How to form a giant epithermal precious metal deposit: Relationships between fluid flow rate, metal concentration of ore-forming fluids, duration of the ore-forming process, and ore grade and tonnage

Abstract

Economic metal deposits associated with hydrothermal systems represent the end product of a series of complex processes the are related to the amount of ore fluid that flows through the system, the concentration of metal in the ore fluid, the depositional efficiency, and the duration of the ore-forming process. These factors, in turn, largely control the size (tonnage) and grade of the deposit that forms. Of these factor, the duration of the mineralizing event is one of the least understood aspects of ore genesis. We have used ore tonnage (produced + reserves) and grade information from a large number of epithermal precious metal (Au ± Ag) occurrences, combined with simplifying assumptions related to the metal solubilities and precipitation efficiency, to estimate the amount of fluid required to deposit the amount of the metal observed in the deposit. Total gold tonnage for 279 epithermal veins, prospects and deposits ranges from 0.0002 T (6 troy oz; Wharekirau-ponga, New Zealand) to 1089 T (35 million troy oz) in the giant Pueblo Viejo deposit, Dominican Republic, with a median value of 5.93 T Au (191,000 troy oz). Total silver tonnage for 246 epithermal deposits ranges from 0.0004 T (12 troy oz; Wharekirau-ponga, New Zealand) to 62,207 T (2.0 billion troy oz; Cerro Rico de Potosi, Bolivia), with a median value of 40.8 T (1.31 million troy oz) Ag. The median gold grade, based on 246 deposits, is 5.29 g/T, and the median silver grade for 216 deposits is 93.3 g/T. The reported metal content and grade data suggest that about ~1012 kg of fluid is required to transport and deposit the amount of Au or Ag observed in a median size deposit, and that the largest deposits may require 1014 to 1015 kg of metal-transporting fluid. The median reported flow rate compiled for 693 modern continental geothermal systems, representing modern analogs of the ore-forming environment for the epithermal deposits, is about 5.78 kg/sec with the highest reported flow rate of ~3000 kg/sec. Based on the median flow rate, the time required to form a median-sized epithermal Au deposit is ~12,900 years, and that required to form a median sized Ag deposit is ~7800 years. The largest “giant” deposits could have formed in ~105 years assuming the median flow rate, or in ~104 years at the maximum reported fluid flow rate of 3000 kg/sec.Results of this assessment of the relationship between precious metal grades and tonnage, combined with reasonable fluid flow rates and metal contents of the ore-forming solutions, indicate that the active mineralization event need not be longer than ~104 years to form a median-sized deposit, assuming continuous deposition. Field and laboratory data suggest, however, that mineralization in epithermal deposits is episodic rather than continuous and is associated with discrete pulses, perhaps triggered by hydrothermal eruption events that lead to fluid immiscibility and the concomitant quantitative precipitation of all metal in solution. As such, gold and/or silver deposition likely occurred over only a small fraction of the total lifetime of the hydrothermal system, as evidenced by discrete bands of high-grade ore separated by barren or lower grade bands within mineralized quartz veins. Results of this study suggest that giant deposits do not necessarily require ore-forming fluids with exceptionally high metal contents, but rather may simply represent hydrothermal systems that were active for longer periods of time or were characterized by higher fluid flow rates, or both, resulting in greater amounts of metal-bearing fluid flowing through the ore horizon.