Everything about Heavy Mineral Sands Ore Deposits totally explained
Heavy mineral sands are a class of
ore deposit which is an important source of
zirconium,
titanium,
thorium,
tungsten,
rare earth elements, the industrial minerals
diamond,
sapphire,
garnet, and occasionally precious metals or
gemstones.
Heavy mineral sands are
placer deposits formed most usually in
beach environments by concentration due to the
specific gravity of the mineral grains. It is equally likely that some concentrations of heavy minerals (aside from the usual
gold placers) exist within
streambeds, but most are of a low grade and are relatively small.
Grade and Tonnage Distribution
|}
The grade of a typical heavy mineral sand ore deposit is usually low. Within the 21st century, the lowest cut-off grades of heavy minerals, as a total heavy mineral (THM) concentrate from the bulk sand, in most ore deposits of this type is around 1% heavy minerals, although several are higher grade.
Of this total heavy mineral concentrate (THM), the components are typically
- Zircon, from 1% of THM to upwards of 50% of THM,
- Ilmenite, generally of 10% to 60% of THM
- Rutile, from 5% to 25% of THM
- Leucoxene, from 1% to 10% of THM
- Trash minerals, typically magnetite, garnet and chromite which usually account for the remaining bulk of the THM content
- Slimes, typically minerals as above and heavy clay minerals, too fine to be economically extracted.
Generally, as zircon is the most valuable component and a critical ore component, high-zircon sands are the most valuable. Thereafter, rutile, leucoxene and then ilmenite in terms of value given to the ore. As a generality, typically the valuable components of the THM concentrate rarely exceed 30%.
Being ancient stranded dune systems, the tonnage of most deposits in excess of several tens of millions of tonnes to several hundred million tonnes. For example, the medium-sized
Coburn mineral sands deposit,
Western Australia, is 230 million tonnes at 1.1% heavy minerals, and is 13km long.
Source
The source of heavy mineral sands is in a hardrock source within the
erosional areas of a river which carries its load of
sediment into the ocean, where the sediments are caught up in
littoral drift or
longshore drift. Rocks are occasionally eroded directly by wave action shed
detritus, which is caught up in longshore drift and washed up onto beaches where the lighter minerals are winnowed.
The source rocks which provide the heavy mineral sands determine the composition of the economic minerals. The source of
zircon,
monazite,
rutile, sometimes
tungsten, and some
ilmenite is usually
granite. The source of ilmenite, garnet, sapphire and diamond is
ultramafic and
mafic rocks, such as
kimberlite or
basalt. Garnet is also sourced commonly from
metamorphic rocks, such as
amphibolite schists. Precious metals are sourced from ore deposits hosted within
metamorphic rocks.
Transport
The accumulation of a heavy mineral deposit requires a source of sediment containing heavy minerals onto a beach system in a volume which exceeds the rate of removal from the trap site. For this reason not all beaches which are supplied by sands containing heavy minerals will form economic concentrations of the minerals.
Trap
The heavy minerals within the source sediments attain an economic concentration by accumulation within low-energy environments within streams and most usually on beaches. In beach placer deposits the lowest energy zone on the beach is the
swash zone, where turbulent surf washes up on the beach face and loses energy. In this zone, heavier grains accumulate because they're denser than the
quartz grains they occur with and become stranded. It is for this reason that beach placer deposits are often referred to as "strand-line deposits".
The size and position of a heavy mineral deposit is a function of the wave energy reaching the beach, the mean grainsize of the beach sediments, and the current height of the ocean.
Anecdotal reports of certain beach placers which are forming in modern times suggest that the greatest enrichment of the sands tends to occur in storm events which are energetic enough to remove most of the beaches sediment load—a process favoring the lighter minerals. The resultant 'clinker' sands which were left behind were mined during low tide following major storm events, suggesting that most beach placer deposits are formed during such cycles.
In most cases,
fossilised
dune systems are exploited for heavy mineral sands because they're from the ocean and because they're often remnants of previous
intraglacial highstands.
Tectonic activity, which results in
coastlines rising from the ocean, may also cause a beach system to become stranded above the high water mark and lock in he heavy mineral sands. Similarly, a beach system which is drowned by the subsidence of a coastline may be preserved, often for millions of years, until it's either covered by sedimentaion or rises from the ocean.
Specific trap sites for heavy mineral sand placer deposits are in beaches on the
leeward side of
headlands, as this forms a low-energy zone which traps sediments carried along by the longshore drift. Also,
sand bars developed at the
mouths of rivers which feed the placer deposits are rich trap sites where the winnowing action of the waves are most efficient, because heavy minerals, if they're going to be too heavy to be moved, will deposit at an
isthmus in preference to drifting too far down the beach.
Diamond sands
The coast of
Namibia is host to economic
diamondiferrous beach sands, which are exploited by building
sea walls and isolating stretches of coastline. The beaches are so isolated that they're sometimes processed in their entirety, down to the
bedrock, in search of diamonds. Such deposits have been sought around the world, with sporadic reports of high-value stones but no instances of economic quantities of sediment.
Environmental concerns
The mining of beach sands and of fossilized beach placers is often controversial because the operation requires the
strip mining of large
acres of land. Often this land is in ecologically sensitive surroundings and contains fragile
ecosystems built up on poor sandy soils.
The mining process is ideally modelled on the extraction operations underway in
Australia, where the strip mining is followed by rehabilitation of the mined areas including intensive re-vegetation with ecologically similar species, re-contouring of the land to its original shape, including dunes, and management of
groundwater resources. Modern mining practises tend to favor
dry mining rather than
dredging operations, due to the advent of electrostatic mineral separation processes.
In practice, not all mining of
sub-Saharan African deposits is carried out in such an environmentally responsible manner, although some
South African mines do practise dune rehabillitation
(External Link
). The mining of the coast of
South America, in particular
Chile and
Ecuador, is carried out in an environmentally responsible manner.
Examples of environmentally sensitive and poltically sensitive mineral sands mining operations which have gained public attention and galvanised environmental activism responses to mining proposals include the Tuart-Ludlow mineral sands mine, Western Australia, and the culmination of conservationist efforts to preserve
Rainbow Beach and
Fraser Island, Queensland, Australia. These latter campaigns successfully lobbied government and saw Fraser Island and Rainbow Beach protected by the High Court of Australia, however the Tuart-Ludlow campaign failed to protect the Tuart forests in coastal Western Australia.
It should also be noted that mineral sand mining operations were carried out successfully for 35 years in and adjacent to National Parks at Hawks Nest, New South Wales, and continue to be carried out on Stradbroke Island, Queensland.
Further Information
Get more info on 'Heavy Mineral Sands Ore Deposits'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://heavy_mineral_sands_ore_deposits.totallyexplained.com">Heavy mineral sands ore deposits Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
