Paper No. 7
Presentation Time: 3:05 PM
TECTONIC CONTROL OF AGES OF EXPOSED HYDROTHERMAL ORE BODIES
WILKINSON, Bruce H., Department of Earth Sciences, Syracuse University, Syracuse, NY 13244 and KESLER, Stephen E., Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, eustasy@umich.edu
Ages of various types of ore bodies now exposed at the Earth's surface primarily reflect characteristic crustal depths at which they originate and the cumulative influence of vertical tectonic movements that serve to displace ores from original depths of formation. Abundances of mid- to upper-crustal ore types such as epithermal gold, porphyry copper, and orogenic gold deposits increase rapidly to some modal age, and then decrease more gradually with increasing age. Paucity of exposures younger than modal ages largely reflects some intrinsic interval of geologic time needed for tectonic movements to exhume rock units from depths at which they formed, while increasing rarity of exposures older than modal ages reflects the overall dispersion (by uplift and erosion as well as by burial and subsidence) from initial emplacement depths. The geologic cycling of crustal ore bodies essentially behaves as a tectonically-driven diffusive system.
Atomic diffusion of isotopes and trace elements is a nearly ideal analogue for tectonic diffusion of ore bodies, and either is readily represented as one-dimensional diffusion governed by Fick's second law. In both cases, concentration of diffusants (ions or lithosomes) at any distance from the system boundary (mineral or Earth surface) is only dependent on numbers of implanted/emplaced ions/lithosomes, the mean depth and variance of implantation/emplacement, and the diffusion coefficient.
Tectonic diffusion coefficients derived from best-fit approximations to age-frequency distributions of epithermal gold, porphyry copper, and orogenic gold deposits are about 10‑8 m2/s. As context for this value, diffusivity in water at room temperature is typically about 10‑9 m2/s, while gaseous diffusion coefficients at Earth surface temperatures are on the order of 10-5 m2/s. The tectonic dispersion of hydrothermal ore deposits in the Earth’s crust occurs at rates approximately the same as that of ions in aqueous fluids over comparable ranges of temperature and pressure.
