Page 245 - Introduction to Mineral Exploration

228 M.K.G. WHATELEY & B. SCOTT

sprinkle, and glue, a sample of rock chips and 1 Mobilization and transport of equipment to
panned concentrates from each sample interval the drilling site. This can vary from movement
on to a board so that a continuous visual repres- along a major road by truck to transport by
entation of the hole can be made. Again, de- helicopter.
scriptions must be systematic and quantitative. 2 Setting up at each site and movement
between successive borehole locations. Again
costs can vary greatly depending on distance
10.3.4 Drilling contracts
and terrain.
Drilling can be carried out with either in-house 3 A basic cost per meter of hole drilled.
(company) equipment or it can be contracted 4 Optional items costed individually, e.g.
out to specialist drilling companies. In the cementing holes, casing holes, surveying.
latter case the conditions of drilling, amount of 5 Demobilization and return of equipment to
work required, and the cost will be specified in driller’s depot.
a written contract. The purpose of drilling is to All items of cost should be detailed in the
safely obtain a representative sample of the tar- contract. There may be a difference of interest
get mineralisation in a cost-effective manner. between the driller and the company repres-
Once the mining company has been assured, in entative on site, normally a geologist, who is
writing, of the drilling company’s safe working there to see that drilling proceeds according
practices, the choice of drilling equipment is to the company’s plan and the contract. The
crucial and much depends upon the experi- driller may be paid by distance drilled during
ence of the project manager. Unless the drilling each shift whilst the geologist in charge is more
conditions are well known test work should, concerned with adequate core recovery and
if possible, be carried out to compare different that the hole is proceeding to its desired target.
drilling methods before any large-scale pro- The company representative usually signs
gram begins (Box 10.3). documents at the completion of each shift
The main items of cost in a contract are as where progress and problems are described
follows: and it is on the basis of these documents that

BOX 10.3 Comparative sampling results from two different drilling methods. (after Springett 1983b.)

If drilling conditions are not known a program of test work to compare and contrast appropriate drilling
procedures may be desirable before any large scale sampling program is commenced, as in the following
example.
Silver mineralization 12.2 m thick consisting of fine-grained dolomite and quartz with minor
chalcopyrite, galena, and sphalerite occurs in limestone. The silver is principally acanthite but with
some native silver. It has been tested by diamond and percussion drilling, and also by mining from a test
shaft.
Diamond drilling: BQ core, diameter 36.5 mm with samples every 1 m over the total length of 12.2 m;
core recovery was 93%. The core was split for assay and the total weight of the overall sample was 10 kg.
The weighted assay average was 62 g t silver. The main concern in diamond drilling was the possibility
−1
of high grade, friable silver mineralization being ground by the diamond bit and washed away by the
circulating water.
Percussion drilling: With a 1.6-cm-diameter hole samples were collected every 1 m; material recovery
was 87%. All the recovered material was taken for the analysis and totalled 255 kg. The weighted assay
−1
average was 86 g t . Whilst this method was cheaper than core drilling, there was the possibility that the
high specific gravity silver minerals might sink in the column of water in the bore hole and not be fully
recovered.
Bulk sample: From a 1.1-m square test shaft all the material was taken for assay, totalling 51.8 t. The
weighted assay average was 78 g t .
−1
Accepting the bulk sample result as the best estimate of silver grade the cheaper percussion drilling
method can be used to sample the deposit rather than the more expensive core drilling.