The geology and geochemistry of the Daisy Creek Prospect, a stratabound copper-silver occurrence in Western Montana

Stratabound copper, lead, and silver minerals at the Daisy Creek prospect are hosted by crossbedded feldspathic quartzites and orthoquartzites of the Bonner Formation, Missoula Group, Belt Supergroup. Detailed float rock-chip mapping of a 60,000 m² soil grid indicates that mineralization occurs in reduced, coarse grained quartzites, over- and under—lain by fine grained, hematitic quartzites and siltites. Sixty km² regional mapping shows that the prospect lies on the eastern limb of a major broad, open syncline, and that two phases of deformation have folded the strata. Paragenetic relationships in float rock—chips indicate that early diagenetic pyrite was replaced successively by galena, chalcopyrite, and argentiferous bornite and chalcocite. Later, middle diagenetic oxidation resulted in the formation of cupriferous goethite, barite, and acanthite. Holocene supergene oxidation formed abundant malachite, cerrusite, and pyromorphite from these sulfides and earlier oxidation products. Soil grid geochemical anomalies of Cu, Pb, Zn, Ag, Ba, and Hg occur as overlapping zones over the prospect. These geochemical zones overlie and correlate well with areas containing different gossan types. The types of different gossan morphologies and anomalous geochemical concentrations suggest that the zones are mineralogically related and that they are characterized by the presence of galena, chalcopyrite, bornite, and chalcocite. The paragenetic relationships, geochemical soil zoning geometries, gossan morphologies, and geochemical and mineralogical constraints on the mineralization suggest that sulfide deposition occurred before the onset of silica cementation. Furthermore, it involved the introduction of metal-chloride complexed, oxidized ground waters along an aquifer into locally reduced sediments. Diffusion of copper, and later oxygen, from the oxidizing ground waters into the reduced zone resulted in the formation of Cu-sulfides and their subsequent oxidation. A new method of statistical analysis involving the regression of stream sediment data from around the Daisy Creek prospect reveals that the prospect can be precisely located using this technique, and that numerous other 'false anomalies' are recognized. Traditional methods for anomaly recognition fall short of providing the information derived from this technique.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat