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Australian Journal of Earth Sciences (1997) 44, 149-183
Evaporites, brines and base metals: fluids, flow and 'the evaporite that was' J. K. WARREN JK Resources Pty Ltd, PO Box 520, Mitcham SA 5062, Australia. <
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Waters in modern evaporite systems are marine, non-marine, or hybrid but mineralogies in most ancient systems are not so simple that marine and non-marine brines can be easily interpreted from the chemistry of their precipitates. Complications arise related to subsurface brine mixing and back-reactions both at the surface and in the subsurface. The precipitation order of ancient bittern salts from seawater may have been dependent on flux rates of river inflow relative to flux rates through mid-ocean ridges. In ancient continental systems the chemistry of the inflow waters was a fundamental control on the subsequent mineral paragenesis. Our lack of hydrogeochemical understanding of ancient evaporite systems has led to the 'potash problem'. Potash evaporites, traditionally interpreted as marine salts, fall into two categories: (i) potash deposits characterised by MgSO4 salts, such as polyhalite, kieserite and kainite; and (ii) potash deposits characterised by assemblages containing halite, sylvite and carnallite and entirely free or very poor in the magnesium-sulfate salts. This latter group makes up more than 60% of ancient potash deposits. The former group may well be marine-derived but the latter group must have precipitated from Na-Ca-Mg-K-Cl brines with compositions quite different from that of concentrated modern seawater. After primary precipitation, ongoing pore-water flow propels near-surface and burial diagenesis, both processes that can dissolve and reprecipitate evaporites and drive the chemical evolution of subsurface brines. The hydrologic framework of a large evaporite basin consists of several regimes: (i) the active phreatic-depositional; (ii) the compactional; (iii) the thermobaric; and (iv) the active phreatic-exhumation/uplift. Boundaries are indistinct and transitional. Nonetheless the regimes show hydrological end-members characterised by distinct relative positions, hydrogeochemistries, flow dynamics and evaporite textures. Basin-scale dissolution and remobilisation of evaporite units can occur within all these hydrological settings with the relatively impervious evaporite unit acting both as a focus for fluid flow and as a source of dissolved ions in the subsurface brines. On a geologic time scale both meteoric and brine-reflux driven circulations are rapid and at least partially convective through the subsiding sediment pile. Evaporite units can also act as pressure seals and instigate convectional flow in the compactional and thermobaric regimes. With the cessation of basin subsidence, compactional and thermobaric flow rates slow down and finally cease. This is followed at some later stage by a phase of emergence of basin fill, perhaps due to a sea-level fall or basin uplift. The resulting depositionally inactive, or hydrologically mature, sedimentary basin is increasingly subject to gravity-driven meteoric circulation and the basin evaporites are increasingly flushed by meteoric waters. Into this hydrological framework fit the various diagenetic textures of former evaporite beds. Textures include: caprocks, basal anhydrites, some rhythmically banded lead-zinc ores, as well as the more usual indicators of 'the evaporite that was'—evaporite dissolution breccias, rauhwacken, silicified evaporite nodules, calcitised evaporites and calcitised dolomites. Beyond the normal sedimentary realm of evaporite studies lies the metamorphic realm. In this context meta-evaporite deposits are discussed, along with indicator processes of: scapolitisation, albitisation, anorthositisation and tourmalinisation. The various indicator textures are then used to discuss the evolution of the earth's surface water, from the Archaean till the Phanerozoic. Halite the mineral or its pseudomorphs characterise areas ofwidespreadchoicesof sedimentation from the Archaean to the present. Calcium sulfate is more enigmatic. Widespread beds composed of calcium sulfate minerals or their pseudomorphs were scarce to absent up to approximately 1.8 Ga ;-.'ii'C then the halite/gypsum precipitates from a concentrating seawater brine reflect the same predictable succession of precipitated salts as is found in modern seawater (with possible complications in the bittern sales). This implies marine water has maintained similar proportions of major ions throughout the Phanerozoic on the Proterozoic back to 1.9-1.8 Ga. Prior to this the Archaean ocean may have been a Na-HCO3 ocean and not the Na-Cl ocean of today. Key words: base metals, brines, evaporite, geochemistry, meta-evaporite. INTRODUCTION •• • •' Part I of this series on the evaporite-base-metal association discussed evaporite salts and how to recognise their various depositional and diagenetic forms (Warren 1996). Part II now discusses the hydrological controls on evap-
orite depnsi'bri ^ ;?•.•serration. A.; well, it exp';Uii;, '.lie varieties of n)!':;>o.c : even whcci the evaporite salt has gone, ..mi tlie c;c A.-vc.u evolution of seawater chemistry based on trie (ecognitioh
