Colorado-Wyoming kimberlite province a rich and largely

The Colorado-Wyoming kimberlite province in the western U.S. includes more than 100 kimberlite intrusives, one of the largest lamproite fields in the world, and dozens of unexplored geophysical, remote-sensing and heavy-mineral anomalies.

The area known as the Colorado-Wyoming State Line District (CWSLD) consists of 35 Early Devonian kimberlite dykes and diatremes, many of which are diamondiferous. Among the companies exploring this district, which extends 3 miles north into Wyoming and 10 miles south into Colorado, are: A&E Resources, Bald Mountain Mining, Colorado Diamond, Fleck Resources, MPH Consulting, Redareum Red Lakes, Royal Star and Union Pacific Resources. To date, the CWSLD has recovered more than 112,000 gem and industrial diamonds, ranging from microdiamonds to a 6.2-carat gemstone, which was recovered last fall by the Colorado Diamond Co. Overall, the quality ratio of industrial to gemstones is favorable, with reports of as much as 50% gemstones being recovered from some deposits.

Kimberlites in the CWSLD consist of hypabyssal and diatreme facies. Estimates suggest the known kimberlites are deeply eroded and as many as half of the original pipes may have been removed. In other words, large amounts of diamond should lie downstream from the eroded kimberlites, although no effort has been made to locate diamond placers.

In 1981, an airborne INPUT survey over the Wyoming portion of the district by Patterson, Grant & Watson identified conductivity anomalies that were not associated with any known kimberlite. The survey also detected several weak to strong magnetic anomalies. One group of prominent magnetic anomalies along the northern edge of the district was interpreted to be the manifestation of undiscovered pipes. To date, these anomalies have not been explored. Several remote-sensing anomalies were also outlined in the district in 1984 by the University of Wyoming’s Department of Geology and Geophysics and the Wyoming Geological Survey (WGS), and some of these are also unexplored. The province extends many miles north and south of the CWSLD. Kimberlites have been found as far south as Boulder and Estes Park, Colo., and as far north as the Iron Mountain and Middle Sybille Creek areas of Wyoming. To date, only the State Line kimberlites have yielded diamonds, although subcalcic G10 pyropes from the Iron Mountain District suggest those kimberlites also tapped the diamond stability field. However, research by Professor M.E. McCallum of Colorado State University indicates groundmass ilmenites from the Iron Mountain kimberlites were formed under oxidizing conditions, suggesting that any diamonds in those rocks would most likely have been destroyed.

North of the CWSLD, the WGS identified nearly 300 kimberlitic, heavy-mineral anomalies (pyrope garnet, chromian diopside, and/or picroilmentite) over a 1,200-sq.-mile area in the Laramie Range. Heavy mineral anomalies have also been identified in the Seminoe Mountains greenstone belt of central Wyoming, and in the Medicine Bow Mountains and the Green River Basin of southern Wyoming. The anomalies in the Green River Basin are scattered over a few hundred square miles in ant hills and road cuts. Most of these anomalies remain unexplored.

The Green River Basin also hosts one of the three major lamproite fields in the world, known as the Leucite Hills. The Leucite Hills are receiving some exploration activity and the WGS is currently processing a small bulk sample of olivine-bearing lamproite for diamond.

— W.D. Hausel is the senior economic geologist of the Wyoming Geological Survey.