Hydrothermal Processes in Impact Structures

Impacts on land will involve volatiles and water of the lithosphere, and those in oceanic environments will directly affect water bodies as well as rock-bound volatiles. Even impact into arid environments will potentially result in release of volatiles through shock dissociation of hydrous minerals of the target. Interaction of aqueous solutions and other volatile components with hot, shocked rock volumes will result in hydrothermal activity, leading to effective alteration processes and potentially large-scale hydrothermal overprint on deformed and shock metamorphosed rocks of the crater floor and impact breccias, as well as deposition of secondary minerals from hydrothermal solutions. In favorable circumstances, these hydrothermal deposits could be of economic value. The size and duration of such a hydrothermal system will critically depend on the magnitude of the impact event, i.e., the amount of deposited kinetic energy, the rock volume affected, and the amount of volatiles released/set into circulation. Possible heat sources include shock heating, frictional melting/heating in the crater floor, the emplacement of a sizable impact melt sheet in the crater structure, and uplift of hot rocks from deeper crustal levels into the central uplift structure (see Fig. 10). The duration of the active hydrothermal process will be governed by the amount of heat available to drive a convective process. Fluid circulation is facilitated by the presence of large volumes of impact melt - as heat source - and shocked and fractured/brecciated target rocks, the unconsolidated nature of the crater fill, and enhanced permeability due to impact-induced fracturing and brecciation of target rocks.

Low-temperature hydrothermal processes may even provide for the

Fig. 10. Model for fluid flow in the Witwatersrand Basin generated by the Vredefort impact event. A schematic complex impact structure is shown, with a blanket of hot impact melt. The central uplift region comprises hot (high temperature = +T) mid-crustal rocks that are subjected to high shock pressures (+Ps). Temperature along a profile away from this hot central uplift decreases, and it also decreases away (downward) from an upper impact melt body (i.e., with depth). Outside of the central uplift, lithostatic pressure increases with depth (or, as indicated, P decreases upwards). In addition, one must assume that secondary (i.e., impact generated) porosity decreases away from the central part of the impact structure. The assumed combination of high T along the impact melt cover and in the volume of the central uplift, as well as decreasing porosity away from the center of the impact structure and decreasing lithostatic pressure with decreasing depth, lead necessarily to the conclusion that fluid flow will be largely laterally away from the central uplift, and enhanced at relatively shallow depths. Also shown is a schematic present depth of erosion, as it would apply to the Vredefort situation. This implies that the currently mined Witwatersrand strata just below this erosion depth would have likely been in the presumed flow channel.

generation of biologically active environments, perhaps creating unique niches for new development of life (e.g., Farmer 2000; Kring 2000; Cockell et al. 2002). Newsom et al. (1986) discussed the chemical effects that impact-hydrothermal alteration could have on regolith and soil development - obviously an issue of major interest in these days of preparation for surface sampling on Mars.

Naumov (2002) compiled a vast amount of mineralogical information from the impact crater literature and presented a detailed synthesis of investigations into low- to intermediate (ca. 50-350 oC) temperature hydrothermal processes as observed in the various lithologies in the Kara, Popigai, and Puchezh-Katunki impact structures. This work demonstrates the impact-triggered generation of hydrothermal cells, in the course of which extensive rock alteration, involving element leaching and redeposition, can take place. These three large impact structures, of 65, 80 and 100 km diameter, respectively, are all characterised by extensive alteration and secondary mineral formation. In his recent review, Naumov (2002) concludes:

• The kinetic energy of an impact event and the pre-impact target characteristics are the most important parameters for the development of a post-impact hydrothermal system.

• As a result of impact, a near-surface high-gradient zone of hot and high permeability rocks is generated. High pressure and temperature, as well as permeability gradients, in impact structures may lead to the formation of hot-water circulation that can last, in some cases, for thousands of years.

• The most extensive hydrothermal alteration has been observed in impact craters formed in shelf or intra-cratonic shallow basin environments.

• Mineral assemblages observed indicate post-impact hydrothermal alteration at 50-350 oC, pH of 6-8 (due to uptake of alkali elements and Ca from strongly deformed rocks), and Eh values of > -0.5.

• The composition of hydrothermal mineralization is determined primarily by the respective target rock composition found at a given structure (the more varied the spatially observed target composition is, the more variable may be the secondary mineral paragenesis), besides the physical properties of rocks after their deformation under shock-metamorphic conditions.

• Post-impact mineralization can be present in all parts of an impact structure and in all types of impact lithologies. But the crater fill (suevites, impact melt rock) is likely to be more affected than parautochthonous impactites of the central uplift and the crater rim.

• In accordance to the findings of fluid inclusion studies by Komor et al. (1987, 1988) on the Siljan structure, Naumov (2002) concludes that crystallization temperatures for secondary mineral parageneses decrease upward in an impact structure and outwards from the center of the structure (compare Fig. 10).

• Meteoric, groundwater, and surface water can be sources for the hot solutions in post-impact hydrothermal cells. The shocked target rocks are the sources for the adsorbed mineral constituents.

• A three-stage development of hydrothermal activity is envisaged: Stage 1, when isotherms are still in pre-impact configuration; Stage 2, after inversion of the thermal field (30 oC gradients in central part to 100 oC at the periphery), and Stage 3, with gradients of <10-30oC.

• Hot-water circulation affects only the upper parts of an impact structure (compare Fig. 10).

The proposed scheme involves that superficial aqueous fluids infiltrate hot rocks of the central uplift as well as impactites of the crater fill. Alkalinity rises due to uptake of K, Na and Ca. Silica is freely available. This represents a very favorable situation for the formation-deposition of Fe-smectites and zeolites, phases typically identified in impact crater settings (e.g., Stoffler et al. 1977). The alkaline components are largely deposited in the upper part of a crater fill. Upon ascent of a fluid, its temperature decreases, OH- is taken up, and, especially in cases where ample carbonate is present, CO2 contents of the fluids may increase. Consequently, solutions may become more acidic.

Clearly, the strong deformation, especially fracturing and brecciation, that affects huge rock volumes in large impact events, and the instantaneous increase in temperature over huge rock volumes provide ideal conditions for the initiation of hydrothermal systems.

So far, it does not seem possible to define parameters that could distinguish unambiguously between the results of an impact-triggered hydrothermal mineralization event and one that is the result of other geological processes such as volcanism or metamorphism. The geological context - impact or endogenic setting - will have to provide the vital clues to allow the determination of a cause of formation of a specific hydrothermal ore deposit.

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