Boiling as a mechanism for colour zonations observed at the Byrud emerald deposit, Eidsvoll, Norway: fluid inclusion, stable isotope and Ar–Ar studies

The B yrud emerald deposit comprises pegmatite veins hosted within C ambrian black shales and L ate C arboniferous quartz syenite sills intruded by a P ermo‐ T riassic riebeckite granite. The emerald deposit genesis is consistent with a typical granite‐related emerald vein system derived from dominantly magmatic fluids with minor contributions from metamorphic source(s). Muscovite from an emerald‐bearing pegmatite at B yrud yielded an excellent A r– A r plateau age of 233.4 ± 2.0 Ma. Emerald display colour zonation alternating between emerald and beryl. Two dominant fluid inclusions types are identified as follows: two‐phase (vapour+liquid) and three‐phase (brine+vapour+halite) fluid inclusions and these are interpreted to represent conjugate fluids of a boiling system. The emerald was precipitated from these saline fluids with approximate overall salinities on the order of 31 mass per cent N a C l equivalent. Raman analyses indicate molar gas fractions for CO 2 , N 2 , CH 4 and H 2 S are approximately 0.8974, 0.0261, 0.0354 and 0.0410, respectively. Formational temperatures and pressures of approximately 160–385°C and below 1000 bars were derived from fluid inclusion data and lithostatic pressure estimates from fluid inclusion studies within the O slo rift. The colour zonation observed in the B yrud emerald crystals is related to alternating emerald and beryl precipitation in the liquid and vapour portions, respectively, of a two‐phase (boiling) system.