W.K. Fletcher

Effects of valley and local channel morphology on the distribution of gold in stream sediments from Harris Creek, British Columbia, Canada

Abstract Large (60 kg, minus 2 mm) sediment samples collected from a 5-km reach of a mature highland stream were used to investigate the effects of bar morphology and channel slope on the downstream dispersion of Au. Results are compared with those for an abundant heavy mineral (magnetite) and are discussed in terms of differential entrapment of light and heavy minerals by gravels and differences in supply of magnetite and Au to the study reach. Gold is selectively trapped in gravels; therefore, highest Au concentrations, a long dispersion train and lowest between-sampling-location variability were obtained with heavy-mineral concentrates of the minus 105+74 μm fraction from sandy gravels. In contrast, sands in bar-tail eddy pools gave the least reliable Au concentrations, due to random (Poisson) sampling errors and local post-depositional winnowing, and show rapid downstream anomaly decay.

Distribution and behaviour of platinum in soils, sediments and waters of the Tulameen ultramafic complex, southern British Columbia, Canada

Abstract This paper reports Pt content of surficial media associated with Pt-rich chromitites in the dunite core of the Tulameen ultramafic complex. Platinum content of the −212 μm fraction of soils and sediments was determined by fire assay-inductively coupled plasma spectroscopy. C-horizon soils on dunite colluvium (mean: 24.2% MgO), locally-derived dunitic till (16.5% MgO) and exotic non-dunitic till (5.7% MgO) have median Pt concentrations of 88 ppb, 36 ppb and 8 ppb, respectively. Corresponding medians in ashed LFH horizons are 65 ppb, 13 ppb and 7 ppb Pt. Platinum values of 8–91 ppb are found in sediments from the small stream that drains the area. Stream and bog waters contain less than 1 ppt to a maximum of 2.45 ppt Pt. Geochemical patterns for Pt indicate that glacial transport and mass wasting are the dominant processes that control distribution of Pt in soils on Grasshopper Mountain. There is also some (slight) evidence for very limited hydromorphic mobility of Pt and its accumulation in bogs. During routine exploration geochemical programmes the considerable local variability in soil parent materials and related variations in background concentrations of Pt need to be taken into account in evaluating the significance of Pt values. This requires careful identification of soil parent materials. Soil MgO content provides a useful index of their dunite content for this purpose.

Platinum distribution in soil profiles of the Tulameen ultramafic complex, southern British Columbia

Abstract Exploration for chromitite-associated Pt deposits is hampered by a poor understanding of the distribution and behaviour of Pt in the surficial environment. This study investigates Pt content, residence sites and PGE mineralogy of soils developed on till and colluvium associated with the Tulameen ultramafic complex in southern British Columbia. Seventy-six soil profiles, as well as sediments, bogs and waters were sampled above the dunite core of the Tulameen complex, within which Pt occurrences consist of massive-to-discontinuous segregations of platiniferous chromitite. Platinum content of the −70 mesh fraction of soils and sediments was determined by lead fire assay-inductively coupled plasma emission spectrometry. Samples from fourteen selected profiles were then examined in detail to determine Pt mineralogy and its distribution between different size, density and magnetic fractions. Platinum concentrations in the −70 mesh fraction of C horizon soils range from 2 to 885 ppb and are closely related to soil dunite content, as estimated from MgO content and verified by XRD mineralogy. Dunite colluvium (mean: 24.2% MgO), locally-derived dunitic till (mean: 16.5% MgO) and exotic non-dunitic till (mean: 5.7% MgO) have median Pt concentrations of 88 ppb, 36 ppb and 8 ppb, respectively. This trend is evident in all size and density fractions. Platinum content of heavy mineral (SG > 3.3) fractions is 10–20× greater than in light mineral fractions. Platinum is most abundant in the heavy magnetic fraction from non-dunitic tills and dunitic tills remote from known mineralization, but the proportion of Pt in the heavy non-magnetic fraction increases with increasing proximity to mineralization. Scanning electron microscopy and microprobe studies of heavy fractions from C-horizons identified Pt-Fe-Cu alloys as free grains, and as inclusions in Mg-silicates and chromites. Chromite occurs as Mg-Cr-rich anhedral fragments and as Fe-rich euhedral to subhedral crystals. The latter are relatively more abundant in the magnetic fraction and are interpreted as Pt-poor grains disseminated throughout the dunite. The Mg-Cr-rich chromite fragments are relatively more important in the non-magnetic fraction and are interpreted as remnants of Pt-bearing massive chromitite segregations. The abundance of chromite fragments in soils near chromitite segregations accounts for the high Pt content of the non-magnetic heavy fractions of these soils. The −270 mesh fraction or the magnetic heavy fraction of C-horizon soils would be the most suitable sample media for reconnaissance geochemical sampling. However, the greater contrast in Pt concentrations, more limited dispersal and Mg-Cr-rich chromite association of the non-magnetic heavy fraction make it a more suitable medium for detailed geochemical sampling.

Organic Matter Scavenging of Copper, Zinc, Molybdenum, Iron and Manganese, Estimated by a Sodium Hypochlorite Extraction (pH 9.5)

Abstract The importance of trace metal scavenging by organic matter in geochemical samples was estimated using an alkaline sodium hypochlorite extraction to leach copper, zinc, molybdenum, iron and manganese from a variety of soils, and stream and lake sediments collected on the Nechako plateau, central British Columbia. The reagent oxidizes or dissolves most forms of organic matter, together with any sulphide minerals, to give strongly coloured extracts containing the associated trace elements at a pH where solution of other sample fractions is at a minimum. Metals precipitated due to alkaline conditions are redissolved by a succeeding distilled-water leach (pH 3.0 ± 0.3). A large fraction of the copper, zinc, molybdenum, and manganese held within the organic fraction of the A soil horizon is liberated whereas only minor amounts of copper, zinc, and manganese are released from inorganic soil (B and C) horizons. Molybdenum, however, is relatively soluble in all soils as the molybdate ion. Despite similar concentrations of organic matter in A horizon soils and stream sediments the latter release a lower proportion of their trace element content. Behaviour of the organic fraction of lake sediments varies from lake to lake and there is great variability in the association of copper, zinc, molybdenum and manganese with organic matter even within the same lake. The presence of organic matter in samples subjected to other partial extractions can be a deleterious factor if the organic fraction is not first removed by a hypochlorite extraction.

Transport of cassiterite in a Malaysian stream: implications for geochemical exploration

It is generally supposed that accumulations of heavy minerals develop on stream beds because heavy minerals are transported more slowly than the sediment as a whole. However, insofar as relative transport rates of heavy minerals and sediments have not been measured in streams, details of the underlying sedimentological processes remain poorly understood. To address this problem, pit traps have been used to measure transport rates of cassiterite and sediments in the Sungai Petal, a cassiterite-rich stream in Perak, Malaysia. Under base level flow conditions (0.5 m3 per sec) no bedload sediment is transported. However, after heavy rain storms flows rise rapidly to discharges that exceed 3.5 m3 per sec. Bedload transport starts at a discharge of approximately 1.0 m3 per sec and thereafter increases exponentially for all size fractions up to discharge of 2.2 m3 per sec when the rate of sediment accumulation in the trap exceeds 5 kg per minute. Concentrations of Sn in sediments caught in the trap also increase as discharge and sediment transport rates increase. This increase results from the preferential retention of cassiterite in the bedload as more sediment is transported in suspension with increasing discharge. Because very fine sediment is swept away in suspension at the onset of bed movement, the smallest differences between Sn concentrations at low and high flows are found in the fine sand and silt fractions. In contrast, because larger particles of sediment and cassiterite are both initially transported as bedload, the coarse fractions of the stream sediments only become enriched in Sn under higher flow conditions. This has important implications for geochemical surveys insofar as enrichment of very fine cassiterite will occur under even relatively low flow conditions and hence have relatively uniform concentrations on the stream bed. Conversely, because coarse cassiterite will only be enriched under higher flow conditions, Sn concentrations in these fractions can be expected to exhibit greater variability on the bed of the stream. It is concluded that the very fine sand and silt fractions will provide the best anomaly contrast and longest dispersion trains for elements dispersed in drainage sediments as the principal constituents of heavy minerals.

Contamination from forestry activities: implications for stream sediment exploration programmes

Abstract Although it is well established that background geochemical patterns in stream sediments generally depend on drainage basin geology, little is known of the effects of watershed disturbance on such patterns. Here we report on the effects of forestry activities on sediment geochemistry on the Interior Plateau of British Columbia. Stream sediment samples were collected from six small streams in 1996, before logging, and in 1997 after clear-cut logging. Samples were sieved and the −212 μm fraction analysed by multi-element ICP after both strong acid and total decomposition procedures. Field and analytical duplicates, and samples from unlogged control sites were used to estimate variations not associated with logging activities. Results show that prior to logging each stream had a distinct multi-element geochemical signature related to drainage basin geology. There was no overall effect of logging on these signatures within or downstream of the cut-blocks — probably because unlogged buffer zones along the stream channels prevented the development of new sediment sources. Logging roads did, however, cause local changes of sediment geochemistry in two ways. (1) Abrasion of zinc from galvanized culverts results in zinc anomalies, with concentrations > 200 ppm. These anomalies can extend several hundred metres downstream and persist for at least five years after installation of culverts. (2) Construction of stream crossings and roadside ditches create new sources of sediment supply to the streams. Although of local origin, the newly exposed and eroding surficial deposits contain more fine-grained material and have multi-element geochemical signatures that differ significantly from those of the mature fluvial sediments found in the stream channel before logging. Input of the new material results in geochemical anomalies that can extend several hundred metres downstream from the source. In this study Co, Mg and Ni were found to have the largest concentration differences between sediment sources and sediments, and hence to give the longest anomalous dispersion trains. The effects of logging roads on sediment geochemistry thus relate to creation of new sources of local sediment as well as to introduction of exotic materials into the stream. No geochemical response could be attributed to the clear-cut forest block. Such changes might develop eventually if soil erosion increased or if changes to the groundwater regime modified input of dissolved elements to the stream channel.

Distribution and dispersion of gold in point bar and pavement sediments of the Huai Hin Laep, Loei, northeastern Thailand

Abstract Gold anomalies in drainage sediments are often erratic, reflecting both the nugget effect and hydraulic effects whereby gold is concentrated at favorable sites along a stream. This study investigates these factors in a stream in northeastern Thailand. Bulk sediment samples, consisting of approximately 40 kg of −12 mm material, were collected from bar and pavement sites along an 8 km study reach. Samples were wet sieved into eight size fractions. The five fractions between 425 μm and 53 μm were then processed to obtain heavy mineral concentrates (SG > 3.3). Gold content of all size and density fractions finer than 425 μm was determined by fire assay-atomic absorption. Concentrations of Au in the heavy mineral concentrates typically range from 10,000 to 50,000 ppb (maximum 198,000 ppb), whereas the corresponding light mineral fractions and the −53 μm fraction generally contain The estimated median number of gold particles in the heavy mineral concentrates increase from less than one in the 212–425 μm fraction to about three in the 53–106 μm size range. However, because of dilution by the light mineral and −53 μm fractions, the probability of a 30 g analytical sub-sample containing a particle of gold is so low that in thirteen out of sixteen −149 μm sediment samples no gold was detected. Insofar as this results from dilution by large quantities of −53 μm sediment, failure of conventional sieved sediment samples to reliably detect the anomaly is probably a consequence of increased erosion caused by deforestation and land usage. Heavy mineral concentrates from pavement and other high energy sites are more reliable than conventional sediment samples for detecting gold anomalies of the Huai Hin Laep type. A low sample density is adequate but, because anomaly contrast may increase downstream, careful interpretation is required.

Enrichment of platinum and associated elements in organic seepage soils of the Tulameen ultramafic complex, southern British Columbia

Abstract Concentrations of Pt have been determined in seepage waters, organic soil horizons and trees in and upslope from a seepage site on the dunite core of the platiniferous Tulameen ultramafic complex. The concentrations of Pt (together with Sb, Cr, Ni, Br, Co, As and Cu) are enhanced twenty-fold in the organic seepage soils compared to Pt concentration in both the LFH horizons of the upslope soils and Pt concentrations found in the ash of twigs and bark of trees growing on the seep. The seepage waters contain 0.4 to 0.8 ng/L Pt. The study provides evidence that weathering under temperate conditions can release Pt from platinum group minerals and that the released Pt can then be transported, either in solution or a colloidal form, to seepage sites where it is accumulated by organic matter. Organic-rich soils from seepage sites might therefore provide suitable sampling media for geochemical surveys for Pt.

Transport of magnetite and gold in Harris Creek, British Columbia, and implications for exploration

Abstract Earlier studies have suggested that difficulties sometimes encountered in following up anomalies can result partly from large seasonal variations in gold content of stream sediments. This possibility has been investigated using pit traps installed in a bar on Harris Creek, British Columbia. Sediments caught in the traps in 1988 and 1989 provide data on variations in concentrations of gold and magnetite in bedload as stream discharge changes. These show that anomalous concentrations of gold are only transported for brief periods when increased discharge, caused by snowmelt floods, disrupts the cobble-gravel pavement. For the purposes of exploration geochemistry, these flood sediments, trapped in the voids of the newly formed pavement as the flood peak passes, provide the best chance of recognizing the presence of anomalous concentrations of gold in a catchment. However, their subsequent burial by post-flood gold-poor sediments may cause seasonal variations in gold content of near-surface sediments.

Aspects of exploration geochemistry in Southeast Asia: soils, sediments and potential for anthropogenic effects

Abstract Compared with the rain forests of the old shield areas of West Africa and the Amazon Basin, the rain forests of SE Asia are characterized by high relief, active tectonism and volcanism. These features, together with heavy tropical rains, result in the major rivers having some of the highest sediment yields in the world. Smaller streams and rivers, even in areas of only moderate relief, have coarse sand and gravel beds. Frequent tropical rain storms mobilize these sediments and rapidly flush very fine sand, silt and clay from the stream bed in suspension. During this flushing process fine-grained heavy minerals, for example gold and cassiterite, lag behind and accumulate on the bed of the stream. Because the finer ( Logging of tropical rain forests and conversion of land to agricultural use greatly increases rates of soil erosion. Studies of Au anomalies in Thailand indicate that, depending on the extent of the soil anomaly within a catchment basin, this can dilute Au concentrations to a point where the Au anomaly in sediments can go undetected. However, if soil erosion is minimized by re-establishing a cover of ground vegetation, as in mature rubber plantations, fine sediments are again flushed from the stream bed and geochemical anomalies return to more natural conditions with accumulations of heavy minerals on the stream bed.