Variations in Isotopic Composition of Oxygen and Carbon in Leadville Limestone (Mississippian, Colorado) and in its Hydrothermal and Metamorphic Phases

Abstract

The isotopic composition of oxygen and carbon in the Leadville limestone and in the hydrothermal dolomites and contact metamorphic marbles that replace the limestone is related to the geologic occurrence of these rocks. The Leadville formation includes widespread uniform beds of limestone, dolomite, and chert that show no obvious secondary alteration. The oxygen isotope composition of these beds approximates that of other apparently "unaltered" carbonate sediments of pre-Mesozoic age. Moreover, isotopic fractionation measured between calcite and quartz also suggests that the beds crystallized at near-surface temperatures. In contrast, Leadville quartz and carbonates in the great halo of hydrothermal dolomite that is spatially and temporally related to sulfide ore at Gilman, Colorado, yield a range of oxygen isotope values, all lower in O¹⁸, that are indicative of much higher temperatures. Differences in oxygen isotopes in quartz, dolomite, and occasional calcite within the halo in the Gilman area are fairly systematic with respect to ore and obvious conduits. Highest temperatures are indicated at ore and at conduits. Successively lower temperatures of formation seem indicated toward the perimeter of the dolomite halo, 2-15 miles away. Consequently, the measured variations in the isotopic composition of oxygen may prove to be a useful guide to ore. They clearly contribute to an understanding of the origin and emplacement of these deposits. The oxygen isotope compositions of quartz, calcite, and dolomite in the Yule marble, a contact-meta-morphic phase of the Leadville formation, and in the Leadville calcite adjacent to quartz-latite sills are intermediate values consistent with the geological evidence that these types of metamorphism were relatively dry.

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