Page 52 - Introduction to Mineral Exploration

3: MINERAL DEPOSIT GEOLOGY AND MODELS 35

FIG. 3.4 Stockwork of molybdenite-bearing quartz
veinlets in granite that has undergone phyllic
alteration. Run of the mill ore, Climax, Colorado.

FIG. 3.3 Diagram of the Vulcan pipe, Herberton, overall shapes of some are cylindrical, others
Queensland. The average grade was 4.5% tin.
(After Mason 1953.) are caplike, whilst the mercury-bearing stock-
works of Dubnik in Slovakia are sometimes
pear-shaped.
pipes of Messina in South Africa (Jacobsen Stockworks most commonly occur in por-
& McCarthy 1976). phyritic acid to intermediate plutonic igneous
intrusions, but they may cut across the contact
Irregularly shaped bodies into the country rocks, and a few are wholly
or mainly in the country rocks. Disseminated
Disseminated deposits. In these deposits, ore deposits produce most of the world’s copper
minerals are peppered throughout the body of and molybdenum (porphyry coppers and dis-
the host rock in the same way as accessory seminated molybdenums) and they are also of
minerals are disseminated through an igneous some importance in the production of tin, gold,
rock; in fact, they often are accessory minerals. silver (see Chapter 16), mercury, and uranium.
A good example is that of diamonds in kim- Porphyry coppers form some of the world’s
berlites. In other deposits, the disseminations monster orebodies. Grades are generally 0.4–
may be wholly or mainly along close-spaced 1.5% Cu and tonnages 50–5000 Mt.
veinlets cutting the host rock and forming
an interlacing network called a stockwork Irregular replacement deposits. Many ore
(Fig. 3.4), or the economic minerals may be dis- deposits have been formed by the replacement
seminated through the host rock along veinlets. of pre-existing rocks, particularly carbonate-
Whatever the mode of occurrence, mineralisa- rich sediments, e.g. magnesite deposits. These
tion of this type generally fades gradually out- replacement processes often occurred at high
wards into subeconomic mineralisation and temperatures, at contacts with medium-sized
the boundaries of the orebody are assay limits. to large igneous intrusions. Such deposits have
They are, therefore, often irregular in form therefore been called contact metamorphic
and may cut across geological boundaries. The or pyrometasomatic; however, skarn is now