Boiling as a mechanism for colour zonations observed at the Byrud emerald deposit, Eidsvoll, Norway

The Byrud emerald deposit was first discovered in the early 1860s, near Lake Mjøsa, southern Norway. From a geological, gemmological, and geochemical perspective, the Byrud emeralds are of great interest as they are vanadium-rich rather than chromium-rich, which is uncommon for the majority of emeralds worldwide. The emerald deposit genesis is consistent with a typical granite-related emerald vein system from dominantly magmatic fluid sources. Emerald mineralization occurs within pegmatite veins which are hosted within Cambrian black shale and Late Carboniferous quartz syenite sills, and intruded by a Permo-Triassic alkaline granite. Muscovite from an emerald-bearing pegmatite at Byrud yielded an Ar-Ar plateau age of 233.4 ± 2.0 Ma. The emerald display colour zonation alternating between emerald and beryl. Two dominant fluid inclusions types are identified: two-phase (vapour+liquid) and three-phase (brine+vapour+halite) fluid inclusions. Raman analyses indicate molar gas fractions for CO2, N2, CH4, and H2S are approximately 0.8974, 0.0261, 0.0354, and 0.0410, respectively. Formational temperatures and pressures of approximately 160 °C to 385 °C and below 1000 bars were derived from fluid inclusion data and lithostatic pressure estimates from fluid inclusion studies within the Oslo Paleorift.The emerald from the Byrud mine displays growth zoning and banding from green emerald to colourless beryl. The use of chemical analyses, fluid inclusions and petrography is consistent with boiling as a mechanism for this distinct ‘striped’ colour zonation or banding. These zones are due to enrichment of vanadium and chromium in the green bands which host salt-saturated aqueous primary fluid inclusions with an overall 31 mass percent NaCl equivalent salinity. Vapour-rich primary fluid inclusions dominate within the colourless bands of the zoned emerald. The chemical compositions of the salt-saturated and the vapour-rich inclusions indicate they represent the coexisting liquid and vapour portions of a two-phase boiling system. This “boiling” formational model has application to not only other emerald deposits, but also other zoned gem deposits worldwide.