Increased solubility of quartz following ferrous–ferric iron reactions
A knowledge of the solubility of quartz and silicates in natural waters is important to the understanding of many gelogical processes. Here we provide experimental data to support an earlier hypothesis1, that during the supergene enrichment of banded iron-formations, redox reactions involving iron result in a much more rapid dissolution of quartz than would be predicted from its known solubility in water. The process is relevant not only to iron ore genesis but also to the much wider field of chemical weathering— for example, to the genesis of laterites and soils from quartz-bearing rocks. In addition, the reactions may offer a mechanism by which quartz is pseudomorphed by iron oxides.
The Role of Water in Grain-Scale Deformation Within the Cove Fault Zone, South Central Pennsylvania
Charles M. Onasch, Advisor John R. Farver, Co-Advisor Fluids, and water in particular, play an integral role in deformation within the Earth’s crust over a wide range of physical conditions. At low temperatures ( 300 C), water-related chemical processes, such as diffusive mass transfer, advective mass transfer, and hydrolytic weakening dominate. In the transition between high and low temperature regimes, both mechanical and chemical processes operate and interact in complex ways. The Cove fault zone in south central Pennsylvania contains several mapscale blocks of quartz arenite, which display a wide range of brittle and ductile microstructures. Abundant evidence of fluids is present from mapto latticescales, including quartz precipitated in microveins, fluid inclusion planes, and zones of cataclasis, and the removal of quartz along dissolution surfaces. Three different aqueous fluids were identified based on the cathodoluminescence color of quartz precipitated by that fluid, and the homogenization (Th), last ice melting temperatures (Tm), and salt species from aqueous, two-phase fluid inclusions. Fourier Transform Infrared (FTIR) spectroscopy data indicate that a large amount of water has become incorporated in the lattice of deformed detrital grains, especially those in the proximity of fluid conduits (e.g., microveins). The fluids, based on the oxygen isotopic composition of the quartz precipitated, appear to
THE ELECTROCHEMICAL MECHANISM OF SULFIDE SELF-POTENTIALS*
Self-potentials associated with a sulfide ore body result from the ohmic potential drop within the country rocks. The electric current is produced by separate but simultaneous reduction of oxidizing agents near the surface and oxidation of reducing agents at depth. The ore does not participate directly in either reaction, but serves as a conductor to transfer the electrons from the reducing agents to the oxidizing agents. The possibility for the above reactions to occur depends upon differences in oxidation potential of ground waters at different depths. In the zone of weathering, the oxidation potential is controlled by the reduction mechanism of oxygen, and ranges in value from 0.2 to 0.7 volt (on the hydrogen scale). If the ore tends to oxidize at some lower potential, then the latter is the available one. In the zone beneath the water table, the potential is probably controlled by the oxidation-reduction equilibria of iron-rich minerals, and ranges in value from 0 to -0.3 volt. The available potential is independent of ore type. The maximum potential difference available to produce natural currents is estimated at: graphite 0.8, pyrite 0.7, covellite 0.6, chalcocite 0.5, galena 0.3 volt. Self-potentials will be large if the ore body (1) is composed of minerals difficult to oxidize, (2) has low electrical resistance (physical continuity together with low resistivity), (3) extends vertically across the water table, and (4) exists close to the surface.