Geological Indicators or Guides
i) Geological Indicators point to the presence of mineralization in the area- float, and black sands with ore minerals, zones of wallrock alteration containing economic minerals, as well as oxidized and leached outcrops, primary and secondary dispersion haloes of host rock ore minerals and elements.
ii) Primary Dispersion Haloes represent the entire complex of wall rock alterations and accumulations in the host rock linked with the concentration of valuable elements during endogenous and exogenous mineralization. Their role is particularly important in the search for buried deposits. Their development is governed by the same laws of endogenous and exogenous deposits. Endogenous processes result not only in the formation of ore bodies and disseminated mineralization around the ore, but also in various host rock alterations under the action of hydrothermal solutions.
iii) The intensity and scale of ore concentration and wall rock alteration depend primarily on the structure of the host rock, ie.,
- on the type of deformation structure,the character of the contacts and porosity,fracturing,texture and structure ,
chemical reactivity or invertness (dispersion haloes are less extensive for example, in carbonate rocks) etc.
In structures of a single ore type the vertical range of the primary halo usually increases with mass of metal in the deposit.
iv) Together with the principal ore elements, their related minerals and associated elements (indicators of mineralization) are equally well expressed in haloes, usually as sulphides, silicates and carbonates. All appear in the same quantity ratio as the ore body, but the haloes of associated elements are often much wider than those of the principal constituents of the deposit. A definite correlation has been observed between, Pb, Zn,and Bi; between Ba, Sr, Hg, and Cu; and between Mo and Cu, and between other elements.
v) The prospector should study vertical zonality (with emphasis on the distribution of Li, and particularly of Na and K) in the development of albitization and griesanization zones Where ore forming and associated elements especially of mobile elements such as Hg, As, F etc. concentrate in the upper part of the halo are the most promising.
vi) Study of primary dispersion haloes are expected to help prospectors in the future to decipher and take practical advantage of the zonal distribution of ore deposition and of the depth of the erosion section. Elements (Pb, Zn, Cu, Mo) with affinity for S wider and much longer dispersion haloes than those with affinity for O (Sn, W, Zr, Th)
vii) It has been established that mercury haloes develop above various metallic deposits, particularly above copper pyrites; As haloes are known over many complex ore deposits; Mn and B haloes are observed above some Sn deposits and Haloes of germanium over others (topazes and axinites)
viii) Primary dispersion haloes originating from endogenous processes probably represent the combined effect of two components; syngenetic concentration which is characteristic of host rocks, and epigenetic accumulation superposed on the host rocks in the process of mineralization.
ix) Hydrothermally altered zones are both indicators of mineralization and carriers of primary haloes. Compared with hydrothermally altered rocks these primary haloes show a more intimate spatial and time relationship with the ore deposits.
x) Secondary Dispersion Haloes develop under exogenous conditions. It is sometimes difficult to distinguish between primary and secondary dispersion haloes as the primary dispersion haloes undergo the same secondary alteration as the ore bodies themselves; for this reason haloes in the supergene zone are probably of mixed origin.
xi) Secondary haloes are almost always greater in area than primary ones due to more intensive migration of elements,
impregnation of host rocks, which had earlier contained these elements only in quantities corresponding to their general percentage in the earth crust.
The dimensions of secondary dispersion haloes largely depend on the degree of erosion. Huge deposits often have large secondary haloes.
Cu, Ni, Co, Mo, U, Ra, Zn, and certain other elements are relatively mobile under supergene conditions.
On the one side they abut on the even more mobile elements- K, Na, Ca, Mg, etc., and on the other side on those of relatively low mobility like Fe+3, Ti, Al, Zr, Hf, Ta, Pt, Au, Sn, W, Hg etc.
Comparison of the migration propensities of these three groups indicate that the most mobile elements have a water migration factor.
xii) Schematically, the formation of secondary haloes may be represented as follows:
Physical chemical and biogenic weathering disintegrates the rocks
A part of the atmospheric water, in the form of flows pours into depression while another part seeps into fissures and rock pores and reaches the water table
A third very small part is absorbed by the roots of the plants
Dissolved elements migrate in all directions (in surface streams they are present as silt and small fragments), appearing in higher concentrations in streams, rivers, ground water and the rocks lying above the water table
finally the plant fissures
xiii) Secondary dispersion haloes in soils are very important for practical purposes. The metal content of soils depends on the chemical and mineral composition of the soil forming rocks and the geographical conditions in which the soil process has taken place.