Drilling Techniques for Resource Estimation of Mineral Sand Deposits

摘要

Drilling mineral sand deposits has specifi c challenges, which are different from drilling hard rockrndeposits:rn1.Drilled sequences are represented by non-consolidated free-fl owing sands to semi-consolidatedrnclayey sands and indurated layers.rn2.Valuable heavy minerals (ilmenite, rutile, zircon, leucoxene) have an average density ofrn4.6 g/cm~3, which is signifi cantly heavier than the hosting sand matrix; mainly represented by quartzrn(density = 2.6 g/cm~3). The large difference in the mineral densities can result in sample segregationrnin the drilling rods and capturing devices causing biased composition of the recovered samples.rn3.The drilled strata can contain beds of consolidated sediments, hard pan lenses and concretions.rnTherefore, chosen drilling equipment should be fl exible and permit a switch from drilling thernfree-fl owing sands to hard rock drilling.rn4.Thickness of the sand hosted mineralisation varies from several metres to more than 170 m.rn5.Mineralisation is distributed above and below the water table.rn6.Evaluation of the mineral sands deposits is based on hundreds of drill holes and many thousandsrnof samples, therefore drilling methods should be fast and cost effective.rnThe drilling methods commonly used for mineral sand deposit evaluation, include aircore, triplerntube diamond and sonic; including its variants, such as vibrocore drilling technique. The ability of thernstudied drilling techniques to properly address the above listed challenges in mineral sand deposits,rnwhilst maximising the quality of the obtained samples has been compared in three deposits: CorridorrnSands (Mozambique), Richards Bay (Republic of South Africa) and Fort-Dauphin (Madagascar).rnThis study has shown that aircore samples tend to be negatively grade biased, underestimating thernheavy minerals content, in particular when drilled intervals are located below the water table.rnTriple tube diamond core method can produce non-biased samples. However, results can be marredrnby incorrectly chosen drilling mud.rnSonic drilling is a relatively new technique based on a high frequency vibratory technology. It isrnthe only production drilling technique that allows a continuous relatively undisturbed sample ofrnunlithifi ed sands to be obtained. The main limitations of this technology preventing its wider use forrndeveloping mineral sand resources are the relatively high drilling costs, in particular when the depthrnof drilling exceeds 100 m, and speed of drilling advance.