Maynard published a textbook (306 p.) in 1983 and indicated that ore deposits come in a bewildering range of types, and their study involves almost all subdisciplines of geology. Accordingly, it has been difficult to cover the subject matter of economic geology satisfactorily in one book or in one course. In practice, some sort of restriction is made. For North American geologists, there has been a tendency, now fading, to regard economic geology as synonymous with the study of hydrothermal deposits. At one time this was carried to the extreme of insisting that almost all deposits were, in fact, hydrothermal. More recently, the importance of sedimentary processes in ore genesis has begun to be appreciated, but the sedimentology necessary for the understanding of these processes is hard to integrate with the standard economic geology curriculum.
It seemed to Maynard in 1983 that a workable solution to this problem is to make divisions along the lines of the traditional areas of geology. Thus, one can approach volcanic ores from the viewpoint of a volcanologist, sedimentary ores from that of a sedimentologist. The ores are then treated within the context of the surrounding rocks, rather than as isolated entities. This book attempts to fill this purpose for sedimentary ores.
Most sedimentary ores are formed by chemical processes, and these processes are the subject of this book. Accordingly, I have largely excluded placers. Ores are defined to be economic accumulations of metals, and Maynard did not discuss non-metallics. Most non-metallics have an evaporite origin, a style of deposition well covered elsewhere (e.g., Braitsch 1971). The remaining metallic deposits form a more or less coherent group whose precipitation is mostly controlled by Eh gradients. That is, a particular metal dissolves at, say, low Eh, then migrates along the gradient to be precipitated at high Eh, or vice-versa. What, then, is meant by "sedimentary ores"? How do they differ from deposits classed as "syngenetic" or "strata-bound"? Maynard used sedimentary in the sense of "formed by sedimentary processes."
Almost all such ores are also strata-bound, but many strata-bound ores seem to have been formed by hydrothermal processes (see Chapter 7). Similarly, many sedimentary ores are syngenetic, that is, formed at the same time as the surrounding sediment. But there is also an important class of syngenetic OreS that is volcanic in origin (see Chapter 8), and some sedimentary ores are epigenetic. It is useful to make the following distinctions (based on Tourtelot and Vine 1976):
- Sedimentary-Formed by sedimentary processes, at low temperatures and pressures near the Earth's surface, hosted by sedimentary rocks or soils.
- Strata-bound-Enclosed by essentially parallel layers of sedimentary rocks. May be cross-cutting within a layer.
- Syngenetic-Formed at the same time the sediment was deposited. Sedimentary examples are iron and manganese ores, volcanic examples are the
Kuroko-type Cu-Pb-Zn ores. - Early diagenetic (or syndiagenetic~Formed after deposition of the sediment, but while still in contact with overlying water via diffusion through
pore fluids. Pyrite in modem sediments obtains its sulfur in this way. - Late Diagenetic-Formed after the sediment is closed to the overlying water, but from metals derived from within the same stratigraphic sequence.
- Some U deposits in tuffaceous rocks may have this origin (Chapter 6).
- Epigenetic-Formed after consolidation of the sediment and usually cutting across the strata; metals supplied from outside the system. Two varieties
may be distinguished:
- Ground water-epigenetic-Formed by cold-warm ground water. An example is roll-front U.
- Hydrothermal-epigenetic-Formed by hotter (> 100°C) solutions, usually from depth. Carbonate-hosted Pb-Zn deposits are mostly of this
type.
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