|
|
Check out |
THE WEATHERED GEOCHEMICAL PROFILE The abundance of many elements is often higher near the surface, commonly from surface to a depth of a metre or so. This often corresponds with a zone of superficial iron enrichment and is largely caused by precipitation of salts during evaporation of ground water. This zone is the preferred sample interval for partial leach sampling. The zone from about 3 to 5 metres below this becomes progressively depleted in most elements. In deeply weathered terrains the mottled and pallid (saprolite) zones beneath can be highly depleted and are of little value for sampling purposes. However it is this zone that has been the target of many explorers seeking to avoid transported surface material and to collect "fresh" bedrock. The geochemical character of weathered profiles is now reasonably well established and exploration sampling, including drilling, should be conducted with bedrock weathering and the transported surface profile taken into consideration. Most important is the recognition of the way in which elements are redistributed in the profile and to test this re-distribution by sampling or drilling through an entire profile into fresh bedrock and below the weathering and redox interface to determine the profile character. Paleo-water tables and fossil redox fronts can produce local enriched horizons within otherwise highly depleted zones in a weathered profile. Supergene processes at a redox front in unmineralised rocks can produce false anomalism. It is also important to know how elements, sourced from mineralisation or the bedrock at depth, redistribute themselves at the surface after being transported from a source. In some instances, due to differential chemical mobility elements can become widely separated from each other and appear disconnected from an obvious and common source. There are both similarities and differences between mineralised and un-mineralised profiles. THE WEATHERED PROFILE AND MOVEMENT OF METALS A weathered profile results essentially from two key geochemical processes, oxidation (redox processes) and hydration. Oxidation largely affects the sulphide component and metal species in the rocks, hydration the silicate species, producing clays and other hydrated species. The depth of formation and re-distribution of the by-products from these processes in the present landscape is controlled by the Paleo- to recent climate and processes and rates of erosion and deposition (Stumm et al., 1981; Trescases, 1992; Thornber, 1992; Appelo et al., 1993).
One of the more important processes responsible for transportation and concentration of many metal species from a zone of weathering sulphide mineralisation appears to be their reaction with or adsorption on to ultra-fine (<10-15 micron) hydrated crystalline aggregates of Fe or Mn oxides which carry them to the surface. These oxide aggregates are actively forming at redox interfaces at or near weathering fronts. Other mechanisms for metal ion transport certainly exist, for example, simple ionic transport in solution, gas transport, biological agents (fauna, flora) etc. but these are probably less important in the exploration context. The hydrated species goethite and pyrolusite, both have significant capacity to absorb or complex other metal species.
Weathering of unmineralised bedrock or that with minor background sulphide also results in development of elevated metal levels at the redox/weathering front by the same redox/hydration processes, with Fe and Mn in this case derived from weathering of Fe-Mg minerals in the soil or rock fragments. The redox front and weathering front may not always be at the same depth. These are controlled by climate and other factors and are related to depth of oxygen penetration and the water table depth (Lawrance, 1999; Gray et al., 1992).
The concentration of amorphous Fe and Mn oxides and subsequent enrichment or release of the metal species absorbed on them in near surface environments is related to local hydrology, Eh and pH characteristics in weathered profile, and climate. Fe-oxides tend to accumulate as ferricrete cements in the matrix of superficial materials, Mn-oxides often accumulate within drainage lines giving rise to drainage delimited anomalism.
PROCESSING DATA FROM DEEPLY WEATHERED OR BURIED TERRAINS There are ways in which data
processing can improve resolution of anomalies. These methods include scaled or
coloured dot plots
and metal factor maps overlain over geology, image plots or stacked line
profiles. Line profiles are best presented as series of stacked graphs of
different elements along a single profile. For regional spatial information the
use of metal (enrichment or abundance) factor maps is very effective. Metal
associations may be defined by using say correlation matrices, factor or
principal component analysis methods or by selecting element associations that
are known to exist in ores mined or prospected locally in the district. The factors may also be used to show metal or
lithogeochemical depletion halos indicative of alteration associated with
mineralisation.
The metal factor approach is often quite productive because it combines the abundance of a number of elements and tends to reflect subtle anomalism better than single element presentations. This method, while highlighting quite subtle features in data, can also de-emphasise the significance of a strong single element response that is unaccompanied by other essential indicator elements that are normally associated with it in mineral occurrences (eg. alluvial gold). The use of metal factors also helps to discriminate between metal associations and thereby different styles of mineralisation or levels of erosion of a mineralised system. Systematic and comprehensive field documentation is necessary to permit effective interpretation of subtle anomalism from buried sources.
|
Send e- mail to rminres@zip.com.au with
questions or comments about this web site.
|
