ROCKS, FOSSILS AND ORES
6.11 Economic Geology
Any geologists worth their salt should be able to recognise the principal economic minerals and rocks, for it is their duty to consider the economic, as well as the purely scientific, aspects of any area they map. To ignore them,
or to consider them beneath their scientific dignity, as some do and freely admit, is intellectual snobbery.
Before going into the field, review any literature concerning the minerals in the region you are about to map, both metalliferous and industrial. Note records of quarries and mines. Find out what ores have been mined, and particularly, whether they were associated with sulphides, for these ores have distinctive outcrops. Also note the rocks the ores were associated with and keep them in mind when mapping.
6.11.1 Types of body
Ore bodies do not necessarily crop out at the surface in easily recognisable form. Some are just rock in which metallic minerals are disseminated and often sparsely disseminated at that. Somestratiformzinc-lead ores are merely shales with finely dispersed zinc and lead sulphides, similar in grain-size to the rock minerals themselves.Porphyry copper deposits, those large stock- like granitoid intrusions which supply more than half the world’s copper, often contain less than one per cent metal, and look much like any other intrusion. Take nothing for granted.
6.11.2 Oxidation
Ore bodies do not stand up out of the ground with fresh shining crystals of ore minerals glinting in the sun. Sulphides, in particular, are usually extensively altered above the water-table by oxidation. Some oxidise to a highly soluble state (copper, zinc and silver ores are examples) and the metals are leached downwards to be re-deposited near the water-table as a zone ofsecondary (supergene) enrichment, leaving the upper part of the ore body depleted in these metals. Those redeposited in the oxidised environment just above the water-table form an enriched zone of oxide and carbonate ores, those rede- posited below it are reduced again by reaction with the other sulphides present to form a zone ofsecondary sulphide enrichment. Native metals, such as sil- ver and copper, may also result from secondary enrichment, depending on conditions (Figure 6.4). The iron sulphides, invariably associated with sul- phide ores, form insoluble oxides which remain at, and close to, the surface to accumulate during erosion resulting in a mass of cellular limonite called gossan. Gossans, and the reddish-brown soils associated with them, form dis- tinctive indications of sulphide mineralisation, but that does not necessarily mean that any useful ores are associated with them; only too frequently you find only pyrite below.
Where small groundwater springs near mineralisation have leaked out, rocks may be stained by the brightly coloured copper carbonates azurite and malachite, or coated with tiny green crystals of the lead chloro-phosphate pyromorphite, so easily mistaken for moss.
ground level
water -table
Leached zone
No leaching
No leaching
No leaching Iron
accumulates as limonite
gossan
Copper minerals
Zinc minerals
oxidise and
oxidise and
metal leaches
leach down
down
Enrichment by Often massive
enrichments of zinc carbonate (smithsonite) Enriched by
secondary
Primary Primary sulphides sulphides
sulphides (bornite,
bornite) chalcocite, etc.)
(chalcopyrite, malachite chrysocolla or sometimes native copper
Silver released from
oxidised galena and leached down
Often major
Enrichment of native
Silver in galena silver and silver sulphide
(acanthite) enrichment of
horn silver and native silver Lead
carbonates and sulphates
present in gossan
Gold and cassiterite occur as minor
enrichments in gossan
or enrichment
no enrichment
no enrichment
No enrichment Galena
No enrichment
No enrichment
No
Native gold cassiterite enrichment Sphalerite
Sulphates and carbonates (anglesirite, cerussite) remain more
or less in place
Pyrite iron oxidises
Iron sulphides to limonite
OXIDISINGREDUCING
Secondary sulphide zone Secondary oxide zone
Primary ore
Figure 6.4 Oxidation of sulphide ore deposits, showing how some ore minerals are oxidised and carried downwards; some to reprecipitate above the water-table; some to be reduced again to deposit as new sulphides, or as native metal, just below it, enriching sulphide ores already there. The insoluble iron-oxides remain at surface as a mass of rather cellular insoluble iron-oxide gossan
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6.11.3 Structural control
Pay particular attention to the fracture pattern in any mineralised district, for ore deposition may have been controlled by faults or joints. However, ore may also be controlled by folds, bedding planes, unconformities, lithological changes, and by contacts where granites and diorites have intruded limestones or dolomites. Ore bodies can be any shape. Some are vein-like, some are irregular masses grading into their host-rocks, some are mineralised rock breccias in collapsed carbonate rock caverns; others are merely an ore-bearing part of an otherwise barren rock, sedimentary, metamorphic or igneous, and these are the most easily missed.
6.11.4 Industrial minerals and rocks
Many of the materials you map have an economic use. The range is extensive and covers rocks themselves, such as: limestone (building, cement, chemical neutraliser); marble (building, monuments, etc.); granite (monuments, build- ing facing, ballast); slate; and superficial deposits such as gravels and sands (concrete aggregate, moulding sands); clays (bricks, ceramics, fillers). The list is extensive. Note the gravel pits, some of them possibly now lakes, and also the quarries in your area. Build up your background of industrial minerals and refer to Knill (1978) and Harbin and Bates (1984), and look around you and note the mineral products in everyday life, from glass in your windows to the buildings in your streets. In one 300 m stretch of Swansea High Street there were nine different types of polished facing stone. In cemeteries, note which type of gravestone has weathered best; just look at the dates on them.
6.11.5 Fuels
Coal is the most likely fuel to be encountered when mapping in Carbonifer- ous areas of the United Kingdom and the coal fields are well-documented.
Overseas, not all coals are Carboniferous, and in some countries brown coals and lignites are important. Oil shales are an often forgotten potential energy source, for when petroleum sources are depleted and/or the price of liquid and gaseous fuels rises too high to be economic; ironically they are largely con- fined to the non-OPEC countries: China, the USSR and Estonia exploit them.
Britain has large potential resources associated with the Jurassic Kimmeridge Clay and, in fact, shale oil was produced from a small Carboniferous field in the Midland Valley of Scotland from 1850 until 1964 (Barnes 1988). Oil shales can be any age from Palaeozoic upwards. However, liquid oil and gas are unlikely resources to be encountered when field mapping unless in OPEC areas.
6.11.6 Water
Water has been described as the ‘essential mineral’ and geologists in many countries spend a considerable part of their time looking for it. Much of the
Figure 6.5 The water profile of soil and rocks
search for water is geological common sense. Note its occurrence in any area you map and learn from it; the general water profile is shown in Figure 6.5.
There are two basic forms of water supply, namely wells and reservoirs.
Shallow wells are common in any country. They are sunk to a water- bearing horizon, sometimes to weathered rock but often to sands or gravels.
The water is raised by a bucket, a hand-pump, or a mechanical pump of some sort. They have the drawback of being subject to contamination unless lined to prevent contamination by soil water. Deep wells (i.e. boreholes) are of two general types; those drilled to an aquifer of water-bearing perme- able rock, and artesian and sub-artesian wells drilled to an aquifer below a
Figure 6.6Wells: well no. 1 penetrates to an aquifer and the water must be pumped. Over-pumping will draw down the rest-level and locally deplete the water-table until it is re-charged. Well no. 2 has penetrated through the aquiclude which confines the water within the aquifer and, because the well- head is below the piezometric surface (the level of the water-table in the aquifer where it crops out), the well is artesian and water flows without pump- ing. Well-head no. 3, however, is above the piezometric surface and needs to be pumped. Note that the piezometric surface slopes downwards towards the centre of the artesian basin
non-permeableaquiclude (Figure 6.6). Deep wells are cased, i.e. lined by pipes to prevent contamination from shallow groundwater.
Carbonate rocks make excellent aquifers, largely because of their joint- ing, often enlarged by solution. However, any well-jointed rock can be an aquifer, and in southern Africa Karroo dolerites are an important water source, whilst in East Africa quartzites confined by phyllites serve as aquifers too;
and yet again in the apparently unpromising African basement, granitoid rocks supply water where open-jointed water-bearing zones are sandwiched between tighter-jointed unaltered rock below and deeply weathered and partly kaolinised granite above (Barnes 1988). Do not be too hidebound over your idea of what an aquifer should be. However, wells in an unconfined aquifer can soon be depleted by over-pumping and, despite their usually far larger reserve, artesian wells can be depleted too so that they are no longer artesian and must be pumped to maintain supply. The London Basin is an example.
Reservoirs depend on being sited in a valley with catchment which will have a sufficient amount of water to fill it: reservoirs which did not fill are not unknown! Geologically, the dam wall must be sited so that it neither
leaks around the walls nor beneath it. Cavernous limestones are not good foundation rocks: again, this has been done. There are two types of dam wall:gravity dams, held in place by earth- or rock-fill supporting a clay core which prevents the leakage of water through the dam wall, or beneath or around its sides; and concretearch dams. Both are matters of engineering geology, beyond our remit here.