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Depositional placer accumulations in coarse-grained alluvial braided river systems

J. P. Burton and Philip Fralick
Depositional placer accumulations in coarse-grained alluvial braided river systems
Economic Geology and the Bulletin of the Society of Economic Geologists (August 2003) 98 (5): 985-1001

Abstract

Depositional paleoplacer accumulations in coarse alluvial braided river systems form by the concentration of heavy minerals during sedimentation of the detritus and, where present, are usually associated with one or more subenvironments of gravel-dominated longitudinal bar complexes. Conditions necessary for the formation of this deposit type were investigated using three separate approaches. Huronian, uraniferous paleoplacers were studied in working faces of mines to gain insight into areas of heavy mineral accumulation. Primary data on processes responsible for the concentration of heavy minerals in the various subenvironments of longitudinal bars were obtained from examination and sampling of matrix and surficial sands of modern gravel bar systems in the North Saskatchewan River, Alberta, and in the Jackpine, Agawa, and Mississagi Rivers, Ontario. Concentration processes were further investigated through a series of experiments using a sediment-water recirculating flume. Sand composed of a mixture of quartz, pyroxene, magnetite, and lead was moved over a gravel bed; flow velocity, depth of flow, grain size of the quartz sand, and clast size systematically changed during 27 experiments. Data from these studies indicate that a number of conditions are necessary, or at least desirable, for heavy minerals to accumulate in coarse-grained alluvium. These are (1) a low proportion of granule to very small pebble-sized lithic fragments; (2) a very heavy mineral population whose hydraulic behavior more closely resembles that of the pebble population than that of the quartz sand; (3) flow velocities capable of creating a suspension cloud from the coarse-grained quartz sand population; (4) a change in the regional slope (inflection point) creating a gradient-parallel zone of energy loss; (5) infrequent major flood events; and possibly (6) preconcentration, i.e., an enriched, erosional lag upslope from the ultimate area of depositional placer accumulation. These conditions, plus the presence of economically significant minerals in the sediment load, control the formation of exploitable placer deposits in longitudinal bars of braided rivers.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 98
Serial Issue: 5
Title: Depositional placer accumulations in coarse-grained alluvial braided river systems
Affiliation: Lakehead University, Department of Geology, Thunder Bay, ON, Canada
Pages: 985-1001
Published: 200308
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 69
Accession Number: 2003-082638
Categories: Economic geology, general, deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, geol. sketch maps
N46°23'60" - N46°23'60", W82°40'60" - W82°40'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200324
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