1.3 APPLIED SEDIMENTOLOGY
11
Table 1.2
Summary of the Main Sedimentary Ores and Their Common Modes of Origin
Name
Process
Examples
I. Syngenetic Originated by direct
precipitation during
sedimentation
II. Epigenetic Originated by postdepositional
diagenesis
III. Placer
Syndepositional detrital
sands
Manganese nodules and
crusts, some oolitic
ironstones
Carnotite, copper-lead-zinc
assemblages, some ironstones
Alluvial gold, cassiterite
and zircon
the genesis, nor aid the exploitation of the ore body. Secondly, direct sensing methods
of prospecting have long been available. Ores can be located by direct geochemical and
geophysical methods. Geochemical techniques began with the old-style panning of alluvium gradually working upstream to locate the mother lode. This is now superseded
by routine sampling and analysis for trace elements. Direct geophysical methods of locating ore bodies include gravity, magnetometer, and scintillometer surveys. For these
reasons sedimentology is not as useful in searching for sedimentary ores as it is for petroleum. Its relevance in mining is twofold. First, it has a large contribution to make to
the problems of ore genesis in sedimentary rocks. Secondly, it is useful as a backup tool
in the search for and development of ore bodies. Three main processes are generally believed to be responsible for the genesis of sedimentary ores (Table 1.2). Detrital placer
deposits are formed where current action winnows less dense quartz grains away to
leave a lag concentrate of denser grains. These heavy minerals may sometimes be of
economic importance.
Syngenetic sedimentary ores form by direct chemical precipitation within the depositional environment. Epigenetic ores form by the diagenetic replacement of a sedimentary rock, generally limestone, by ore minerals. The origin of sedimentary ores has always excited considerable debate. This generally hinges on two criteria. First, how may
epigenetic and syngenetic ores be differentiated? Secondly, to what extent are the mineralizing fluids of epigenetic ores derived from normal sedimentary fluids, and to what
extent are they hydrothermal in origin? In recent years it has been increasingly realized
that ordinary sedimentary processes can generate both syngenetic, and epigenetic ores
in sediments. Conversely, less credence is now given to a hydrothermal origin for many
epigenetic ores. The following account of detrital, syngenetic, and epigenetic sedimentary ore attempts to show how sedimentology helps to decipher problems of ore genesis and exploitation.
1.3.2.1 Placer Ores
Detrital sediments are generally composed of particles of varying grain density. For example, most sandstones contain a certain amount of clay and silt. The sand particles are
largely grains of quartz, feldspar, and mafic minerals with specific gravities of between
11
Table 1.2
Summary of the Main Sedimentary Ores and Their Common Modes of Origin
Name
Process
Examples
I. Syngenetic Originated by direct
precipitation during
sedimentation
II. Epigenetic Originated by postdepositional
diagenesis
III. Placer
Syndepositional detrital
sands
Manganese nodules and
crusts, some oolitic
ironstones
Carnotite, copper-lead-zinc
assemblages, some ironstones
Alluvial gold, cassiterite
and zircon
the genesis, nor aid the exploitation of the ore body. Secondly, direct sensing methods
of prospecting have long been available. Ores can be located by direct geochemical and
geophysical methods. Geochemical techniques began with the old-style panning of alluvium gradually working upstream to locate the mother lode. This is now superseded
by routine sampling and analysis for trace elements. Direct geophysical methods of locating ore bodies include gravity, magnetometer, and scintillometer surveys. For these
reasons sedimentology is not as useful in searching for sedimentary ores as it is for petroleum. Its relevance in mining is twofold. First, it has a large contribution to make to
the problems of ore genesis in sedimentary rocks. Secondly, it is useful as a backup tool
in the search for and development of ore bodies. Three main processes are generally believed to be responsible for the genesis of sedimentary ores (Table 1.2). Detrital placer
deposits are formed where current action winnows less dense quartz grains away to
leave a lag concentrate of denser grains. These heavy minerals may sometimes be of
economic importance.
Syngenetic sedimentary ores form by direct chemical precipitation within the depositional environment. Epigenetic ores form by the diagenetic replacement of a sedimentary rock, generally limestone, by ore minerals. The origin of sedimentary ores has always excited considerable debate. This generally hinges on two criteria. First, how may
epigenetic and syngenetic ores be differentiated? Secondly, to what extent are the mineralizing fluids of epigenetic ores derived from normal sedimentary fluids, and to what
extent are they hydrothermal in origin? In recent years it has been increasingly realized
that ordinary sedimentary processes can generate both syngenetic, and epigenetic ores
in sediments. Conversely, less credence is now given to a hydrothermal origin for many
epigenetic ores. The following account of detrital, syngenetic, and epigenetic sedimentary ore attempts to show how sedimentology helps to decipher problems of ore genesis and exploitation.
1.3.2.1 Placer Ores
Detrital sediments are generally composed of particles of varying grain density. For example, most sandstones contain a certain amount of clay and silt. The sand particles are
largely grains of quartz, feldspar, and mafic minerals with specific gravities of between