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Bogie I., Lawless J.V., Rychagov S. and Belousov V.


CLASSIFICATION OF THE TYPES OF GEOTHERMAL SYSTEMS AND THEIR ORE MINERALIZATION Bogie I. 1, Lawless J.V. 1, Rychagov S. 2 and Belousov V. 2 1 Sinclair Knight Merz Limited, Auckland, New Zealand, E-mail: Jlawless@skm.co.nz 2 Institute of Volcanology and Seismology FED RAS, Petropavlovsk-Kamchatsky, Russia, E-mail: rychsn@kcs.iks.ru Magmatic arc-related hydrothermal systems form where both the heat that drives them and some proportion of their fluid constituents are derived from solidifying magmatic intrusive bodies. A range of types of active systems can form due to variation in the depth and age of the magmatic body and variations in the near-surface hydrological environment.

Subaerial hydrothermal systems can be broadly divided into basinal systems and stratovolcano types. A third type, giant vapour dominated systems, are rare. Intrusives tend to be deep in basinal systems with limited variation in the type of system in this setting. Stratovolcanoes can have much shallower intrusions and can evolve over time to produce a range of active systems. Submarine hydrothermal systems have been divided into arc/back arc and spreading centre systems.

There is potential for Cu-Au porphyry-style mineralisation to be directly associated with the intrusives causing hydrothermal systems in arc type settings. Basinal systems and the associated bimodal volcanism, can produce low-sulphidation epithermal deposits. Young stratovolcanic systems with shallow intrusives can produce high-sulphidation epithermal deposits. Older stratovolcano systems, or those with deeper intrusives, can form intermediate-sulphidation epithermal deposits. Submarine back arc/arc systems produce Kuroko-style deposits and spreading centres produce Cyprus-style deposits.

1. Introduction Both active and extinct hydrothermal systems can be identified either by current thermal activity or the effects past thermal activity has had on the host rocks. Many of these systems have close spatial associations with magmatic activity and contain evidence, usually of an isotopic nature (Giggenbach, 1992), of their derivation from fluids that contain at least a component derived from magmatic activity. Parts of older systems (and more rarely some young systems) may contain economic hydrothermal ore deposits (Hedenquist et al., 1996; Corbett, Leach, 1998). All these systems can be grouped together as magmatic-related hydrothermal systems (MRHS). Other tectonicrelated hydrothermal systems without such a direct magmatic connection can also host ore deposits, but of different types (for example orogenic gold deposits) and are not further addressed herein.

2. Overview of system types Since currently active MRHS are intact with clearly identified tectonic and hydrological settings and there is potential for more than one type of ore deposit to form in some hydrothermal systems, any genetic classification scheme should be based on 1. , active systems.

A problem arises however in that there is a disproportionate amount of information available from the active hydrothermal systems around the world upon which to base a classification, with much more information available from subaerial systems developed for power generation. This introduces a bias against submarine systems and those types of subaerial system that are not developable for geothermal energy production, most notably those with magmatic solfataras which, of any of the systems, are most obviously magmatically-related.

A further bias arises through the greater degree of development of systems in certain countries for historical reasons. Some countries with a large amount of information available from geothermal energy drilling, for example Italy, have highenthalpy geothermal systems in an unusual or unique tectonic environment (continental collision zone/back arc). Bearing in mind these problems and that these developed systems are estimated to reflect less than 10 % of subaerial geothermal systems. Table presents a compilation of all the high-enthalpy geothermal systems developed for geothermal energy (up to 2004) divided up into their tectonic setting. If this compilation can be taken to reflect the worldwide distribution of all subaerial MRHS, arc stratovolcanic settings predominate.

Table 1: Tectonic settings of developed high-enthalpy magmatic-related systems (to 2004) Tectonic Setting # % Continental collision zone 3 Continental rift 2 Oceanic rift 7 Hot spot 1 Arc basin 7 Arc stratovolcano 41 Back arc 5 Transform basin 5 Total 71 The vast majority of magmatic solfataras occur in andesitic stratovolcanoes and while the proportion of andesitic stratovolcanoes with magmatic solfataras to those that host developable geothermal systems is not accurately known, there is possibly a similar number of both. If this is the case, subaerial MRHS are overwhelmingly found in andesitic stratovolcanoes. However the case of Mt Sabalan in Iran, that has recently Bogie I., Lawless J.V., Rychagov S. and Belousov V.

been successfully drilled (Bogie et al., 2005), shows that they occur in intra-plate trachyandesite stratovolcanoes as well.

Hence, no other subaerial environment is as important for hosting MRHS as stratovolcanoes and some of the other environments are spatially restricted to specific areas of the world. For example the transform basin systems all lie in SW USA and NW Mexico. The main unifying aspect of the other systems is that they are, for the most part, located in basins/rift settings.

If submarine geothermal systems are considered however, the predominance of andesitic stratovolcano hosted MRHS may be balanced out by oceanic rift systems.

There is approximately 55,000 km of mid-ocean rifts and back arc spreading centres.

Accordingly there could be of the order of 1000 or more rift-related submarine geothermal systems.

There are also active submarine andesite stratovolcano hosted MRHS.

Approximately half of the submarine andesitic stratovolcanoes examined so far have hydrothermal activity (de Ronde et al., 2003). There is approximately 22,000 km of submarine arcs, which in combination with about 500 active andesitic stratovolcano systems on approximately 25,000 km of subaerial arcs, may provide an overall worldwide total of andesitic stratovolcanoes similar to, but possibly less, than that existing in oceanic rift settings. Submarine MRHS have also been found in back-arc settings away from spreading centres (Binns, 1991), however there is insufficient information to estimate how many there are worldwide. Since submarine back-arcs are more common than subaerial back-arcs there is likely to be more back arc submarine hydrothermal systems than subaerial ones. Due to the low proportion of subaerial back arc systems to subaerial andesitic systems, submarine back arc systems are unlikely to come close to the number of rift or andesitic stratovolcano systems.

The strong chemical and hydrological control provided by the sea in submarine systems, most particularly pressure, does however mean that subaerial and submarine systems are very different.

The high-temperature MRHS under study belong to the systems associated with insular-arch andesite volcanism according to the geological-hydrochemical classification by R. Henley and A. Ellis (Henley, Ellis, 1983). The near-surface geological structure and local hydraulic gradients are of great importance for the formation of hightemperature system discharge centers. At the same time it is well known that the hydrothermal cell abyssal part is concentrated around subvolcanic bodies (intrusions) 1. , located within the boundary of the tectonic-magmatic structure axial zones (the Vernadskogo and Karpinskogo volcanic ridges in Paramushir island, the Ivana Groznogo volcanic ridge in the central part of Iturup island and the Kambalny one in the south of Kamchatka). As a rule small intrusions of andesite volcanoes are manifested in the form of circular structures tracing the ridge axial zones. The volcanic ridge geological structures determine space distribution of supply, heating, drainage and discharge areas of the thermal water. Interaction between hydrothermal solutions and including rocks causes increase of their mineralization (The structure, 1993).

Hydrothermal chemical compositions as well as their temperature are the basic factor controlling solubility of mineral and gases. Besides, it also influences upon the type and mineralogy of hydrothermal reactions. Sulphur plays an important part in the composition of hydrothermae and minerals formed by hydrothermal solutions. The series of geologists studying the recent and paleohydrothermal ore-forming systems mark out two types of high-temperature (>1500) hydrothermal systems as per sulphur oxidation condition: low sulfidation and high sulfidation (Hedenquist, Houghton, 1987).

In more details this type of MRHS is considered in a papers by S. Rychagov et al., publication in this book.

3. Classification by hydrology As discussed above, a tectonic classification of systems is complicated by the occurrence of both marine and submarine types of systems and by the occurrence of some MRHS located in unique tectonic environments around the world. A simpler and more useful system is to divide them hydrologically, firstly distinguishing between subaerial and submarine systems.

Subaerial systems can be divided further into basinal, stratovolcano (Henley and Ellis, 1983) and giant vapour dominated systems. It is tempting to apply the same distinction between stratovolcano and basinal types to submarine systems. This is because mid-ocean ridges have central grabens within which the MRHS form. However, there are also examples of submarine stratovolcanoes where the system is hosted within a summit caldera (de Ronde et al., 2005), which also constitutes a basin. Hydrologically, submarine basins are not as important as they are in subaerial settings.

The key difference between the two types of submarine systems is the nature of the hydrothermal fluid. Mid-ocean ridges MRHS and back arc spreading centres have seawater as the primary source of hydrothermal fluid, whereas in andesitic stratovolcano Bogie I., Lawless J.V., Rychagov S. and Belousov V.

and back-arc hosted systems not related to spreading centres it is mainly water of magmatic origin (Urabe, 1987). Thus, submarine MRHS can be divided into magmatic and seawater type systems.

Subaerial basinal type systems can be subdivided into high and low salinity systems with the former being rare and geographically localised, however the known existence of this type of system justifies inclusion as a sub-category. Subaerial andesitic stratovolcano systems can be divided into immature (more magmatic) and mature systems. Figure 1 outlines this classification. Each type of system will be discussed in turn with emphasis given to the dominant stratovolcano type systems.

Chart Title Magmatic Related Hydrothermal Systems Subaerial Submarine Basinal Stratovolcano Giant Vapour-dominated Magmatic Seawater High Salinity Low Salinity Immature Mature Figure 1: A classification scheme for magmatic-related hydrothermal systems 3.1 Subaerial Systems 3.1.1 Basinal Systems As the name suggests these systems are located in topographic basins, mainly rifts.

They can also be found in localised pull-apart basins on arcs, for example Suoh, Sumatra, Indonesia. These basins are zones of accumulation of low density material, and because most basins occur on thick, comparatively low density continental crust means that magmatic intrusions are usually relatively deep, because there is insufficient density difference to drive the intrusives diapirically to shallow levels. The presence of a dilatant structural pathway through basin creation does however mean that magmas are channelled into the centre of the basin (fig. 2). There is scope for magmatic volatiles, released from the deep intrusion, to be neutralised and reduced by rock reaction and diluted by meteoric waters before they can reach the surface. The release of volatiles under high pressure, because the intrusions are deep, also has an affect on the chemistry of the released volatiles. This favours CO2-rich, less-oxidising volatiles. Shallower released volatiles tend to be more Cl-rich, more acid and oxidising (Fournier, 1999).

1. , As there is a surrounding high elevation area to provide artesianal recharge and there is usually no volcanic edifice above the centre of the system, it is usual for boiling point with depth conditions to extend from shallow depths and for the deep convective hydrothermal reservoir waters to reach the surface. Hence, active systems of this type are easily recognised and can be readily studied geochemically. For example, Broadlands/Ohaaki in New Zealand has served as a test case for many studies of mineralisation in active systems (Simmons and Browne, 2000), however this can also be somewhat misleading since it is representative of only one type of system. High salinity systems These systems are uncommon and are geographically limited to the SW USA and NW Mexico (although basinal brines, warmed by the normal geothermal gradient, rather than magmatic activity, are relatively common world-wide and responsible for the formation of many mineral deposits of other types, e.g. Mississippi Valley type polymetallic deposits).

These systems include Cerro Prieto and Salton Sea. They are hosted in deltaic and lacustrine sediments within a transform basin and have associated bimodal basalt-rhyolite magmatism. Extremely high salinity (many times that of seawater) can be attributed to evaporites occurring within the lacustrine sediments (Mckibben and Hardie, 1997).

Because they are well capped by the sediments and contain a denser than average reservoir water which impedes convection, they generally have very limited surface expression comprising only mud pots and fumaroles, or nothing at all. Some fields are only found when drilling for oil. The reservoir fluids have a high salinity and as a consequence of hydrothermal mineral buffering of the cation/hydrogen ion ratio are slightly acid. Consequently, reservoir waters have relatively high concentrations of silver and base metals which have higher solubilities in acidic Cl-rich waters due to the formation of Cl complexes (Seward and Barnes, 1997), rather than bisulphide complexes. Low salinity systems These are the most common basinal type systems, as exemplified by the majority of systems within the Taupo Volcanic Zone of New Zealand such as Wairakei and Ohaaki, with boiling neutral-Cl hot springs and geysers occurring at the surface. Since boiling point with depth conditions extend to the surface, they are prone to hydrothermal eruptions and many hot pools occupy hydrothermal eruption craters. Significant areas of silica sinter can form around the springs. Areas of higher ground can contain fumaroles and acid steam-heated features. These differ from magmatic solfataras because they Bogie I., Lawless J.V., Rychagov S. and Belousov V.

contain H2S and its associated oxidation products, rather than SO2. These reservoirs contain low salinity, reduced, neutral-Cl waters. Dissolved base metals are comparatively low, but gold has been estimated to be close to saturation, in some instances (Brown, 1986), as bisulphide complexes.

Figure 2: A schematic model of a basinal type hydrothermal system 3.1.2 Stratovolcano Systems These are hosted by stratovolcanoes that allow intrusives to reach shallow levels, possibly as shallow as one kilometre. The minimum depth is controlled by how effectively volatile-bearing melts can be contained by the volcanic pile without having a volcanic eruption (fig. 3).

As stratovolcanoes can form on continental crust of varying density and thickness there is also scope for much deeper intrusions to form due to variations in the density contrast between the melt and the crust, and the volatile content of the melt. Intrusives with a higher volatile content have a density favourable to upward movement however also require the highest pressure to contain the volatiles. Upward movement triggers volatile loss resulting in the melt freezing within the crust (Burnham, 1967) and forming typical porphyritic textures.

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