Heavy Minerals

Abstract

Heavy minerals are dense grains found not only in rocks, but also in different types of sand. After being collected from different areas along the beach at Fort Fisher, the sand grains were brought back to the lab to be examined. Each sample was cleaned, and placed in a dense liquid. The separation of the heavy minerals from the rest of the sand then took place and was later examined under a petrographic microscope. The purpose of the investigation was to recognize the different minerals, their percentage within the sample and to determine each minerals origin. Results varied with each soil sample. There was a greater abundance of some minerals than others in the different mineral samples. This was due to location of where the sand was collected. Many of the heavy minerals found were known to be familiar to the beach area.

Background

Heavy minerals are minor, high-density sandstone minerals, which have a higher average of specific gravity than other minerals. These minerals are present in the parent rocks as either rock forming minerals, like mica, or as accessory minerals, such as zircon and tourmaline ( Mange, Maria A. and Heinz F.W Maurer, 1992). Heavy minerals usually make up only one percent of the parent rock from which it came from.

The number and the type of heavy minerals vary within wide limits. Only a few species are common in sands. Some of these minerals include kyanite, garnet, tourmaline, magnite, and others. The heavy minerals are often useful guides to the source rock where the sediments came from. Quartz, which is considered to be a heavy mineral, is an exception because it is found almost in all parent rocks ( Pettijohn, F.J., P.E. Potter, R. Siever, 1987).

In order to separate the heavy minerals from the other sand grains, they must be concentrated by gravity separation using a liquid with a density of 2.89 or 2.96. The heavy minerals, because they are more dense than the liquid, will then sink to the bottom of the funnel. There are certain factors that affect the reliability of heavy mineral studies. They include such factors as the climate of the source area. This controls the pre-selection of mineral grains during weathering. The weathering of the minerals determines the original input of the heavy minerals into the sedimentary system. Another factor deals with hydraulics. It affects the sorting of grains with the size, form, and densities of the mineral grains. The heavy minerals have a direct relationship to the grain size. It helps reflect the initial size of the source rock.

Petrologists use heavy minerals to find provenance (Mange, Maria A. and Heinz F.W. Maurer, 1992). The word provenance means origin or birth, which deals with the source rock the mineral came from. Petrologists also use the minerals to trace sediment transport paths, map sediment transport paths, and to outline sand bodies (Pettijohn, F.J., P.E. Potter, R. Siever, 1987).

Research Question

Heavy minerals are dense sediments, which are found in rocks and sands. Research on these minerals has tapered off considerably due to more advanced technology on rock fragments and minerals. The investigation is prepared to answer what type and the percent composition of the minerals found in the sand samples. Also, the experiment will help determine where the minerals originated.

Methods

During our trip to Fort Fisher, sand samples were collected from three different areas: sand located between the seawall, the sand along the shoreline, and the protected sand located near the sand dunes. The sand was then brought back to the lab for further investigation.

The first step was to measure out 30 grams of each sample on an electronic scale, which measured up to a hundredth of the number. Each sample was then washed in order to extract the salt from the sand. The salt, from the beach, might have affected the density of the liquid, allowing the results to be inaccurate. The sand was then set a side to dry.

In order to set up for the main portion of the experiment, several items were used. A funnel with a small rubber tube attached at the bottom was held by a clamp, which was attached to a stand. A small beaker was placed under the funnel and tube, to catch any excess waste. A pinch clip was clamped to the rubber tube to catch the heavy minerals and liquid.

The liquid, sodium polytongstate, which has a density of 2.86, was poured ¾ full into the funnel. The first sand sample was then poured into the liquid and began the process of separating, the heavy minerals sank and the light minerals stayed afloat. The entire separating process took around 4 hours.

In order to extract the heavy minerals, a number of steps were used. First, filter paper was placed into a second funnel and then into a small beaker. Then the first funnel was placed over the second, and the pinch clip was slowly released, allowing only the heavy minerals to be released. The pinch clip was then closed, trapping the light fragments in the first funnel. The heavy minerals collected were washed thoroughly to filter out all the polytongstate liquid. The sample was then set a side to dry.

Before the minerals could be mounted onto the slide, they were first measured on the electronic scale. This was done in order to find the percent composition of the individual minerals found later in the experiment. In order to mount the mineral grains onto a slide, a glue mixture, known as epoxy, was mixed until it became sticky. It was then spread carefully, eliminating air bubbles, onto the slide. The minerals from the sand were sprinkled onto the slide and set aside to dry.

Using a petrographic microscope at 3x, the heavy minerals were examined to determine their type. The book was used to compare the minerals under the microscope with the pictures of the minerals in the book (Mange, Maria A. and Heinz F.W. Maurer, 1992). Sections of the slide were viewed, drawn, and counted. The process continued until 100 grains were counted, drawn, and each mineral had been recognized. Each heavy mineral type was then counted individually, and the percent composition for each was determined. The experiment was then repeated for the last two samples. On the third sand sample, the mineral grains were viewed at 10x because the grains were finer and smaller than the minerals in the other two samples. Data charts and graphs were then composed to show the results.

Results

The results within the experiment varied with each sand sample. The mineral distinctions within each sample helped categorize each sample with its on area. Each sample was carefully drawn and identified to make sure the results were as accurate as possible.

The first heavy mineral sample was from the sand found between the sea wall at Fort Fisher. The heavy minerals extracted from the sand weighed a total of 0.074 grams. The percent composition was then calculated to compare its mass to the rest of the 30 gram sand sample. The heavy minerals in the first sample made up only 0.247 percent of the total sand sample. Eight different heavy minerals were found within this sample. The majority of the heavy mineral sample was made up of monazite, making up 37 percent, and magnetite, which made up 33 percent of the sample. The other minerals included garnet, tourmaline, apatite, epidote, chloritoid, and calcite. For a description of the previous minerals, turn to A4. The pie chart located on A1 shows a comparison of the percentages of the individual minerals. Each mineral found in this sample is commonly found in beach sands. This means the source rocks from which the minerals came, are located at or near the beach area.

The second sand sample was collected from the shoreline on the beach at Fort Fisher. This area is where the water washes over the sand. The heavy minerals extracted from this area weighed a total of 0.085 grams. The percent composition of sample two was then taken to compare its mass with the rest of the sand. The heavy minerals in this sample made up 0.285 of the total sand sample from the beach. There were a total of nine different minerals identified in the sample. Again, monazite, making up 25 percent of the sample, and magnetite, which made up 29 percent, were the dominant minerals found. The other minerals identified were garnet, tourmaline, apatite, epidote, chloritiod, calcite, and quartz. The quartz is usually considered to be a light mineral, so it may have got caught up with the heavy minerals during the separation. To look at percentages of the minerals within the second sand sample, refer to the chart on A2. The same heavy minerals that were located in the first sample were found in the second sample. This means the heavy minerals also came from source rocks located at or near the beach.

The third sand sample was taken from the protected area of the beach at Fort Fisher. This is the area on the beach close to the sand dunes. The water is unable to reach this type of sand. After separation occurred, the isolated heavy minerals weighed 3.003 grams. There was a bigger sample of heavy minerals in this sand due to the increase in the abundance of magnetite. The percent composition of the mineral sample was taken to compare its mass with the rest of the sand fragments. The heavy minerals in sample 3 made up 10.01 percent of the sand. This is a greater increase in percent composition compared with the first two samples. Magnetite was the dominant mineral, making up 58 percent of the mass of the heavy mineral sample. The other minerals identified in this sample included garnet, monazite, apatite, epidote, quartz, calcite, muscovite, and kyanite. The last two minerals, muscovite and kyanite, were found only in the third sample. The reason for this discovery is unknown. Kyanite is usually found in the mountain region. There is a possibility that it was swept down by the water hundreds of years ago, or its source rock could have been brought to the beach area from an outside factor. The graph on A3 shows a comparison of the percent compositions of the heavy minerals. Excluding the kyanite and muscovite, the minerals found in this lab experiment are commonly found within the beach sands.

Conclusion

In this investigation, each heavy mineral was examined under the microscope to determine its type, its percentage within the mineral sample and the origin of each mineral. In each sample, the minerals proved to be similar. There was an exception in the third sample when kyanite and muscovite were found. These two heavy minerals were not common to the area. The investigation also proved that two minerals, magnetite and monazite were the two dominant minerals found in all three samples. In the third sample, magnetite made up over half of the composition of the heavy minerals. This was the reason the sand tested had a black tint. Each mineral was studied and read about to determine the origin. Almost all the minerals were found to have originated from beach sands (Pellant, Chris and Roger Phillips, 1990). It would be interesting to further investigate heavy minerals by collecting samples of sand on the different beaches around North Carolina. Then one would be able to compare the different types of heavy minerals at each location and compare the differences and similarities. This would enable a person to obtain a clearer understanding of the heavy mineral composition along the coast of North Carolina.

References

Mange, Maria A. and Heinz F. W. Maurer. Heavy Minerals in Colour. London:

Chapman and Hall, 1992.

Pellant, Chris and Roger Phillips. Rocks, Minerals, and Fossils of the world. Boston:

Little, Brown and Company, 1990.

Pettijohn, F.J., P.E. Potter, R. Siever. Sand and Sandstone. New York: Springer-Verlag,

1987.

THE USE OF HEAVY MINERALS IN MINERAL EXPLORATION

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.
  
  
• Utilizing the same bulk sample, precious metals, base metals and diamond indicator minerals can all be effectively prospected for, thereby reducing the risk associated with exploration for a single commodity.

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.