Ultimately these heated waters, together with those of mag matic origin, ascend, and as they cool and mingle with the descend ing vadose waters deposit their mineral burden, either in the main channels of circulation, thus forming ore-veins, or in the pores of the rocks forming the so-called cemented ore-bodies.
As the openings become closed by cementation, however, the circulation becomes feeble and ultimately completely stops, al though earth movements may refracture the rocks and inaugurate a new circulation. The vertical movement of the water is con fined to the zone of fracture and cementation. Below this zone, the pressure is sufficiently great to produce a differential move ment of the rock particles, and thus to close all openings by rock flow. For free upward movement the ground must be fissured, fault-fractures, especially when brecciated, and shear-zones are preferentially used by the ground waters, and these, therefore, frequently become the seat of the deposition of ores. The exist ence of impervious formations is an important factor in directing the general circulation of underground waters, and in the localiza tion of ore deposits.
The deposition of minerals from solution is brought about in a variety of ways. Solutions which are unsaturated at depth be come, as they ascend, saturated by decrease of temperature and pressure. Chemical reactions also come into play—for instance, between solutions of different origin, between solutions and solids and between gases and solutions. Under one or more of these conditions the metallic burden is precipitated, lining and ulti mately filling the channels traversed by the solutions. But not only are open channels filled : the wall rocks of the fissures may be affected or the constituent minerals of the rocks attacked by the penetration of the circulating waters. Consequently, two dis tinct classes of hydatogenetic ore-bodies must be distinguished, namely cavity-fillings and metasomatic replacements.
Cavities in rocks are due to a variety of causes. The openings may be those of discission, i.e., fractures of dislocation (faults), or contraction-joints; they may be the interstices of sediments or gas-pores in lavas ; they may be due to the folding of sedimentary beds (saddle and trough openings) ; or they may be solution cavities in limestone and dolomite, i.e., caves, sink-holes and en larged joints and bedding planes. The filling of fault-fissures, zones of brecciation, interstitial openings and the vesicular cavities of lavas, gives rise to important ore deposits.
Metasomatic replacement is responsible for the formation of perhaps the largest class of ore-bodies. By metasomatism is meant
the replacement molecule by molecule of a constituent of a rock by new mineral matter. Such replacements can be effected not only by water below the critical temperature, as in the hydrother mal circulation, but also by gases, as in the case of pneumatolytic replacement.
Since different minerals are differently affected by solutions, rocks will be more or less completely replaced according as they are made up of aggregates of the same or of different minerals. Limestones and dolomites, which are composed almost entirely of either calcite or the mineral dolomite, are far more liable to ex tensive and complete replacement than any other type of rock. They lend themselves to replacement both by the facilities they offer for circulation and by the solubility of the carbonates of which they are composed.
Where rocks of differing susceptibility to replacement occur in alternate beds the more soluble will be preferentially replaced, thus giving rise to a bedded type of ore deposit. Impervious beds crossing the paths of vertical fissures retard or stop the flow of solution, with the result that the latter, by spreading out beneath or above the impervious layer, tends to form characteristic pear shaped replacements. In cases where there is a rapid alternation of impervious and permeable layers, selective action leads to the formation of serrated or tooth-shaped ore-bodies.
In sandstones whose cement is calcareous, the replacement of the calcite by ores is common ; but it is sometimes difficult to say whether the disseminated ore particles have been produced by replacement, or by the filling of inter-granular spaces. Dissem inated ore-bodies may also be produced by the differential alter ation of igneous rocks, since these are aggregates of minerals varying in susceptibility to alteration.
Among the best examples of metasomatic replacement are the lead and zinc deposits found in limestone. Sulphides of lead and zinc are soluble in alkaline sulphides without decomposition. In contact with limestone such solutions deposit these sulphides as galena and zinc blende. Two types are common, viz., flats and pitches. A flat is a replacement which has progressed outwards into the limestone from bedding planes. On the other hand, a pitch is a replacement which has been effected from joints in the limestone. The names refer to the horizontality, on the one hand, and to the steep dip, on the other, of the respective deposits.