It is essential that detailed records are kept as they will be used to determine the value of the deposit as well as to determine its minability. Information included in the drill logs will also be used in selecting a mining method.
In the United States, the volume of placer material is always reported as bank cubic yards. Internationally, grams or milligrams per cubic meter are used. The value of ground in gold placers is reported in cents per unit of volume, as well as in milligrams of gold per unit of volume. The vein miners tend to use cents, troy ounces, milligrams or carats per ton. It is a wrong unit to use for a placer grade. The fineness of gold and the price used in the value determination are also reported. Minerals, other than precious metals, are reported in kilograms per cubic meter (pounds per cubic yard), percent, or the particular unit customarily used for the commodity in question. For diamonds, carats and points per unit of volume are used.
In some cases, like the deposit estimation by intersections, is used the amount or value per unit of surface.
When, a gold or other high value mineral sample is taken, it should never be split or reduced prior to assay determination. Any attempt to divide a sample to reduce its volume will yield erratic assay results. When dealing with fine size placer minerals having a low unit value, a reduction of the sample size by mixing and splitting is an acceptable procedure.
At this point, the procedures determining the value of a sample need to be discussed. The only correct way to determine the amount of valuable mineral in a placer sample is by weighing. After the volume of the total sample taken has been determined, the amount of valuable mineral in the sample is measured by weighing. After the sample volume and valuable mineral quantity have been determined, a value can then be placed on the ground represented by that sample. It should be noted that placer samples should never be fire or AA assayed. The only time fire assaying is acceptable is to determine the fineness of the gold.
RESERVE ESTIMATION AND PLACER VALUATION
After samples have been collected, washed, and assayed, reserves for a deposit can be estimated. There are many placer reserve estimation methods available. Some of these are the block, triangle, polygonal, traverses, and diamond methods.
The value using the block method is calculated as follows:
1) Find the volume of each block, length times width times depth.
2) Multiply the volume by the value per cubic meter (cubic yard) in each block,
3) Find the sum of all weighed values to obtain the total value.
4) Find the average grade by dividing the total in step 3 by the total volume to obtain the value per cubic meter (cubic yard).
If this method is used, the value of deposit is calculated as follows:
1) The volume equals the average of depth of the three drill holes times the area of the triangle.
2) The weighed average gold content is equal to the gold value of each of the three holes times the depth of each of the holes divided by the sum of the depths of the three holes.
3) Total volume equals the summation of values in step 1.
4) Total gold content equals the total volume in step 3 times the average from step 2.
5) The average grade equals the total gold content in step 4 divided by the total volume in step 3.
Either method may be used in finding the value using polygons.
The grade for regular polygons is calculated as follows:
1) The total volume equals the sum of the volumes of the individual polygons. Find the volume of the polygons by multiplying the area of the polygon times the depth of the drill hole.
2) Total gold content equals the sum of the grades of each hole times the volume of each polygon.
3) Average grade equals the value in step 2 divided by the value in step 1.
The area of each irregular polygon may be found by using a planimeter or by dividing the polygon into triangles or rectangles.
This method is very similar to that of the triangle and may be used as a check:
1) Area of a traverse is the average of the depths of the boreholes times the distance between the boreholes.
2) The total volume of a section between two traverses is one-half the sum of the areas of all individual traverse, A and B, times the distance between the traverses.
3) The gold content of a section is the volume of the section times the average value between two traverses.
4) The total gold content is the sum of the gold contents of each section.
5) The average grade is the value in step 4 divided by the value in step 2.
Method of Diamonds
The method of diamonds is much the same as that of triangles. The drill hole is located at the centre of the diamond and apexes midway between drill holes on adjacent lines. This method is best used for regularly spaced holes. The total area is equal to the sum of all diamonds that may be treated as right triangles for all practical purposes:
1) Total volume equals the area of each diamond times the depth of each hole through the pay zone.
2) Total gold value equals the sum of the gold value in each hole times the volume of each diamond.
3) To find the value per cubic meter (cubic yard), divide the value in step 2 by the value in step 1.
There are many ways to sample and many methods to calculate the value of a placer deposit. It is important to remember to use care in sampling and to select the method that best suits the type of occurrence that is being sampled. Also, use the ore reserve calculation method that best applies to the configuration of the deposit and mode of deposition.
Nothing replaces experience, not a costly book or advanced software. Gold, diamond or tin placer has a personality of its own, but all placers in a geological province area are similar. Also, placers formed in cold climates like Alaska or Yukon, are very different from tropical placers formed under laterization processes. A point often missed (for a time) by a placer specialist (geologist) shifting to new hunting grounds.
Many systems exist but an off-the-shelf application will lead to a gross under or over estimation of the deposit. The acceptable is to be within 10% of underestimation, so keep the optimism down.
Finding more gold is not a problem, finding less is.
1m3 = 1.3 yd3
1g/m3 = 11.8 grains/yd3
1ozt. = 31.1 g
Banka drill factors
4 inch 11.5 l/m
6 inch 22 l/m
COST OF ALLUVIAL EXPLORATION
It’s very logical to want to have a good idea where the bedrock is before starting a sampling program. The seismic offers excellent depth-distance profiles. Its cost is low compared with the costs of drilling.
“Drill” the land with seismic to find where to put the least amount of holes for the most amount of information.
Follow-the-pattern blind drilling costs too much. Alluvial deposit pays in paleochannel(s). Missing one, in exploration phase, makes a bonanza look like a bust.