Blue lace agate and chalcedony pseudomorphs from Ysterputs in southern Namibia

Abstract

The Blue Lace Agate mine exploited a hydrothermal vein deposit at Ysterputs in southern Namibia until 2017. The deposit occupies a shear zone and associated fractures in a Jurassic dolerite sill that intruded a Permian marine deposit. Much of the chalcedony constituting the agate is the length-slow variety ‘quartzine’, indicating deposition in a saline environment, presumably derived from the marine sediments. Associated minerals are dolomite, forming an initial coating on the fissure wall and brecciated dolerite fragments, as well as late-stage crystals of ankerite, calcite, dolomite, gypsum and quartz. Ysterputs is also the source of cubic chalcedony pseudomorphs, originally thought to be after fluorite, but probably after melanophlogite.