Ore Deposits and Hydrothermal Systems

Stable isotopes have become an integral part of ore deposits studies. The determination of light isotopes of H, C, O, and S can provide information about the diverse origins of ore fluids, about temperatures of mineralfration and about physico-chemical conditions of mineral deposition. In contrast to early views, which assumed that almost all metal deposits owed their genesis to magmas, stable isotope investigations 

In as much as water is the dominant constituent of ore-forming fluids, knowledge of its origin is fundamental to any theory of ore genesis. There are two ways for determining 5D- and 5 0-values of ore fluids  

By direct measurement of fluid inclusions contained within hydrothermal minerals 

By analysis of hydroxyl-bearing minerals and calculation of the isotopic composition of fluids from known temperature-dependent mineral-water fractionations, assuming that minerals were precipitated from solutions under conditions of isotope equilibrium. 

The mineral alunite, and its iron equivalent jarosite, is a special case. Alunite (KAl2(S02)2(OH)2) contains four sites where elements containing stable isotopes are found and both the sulfate and hydroxyl anionic groups may provide information on fluid source and condition of formation. 

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Helgeson, H. C., 1970, A chemical and thermodynamic model of ore deposition in hydrothermal systems. Mineralogical Society of America Special Paper 3, 155-186. 

Epithermal ore deposits are hydrothermal deposits that form at shallow crustal levels. A wide spectrum of ore deposits of a different nature occurs in this category. Typical temperatures of mineralization range from 150 to 350°C with variable salinities. Individnal deposits often reveal that more than one type of fluid was involved in the formation of a single ore deposit. One of the fluids involved often appears to be of meteoric origin. In many deposits different fluids were alternatively discharged into the vein system and promoted the precipitation of a specific suite of minerals, snch as one fluid precipitating snlfides and another precipitating carbonates (Ohmoto 1986). 

The transition elements are useful tracers in many geological systems. They are industrially important and form economic ores, especially in hydrothermal systems where they are often present as sulfide minerals. Cd, Hg, Zn, and Pb are persistent industrial pollutants and determination of low levels of these elements in ores and fossil fuels is critical as processing of ores or burning of fuels may concentrate and release toxic elements. The concentration of such toxic trace elements may affect the economic value of an ore or fossil fuel deposit significantly. 

There are many types of hydrothermal system and associated hydrothermal ore deposits. Formation of hydrothermal ore deposits and mass transfer in seafloor hydrothermal system will be considered below. 

Hydrothermal system at discharge zone is composed of five reservoirs such as ore deposit/zone IV boundary, zone III/II boundary, zone II/I boundary, zone I/fresh country rocks boundary and temperature of each reservoir is 250°C, 220°C, 150°C, 100°C, and 25 °C, respectively. 

Giggenbach, W.F. (1997) The origin and evolution of fluids in magmatic-hydrothermal systems. In Barnes, H.L. (ed.). Geochemistry of Hydrothermal Ore Deposits. New York John Wiley and Sons, pp. 699-... 

White, D.E, (1981) Active geothermal systems and hydrothermal ore deposits. Econ. Geol, 75th Anniv. Vol., 392-423. 

In and near the Japanese Islands many Neogene hydrothermal ore deposits have been formed from the middle Miocene to the present time, and many subaerial active geothermal systems occur. Some of them are associated with base-metal (Cu, Zn, Pb, Fe, Mn) and precious-metal (Au, Ag) mineralizations. 

Constraints on Li isotopic fractionation from equilibrium laboratory experiments in magmatic-hydrothermal systems have been examined in only one instance (Lynton 2003). This study examined Li in a quartz-muscovite-aqueous fluid system at conditions of formation of magmatic hydrothermal porphyry-type deposits (400-500°C, 100 MPa). In the presence of a fluid containing L-SVEC (5T i = 0), quartz showed rapid shift from 5 Li = +27 in the starting material to c. +10 at both run temperatures. Muscovite (initial 5 Li = +9), shifted more sharply at 500°C (to c. +20) than at 400°C (to c. +13). Although the results are difficult to put directly into the context of a natural mineral deposit, they do indicate that over geologically relevant time scales, minerals in magmatic-hydrothermal systems should show appreciable Li isotopic fractionation, and that this may permit the composition and/or temperature of ore... 

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