an Old Continent, pp. 149-154. CRC LEME, Perth. Rutherford N.F. 2000. Geochemistry in the weathered profile, Cobar, NSW. Extended Abstract In: Central West Symposium Cobar 2000: Geology, Landscapes and Mineral Exploration. Cobar 17th-19th April 2000. Published by CRC LEME, Perth, CSIRO Expln & Mining, Wembley, WA. Salt, C.J. & Donnelly, T. 1996. RAB drilling drogram over the Ghost Grid. EL 3232: Nullawarra. Internal Report Burdekin Resources NL. Schmidt, B.L. 1991. Peko Exploration Ltd. Cobar Supergroup Project. EL's 3401-3405. First annual report to 9/12/91. Unpubl. report by Geopeko Limited to the Department of Mineral Resources, Sydney, NSW. Thornber M.R. 1992. The chemical mobility and transport of elements in the weathering environment. In: Butt C.R.M. & Zeegers H. eds Regolith Exploration Geochemistry in Tropical and Subtropical Terrains, Handbook of Exploration Geochemistry Volume 4., pp. 79-96. Elsevier, Amsterdam. Webster, J.G. & Mann, A.W. (1984) The influence of climate, geomorphology and primary geology on the supergene migration of gold and silver. Journal of Geochemical Exploration, 22: 21-42. |
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