Part one
Part two
Part three
Part four
abstract........
1- Eluvial placer deopsitsTHE USE OF HEAVY MINERALS IN MINERAL EXPLORATION
Sunday
resarch 2
Geology department
Faculty of scince
Zagazig university.
Heavy minerals
And Uses
By
Alaa Fathi Ali Mohammed
(4th year - gology special)
Supervised by
Dr. Mohamoud Ragab Blasy
2010
CONTENT
Abstract
Introduction
Heavy mineral reconnaissane
Field work of heavy mineral reconnaissance
Laboratory study
Presentation of data
Relation to other geological method
Some principles of placer formation and its types
Types of placer
Elluvial placer deposits
Stream or alluvial placer deposits
Beach placer formation
Eolian placer formation
Examples of some placer
Gold placer formation
Platinum placer formation
Tin placer formation
Diamond placer formation
Other gemstones
Some egyptian placer deposits
References
introduction....
Heavy mineral reconnaissance
Heavy mineral reconnaissance is a method of geochemical exploration and is used for search for ores associated with crystalline rocks and also reveals the possible presence of placer deposits
a-Field work of heavy mineral reconnaissance.
Fieldwork and part of the laboratory work can be performed technically untrained persons guided by junior scientists supervised by a senior scientist. Among the
Some principles of placer formation and its types.
The gravity separations of heavy minerals from light one give the placer deposits by action of water or air. This process can only occur for mineral with high specific gravity, chemical resistance to weathering and durability as hardness. These placer formed come from lode deposits as gold viens, vienlets, disseminated in crystalline rock, or from former placer deopsits as beach stream gravels or buried placers.
The placer deposits takes different shapes as pockets, strings, lenticular, ribbon, sheet and mantle.
Types of placers
They are formed without stream action, upon hill slopes from materials relased from weathered lodes that outcrop above them, where the heavier, resistant minerals collect below the outcrop at the lighter nonresistant products of decay are dissolved or swept downhill by rain wash or are blown away by the wind. This brings partial concentration and so fairly rich lodes are necessary to yield work able deopsits by this incomplete concentarion. The most Eluvial deposits are gold and tin, minor deposits include manganise, tungesten, kyanite, barite and gemstones.
2- Stream or alluvial placer deposits.
This is by far the most important type of placer deposits and yielded the great quantity of placer gold, tin stone, platinum and precious stones. They have been the cause of some of the great gold and diamond of the world. Stream water flow down stream. It rushes through canyons sweeping every thing along with it, it slackens in wide places, it swirls around the outside bends, creating back eddies on the inside, laps up over bottom projections forming quiet eddies on the lee side. In these slack waters the heavy substances drop to the bottom. In streams, jigging action is effective in concentrating placer minerals in the bottom gravels. During dry seasons of low stream velocityh the placer minerals remain at rest, but in flow time, the minerals an gravels may all swept farther down stream and reconcentrated on bars, stream margins of flood plains minute particles of gold called "colors" may be carried far down stream
Introduction
As the discovery of mineral resources becomes more elusive, more sensitive detection techniques are required. One of the most important techniques is the use of heavy minerals.
This idea is not new, although in recent years sample preparation and analysis have become much more sophisticated. The prospector of 100 years ago with his gold pan used the occurrence of gold, sulphide and gossan grains as a vital exploration guide. Today, the value of advanced technology and heavy mineral surveys has been well demonstrated with the significant diamond discoveries in northern Canada.
This article illustrates how heavy mineral techniques can be of vital importance to the exploration geoscientist. Methods of sample collection, preparation and analysis will be explained, along with a discussion of some case histories.
Scope of Heavy Mineral Surveys
The following points demonstrate the wide scope and the advantages of effective heavy mineral exploration.
The ability exists to test various geochemical sample media, including drainage sediments, till and rock.
Concentrates from drainage sediment and till sampling have proven to be beneficial as geochemical and mineralogical guides to mineralization.
The heavy mineral technique overcomes one of the main problems in gold and diamond exploration - the nugget effect. The collection of a large sample and then concentration of the desired minerals into a small sub-sample effectively overcomes this effect.
Diamond-bearing kimberlite pipes can be distinguished from barren pipes by microprobe analysis of selected heavy mineral grains.
A very select sample treatment process, such as described below, elevates the sensitivity of heavy mineral surveys. It can in many cases enable the use of surface till sampling, at a considerable cost saving when compared to base-of-till sampling. It can also increase the downstream or down-ice detection of mineral grains, greatly reducing the sample density.
The use of heavy minerals can overcome specific problems, such as the poor or non-detection of zinc mineralization in carbonate rocks by standard silt sampling techniques.
The production and analysis of heavy minerals from rock samples is not yet a common exploration method in Canada. However, Soviet geologists have used these methods to detect anomalous primary halos around ore bodies.
Microscope examination of heavy mineral grains can be used as an aid to geological mapping.
The occurrence, in till, of specific minerals that can be related to distinctive rock types, can help confirm ice direction(s).
The characteristics of the shape and composition of gold and sulphide grains (as determined by scanning electron microscope), already utilized to some degree, have a much larger potential application in exploration.
Sample Collection
Most commonly, if the topography is suitable, stream sediments are the best sample medium. Stream sampling surveys encompass a wide variety of environments, from large gravel bars in rivers, to tiny pools of sediment in rocky narrow creeks, to dry washes in arid climates. It is paramount to conscientiously choose an appropriate sample site. If a sand or gravel bar is present, a concentration of heavy minerals typically occurs in specific areas. In contrast to the classic base metal silt sampling procedure, where very fine grained particles of silt or clay are collected from quiet water sedimentation, high energy environments within the sediments provide the best material for heavy mineral sampling.
The preferred procedure is to wet-sieve the sample by carefully shoveling the sediments into a -20 mesh stainless steel sieve (diameter 36 cm, depth 17 cm) resting in a large aluminum pan containing water. Some liquid detergent is added to prevent flotation of metallic minerals.
Using handles on the sieve, a washing-machine type motion is used to sieve the sediments. In this manner approximately 10 kg of -20 mesh material is collected. Care must be taken to clean the sieves and pans to prevent contamination. In arid areas, the drainage sediments are usually sufficiently dry to permit dry sieving at -20 mesh. However, if samples are damp and no water is available locally, 20 kg of coarser material can be gathered by using a -6 mesh screen.
For till sampling, the collection of 10 kg of -6 mesh material is generally recommended, although some - 20 mesh sampling should be carried out as part of any orientation program.
Preparation of Heavy Mineral Concentrate
Heavy mineral concentrates are best produced through the application of heavy liquid separation. This method may be more expensive than jigs, centrifuges, shaker tables or Magstream separators, however, if done correctly it can be very accurate. By accuracy we mean good reproducibility of results with very little loss of heavy mineral grains, even in the -150+400 mesh range.
Discovery Consultants has collected about 4,000 heavy mineral samples which have been processed through C.F. Mineral Research Ltd. in Kelowna, B.C. We believe that this laboratory, owned by Mr. Charles Fipke, is second to none in North America for the processing of heavy mineral samples.
The following is a brief, simplified description of the lab procedure. First, the samples are wet sieved into several fractions, dried, and further sieved if necessary. A chosen size fraction(s) is then slowly fed into the middle of a column of tetrabromethene (TBE), specific gravity 2.96. The resultant heavy minerals are then further separated by methylene iodide (MI), specific gravity 3.27. The specific gravity of the heavy liquid can be lowered to ensure that particular minerals are in a unique fraction. For example, in diamond exploration, a liquid with a specific gravity of 3.2 can be used, to include chrome diopside. A Frantz electromagnetic separator is then used to generate distinct fractions based on variations in magnetic susceptibility (usually magnetic, para-magnetic and non-magnetic fractions). In the case of diamond indicator prospecting, four fractions are generated, separating most regional garnets from kimberlitic pyropes. Electrodynamic separation can be utilized to concentrate (picro)ilmenite from nonmetallic gangue.
Analysis of Heavy Mineral Fractions
The heavy mineral fractions commonly weigh less than 10 g, and may, in highly selective cases, be in the 0.5 g to 2 g range. For gold exploration, analysis by neutron activation is advocated. This method has the benefit of obtaining values for 30 additional elements, which may be effectual in identifying the type of mineralization present. The analysis by neutron activation does not depend on acid extraction and therefore gives a 'total' value, which is notably useful for barium and tungsten. The analysis is non-destructive and once the sample has 'cooled' additional analysis or a study of gold morphology can take place.
If base metal values are required then atomic absorption (AA) or inductively coupled plasma (ICP) analysis, following acid extraction, is recommended.
In diamond exploration, the initial evaluation of samples is mineralogical, leading to the analysis of specific mineral grains. The selected fraction is examined under an optical microscope for the presence of possible kimberlite or lamproite indicator minerals. The picked grains are mounted for SEM (scanning electron microscope) scanning, a semiquantitative analysis. Grains confirmed as denotative of kimberlites are then submitted to SEM probe analysis. This precise analysis can distinguish among minerals from diamond-bearing, weakly diamond-bearing and barren kimberlite.
Interpretation of Results
Heavy mineral techniques should not be used to the exclusion of other geochemical methods. The case histories, which are described below, demonstrate the importance of combining complementary methods, especially during the follow-up of anomalies. It is also crucial to carry out orientation field and lab studies before processing the samples. Decisions need to be made as to which size, specific gravity and magnetic susceptibility fractions to produce. For example, for gold exploration in Nevada, the fine size fraction (-150 mesh) gives more meaningful results; the coarse fraction may contain more gold but the gold content correlates with the total weight of the concentrate, that is, with hydraulic (placering) processes. For base metals, limonitic grains can be important, necessitating a different fraction. In diamond exploration, a magnetic separation that can distinguish between regional metamorphic garnets and pyrope garnets would be most useful. Generally one chooses a fraction that casts as broad a mineralogical/geochemical net as possible, without significantly diluting the target elements or minerals. However, in some circumstances more than one fraction is required for each sample.
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