|
|
Check out |
BACKGROUNDS, ANOMALISM AND GEOCHEMICAL TARGETS A background level of an element is normally considered to be a typical range of values found in a particular un-mineralised rock type in an area. A background for any given rock type will vary depending upon the degree of weathering, changes in its geomorphic setting, the local hydrology and climate in an area, and most significantly analytical method. In
reality background is what you want it to be. It is scalable and can be
qualitative. It is largely determined by analytical method rather than any other
factor. Different analytical
techniques have different "backgrounds", for example some partial
leaches can easily measure to parts per billion, total analyses only make it to
a thousand times worse than this 1 ppm. The
choice of analytical method makes a large difference to the resolution of the
variability in background in many sample media.
Partial leaches resolve background variation much better than total
leaches because of their much lower detection limits. An
anomaly is created when some component of
interest is added to an arbitrary background from some external source. There is
therefore not necessarily any real relationship between a background value and
an anomalous value, despite the frequent assertion that there is one in order to
process geochemical data. This
especially true when dealing with areas of transported cover or deeply buried
mineralisation. The
relationship between a background value and an added anomalous value is often
obscured by analytical detection limit, assay method and the degree of smoothing
imposed on the sample media. For example, stream sediments in regional
surveys from catchments with mixed source lithologies are considered often to
have a single background for the purposes of statistical analysis. It is
only acceptable to consider this the case due to sediment mixing during
transport that “normalises” the variability. Anomalism will be, in
this case, an addition to this smoothed or normalised background despite the
variability of the original source components. Conversely, near the heads of catchments in highly variable
geological terrains background will change frequently making the determination
of potential anomalism more difficult.
In exploration geochemistry we should try to measure the anomalous component above a (variable) background. In practice this can be a difficult task. Partial and selective leach methods attempt to do this. Potentially they offer more opportunity for detection of anomalism from unexposed or blind mineralisation than total analytical methods. They attempt to do this by partially or selectively removing the more soluble or mobile components from the surface of grains and the weathered Arind@ about grains leaving the un-pulverised (variable) background matrix component largely untouched. Total methods do not differentiate between variable background levels and an introduced anomalous component with the leachate attacking a pulverised sample media. Partial leach substrates however, can themselves be locally quite variable in terms of their absorptive capacity and composition, (more or less clay, Fe-oxide or organic component etc.) and this will introduce a degree of variability.
The form of a partial or selective leach anomaly will not necessarily "mirror" that of a total leach anomaly, although it generally does where mineralisation is outcropping or shallowly buried. Where mineralisation is buried at some depth there might be significant differences in geochemical patterns. The total leach assays will most closely reflect surface rock geochemistry rather than a subtle transported anomaly signature that may lie close to or be within the typical background range for the rocks capping the mineralisation. A partial anomaly may delineate zones of ground water flow egress from joints or faults cutting mineralisation rather than directly reflect mineralisation.
A highly elevated value of an ore element is only likely to occur if mineralisation outcrops. High values are unlikely to reflect the presence of unexposed mineralisation. In most geochemical surveys it is assumed that anomalism and mineralisation go hand in hand and that mineralisation is at least shallow beneath an anomaly peak. This is a reflection of the simple methods by which data are processed without due regard to sample environment setting and variation. We are constrained by the fact that our interpretive experience is largely based on dealing with outcrop sourced geochemical anomalism. To successfully interpret geochemical data from areas with cover the interpretive strategy needs to change. Our outcrop geochemical anomaly models may need to be extensively modified or abandoned in areas of deep cover or in deeply weathered terrains. Big numbers are not always the best numbers. The numbers we are looking for might, for example, represent the transported part of a subtle anomaly or be a low order remote leakage expression of a buried source. In deeply weathered terrains we need to adjust our approach in data assessment to use low assay values and to accurately identify anomalism within them. By using partial leach methods and analysing for a range of analytes, it is possible to use co-anomalism of associated elements rather than the absolute magnitude of an element to identify anomalism. Interpretation strategies that
utilise a uniform background value, for example, a standard deviation cut-off, setting below limit of
detection values to half the detection limit, using
average 25th quartile value, or smoothing procedures, while
convenient to improve the visual result from imaging or contouring processes,
severely degrade the ability to resolve low order anomalism sitting at or near
the detection limit, especially in partial leach
surveys. In the search for buried mineralisation, especially in deeply weathered terrains, or terrains with thick superficial cover, we might need to look for an expression of an alteration halo characterised by elements other than those from ore minerals in an ore body. These may define zones of depletion rather than enhancement. We need to assay for a range of elements that will identify both the ore minerals and the associated alteration halo mineralogy. It often requires low levels of detection to separate these respective signatures from a variably weathered unmineralised and unaltered bedrock signature. The expression of anomaly geochemistry and alteration may be confined to narrow joints or shears rather than be pervasive through the bulk of a rock, or be expressed in an erratic manner along a vein or fault system. In areas with deep cover we might anticipate anomalism expressed as zones of dispersion from buried “outcrop” of faults or shears that have channelled anomalism into the overburden. The anomalous pattern, as detected at the surface, may bear no relation to the form of the original mineralisation. |
Send e- mail to rminres@zip.com.au with
questions or comments about this web site.
|
