Page 95 - Introduction to Mineral Exploration

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

Element selection of experience of the recognition of weathered
outcrops and the use of a number of simple
Before samples are submitted to the laboratory, field tests.
discussions between the project manager, chief Many deposits which crop out have been
geologist, and the field staff should take place recognized because they have a very different
to ensure that all the elements that may be appearance from the surrounding rocks and
associated with the mineral deposit in question form distinct hills or depressions. A classic
are included on the analytical request sheet example of this is the Ertsberg copper deposit
and that analysis includes possible pathfinder in Irian Jaya, Indonesia. It was recognized
elements. Typical elemental associations are because its green-stained top stood out through
discussed in detail in Chapter 8.
the surrounding jungle. Its presence was first
noted by two oil exploration geologists on a
Analytical techniques mountaineering holiday in 1936 and reported
in a Dutch university geological journal (Dozy
There are a wide variety of analytical tech- et al. 1939). A literature search by geologists
niques available to the exploration geologist. working for a Freeport Sulfur–East Borneo com-
The method selected depends upon the ele- pany joint venture found the report and invest-
ment which is being analysed and upon the igated the discovery resulting in one of the
amount expected. Amongst the instumental largest gold deposits outside South Africa. The
methods available are atomic absorption spec- Carajas iron deposits in the Brazilian Amazon
trometry (AAS), X-ray fluorescence (XRF), were also recognized because they protruded
X-ray diffraction (XRD), neutron activation through rain forest (Machamer et al. 1991). On
analysis (NAA), and inductively coupled a smaller scale silicification is characteristic
plasma emission or mass spectrometry (ICP- of many hydrothermal deposits, resulting in
ES/MS). These analytical methods and their slight topographical ridges. It is said that the
application to different mineral deposit types silicification of disseminated gold deposits is
are discussed in Chapter 8. AAS is a relatively so characteristic that one deposit in Nevada
inexpensive method of analysis and some was discovered by the crunchy sound of a geo-
exploration camps now have an instrument logist’s boots walking across an altered zone
in the field; this ensures rapid analysis of the after dark. Karst-hosted deposits in limestone
samples for immediate follow-up. The other terrain often form distinct depressions, as do
methods involve purchasing expensive equip- many kimberlite pipes, which often occur
ment and this is usually left to specialist under lakes in glaciated areas (see section 17.2).
commercial laboratories. Besides forming topographical features most
Detailed identification of individual min- outcropping mineral deposits have a charac-
erals is usually undertaken using a scanning teristic color anomaly at the surface. The most
electron microscope (SEM) or an electron common of these is the development of a red,
microprobe, and these and other techniques are yellow or black color over iron-rich rocks,
discussed in section 2.2.2. particularly those containing sulfides. These
altered iron-rich rocks are known generic-
ally as ironstones, and iron oxides overlying
5.1.6 Prospecting
metallic sulfide deposits as gossans or iron
One of the key exploration activities is the hats (an example is shown in Fig. 5.4). These
location of surface mineralisation and any old relic ironstones can be found in most areas of
workings. Although this often results from the world, with the exception of alpine moun-
the follow-up of geochemical and geophysical tains and polar regions, and result from the
anomalies or is part of routine geological map- instability of iron sulfides, particularly pyrite.
2−
ping, it can also be the province of less formally Weathering releases SO 4 ions, leaving relic red
trained persons. These prospectors compensate iron oxides (hematite) or yellow-brown oxy-
for their lack of formal training with a detailed hydroxides (limonite) that are easily recognized
knowledge of the countryside and an acute eye in the field. Other metallic sulfides weather
– an “eye for ore.” This eye for ore is the result to form even more distinctively colored oxides