Dispersal Trains Research Articles

Glacial landform-sediment assemblages in the Canadian High Arctic and their implications for late Quaternary glaciations

AbstractModern terrestrial glaciers in the Canadian High Arctic range from polythermal to cold-based. Where polythermal glaciers override thick unconsolidated sediment, longitudinal compression and glaciotectonic thrusting produce thrust-block moraines. In contrast, the dominant geomorphic record of cold-based glaciers consists of lateral and proglacial meltwater channels. Geomorphic and sedimentary evidence indicates that late Quaternary fiord glaciers were also characterized by variations in basal thermal regime. Erratic dispersal trains and striated bedrock record the flow of warm- based ice during the Last Glacial Maximum. Emergent grounding-line fans and morainal banks, deposited during deglaciation, consist of heterogeneous glaciomarine deposits that record well-developed subglacial drainage and high sedimentation rates. However, in other fiords, subaqueous outwash and fine-grained glaciomarine deposits are absent and deglaciation is recorded by lateral meltwater channels graded to raised glaciomarine deltas, suggesting these glaciers were predominantly cold-based during retreat. Regionally, deglacial depocentres are located at pinning points within fiords and a prominent belt of glaciogenic landforms at fiord heads records stabilization of ice margins during early Holocene retreat, rather than the limit of late Quaternary glaciation. Collectively, these observations refute previous reconstructions which inferred a climatically controlled switch from cold- to warm-based thermal conditions in fiord glaciers during early Holocene deglaciation, and indicate that the dominant controls on thermal regime were glaciological.

Detrital transport of metals by glaciers, an example from the Pinchi Mine, central British Columbia

Abundant cinnabar (HgS) mineralization is associated with the Pinchi Fault in central British Columbia. Two formerly producing mercury mines have been developed on this fault: Pinchi and Bralorne Takla. The mercury content of till (a sediment type directly deposited by glaciers) in the area of this fault is primarily controlled by the occurrence of cinnabar mineralization in bedrock and the direction of ice flow. Cinnabar-bearing bedrock was eroded by glaciers, transported in the direction of ice flow, and deposited "down-ice" from its source. An example of such a dispersal train is documented for the Pinchi Mine area where mercury ore was transported over a distance of 12 km, as measured in the clay-sized fraction ( 3.3) of this same sediment. Antimony, chromium, and nickel dispersal trains were also detected in the region. These data indicate that natural glacial processes can result in the "mobilization" of metals in the surficial environment, a factor which has to be considered at mine sites in glaciated terrain, where mine reclamation and remediation measures are now required.

Stream sediment geochemistry of Sn, Au and associated elements in Southeast Asia

This paper summarises and reviews some detailed orientation surveys, case histories and experimental studies of the behaviour of cassiterite and gold in streams in SE Asia as a basis for recommendations on the design and interpretation of exploration geochemical surveys for these elements. In the tropical rain forests of SE Asia the frequent rain storms mobilize stream sediments and rapidly flush very fine sand, silt and clay from the stream bed in suspension. This flushing process causes fine heavy minerals such as cassiterite and gold to preferentially accumulate on the bed of the stream. The most consistent and longest geochemical dispersion trains for elements transported as heavy minerals are thus found in the very fine sand fractions « 100 l1m). Use of coarser fractions, collected at heavy mineral trap sites, gives more erratic geochemical anomalies that are often displaced downstream away from their source. During interpretation these displaced anomalies can be recognised by: (i) ratioing Au or Sn values to either abundance of a ubiquitous heavy mineral (e.g., magnetite) or the transport equivalent size of the stream sediment; (ii) by absence of mobile pathfinder elements (e.g., As and base metals); or (iii) from field observations of stream width, velocity, bed roughness and gradient that permit favourable sites for heavy mineral accumulation to be identified. Where removal of the rain forest and conversion ofland to agricultural production greatly increases rates of soil erosion, the accumulations of heavy minerals can be diluted to the extent that Au anomalies may go undetected. Possible answers to this problem are: (i) use of more sensitive analytical procedures; (ii) elimination of the silt-clay fraction and analysis offine sand (53-100 l1m) fraction; or (iii) use of heavy mineral concentrates. Once 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.

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

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 (< 100 μm) light mineral particles are most readily flushed away in suspension, the most consistent and longest geochemical dispersion trains for elements transported as heavy minerals are found in the very fine sand and silt fractions. Use of coarser fractions gives shorter, more erratic geochemical anomalies. 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.

Discovery of the Mt Bini porphyry copper-gold-molybdenum deposit in the Owen Stanley Ranges, Papua New Guinea — A geochemical case history

The geochemical dispersion train of a porphyry Cu-Au-Mo deposit with overprinted epithermal Au-Ag in a rugged tropical environment 50 km northeast of Port Moresby, Papua New Guinea was successfully located by systematic stream sediment and rock float sampling. A reconnaissance sampling density of 1 site per 10 km 2 was sufficient to locate the deposit. Panned concentrate (PC) gold and bulk cyanide leach (BCL) gold clearly define significant anomalous drainage basins at the regional scale. A diagnostic Pb signature in material split from the BCL characterised the anomaly. The Au anomaly in the PC's extended farther downstream than that in the BCL. Results of analysis for Au, Cu, and Mo in ridge and spur samples of residual soils, together with rock-chip samples in drainage outcrop, effectively delineated mineralisation. A coincident Au-Cu-Mo anomaly is explained by porphyry-style mineralisation consisting mainly of chalcopyrite (with associated gold) and molybdenite within a potassium-rich quartz diorite. The tenor of the geochemical expression varies as a function of the pyrite content. Quartz veining intensity is seen to be a useful measure of Cu-Au mineralisation. Trenching of the most anomalous soil samples delineated targets which were successfully tested by diamond drilling.

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 m 3 per sec) no bedload sediment is transported. However, after heavy rain storms flows rise rapidly to discharges that exceed 3.5 m 3 per sec. Bedload transport starts at a discharge of approximately 1.0 m 3 per sec and thereafter increases exponentially for all size fractions up to discharge of 2.2 m 3 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.

Palimpsest glacial dispersal trains and their significance for drift prospecting

Palimpsest glacial dispersal trains are residual trains that are produced when the lithic components of an earlier dispersal trains are incompletely re-entrained by a subsequent glacial movement in a different direction. As evidence for multiple ice-flow directions comes from an ever growing number of regions, palimpsest trains increasingly appear as common features of glacial dispersal patterns produced by Quaternary ice sheets. The major characteristics of these trains, particularly their trend and outline, are predominantly related to an earlier glacial movement and only partly related to the glacial movement that produced the till sheet in which they occur. Palimpsest dispersal trains owe their characteristics to the cumulative effects of two or more succeeding ice-flow directions. The concept of palimpsest trains leads not only to a better understanding of the factors which control glacial dispersal patterns but also to an integrated classification of dispersal trains. The three main types of glacial dispersal trains, ribbon-shaped, fan-shaped and amoeboid, owe their characteristics features to the direction, number and succession of former glacial movements. The concept of palimpsest dispersal trains is supported by an empirical glacial dispersal model which do not only constrains the conditions under which these trains may form but also identifies two key factors of subglacial clastic dispersal: (1) the travel distance of basal ice and (2) the rate of subglacial erosion. The basis of this glacial dispersal model is a corollary of Krumbein's exponential decay model: the concentration of lithic components of a rock type in subglacial debris increases exponentially until its distal contact is crossed, at which point the exponential increase of lithic components of rock types lying downglacier will cause the concentration of lithic components of the first rock type to decrease exponentially. The proposed model carries out multiple iterations during each of which compositional data obtained by the previous iteration are redistributed through an exponential decay function. Our work shows that not only is it important to know the sequences of ice-flow directions which occurred in glaciated terrains but also to recognize that older glacial movements are most likely to have residual effects on dispersal patterns produced during subsequent glacial movements. The recognition of palimpsest dispersal trains provides a new tool to better assess these residual effects on regional geochemical patterns in glacial sediments and thus to adjust drift prospecting strategies.

Mapping soil depth classes in dry Mediterranean areas using terrain attributes derived from a digital elevation model

Modern land management increasingly demands quantitative information on spatially variable soil properties. Traditional soil survey maps do not provide this information. In this paper we report on the application of terrain attributes to mapping soil depth classes at high spatial resolution over large areas under dry Mediterranean conditions. Soil data were collected in 111 georeferenced field plots of 30 m × 30 m, more or less equally distributed over three lithological units, phyllites, shales and limestones. Topographic attributes were computed from a digital elevation model at 30 m resolution. A principal components analysis was carried out on the map overlays of the terrain attributes in order to obtain uncorrelated topographic factors that enabled us to apply a probability approach. Using the maximum likelihood classifier with the field plots as a dispersed training area, predictions were made of mean soil depth class and the probability of occurrence of shallow or deep soils. A cross-validation revealed a 65%, 81% and 61% accuracy for the three maps of the shale area, a 50%, 55% and 40% for the maps of the phyllite area, and a 78%, 72% and 75% accuracy for the maps of the limestone area. Explanations for both the good results in the shale and limestone areas, and the poor results in the phyllite area, focus on the effects of the spatial scale of topographic variation, sediment transport mechanisms, and the impact of land use.

Ice-flow history and glacial dispersal patterns, southeastern Cape Breton Island, Nova Scotia: implications for mineral exploration

Regional till sampling and stratigraphic studies were completed in southeastern Cape Breton Island, Nova Scotia, in 1990 and 1991 to document the directions and distances that successive ice-flow events have dispersed material and to document regional till geochemical patterns to aid mineral exploration in this heavily drift-covered area. Three major Wisconsinan ice-flow events affected the area: an early eastward flow, followed by a northeastward flow, and finally a southward flow. The shape and magnitude of dispersal trains in the area are the net result of the three ice-flow events, which are variable from north to south. In the north, the northeast ice flow was the dominant erosive and depositional event, producing a large northeast-trending drumlin field. Salmon River porphyry clasts and Pb- and Zn-rich till from the Yava Pb mine were dispersed more than 15 km to the northeast. In the south, eastward ice flow was the dominant erosive and depositional event, producing large east-trending till ridges. Dispersal in this area is difficult to detect because exotic red silty till in the giant till ridges masks underlying mineralized bedrock and dilutes the locally derived glacial debris. Many precious and base metal anomalies and dispersal trains were identified in the regional till samples, several of which are related to unknown mineralized sources.

Theory of glacial erosion, transport and deposition as a consequence of subglacial sediment deformation

AbstractA theory of erosion, transport and deposition of unlithified sediments by glaciers is presented. It predicts the large-scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle, together with the resultant intensity of large-scale lineations (drumlins) which will be incised in the landscape. The theory also predicts the dispersal patterns of subglacial lithologies, together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till. It predicts major erosional discontinuities within tills and the formation of boulder pavements. It suggests that the dominant proportion of the lowland tills produced by Pleistocene mid-latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine-grained.The theory is based on an analysis of glacier-dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition.

Theory of glacial erosion, transport and deposition as a consequence of subglacial sediment deformation

AbstractA theory of erosion, transport and deposition of unlithified sediments by glaciers is presented. It predicts the large-scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle, together with the resultant intensity of large-scale lineations (drumlins) which will be incised in the landscape. The theory also predicts the dispersal patterns of subglacial lithologies, together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till. It predicts major erosional discontinuities within tills and the formation of boulder pavements. It suggests that the dominant proportion of the lowland tills produced by Pleistocene mid-latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine-grained.The theory is based on an analysis of glacier-dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition.

Natural Background Determination and Impact Quantification in Trace Metal-Contaminated River Sediments

Historic and current mining activities have contaminated stream sediments around the world with toxic heavy metals. A general lack of premining baseline data makes it difficult to quantify the extent of contamination and to set realistic remediation goals. These problems can be solved by modeling the downstream dispersion of metal anomalies based on dilution mixing of anomalous and tributary sediments. The model allows calculation of the dispersion curves of metals in stream sediments both before and after mining and also allows the quantification of any anthropogenic exaggeration of an anomaly. Mining activities were found to amplify naturally occurring metal anomalies up to 3 orders of magnitude, extending downstream dispersion trains from a natural limit of around 20 km to as much as 500 km. The dilution mixing model provides a useful tool for calculating premining dispersion trains and quantifying the effects of mining on a river basin. Such information is relevant to the understanding, litigation, and remediation of contaminated rivers around the world.

Concepts for geochemical gold exploration based on the abundance and distribution of ultrafine gold

Conventionally, geochemical exploration for gold is based on the assumptions that (1) gold is chemically inert in surficial environments; (2) gold occurs mainly in discrete grains; and (3) gold is transferred by mechanical means to form clastic dispersion halos and dispersion trains. Consequently, the commonly adopted methodology has been (1) to determine gold in heavy mineral concentrates; (2) to use large samples in order to improve the reproducibility of gold analyses; (3) to use high detection limits and thresholds; and (4) to determine total gold contents and pathfinder elements in the samples. However, these methods are not always successful in locating gold deposits, and they have limited application in the search for buried or blind deposits. In China, studies of the distribution and migration of particulate and ultrafine gold indicated that (1) gold is active and mobile in surficial environments; (2) gold occurs not only as discrete grains, but also as ultrafine particles and other complex forms; and (3) regional low-concentration gold anomalies as well as local anomalies over buried gold deposits originate from ultrafine gold and other complex forms of gold. The methodology developed in China for regional and local geochemical gold exploration is based on this experience. Results of investigations around two gold deposits in China are presented.

Ice-flow patterns and glacial transport in the eastern Hudson Bay region: implications for the late Quaternary dynamics of the Laurentide Ice Sheet

Recent field surveys in the eastern Hudson Bay region have led to the discovery of regional ice-flow sequences that require a significant reassessment of the late Quaternary dynamics of the Laurentide Ice Sheet. Two regional ice-flow phases can be recognized from till compositional data and from crosscutting relationships observed on striated bedrock surfaces: the oldest is directed toward the northwest and north-northwest, while the youngest is directed toward the west and includes a late-glacial deflection toward the southwest. The wide regional distribution of striae formed during the early northwestward glacial movement together with the recognition of palimpsest glacial dispersal trains associated with this phase suggest that it was a long-lived, time-transgressive regional event. The ensuing glacial movement is a regionally dominant westward ice-flow phase during which several large glacial dispersal trains were formed downglacier from distinctive bedrock sources. The largest of these trains extends westward over a distance of 120 km from Lac à l'Eau Claire to Hudson Bay. Regional glacial transport data as well as glacial and deglacial landforms indicate that this was a long-lived glacial phase, likely lasting throughout the Late Wisconsinan glacial maximum and until déglaciation about 8000 BP. The erosional and depositional record of the northwestward ice-flow event is quite comparable to that of the ensuing glacial phase, and it is thus thought to represent the Early Wisconsinan glacial maximum. In view of the large regional extent of the northwestward ice-flow phase, it must postdate the early buildup of the ice sheet. Along the southeastern Hudson Bay coast, the Late Wisconsinan westward glacial movement was followed by a southwestward deflection that was likely caused by glacial streaming prior to 8000 BP in James Bay, in response to calving and surging into Glacial Lake Ojibway.

A shear‐diffusion model of till genesis based on the dispersal pattern of indicator rocks in the Grand‐Volume Till of central Gaspésie, Québec, Canada

Dispersal patterns of indicator rocks in central Gaspésie reveal that glacial debris is entrained in a basal debris‐rich zone of shearing where clast diffusion takes place. The Grand‐Volume Till forms a thin till sheet over the high plateaus of Gaspésie Peninsula and resulted from a succession of two Wisconsinan ice flows of distinct orientations (SSE and NE). The lithological composition of this till determined by pebble counts and the three‐dimensional dispersal patterns of indicator rocks in it suggest that debris transport occurred principally by simple shear deformation of glacial debris. In addition, the intermixing of clasts at the intersection of two lithologically distinct dispersal trains of SSE and NE orientations, respectively, suggests that extensive mixing takes place during shearing. Physical interactions among the clasts lead to both upward and downward movements which cause the clasts to diffuse across the zone of shearing. This process of shear‐diffusion results in continuous incorporation and mixing of the newly encountered rock types during glacial transport.

The significance of various garnet types in surficial materials in southeastern Gaspé Peninsula, Quebec

The distribution patterns of four types of garnet in surficial materials of southeastern Gaspé Peninsula are interpreted in terms of the glacial history of the area, the lithostratigraphy of the bedrock, and mineral occurrences. Each type is derived from a distinct source, and all have undergone at least some southeastward glacial dispersion. Type 1 garnets, anhedral Ca-garnet fragments, are from the McGerrigle granite metamorphic aureole and were brought into the region by the same ice flow that caused the well-documented southward dispersal of granite debris. Their distribution within the study area depicts the southern part of this important dispersal train with an improved degree of precision. Type 2 garnets, Ca-garnet crystals, seem to be related to skarns within the boundaries of the study area; some patterns cannot be linked to any known source, thus providing challenging exploration opportunities. Type 3 garnets, colored pyralspite garnet crystals, are related to a tectonic breccia within the Maquereau–Mictaw contact zone. Type 4 garnets, Grenville-type anhedral garnets, were not brought into the region by Laurentide ice, as one might suspect, but rather seem to be related to specific horizons within the Silurian Lower Chaleurs Group. On the basis of a palinspastic reconstruction of southern Gaspé Peninsula and a plate tectonic model for the Paleozoic of eastern North America, we propose that these garnets were derived from the erosion of a metamorphic Grenvillian terrane, which presently forms the Northwestern Highlands Zone of Cape Breton Island. The distribution data for this garnet type also lends support to the concept put forth in a previous study whereby a local Late Wisconsinan ice mass penetrated some 5–6 km into the region from the southeast near Saint-Godefroi.

Diamond exploration in glaciated terrain: a Russian perspective

Glacial and glaciofluvial deposits are widely developed in the Arkhangelsk and Middle Timan areas of the North Russian craton, where numerous kimberlite pipes have been found during the last two decades. These deposits often overlie the pipes and make diamond exploration (particularly mineralogical prospecting) very difficult. They include ground moraine (monolithic till and scaly till facies), end moraines, and glaciofluvial and lake sediments. Kimberlite indicator minerals define haloes of different types in these deposits. Short-distance haloes (1–3 km) form in basal till in the immediate vicinity of pipes, and in glaciofluvial sediments of the alluvial stream facies. They are very small, which makes them difficult to find. Long-distance dispersal trains, extending up to 50–70 km from the sources, are observed in glaciofluvial sediments such as eskers, related to streams overloaded with clastic material. Detached haloes, which have lost their relation to the primary sources, usually are observed in end moraines and modern beach sediments. They reflect the averaged mineralogical composition of all the moraines of the area and thus are useful primarily during reconnaissance exploration. Pyrope and chromite grains in glacial and glaciofluvial sediments show specific surface features reflecting mechanical and chemical alteration during their transport. These features may be used to recognise the mode of transport and possible distance from the primary (kimberlitic) source.

Glacial dispersal of rock debris in central Gaspésie, Quebec, Canada

The lithological content of tills in central Gaspésie is evaluated by pebble counting of 231 samples collected in excavation pits and containing 200 pebbles each. The results are used here to establish the pattern of debris dispersal and to infer the glacial history of the area. The dispersal pattern is characterized by well-defined southeasterly (160–170°) and northeasterly (40–60°) trending trains. Half-distance values of glacial transport along the trains range from 5 to 9 km for both directions, suggesting ice flow events of considerable magnitude. The volume of material in the trains represents 1–6 m of glacial erosion of the bedrock. Glacial cirques and short U-shaped valleys, about 100–200 m deep, are incised into the McGerrigle Mountains granite pluton as well as the adjacent metabasalt. The corresponding trains are aligned with these erosional features, indicating that their clast content was derived from those features during an early Alpine Glacier Phase. The southeasterly trending dispersal trains are associated with an invasion of central Gaspésie by the Laurentide Ice Sheet during the Early Wisconsinan, whereas the northeasterly trending trains are associated with a local centre of outflow over Gaspésie during the Late Wisconsinan.

Distribution and behavior of gold in soils and tills at the Nickel Plate Mine, southern British Columbia, Canada

The Nickel Plate deposit, in which gold occurs as <25 μm blebs associated with arsenopyrite in garnet-pyroxene skarns, is in the subalpine zone near the southern limit of the Thompson Plateau. During the last glaciation the Cordilleran ice sheet moved south-southwest across the deposit and deposited a stony basal till. A dispersion train with anomalous concentrations of gold in tills and soils now extends 2 km down ice from the deposit. Gold contents of samples of humus (LFH horizon) and the −212 μm fraction of mineral soils (A, B and C horizons) was determined by instrumental neutron activation and fire assay-atomic absorption, respectively. Selected samples were examined in detail to determine distribution of gold between different size and density fractions. Despite erratic variability, Au contents of the −212 μm fraction generally decrease from 200–400 ppb close to the mine site to <50 ppb at distal sites. At most sites there is also a twofold increase of gold values down the soil profile. Within samples concentrations of Au in the −420 + 212 μm, −212 + 106 μm, −106 + 53 μm and −53 μm fractions are usually roughly constant. However, because of its abundance, the −53 μm fraction contains more than 70% of the gold. Amenability of gold in this fraction to cyanidation suggests that it is largely free gold. For size fractions > 53 μm the contribution of the heavy mineral (SG > 3.3) fraction to total gold content increases with decreasing grain size. Distribution of gold between size and density fractions is consistent with its release from the bedrock or pre-glacial regolith by glacial abrasion. The bulk of the gold was incorporated into the fine fractions of the till at or close to the source. However, differences between down ice dilution ratios for gold in different heavy mineral size fractions suggest that comminution of host minerals continued to transfer gold to the finer size fractions during glacial transport. For exploration purposes, B and C horizon samples provide the best anomaly contrast. Estimates of the abundance of gold particles in different size fractions indicate that the nugget effect, which causes erratic gold values in the −212 μm fraction, can be avoided by analysis of 30 g of −53 μm material.

A note on the numerical and N-soliton solutions of the Benjamin-Ono evolution equation

An efficient pseudospectral method is applied for the numerical solution of the weakly non-linear Benjamin-Ono equation for arbitrary initial conditions. The general solution at large time is shown to evolve into an ordered pattern of Lorentzian solitons and a dispersive train moving in opposite directions. A practical new relationship for estimating the number of solitons is proposed in terms of the soliton-number (representing the ratio between nonlinearity and dispersion effects) and the shape of the initial disturbance.