Mining in Manitoba |
Structural Geology |
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Structures Favourable to Mineral Deposition Many important ore deposits are magmatic in origin and occur in areas around volcanic necks, where intrusive and extrusive igneous activities merge. Batholiths of magmatic material have produced the volcanic rock that is found deposited above them. Rising slowly into this volcanic cover, the batholith intrudes material of its own making. The fluids (gas and liquid) that are given off yield the ores, whose presence is suggested by breccia (broken rock) and porphyry. Where the volcanic cover is thick, the existence of mineral bodies below can only be guessed at, or determined by geophysical means. Where the volcanic rock has been eroded to sufficient depth, ores may be exposed if they are present. Breaks in the surrounding rock may either be partly the cause of igneous intrusions or the result of it. The primary cracks in rock is the result of cooling of the magma and the shrinkage of drying sediment. These yield joints in fairly regular patterns. Later cracks are produced in igneous, sedimentary, and metamorphic rock by earth movement, volcanism, or solution action. Fractures can become fissures and extend down for miles. The contact between a vein and the wall rock is often sharply defined. The veins may contain minerals that are different from those that have penetrated the wall rock and partly replaced it. Openings in veins are known as vugs.
Where the alignment of the surrounding rock is such that the more porous and permeable structures rise away from the intrusive body, they act favourably to carry the mineralizing solutions and direct them to new areas of deposition. Movement along a fault also may serve as passageways for mineral solutions and may become a place for them to settle. In mountainous regions, rocks are generally first folded (forming fractures and faults), then intruded by igneous bodies, then faulted and finally invaded by metal-bearing solutions. Basins of sedimentation, including beds of minerals that were deposited by evaporation, have rims that are often easy to map. Local variations in the size and shape of rock layers, and in their porosity and permeability, may control the accumulation of minerals. The presence of a caprock that has trapped mineral solutions may have prevented them from rising further. Minerals can also gather at the crests of folds or the troughs. The chemical nature of rock may influence mineral deposition; making carbonate rocks especially favourable hosts. Secondary deposits of minerals form in a number of ways. Atmospheric weathering, together with related chemical changes that are due to groundwater, oxidizes certain minerals and yields solvents that dissolve other minerals. These are carried downward by percolating water, and so porous or broken rock is a favourable condition. They may be deposited above the water table as oxidized ore or they may descend below the water table and be deposited as enriched sulfide ores. Mechanical erosion of surface rocks removes durable minerals so that they can be transported by gravity and stream action, to be laid down as alluvial deposits or placers.
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