|
|
Check out |
The Geochemical Profile in Deeply Weathered or Buried Terrains. The figures given below indicate a number of aspects of geochemical differentiation of elements in weathered profiles. Changes in erosion characteristics across an area may truncate a profile at different levels. Similarly, deposition of transported material across a weathered surface will influence the character of the profile by changing the water table depth and depth of active oxidation. The drill profiles (3 metre composite assays) show the importance of testing a complete profile in deeply weathered environments early in an exploration program in order to understand the geochemical distribution induced by the weathering process in an area under investigation. This testing is necessary to ensure detection of mineralisation whose geochemical expression might be entirely removed in the near surface parts of a profile by erosion truncation or be capped by transported cover. Of note is the low order enrichment of mineralisation sourced elements in the upper superficial environment (0-1 metre depth) that can be effectively assayed by low level analytical methods (ppb range ICP-MS) using partial or selective leach techniques.
Au, Pb, As, Sb re-distribution in weathered profile above pyrite-rich quartz vein-hosted mineralisation. Of note is the precipitation of Au and insoluble Pb salts at an old paleo-water table level. Soluble elements do not persist in such zones. The low concentration of As (and in fact Sb) at the surface associated with Fe-oxides is notable and offers a geochemical target for partial leach surveys. Low bedrock and groundwater pH from sulphide weathering gives rise to high cation ion mobility away from source.
Examples of geochemistry
in weathered profiles associated with un-mineralised bedrock and base metal vein
mineralisation in a deeply weathered terrain. Note should be made of the scales. Although the profile forms are similar the relative magnitude
of values down each profile is markedly different. Of note is the apparent greater depletion of base metal
values associated with the base metal sulphide profile – this is in part scale
but also reflects the greater mobility of metal ions where pH is low (from
sulphide oxidation). These sections
lack a paleo-water table and fossil redox zone that would have given rise to Pb
anomalism part way down the profile. Where ferricretes and ferruginous soils cap a deeply weathered profile they can "fix" Cu, Pb, Zn, Au, As, Sb, Tl and other elements from mineralisation to form discrete but low order anomalies that reflect the dispersion of geochemistry away from a source by ground water and other agents even though the intervening pallid zones are highly depleted. Rainfall may deplete the immediate surface soil horizons (A horizon) with salts either being entirely removed or precipitated slightly deeper in the profile. Whilst organic enrichment in the A-soil horizon may occur this is often transient, dispersed by sheet wash erosion and has the potential to be contaminated by wind borne contaminants or other agencies. The best sample depths for soils in such environments is often at depths of about 20-30 centimetres. In arid terrains dominated by evaporative processes where calcretes are prominent in the profile care must be taken to carefully select samples from a zone that is coincident with a zone of precipitation and accumulation. This is often marked by colouration changes due to subtle increases in Fe-oxide accumulation. The depth at which this occurs can vary along a sample traverse so no fixed sample depth should be defined. The character of a calcrete-rich terrain may be similar to that illustrated below. Changes in sample depth, marked by colouration changes can be subtle.
|
Send e- mail to rminres@zip.com.au with
questions or comments about this web site.
|
