Page 101 - Introduction to Mineral Exploration

84 C.J. MOON & M.K.G. WHATELEY

the use of computerized methods (GIS) which mental in demonstrating the mineralogy of
allow rapid integration and interrogation of the the mineralized structures. The quartz veins
databases and are discussed in detail in section contain significant sulfides, mainly pyrite
9.2. A straightforward example of the overlay- but with lesser chalcopyrite, galena, sphalerite,
ing of different data types is shown in Fig. 5.10 native copper, and tetrahedrite–tennantite. By
taken from the work of Ennex plc in Northern contrast, the shears are richer in arsenopyrite
Ireland as detailed in Clifford et al. (1990, with chloritization often observed. Thus geo-
1992). The area was chosen for investigation chemistry defined the approximate location
because of the presence of favorable geology, of the quartz veins and auriferous shears, geo-
placer gold in gravels and anomalous arsenic physics the location of the shears, and tren-
geochemistry discovered during government ching the subcrop of the quartz veins.
surveys. A licence was applied for in 1980. Once the gold-bearing structures had been
Ennex geologists investigated the anomalous defined their depth potential was tested by dia-
areas and by careful panning and float mapping mond drilling (Fig. 5.10c). The diamond drilling
identified the source of the gold as thin quartz (6490 m in 63 holes) took place during 1985–86
−1
veins within schist. Although the veins iden- and defined a resource of 900,000 t at 9.6 g t .
−1
tified assayed about 1 g t Au and were not Subsequent underground exploration con-
of economic grade, they were an indication firmed the grades and tonnages indicated by
that the area was mineralized and prospect- drilling but the deposit is undeveloped because
ing was continued over a wider area. In late of a ban on the regular use of explosives in
1983 economically significant gold in bedrock Northern Ireland by the security authorities.
was found and subsequently three other gold-
bearing quartz veins were found where a small
stream has eroded to bedrock. More than 1300 5.2 WHEN TO DRILL AND WHEN TO STOP
mineralized boulders were mapped over a
strike of 2700 m. Channel samples of the first One of the hardest decisions in exploration is
−1
identified vein gave assays of 14 g t Au over to decide when to start drilling, and an even
3.8 m with selected samples assaying over harder one is when to stop.
−1
150 g t Au. The pressure to drill will be evident when
Detailed follow-up concentrated on building the program has identified surface mineralisa-
up a systematic picture of the area where there tion. Management will naturally be keen to
is no outcrop. This focused on boulder mapping test this and gain an idea of subsurface miner-
and sampling, deep overburden geochemistry alisation as soon as possible. However, this
and geophysical mapping using very low fre- pressure should be resisted until there is a
quency electromagnetic resistivity (VLF–EM/ reasonable idea of the overall surface geology
R) surveys. Deep overburden geochemistry and the inferences that can be made from this
was undertaken using a small core overburden knowledge concerning mineralisation in depth.
drill to extract 250 g samples on a 50 × 20 m
grid. This spacing was decided on follow-
ing an orientation study over a trenched vein. 5.2.1 Setting up a drilling program
Geochemical sampling defined 33 anomalies The geologist in charge of a drilling program is
(>100 ppb Au) (Fig. 5.10a). A variety of geo- faced with a number of problems, both logist-
physical methods were tested including in- ical and geological. There must be a decision on
duced polarization and VLF–EM/R. No method the type of drilling required, the drillhole spac-
located the quartz veins but the VLF–EM/R ing (see section 10.4.4), the timing of drilling,
method did define previously unidentified gold and the contractor to be used. The logistics of
bearing shears (Fig. 5.10b) and the subcrop of drilling should be considered carefully as the
graphitic pelites. The nature of the geophysical drill will need drill crews, consumables, and
and geochemical anomalies was defined by a spare parts; this will require helicopter support
2850 m trenching program (Fig. 5.10c). Besides in remote areas and vehicle access in more
increasing the number of gold-bearing struc- populated areas. Many drills require vehicle
tures to 16, this trenching was also instru- access and access roads must be made and pads