Page 55 - Introduction to Mineral Exploration

38 A.M. EVANS & C.J. MOON

Chapter 14). Figure 3.7 shows the distribution
of the gold orebodies in the East Rand Basin
where the vein quartz pebble conglomerates
occur in quartzites of the Witwatersrand
Supergroup. Their fan-shaped distribution
strongly suggests that they occupy distributary
channels. Uranium is recovered as a byproduct
of the working of the Witwatersrand goldfields.
In the very similar Blind River area of Ontario
uranium is the only metal produced.

Chemical sediments. Sedimentary iron and
manganese formations and evaporites occur
scattered through the stratigraphical column
where they form very extensive beds conform-
able with the stratigraphy.

Igneous host rocks
Volcanic hosts. The most important deposit
Pay streaks
type in volcanic rocks is the volcanic-
associated massive sulfide (see Chapter 15) or
oxide type. The sulfide variety often consists of
0 5 km
over 90% iron sulfide usually as pyrite. They
are generally stratiform bodies, lenticular to
sheetlike (Fig. 3.8), developed at the interfaces
between volcanic units or at volcanic–
FIG. 3.7 Distribution of pay streaks (gold orebodies)
in the Main Leader Reef in the East Rand Basin of sedimentary interfaces. With increasing
the Witwatersrand Goldfield of South Africa. The magnetite content, these sulfide ores grade
arrows indicate the direction of dip at the outcrop or into massive oxide ores of magnetite and/or
suboutcrop. For the location of this ore district see hematite such as Savage River in Tasmania,
Fig. 14.2. (After Du Toit 1954.) Fosdalen in Norway, and Kiruna in Sweden

Andesite
Bedded chert, often
ferruginous
Massive sulfides
Py - sp - ga - cp (+ Ag, Au)

Sulfidic tuff
Stockwork Py-cp low in Rhyolite
sp, ga, Ag, Au

FIG. 3.8 Schematic section through an idealized volcanic-associated massive sulfide deposit showing the
underlying feeder stockwork and typical mineralogy. Py, pyrite; sp, sphalerite; ga, galena; cp, chalcopyrite.