A comparison of rock and soil samples for geochemical mapping of two porphyry-metal systems in Colorado

Abstract

Paired rock and soil samples were collected at widely spaced locations in large segments of the porphyry-metal systems of the Montezuma district in central Colorado and of a northwestward extension of the Summitville district into Crater Creek in southern Colorado. The paired samples do not covary closely enough for one sample medium to proxy for the other. However, the areal distributions of elements in both rocks and soils in these two districts conform to alteration zoning as defined by mineralogy. Differing geochemical patterns of rocks and soils reflect species-dependent responses to weathering. Soils appear to be statistically enriched in ore elements and depleted in rock elements as compared to the matching rocks. These differences are largely artificial, owing to different methods of sample preparation and chemical analysis for rocks and for soils. The distributions of metals in soils delineate the occurrence of ore-metal minerals, mostly from vein deposits, whereas the distributions of metals in rocks conform to zones of pervasive hydrothermal alteration and to the distribution of varied mineral deposits among these zones. Rock and soil samples are equally useful, of comparable map resoluton, and complement one another as a basis for geochemically mapping these porphyry-metal systems.

Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA

“…The large MSWD suggests an open system or substantial underestimation of the error associated with the determined date. Patton (1917), Steven and Ratté (1960), Neuerburg et al (1978), and Neuer burg (1978) documented porphyry-epithermal-style mineralization in the Crater Creek area. Alteration and mineralization are localized, and all the intrusions have unaltered domains.…”

“…Hydrothermal alteration and mineralization are spatially associated with several postcaldera intrusions of the Platoro caldera magmatic system, most notably the Alum Creek, Alamosa River, and Summitville intrusions. Previous studies (Steven and Ratté, 1960;Mehnert et al, 1973;Lipman, 1975;Neuerburg, 1978;Bethke et al, 2005) asserted causative and/or temporal association of mineralization with individual plutons in area mineralization districts. However, the lack of geochronologic data limits determination of both spatial and temporal interpretations of genetic links to intrusion emplacement.…”

“…Mineralization in the Summitville, Platoro, Stunner, and Jasper districts has been interpreted as being related to postcaldera structures and postcaldera magmatic activity of the Platoro caldera complex (Patton, 1917;Steven and Ratté, 1960;Mehnert et al, 1973;Lipman, 1975;Neuerburg, 1978;Bethke, 2011). The largest deposit, the Summitville Au-Ag-Cu deposit, is a 22.5 ± 0.5 Ma high-sulfidation, epithermal deposit associated with the quartz dacite volcanic dome at South Mountain and underlying quartz monzonite porphyry intrusion (Bethke et al, 2005).…”

“…Largely subeconomic mineralization in the other districts has been minimally studied, and relationships to postcaldera magmatic activity are not well established; however, mineralization in the Jasper district occurs within a postcaldera pluton at the intersection of two caldera-related faults (Lipman, 1974(Lipman, , 1975. Molybdenite vein stockwork and copper-lead-zinc vein mineralization (Neuerburg, 1978) and alteration in the Crater Creek area are localized around plutons and along northwesttrending faults and dikes.…”