The fault-controlled kimberlitic Sloan diatreme penetrates Precambrian granitic rocks near Prairie Divide in northern Colorado. The kimberlite is predominantly an intrusive breccia in which clasts consist of serpentine pseudomorphous after olivine and pyroxene, with variable amounts of magnesian ilmenite, perovskite, pyrope, chrome diopside, phlogopite, biotite, chromite, picotite, and magnetite. The matrix of the breccia consists of finely crystalline serpentine, calcite, dolomite, phlogopite, hematite, chlorite, and talc.

Chemical analyses of the kimberlite show low concentrations of SiO2, Al2O3, and K2O and relatively high amounts of MgO and H20. These chemical trends and a low phlogopite content suggest a basaltic kimberlite affinity.

Included in the pipe are numerous xenoliths of Precambrian felsic rocks, Upper Ordovician to Silurian (?) carbonates, kimberlite and lherzolite nodules (and mineral inclusions), and phlogopitic carbonate nodules. Stable carbon and oxygen isotope analyses of carbonate inclusions suggest a magmatic origin for the phlogopitic and a few other carbonates, which probably represent a carbonatite liquid that was associated with the original kimberlitic magma.

A very Late Silurian to Early Devonian age of emplacement is postulated.

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