Composition and origin of crustal brines

摘要

The significance of brines and evaporites in the genesis of ore deposits has been recognized increasingly in recent years, and they have been implicated in the formation of a wide range of deposits from base metals to emeralds. In general terms this is not surprising; many metals are complexed in aqueous solution by chloride, and so relatively small amounts of brine can transport relatively large masses of metal. Highly saline fluids can evolve in four distinct geological settings, the first three of which are of potential economic significance: (1) exsolution from crystallizing magma; (2) evaporation of sea water, which leads to the formation of bittern brines, enriched in Br, once halite begins to precipitate; (3) redissolution of halite deposits; and (4) removal of water from chloride-bearing solutions during hydration reactions within the crust.The original chemical characteristics of brines change as they migrate through the crust, since brines equilibrate readily with their hosts for most chemical components. Low-temperature, sedimentary brines are typified by high, but variable contents of Ca relative to Na, low levels of such elements as K, Mg, Fe and Mn and variable concentrations of the alkaline earth elements, close to saturation with sulphate minerals. Bromide levels relative to chloride are often elevated by comparison with sea water, but some very Br-depleted brines also occur.