Description

Gold deposits are categorized as placer or hard-rock. Placer deposits hold liberated gold and are comparatively easy to access; However placer exploration is challenging, as the natural process involved in the formation of gold placers is complex. Even under the most favorable circumstances it is generally not possible to estimate the potential gold grade of a placer deposit without a systematic sampling campaign.

Figure 1: Auger drill in placer gold exploration

Figure 2: Extracted samples, separated by depth

In practice, samples from drills or dug excavations are washed over portable sluices or panned to concentrate the gold for visual inspection. But clean gold concentrates that could possibly be weighed on a fine scale cannot be produced without accepting considerable losses.

Once a sample is concentrated by panning the operator’s experienced eye is most miners’ only tool to qualify a deposit worth mining or not. Shown below is an example of a pan concentrate from bulk sampling. The operator counted 130 gold “colors” but no information on the gold grade is generated from visual examination. Furthermore, this method is highly dependent on operator skills and motivation which is not appropriate for a systematic exploration program.

Figure 3: Pan concentrate

To be more precise the sample can be submitted for fire assay to determine the gold content in milligrams or g/t for example. A fire assay at one of the commercial assay labs takes about 2 weeks and may cost $30 per sample. Add sample shipping, sample preparation and disposal.

There are other options but they are either outdated (mercury amalgamation) or very labor intensive (manual sorting of gold colors from pan concentrates) and therefore no practical solutions in professional placer exploration.

Stepping forward into the 21st century where camera systems have become part of the daily life: Barcode scanners, Cell phones verifying access through facial recognition, vehicles scanning traffic signals and highway markings to name a few.

If the human eye can identify gold based on the characteristic yellow color then applying camera systems with computer-aided image processing is a plausible approach to delivering high accuracy, high repeatability and detailed information content.

Figure 4: Pan concentrate under microscope

While camera systems for gold detection and analysis were previously reserved to scientific research, the GOLD DETECTOR is built for industrial exploration use at a fraction of the cost of scientific tools. No software installation is required and the machine is designed for use by anyone even without computer knowledge. There is no cost for consumables, limiting the cost per analysis to labor alone.

That being said, there is a powerful hardware and software package hiding behind a simple user interface: Pan concentrate is presented to a high resolution flatbed scanner with an optical resolution of over 100 Megapixels (MP). The 2-dimensional image captures the flat side of gold particles as they naturally rest on a glass sample holder. The software performs a search for golden colored particles and presents gold count, particle size, particle shape and calculates the gold grade in grams/ton or ounces/ton.

Figure 5: GOLD DETECTOR scanner and sample holder

Figure 6: Detected gold particles (left) and native scan prior to gold search (right)

In the calibration phase the GOLD DETECTOR is adjusted to samples from the deposit, mainly to confirm the thickness of gold flakes. The thickness varies by deposit but is typically consistent within a deposit or area for particles of similar size. For an exemplary deposit particles of 100 micron (150 mesh) in size may be 20 micron (600 mesh) thick and particles of 1000 micron (18 mesh) in size may be 200 micron (70 mesh) thick. In this case the thickness can be calibrated as 10% of the particle size or as an assigned thickness for each particle size bucket.

The GOLD DETECTOR technology is proven accurate for alluvial gold that is fairly uniform in thickness and best suited for typical alluvial/placer gold that is found in grain sizes between 75 micron (200 mesh) and 2000 micron (10 mesh). The sample holder that coming with the tool is designed for pan concentrates containing anywhere between 0 to 5000 gold particles.

The manufacturer offers an accuracy and calibration test. For this service a small sample of pan concentrate from the deposit is submitted, which the manufacturer divides into 5 subsamples for scanning with the GOLD DETECTOR followed by fire assays. Alignment with the diagonal line across the chart indicates that the GOLD DETECTOR measurement accurately correlates with the fire assay result.

Once the initial setup and calibration is completed the GOLD DETECTOR is a “one button” operation. In practice it takes approximately 5-10 minutes to complete the analysis of one pan concentrate sample. That includes the placement of the sample on the sample holder, scan process and removal of the sample. The output is a printable “one page” report that includes

• Sample Designation (entered by operator)
• Information about the sample location such as GPS coordinates or section depth (entered by operator)
• Mass or volume of the extracted sample prior to panning (pounds, kilograms, tons, cubic meters, cubic yards or cubic foot) (entered by the operator)
• Measured gold count (number of gold particles detected)
• Measured gold particle size distribution (particle size buckets can be defined by operator, typically 0-125 micron, 125-250 micron, 250-500 micron, 500-1000 micron, 1,000-2,000 micron)
• Measured gold mass (in milligrams) based on calibrated thickness
• Measured gold grade (in grams or ounces per ton or cubic meter for example)

An exemplary analysis report is shown below. The processed “Sample 1” comes from a 0.1 ton extracted gravel sample and the GOLD DETECTOR found it to contain a total of 21 gold particles with a total gold weight of 12.98 mg, making for 129.85 mg/t gold grade of the tested ground. The analysis shows that the samples contains minimal fine gold, but instead is dominated by coarse gold (500-1,000 micron and 1,000-2,000 micron). This information is valuable in assessing the deposit and forecasting the profit because coarse gold can more efficiently be recovered than fine gold.

No other assay method can provide the same information depth.

Figure 8: Sample Analysis report

The GOLD DETECTOR has been described as a game changer by industry experts. Key features such as rapid turnaround time and unmatched data depth make it a practical tool for every placer exploration campaign.

Basic Exploration of Alluvial / Eluvial Gold Deposits

The topic of this short information is the exploration of deposits of not consolidated (loose) gold bearing material with free gold, which is not enclosed in rock particles. Gold in hard rock (primary gold) has to be liberated by crushing and milling and is not topic of this information.

There are two types of non-consolidated gold deposits:

Eluvial Deposits

These deposits occur in close proximity to the primary deposit (mainly gold bearing quartz veins). In flat terrain, they are as loose material around or just above the quartz veins and in sloping terrain they can also lie on a slope and at the foot of a mountain, where gold-bearing quartz veins are situated. The thickness of the material is often only a few decimeters to less than one meter. At a foot of a mountain, the material can pile up to several meters.

Eluvial deposits are characterized in the way that

• the material (quartz pebbles, host rock pebbles) is sharp-edged and not or only slightly rounded
• the material usually contains a lot of fine dust
• the material is not or only slightly layered
• the material contains little heavy minerals (black sand)
• the gold content begins on the surface and changes little down
• the gold is sharp edged, irregular, and is often grown together with some quartz
• the particle size distribution is wide (from very fine gold to sometimes large nuggets)

If there was no mechanical transport of the material e.g. by a seasonal river, the grade of the material is decreasing sharply with increasing distance from the primary deposit.

Such material can therefore be found directly above, below and in the close vicinity of the gold-bearing quartz veins. Often the quartz chunks contain ingrown gold, which can be recovered after grinding. The grade of the quartz rock has to be tested by fire assay.

Alluvial Deposits

These deposits were formed and deposited by ancient, actual or temporarily active rivers. They can often extend many kilometers away from the primary deposit.

Alluvial deposits are characterized in that

• The material is more or less well rounded (gravel)
• the material occurs in layers (finer material/sand/small gravel above, coarse material (large gravel, boulders below). There also exist intermediate coarse material layers, often with gold
• the gold mostly starts only at a certain depth, there is often a barren or poor overburden, which requires removal without washing
• there is a large quantity of heavy minerals (black sand)
• the gold is smooth, rounded or flat
• the particle size distribution of the gold gets more uniform and narrow with more distance from the source
• by the action of the water, the gold is washed into the cracks and crevices of the underlying bedrock. It is therefore necessary to dig into the bedrock, at least, if possible, about 30-50cm and wash this material. If there are cracks in hard bedrock like granite they may be cleaned manual (i.e. by dredge)

The gold grade of alluvial material can be constant over many kilometers from the primary source, but the gold gets finer and flatter with distance. Both deposit types (alluvial and eluvial) can also be mixed if, for example, eluvial material has slid down to an alluvial deposit.

Exploration Objectives

• Determine the reserves (gold grades and quantities of material)
  Gold grades and particle size distribution are relevant factors to determine the expected gold recovery rates per ton or cubic meter/yard of material. The relevant metrics can be determined by computerized image analysis, applicable to most alluvial placer gold deposits.
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• Map the deposit to provide data for mine planning
  layers (depths and thickness of overburden and gold bearing layers, slopes, etc.)
  barren areas (for waste dumps, water ponds, etc.)
  A current topographic map of the exploration area is essential. Drone mapping is a modern and cost effective solution.
• Determine the duration of the project
• Calculate the profitability of the project

The exploration of a very large area is expensive and takes a long time. One can, for example, explore the entire deposit with 500m distance between the lines, and decrease the line spacing, for example, to 250m or less for detailed production planning. The more irregular the deposit is, the more narrow the grid has to be (e.g. 30m or even 10m distance between holes).

For shallow deposits (i.e. a few meters depth), manual pits or pits using an excavator can be used to extract samples. If the material layer is deeper than about 5-6m, one can think about using a mechanized auger or percussion drill with casing.

Typical section of the valley (alluvial deposit), here: barren overburden and deep gold-bearing layer (“pay dirt”) with rich old channel

If the material is layered, it is necessary to sample the various pits layer per layer in order to determine the thickness of the barren overburden which has to be removed without washing and to determine the thickness of the gold bearing layer(s). It is therefore necessary to note for all pits the overall depth from surface to bedrock, the thickness of the barren overburden and the thickness of the gold bearing material.

Example

A) Sampling drill-holes or pits (distance between pits 30m)
With additional sample pits in eluvial material close to the veins and down the slope

Mapping The Grades

Production Planning

Reserve estimation example (simplified to one layer)

HiGrade DPI is a novel analysis method for placer gold that relies on the distinct color of gold. Analyse surface or drill samples from a placer claim . Determine the gold grade and properties. Produce instant printable reports for each sampling location of the claim.

The pan concentrate is placed on a specially designed sample holder from where a high resolution image is created with an optical scanner. Then the automatic HiGrade DPI particle analyzer detects gold particles based on their golden color and calculates the gold grade of the sample. The automatically generated report includes unique statistics on the particle size distribution and particle shape. The analysis takes less than 5 min, requires no consumables and is non-destructive.

We offer a comprehensive tool kit for industrial and scientific applications, including dedicated prospectors, mineral laboratories, exploration firms and placer operations.