SOLUBILITY OF METALS AND NUTRIENTS IN BRINES: IMPLICATIONS FOR ORE DEPOSITS, BIOPRODUCTIVITY, AND ANOXIA IN SEDIMENTARY BASINS

The concentrations of metals and nutrients in brines constrain models of ore formation, bioproductivity, and anoxia in sedimentary basins where they discharge. We use information on the equilibrium solubility of metals and nutrients in brines to evaluate their impact on sediment composition and bioproductivity in basins. The solubility models facilitate comparisons with contrasting inputs by petroleum, and conventional sources.

The element enrichments in syngenetic and subseafloor ore deposits mined for Au, Ag, Pb, Zn, Cu, Co, Sn, Fe, Mn, or Ba are fairly well explained by their solubility as chloride vs. sulfide vs. other complexes and variations in the redox (±pH) state of hot brines (T>100°C, NaCl 3m, SS 10^-2m) and overlying seawater. In anoxic basins, introduced Fe, Mn, and Ba will precipitate at redox interfaces distant from seafloor vents.

The element enrichments in V-Ni-Cr-Mo-U-PGE-Au black shales are not explained by hot brines. Although some of these elements are soluble in cooler brines or connate waters as ammonia, organic acid, polysulfide, thiosulfate, phosphate, cyanide, or other complexes, the transport medium that best explains them is petroleum with metals bound to porphyrin, thiol, amine, methyl, or other functional groups. As petroleum is metabolized and metals are liberated, a variety of ore minerals will precipitate.

Phosphorites occur with both styles of mineralization permitting that P was introduced by brine and petroleum. Yet, P is relatively insoluble in brine and its concentration in petroleum is uncertain. Both fluids contain other nutrients that will stimulate bioproductivity and extraction of P (and metals) from seawater by assimilation within organisms and adsorption to settling particles. As organic matter is metabolized and ferric oxyhydroxide is reduced and sulfidized, P will be released to form phosphorite.

If inputs of brine or petroleum are large and brief, basin chemistry will change and be evident in sedimentary sequences.