Origin of ore fluids

Ore fluids may be generated in a variety of ways. The principal types include (1) sea water, (2) meteoric waters and (3) juvenile water, all of which have a strictly defined isotopic composition. All other possible types of ore fluids such as formation, meta-morphic, and magmatic waters can be considered recycled derivatives or mixtures from one or more of the three reference waters (see Fig. 3.11). 

The isotopic composition of present day ocean water is more or less constant with 5-values close to 0%c. The isotopic composition of ancient ocean water is less well constrained, but stiU should not be removed from 0 by more than 1 or 2%c. Many volcanogenic massive sulfide deposits are formed in submarine environments from heated oceanic waters. This concept gains support from the recently observed hydrothermal systems at ocean ridges, where measured isotopic compositions of fluids are only slightly modified relative to 0%c. 8 0 and 5D-values of vent fluids are best understood in terms of sea water interaction with the ocean crust (Shanks 2001). 

Bowers and Taylor (1985) have modeled the isotopic composition of an evolving sea water hydrothermal system. At low temperatures, the 5 0-value of the fluid decreases relative to ocean water because the alteration products in the oceanic crust are 0-rich. At around 250°C, the solution returns to its initial sea water isotopic 

Heated meteoric waters are a major constituent of ore-forming fluids in many ore deposits and may become dominant during the latest stages of ore deposition. The latter has been documented for many porphyry skam-type deposits. The isotopic variations observed for several Tertiary North American deposits vary systematic with latitude and, hence, palaeo-meteoric water composition (Sheppard et al. 1971). The ore-forming fluid has commonly been shifted in 0-isotope composition from its meteoric 5 0-value to higher 0 contents through water-rock interaction. Meteoric waters may become dominant in epithermal gold deposits and other vein and replacement deposits. 

The concept of juvenile water has influenced early discussions about ore genesis tremendously. The terms juvenile water and magmatic water have been used synonymously sometimes, but they are not exactly the same. Juvenile water originates from degassing of the mantle and has never existed as surface water. Magmatic water is a non-genetic term and simply means a water that has equilibrated with a magma. 

Origin of ore fluids is constrained by (1) chemical compositions of ore fluids estimated by thermochemical calculations (section 1.3.2) and by fluid inclusion analyses, (2) isotopic compositions of ore fluids estimated by the analyses of minerals and fluid inclusions (section 1.3.3), (3) seawater-rock interaction experiments, (4) computer calculations on the seawater-rock interaction, and (5) comparison of chemical features of Kuroko ore fluids with those of present-day hydrothermal solutions venting from seafloor (section 2.3). 

During the last two decades, many experimental studies on the seawater-rock interaction at elevated temperatures (100-400°C) have been conducted. Particularly, detailed seawater-basalt interaction experiments have been done. Several experimental studies on seawater-rhyolite interaction and seawater-sedimentary rock interaction are also available (Bischoff et al., 1981). Examples of chemical compositions of modified seawater experimentally interacted with various kinds of rocks are shown in Table 1.9. 

Several factors such as Cl concentration, water/rock ratio and temperature are important in controlling the chemical composition of the hydrothermal solution interacted with the rocks. For example, water/rock ratio affects the alteration mineralogy (Mottl and Holland, 1978 Seyfried and Mottl, 1982 Shikazono, 1984). For example, at low water/rock ratio, epidote is stable, while chlorite at high water/rock ratio (Shikazono, 1984 Shikazono and Kawahata, 1987). 

In Fig. 1.59 the relationship between temperature and concentration of elements (Zn, Ba) at constant Cl concentration which is equal to that of seawater obtained by the experimental studies and analytical data on natural hydrothermal solution (geothermal water) are shown. It is seen that the concentrations of base-metal elements (Zn, Fe, Mn, Cu, Pb) and Ba increase with increasing of temperature. Concentrations of these 

Isotopic compositions of Kuroko ore solution (K.O.), seawater (S.W.) and magmatic water (M.W.) (Shikazono, 1978) 

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These detailed studies on individual mine district suggest that carbon in carbonates was derived from the country rocks underlying the ore deposits and oxygen in ore fluids is controlled by origin of ore fluids (mostly meteoric water) and boiling of ore fluids. 

Based on these summaries, the formation mechanism of ore deposits and origin of ore fluids were considered. Mixing of ascending hydrothermal solution and ambient cold water (seawater, groundwater) is considered to be an important depositional mechanism. 

Characteristic features of these deposit types also indicate that geologic and tectonic environments affect the origin of ore fluids and formations of these deposits. 

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... 

Numerous geochemical data (fluid inclusions, stable isotopes, minor elements) on the epithermal vein-type deposits in Japan are available and these data can be used to constrain geochemical environment of ore deposition (gas fugacity, temperature, chemical compositions of ore fluids, etc.) and origin of ore deposits. 

These deposits are characterized by polymetallic (Cu, Pb, Zn, Au, Ag, etc.) mineralization and formation in extensional stress fields. Ore fluids responsible for these ore deposits are dominated by seawater origin, considering isotopic and chemical composition of ore fluids. 

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 ... 

Munoz M., Boyce A. L, Courjault-Rade P., Fallick A. E., and Tohon E. (1991) Continental basinal origin of ore veins from southwestern Massif Central fluorite veins (Albigeois, Erance) evidence for fluid inclusion and stable isotope analysis. Appl. Geochem. 14, 447-458. 

Shelton KL (1983) Composition and origin of ore-forming flnids in a earbonate-hosted porphyry eopper and skam deposit A fluid inelusion and stable isotope study of Mines Gaspe, (Jnebee. Eeon Geol 78 387-421... 

There is another opinion on the origin of Kuroko ore fluids. Sawkins (1982) thought that intrusive felsic magmas were the source of the metals and heat in Kuroko hydrothermal systems. He stressed the contributions of magmatic fluid and seawater in... 

High salinity of Kuroko ore fluids does not solely mean magmatic contribution. Instead salinity variation can be reasonably explained by subcritical boiling of fluids of seawater origin. 

However, it cannot be decided at present which processes (degree of seawater-rock interaction or mixing ratio of seawater, igneous water and meteoric water) are important for the generation of Kuroko ore fluids solely from the isotopic studies. But experimental and theoretical considerations on seawater-volcanic rocks interaction and origin of hydrothermal solution at midoceanic ridges suggest that Kuroko ore fluids can be produced dominantly by seawater-volcanic rock interaction. 

Watanabe, M., Hattori, K. and Sakai, H. (1976) Origin of the ore-forming fluids from hydrogen isotopic ratio. Mining Geology Special Issue, 7, lOl-116 (in Japanese). 

This submarine vs. subaerial hypothesis for the origin of the two types of deposits (Kuroko deposits, epithermal vein-type deposits) can reasonably explain the difference in metals enriched into the deposits by HSAB (hard-soft acids and bases) principle proposed by Pearson (1963) (Shikazono and Shimizu, 1992). Relatively hard elements (base metal elements such as Cu, Pb, Zn, Mn, Fe) are extracted by chloride-rich fluids of seawater origin, while soft elements (Au, Ag, Hg, Tl, etc.) are not. Hard elements tend to form chloro complexes in the chloride-rich fluid, while soft elements form the complexes in H2S-rich and chloride-poor fluids. Cl in ore fluids is thought to have been derived from seawater trapped in the submarine volcanic and sedimentary rocks. 

Because little mass can precipitate from it, the brine, if related to deposition of the metalliferous muds, is likely to be a residuum of the original ore fluid. As it discharged into the deep, the ore fluid was richer in metals than in reduced sulfur. Mineral precipitation depleted the fluid of nearly all of its reduced sulfur without exhausting the metals, leaving the metal-rich brine observed in the deep. 

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). 

Studies of hydrothermal alteration products associated with ore mineralization in acidic rocks have established the general propensity for the original minerals to be replaced by illite, sericite or hydromica in the innermost zone near the source of hydrothermal fluids and by kaolinite or expandable minerals further from the vein or center of fluid emanation. The newly-formed "mica" can be 2M, 1M, or lMd in polymorph and range compositionally from muscovite to a low potassium, silicic species which can be assimilated in the term illite (Lowell and Guilbert, 1970 Schoen and White, 1966, 1965 Kelly and Kerr, 1957 Bonorino, 1959 Tomita, e al., 1969 Yoder and Eugster, 1955 Meyer and Hemley, 1959, among many authors). 

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