HEAVY MINERAL CONCENTRATES

Heavy minerals can be concentrated from glacial till samples or from stream sediments.

The theory behind the use of heavy mineral concentrates relies on the fact that by concentrating the heavy mineral fraction from the sample, the dilutant light minerals which may obscure an anomaly derived from a deposit are eliminated, providing a clearer picture. This allows the minerals to be examined under a binocular microscope, SEM or electron microprobe for particle shape or composition. Particle shape of gold grains will commonly identify distance to source. Very abraded grains may have travelled a long distance whereas delicate angular grains are probably more local. Similarly for diamond exploration, the presence of certain mineral phases may indicate the distance to the source kimberlites. Bulk analysis of heavy minerals provides a rapid, low cost method to screen large volumes of samples. Samples with appropriate chemistry can then be further evaluated as the INAA technique is non-destructive.

Heavy mineral concentrates are usually prepared by tabling or wet sieving a very large sampleof till or stream sediments (up to 20 kg may be routine). The heavy mineral concentrate collected at this stage is then further processed with heavy liquids using methylene iodide (SG = 3.3). The resultant concentrate then is separated into magnetic and non-magnetic fractions and it is the non-magnetic fraction which is usually analyzed. In some cases more elaborate separations include separating paramagnetic fractions and sieving to various particle sizes before analysis. This may provide further insight into identifying the source of the heavy minerals. Both gold and diamond mines have been discovered using this technology.

INAA (Instrumental Neutron Activation Analysis) is an analytical technique dependent on measuring gamma radiation induced in the sample by irradiation with neutrons. The primary source of neutrons for irradiation is usually a nuclear reactor. Each activated element emits a “fingerprint” of gamma radiation which can be measured and quantified. Routine multi-element analyses by INAA are performed on practically any material from the smallest sample which can be weighed accurately to very large samples.

A 30 g aliquot, if available, is encapsulated in a polyethylene vial and irradiated along with flux wires at a thermal neutron flux of 7 x 10 12 ncm-2 s-1. After a 7-day period to allow Na-24 to decay the samples are counted on a high purity Ge detector with resolution of better than 1.7 KeV for the 1332 KeV Co-60 photopeak. Using the flux wires and control standards, the decay-corrected activities are compared to a calibration developed from multiple certified international reference materials. For values exceeding the upper limits, assays are recommended. One standard is run for every 11 samples. One blank is analyzed per work order. Duplicates are analyzed when sample material is available.

3A (Heavy Mineral Concentrates) Elements and Detection Limits (ppm)

ElementDetection LimitUpper Limit
Ag510,000
As2100,000
Au5 ppb30,000 ppb
Ba200-
Br510,000
Ca1%50%
Ce310,000
Co510,000
Cr10100,000
Cs2-
Eu0.22000
Fe0.02%75%
ElementDetection LimitUpper Limit
Hf15000
Hg5-
Ir50 ppb-
La110,000
Lu0.051000
Mo2010,000
Na0.05%-
Nd1010,000
Ni20010,000
Rb50-
Sb0.210,000
ElementDetection LimitUpper Limit
Sc0.11000
Se20-
Sm0.110,000
Sr0.20%-
Ta110,000
Tb2-
Th0.510,000
U0.510,000
W410,000
Yb0.21000
Zn200100,000

Reference:
Hoffman, E.L., 1992. Instrumental Neutron Activation in Geoanalysis. Journal of Geochemical Exploration, volume 44, pp. 297-319.

0.5 g of sample is digested with aqua regia for 2 hours at 95oC. Sample is cooled then diluted with deionized water and homogenized. The samples are then analyzed using a Perkin Elmer ICP.

3C (HMC Base Metals) Elements and Detection Limits (ppm)

ElementDetection LimitUpper Limit
Ag0.2100
Cd0.55,000
Cu110,000
Mn2100,000
Mo210,000
Ni110,000
Pb25,000
S0.01%10%
Zn110,000

Notes:

Extraction of each element by Aqua Regia digestion is dependent on mineralogy.
Assays are recommended for values which exceed the upper limits.

Download Our Price List