Abstract

The early Proterozoic (Birrimian) greenstone belts of Boromo and Hound, Burkina Faso, were studied by the geology and mining projects of the United Nations Development Program in Burkina Faso. The southern extremities of both belts are north-northeasterly trending, but farther north in the center of the country the trend changes to northeasterly. The belts consist of steeply dipping volcanic and volcano-sedimentary units intruded by Eburnean granitoids (ca. 2,000 m.y.), all of which have undergone incipient to low greenschist facies metamorphism.Many former gold prospects and mines are known from both greenstone belts, including the Poura vein gold deposit, the Dossi shear zone prospects, the silicified gold-bearing zone of Kiere, and the gold- and scheelite-bearing quartz veins of Bagassi. In addition, more recently, gold anomalies have been found at Kwademen, Zoulou, Semapoum, Koupela-Nagsene, and Koudma. Most of the primary gold occurrences were considered to be of metamorphogenic origin, having been derived from auriferous source rocks of the older volcanic and volcano-sedimentary units.Studies were made of the distribution of gold in Birrimian basalts, andesites, rhyodacites or rhyolites, and exhalites and in Eburnean granites and granodiorites. Samples weighing 1.5 to 2.0 kg were collected from 109 bed-rock exposures of these rock types from beyond the alteration areas and zones of favorable gold mineralization. These were then analyzed by atomic absorption spectrometry to determine the background gold content of the unmineralized rocks of the greenstone belts. Copper, nickel, and zinc contents of the Birrimian volcanic rocks and of the Eburnean granitoids were also determined.The analytical data showed that gold was more abundant in the Birrimian volcanic rocks and exhalites than in the Eburnean granites and granodiorites. Furthermore, the gold content was shown to increase from basalts to andesites to rhyodacites to exhalites. Among different types of exhalites, including chert and tuffaceous exhalites rich in manganese and iron, and in quartzite, the gold content varied from 0.1 to 9.7 ppb (mean 2.97 ppb). Because of the petrographic variability of these rocks further detailed studies of their gold contents are required.The gold contents of rocks surrounding zones of confirmed gold mineralization were generally higher than elsewhere, ranging from 3 to 10 ppb, except in the mineralized zones themselves where it exceeded 10 ppb.

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