AN INVESTIGATION OF THE FLUORSPAR VEINS AND LOST GOLD MINES FOUND WITHIN THE ALLARD STOCK, LA PLATA MINING DISTRICT, COLORADO, U.S.A

The ~68 Ma Allard stock in the La Plata Mountains has a long history of exploration for copper mineralization. Fluorite-rich quartz veins exposed proximal to historic gold and copper mines hold promise for precious metals mineralization but were not extensively examined in previous studies. In this investigation, field mapping combined with petrologic and petrochemical studies were employed on the veins to determine their distribution, mineral assemblage, paragenesis, and potential for Au.

Fluorite veins in the study area are up to 80 cm wide and trend on strike for tens of meters. The dominant trends of the veins are ~N 296° W and ~N53° E. Veins vary from massive quartz + fluorite to zoned veins with fluorite-rich cores and dark fine-grained margins. The dominant mineral assemblage in the veins is fluorite + polygranular quartz + chalcedony ± apatite ± (Cu, Fe, Pb, Zn sulfides). In some sample there is a distinct zonation of fluorite rich zones to chalcedony + quartz or carbonate rich zones.

Geochemical analyses of 16 selected vein samples reveal Ag concentrations from 585 to 13,696 ppb and Au from 2.1 to 147 ppb. Copper concentrations range from 43 to >10,000 ppm with a mean of 1,528 ppm.

The fluorite veins in the Allard stock cut all other rock units and structures and are interpreted to have formed from late-stage fluids associated with Laramide magmatism. Overall, these veins themselves do not have economic potential as they are limited to certain zones of the stock and do not contain high levels of precious metals. However, through examination of the historic Allard Tunnel, Copper Age Mine, Broadway Mine, and Esmerelda Mine, it was found that fluorite veins and breccia dikes are running through each. Mapping suggests that this spatial correlation is related to NE trending fracture zones. These fracture zones were the structures and conduits likely reactivated by the fluorite event which carried the gold and copper impregnated fluids culminating in deposition of rich tellurides.