A Quaternary geological perpective on geochemical exploration in glaciated terrain

Abstract

The application of Quaternary geology and glacial sedimentology is given as a broad guide for geochemical exploration in glaciated terrain. Predictive models of glacial dispersal provide an important basis for tailoring drift prospecting methods to suit regional variations in ice flow history and dynamics. The models relate compositional variations in glacial dispersal trains to ice flow direction, glacial history and subglacial processes. They are continually refined with reference to the geological and physical properties of the ice bed; new empirical field evidence constraining particle trajectories; and knowledge of subglacial processes affecting glacial erosion, transport and deposition. Transport at the ice bed leads to an exponential decrease in indicator concentrations with increasing distance of glacial transport, whereas linear decrease is associated with englacial transport, and may be characteristic of ice streams. The partitioning of rock and mineral fragments through subglacial comminution leads to compositional differences among size fractions that can reflect intensity of subglacial process, distance of transport, and provenance; hence, the choice of size fraction is important to drift prospecting by geochemical methods.Drift prospecting is based on the premise that lithological, mineralogical or geochemical indicators of economic mineralization can be traced in glacial deposits to locate their bedrock source. Of the varied types of glacial sediment, till is most directly related to bedrock composition and has been successfully used as a sample medium for mineral exploration. Till is mechanically derived from bedrock and preglacial sediments along the path of ice flow, and modified by glacial processes of erosion, transport and deposition. Till geochemistry is determined by its constituent rock fragments and mineral grains, including both common rockforming minerals and economic indicators such as sulphides. Hence, in glaciated terrain the practice of geochemical exploration is firmly linked to our understanding of clastic sedimentology and glacial process. As such,

Principal component analysis of the geochemistry of soil developed on till in Northern Ireland

“…This technique has its roots in mineral prospecting where geochemistry is used to establish the provenance and dispersal pattern of valuable mineral deposits that have underwent transport by ice during glaciation (e.g. Aario & Peuraniemi, 1992;Kauranne, 1958;Klassen, 2001;Klassen & Thompson, 1993;McClenaghan, Thorleifson, & DiLabio, 2000;Parent, Paradis, & Doiron, 1996;Peuraniemi, Aario, & Pulkkinen, 1997;Sarala, Rossi, Peuraniemi, & Ojala, 2007;Veillette, 2004). The geochemical signature of the till is inherited from the bedrock parent material which allows till provenance to be established (Boston, Evans, & Ó 'Cofaigh, 2010).…”

Lithology and geochemical dispersal in till: Petitcodiac area, New Brunswick

“…This is probably due to the occurrence of these lithologies as clasts in the underlying conglomerate units, discussed previously by Broster et al (1997). Klassen (2001) noted that the distinction of glacial dispersal concentration patterns depends on the physical and geochemical properties of the indicator, glacial process (e.g. ice dynamics), and the size fraction chosen for study.…”

“…Mathematical techniques can also be used to quantify the transport distance and reduction in glacial load. For example, half-distance length (Gillberg 1967;Bouchard and Salonen 1989) and the empirical exponential decay relationship between distance and load, based on the change in concentration between local and distal components, are discussed by Klassen (2001), and by Stea and Finck (2001). However, entrainment, erosion, mixing and transport (and thus, the dispersal pattern) are affected by several factors, including: (1) grain size fraction and volume of sample investigated; (2) lithology and relative mineral hardness; (3) characteristics of the source unit (e.g.…”

“…attitude and size of outcrop, both related to the amount of erosion); (4) inheritance, overprinting and dilution of load; (5) glacial dynamics (e.g. basal thermal conditions, velocity and fl ow direction, centres of ice-growth); (6) erosional and depositional history of the till fraction; (7) local subglacial topography; and (8) the degree of variation of the preceding factors (Dreimanis and Vagners 1971;Shilts 1976;Broster 1986;Hornibrook et al 1991;Balzer and Broster 1994;Broster and Huntley 1995;Broster et al 1997;Stumpf et al 2000;Klassen 2001;Stea and Finck 2001).…”

Glacial indicator dispersal processes: a conceptual model

“…Exponential uptake and decay functions have been used to describe both indicator erosion and entrainment, and dilution of indicator concentration during transport (Klassen 2001). The Klassen (2001) model describes erosion and entrainment of indicator material into the glacial debris load over the indicator source (flowline interval T = ÀL to 0), and the resulting increasing indicator concentration in till by:…”