Page 242 - Introduction to Mineral Exploration

10: EVALUATION TECHNIQUES 225

300 m long and from $US70 to 150 a meter than 85–90% the value of the core is doubtful
for lengths up to 1000 m in accessible areas. as mineralized and altered rock zones are fre-
Standard references for diamond drilling are quently most friable and the first to be ground
Australian Drilling Industry Training Com- away and lost during drilling. The core is not
mittee (1997), Cummings and Wickland (1985), then representative of the rock drilled, it is not
the Australian Drillers Guide (Eggington 1985) a true sample, and it is probably misleading
and The Management of Drilling Projects (Fig. 10.12).
(1981).
Core drilling
Often, initial, rapid core logging is done at the
Sonic core drilling
drill site. This information is used to decide
Sonic drilling retrieves core but without the whether the hole is to be either continued or
contamination caused by drilling muds. Sonic abandoned. Wetted core is more easily exam-
drilling applies the principle of harmonics to ined, using either a hand lens or a binocular
drill and case a borehole (Potts 2003). It uses a microscope. Most organizations have a stand-
variable-frequency drill head to transmit vibra- ard procedure for core logging and a standard
tion energy through the drill pipe and core terminology to describe geological features.
barrel to allow continuous core sampling to Field data loggers are now used to gather com-
take place. Sonic drilling can penetrate over- pany standardized digital data, which are down-
burden, fine sand, boulders, and hard rock. It loaded to the central database upon return to
can collect samples up to 254 mm in diameter the field base or office (see section 9.1). Onions
and can drill up to 200 m vertically or in in- and Tweedie (1992) discuss the time and costs
clined holes. The big advantage of sonic drilling saved using this integrated approach to data
is that uncontaminated, undisturbed samples gathering, storage, and processing.
can be collected because no air, water, or other Once the initial logging at a drill site is com-
drilling medium is used. Sonic drilling can plete, the core is moved to a field base, where
realize 100% core recovery, even in glacial till, a more detailed examination of the core takes
clay, sands and gravels as well as hard rock. It place at a later date. Nevertheless, the main
offers rapid penetration, reduced on-site costs structural features should be recorded (frac-
and minimal environmental impact. In the ture spacing and orientation) and a lithological
later case the clean up and waste disposal costs description (colour, texture, mineralogy, rock
are significantly lower. alteration, and rock name) with other details
such as core recovery and the location of exces-
sive core loss (when say >5%). The descrip-
10.3.3 Logging of drillhole samples
tion should be systematic and as quantitative
Information from drillholes comes from the as possible; qualitative descriptions should be
following main sources: rock, core, or chips; avoided. These data are plotted on graphical
down-the-hole geophysical equipment; instru- core logs (see Fig. 5.13) and used as an aid in
ments inside the hole (see MWD above); and interpreting the geology of the current and next
performance of the drilling machinery. In this holes to be drilled.
section we are only concerned with geolo- Core is stored in slotted wooden, plastic or
gical logging, but the geologist on site at a metal boxes short enough to allow two persons
drill location must be familiar with all sources to lift and stack them easily (Fig. 10.13; see
of information. The collection of geotechnical Fig. 13.10). Core is collected for a variety of
data from core is discussed in section 10.6. purposes other than geological description, e.g.
metallurgical testing and assaying. For these
latter purposes the core is measured into appro-
Geological logging
priate lengths (remembering the principle of
Effective core recovery is essential – that is, the stratified sampling) and divided or split into
length or volume (weight) of sample recovered two equal halves either by a diamond saw or a
divided by the length or volume (weight) drilled mechanical splitter. Half the core is sent for
expressed as a percentage. If recovery is less assay or other investigations whilst the other