Resources of Gold in Phanerozoic Epithermal Deposits

Abstract

We have estimated Earth’s endowment of gold in Phanerozoic epithermal deposits using a tectonic-diffusion model, which simulates the emplacement of deposits at a shallow crustal depth and their subsequent vertical tectonic migration in the crust. The calculation was calibrated by least-squares comparison of a calculated age-frequency distribution to the age-frequency distribution for 448 epithermal deposits of Phanerozoic age. Results indicate that ~17 percent of the epithermal deposits that formed through Phanerozoic time remain in the crust today whereas ~83 percent have been removed by erosion. Assuming a similar age distribution for all 1,181 epithermal deposits in our compilation indicates that ~307,000 deposits formed throughout Phanerozoic time, that ~63,000 of these remain in the crust, and that ~244,000 have been eroded. Grade and tonnage data of gold in 757 epithermal deposits in the compilation have an arithmetic average of 34.7 t and yield an estimate of 2.2 × 106 t of gold for epithermal deposits remaining in the crust. Using the model-based vertical distribution of deposits remaining in the crust, we estimate that epithermal deposits in the upper kilometer of the United States contain 2.9 × 104 t of gold. Adjusting for areas of outcropping volcanic rocks in each continent increases our estimate to 4.0 × 104 t. These estimates are similar to the 1.7 × 104 t gold resource estimated by the U.S. Geological Survey using completely different methods. Several comparisons based on these results provide insights into the efficiency of processes that form epithermal deposits and the outlook for sustainability. For instance, all of the epithermal deposits that formed through Phanerozoic time represent only about 0.03 percent of the gold in the crust. Only about 0.007 percent of crustal gold remains in epithermal deposits; the rest has been eroded and recycled. This is consistent with comparisons based on the flux of gold in the Taupo Volcanic Zone, which indicate that epithermal deposits trap only 0.1 to 0.2 percent of the gold that moves through convergent margin hydrothermal systems. Epithermal gold deposits that remain in the crust can supply current production from them for 1,400 to 5,000 years, depending on success of exploration and depth of mining. Finally, we are consuming gold from epithermal deposits about 17,000 times faster than Earth is replenishing the supply.