Compared with other types of petrologists, ore petrologists spend a large fraction of their microscope time looking at the ``opaques.'' However, with the decline in domestic economic geology programs, fewer courses on reflected-light microscopy will probably be available to geology students. This is unfortunate, because oxide and sulfide minerals often record critical information on the conditions of rock genesis. Craig [1990] and Barton [1991] reviewed examples of textures in ores and their interpretation. Barton illustrated how careful interpretation of disequilibrium textures can reveal many aspects of mineralizing processes, including the duration of geological processes. Craig and Vaughan [1994] produced a second edition of their widely-used textbook on ore microscopy and petrography; perhaps a future edition will contain more examples illustrating the growing importance of elemental and isotopic microanalytical methods in ore petrology. Murowchick [1992] summarized textural criteria that can be used in assessing the ancestry of pyrite and marcasite, as well as the pH and temperature of their formation.
Economic geologists continue to integrate the use of new microanalytical
techniques into ore petrology. The microscopic distribution of metals,
ligands and their isotopes within and among crystals in ores is
relevant not only to understanding ore genesis, but also to ore
beneficiation and processing. For example, because the specific crystal
chemical hosts for Au in sediment-hosted disseminated gold deposits are
uncertain, Arehart et al. [1993a] used backscattered electron and
secondary ion imaging techniques to document a direct correlation of Au
with metastable arsenian pyrite on a microscopic scale. They proposed
that Au is present as Au
in solid solution, having been deposited
from aqueous bisulfide complexes by coupled Au oxidation and As
reduction. Such data have important ramifications for processing and
extracting the gold from these types of ores.
Sphalerite is the only common ore mineral whose composition can record the pressure of mineralization; it is thus an important geobarometer. Toulmin et al. [1990] reviewed the basis and status of the sphalerite geobarometer, noting that experimental and theoretical discrepancies at low temperatures and high pressures need to be resolved, but also that the geobarometer could be successfully applied to equilibrium sphalerite-pyrrhotite-pyrite assemblages which have not suffered retrograde effects. The phase equilibria experiments of Lusk et al. [1993] have since filled in an important pressure-temperature region relevant to this geobarometer.