Ore Deposits formed by Colloidal Deposition
A colloidal system consists of two phases:
1. The dispersed phase - is the diffused phase
2. The dispersion medium - in which the diffused phase is dispersed.
Colloidal particles range in size from those in true solutions to those in coarse suspension. The limit of size are 10-3 to 10-7 cm (> solution < coarse suspension). The colloidal material may be solid, liquid or gas and may be dispersed in one of these same phases. In the study of ore transport, we are concerned with solids suspended in liquids or a gaseous medium.
A colloidal system consisting of solids dispersed in a liquid is called a Sol. Colloidal particles have large surface areas per unit volume. Ions absorbed on the surfaces of such particles control their behaviour. If the particles absorb cations they become positively charged, if they absorb anions they become negatively charged. These charges prevent the particles of the sol from coagulating or flocculating, but if an electrolyte is added the particles neutralize and flocculate.
Most sulfides and organic sols are negative, whereas most oxide and hydroxide sols are positive. There are some exceptions, eg. colloidal silica is negative. Colloids are most stable in cool, dilute solutions and in the presence of a second (protective) colloid. Eg. colloidal gold is stable below 150oC and coagulates between 150-250oC. In the presence of colloidal silica this colloidal gold is stable upto 350oC.
It is difficult to explain the colloidal migration of metals in depth because the rocks at depth are dense and relatively impermeable. Some geologists suggest that ore fluids change from solutions at depth to colloidal sols in the near surface environments.
Precipitation from colloids involves two distinct stages:
1. Nucleation (the formation of centres of crystallisation), and
2. Crystal growth
Leaving aside the question of stability, it is the relative rates of these processes, which determine the particle size of the precipitate so formed. A high degree of dispersion and large number of crystals is obtained when the rate of nucleation is high and the rate of crystal growth is low.
The initial rate of nucleation depends on the degree of supersaturation, which can be reached before phase separation occurs, so that colloidal sols are most easily obtained when the substance in question has a very low solubility. With material as soluble as, for example, calcium carbonate, there is a tendency for the smaller particles to dissolve and recrystallize on the larger particles as the precipitate is allowed to age.
The rate of particle growth depends mainly on the following factors:
1. The amount of material available.
2. The viscosity of the medium, which controls the rate of diffusion of material to the particle surface.
3. The ease with which the material is correctly orientated and incorporated into the crystal lattice of the particle.
4. Adsorption of impurities on the particle surface, which act as growth inhibitors.
5. Particle-particle aggregation.
Evidence of precipitation from colloids is furnished by certain minerals which form colloform textures that are indicative of flocculation from a sol. Colloform textures have the following characteristics:
1. Colloform textures occur in a series of concentrically curved or scalloped layers in which the curvature is always convex towards the younger or free surface.
2. The free surface is botryoidal, reniform or stalactitic.
3. Colloform textures are typically exhibited by agate, chalcedony, malachite, azurite, collophane, manganese oxides, spodument, wurtzite, lead and zinc ores, pyrite, cassiterite and wolframite.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)