HYDROTHERMAL ORE DEPOSITS

 

HYDROTHERMAL ORE DEPOSITS

 

 

                                             Figure 1.1 HYDROTHERMAL PROCESS

 

 Concentration by hot aqueous (water-rich) fluids flowing through fractures and pore spaces in rocks.

 The fluid temperature ranges from 50 to 500 degree C. These fluid can be the final product of volcanic differentiation or the ground water got heated by contact of magma or chamber.

 Hydrothermal deposits are produced when groundwater circulates to depth and heats up either by coming near to the hot igneous body at depth or by circulating to great depth along the geothermal gradient.

 Such hot water can dissolve valuable substances throughout a large volume of rock. As the hot water moves into cooler areas of the crust, the dissolved substances are precipitated from the hot water solution. If the cooling takes place rapidly,

 They might occur in open fractures or upon reaching a body of cool surface water, then precipitation will take place over a limited area, resulting in a concentration of the substance attaining a higher value than was originally present in the rocks through which the water passed.

  Examples :-

 1.  Massive sulphide deposits at oceanic spreading centers . Hot fluids circulating above the magma chambers at oceanic ridges can scavenge elements like Sulphur, Copper, and Zinc from the rocks through which they pass.

 2.   As these hot fluids migrate back toward the seafloor, they come in contact with cold groundwater or sea water and suddenly precipitate these metals as sulfide minerals like sphalerite (zinc sulphide) and chalcopyrite (Copper, Iron sulphide).

 3.   Vein deposits surrounding igneous intrusions. Hot water circulating around igneous intrusions scavenges metals and silica from both the intrusions and the surrounding rock.

 4.  When these fluids are injected into open fractures, they cools rapidly and precipitated mainly as quartz, but also a variety of sulphide minerals, and sometimes gold, and silver within the veins of quartz.

 5.   Rich deposits of copper,zinc, lead, gold, silver, tin, mercury, and molybdenum result.

 6.   Strata bound ore deposits in lake or oceanic sediments. When hot groundwater containing valuable metals scavenged along their flow paths enters unconsolidated sediments on the bottom of a lake or ocean, it may precipitate ore minerals in the pore spaces between grains in the sediment.

 7.   Such minerals may contain high concentrations of lead, zinc, and copper, usually in sulphide minerals like galena (lead sulphide), sphalerite (zinc sulphide), and chalcopyrite (copper-iron sulphide).

 Since they are included within the sedimentary strata they are called strata bound mineral deposits.

 Prerequisites :-

 The prerequisites for hydrothermal deposits are  :-

 (i) The availability of enough metal content in the hydrothermal solution,

 (ii) Presence of solution capable of dissolving and transporting mineral matter,

 (iii) Available openings in the rocks permitting movement of hydro thermal solution from the source to the site of deposition,

 (iv) chemical reaction causing deposition of ore.

 Openings :-

          Pore spaces, crystal lattices, bedding planes, vesicles, cooling cracks, breccias, fissures, shear zones, foldings and warpings, volcanic pipes, solution and rock alterations etc. are the various types of openings in the rocks permitting movement of solution or deposition of ore-minerals.

 For large deposits vast quantities of solution and fairly large confined channel ways are needed. The flow of solution must be confined to avoid dispersal of mineral matter. Fissures, shears and permeable beds may provide confined ways

Wide spread permeability and the mineralising solution are spread over a large area which result in dispersed ore.

Crystal lattices permit diffusion which is a slow process and may not generate large deposits.

Host Rock :-

        Reactive host rocks like carbonate rocks, greenstones etc. are congenial to ore deposition, particularly in the case of replacement deposits.

  Deposition :-

         The deposition from hydrothermal solutions is influenced due to chemical changes in solution, reactions between solution and wall rocks or vein matter and changes in temperature and pressure. The reaction between mineralising solution and wall rock gives rise to chemical changes, accompanied by deposition. The solution in its journey loses temperature and pressure which decreases solubility and promotes precipitation. The heat loss is also influenced by nature of opening. Open fissure with straight wall would cause less heat loss than the intricate openings of breccia with large exposed area.

 paragenesis :-

            In the formation of ore-minerals, the individual minerals are formed in orderly sequence with quartz coming first, followed by iron sulphides or arsenides, sphalerite, enargite, chalcopyrite, bornite, galena, gold and silver minerals. This sequential arrangement is called paragenesis.

  Wall Rock Alteration :-

     The wall-rock alteration is quite common in case of hydrothermal deposits. The nature of mineralising solution like its chemical

 Control of Ore Localisation :-

   The ore localization is controlled by the following factors :

              (i) Chemical and physical characters of host rock.

              (ii) Structural features.

             (iii) Intrusives

Cavity filling and replacement are the two types of deposits formed due to hydrothermal processes.  In general, replacement deposits are formed at high temperature and cavity filling in low temperature.Cavity filling is due to deposition of minerals in various types of openings, while in metasomatic replacement replacement deposit the earlier formed mineral is replaced by the new mineral.

  1. CAVITY FILLING :-

                  The precipitation of minerals from mineralizing solution in the cavities or the open spaces in rock forms cavity filling deposit. The walls of the cavity are lined first by the first mineral to be deposited. The minerals usually grow inward with development of crystal faces pointed towards the supplying solution in the form of comb structure.

  Successive crusts of different minerals may be precipitated upon the first one until filling is complete. This gives rise to crustification and if the cavity is a fissure, a crustified vein is formed. Symmetrical crust. May result with similar precipitation on both the walls of the vein, and asymmetrical with unlike crustification on each side. In case of breccia, the crusts surround the breccia and cockade ore is formed.

 The cavity filling may also give rise to ribbon structure with narrow layers of quartz separated by thin dark seams of altered wall rock. The following types of deposits may the

 (1) Fissure vein :-

                    It is a tabular type of deposit, involving formation of fissure itself by stresses operating within earth's crust, and ore forming processes. These fissure veins may be massive or crustified. They may be simple, composite, linked, sheeted, dilated and chambered. They may be vertical or inclined. Pinches and swells produced by movement along irregular fissures may occur. Several minerals, both ore and gangue, may fill in the fissure. Fissures may occur in groups, and may have formed at the same time or may be of different ages. The depth of fissure veins is quite variable. Some of them continue to depth of several thousand metres like those at Kolar gold mine.

 (ii) Shear zone deposit :-

                      A shear zone with sheet like connected openings, and large exposed surfaces serves as excellent channel ways for mineralising solutions and precipitation takes place as thin plates of minerals or in the form of fine grains, e.g Singhbhum shear zone deposit.

 (iii) Stockwork :-

                      It signifies a network of small ore bearing vein lets and stringers traversing a mass of rock. The vein lets show crustification, comb-structure and druses, and represent open space fillings.

 (iv) Saddie reef :- 

                   The disseminated deposits represent multiple centres of replacement and consist of altered host rock and disseminated ore grains e.g. porphyry copper deposits. The mineralising solution spreads in the host rock and gives rise to scattered specks, grains, blebs and small vein lets of ore scattered within the host rock. The boundaries between the ore and the host rock is vague and gradational. This type of deposit being of low grade and huge, requires large scale mining.

    The replacement lodes may be massive or high-grade ore flanked by a fringe of disseminated ore. Here, replacement may start from a fissure with bold front or some prominent centres followed by replacement at many small centres. Like fissure veins, they occurs as replacing walls of fissures or thin beds. The outlines of the ore body may be wavy, irregular and gradational with the country rock.

 Criteria for Identification of Replacement Deposits :-

 The various criteria for recognition of replacement deposits are as follows:

  (a) Presence of unsupported residuals of host rocks: The country rock that escaped replacement remained as isolated body with the ore-mass. The residual rock may show bedding or other structural features which are in conformity with those in the wall rock. This constitutes supporting evidence of replacement.

  (b). Preservation of rock structures: The structure of the replaced body is sometimes faithfully retained in the ore. This constitutes the conclusive evidence of replacement. Such inherited structural elements may be stratification, cross bedding, schistosity, folding, faults, or joints. Fossils and dolomitization rhombs are also sometimes preserved.

 (c) Doubly terminated crystals of ore, transecting rock grains of the... enclosing rock Such crystals may be microscopic or megascopic in size

  (d) Absence of crustification: This is also one of the criterias to recognise replacement deposits

  (f) Intersection of diversely oriented host crystals: Small wavy veins of irregular width which transect host rock indicate replacement.

  (g) Irregular outlines of the ore: The wavy outlines of the ore with protuberances and embayments into the host rock and extreme irregularity are indicative of replacement.

  (h) Mineral pseudomorph : A mineral after another of different. composition is typical of replacement, e.g. chalcecite may replace pyrite cubes and form pseudomorphs.