27. Giuliani, G; Groat, LA; Marshall, D; Fallick, AE; Branquet, Y.Emerald Deposits: A Review and Enhanced Classification.Minerals, 2019, 9 Emerald Deposits: A Review and Enhanced Classification
emerald deposits; classification; typology; metamorphism; magmatism; sedimentary; alkaline metasomatism; fluids; stable and radiogenic isotopes; genetic models; exploration
Although emerald deposits are relatively rare, they can be formed in several different, but specific geologic settings and the classification systems and models currently used to describe emerald precipitation and predict its occurrence are too restrictive, leading to confusion as to the exact mode of formation for some emerald deposits. Generally speaking, emerald is beryl with sufficient concentrations of the chromophores, chromium and vanadium, to result in green and sometimes bluish green or yellowish green crystals. The limiting factor in the formation of emerald is geological conditions resulting in an environment rich in both beryllium and chromium or vanadium. Historically, emerald deposits have been classified into three broad types. The first and most abundant deposit type, in terms of production, is the desilicated pegmatite related type that formed via the interaction of metasomatic fluids with beryllium-rich pegmatites, or similar granitic bodies, that intruded into chromium- or vanadium-rich rocks, such as ultramafic and volcanic rocks, or shales derived from those rocks. A second deposit type, accounting for most of the emerald of gem quality, is the sedimentary type, which generally involves the interaction, along faults and fractures, of upper level crustal brines rich in Be from evaporite interaction with shales and other Cr- and/or V-bearing sedimentary rocks. The third, and comparatively most rare, deposit type is the metamorphic-metasomatic deposit. In this deposit model, deeper crustal fluids circulate along faults or shear zones and interact with metamorphosed shales, carbonates, and ultramafic rocks, and Be and Cr (+/- V) may either be transported to the deposition site via the fluids or already be present in the host metamorphic rocks intersected by the faults or shear zones. All three emerald deposit models require some level of tectonic activity and often continued tectonic activity can result in the metamorphism of an existing sedimentary or magmatic type deposit. In the extreme, at deeper crustal levels, high-grade metamorphism can result in the partial melting of metamorphic rocks, blurring the distinction between metamorphic and magmatic deposit types. In the present paper, we propose an enhanced classification for emerald deposits based on the geological environment, i.e., magmatic or metamorphic; host-rocks type, i.e., mafic-ultramafic rocks, sedimentary rocks, and granitoids; degree of metamorphism; styles of minerlization, i.e., veins, pods, metasomatites, shear zone; type of fluids and their temperature, pressure, composition. The new classification accounts for multi-stage formation of the deposits and ages of formation, as well as probable remobilization of previous beryllium mineralization, such as pegmatite intrusions in mafic-ultramafic rocks. Such new considerations use the concept of genetic models based on studies employing chemical, geochemical, radiogenic, and stable isotope, and fluid and solid inclusion fingerprints. The emerald occurrences and deposits are classified into two main types: (Type I) Tectonic magmatic-related with sub-types hosted in: (IA) Mafic-ultramafic rocks (Brazil, Zambia, Russia, and others); (IB) Sedimentary rocks (China, Canada, Norway, Kazakhstan, Australia); (IC) Granitic rocks (Nigeria). (Type II) Tectonic metamorphic-related with sub-types hosted in: (IIA) Mafic-ultramafic rocks (Brazil, Austria); (IIB) Sedimentary rocks-black shale (Colombia, Canada, USA); (IIC) Metamorphic rocks (China, Afghanistan, USA); (IID) Metamorphosed and remobilized either type I deposits or hidden granitic intrusion-related (Austria, Egypt, Australia, Pakistan), and some unclassified deposits. DOI
26. Verbaas, J; Thorkelson, DJ; Crowley, J; Davis, WJ; Foster, DA; Gibson, HD; Marshall, DD; Milidragovic, D.A sedimentary overlap assemblage links Australia to northwestern Laurentia at 1.6 Ga.Precambrian Res., 2018, 305: 19-39 A sedimentary overlap assemblage links Australia to northwestern Laurentia at 1.6 Ga
Columbia; Nuna; Proterozoic; Paleogeography; Laurentia; Australia supercontinent; Detrital zircon; Wernecke Breccia; Olympic Dam
The Columbia (Nuna) supercontinent existed from approximately 1.9 Ga to 1.3 Ga. Laurentia was part of Columbia, and the western edge of Laurentia (current coordinates) was likely proximal to a large landmass during parts of this interval. Here, we present detrital zircon ages of a Paleoproterozoic sedimentary succession in northern Yukon, Canada, that bear on the evolution of Columbia. The sedimentary succession is preserved as lasts within 1.60 Ga hydrothermal megabreccias. Analyses of detrital zircon reveal abundant 1.78-1.68 Ga zircon with evolved Hf isotope values (-16.1 < epsilon Hf(t) < +1.4). Sm-Nd isotope analysis on clasts yields epsilon Ndi from -5.3 to -5.5 and model ages from 2.4 to 2.2 Ga. The detrital zircon age distribution is strikingly similar to those from sedimentary megaclasts in the ca. 1.59 Ga Olympic Dam Breccia Complex on the Gawler Craton of Australia. The whole rock Sm-Nd ratios are consistent with derivation from the Gawler Craton. We propose that the sedimentary material contained in both breccia complexes was derived from an overlap assemblage deposited on Australia and Laurentia at ca. 1.6 Ga. This model supports a previous hypothesis that the Gawler Craton was connected to northwestern Laurentia at ca. 1.6 Ga, and that these regions shared a single hydro thermal province that is recognized in northwestern Laurentia as the Wemecke Breccia and in the Gawler Craton as the Olympic Dam Breccia Complex and associated IOCG deposits. The sedimentary overlap succession was deposited after collision between Australia and Laurentia. Australia was subsequently translated southward along the Laurentian margin, placing the Gawler Craton next to southwestern Laurentia and the Mt. Isa Inlier adjacent to northwestern Laurentia by 1.5 Ga. DOI
25. Verbaas, J; Thorkelson, DJ; Milidragovic, D; Crowley, JL; Foster, D; Gibson, HD; Marshall, DD.Rifting of western Laurentia at 1.38 Ga: The Hart River sills of Yukon, Canada.Lithos, 2018, 316: 243-260 Rifting of western Laurentia at 1.38 Ga: The Hart River sills of Yukon, Canada
The Hart River sills are a set of mafic to intermediate intrusions that occur in northern Yukon, Canada. The largest sills are over 500 m thick and over 200 km long. New U-Pb dates of 1382.15 +/- 0.39 Ma and 1382.14 +/- 0.36 Ma were obtained via chemical abrasion thermal ionization mass spectrometry on zircon. Whole rock initial neodymium isotopic compositions of the Hart River sills are juvenile and have epsilon Ndi from +1.5 to +4.0. The primary mineralogy of the Hart River sills is predominated by clinopyroxene and plagioclase. Geochemical modeling indicates that the Hart River sills lie on a common liquid line of descent defined by a fractionating assemblage of plagioclase, clinopyroxene and minor olivine. The Hart River sills have rare earth element and high field strength abundances similar to normal mid-ocean ridge basalts (N-MORB) but are enriched in large ion lithophile elements. The Sm/Yb and Dy/Zr ratios indicate >8% partial melting of spinel-bearing mantle. During the emplacement of the Hart River sills, western Laurentia was juxtaposed with Australia and eastern Antarctica within the supercontinent Columbia. The degree of partial melting, similarity to N-MORB, and juvenile isotopic signature are consistent with an episode of rifting at 1.38 Ga. Coeval magmatism and intracontinental rift basins farther south on Laurentia provide additional evidence for rifting of supercontinent Columbia at 1.38 Ga. (C) 2018 Elsevier B.V. All rights reserved. DOI
24. Lake, DJ; Groat, LA; Falck, H; Mulja, T; Cempirek, J; Kontak, D; Marshall, D; Giuliani, G; Fayek, M.Genesis of emerald-bearing quartz veins associated with the Lened W-skarn mineralization, Northwest Territories, Canada.Can. Mineral., 2017, 55: 561-593 Genesis of emerald-bearing quartz veins associated with the Lened W-skarn mineralization, Northwest Territories, Canada
emerald; tungsten skarn; mineral chemistry; geochemistry; stable isotopes; Lened; Northwest Territories
Emerald at the Lened occurrence in the western Northwest Territories is hosted by quartz veins cutting skarn near the Lened granite pluton and older Selwyn Basin strata. Euhedral beryl crystals (<0.5 x 5 cm) are present in approximately half of the 26 outcropping veins. Most of the crystals are opaque to translucent and colorless to yellowish and grassy green. Less than 5% of the beryl is transparent, bluish green, and can be considered pale emerald. Using field relationships, Ar-Ar dating, whole-rock geochemistry, stable isotopes (O, H, C, and B), and mineral chemistry, the sources of the emerald-forming fluids and chromophores have been assessed; the results clearly show that the ca. 100 Ma (Ar-Ar muscovite) Lened emerald occurrence is a Type I (igneous) skarn-hosted emerald deposit related to the proximal ca. 100 Ma (Ar-Ar biotite) Lened pluton. Beryllium and other incompatible elements (i.e., W, Sn, Li, B, and F) in the emerald, vein minerals, and surrounding skarn were derived during the terminal stages of crystallization of the proximal Lened pluton. Decarbonation during pyroxene-garnet skarn formation in the host carbonate rocks probably caused local overpressuring and fracturing that allowed ingress of magmatic-derived fluids and formation of quartz-calcite-beryl-scheelite-tourmaline-pyrite veins. Channel-water delta D values in emerald are depleted and range between -87.4 parts per thousand and -62 parts per thousand, similar to other granite-related emerald occurrences. The calculated delta O-18(H2O) for the vein fluid (similar to 10 parts per thousand, vein quartz, 350 degrees C) is compatible with a peraluminous granitic fluid source. The calculated delta O-18(H2O) of granite-derived fluids (similar to 12 parts per thousand; magmatic quartz, 600-700 degrees C) is slightly isotopically heavier than the vein fluid, which can be explained by fractionation during vein crystallization or mixing with O-18-depleted meteoric fluid. The delta B-11 values of accessory dravite in the emerald veins averages -4.9 + 0.3 parts per thousand (1 sigma, n = 10), which is compatible with a magmatic source, and the Al-Fe-Mg composition is that of tourmaline formed in sedimentary environments, with Mg likely derived from metasomatism of local marine carbonates. The vein fluid was largely igneous in origin, but the dominant emerald chromophore V (emerald vein = avg. 1560 ppm V versus 75 ppm Cr) was mobilized by metasomatism of V-rich sedimentary rocks (avg. 2000 ppm V) that underlie the emerald occurrence. Lened is a unique igneous skarn-hosted emerald occurrence that contributes to the understanding of emerald deposits and emerald exploration criteria in Canada and globally. DOI
23.Marshall, D; Meisser, N; Ellis, S; Jones, P; Bussy, F; Mumenthaler, T.FORMATIONAL CONDITIONS FOR THE BINNTAL EMERALD OCCURRENCE, VALAIS, SWITZERLAND: FLUID INCLUSION, CHEMICAL COMPOSITION, AND STABLE ISOTOPE STUDIES.Can. Mineral., 2017, 55: 725-741 FORMATIONAL CONDITIONS FOR THE BINNTAL EMERALD OCCURRENCE, VALAIS, SWITZERLAND: FLUID INCLUSION, CHEMICAL COMPOSITION, AND STABLE ISOTOPE STUDIES
fluid inclusions; emerald; beryl; stable isotopes; Alps; Switzerland
Emerald from the Binntal occurrence in the Canton of Valais in Switzerland has been studied to determine its chemical zonation, stable isotopic signatures, depositional-fluid characteristics, pressure-temperature emplacement conditions, and formational model. The emerald is vanadium-rich, with optical and blue cathodoluminescence zoning related to chemical variations, primarily in V2O3 concentrations. The hydrogen isotope signature of the emerald channel fluids is unique and in agreement with previously identified high-altitude (deuterium-depleted) Alpine-age meteoric fluids. Field studies, fluid inclusion analyses, and oxygen isotope thermometry are consistent with a metamorphic formational model for the Binntal emerald at temperatures and hydrostatic pressures ranging from 200 to 400 degrees C and 100 to 250 Mpa, respectively. This corresponds to formational depths on the order of 4 to 9 km and fluids consistent with a 10-20 Ma CO2-dominant fluid with approximate mole percentages of 84.0, 11.9, 1.5, 1.3, 0.3, and 0.5 for CO2, H2O, CH4, N-2, H2S, and NaCl, respectively. DOI
22. Furlanetto, F; Thorkelson, DJ; Rainbird, RH; Davis, WJ; Gibson, HD; Marshall, DD.The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China.Gondwana Res., 2016, 39: 14-40 The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China
Wernecke Supergroup; Laurentia; Columbia; Paleoproterozoic; Detrital zircon geochronology; Geochemistry
The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U-Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 +/- 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850-1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan-Dahongshan-Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan-Dongchuan-Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia. (C) 2016 International Association for Gondwana Research. Published by Elsevier B. V. All rights reserved. DOI
21.Marshall, D; Downes, PJ; Ellis, S; Greene, R; Loughrey, L; Jones, P.Pressure-Temperature-Fluid Constraints for the Poona Emerald Deposits, Western Australia: Fluid Inclusion and Stable Isotope Studies.Minerals, 2016, 6 Pressure-Temperature-Fluid Constraints for the Poona Emerald Deposits, Western Australia: Fluid Inclusion and Stable Isotope Studies
fluid inclusions; emerald; beryl; stable isotopes; yilgarn craton; Australia
Emerald from the deposits at Poona shows micrometre-scale chemical, optical, and cathodoluminescence zonation. This zonation, combined with fluid inclusion and isotope studies, indicates early emerald precipitation from a single-phase saline fluid of approximately 12 weight percent NaCl equivalent, over the temperature range of 335-525 degrees C and pressures ranging from 70 to 400 MPa. The large range in pressure and temperature likely reflects some post entrapment changes and re-equilibration of oxygen isotopes. Secondary emerald-hosted fluid inclusions indicate subsequent emerald precipitation from higher salinity fluids. Likewise, the delta O-18-delta D of channel fluids extracted from Poona emerald is consistent with multiple origins yielding both igneous and metamorphic signatures. The combined multiple generations of emerald precipitation, different fluid compositions, and the presence of both metamorphic and igneous fluids trapped in emerald, likely indicate a protracted history of emerald precipitation at Poona conforming to both an igneous and a metamorphic origin at various times during regional lower amphibolite to greenschist facies metamorphism over the period -2710-2660 Ma. DOI
20. Medig, KPR; Thorkelson, DJ; Davis, WJ; Rainbird, RH; Gibson, HD; Turner, EC; Marshall, DD.Pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic.Precambrian Res., 2014, 249: 88-99 Pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic
Yukon; Mesoproterozoic; Columbia; Nuna; Supercontinents; Geochronology
Two supercontinents have been proposed for the latter half of the Precambrian: Columbia (or Nuna) from ca. 1.9 to 1.3 Ga, and Rodinia from ca. 1.1 to 0.75 Ga. In both supercontinents, Laurentia and Australia are regarded as probable neighbours, although their relative positions are contentious. Here we use detrital zircons ages from unit PR1 of the lower Fifteenmile group in Yukon, Canada, to demonstrate that northeastern Australia and northwestern Laurentia were firmly connected in the Mesoproterozoic. The zircon ages define a near-unimodal population with a peak at 1499 +/- 3 Ma, which lies in an interval of magmatic quiescence on Laurentia, known as the North American magmatic gap (NAMG), and abundant magmatism in Australia. Sediment compositions and textures suggest the sediment was derived from a proximal metaplutonic source. We suggest that the Williams and Naraku batholiths in the Mt. Isa inlier in northeastern Australia, with crystallization ages ranging from 1493 +/- 8 Ma to 1508 +/- 4 Ma, are the most probable sources of sediment for the PR1 basin. The plutons were exhumed between 1460 and 1420 Ma, and likely formed an active, eroding highland in the Australian part of Columbia. Sediment derived from these plutons was carried eastward by a short, direct river system and deposited into the PR1 marine basin. Formation of the PR1 basin coincides with the formation of the southern Cordilleran Belt-Purcell, Hess Canyon, and Trampas basins. These basins, formed on the western margin of Laurentia, also have detrital zircon populations that fall into the NAMG, suggesting that sediment was derived from a non-Laurentian westerly source. The PR1 basin is herein correlated with the Belt-Purcell, Hess Canyon, and Trampas basins to the south, and together these basins record the onset of Columbia breakup along the length of the western margin of Laurentia from as far north as Yukon to as far south as Arizona. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved. DOI
19. Furlanetto, F; Thorkelson, DJ; Gibson, HD; Marshall, DD; Rainbird, RH; Davis, WJ; Crowley, JL; Vervoort, JD.Late Paleoproterozoic terrane accretion in northwestern Canada and the case for circum-Columbian orogenesis.Precambrian Res., 2013, 224: 512-528 Late Paleoproterozoic terrane accretion in northwestern Canada and the case for circum-Columbian orogenesis
Wernecke Supergroup; RackIan; Paleoproterozoic; Bonnetia; Columbia
The reconstruction of the paleocontinental configuration involving ancestral North America (Laurentia) at the Paleoproterozoic-Mesoproterozoic boundary has been developed in the last 30 years with different scenarios being proposed and different combinations of landmasses assembled together. However, the lack of information for the northwestern side of the North American craton has so far been an obstacle for the complete paleocontinental reconstruction and its tectonic history. Here we provide new age determinations on rocks of the Wemecke Supergroup and of the Wernecke Breccia of the Wernecke Mountains in Yukon to provide a more complete picture of the entire North American craton and its possible conterminous at 1600 Ma. The six youngest U-Pb ages of the detrital zircon from quartz sandstones of the Wernecke Supergroup suggest that the sedimentary succession is as old as 1640 Ma. Lu-Hf garnet ages on garnet bearing schists of the Fairchild Lake Group (lower Wemecke Supergroup) give a bimodal population of ages of approximately 1600 Ma and 1370 Ma: the first age is related to the Racklan Orogeny, and the younger event is likely attributable to a reheating episode (Hart River Sills emplacement). The younger age of the Wernecke Supergroup puts into question the previous model concerning the emplacement of the Bonnet Plume River Intrusions, and requires the development of a new tectonic model for the northwestern margin of Laurentia. This new model involves obduction of an exotic terrane on top of the Wemecke Supergroup during the latest phases of the Racklan Orogeny (ca. 1600 Ma). This exotic terrane, herein called Bonnetia, contains rocks of the Bonnet Plume River intrusions and of the Slab volcanics. During the hydrothermal event that led to the emplacement of the Wernecke Breccia, clasts and megaclasts of the overlying Bonnetia foundered down to the breccia pipes to the level of the Wernecke Supergroup, and this dynamic explains the existence of older rocks engulfed within a younger sedimentary succession. The Racklan Orogeny is now interpreted as a northwestern expression of the Mazatzal Orogeny of southwestern United States, and of the Labradorian Orogeny of eastern Canada which was in turn connected with the Gothian Orogeny of Scandinavia. The connection among these orogenic events makes plausible the hypothesis of a circum-Laurentian orogenic belt with possible extensions in other landmasses (Australia, Antarctica, Siberia, or China) where coeval deformation belts are present. (c) 2012 Elsevier B.V. All rights reserved. DOI
18. Hewton, ML; Marshall, DD; Ootes, L; Loughrey, LE; Creaser, RA.Colombian-style emerald mineralization in the northern Canadian Cordillera: integration into a regional Paleozoic fluid flow regime.Can. J. Earth Sci., 2013, 50: 857-871 Colombian-style emerald mineralization in the northern Canadian Cordillera: integration into a regional Paleozoic fluid flow regime
Emerald in the Mackenzie Mountains is hosted in extensional quartz-carbonate veins cutting organic-poor Neoproterozoic sandstones and siltstones within the hanging wall of a thrust fault that emplaced these strata above Paleozoic rocks. Isotopic compositions of water extracted from emerald are typical of evolved sedimentary sulphate brines. Fluid inclusion studies indicate two saline fluid populations: a CO2-N-2-bearing, high-salinity brine (20.4-25.8 wt.% NaCl equivalent), and a gas-free, saline brine (7.6-15.3 wt.% NaCl equivalent). Both populations display evidence of post-entrapment volume changes. delta O-18(VSMOW) (VSMOW, Vienna standard mean ocean water) values for emerald, quartz, and dolomite yield averages of 17.3 parts per thousand (+/- 0.9), 19.6 parts per thousand (+/- 1.5), and 18.1 parts per thousand (+/- 1.0), respectively. Dolomite delta C-13(VPDB) (VPDB, Vienna Pee Dee belemnite) averages -6.8 parts per thousand (+/- 1.0). Two pyrite samples returned delta S-34(CDT) (CDT, Cafion Diablo troilite) values of 5.1 parts per thousand and 11.2 parts per thousand. Triply concordant mineral equilibration temperatures determined from mineral pair delta O-18(VSMOW) equilibration (quartz-emerald, quartz-dolomite, emerald-dolomite) range from 380 to 415 degrees C. Depth calculations based on mineral pair isotope equilibration and typical geothermal gradient indicate vein formation at 6-11 km depth. A Re-Os isochron age of 345 +/- 20 Ma from pyrite indicates that mineralization was contemporaneous with estimated ages of some northern Cordilleran Zn-Pb occurrences. Emerald mineralization resulted from inorganic thermochemical sulphate reduction via the circulation of warm basinal brines through siliciclastic, carbonate, and evaporitic rocks. These brines were driven along deep basement structures and reactivated normal faults during the development of a trans-tensional back-arc basin during the late Devonian to middle Mississippian. The Mountain River emerald occurrence thus represents a variant of the Colombian-type emerald deposit model requiring thermochemical sulphate reduction. DOI
17. Loughrey, L; Marshall, D; Ihlen, P; Jones, P.Boiling as a mechanism for colour zonations observed at the Byrud emerald deposit, Eidsvoll, Norway: fluid inclusion, stable isotope and Ar-Ar studies.Geofluids, 2013, 13: 542-558 Boiling as a mechanism for colour zonations observed at the Byrud emerald deposit, Eidsvoll, Norway: fluid inclusion, stable isotope and Ar-Ar studies
boiling; Byrud; colour zonation; emerald; fluid inclusion; Norway
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 +/- 2.0Ma. 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, N-2, CH4 and H2S are approximately 0.8974, 0.0261, 0.0354 and 0.0410, respectively. Formational temperatures and pressures of approximately 160-385 degrees 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. DOI
16. Nielsen, AB; Thorkelson, DJ; Gibson, HD; Marshall, DD.The Wernecke igneous clasts in Yukon, Canada: Fragments of the Paleoproterozoic volcanic arc terrane Bonnetia.Precambrian Res., 2013, 238: 78-92 The Wernecke igneous clasts in Yukon, Canada: Fragments of the Paleoproterozoic volcanic arc terrane Bonnetia
Geochemistry; Tectonics; Wernecke; Proterozoic; Yukon; Metasomatism
The Wernecke igneous clasts consist of blocks of plutonic and volcanic rock that range up to hundreds of metres in size. These clasts occur exclusively within zones of hydrothermal breccia (Wernecke Breccia) which are widespread in central and northern Yukon. The breccia zones are hosted by the Wernecke Supergroup and have been dated by U-Pb titanite at 1599 Ma. Four U-Pb zircon ages on the Wernecke igneous clasts (1714-1706 Ma) demonstrate that the clasts are older than the Wernecke Supergroup (< 1.64 Ga) and indicate that the clasts were not derived from dykes within the Wernecke Supergroup. Instead, the clasts were derived from an obducted terrane named Bonnetia. Geochemical characteristics of the Wernecke igneous clasts infer that Bonnetia formed as a volcanic arc with a component of within-plate magmatism. Neodymium mantle depletion ages of 2080-2760 Ma suggest that the arc was built on older continental crust. Consequently, Bonnetia may have been a volcanic arc, possibly Wilt on a rifted fragment of Laurentia, on another continental fragment, or possibly on the leading edge of another continent. The subsequent event of breccia-formation may represent a hydrothermal response to abduction-caused tectonic loading of the crust. The characterization of Bonnetia as a volcanic arc complex that underwent obduction requires that northwestern Laurentia was flanked by an ocean basin in the late Paleoproterozoic. (C) 2013 Elsevier B.V. All rights reserved. DOI
15. Brown, SR; Gibson, HD; Andrews, GDM; Thorkelson, DJ; Marshall, DD; Vervoort, JD; Rayner, N.New constraints on Eocene extension within the Canadian Cordillera and identification of Phanerozoic protoliths for footwall gneisses of the Okanagan Valley shear zone.Lithosphere, 2012, 4: 354-377 New constraints on Eocene extension within the Canadian Cordillera and identification of Phanerozoic protoliths for footwall gneisses of the Okanagan Valley shear zone
The Okanagan Valley shear zone delineates the SW margin of the Shuswap metamorphic complex, the largest core complex within the North American Cordillera. The Okanagan Valley shear zone is a major Eocene extensional fault zone that facilitated exhumation of the southern Shuswap metamorphic complex during the orogenic collapse of the SE Canadian Cordillera when convergence at the western margin of North America switched from transpression to transtension. This study documents the petrology, structure, and age of the Okanagan gneiss, the main lithology within the footwall of the Okanagan Valley shear zone, and constrains its history from protolith to exhumed shear zone. The Okanagan gneiss is an similar to 1.5-km-thick, west-dipping panel composed of intercalated orthogneiss and paragneiss in which intense ductile deformation of the Okanagan Valley shear zone is recorded. New U-Pb zircon ages from the gneiss and crosscutting intrusions constrain the development of the Okanagan gneiss to the Eocene, contemporaneous with widespread extension, intense deformation, high-grade metamorphism, and anatexis in the southern Canadian Cordillera. Thermobarometric data from the paragneiss domain indicate Eocene exhumation from between 17 and 23 km depth, which implies 64-89 km of WNW-directed horizontal extension based on an original shear zone angle of similar to 15 degrees. Neither the Okanagan gneiss nor its protolith represents exhumed Proterozoic North American cratonic basement as previously postulated. New U-Pb data demonstrate that the protolith for the gneiss is Phanerozoic, consisting of Mesozoic intrusions emplaced within a late Paleozoic-Mesozoic layered sequence of sedimentary rocks. DOI
14. Galicki, M; Marshall, D; Staples, R; Thorkelson, D; Downie, C; Gallagher, C; Enkin, R; Davis, W.Iron Oxide +/- Cu +/- Au Deposits in the Iron Range, Purcell Basin, Southeastern British Columbia.Econ. Geol., 2012, 107: 1293-1301 Iron Oxide +/- Cu +/- Au Deposits in the Iron Range, Purcell Basin, Southeastern British Columbia
The Iron Range iron oxide +/- Cu +/- Au deposits in southeastern British Columbia comprise massive lenses and veins of hematite and martite with lesser magnetite in the Iron Range fault zone, which crosscuts the Proterozoic Aldridge Formation and Moyie sills. The mineralized zones are flanked by albite-quartz-iron oxide breccia within sedimentary rocks and by chlorite-altered iron oxide breccia where they are in contact with Moyie sills. Oxygen isotope analyses indicate 340 degrees to 400 degrees C precipitation temperatures for the albite-quartz-magnetite assemblages in the mineralized zones. Magnetite trace element compositions closely compare with those in iron oxide-(copper-gold) (IOCG) to porphyry-type mineralization worldwide. Paleomagnetic studies show consistent paleopole orientations concordant with Cretaceous poles and support links to a porphyry-type genesis associated with phases of the nearby SO to 105 Ma magnetite-bearing Bayonne Suite plutonic rocks. The Iron Range iron oxide mineralized zones share many characteristics of major IOCG deposits, with the exception of economic Cu (+/- Au) concentrations in the exposed rocks; however, recent drilling intersected minor chalcopyrite, pyrite, and gold. DOI
13. Al-Atar, U; Bokov, AA; Marshall, D; Teichman, JMH; Gates, BD; Ye, ZG; Branda, NR.Mechanism of Calcium Oxalate Monohydrate Kidney Stones Formation: Layered Spherulitic Growth.Chem. Mat., 2010, 22: 1318-1329 Mechanism of Calcium Oxalate Monohydrate Kidney Stones Formation: Layered Spherulitic Growth
The morphology of calcium oxalate monohydrate (COM) kidney stones is studied using polarized light microscopy and X-ray diffraction. We show that polycrystalline structure of COM stones exhibits spherulitic texture where the arrangement of crystallites indicates that their fast growth direction is perpendicular to the corresponding radius of spherulite, resulting in the layered morphology. This is in contrast to "normal" spherulites, where the crystal growth process leads to the formation of a radiating array of fiber crystallites. We demonstrate that COM stories consist of spherulitic domains. The domains have the shape of comparatively narrow randomly distorted cones in which the crystallites form strong texture, so that their crystallographic axes have almost the same directions and the [100] crystallographic planes are nearly perpendicular to the radial direction of the domain. However, the order among the domains does not exist. Deviations of their radial directions from the corresponding radial directions of the whole stone are not large as a rule, while the other crystallographic directions of the domains are randomly distributed. A model of layered spherulitic growth explaining the observed morphology is proposed. The model suggests that every domain is formed by means of a continuous crystallization process periodically inhibited by precipitation of organic material so that alternating organic and polycrystalline layers appear. Fine crystalline channels remaining in organic layers connect neighboring crystalline layers and maintain, thereby, the coherence of crystal structure all over the domain. Preformed COM microcrystals occasionally adsorbed from urine on the surface of the growing organic layer serve as seeds for new spherulitic domains. The results are important for understanding the general principles of biomineralization, and spherulitic crystallization and could lead to the development of new strategies for preventing kidney stone formation. DOI
12. Xue, G; Marshall, D; Zhang, S; Ullrich, TD; Bishop, T; Groat, LA; Thorkelson, DJ; Giuliani, G; Fallick, AE.Conditions for Early Cretaceous Emerald Formation at Dyakou, China: Fluid Inclusion, Ar-Ar, and Stable Isotope Studies.Econ. Geol., 2010, 105: 339-349 Conditions for Early Cretaceous Emerald Formation at Dyakou, China: Fluid Inclusion, Ar-Ar, and Stable Isotope Studies
The Dyakou emerald occurrence is located in Malipo County in the province of Yunnan, southern China. The occurrence lies in the northern part of the Laojunshan-Song Chay metamorphic core complex, which is exposed in an area of approximately 2,000 km(2) and extends across the border between China and Vietnam. Emerald mineralization is hosted by pegmatite and associated quartz veins that intrude deformed Proterozoic biotite-muscovite granofels and schist. Hydrogen and oxygen isotope results from the emerald channel waters and emerald, respectively, are consistent with an igneous fluid source. The delta(18)O fractionation between emerald and quartz yields vein temperatures of 365 to 420 degrees C. Fluid inclusions indicate that the emerald precipitated from saline brines ranging from almost pure water to 10.5 mass percent NaCl equiv. Fluid inclusion isochores intersected with delta(18)O data yield pressures changing along the geothermal gradient from 1,500 to 3,300 bars. Ar-Ar geochronology of biotite and muscovite from the emerald veins yields consistent ages of 124 +/- 1 Ma. These constraints combined with field observations indicate that the Dyakou emerald deposit is consistent with the igneous-related model for emerald formation. DOI
11. Ickert, RB; Thorkelson, DJ; Marshall, DD; Ullrich, TD.Eocene adakitic volcanism in southern British Columbia: Remelting of arc basalt above a slab window.Tectonophysics, 2009, 464: 164-185 Eocene adakitic volcanism in southern British Columbia: Remelting of arc basalt above a slab window
Adakite; Slab window; Volcanism; Eocene; Geochemistry; British Columbia
The Princeton Group is an assemblage of terrestrial volcanic and clastic sedimentary rocks in south-central British Columbia, and is part of the Challis-Kamloops belt that stretches from central British Columbia to the northwestern United States. The volcanic rocks were largely deposited as cinder cones and composite volcanoes, and are composed of basaltic andesite (olivine+clinopyroxene), andesite and dacite (hornblende + plagioclase+clinopyroxene), and rhyolite (biotite +quartz+ K-feldspar), with calc-alkaline affinity. New Ar-40/Ar-39 dates on homblende and groundmass separates, and whole rock indicate that magmatism took place during the Early to Middle Eocene, from 53-47 Ma. New neodymium isotopic measurements, in conjunction with previously published results, indicate that the Princeton Group has an epsilon Nd-50 = 1.2-6.4 and therefore represents primarily juvenile additions to the continental crust. The major and trace element abundances of Princeton Group rocks resemble those of many modem continental arcs. The compositions are notable, however, because they have an "adakitic" signature that extends throughout their entire compositional range, including high-Mg# basaltic andesite. Trace element modelling indicates that this signature was not derived from anatexis of normal oceanic crust, but from an "arc-like" source enriched in large-ion lithophile elements. This source may have been basaltic dykes that were emplaced into the lithospheric mantle during Mesozoic arc magmatism and subsequently partially melted during an event of lithospheric heating in the Eocene. The heating may have been caused by upwelling asthenosphere related to a slab window or slab tear. (C) 2007 Elsevier BY All rights reserved. DOI
10. Groat, LA; Giuliani, G; Marshall, DD; Turner, D.Emerald deposits and occurrences: A review.Ore Geol. Rev., 2008, 34: 87-112 Emerald deposits and occurrences: A review
Emerald deposits; Emerald occurrences; Crystal chemistry; Geochemistry; Production; Stable isotopes; Classification; Exploration
Emerald, the green gem variety of beryl, is the third Most valuable gemstone (after diamond and ruby). Although it is difficult to Obtain accurate statistics, Colombia supplies most (an estimated 60%, worth more than $500,000,000 per year) of the world's emeralds. However there is speculation that the emerald mines in Colombia are becoming depleted. Brazil currently accounts for approximately 10% of world emerald production. Emeralds have also been mined in Afghanistan, Australia, Austria, Bulgaria, China, India, Madagascar, Namibia, Nigeria, Pakistan, South Africa, Spain, Tanzania, the United States, and Zimbabwe. Because it is difficult to obtain accurate analyses of beryllium, most published analyses of beryl are renormalized on the basis of 18 oxygen and 3 Be atoms per formula unit. The color of emerald is due to trace amounts of chromium and/or vanadium replacing aluminum at the Y site; in most cases the Cr content is much greater than that of V. To achieve charge balance, the Substitution of divalent cations at the Y site is coupled with the substitution of a monovalent cation for a vacancy at a channel site. Beryl is relatively rare because there is very little Be in the upper continental Crust. Unusual geologic and geochemical conditions are required for Be and Cr and/or V to meet. In the classic model, Be-bearing pegmatites interact with Cr-bearing ultramafic or mafic rocks. However in the Colombian deposits there is no evidence of magmatic activity and it has been demonstrated that circulation processes within the host black shales were sufficient to form emerald. In addition, researchers are recognizing that regional metamorphism and tectonometamorphic processes Such as shear zone formation may play a significant role in certain emerald deposits. A number of genetic classification schemes have been proposed for emerald deposits. Most are ambiguous when it comes to understanding the mechanisms and conditions that lead to the formation of an emerald deposit. Studies of individual emerald deposits show that in most cases a combination of mechanisms (magmatic, hydrothermal, and metamorphic) were needed to bring Be into contact with the chromophores. This Suggests the need for a more flexible classification scheme based on mode of formation. Stable isotopes can be used to estimate the contribution of each mechanism in the formation of a particular deposit. Such estimates could perhaps be more precisely defined using trace element data, which should reflect the mode of formation. Emerald may be identified in the field by color, hardness, and form. It will tend to Show LIP in stream sediment samples but because its specific gravity is relatively low, it will not concentrate in the heavy mineral fraction. In Colombia, structural geology, the sodium content of stream sediment samples, and the lithium. sodium, and lead contents of soil samples have all been used to find emerald occurrences. Exploration for gem beryl could result in the discovery of new occurrences of non-gem beryl or other Be minerals that Could become new sources of Be and Be oxide. Future efforts should go towards creating a comprehensive data base of emerald compositions (including trace elements), determination of the role of metamorphism in the formation of some emerald deposits, improved classification schemes, and more effective exploration guidelines. (C) 2008 Elsevier B.V. All rights reserved. DOI
9.Marshall, D.Economic Geology Models 2. Melt Inclusions of Native Silver and Native Bismuth: A Re-examination of Possible Mechanisms for Metal Enrichment in Five-Element Deposits.Geosci. Can., 2008, 35: 137-145 Economic Geology Models 2. Melt Inclusions of Native Silver and Native Bismuth: A Re-examination of Possible Mechanisms for Metal Enrichment in Five-Element Deposits
This paper presents preliminary observations on veinlets and trails of native bismuth and silver melt inclusion that cross-cut silicate and carbonate vein fill and alteration minerals in the five-element veins at Cobalt, Canada. The low melting temperature of bismuth (271 degrees C) is consistent with the current estimates of vein formation at Cobalt, and melt textures are displayed in native bismuth inclusions and trails. Native silver displays identical textures and these are also interpreted to have formed from a melt. However, native silver melts above 950 degrees C, which is in direct conflict with current estimates of silver deposition within the Cobalt camp. In fight of the similarities in textures, existing temperature evidence, the lack of experimental studies in the Co-As-Ag ternary, and recent advances in the study of melt inclusions in sulfide deposits, the native saver textures are also interpreted to have formed at temperatures as low at 350 degrees C. A primary three-phase fluid inclusion assemblage contained within growth-zoned quartz crystals in the granophyric phase of the Nipissing diabase was chosen as representative of the highest temperature fluids responsible for ore deposition at Cobalt. This fluid inclusion assemblage displays microthermometric behaviour similar to the hypersaline fluid inclusions previously determined as the transporting medium for the silver mineralization at Cobalt and are consistent with depositional temperatures of about 350 degrees C. These temperatures, although sufficient to produce melt inclusions of native bismuth, are insufficient to melt silver. petrography and solid inclusion textures are consistent with metallic silver melts, indicating that Ag-Sb-Hg ternary or more complex silver-bearing systems containing H(2)O, H(2)S and salts may have eutectics at temperatures below 350 degrees C. This is interpreted as a potential mechanism for silver mobilization and enrichment, and has potential applications to other types of vein mineralization.
8. Al-Suwaidi, M; Ward, BC; Wilson, MC; Hebda, RJ; Nagorsen, DW; Marshall, D; Ghaleb, B; Wigen, RJ; Enkin, RJ.Late Wisconsinan Port Eliza Cave deposits and their implications for human coastal migration, Vancouver Island, Canada.Geoarchaeology, 2006, 21: 307-332 Late Wisconsinan Port Eliza Cave deposits and their implications for human coastal migration, Vancouver Island, Canada
Sediments of Port Eliza Cave provide a record of the Last Glacial Maximum (LGM) on Vancouver Island that has important implications for human migration along the debated coastal migration route. Lithofacies changes from nonglacial diamict to glacial laminated silt and clay and till, then a return to nonglacial conditions with oxidized clay, colluvial block beds, and speleothems, along with radiocarbon and U/Th dates, define glacial-nonglacial transitions. Scanning electron microscope studies and clay mineralogy confirm that the laminated fines represent glaciation. Preglacial faunal evidence shows a diverse range from small species, including birds, fish, vole, and marmot, to larger species, such as mountain goat. Pollen data from the same unit show a cold, dry tundra environment with sparse trees. Deglaciation occurred prior to an age of 12.3 ka B.P. based on dated mountain goat bone. These data support the viability of the coastal migration route for humans prior to similar to 16 ka B.P. and then as early as similar to 13 ka B.P. (c) 2006 Wiley Periodicals, Inc. DOI
7. Kontak, DJ; Kyser, K; Gize, A; Marshall, D.Structurally controlled vein barite mineralization in the Maritimes Basin of eastern Canada: Geologic setting, stable isotopes, and fluid inclusions.Econ. Geol., 2006, 101: 407-430 Structurally controlled vein barite mineralization in the Maritimes Basin of eastern Canada: Geologic setting, stable isotopes, and fluid inclusions
The Brookfield barite deposit occurs in the Carboniferous Maritimes basin of eastern Canada, an area known for ca. 300 Ma Mississippi Valley-type (MVT) Zn-Pb-Ba mineralization, including the past-producing 'Walton barite and Gays River Zn-Pb deposits. In contrast to most of MVT mineralization in the Maritimes Basin that is hosted by Visean carbonate rocks of the Windsor Group, the Brookfield deposit is hosted by red terrestrial clastics of the underlying Tournaisian Horton Group. The barite mineralization occurs as <= 25-m-wide fault fill with both coarse-textured, pristine barite and mylonitic-textured barite situated close to the Cobequid-Chedabucto fault system, separating the Meguma and Avalon terranes. The breccia vein mineralization has a simple mineralogy and consists of barite with up to 1.3 wt percent Sr, minor siderite, and trace amounts of late-stage quartz and calcite. The host rocks close to the barite mineralization are intensely altered, with quartz and kaolinite replacing muscovite and complete removal of diagenetic hematite. A Grant-type isocon plot indicates mass loss (C-A = 0.82 degrees C-O) during wall-rock alteration with BEE profiles maintained but relative chondrite-normalized abundances reduced. Although an Fe-Mn zone occurs a few 100 m along strike, no such mineralization occurs in the barite deposit. Fluid inclusion studies indicate that vein fluids were saline brines (20-30 wt % NaCl + CaCl2 equiv) that coexisted with an immiscible N-2-CO2-CH4 gas. Laser Raman analysis of the most common gas-rich inclusions indicates a uniform fluid chemistry with 66 mol percent N-2 and 34 mol percent CO2 and is consistent with compositions inferred from thermometric measurements. Homogenization temperatures for primary, aqueous fluid inclusion asemblages in quartz intergrown with barite indicate minimum trapping temperatures of 210 degrees C. Conditions at the time of entrapment, based on intersection of aqueous and gaseous isochores, are estimated at similar to 775 bars and 250 degrees C. Stable isotope data for vein minerals (barite, siderite, quartz) are uniform in nature and indicate delta O-18(fluid) = +12 per mil at 250 degrees C for siderite and +9.8 per mil for quartz, delta C-13(fluid) = -4 to -6 per mil from siderite, and delta S-34(fluid) = +12 per mil from barite. Fluid inclusion extracts indicate delta D values of -47 to -71 per mil. Collectively, these isotopic compositions are consistent with a basinal-type fluid derived from modified meteoric water with sulfur derived from Carboniferous evaporites and carbon of mainly marine limestone origin (i.e., Carboniferous Windsor Group) with a minor biogenic component from the Horton Group. Mineralization at Brookfield resulted from focusing of heated, overpressured brines of modified basinal origin into an active fault zone environment. The association of the barite mineralization with intensely altered wall rock represents a rare example of such alteration in the MVT metallogenic domain of the Maritimes Basin. However, similar alteration associated with Ba-Fe-Mn mineralization along the Cobequid-Chedabueto fault system raises the possibility that the Brookfield deposit may instead be part of another mineralizing event that was distinct from the 300 Ma MVT deposits. DOI
6. Madsen, JK; Thorkelson, DJ; Friedman, RM; Marshall, DD.Cenozoic to Recent plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in western North America.Geosphere, 2006, 2: 11-34 Cenozoic to Recent plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in western North America
tectonics; magmatism; geochronology; forearc; slab window; ridge subduction; western North America; Cordillera
Forearc magmatic rocks were emplaced in a semicontinuous belt from Alaska to Oregon from 62 to 11 Ma. U-Pb and Ar-40-Ar-39 dating indicates that the magmatism was concurrent in widely separated areas. Eight new conventional isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from forearc intrusions on Vancouver Island (51.2 +/- 0.4, 48.8 +/- 0.5 Ma, 38.6 +/- 0.1, 38.6 +/- 0.2, 37.4 +/- 0.2, 36.9 +/- 0.2, 35.4 +/- 0.2, and 35.3 +/- 0.3 Ma), together with previous dates, indicate that southwestern British Columbia was a particularly active part of the forearc. The forearc magmatic belt has been largely attributed to ridge-trench intersection and slab window formation involving subduction of the Kula-Farallon ridge. Integration of the new and previous ages reveals shortcomings of the Kula-Farallon ridge explanation, and supports the hypothesis of two additional plates, the Resurrection plate (recently proposed) and the Eshamy plate (introduced herein) in the Pacific basin during Paleocene and Eocene time. We present a quantitative geometric plate-tectonic model that was constructed from 53 Ma to present to best account for the forearc magmatic record using ridge-trench intersection and slab window formation as the main causes of magmatism. The model is also in accord with Tertiary to present inboard magmatic and structural features. DOI
5. Laughton, JR; Thorkelson, DJ; Brideau, MA; Hunt, JA; Marshall, DD.Early Proterozoic orogeny and exhumation of Wernecke Supergroup revealed by vent facies of Wernecke Breccia, Yukon, Canada.Can. J. Earth Sci., 2005, 42: 1033-1044 Early Proterozoic orogeny and exhumation of Wernecke Supergroup revealed by vent facies of Wernecke Breccia, Yukon, Canada
In the Yukon, the oldest known supracrustal succession, the Wernecke Supergroup, was deposited in a marine basin before 1.71 Ga. The earliest orogenic event to disturb these strata was the Racklan orogeny, which produced folds and fabrics at peak temperatures of 450-550 degrees C. These features and those of the correlative Forward orogeny are recognized at the surface and in the subsurface throughout much of northwestern Canada. Zones of Wernecke Breccia (hydrothermal breccias, 1.60 Ga) were emplaced into the Wernecke Supergroup after Racklan deformation and metamorphism. Two main types of breccia are recognized: grey sodic breccias and colourful potassic breccias. In the Slab Mountain area, a belt of grey breccias contains abundant megaclasts of country rock including blocks of a subaerial lava succession, the Slab volcanics. These grey breccias are interpreted as a vent facies of Wernecke Breccia, and their emplacement into the stratigraphically lowest unit of the Wernecke Supergroup infers that at least 9 km of exhumation occurred in the core of a major Racklan anticline prior to brecciation. The Slab volcanics are preserved only as clasts in Wernecke Breccia and are interpreted as fragments of a former valley-filling basalt succession which overlay deformed and deeply incised strata of the Wernecke Supergroup. DOI
4.Marshall, DD; Groat, LA; Falck, H; Giuliani, G; Neufeld, H.The Lened emerald prospect, Northwest Territories, Canada: Insights from fluid inclusions and stable isotopes, with implications for northern Cordilleran emerald.Can. Mineral., 2004, 42: 1523-1539 The Lened emerald prospect, Northwest Territories, Canada: Insights from fluid inclusions and stable isotopes, with implications for northern Cordilleran emerald
Lened prospect; emerald; fluid inclusions; stable isotopes; geochemistry; Northwest Territories
Vanadium-rich emerald at the Lened occurrence, Northwest Territories, is hosted within a fractured garnet-diopside skarn. The emerald is generally found in quartz-carbonate veins with mm-scale alteration haloes of muscovite and carbonate. The skarn is hosted within the Cambro-Ordovican Rabbitkettle Formation, which overlies black shales of the Devonian Earn Group. Skarn and subsequent quartz-carbonate veins are the result of contact metamorphism related to the emplacement of the adjacent 93 Ma Lened pluton of the Selwyn Plutonic Suite. Fluid-inclusion studies reveal the presence of two distinct fluids, a CO2-bearing fluid related to emerald precipitation and a brine limited to subsequent crystallization of quartz. The CO2-bearing fluid is dominantly a dilute aqueous brine with approximately 4.5 mole % CO2 and minor amounts of CH4, N-2, and H2S. The pressure of formation is limited by estimates of the maximum lithostat to less than 320 MPa. The temperatures of formation, in the interval 200-610degreesC, are constrained by temperatures of fluid-inclusion homogenization and isochore intersections with the 320 MPa maximum lithostatic pressure. Preliminary stable isotope data from Lened are consistent with isotopic data for fluids originating from other nearby plutons of the Selwyn Plutonic suite, indicating that emerald at Lened may be derived from a magmatic source. Geological constraints are consistent with Lened being a Type-I (igneous-activity-related) occurrence of emerald. Oxygen isotope data, fluid compositions, pressures and temperatures of formation similar to those of the Regal Ridge occurrence in the Yukon suggest that emerald in the northern Cordillera is likely igneous-activity-related, although the Regal Ridge and Lened occurrences do display some characteristics of schist-type occurrences as well. DOI
3.Marshall, D; Groat, L; Giuliani, G; Murphy, D; Mattey, D; Ercit, TS; Wise, MA; Wengzynowski, W; Eaton, WD.Pressure, temperature and fluid conditions during emerald precipitation, southeastern Yukon, Canada: fluid inclusion and stable isotope evidence.Chem. Geol., 2003, 194: 187-199 Pressure, temperature and fluid conditions during emerald precipitation, southeastern Yukon, Canada: fluid inclusion and stable isotope evidence
fluid inclusions; stable isotopes; emerald; thermobarometry; Yukon
The Crown emerald veins are somewhat enigmatic, displaying characteristics that are common to emerald deposits of tectonic-hydrothermal origin and of igneous origin. The veins cut the Fire Lake mafic meta-volcanic rocks, occurring within 600 m of an outcrop of Cretaceous S-type granite. Field work and vein petrography are consistent with a polythermal origin for the veins. The primary vein mineralogy is quartz and tourmaline with variable sized alteration haloes consisting of tourmaline, quartz, muscovite, chlorite and emerald. The veins weather a buff brown colour due to jarosite, scheelite and minor lepidocrocite, which were precipitated during the waning stages of vein formation. Microthermometic studies of primary fluid inclusions within emerald growth zones are consistent with emerald precipitation from H2O-CO2-CH4 ( +/- N-2 +/- H2S) bearing saline brines. The estimated fluid composition is approximately 0.9391 mol% H2O, 0.0473 mol% CO2, 0.0077 mol% CH4 and 0.0059 mol% NaCl ( similar to 2 wt.% NaCl eq.). Fluid inclusion and stable isotope studies are consistent with vein formation in the temperature range 365-498 T, with corresponding pressures along fluid inclusion isochore paths ranging from 700 to 2250 bars. These data correlate with a very slow uplift rate for the region of 0.02-0.07 mm/year. Emerald deposits are generally formed when geological conditions bring together Cr (+/- V) and Be. Cr and V are presumed to have been derived locally from the mafic and ultramafic rocks during hydrothermal alteration. The Be is most likely derived from the nearby Cretaceous granite intrusion. (C) 2002 Elsevier Science B.V. All rights reserved. DOI
2. Groat, LA; Marshall, DD; Giuliani, G; Murphy, DC; Piercey, SJ; Jambor, JL; Mortensen, JK; Ercit, TS; Gault, RA; Mattey, DP; Schwarz, D; Maluski, H; Wise, MA; Wengzynowski, W; Eaton, DW.Mineralogical and geochemical study of the Regal Ridge emerald showing, southeastern Yukon.Can. Mineral., 2002, 40: 1313-1338 Mineralogical and geochemical study of the Regal Ridge emerald showing, southeastern Yukon
emerald deposit; geology; mineralogy; geochemistry; fluid inclusions; stable isotopes; Finlayson Lake; Yukon; Canada
In September, 1998, one of the authors (WW) discovered a major occurrence of emerald in the Finlayson Lake district of southeastern Yukon. The Regal Ridge showing occurs in complexly deformed metavolcanic rocks in the Yukon-Tanana Terrane, near their contact with a mid-Cretaceous granitic pluton. The emerald crystals occur where quartz veins cut mica-rich layers in a shallowly dipping mica schist of the Upper Devonian Fire Lake mafic metavolcanic unit. At least eight such veins have been found. Most are surrounded by a much more extensive, overlapping mass of fine, dark tourmaline crystals. The tourmaline crystals are locally associated with minor amounts of scheelite, and small amounts of sulfides have been observed to occur within the quartz veins. A zone of sparse, disseminated sulfides apparently coincides with the tourmaline zone, which is surficially marked by ochreous products of oxidation. Green beryl crystals up to 4 cm in length occur in tourmaline zones and, rarely, in the quartz veins. Some of the smaller crystals, and sections of larger crystals, are of gem quality. The Cr content (average 3208 ppm) shows that it is the predominant chromophore. Fluid-inclusion data indicate that the emerald precipitated from a fluid whose maximum salinity was 3 wt.% NaCl equivalent. The oxygen isotopic composition of the emerald is highly variable (12.3 to 14.8parts per thousand), but there is little difference in corresponding deltaD values (-57.3 and-59.8parts per thousand, respectively), which suggests the presence of an isotopically homogeneous fluid that underwent isotopic exchange with the host rocks without achieving homogenization. The delta(18)O values for coexisting quartz and tourmaline from the quartz veins yield temperatures of formation of approximately 365 and 498degreesC. Based on fluid-inclusion isochoric data, these temperatures correspond to pressures of 1.0 to 2.5 kbar, and inferred depths of 3 to 7.7 km. The close proximity of the granite suggests that it is the source of the Be, although the Be content is low (12 and 13.2 ppm). The source of the Cr is the schist (520 ppm Cr). An Ar-40/Ar-39 age of 109 Ma for a mica sample from the schist could either reflect a thermal overprint age related to the event that produced the emerald crystals, or cooling following intrusion of the adjacent pluton, or both. DOI
1.Marshall, D; Pfeifer, HR; Hunziker, JC; Kirschner, D.A pressure-temperature-time path for the NE Mont-Blanc massif: Fluid-inclusion, isotopic and thermobarometric evidence.Eur. J. Mineral., 1998, 10: 1227-1240 A pressure-temperature-time path for the NE Mont-Blanc massif: Fluid-inclusion, isotopic and thermobarometric evidence
Alpine pressure-temperature-time paths; fluid inclusions; thermobarometry; Ar-40/Ar-39 dating
The Mont-Chemin region at the NE extreme of the Mont-Blanc massif; Canton Valais, Switzerland is predominantly comprised of the granitic rocks of the Mont-Blanc intrusive rock suite and the Mont-Blanc basement gneisses. Fluid inclusions, fluid-mineral equilibria, stable-isotope and radiogenic-isotope studies have been used to derive pressure, temperature, age (PTt) and fluid-composition constraints for a number of Alpine events. The earliest of these events is recorded in a paragonite-katophorite schist hosted within the basement gneisses. The paragonites yield a Ar-40/Ar-39 age of 47 Ma. Mineral thermobarometry is consistent with formation temperatures in excess of 300 degrees C, with minimum pressures of 1500 bars. A well-defined pressure-temperature uplift path is recorded in minerals hosted by veins of different generations. The overall PTt path defines a geothermal gradient of 25 degrees C/km, but the younger portions of this PTt path are consistent with geothermal gradients slightly in excess of 50 degrees C/km, similar to those observed to the East along the Rhone-Simplon line.
