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

Abstract

The Byrud emerald deposit comprises pegmatite veins hosted within Cambrian black shales and Late Carboniferous quartz syenite sills intruded by a Permo‐Triassic 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 Byrud yielded an excellent Ar–Ar 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 NaCl equivalent. 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 160–385°C and below 1000 bars were derived from fluid inclusion data and lithostatic pressure estimates from fluid inclusion studies within the Oslo rift. The colour zonation observed in the Byrud emerald crystals is related to alternating emerald and beryl precipitation in the liquid and vapour portions, respectively, of a two‐phase (boiling) system.

Pressure–Temperature–Fluid Constraints for the Poona Emerald Deposits, Western Australia: Fluid Inclusion and Stable Isotope Studies

“…It is also possible that our limited sample size for emerald and quartz sampled various proportions of two mismatched precipitation events of the quartz and beryl (cf. [7]). The range in δD VSMOW values from −22 to −55 shows two small but distinct data sets, three points with higher δD values corresponding to a distinctly metamorphic fluid, while the second data set of two points of lower δD values overlaps both the igneous and metamorphic fluid fields (Figure 11).…”

“…Na concentrations are important as they are proportional to water contents within emerald [26]. [6,7,25,27]. Data are normalized from wt % microprobe analyses (Table 1), with Fe data reported as FeO.…”

“…The FIs studied occur as planes of inclusions along healed fractures, growth zones or as isolated inclusions. Growth zones observed in zoned emerald (Figure 3) make it possible to identify emerald-hosted primary fluid inclusions belonging to FIT-1, based on timing relationships between fluid inclusions and mineral growth characteristics [17,18,35] or with c-axis parallel primary-fluid-inclusions ( Figure 8) as in [7,25]. FIT-1 primary fluid inclusions occur in both the Cr-enriched (emerald) and clear beryl zones within the same crystal.…”

“…However, as these could be identified as necked-down [17], metastable, or of indeterminable origin [18], our efforts concentrated on the FIT-1 FIs as these were the only FIs unambiguously identifiable as primary and thus capable of yielding valid pressure, temperature, and fluid compositional data. The microthermometric measurements from FITs 1, 2 and 3 are presented in Table 3 [7,25]. Photomicrograph taken in plane polarised light.…”

Major and Trace Element Geochemistry of Dayakou Vanadium-Dominant Emerald from Malipo (Yunnan, China): Genetic Model and Geographic Origin Determination

“…V/Cr ratios of some samples are greater than 110. The highest Li, Cs, and LILE contents indicate a magmatic origin of fluid for emerald mineralization which is comparable to the Lened in Canada and Eidsvoll in Norway [14,43]. The Lened emerald shows a similarly high concentration of Cs but depletes in Rb (below the detection limit).…”