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The majority of large placer deposits are in South America, Africa or Asia.
When one operates in a foreign, remote area, it is essential to use reliable methods, local labour, and tested equipment.
Photogeology, at the initial stage, offers a 2D (surface) definition of the area and its geology.
Seismic adds a 3rd dimension to lead the sampling program. It provides cross-sections of alluvial terraces with less than 5% depth error.
The drilling, or pitting, in strategically selected areas, tests economic reliability of the deposit.
If grades are interesting, a more systematic sampling program follows this initial evaluation.
This approach saves money, time, and often... a project.
ALLUVIAL DIAMOND DEPOSITS
My speciality is exploration / mining of South American, Precambrian Guyana Shield related diamond or gold deposits.
EVALUATING ALLUVIAL DEPOSITS
After many years of exploring alluvial deposits, here are my suggestions.
One of the most difficult task associated with placer mining is the sampling of the deposit. More placer projects have failed due to inaccurate assessment of the reserves than to any other reason. Within the realm of placers, those containing valuable minerals with a high unit value (diamonds) are more difficult to sample than those with larger bulk, lower unit value minerals (tin).
Some items to consider when sampling a placer deposit are:
1) A relatively large size sample is needed for accurate valuation of the ground being tested. Placers are composed of many sizes of gravel that make a representative sample difficult to obtain.
2) When sampling placers for high unit value minerals such as gold, any error in mineral content of the sample will be magnified in the calculation of reserves.
3) Values usually are erratically distributed within the gravel mass. Therefore, some placers with a more uniform value distribution may be adequately assessed with a minimum number of samples, while a deposit with a high erratic distribution of values may not be adequately sampled regardless of how many samples are taken.
4) The investigation of a placer deposit should be made by or be under the direction of a person experienced in the art of placer sampling.
5) During a sampling program, items that must be observed and noted in addition to the sample size and valuable mineral content should include boulder size and number, clay content, bedrock conditions, water, frozen ground, false bedrock, and any other physical characteristics that would affect mining of the deposit.
The steps to be followed in approaching a placer sampling program are outlined below:
(1) check status of land ownership, (2) physical characteristics of area, and (3) research mining history of the area.
(1) photogeomorphology, (2) surface grab sampling over all exposures of gravel, (3) few seismic cross section, (4) geobotanical study, and (5) survey for old workings.
Choosing a Sampling Method
The main methods to consider are ( 1) existing exposures, (2) hand-dug pits or shafts, (3) backhoe trenches, (4) bulldozer trenches, (5) other machine-dug pits or shafts, (6) churn drill holes, (7) other drilling methods, or (8) bulk samples.
Special Problems Associated with Placer Sampling:
These are (1) large rocks and boulders, (2) erratic high values, (3) uncased holes, (4) small diameter holes, and (5) salting.
Sample Processing or Washing:
The consideration is: if the actual sample washing should be manual or by some sort of equipment.
Data processing consists of record keeping, reporting values, and assay procedures.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.