The Kalana vein-hosted gold deposit, southern Mali

• First detailed description of the Kalana deposit in English
• One of few documented cases of late-tectonic gold deposits in these greenstone belts
• It presents structural, mineralogical and sulfide crystallo-chemical data on a very atypical deposit.
• Gold precipitates during two distinct episodes, with different parageneses.

The Kalana gold deposit occurs within metamorphosed Paleoproterozoic volcano-sedimentary series of the Birimian Baoulé-Mossi domain of the West African Craton, in the south of Mali. These formations are intruded successively by small dioritic bodies, as well as andesitic and tonalitic dykes, which exhibit volcanic arc-setting signatures. Mineralization is hosted by two sets of quartz veins that intersect the regional schistosity. The first set of veins is the most important in terms of grades and size, and consists of thick veins (up to meter size) that range in trend from N–S to E–W with shallow plunges. The second set consists of much thinner (centimeter size) sub-vertical quartz veins oriented NE–SW. The two sets of veins are interpreted to have formed during the evolution of late, gently dipping thrust faulting. Two episodes of gold precipitation are recognized: a first episode, during early stages of vein growth, formed micron-size native gold inclusions in arsenopyrite; a later episode, during vein shearing and fracturing of the quartz lodes, precipitated native gold in free form in quartz, in fractured arsenopyrite, and associated with chalcopyrite, galena, sphalerite, Bi sulfides and native bismuth. All evidence suggests that the Kalana deposit represents orogenic gold mineralization formed during a relatively long-lived hydrothermal system, at a late stage of the tectonic history of the greenstone belt.

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