The University of Southampton
University of Southampton Institutional Repository

The dispersion of magnetite bedload tracer across a gravel point-bar and the development of heavy-mineral placers

The dispersion of magnetite bedload tracer across a gravel point-bar and the development of heavy-mineral placers
The dispersion of magnetite bedload tracer across a gravel point-bar and the development of heavy-mineral placers
Economic concentrations of heavy-minerals are often associated with fluvial point-bars but prospecting models identifying the heaviest concentrations are poorly developed. Consequently, the dispersal and storage of a heavy-mineral bedload tracer–magnetite–across a rapidly evolving point-bar was studied using magnetic susceptibility as a surrogate measure of magnetite concentration. The bar-head was a preferential area for the development of a placer owing to a lag accumulation of magnetite over an armoured bed surface. In contrast, when viewed in plan, the bar-platform and bar-tail were regions of tracer dilution owing to downstream dispersion and mixing with shale in the vertical as the bar-top rapidly aggraded. However, in section, false-bottom placers developed along bedding planes were evident. The latter consisted of thin layers of concentrated magnetite resulting from the passage of bedload sheets, consisting of a mix of shale and magnetite, moving repeatedly from the bar-head to accrete over the bar-tail. Differential density-sorting of magnetite and shale occurred during transport and deposition, such that the heavier magnetite tended to accumulate as a visible concentrated bed-layer, later to be over-run by layers of shale-sediment in which magnetite was present diffusely. However, the placer thickness was greater than that visible because finer fractions of magnetite from each concentrated layer infiltrated the interstices of the top of the shale bed below. The placer thickness, the infiltration potential of the sediment bed and the actual rate of infiltration of the tracer were determined by fitting a mathematical function to measured variation in magnetic susceptibility with depth in the sediment body. Finally, a simple mathematical model, described in the literature as reproducing the plan-view of flow and topographic patterns in river bends, was found to reproduce patterns of depth, velocity, shear stress and competence in the point-bar environment which were in accordance with the interpretation of the field data. It was concluded that such a model when linked to entrainment functions for sediments of mixed density might be suitable for prospecting for economic heavy minerals in the point-bar environment.
fluvial systems, heavy minerals, magnetite, placers, prospecting, point-bar environment
0169-1368
402-416
Carling, P.A.
8d252dd9-3c88-4803-81cc-c2ec4c6fa687
Orr, H.
038077d8-4acf-4c6b-aef1-65280eae986c
Kelsey, A.
57537081-4b7f-4dd4-a3e1-f5b08762e0e7
Carling, P.A.
8d252dd9-3c88-4803-81cc-c2ec4c6fa687
Orr, H.
038077d8-4acf-4c6b-aef1-65280eae986c
Kelsey, A.
57537081-4b7f-4dd4-a3e1-f5b08762e0e7

Carling, P.A., Orr, H. and Kelsey, A. (2006) The dispersion of magnetite bedload tracer across a gravel point-bar and the development of heavy-mineral placers. Ore Geology Reviews, 28 (4), 402-416. (doi:10.1016/j.oregeorev.2005.02.003).

Record type: Article

Abstract

Economic concentrations of heavy-minerals are often associated with fluvial point-bars but prospecting models identifying the heaviest concentrations are poorly developed. Consequently, the dispersal and storage of a heavy-mineral bedload tracer–magnetite–across a rapidly evolving point-bar was studied using magnetic susceptibility as a surrogate measure of magnetite concentration. The bar-head was a preferential area for the development of a placer owing to a lag accumulation of magnetite over an armoured bed surface. In contrast, when viewed in plan, the bar-platform and bar-tail were regions of tracer dilution owing to downstream dispersion and mixing with shale in the vertical as the bar-top rapidly aggraded. However, in section, false-bottom placers developed along bedding planes were evident. The latter consisted of thin layers of concentrated magnetite resulting from the passage of bedload sheets, consisting of a mix of shale and magnetite, moving repeatedly from the bar-head to accrete over the bar-tail. Differential density-sorting of magnetite and shale occurred during transport and deposition, such that the heavier magnetite tended to accumulate as a visible concentrated bed-layer, later to be over-run by layers of shale-sediment in which magnetite was present diffusely. However, the placer thickness was greater than that visible because finer fractions of magnetite from each concentrated layer infiltrated the interstices of the top of the shale bed below. The placer thickness, the infiltration potential of the sediment bed and the actual rate of infiltration of the tracer were determined by fitting a mathematical function to measured variation in magnetic susceptibility with depth in the sediment body. Finally, a simple mathematical model, described in the literature as reproducing the plan-view of flow and topographic patterns in river bends, was found to reproduce patterns of depth, velocity, shear stress and competence in the point-bar environment which were in accordance with the interpretation of the field data. It was concluded that such a model when linked to entrainment functions for sediments of mixed density might be suitable for prospecting for economic heavy minerals in the point-bar environment.

This record has no associated files available for download.

More information

Published date: May 2006
Keywords: fluvial systems, heavy minerals, magnetite, placers, prospecting, point-bar environment

Identifiers

Local EPrints ID: 58071
URI: http://eprints.soton.ac.uk/id/eprint/58071
ISSN: 0169-1368
PURE UUID: 454e88cc-eb4c-479e-be60-5d0f50be674c

Catalogue record

Date deposited: 12 Aug 2008
Last modified: 15 Mar 2024 11:09

Export record

Altmetrics

Contributors

Author: P.A. Carling
Author: H. Orr
Author: A. Kelsey

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×