12
1 INTRODUCTION
2.0 and 3.0. At the same time most sands contain traces of minerals with specific gravities between 4.0 and 5.0. The heavy minerals, as these are called, commonly consist
of opaque iron ores, tourmaline, garnet, zircon, and so on. More rarely they include
ore minerals such as gold, cassiterite, or monazite. It is a matter of observation that the
heavy mineral fraction of a sediment is much finer grained than the light fraction. There
are several reasons for this. First, many heavy minerals occur in much smaller crystals
than do quartz and feldspar in the igneous and metamorphic rocks in which they form.
Secondly, the sorting and composition of a sediment is controlled by both the size and
density of the particles- this is spoken of as their hydraulic ratio. Thus, for example, a
large quartz grain requires the same current velocity to move it as a small heavy mineral. It is for this reason that sandstones contain traces of heavy minerals which are finer
than the median overall grain size of the less dense grains (see Fig. 8.15). In certain flow
conditions, however, the larger, less dense grains of a sediment are winnowed away to
leave a residual deposit of finer grained heavy minerals. The flow conditions that cause
this separation are of great interest to the mining industry, both as a key to understanding the genesis of placer ores, and in the flotation method of mineral separation in which
crushed rock is washed to concentrate the ore. Placer deposits occur in nature in alluvial channels, on beaches and on marine abrasion surfaces (Fig. 1.2A). Placer sands are
frequently thinly laminated with occasional slight angular disconformities and shallow
troughs. This is true of both fluvial and beach-sand placers. In Quaternary alluvium
placers occur in the present channel system, notably on the shallow riffles immediately
downstream of meander pools, and also in river terrace deposits and buried channelfill alluvium below the floor of the present river bed. Gold is a characteristic alluvial
placer ore mineral occurring in the Yukon and Australia. The Pre-Cambrian gold- and
uranium-bearing fluvial conglomerates of the Witwatersrand appear to be paleoplacers
deposited on a braided alluvial fan though this has been disputed (see Section 6.3.2.2.4).
Recent marine placers, like their fluvial counterparts, occur at different topographic levels due to Pleistocene sea level changes (Fig. 6.46).
Essentially, placer ores are controlled by the geochemistry of the sediment source,
by the climate, and hence depth of weathering, by the geomorphology, which controls
the rate of erosion and topographic gradient, and by the hydrodynamics of the transportational and depositional processes. The genesis of placer deposits is seldom in dispute because the textures and field relationships of the ores clearly indicate their detrital origin. Furthermore, the processes of placer formation can be observed at work on
the earth today. The origins of syngenetic and epigenetic sedimentary ores are more
equivocal.
1.3.2.2 Syngenetic Ores
Syngenetic ores are those that were precipitated during sedimentation. This phenomenon has been observed on the earth's surface at the present time. One of the best known
examples of this process is the widespread formation of manganese nodules and crusts
on large areas of the beds of oceans, seas, and lakes (see Section 6.3.2.10.1). Goethite
ooliths are forming today in Lake Chad in Central Africa (Lemoalle and Dupont, 1971).
More spectacular examples of modern sea floor metallogenic processes occur associated
with volcanic activity as at Santorini and in the "hot holes" of the Red Sea (Puchelt,
1971; Degens and Ross, 1969).
1 INTRODUCTION
2.0 and 3.0. At the same time most sands contain traces of minerals with specific gravities between 4.0 and 5.0. The heavy minerals, as these are called, commonly consist
of opaque iron ores, tourmaline, garnet, zircon, and so on. More rarely they include
ore minerals such as gold, cassiterite, or monazite. It is a matter of observation that the
heavy mineral fraction of a sediment is much finer grained than the light fraction. There
are several reasons for this. First, many heavy minerals occur in much smaller crystals
than do quartz and feldspar in the igneous and metamorphic rocks in which they form.
Secondly, the sorting and composition of a sediment is controlled by both the size and
density of the particles- this is spoken of as their hydraulic ratio. Thus, for example, a
large quartz grain requires the same current velocity to move it as a small heavy mineral. It is for this reason that sandstones contain traces of heavy minerals which are finer
than the median overall grain size of the less dense grains (see Fig. 8.15). In certain flow
conditions, however, the larger, less dense grains of a sediment are winnowed away to
leave a residual deposit of finer grained heavy minerals. The flow conditions that cause
this separation are of great interest to the mining industry, both as a key to understanding the genesis of placer ores, and in the flotation method of mineral separation in which
crushed rock is washed to concentrate the ore. Placer deposits occur in nature in alluvial channels, on beaches and on marine abrasion surfaces (Fig. 1.2A). Placer sands are
frequently thinly laminated with occasional slight angular disconformities and shallow
troughs. This is true of both fluvial and beach-sand placers. In Quaternary alluvium
placers occur in the present channel system, notably on the shallow riffles immediately
downstream of meander pools, and also in river terrace deposits and buried channelfill alluvium below the floor of the present river bed. Gold is a characteristic alluvial
placer ore mineral occurring in the Yukon and Australia. The Pre-Cambrian gold- and
uranium-bearing fluvial conglomerates of the Witwatersrand appear to be paleoplacers
deposited on a braided alluvial fan though this has been disputed (see Section 6.3.2.2.4).
Recent marine placers, like their fluvial counterparts, occur at different topographic levels due to Pleistocene sea level changes (Fig. 6.46).
Essentially, placer ores are controlled by the geochemistry of the sediment source,
by the climate, and hence depth of weathering, by the geomorphology, which controls
the rate of erosion and topographic gradient, and by the hydrodynamics of the transportational and depositional processes. The genesis of placer deposits is seldom in dispute because the textures and field relationships of the ores clearly indicate their detrital origin. Furthermore, the processes of placer formation can be observed at work on
the earth today. The origins of syngenetic and epigenetic sedimentary ores are more
equivocal.
1.3.2.2 Syngenetic Ores
Syngenetic ores are those that were precipitated during sedimentation. This phenomenon has been observed on the earth's surface at the present time. One of the best known
examples of this process is the widespread formation of manganese nodules and crusts
on large areas of the beds of oceans, seas, and lakes (see Section 6.3.2.10.1). Goethite
ooliths are forming today in Lake Chad in Central Africa (Lemoalle and Dupont, 1971).
More spectacular examples of modern sea floor metallogenic processes occur associated
with volcanic activity as at Santorini and in the "hot holes" of the Red Sea (Puchelt,
1971; Degens and Ross, 1969).