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Open File Report 108

Regolith-landscape evolution and geochemical dispersion about the Bronzewing Gold Deposit, WA

Varga, Z.S., Anand, R.R. and Wildman, J.E.

The Bronzewing deposit is in the Archaean Yandal Greenstone Belt. Gold mineralisation occurs within a sequence of mafic volcanics (basalts, dolerites) and minor sediments, which are intruded by felsic porphyries. The mineralisation is associated with a dense stockwork of quartz veining, and alteration of the host sequence, and is accompanied by pyrite, pyrrhotite and minor chalcopyrite and scheelite. Gold, W and Cu are the most significant indicators in mineralised bedrock. Regolith Regolith-landform relationships over the 1000 km2, centred on the Bronzewing deposit, were mapped at 1:50 000. This was based on interpretation of aerial photography, image enhanced Landsat TM images and field traverses. The Bronzewing Au deposit was covered by a blanket of soil and colluvium. Mapping regolith relationships and distributions using drilling and pit faces, has revealed the details of the sub-surface regolith and palaeolandscape, from which the weathering history and likely origins of anomalies in the residuum and transported cover can be deduced. A wide range of sediments overlie older, residual regolith. Colluvial and alluvial sediments are 20-30 m thick in the Discovery pit and directly overlie saprolite. In the southwest of the Central pit, the sediments are 15-20 m thick and overlie lateritic residuum in places. The sediments thin towards the northeast to less than 5 m thick in the Laterite pit. Alluvium is most likely derived from the south, and the colluvium from the east where there is a subdued breakaway. The palaeochannels have been infilled with kaolinite-smectite sediments derived from the erosion of pre-existing red soils and saprolites. On a regional scale, palaeochannel sediments reach 120 m in thickness. Dolocretes, calcretes, pisoliths and megamottles have developed in these sediments. Mobilisation and segregation of Fe by a combination of roots and reduced groundwaters in porous, vegetated red clays was probably responsible for the formation of the megamottles. Dolocretes are likely to be relict forms equivalent to those in deep sediments in the Roe palaeodrainage. They are confined to the base of the channel above the lateritic residuum and may have formed by evaporation of Mg-rich lakewaters so their formation appears to be related to a period of high evaporation rates. The dolocretes differ from valley calcretes of the region, which are surficial deposits in major, active drainages. Considerable palaeotopography (60 m) around Bronzewing suggests that lateritic residuum did not form a simple, extensive, peneplained surface but a discontinuous cover on an undulating plateau. The pre-Eocene landscape at Bronzewing was not only mantled with lateritic residuum but also with thick blankets of kaolinite-hematite red soils. Ferruginous duricrusts and red soils developed in different sites in response to contrasting geological and topographic conditions. Duricrusts were developed on mafic and ultramafic rocks and red soils were probably restricted to felsic lithologies on well-drained upper slopes. Where fully preserved, the residual profile beneath the colluvium and alluvium has a 2-5 m thick lateritic residuum consisting of lateritic nodules and some pisoliths, set in a silty clay matrix. The nodules were formed by the fragmentation and collapse of the underlying ferruginous saprolite. The ferruginous saprolite, a few metres thick, grades downwards into saprolite. Fresh rock is encountered at 80 to 120 m depth. Geochemical dispersion in the regolith of the residual materials (ferruginous saprolite and lateritic residuum) showed significant Au, W and Cu anomalies in the vicinity of the Bronzewing deposit. The lateritic duricrust and nodules and pisoliths of the Laterite and Central pits, where developed over primary mineralisation, contain significant amounts of Au. Elements associated with the Au mineralisation in the lateritic residuum of the Laterite pit are Ag, Ba, Ce, W, Mo, As, Sb and Cu. The Au anomalies are not as consistent in the Discovery pit, and the lateritic residuum of both the Central and Discovery pits are enriched in Cu and W close to the primary mineralisation. Wide spaced sampling would be adequate to sample areas of buried lateritic residuum and ferruginous saprolite. The buried lateritic residuum contains Au to ore grade, which sometimes extends into the colluvium. This relationship is particularly apparent in the Central pit area where extensive Au anomalies occur across the unconformity. Size fractionation of the gravelly colluvium that occurs within a metre of the residuum/colluvium interface, indicates that Au is concentrated in the plus 2000 µm fraction and depleted in the minus 75 µm fraction, relative to the bulk sample. The enrichment in the coarse fractions represents clastic dispersion of lateritic detritus. There is no evidence to show that hydromorphic dispersion has accumulated Au in the fine fraction. Gravelly colluvium is a useful sampling medium in situations where lateritic residuum or ferruginous saprolite are missing. Mottles extracted from the palaeochannel sediments contain no significant enrichment of pathfinder elements, and Au contents are below detection. No significant concentrations of Au or pathfinder elements are present in bulk soils collected from 0.3-0.5 m depth; the Au content barely exceeded the detection limit of 5 ppb. No dispersion of Au into the soil was detected by partial extraction of the soil fine fraction (<250 µm).

Last updated: Sunday, August 05, 2001 14:04:21


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