Isotopic and geochemical studies of a Pliocene porphyry-mo system, Rico, Colorado
The historic mining district of Rico in the southern Colorado Mineral Belt contains a Pliocene porphyry-Mo deposit and peripheral epithermal Pb-Zn-Ag deposits, and hot-springs. The porphyry-Mo mineralisation is confined almost exclusively to Precambrian greenstone. The epithermal mineralisation is hosted by a Palaeozoic and Mesozoic sequence dominated by carbonates, but containing evaporites. The system is geologically complex and focused on a resurgent dome which is cored by a horst of Precambrian strata, and cut by reactivated Precambrian basement shears. The shears have controlled the emplacement of the Laramide and Pliocene granitoids in area. The mineralisation is associated with more evolved members of the latter suite. Sr-Nd-Pb-O isotope and minor element data on the granitoid intrusions indicate that they are not simply differentiates of mantle magmas. However, Nd model ages indicate that they are not purely remelts of 1800Ma Precambrian crust. Realistically it is impossible to quantify the relative proportions of crust and mantle material involved in the genesis of the intrusions. Notwithstanding this, the Rico granitoids are isotopically distinct from those associated with Climax-type porphyry-Mo deposits in Colorado. Sr-Nd-Pb isotope and trace element data on a lamprophyre suite in the area suggest two episodes of mafic magmatism; a period whereby the source was predominantly subduction modified lithospheric mantle, and a period whereby the source was predominantly, OIB-type, asthenospheric mantle. δ-34S data and ore deposit paragenesis indicate that the sulphide S has an igneous origin and that the sulphate S was derived by mobilisation of Pennsylvanian evaporites. Modelling of the S isotopic data indicates a common S source for the epithermal and porphyry mineralisation. The sulphate and sulphide S reservoirs remained essentially decoupled during mineralisation. δ13C data are compatible with the hydrothermal C having been derived by the dissolution and re-precipitation of host sequence carbonate. Pb and Sr isotope analyses of ore and gangue minerals support the conclusions reached in the stable isotope study. The Sr isotope study indicates that the Sr isotopic composition of the hydrothermal fluids varies considerably. Pb isotope ratios measured on molybdenite, pyrite, and galena indicate that the mineralisation at Rico derived its Pb from source with a higher Th/U ratio than that involved in the genesis of Climax-type porphyry-Mo deposits. Sr and O isotope analyses of Laramide sills present throughout the Rico area indicate that the Sr isotopic composition of the hydrothermal fluids varied and that the Sr isotopic composition of altered sills is not solely a function of alteration temperature; the proportion of altered primary feldspar and the growth of Sr-rich secondary minerals is also important. REE data on these sills indicate that hydrothermal alteration mobilised and fractionated these elements, but that mobility is not simply a reflection of alteration temperature. [SO4]4- appears to have been an important REE complex in the near surface oxidising environment.