ABSTRACT: The Emet borate deposits are shown to be older and mineral - ogically more complem than was previously ihoughtj and to have formed within the muds of play a lakes during the Middle Oligocène. The borate minerals formed in two geochemically distinct sedimentary basinsj and are inierbedded with mmls^ volcanic tuffs and days? which appear to have been derived mostly from a volcanic terrain. Oolemanite predominates^ but other borates include meyerhofferiteß ulemiteß iunellite; teruggite, cahnite? hydroboraci « te and a form of veatchite. Calcitej gyp - sum; celesiiie* native sulphurj realgar and orpiment are the principal non-borates* The genesis of colemanite is problematicaL Evidence form elsewhere suggests that it may form by replacement of uleoeite or by dehydration of inyoite after burial. At Emetß no field evidence has been found which unambiguously supports either mode of formation. Later generations of colemanite occur in vughs and veins j and as fibrous margins to colemanite nodules. Other diageneiic changes include the par- Hal replacement of colemanite is often almost completely replaced by caleiie. Reserves are ample, but the presence of arsenic in both the sulphide and borate phases can present problems during mine* ml processing.