Merumite occurrence in Guyana

Merumite was discovered with associated diamonds and gold in 1937 in gravels of the Merume River in Guyana. It was described as essentially a hydrous chromium oxide that contains more than 80 percent Cr2Oa. Milton and Chao in 1958 found it to be a complex aggregate, mainly eskolaite (Cr2Oa) with five or more new chromium minerals which have recently been identified. The deposit is unique. The richest gravel, averaging several ounces merumite per cubic yard, extends about 2 miles along the base of an east-dipping (35°) ridge of sandstone and ash beds, perhaps an outlier of the Precambrian Roraima Formation that forms bold mountainous scarps a few miles south. No chromium mineralization has been observed in the ridge or anywhere in the region, other than the merumite in the placer gravel. Merumite commonly occurs as grains a few millimeters across, but specimens as large as 10 cm have been found. Almost all the merumite is in rounded grains, many with broken worn edges, indicating wear in transport. Granular gold is enclosed in merumite, as is chromian pyrophyllite. Many merumite grains have impressions, and some contain crystals, of doubly terminated "needle" quartz. Water-worn gorceixite, rutile, tourmaline-quartz fels, jasper, euhedral glassy quartz (as much as several centimeters long), and fragments of basaltic rock accompany merumite at all localities; rarely, gold and diamonds are associated in the placers. Merumite was probably derived from a local moderate-temperature hydrothermal deposit possibly formed in the adjacent sandstone-volcanic ash bed from solutions related to local ash deposits and massive gabbro-dolerite intrusive bodies in the Roraima Formation. If the deposit was formed at depths of less than a few thousand feet, erosion may have reached it and re-deposited the merumite and accompanying resistant minerals, mostly'quartz and jasper, in local stream beds. © 1969 Society of Economic Geologists, Inc.