Placer mining or mining of a stream bed for alluvial deposits
After centuries, alluvial mining activity is still very much alive. Nowadays more precise studies are undertaken to determine where to explore, and the exploration process has been significantly optimised and professionalised. After a refraction seismic study is performed, samples are taken from the most significant identified areas. This alluvial deposit exploration is a very important part of the mining process. When completing this phase, you will be able to calculate the costs for the removal of the overburden formations, know how much placer deposit needs to be processed and what the yield should be.
In many topical areas where the gold or diamond alluvial deposits are found, ‘false-bottom’ layers are prone to showing up. The most valuable layers are found under these ‘false bedrock’ (endured clay) layers. In some cases, several of these false-bottom layers were identified before hitting the actual bedrock. Experienced personnel and good tools to perform the geotechnical exploration are a must to be able to tell a false bedrock from the real thing.
So the study of the exact composition of the soil is vital to determine if the identified mining site is economically feasible. With the right exploration tools and a well-trained crew, you can conclude from your research exactly where and how deep the alluvial deposits are. These four items, regarding exploration drilling, should be looked at closely and will make all the difference to your operation:
- Time required;
- Core recovery rate;
- Core sample quality;
- Operational cost.
Core drilling is employed by most of the mining companies. The main disadvantages are the cost and the time required to complete the programmes, however the obtained information is vital.
By applying the sonic drilling technique you reduce the required time to extract the core. This greatly accelerates the exploration process. Depending on the overburden formation you can have your sonic drilling rig built to core is as efficient as drilling rigs get. For example with the CompactRotoSonic crawler CRS-XL Duo you can take high-quality core samples at speed in any soil type by utilising its sonic technology. But when hitting hard rock you can switch to conventional rotary diamond coring tools.
Having one rig that does it all greatly speeds up the exploration process and cuts down on operational costs.
Core recovery rate
Providing accurate samples of a mineral deposit, the rock types, mineral types and rock structures all starts with being able to extract the core sample to the surface. This means applying the right techniques and using the right tools.
The Eijkelkamp SonicSampDrill has developed a variety of tools and techniques over the course of years, with intense and close co-operation with our customers and partners in our development process. We are now able deliver over 95% core recovery rate for alluvial exploration activities.
Core sample quality
The main disadvantages from traditional drilling and coring techniques are that samples break up into small chips that don’t show the structure of the bedrock, and samples are not very realistic due to the disturbance of conventional drilling and coring techniques.
Our sonic drilling technique is a soil penetration method that strongly reduces friction on the drill string and drill bit due to liquefaction, inertia effects and a temporary reduction of porosity of the soil. The combination makes penetrating a large range of soils easy for our sonic rigs and tooling.
The liquefaction and inertia effects enable you to collect very long and continuous samples. Due to the vertical, high-frequency movement the drill string stays straight, with a variance of no more than a few centimetres over the full length of the borehole. In alluvial material, vertical vibrations are generally enough to drive down a drill string for many metres without the injection of any water or air.
The result is a largely undisturbed soil sample, taken in dry or saturated soil layers.
Sonic drilling is a highly efficient drilling method due to its extremely fast penetration speeds in alluvial materials. By breaking down the costs and calculating the price per borehole or drilling depth in metres or feet, you will see that the operational expenditure (OPEX) of the sonic drilling rig is very competitive.
Combine a very competitive, if not a better, operational cost per borehole, with a shorter concession or plot exploration time and a great core recovery and core sample quality, and you will find the recipe for the best possible alluvial exploration process. Consequently your mining operation can be more focused and more profitable. Because you know exactly where the interesting placer deposits are, you do not need to excavate the entire plot. This also implies that you reduce the impact your mining operation has on the environment.
Four successful case studies from subsurface ore deposits in arctic, desert and tropical climates, are presented in the context of the main problems related to drilling in sub-surface ore deposits.
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