Abstract

Age-frequency data are used to determine amounts of vertical tectonic displacement experienced by the global population of orogenic gold deposits following their emplacement at a crustal depth of 10 ± 1 km. Knowledge of this tectonic dispersion allows for an estimate of the total crustal endowment of gold in orogenic gold deposits. This estimate is based on a compilation of data on 365 orogenic gold deposits, of which gold contents are available for 314 deposits and good age control is available for 126 deposits. Ages of well-dated deposits form three clusters that range in age from 47 to 850 Ma (primarily Phanerozoic; 69 deposits), 1730 to 2090 Ma (Proterozoic; 13 deposits), and 2540 to 3107 Ma (Archean; 44 deposits). Gold endowments do not vary significantly among the three age groups and average 155 tonnes (t) per deposit for the entire population for which data on gold tonnages are available. Owing to the distinct tripartite nature of the age frequency distribution, it is not possible that orogenic gold deposits represent largely invariant rates of ore formation and destruction (uninterrupted tectonic dispersion would have resulted in the destruction of essentially all Proterozoic and Archean deposits). Separate tectonic-diffusion calculations for each of the three deposit age populations were carried out while constraining biases to tectonic diffusion (i.e., the most suitable ratios of uplift and erosion versus subsidence and burial), with continental crustal thicknesses on the order of several tens of kilometers.

The results of these calculations indicated that a total of ~25,236 orogenic gold deposits formed over the full span of geologic time, that ~23,664 of these (~94%) were destroyed during subsequent uplift and erosion, and that ~1,572 deposits remain at depth in Earth’s crust. If all 365 known orogenic gold deposits have the same mean gold content as the deposits for which gold contents are available, then ~706,439 t of gold remain at depth in Earth’s crust. Gold in orogenic deposits constitutes about 0.0022 percent of the total gold content of the crust. Using estimates of gold released by prograde metamorphism of the Otago Schist, gold in the deposits that remain in the crust could have been released from about 1,413,000 km3 of amphibolite facies rocks, which constitute only about 12 percent of currently exposed metamorphic rocks.

If we assume a maximum depth of ~3 km for future mining of these deposits, the model results indicate that deposits containing almost 113,000 t of gold exist within crustal rocks above this depth. Discovered deposits make up about 8 percent of this endowment. In the context of spatial density, the areal concentration of orogenic gold deposits along Phanerozoic metamorphic belts is about ten times that in Proterozoic terranes and about three times higher than in Archean belts. This suggests that exploration for the next generation of orogenic gold deposits should be focused on Phanerozoic terranes.

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