SECONDARY ENRICHMENT OF COPPER AT THE MADISON GOLD SKARN DEPOSIT, SILVER STAR DISTRICT, MONTANA

The Madison Gold skarn is located near the SE margin of the Highland Mountains near Silver Star, Montana. This Au-Cu skarn developed along the contact between the late Cretaceous Rader Creek granodiorite and the Mississippian Madison Group limestone. Primary sulfide minerals include pyrite, pyrrhotite, chalcopyrite and minor bornite, whereas the gangue is dominated by garnet and diopside-hedenbergite. Gold was emplaced during the original skarn protore and occurs as free gold and as microscopic grains within sulfide and pyroxene. A later, lower-temperature event resulted in oxidation of the skarn protore and formation of a vertical, tabular body of hematitic jasperoid. The jasperoid has no copper but locally carries very high gold grades (0.1 to 20.0 opt). The jasperoid is cut by a stockwork of anastomosing veins of calcite. The origin of the hematitic jasperoid and calcite veins is explained by the following reaction:

2CaFeSi₂O₆ + 2CO₂ + ½O₂ → Fe₂O₃ + 4SiO₂ + 2CaCO₃

Thus, the jasperoid formed from siliceous fluids liberated from the breakdown of the primary hedenbergite skarn. To explain the abundance of hematite, these fluids must have been oxidized, and therefore would have been incapable of mobilizing gold. Instead, the distribution of gold within the jasperoid most likely reflects the grades and distribution of gold within the skarn protore. In contrast, copper-bearing sulfide minerals were destroyed during jasperoid development, resulting in liberation of Cu²⁺. Cu-rich fluids replaced pyrite and other sulfide minerals outside the main jasperoid body, forming high-grade chalcocite pods. In other locations where no sulfide minerals were present, Cu²⁺ precipitated as native copper at an advancing reduction front. Some of this native copper occurs as pods and nuggets, up to 21 kg in size. Elsewhere, copper forms impressive stockworks with a geometry similar to the calcite veins within jasperoid. Overall, the secondary copper mineralization at Madison Gold appears to have formed during a low-temperature, oxidized geothermal event, which differs from conventional supergene models for formation of chalcocite and native copper.