Geochemistry at a Regional Scale
Regional reconnaissance geochemistry has historically led to a large percentage of the discoveries of mineral deposits. Large scale reconnaissance surveys were a major component of exploration activities in the 1960’s and 1970’s. At that time, it was possible to analyze only a handful of elements (Au, Cu, Pb, Zn, As) at commercial assay laboratories. The assay techniques available at the time were not that sensitive for the purpose for which they were being used, and it is only the really obvious mineral systems that would have been detected. Nevertheless, this was a hugely successful period of exploration.
ICP-MS analyses a much larger suite of elements with 3 or 4 orders of magnitude lower detection limits, and is ideally suited to regional reconnaissance programs. However now that we have suitable tools, greenfields exploration is now out of favour.
The big advantage of the ICP analytical suites is that the additional elements can be used to characterize the sample media. As well as mapping the pathfinder anomalies, the other elements can be used to map bedrock lithologies and to give information about regolith processes. Metals like Au, Cu, Zn and Pb that form chloride complexes are very mobile and readily depleted in lateritic weathering profiles. Elements like Mo, Bi, Te, As, Sb and W form oxyanion complexes that are stable in the regolith. There are still great opportunities for new discoveries in places with leached regolith profiles like Western Australia or Chile, by using the full suite of pathfinder elements, and using the major elements to characterize the sample media. Mineral Mapping is experienced and well qualified to advise clients on the planning and interpretation of regional geochemical surveys.
With the experience of having worked on literally thousands of these data sets, Mineral Mapping has developed a systematic approach to characterizing the host rocks from the immobile trace element signatures, mapping alteration vectors by designing major element scatterplots based on the stoichiometry of the alteration minerals, mapping sulphide mineral speciation from binary and ternary scatterplots, and then looking at the chemical and spatial controls on pathfinder element patterns and alteration mineralogy. Determination of the silicate mineralogy using a SWIR instrument adds another layer of constraint to the geochemical interpretation. Data sets with tens of thousands of analyses can be interpreted and modelled using software packages like ioGAS and Leapfrog. The aim is to be able to predict ore body geometry and clearly define geochemical criteria that will identify a “near-miss” drill hole.
All hydrothermal systems are chemically and mineralogically zoned. Metals require specific conditions of temperature, acidity, redox, aH2S etc, in order to be soluble in a hydrothermal fluid. A change in one or more of those parameters is required to cause metal precipitation. This will leave an imprint in the rock in the form of a metal zoning pattern or alteration in the alteration mineralogy that extends far beyond the ore body grade shells.
SWIR mapping of Mineral Systems
One of the biggest problems in generating realistic geological and alteration models is the lack of consistency in the logging data.
Mine site drill hole data bases typically contain logging data collected over a period of many years. In most cases the logging will have been completed by tens of different geologists. Consistent logging requires a significant level of skill and experience, but this task is generally assigned to the most junior geologists. As a result, there is a large degree of subjectivity in the recognition of basic rock types, and an even greater lack of consistency in correctly identifying alteration mineralogy, particularly when it involves logging RC chips.
Systematic logging with portable analytical devices and spectrometers makes it possible to collect quantitative geochemical and mineralogical data that is independent of operator bias.
With modern portable handheld spectrometers it is possible to measure >1000 meters of RC chips or drill core per day. The data are rapidly interpreted using the CSIRO spectral software “The Spectral Geologist”. This takes a lot of the guess work out of logging. Solid solution variations in sericite and chlorite are easily mapped; things that could never be recognised visually. Spectral logging adds a completely new dimension to your mapping and understanding of mineral systems. Whole core systems such as CoreScan add amazing textural context to the mineralogy. A limitation is that framework silicates which may constitute the majority of the rock mass, do not respond in the SWIR range. Merging spectral data with 4 acid digest ICP geochemistry is a major advantage.
Calculated Quantitative Mineralogy from 4 acid digest ICP Geochemistry
Over the years quite a few different approaches have been used to quantify mineralogy from assay data. Herrmann, W and Berry, RF (2002) used a least squares spreadsheet method for calculating mineral proportions from whole rock major element analyses. This has the limitation that there has to be more constraints than variables to obtain a unique solution. They later modified this to a weighted least squares routine to improve the precision on minerals with lower abundances. Berry (2011) used a second method of calculating the modal mineralogy from chemical analyses based on linear programming using the Simplex method (Press et al., 1986). This required a QXRD or QEMSCAN training data set. The latest version of the linear programming method uses Gibbs Free Energy values per unit mass of minerals to constrain the allowable mineral assemblage. We can now make very realistic estimations of mineral percenatges from a 4 acid digest ICP-AES assay table. This has massive applications for creating gangue mineral models of ore deposits, with applications to predicting physical rock properties and mining and metallurgical characteristics.