Heavy minerals from ore guide to the deposit

Heavy minerals (HM) are minerals with a specific gravity of greater than 2.9 g/cm³. They are studied in sedimentary rocks, of which they usually make up <2% by weight, excluding special sites of HM accumulation which are called placer deposits. To use these HM in the field of genetic and applied economic geology they must be separated and concentrated from the less dense trash or gangue minerals, commonly by heavy liquid processing, magnetic separation, jigging and panning, or flotation. This can be done at a laboratory scale prior to EMPA, SEM-EDX/WDX combined with MLA and more sophisticated methods such as those from geochronology, or at an industrial scale in a wet rubble beneficiation plant.

Heavy minerals can be subdivided into two principal groups, relating to the site and process of their formation. Autochthonous HM develop in a host rock environment during supergene and hypogene processes whereas allochthonous HM derive from a source rock only subjected to chemical and mechanical alteration on transport and deposition. Another classification is based on geogenic (rutile, Coltan, cassiterite) and anthropogenic processes (amalgam derived from gold processing, fayalite as a smelting residue). Geogenic HM concentrations may constitute economic deposits, or in other circumstances act as guides to locate economic deposits from their clastic halos. Anthropogenic haloes may help locate ancient smelting sites.

Examples of allochthonous HM used to locate mineral deposits include: cassiterite and topaz (e.g. highly-fractionated Sn granites), columbite s.s.s (e.g. pegmatites and alkaline magmatic rocks), spinel (sapphire and ruby deposits), gold (e.g. Au deposits), platinum-group minerals (e.g. PGE-bearing ultrabasic rocks). Their resistance to chemical weathering and mechanical abrasion during transit, the hydraulic conditions and the interstices for entrapment in specific environments are crucial in determining distance of transport and the settling of HM at a proximal or distal position relative to the source deposit. Ultrastable to stable HM may shelter less resistant HM forming armored relics and so may give rise to labile minerals far from the provenance area. HM aggregates can tell a story about the genesis of the source deposit. This is also true for morphological studies of stable and ultrastable HM, e.g. zircon, monazite, gold and PGM. Anthropogenic HM can only be used for modern-day stream sediments (e.g. Au-Hg alloys, Pb-Ba slags, stolzite, plattnerite) and often give an overview of what element mix might be expected in the catchment area of a drainage system.

Autochthonous geogenic minerals have a role mainly in so-called palaeoplacers. Deep-seated and lithified older placer deposits, e.g., Witwatersrand, South Africa, Blind River Area, Canada, and Serra de Jacobina, Brazil are the main targets. Typical minerals such as sulphides (e.g. sphalerite, galena), sulphates (e.g. barite, celestite), ferroan carbonates (e.g. siderite) or fluorite formed in situ by diagenetic and hydrothermal processes. Alteration of HM during deep burial provokes corrosion and etch pits observed on the surface of HM and newly formed secondary HM species that furnish clear evidence for the effect of intrastratal solutions during diagenesis on deep burial. Infiltration of hydrocarbons may inhibit alteration or complete dissolution of HM, as it may also for lighter minerals (e.g. the conversion of aragonite into calcite). The value of these minerals lies mainly in the field of hydrocarbon exploration. Heavy minerals newly formed during supergene processes are common to mining and smelting residues where they may locally be identified in stream sediments (pyromorphite, schneiderhoehnite, P mimetesite, sewardite).

Terrain analysis, consisting of the integration of applied sedimentology and geomorphology backed by remote sensing, is important in seeking both primary deposits and placers. Terrain analysis forms a pre-stage and allows for a rough classification of what type of placer deposit is to be expected. Placer deposits occur in clastic host rocks of the following terrigenous depositional systems:

saprolite and residual deposits (residual and eluvial placers)

Residual and eluvial (plus colluvial) placer deposits may host accumulations of cassiterite, gold, chromite, magnetite, ilmenite that possess a high resistance to weathering and may concentrate immediately above a bedrock source by the chemical decay and removal of lighter rock-forming materials.

Alluvial fans are conical, lobate, or arcuate accumulations of sediments that have a focused source of sediment supply, usually an incised canyon or channel from a mountain front or escarpment. As the stream widens and the gradient decreases at the exit of the canyon, the water flow slackens and HM tend to be deposited whilst other minerals are winnowed away. Braided and sandy meandering drainage systems proximal to the source area are prospective areas in the search for placer deposits.

Linear terrigenous shoreline and point source (delta) depositional systems in a microtidal regime have proved to be the most productive environments with respect to HM concentrations. Washover fans are excellent traps for HM in microtidal coastal plains sheltered by barrier islands. HM accumulations need to be preserved due to subsequent wave and wind action. The interrelation of transgression and regression accompanied by wind action is decisive for the built-up of HM concentrations in a coastal environment.

Placer deposits constitute a major proportion of world production of gold, tin, tantalum, titanium, zircon and monazite. For practical usage a tripartite subdivision of placer deposits based on the density of HM can be given:

placers containing HM of densities >6.8 mainly hosting gold, platinum, cassiterite (dominated by fluvial-alluvial mineral sands)