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Mantle Indicator Minerals in Ant Mounds and Conglomerates of the Southern Green River Basin, Wyoming
Minerals of upper mantle origin occur in conglomerates, pediments and ant mounds in the southern Green River Basin. Chrome-bearing pyrope, pyrope-almandine, diopside, spinel and picroilmenite 6 mm in size are found on ant mounds with similar minerals as large as 12 mm in diameter in the Bishop Conglomerate. The minerals are from disaggregated eclogite and peridotite, similar to minerals found in mantle-derived igneous rocks such as kimberlite and lamproite, the primary host rocks for diamond. No kimberlites or lamproites are known in the immediate area, although the region is above the Archean Wyoming craton which is a conducive setting for diamond occurrence. Current assessment of the minerals using established geochemical criteria suggests that prior coexistence with diamond is unlikely. The ant mound and pediment materials are derived from erosion of the Bishop Conglomerate, whose source area is the Uinta Mountains. Extensive sampling of present-day streams in the central Uinta Mountains produced a few anomalies of only 1-2 grains 0.25 mm or less in size; no continuous mineral train exists between the occurrences in the Green River Basin and those in the Uintas. Minerals of similar chemistry do occur in the Leucite Hills lamproites in the Rock Springs uplift, and in the Kamas lamproites in the western Uinta Mountains. The erosional history of the basin prohibits either area to be the source of the Green River Basin occurrences. Dramatic drainage pattern shifts and extensive glaciation since the Oligocene have hindered efforts to locate a bona fide igneous host for the minerals.
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