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The discovery of diamonds in kimberlite diatremes in 1975 led to a joint effort by Colorado State University and the Wyoming Geological Survey to explore for additional kimberlite occurrences within Colorado and Wyoming. Presently, more than 90 separate kimberlite localities are known in the Colorado-Wyoming State Line district and the Iron Mountain district of Wyoming. Additionally, an isolated kimberlite pipe is present west of Boulder, and there is a kimberlite dike in the Estes Park area of Colorado, extending the known kimberlite occurrences in a roughly north-south trend over approximately 120 mi (192 km). Diamonds have been recovered only from diatremes in the State Line district, except for an isolated occurrence of placer diamonds recently identified in stream-se iment concentrates from the Medicine Bow Mountains.
Exploration continues with the examination of the Front Range by available remote-sensing imagery. Target areas given highest priority are those showing apparent relations and similarities to known kimberlite districts. Drainages in these areas are systematically sampled for heavy mineral indicators (i.e., pyrope garnet, magnesium ilmenite, chrome diopside), and the heavy mineral "trains" are traced upslope to potential kimberlite sites. Detailed ground surveys are conducted over several miles around all new discoveries, with special emphasis placed on associated linear trends (faults, dikes, joints, etc). Limited soil and alluvial geochemical sampling has been used with variable success.
Several geophysical methods have been used, but electrical resistivity and magnetics appear to be the most useful. Electrical resistivity methods show that weathered kimberlite is highly conductive (80 to 250 ohm-ft) compared to the enclosing Precambrian granitic host rocks (300 to 7,400 ohm-ft) and that magnetics are variable, showing only small dipolar anomalies (±30 to 150 ^ggr).
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