Sampling overburden

Mineral exploration, the search for economic ore deposits, requires somewhat different reference samples than those used in ore valuation. Soil or sediment and water samples are frequently used in the search when mineralized areas of abundant outcrop or those covered only by thin locally derived overburden are being evaluated. In such cases, it is virtually impossible not to detect the mineralization from an analysis of ore elements in these types of samples. Later, as the mineral deposits closest to the surface were exploited and then played out, new deposits occurred at progressively greater depths, and these sample types were less and less effective as markers in the search (Hoffman 1989). 

For shallow aquifers in the overburden it is common to drive slim steel pipes that are perforated in the lower meter or so. This method has proven to be an excellent way of taking samples for the design of screened production wells. 

XRF analyses of the profile till sampling reflects lower variation of trace element content. However, it is possible to see indications of mineralized bedrock as relatively higher element concentrations particularly, if overburden is thin. 

Groundwater samples are collected from domestic wells on a regular 10x10 km grid. One sample within each grid node is collected from a well finished in bedrock and another from overburden. 

The ferromagnetic fraction of the till samples was separated by standard methods by Overburden Drilling Management Ltd (ODM). Subsamples containing 22 to 110 randomly selected grains were prepared by ODM. To test for heterogeneity, two sets of 5 subsamples, from 2 till samples, were prepared from the 0.5-1.0 mm size fraction using a plastic riffle at Universite Laval. 

Govett Atherden (1987) show Pb and Zn shallow soil geochemical data collected over the Thalanga Pb-Zn-Cu deposit. Four lines were sampled that represent burial depths of 0, 1, 30 and 50 m. The anomalies display a rabbit-ear pattern centred on the deposit regardless of overburden thickness. The magnitude of the anomalies diminishes with increasing cover and Pb diminishes more rapidly than Zn. 

Abstract Mobile Au in soil has been postulated for many years. It has been used by the mineral exploration industry in areas of transported overburden as a vector towards buried deposits. Until now, the nature of this mobile Au has not been known or investigated. Soil samples from a colluvial area above the Bounty Deposit (Yilgarn Craton, Western Australia) investigated by analytical techniques including laser ablation inductively coupled mass spectrometry (LA-ICP-MS) and synchrotron x-ray fluorescence (SXRF) combined with X-ray absorption spectrometry (XAS) have allowed us to map the invisible Au in these soils and suggests that at least some of it occurs in an ionic form. 

For Ni exploration in the TNB and surrounding areas, till samples in thin drift areas can be collected from the flanks of bedrock outcrops, and from till exposures in road cuts and along lake shorelines and river banks. In areas of thicker cover, backhoe trenching and overburden drilling... 

DBCP has been found In ground water In Hawaii, California, Arizona, South Carolina, and Maryland (5-7,19,77-79). Typical positives are 0.02-20 ppb. Areas with the highest frequency of positives and the highest well concentrations are the San Joaquin Valley In California and the region southwest of Phoenix, Arizona. The Hawaii contamination has occurred despite several hundred feet of overburden between the basal aquifer and the surface. One set of California soil core results show that ppb amounts of DBCP has leached about 15 m through the unsaturated zone ( ), whereas DBCP was not detected In another set of California soil cores sampled as deep as 10 m and five years after the last application (80). The latter results can possibly be explained by rapid movement of DBCP down the soil profile to depths greater than 10 m. 

The upper part of the overburden is continuously subject to a variety of natural processes, which include seasonal changes to the element contents and ratios of elements in sampled material. It is necessary to take these changes into account when interpreting the results. Therefore, certain observations must be recorded during the fieldwork. A special field form is normally designed for collecting the relevant data. Portable computers are recommended to avoid a potential source of error when transferring the data from the forms to computer files at the office. 

Generally speaking, the concentration of minerals in the coal is highest near the overburden and the interseam sediment layers. Because of the discrete nature of the minerals their sporadic distribution in a seam cannot be accurately assessed from a single traverse of sampling through the seam. 

A study of the variation in the distribution of major, minor, and trace elements in lignite-bearing lithologic sequences is critical for both interpretation of deposi-tional environments and utilization of the coal. Samples of lignite of the Kinneman Creek Bed, underclay and overburden from the Center Mine were analyzed by neutron activation, x-ray fluorescence, and x-ray diffraction techniques. Major patterns of element distribution in the lignite include (1) concentration in the margins of the seam (Al, Si, S, Sc, Fe, Co, Ni, Zn, As, Zr, Ag, Ba, Ce, Sm, Eu,... 

Samples of coal, overburden, and underclay were analyzed by neutron activation (NAA) and x-ray fluorescence analysis (XRF). NAA analysis... 

Many of the minor metallic elements have concentrations lying within 20% of the average values for shales. This comparison is illustrated by the data in Table VI, which compares the average concentrations in the underclay and overburden samples, after statistical rejection of outliers by the Dixon method (17), with tabulated values from the literature (18). The most notable exception is manganese, for which the average value in shales is 850 ppm but the observed average in the overburden and underclay was 269 ppm. 

Table VI. Comparison of Average Minor Element Concentrations (ppm) for Overburden/Underclay Samples with Average Concentrations for...

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