A LEAD, STRONTIUM, AND SULFUR ISOTOPE STUDY OF LARAMIDE-TERTIARY INTRUSIONS AND MINERALIZATION IN THE COLORADO MINERAL BELT WITH EMPHASIS ON CLIMAX-TYPE PORPHYRY MOLYBDENUM SYSTEMS PLUS A SUMMARY OF OTHER NEWLY ACQUIRED ISOTOPIC AND RARE EARTH ELEMENT DATA.

摘要

The Colorado Mineral Belt (COMB) is characterized by Late Cretaceous to late Tertiary felsic to intermediate intrusions with related base and precious metal mineralization. The COMB hosts three Climax-type molybdenum deposits, associated with young (Oligocene-Miocene), composite granite porphyry stocks.;Climax-type granites have low-radiogenic ('206)Pb/('204)Pb (as low as 17.3) and near normal ('208)Pb/('204)Pb ratios, which indicate that their source contains low U/Pb and average Th/Pb ratios. Older intermediate intrusions have similar lead isotope patterns, but in any given region, Climax-type granites have the lowest ('206)Pb/('204)Pb ratios. Lead isotopic data for all COMB intrusions plot below ('207)Pb-('206)Pb and above ('208)Pb-('206)Pb average growth curves. Secondary Pb-Pb isochron ages range from about 1400 to 2300 m.y. and suggest a component of older lead, possibly from Archean detritus, in some Proterozoic source rocks.;Initial ('87)Sr/('86)Sr ratios increase and subsequently decrease with decreasing geologic age. Older intermediate intrusions have the lowest ratios (0.707). After peaking during the formation of Climax-type granites (0.710 to 0.740), initial ratios for younger felsic rocks decrease (0.709).;Estimated whole rock (delta)('18)O values for all COMB intrusions range from +7 to +9('o)/oo. Climax-type granites tend to be isotopically heavier (+8 to +9('o)/oo).;Sulfur isotope data from Climax-type molybdenites show little variation within or between deposits and indicate a magmatic sulfur source enriched in ('34)S. Hydrothermal processes alone cannot produce the ('34)S enrichments ((delta)('34)S to +5.3('o)/oo). Molybdenites from COMB prospects are not enriched ((delta)('34)S (TURNEQ) 0('o)/oo).;Chrondrite-normalized rare earth element patterns for Climax-type granites are steep-sided and U-shaped (La/Yb(,N) (TURNEQ) 6), and display large negative europium anomalies. Older granodiorites and quartz monzonites generally have straight-line patterns with moderate negative slopes (La/Yb(,N) (TURNEQ) 18).;Sequential samples of host rock outward from Climax-type systems document a systematic isotopic overprinting by the evolving granite complex and ore fluids. No measurable isotopic contamination of the granite complex by host rocks is observed.;Older, intermediate composition plutons and batholiths in the COMB are not genetically related to, nor did they produce, younger Climax-type granite complexes associated with economic molybdenum deposits. Isotopic data preclude a mantle source and suggest lower crustal origins for both felsic and intermediate COMB igneous rocks.