THE USE OF HEAVY MINERALS IN MINERAL EXPLORATION

Bill Gilmour, P.Geo.

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.


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.



Case Histories
The case histories in this article describe four gold and one zinc geochemical targets. They have been chosen to represent some of the problems and solutions encountered during reconnaissance geochemical mineral exploration.


US-1
An orientation survey of the previously discovered Mule Canyon Carlin-type gold deposit in northern Nevada, not yet developed at the time of sampling, yielded the following dispersal pattern. The gold values in the -150 mesh, heavy, non-magnetic fraction are present for some 6 kilometres downstream (Figure US1a).

The two samples closest to the deposit are not the most strongly anomalous in gold. This is likely due to the non-liberation and/or non-degradation of mineralized grains to -150 mesh size, proximal to the deposit; anomalous amounts of gold occur in the coarser size fractions.

Note that the arsenic and silver values are good distal indicator elements as well (Figures US1b and US1c). It is uncertain why silver values are not more strongly anomalous adjacent to the deposit. This may also may be a factor of the non-liberation of silver from its host minerals


US-2
A gold deposit in northern Nevada, containing about 100,000 oz, the North Peak deposit, was found by Discovery using detailed heavy mineral drainage sampling in what was deemed by Santa Fe Gold, the property optionor, not to be a geologically favourable area. Prior exploration in the area, south of the Marigold Mine, had failed to detect any significant anomalies.

Figures US2a, US2b, US2c, US2d, US2e, US2f, US2g and US2h demonstrate how two drainages anomalous in gold lead to a mineralized zone which was discovered by systematic follow-up soil, trenching and systematic rock geochemistry.

Other reconnaissance geochemical methods including grab rocks samples, silts, BLEGs, and biogeochemical sampling, failed to detect any anomalies. It is worthy of note that the host sandstone was recessive and formed no readily sampleable outcrops.


US-3
This target is located several miles away from US-2 was contained within a different rock package. Detailed heavy mineral sampling in what was believed by the optionor, Santa Fe Gold, not to be a geologically favourable area, detected strong anomalous values in a major creek (Figure US3a) but little of significance in the short, usually dry, poorly defined tributaries (Figure US3b). However, follow-up sampling using silt (Figure US3c) and BLEG methods (Figure US3d) discovered anomalous gold values.

Systematic soil and rock sampling and prospecting located significant gold mineralization in outcrop (Figures US3e, US3f, and US3g). In the immature drainage sediments in the small gulleys draining the mineralization, the mineralized grains have not broken down to -150 mesh size, as in the main creek.

This is an example of the importance of not giving up on unexplained anomalies, but of seeking other methods to continue exploration and to discover the source. The heavy minerals highlighted the potential of the area, but in this case other techniques were needed to direct the exploration to the pertinent portion of the catchment area.

The work performed by Discovery in identifying the North Peak deposit and the US-3 target brought the area south of Marigold into exploration focus for Santa Fe Gold. This in turn lead to the identification of other gold mineralization associated with a series of north-south and north-northwest faults linking the Marigold Mine, the Trenton Canyon Mine and a number of other gold shows.

Overall, this gold mineralization stretched over several miles of strike length and included over 1.5 million ounces of drilled gold resource by the time Santa Fe Gold committed to production. Interestingly, the Millennium deposit discovered by Glamis/Goldcorp containing over 2,000,000 ounces of gold, and now in production, was found in 2000 within the same structural zone.


BC-1
In southern British Columbia, a regional heavy mineral drainage sampling program was carried out in a geologically favourable area. Previously, one mineral showing for gold was known in the area of Figure BC1a.

Background-level samples and anomalous samples were followed upstream on all tributaries over two more passes designed to avoid dilution from sediments in the major river valley (Figures BC1b and BC1c). The northernmost anomaly was then covered by a mineral claim via staking. Two phases of soil sampling along with prospecting led to the discovery of multiple areas of gold mineralization related to structurally controlled vein systems (Figures BC1d, BC1e and BC1f).


BC-2
In southern British Columbia, a regional heavy mineral drainage sampling program was carried out in a geologically favourable area. Previously, no mineral showings were known from the area of Figure BC2a.

The background-level sample downstream from the two anomalies on two tributaries may not have been taken far enough upstream to avoid dilution from sediments in the major river valley. The southernmost anomalous sample had a large content of heavy minerals, lessening the significance of the metal values (Figure BC2b).

Ensuing rock and soil sampling along with prospecting led to the discovery of significant gold mineralization related to a structurally controlled vein system (Figures BC2c, BC2d, BC2e and BC2f).

The area was later enclosed within a provincial park and no further work was carried out.

BC-3
In southern British Columbia, a reconnaissance heavy mineral drainage sampling program for zinc/lead deposits has proven effective in discovering significant soil anomalies. To be cost effective in the initial program, samples were collected where major creeks crossed logging roads, usually near the valley bottom (Figure BC3a).

Anomalous creeks were followed up first by silt sampling and then by reconnaissance soil sampling. Figures BC3a and BC3b demonstrate that the heavy mineral sampling detected zinc mineralization at a significantly greater distance from the source than silt sampling, in spite of zinc's noted mobility.

The possible effects of reduced zinc mobility in the presence of abundant carbonate rocks does not seem to be a factor in this situation. It is worthwhile to note that although zinc values in the heavy mineral samples were only slightly anomalous; the subsequent exploration that was carried out did find the source of the anomaly (Figures BC3b, BC3c and BC3d).



Summary
The case histories described above establish the merit of the proper planning and execution of regional geochemical programs. Choosing the correct techniques; orientation surveys, efficient sampling methods, concentration procedures, analysis, as well as systematic follow-up strategies should be the goal of all mineral explorationists.

The implementation of heavy mineral reconnaissance surveys can play a significant role in the discovery of mineral resources.

Discovery Consultants would be pleased to discuss with you the design and implementation of regional and property scale stream sediment sampling programs that will fit your exploration needs.