Ore Deposits Formed by Replacement

Replacement is the process of almost simultaneous solution and deposition whereby one mineral replaces another. It is an important process in the formation of epigenetic (those formed after the formation of the host rock) mineral deposits, in the formation of high- and intermediate-temperature hydrothermal ore deposits, and in supergene sulfide enrichment (enriched by generally downward moving fluids). Replacement is the method whereby wood petrifies (silica replaces the wood fibres), one mineral forms a pseudomorph of another, or an ore body takes the place of an equal volume of rock.  Hence replacement deposits are those in which solutions have reacted with host rocks, leading to replacement of silicates and carbonates by new gangue and ore minerals.

Replacement occurs when a mineralizing solution encounters minerals unstable in its presence. The original mineral is dissolved and almost simultaneously exchanged for another. The exchange does not occur molecule for molecule, but volume for volume.  This means that  fewer molecules of a less dense mineral will be replaced by more molecules of a denser mineral.  Replacement takes place first along major channels in a host rock through which the hydrothermal fluids enter it.  Replacement along smaller openings follows later.  Eventually, rocks or at least some of their constituent minerals are replaced even along capillary sized openings.  Replacement occurs even in those parts of the rock where fluids cannot flow.  This happens by diffusion of ions at the replacement front.  

Early-formed replacement minerals are themselves replaced by later minerals, and definite mineral successions have been established. The usual sequence among the common hypogene (deposited by generally ascending solutions) metallic sulfide minerals is pyrite, enargite, tetrahedrite, sphalerite, chalcopyrite, bornite, galena, and pyrargyrite.

Although replacement can occur at any temperature or pressure, it is most effective at elevated temperatures, at which chemical activity is enhanced. Replacement by cold circulating waters is mostly confined to soluble rocks, such as limestone. These may be replaced by iron oxides, manganese oxides, or calcium phosphates. 

Many ore deposits have been enriched in terms of their metal content where supergene (downward moving) fluids have dissolved metallic minerals from the near-surface parts and redeposited these in the lower parts by replacement of other (gangue) minerals.  At higher temperatures the degree of replacement of preexisting minerals increases, till in extreme cases, there is complete replacement.  Mineralizing solutions at intermediate temperatures form simple sulfides and sulfosalts while those at higher temperatures form sulfides and oxides. Replacement deposits are the largest and most valuable of all metallic ore deposits except those of iron.  Large deposits of lead-zinc deposits have formed where carbonates rocks (limestone, marl, dolomite) have been replaced.  The orebodies range in size from 0.5 million tonnes to 20 million tonnes with over 14 % combined metal content.  Replacement deposits commonly show an envelope of weak mineralization around the ore, giving a larger dispersion pattern and therefore a larger exploration target.  The dispersion is controlled by degree of disequilibrium between fluids and host rocks and level of diffusion/interaction.  This also gives rise to zoning of replacement deposits.

Replacement deposits commonly occur as massive (containing more than 50 % sulfides) lenses, pipes, veins and disseminations often associated with intrusives. 

In general, it has been observed that certain minerals replace others preferentially. Accordingly, a set of "rules" has been proposed:

  1. Sulfides replace gangue or ore minerals

  2. Gangue minerals replace host rock, but not the ore minerals

  3. Oxides replace host rock and gangue, but rarely replace sulfides.

Evidence of Replacment:

  1. Pseudomorphs.

  2. Widening of fractures

  3. Vermicular unoriented intergrowths

  4. Islands (of the host or replaced mineral) having the same optical orientation and surrounded by the new mineral

  5. Relicts

  6. Cusp and caries texture: (host or replaced mineral). Cusps are relict protuberances of the replaced mineral or host rock between “caries”. The caries are embayed surfaces concave towards the replacing mineral into the replaced one.

  7. Non-matching walls of a fracture. This is a feature common when replacement works outward from a central fissure (compare with open space filling textures) (Fig. 6).

  8. The occurrence of one mineral crosscutting older structures.

  9. Selective association: Since replacement is a chemical process, specific selective associations of pairs or combinations of minerals can be expected. For example, chalcopyrite is more likely to replace bornite by a change in the Cu/Fe ratio or in fS2 than it is to replace quartz. Therefore, the occurrence of minerals with some chemical similarity in some textural relationship is often a good indication of replacement.

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