Precipitation of Mississippi Valley-Type Ores: The Importance of Organic Matter and Thiosulphate

Abstract

Certain constraints that apply to many Mississippi Valley-type deposits may be used to develop a model for the genesis of this group of deposits. These constraints include: (1) the occurrence of a wide variety of mineral types together at specific sites in a huge volume of potential host rock; (2) the minus-one oxidation state of sulphur in disulphides in the ores; (3) the ubiquitous occurrence of organic matter in the ores; and (4) the evidence of episodes of dissolution of the ore and gangue minerals. The most likely model for the genesis of Mississippi Valley-type ores involves the transport of metals and sulphur together in one solution in which sulphur is in the form of thiosulphate (S2O 3 2 -). Where this hot mineralizing solution encounters organic matter in the host rock, thiosulphate is reduced to provide the minus-one valent sulphur for disulphide (pyrite and marcasite) precipitation and the minus-two valent sulphur for sulphide (galena, sphalerite, etc.) precipitation. Reversal of some reactions among partly oxidized sulphur species may account for the oscillations between precipitation and dissolution of the sulphide minerals in the ores. Heating of the organic matter at the sites of mineralization produces organic acids, which initially dissolve carbonate host rocks. At somewhat higher temperatures, some organic acids slowly degrade to produce carbon dioxide, whereas others act as pH buffers. This addition of carbon dioxide to a solution with an organic acid pH buffer causes carbonates to precipitate. At higher temperatures still, organic acids quickly degrade so that the pH buffer is destroyed. In the absence of a buffer, the addition of carbon dioxide lowers the solution’s pH and causes carbonates to dissolve. Carbon dioxide from organic acid degradation and from carbonate dissolution can trigger fluorite precipitation by forming the complex MgHCO 3 + at the expense of the complex MgF+. Because MgF+ is an important species in the transport of fluorine, breaking the complex causes fluorite precipitation. In the cool late stage of mineralization, bacterial metabolism of thiosulphate in the presence of organic matter produces carbon dioxide and isotopically heavy sulphate. The former precipitates as late-stage calcite and the latter as late-stage baryte. Thus, the entire paragenesis can be linked to the presence of organic matter at the sites of mineralization, and the interaction of a hot thiosulphate-bearing solution with organic matter in the host rock may accommodate the widely applicable constraints on the genesis of the deposits.