METALLOGENIC PROVINCES AND EPOCHS: DEFINITIONS AND ORIGINS

Despite widespread use of the terms metallogenic province and metallogenic epoch, definitions are surprisingly vague. Quantitative definition of the terms is important if we are to understand their role in the geochemical evolution of Earth.

We propose here that metallogenic provinces and epochs be defined as regions and time periods, respectively, in which the number or size of deposits of interest is at least two times greater than that produced by a steady-state Earth. As such, they can be identified for any type of ore deposits by comparing actual age-frequency distributions for known deposits to ideal age-frequency distributions representing a steady-state Earth. Ideal age-frequency distributions can be generated by a computational model that we have developed, in which an infinitely large number of ore bodies of average size forms at a specified depth and then experiences uplift, stasis, or subsidence during any time step for any specified total number of time steps.

This model-based approach allows enquiry into the origin of metallogenic provinces and epochs. Are they regions and time periods in which deposits were FORMED or were PRESERVED in unusually large numbers or sizes? Application of the model to global porphyry copper deposits of Phanerozoic age (Singer et al., 2005) shows that time periods from 3-4, 13-14, 32-36, 55-64, 289-315 and 439-447 Ma are metallogenic epochs. Although deposits in these age-groups cluster at a global scale, most occupy entire orogens and do not define metallogenic provinces. Time periods with anomalously high total copper contents are similar patterns and also show limited provinciality. Metallogenic epochs of both types have no special relation to periods of plate reorganization or changes in rates of seafloor spreading. These observations in combination with local geologic information indicate that metallogenic provinces and epochs, at least for porphyry copper deposits, are more strongly controlled by processes involving preservation than by processes of formation. Although this does not limit their importance as geochemical features, it requires caution in their use as indicators of variations in Earth's ore-forming processes.