HOW LONG DOES IT TAKE TO FORM AN EPITHERMAL PRECIOUS METAL DEPOSIT?

While the longevity of magmatic-hydrothermal systems associated with ore formation is reasonably well constrained by geochronology, the duration of the ore-forming event(s) is less well understood. Most radiogenic age-dating methods lack the resolution to determine the duration of mineralization. One means of estimating the duration of the ore-forming event is to determine the amount of fluid required to precipitate the metal in the deposit and then to use reasonable estimates of fluid flux to calculate the duration of ore formation. In this study, we use reported ranges in fluid flux obtained from studies of natural springs, geothermal wells, and volcanic fumaroles, along with estimates of the metal content in the fluid and ore grades and tonnages to estimate the amount of time required to form a typical precious metal (Au-rich and Ag-rich) epithermal deposit.

Data compiled for 281 gold-rich epithermal deposits indicate a range from 132 and 479,000 yr, with an average time of 7,940 yr. and Data for 255 silver-rich epithermal deposits indicate a range from 120 to 724,000 yr, with an average of 9,330 yr. Our analysis is based on continuous flow and precipitation of metal in the hydrothermal system over the entire ore-forming event. However, epithermal precious metals deposits form in an environment similar to that of modern continental geothermal systems or hot spring environments. Many of these systems are characterized by episodic hydrothermal eruptions resulting from fluid overpressures in the subsurface that are generated as minerals precipitate and fill the open space. Assuming a flow of 200 kg/sec and Au content in the fluid of 0.006 mg/kg (6 ppm), and assuming each hydrothermal eruption lasts 10 minutes and that one eruption occurs on average each year, about 52,000 years would be required to deposit 1.2 million ounces of gold. The required time decreases to 8,600 years if each eruption lasts one hour, and decreases further to about 360 years if each eruption lasts one day.