Tectonism and Exhumation in Convergent Margin Orogens: Insights from Ore Deposits

Three widespread and abundant types of ore deposits, epithermal silver‐gold (Ag‐Au), porphyry copper (Cu), and orogenic gold (Au), form by hydrothermal activity during tectonic convergence along continental margins, and their preservation provides new insights into rates of those processes that form and destroy ore deposits as well as overlying rock. Ages for each of the three deposit types define lognormal age‐frequency distributions that are nearly identical in form but differ in their modal ages. Epithermal Ag‐Au, porphyry Cu, and orogenic Au deposits exhibit age modes at ∼3, 11, and 199 Ma, respectively. These ore types form at depths of ∼0.5, 1.9, and 10 km, respectively, such that modal age increases with depth of ore emplacement. Qualitatively, the dearth of deposits younger than their modal age reflects the fact that ore deposits form at depth within the crust, and time is required for exhumation to expose them. Continued erosion serves to destroy ore bodies, giving rise to fewer deposits with increasing time beyond modal ages. These relations suggest that ore deposit age frequencies should serve as monitors of rates of tectonism and exhumation along convergent continental margins. Quantitatively, the lognormal aspect of age frequencies is closely approximated by a steady state model of ore formation and crustal deformation. The model presumes that the population of exposed deposits of each type results from ore emplacement at a constant rate and depth and that, after emplacement, these deposits are dispersed vertically (upward or downward) by tectonic uplift and/or subsidence. The aggregate paths of all deposits of some particular type behave as a two‐dimensional random walk in vertical (depth) and lateral (time) space. Moreover, the lognormal nature of ore deposit age‐frequency distributions requires that amounts of uplift and subsidence are generally subequal (there is no bias to the random walk) and necessitates that tectonic “steps” taken during this random walk are on the order of a few hundreds of meters per million years. Modal rate of ore deposit exhumation (emplacement depth/modal age) decreases as a power function with increasing modal age (exhumation distance $$[ \mathrm{mm}\,] =5.12\times \mathrm{time}\,$$ interval0.799 [yr]). Lower rates of exhumation with greater emplacement depth and modal age, however, merely reflect greater random‐walk path lengths (and durations) with greater emplacement depth and are no less than rates that would be anticipated for a steady state system with invariant rates of ore mineralization and tectonic deformation. In fact, exhumation histories of epithermal Ag‐Au, porphyry Cu, and orogenic Au deposits scale in a similar manner and therefore record largely indistinguishable styles of tectonic uplift and subsidence among the ∼700 ore deposits considered here. Agreement between model and observed ore deposit age‐frequency distributions also allows the formulation of a general inventory of ore body abundances in continental crust. For example, if currently known occurrences of epithermal Ag‐Au reasonably approximate the actual number of these deposits that now exist at the Earth’s surface, then less than 1% of all such deposits are now exposed; tectonic uplift and erosion has served to remove ∼93% of all such epithermal Ag‐Au deposits that have formed over Phanerozoic time.