Ore-forming processes

Some applications of the coupled fluid flow-reaction model were carried out to the ore-forming process (e.g., Lichtner and Biino, 1992). However, a few attempts to understand quantitatively the precipitations of minerals from flowing supersaturated fluids in the submarine hydrothermal systems have been done (Wells and Ghiorso, 1991). Wells and Ghiorso (1991) discussed the silica behavior in midoceanic ridge hydrothermal system below the seafloor using a coupled fluid flow-reaction model. 

Se is concentrated in certain ore deposits, such as roll-front U complexes and Carlin-type gold deposits. Roll-front U deposits occur where U(Vl)-bearing groundwater migrates into reducing zones which precipitate both U(TV) and Se(0) or Se(-II). It may be possible to constrain Se sources and/or ore-forming processes using Se isotope variations. 

In conclusion, experimental and theoretical thermodynamic and geochemical data are compared and possible factors causing the formation of concentrations of iron and silica sediments in various conditions, including the specific setting of the Precambrian, are examined. The main purpose of this was not to create any universal hypothesis of the formation of the Precambrian BIF, but to determine the conditions which, when fulfilled, make the ore-forming process possible. 

Therefore the formation of magnetite in that way could hardly be of essential importance in the metamorphism of iron-formations, and martitiza-tion is still less hkely. However, in deposits of other genetic types, for instance skam deposits, oxidation of iron silicates to magnetite at the contact with large masses of carbonate rocks (dolomite, magnesite) can be considered an ore-forming process. The last conclusion is still feasible because the carbon dioxide released in the dissociation of carbonates probably had an undisturbed CO O2 ratio. 

Hemley J. J. and Hunt J. P. (1992) Hydrothermal ore-forming processes in the tight of studies in rock-buffered systems 11. Some general geologic applications. Econ. Geol. 87, 23-43. 

Mossman D. J. and Farrow C. E. G. (1992) Paleosol and ore-forming processes in the Elliot Lake district of Canada. In Early Organic Evolution Implications for Energy and Mineral Resources (eds. M. Schidlowski, S. Golubic, M. M. Kimberley, D. M. McKirdy, and P. A. Trudinger). Springer, Berlin, pp. 67-76. 

In summary, there is no evidence that the higher volatility of Hg relative to the target elements, Pb and Zn, has resulted in its wider dispersion during the ore-forming processes of the sediment-hosted massive sulphide deposits described here. 

Fig. 10.2. The bimodal curve represents the probable distribution of a geochemically scarce metal in the earth s crust. The large peak is the distribution in common rocks, where scarce metals occur not as separate minerals but as atomic substitutes for abundant metals the small peak represents deposits produced by ore-forming processes such as those resulting from the circulation of brines in the crust. Current mining has already reached the point where the distribution curve for easily accessible scarce minerals turns downward. Further reductions in the grade of ore mined will produce declining tonnages of ore and will eventually bring us to a mineralogical barrier — which lies somewhere in the area between the two humps.

On the very crest of the East Pacific Rise, in equatorial latitudes—particularly 12° to 16°S, the sediments are enriched in Fe, Mn, Cu, Cr, Ni, and Pb. The correlation of these areas of enrichment to areas of high heat flow is marked. It is believed that these precipitates are caused by ascending solutions of deep-seated origin, which are probably related to magmatic processes at depth. The Rise is considered to be a zone of exhalation from the mantle of the earth, and these emanations could serve as the original enrichment in certain ore-forming processes. ... 

Hemley, J.J., Cygan, G.L., Fein, J.B., Robinson, G.R. and D Angelo, W.M., 1992. Hydrothermal ore - forming processes in the light of studies in the rock-buffered systems 1. Iron-copper-zinc-lead sulfide solubility relations. Economic Geology, 87 1-22. 

An example of the application of the granite model described here to uranium province formation in the Archaean-Early Proterozoic is presented below to illustrate the potential of this approach for the general understanding of ore-forming processes and genetic modelling in mineral exploration. 

The evidence reviewed above suggests that cystine, cysteine, mercaptocarboxylic acids, and other organosulfur ligands may contribute to the speciation of metals and sulfur in ore-forming processes responsible for syngenetic deposition of metal sulfides in low-temperature subaqueous... 

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