Centre for Teaching and Research in Strategic Mineral Deposits, Department of Geology and Geophysics, The University of Western Australia, Perth, Western Australia 6907
The Osborne Cu-Au mineralised zone sub-crops as a gossan within Precambrian basement rock at 30-45 m depth beneath Mesozoic sediments. An investigation of the regolith profile and geochemistry on section 21937.5 mN through the deposit, and a comparison with the Thalanga Pb-Zn-Cu-Ag deposit, northern Queensland, has highlighted the importance of a combination of sub-vertical diffusion and sub-horizontal dispersion and accumulations of trace elements associated with redox processes in semi-watersaturated buried terrain.
At Osborne, the sedimentary pile comprises a basal coarse sand of low energy fluvial quiescent flood plain and swamp origin, that wedges out around the subcrop of the mineralised zone, and an upper silt- and mudstone horizon of marine origin. A sharp redox front, almost coincident with the inter-sedimentary contact, separates the sedimentary pile into a grey water-saturated reduced lower saprolitic profile and a water-unsaturated ferruginous oxidised upper saprolitic profile.
Trace element anomalism within the Mesozoic sediment is associated with five distinct zones.
(1) A local dispersion plume directly over the buried mineralised zone.
(2) A subtle goethitic zone within the lower reduced profile beneath the redox front.
(3) A strong sub-horizontal goethitic zone just above the redox front in oxidised sediment.
(4) A subtle sub-horizontal haematitic zone within the oxidised upper saprolitic sediment well above the main redox front.
(5) The top four metres of the near-surface profile over the buried mineralised zone. This zone is strongly silcreted and tails off into remnant ferricrete of a palaeo-land surface preserved to the west of the buried mineralised zone.
Gold, Cu, Ag, As, Sb, Bi, Pb, Zn, Co, Cd, Mo, Se, and Mn are associated with one or more of these zones. The redox zone, (2) above, is the most consistently anomalous and laterally continuous of the five zones. In addition to trace element accumulation associated with the redox zone, trace elements have a distinct tendency to be concentrated either in the reduced profile beneath the redox front or in the oxidising environment above the redox front. Gold, Cu, Ag, Mo and Se are most concentrated in the oxidised profile, particularly in the near-surface environment, and of these, Se is additionally anomalous in the reduced zone. Zinc, Cd, Se, Co and Mn are strongly associated with redox zone and in the reduced profile and are not anomalous in the near-surface. Arsenic, Pb, Bi and Sb were not sufficiently anomalous to confidently define their dispersion preference.
The distribution of trace elements suggests that water-saturated reducing conditions within the Mesozoic sedimentary pile have resulted in the sub-vertical diffusion of trace elements into the near-surface with limited lateral dispersion, similar to that shown at the base of the Thalanga profile. A subsequent fall in the watertable has allowed oxidation of the profile and the modification of the dispersion plume by redox processes into sub-horizontal zones associated with redox fronts, where trace elements are primarily associated with iron oxides. The redox zones become increasingly anomalous, broader and less coherent with depth. They are probably sourced from above by leaching of the upper profile with downward movement of groundwater, and from below due to diffusion processes from the basement mineralisation under water-saturated reducing conditions. Under extended dry conditions, the net movement of groundwater into the near-surface with an excess of evaporation over precipitation has caused the subtle migration of trace elements back into the near-surface. However, it is likely that the formation of silcrete in the upper profile under arid phase conditions has prevented further Recent near-surface accumulation of the elements as soil anomalies.
It was previously proposed that to initiate element dispersion into transported cover, redox processes were required to intersect the basement source. This study has further defined the influence of redox processes on element dispersion to included situations where the source is at or below the redox front. Importantly, trace elements can efficiently migrate in water-saturated reducing environments. The relative associations of the trace elements in the supergene environment is controlled by the oxidation state they are forced to adopt under conditions of specific profile geochemistry, particularly Eh and pH. Thus the regolith acts as a chromatograph in which elements are separated based on their quantum chemistry and therefore their behaviour is predicable. This aspect of the study requires further investigation.
These findings have significant implications for exploration. Large areas of inland Australia have been inundated by water under marine environments (e.g. Interior Lowlands of central eastern Australia) or extensive lacustrine and palaeochannel environments (e.g. Yilgarn Block, Western Australia). The ability of trace elements to migrate sub-vertically under water-saturated reducing conditions brings anomalism from mineralised zones, exposed at the unconformity or palaeo-land surface, into the near-surface in these environments and greatly reduces the depth of the target zone in regional exploration. The subsequent modification of these zones by sub-horizontal redox processes with drying of the profile has broadened the target anomaly. Thus the location of the mineralised zone as a subcrop into the oxidising environment (hence intersection of the mineralised zone by redox processes) will tend to produce broader dispersion anomalies than those which remain under reducing conditions.
The redox conditions of the regolith can be identified through field logging of the profile and potential zones of trace element anomalism targeted. The distance of trace element dispersion is dependent on the style and size of primary mineralisation and profile hydrology. These redox zones can be tested by broad spaced (an estimated 500 m by 1000 m grid) vertical RAB drilling to a depth to intersect the lower most redox zone along the perceived strike of the mineralisation. As the anomaly dispersion direction is down slope, element responses corresponding to the redox zone can be used as vectors for the location of buried primary mineralised zone. A major hurdle in exploration in buried terrains, remains the generation of regional exploration targets currently done in areas of shallow cover using geophysical techniques.