A strontium and oxygen isotope study of Laramide magmatic and hydrothermal activity near Central City, Colorado

Twelve Laramide intrusions, eleven country rocks, and over thirty hydrothermal minerals from the Central City and Idaho Springs mining districts, Colorado, have been analyzed for Sr isotope compositions. Intrusions have also been analyzed for oxygen isotopes. Precambrian country rocks have an average 87 Sr/ 86 Sr ratio at 59 m.y. ago of ca. 0.73, but with a high degree of local isotopic heterogeneity. Strontium contents and initial ratios in the Laramide intrusions define a roughly hyperbolic array from 0.705 (4,020 ppm Sr) to 0.722 (32 ppm Sr), whereas whole-rock delta 18 O compositions vary from 7.7 to 12.4 per mil (up to 16ppm in carbonate-altered rocks). Various mixing models which might give rise to these compositions are examined. Intermediate rocks probably contain a large fraction of lower crustal Sr, but a mantle-derived component cannot be ruled out. The more evolved quartz bostonites display assimilation processes during fractional crystallization of plagioclase in the upper crust.Hydrothermal sericites and ores have a very wide range of initial Sr isotope ratios (0.708-0.769), which are indicative of varying proportions of Sr from magmatic and country-rock sources contributing to the five different stages of hydrothermal activity. Low initial ratios found in sericite, pyrite, and fluorite from early and late molybdenite stages indicate a large contribution of magmatic Sr and hence possibly of other cations to these hydrothermal fluids. In contrast, the radiogenic Sr characterizing the uraninite, base metal, and late telluride stages is probably derived, along with other cations, from country rocks.