Ten placer deposit models from five sedimentary environments

Abstract

Placers deposits are now known from five sedimentary environments; washout, river, aeolian, beach, and continental shelf. In each environment, the concentration of mineral grains, or sorting, takes place either by removal of gangue grains (denudation) or by addition of valuable grains (accumulation). Any given deposit will result from both processes but one will usually predominate. Denudation placers all sit on or just above erosive scour surfaces. They arise from a two-step process; initial particle deposition followed by selective removal of gangue particles. For example, deposits from a waning flood-stage river will include many different size, shape and density particles but a subsequent lower energy normal river flow might remove only the smaller, flatter or the less dense particles. The second fluid flow can be quite different from the first as, for example, when the wind selectively removes sand grains deposited by waves. Repeating these two steps, transportation from source and selective entrainment of grains results in a high placer mineral flux allowing denudation placers to achieve high concentrations of particular minerals. Denudation placers have a small thicknesses or vertical dimension, and so they are essentially condensed sections. To be economic, they must have a high value mineral, a large surface area, a long linear dimension, or exceptional grade, and preferably several of these features. Accumulation placer formation does not involve later partial rework and selective grain removal. Concentration grade depends on maximum availability of a valuable mineral and minimal availability of gangue grains capable of being carried with, and deposited from, a given fluid flow condition. Such placer deposits tend to be lower grade compared to denudation placers because the placer mineral flux does not focus on a single two-dimensional surface. Instead repeated favourable flow energy episodes superimpose placer grain enriched-sediment in situations of accumulation with minimal scour. Their large volume makes them economically valuable.

The Potential for REE and Associated Critical Metals in Coastal Sand (Placer) Deposits of Greece: A Review

“…Coastal heavy mineral sands, also widely referred to as "black sands" (due to their common enrichment in Fe-Ti oxides) or simply placers, are economic deposits of heavy minerals and have diverse economic interest as a result of a wide-ranging provenance control from one deposit to another. These deposits form as a result of processes of mechanical erosion, transport, hydrodynamic (or wind-driven in the case of aeolian transport) sorting and ultimately accumulation at suitable sites, leading to the preferential concentration of higher density minerals that are resistant to chemical weathering (such as ilmenite, zircon and monazite), relatively to lighter minerals [51][52][53][54].…”

Age of the Archaean Murchison Belt and mineralisation, South Africa

“…Economic factors in favour of the Witwatersrand are their high-tonnage, high gold-grade and the remarkable lateral continuity of the ores over hundreds to thousands of metres. This lateral continuity is absent in all recent placers globally where meso-scale variations in grade in placers are controlled by subtle meanders in river systems or marine processes in coastal environments (Becker & Batt, 2016;Phillips & Sumpter, 1990;Stanaway, 2012). In the Witwatersrand grade continuity is such that major mines have been opened on less than ten drill holes.…”

Fate and consequence of nutrients at an abandoned feedlot, Glacial Ridge National Wildlife Refuge, Minnesota, USA

“…Beach wave action can concentrate relatively inert minerals with high specific gravity, commonly magnetite and hematite. These heavy sand placers are well known and have been mapped along a wide range of shore line environments (e.g., Stanaway 2012;Komar 2007), although in the glacial Lake Agassiz region placers are likely small and of little value. For example, at several locations along the east coast of Australia, heavy sands accumulated in a "backbarrier wash-over facies, at the rear of swash-aligned barriers in coastal embayments" (Roy 1999), which describes features similar to the glacial Lake Agassiz-age beach sediments at the feedlot.…”