Tourmalinites occur at five stratigraphic levels within the low metamorphic grade multiply deformed Lower Proterozoic metasediments and metavolcanics of the Golden Dyke Dome. The geological setting, exhalite associations, sedimentary structures and tourmaline chemistry all preclude a granitic origin. Tourmalinite derived from the isochemical metamorphism of an exhalite comprising silica and tourmaline. Tourmalinite and other exhalites are hosted by carbonaceous pelitic metasediments and were deposited in deeper basinal areas. The ore fluid was probably dolerite-heated seawater which leached a thick pile of argillaceous sediments and acquired metals and boron by leaching. The first and second increase in geothermal gradient produced tourmalinite, the third produced auriferous tourmalinite and iron formations with tourmalinite and exhalative Pb-Zn deposits forming during the period of maximum geothermal gradient. More tourmalinite formed during the subsequent decline in geothermal gradient. Tourmalinites are regarded as B-rich iron formations which can be the host for or associated with exhalative deposits of Au and base metals.