Economic geology of the Madison Gold Gold-Copper skarn, Silver Star, Montana

The Madison Gold skarn is located near the southeast margin of the Highland Mountains near Silver Star, Montana. This Au-Cu-Ag skarn developed along the contact between the late Cretaceous Rader Creek Granodiorite and the Mississippian Madison Group limestone. Primary sulfide minerals include pyrrhotite, pyrite, chalcopyrite, and minor bornite. Sulfur isotope data collected from primary sulfides suggest a magmatic source of S and a high temperature of formation. With a drop in temperature, the main gangue minerals in the skarn shifted from an anhydrous assemblage (garnet, diopside-hedenbergite) to a hydrous assemblage (phlogopite, chlorite). At the same time, early pyrrhotite was replaced by pyrite. Gold was emplaced during the original skarn protore and occurs as microscopic grains of electrum found within primary sulfides and calc-silicate minerals. Also introduced during this stage were minor amounts of galena, a variety of bismuth and silver telluride minerals, and trace amounts of scheelite and uraninite.;A much later, lower-temperature event, referred to as the hypogene oxidation event, resulted in intense oxidation of the skarn protore, and formation of a sub-vertical, cylindrical body of goethitic jasperoid. The jasperoid has no copper values 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 jasperoid formed during oxidation and breakdown of the primary skarn. Fluids that formed the jasperoid body must have been oxidized, which explains the abundance of goethite, and therefore would have been incapable of mobilizing gold. The distribution of gold within the jasperoid most likely reflects the grade 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, massing up to 47lbs. Elsewhere, copper forms impressive stockworks with geometries similar to calcite veins found within the jasperoid.;Stable isotope analysis of calcite veins collected at different depths of the mine shows that the fluids responsible for calcite deposition were shallow groundwaters, and not deep geothermal fluids. Oxygen-isotopes indicate an increase in temperature of the groundwater with increase in elevation in the mine, which is opposite to the usual case. It is hypothesized that groundwater was heated near the top of the water table by the oxidation of primary sulfides (pyrite, pyrrhotite) in the skarn ore, a process sometimes referred to in the mining industry as "self-heating." Convection of heated groundwater was centered directly over the main mass of sulfide ore, and resulted in the observed intense oxidation and alteration of the primary skarn. This idea of hypogene oxygen from self-heated meteoric water differs from conventional supergene models for formation of chalcocite and native copper.;Keywords: skarn, copper, gold, silver, jasper, secondary enrichment.