Daniel D. Marshall

Cenozoic to Recent plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in western North America

Forearc magmatic rocks were emplaced in a semicontinuous belt from Alaska to Oregon from 62 to 11 Ma. U-Pb and 40 Ar39 Ar 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 Pacifi c 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.

Elemental analysis of individual fluid inclusions in minerals by Secondary Ion Mass Spectrometry (SIMS): Application to cation ratios of fluid inclusions in an Archaean mesothermal gold-quartz vein☆

Abstract A SIMS technique using a CAMECA-4F Ion Microprobe has been developed to quantitatively analyse element ratios of individual, preselected fluid inclusions as small as 3 μm in diameter and as dilute as 0.37 mol% salts. The method, first attempted by Nambu et al. (1977), is destructive; fluid inclusions are opened under vacuum by sputtering through the host crystal with a 12.5 KeV beam of O − ions. Emitted secondary ions are analysed in a mass spectrometer, their intensities being proportional to the concentration of parent elements in the target inclusion. Whereas Nambu et al. (1977) analysed frozen inclusions, K Na and Ca Na ratios have been obtained in this study from inclusions in the liquid state, with a relative reproducibility ( 2σ x ) of 50–65 and 80–200%, respectively. The secondary ion yields of Na + , K + , and Ca + have been calibrated empirically using fluid inclusions synthesized with bulk salinities of 0.37–3.3 mol% and K Na and Ca Na ratios of 0.005–1.0. Thus individual gold-bearing fluid inclusions in quartz from the San Antonio Archaean gold deposit, Bissett, SE Manitoba, yield K / Na = 0.036 + 0.04/−0.02 and Ca / Na = 0.034 + 0.07/−0.03. The excellent spatial control of the primary ion beam permits individual inclusions to be opened without interfering with adjacent inclusions. The most powerful application of the method is therefore in the analysis of natural samples that contain multiple generations of fluid inclusions.

Initial isotopic heterogeneity and secondary disturbance of the SmNd system in fluorites and fluid inclusions: A study on mesothermal veins from the central and western Swiss Alps

Abstract This paper presents SmNd data on hydrothermal fluorites, fluid inclusions and leach experiments on samples from the Mont-Blanc and Gotthard units, Swiss Alps. Both areas have experienced Alpine greenschist-facies overprint (≤350° and ∼450°C, respectively). Mont-Blanc samples represent late Variscan fluorites from three individual veins whereas Gotthard samples were taken from one hand-specimen of probable late Variscan age. Nd concentrations of fluorites range from 3.6 to 23 ppm, 147 Sm 144 Nd ratios from 0.12 to 0.35. None of the samples sets yielded an isochron relationship. Imperfect correlations indicate apparent ages between late Variscan and Alpine, reflecting neither the formation nor the age of metamorphic overprint. Reasons are wall-rock influence during formation and post-formation disturbance. Secondary fluorite (Gotthard) shows the highest Nd concentration and a low Sm/Nd ratio, close to that of fluid inclusion fractions. A leach experiment showed the same trend even more pronounced: ∼7% of Nd and ∼2% of Sm were removed from fluorite by 0.18 N HCl at 100°C. Hence, secondary fluids are likely to leach and fractionate Sm and Nd, and may easily cause dissolution and reprecipitation of fluorite. Consequently, the system is highly suspicious if secondary fluid flow cannot totally be ruled out. This is in sharp contrast to previously published conclusions. However, evidence for rejuvenation of the SmNd system in fluorites is also found in published data sets.

Magmatic and tectonic history of the Leech River Complex, Vancouver Island, British Columbia: Evidence for ridge-trench intersection and accretion of the Crescent Terrane

The Leech River Complex, part of the Pacifi c Rim Terrane, is a Cretaceous metasedimentary and metaigneous assemblage on southern Vancouver Island. The Leech River Complex is fault-bounded between the Eocene Metchosin Igneous Complex to the south (part of the Crescent Terrane) and the Paleozoic to Jurassic Wrangel Terrane to the north and provides critical information on the evolution of the central part of the western North American forearc in Cretaceous through Eocene time. Single detrital zircons from the metasedimentary component, known as the Leech River Schist, give U-Pb interpreted ages that range from Precambrian to ca. 103 Ma, indicating a varied source region and a probable Early Cretaceous depositional age. U-Pb geochronology and fi eld investigations indicate at least two magmatic-metamorphic events in the Leech River Complex: one during the Late Cretaceous, and the other during the early Middle Eocene. The peraluminous Jordan River metagranodiorite, a fi ne-grained biotitic stock and related dikes intruded the Leech River Complex at ca. 88 Ma, during the older magmatic event. Metamorphic pressure-temperature conditions of 525–550 °C and 2–3 kbar are recorded in the contact aureole. The later event occurred during emplacement of the Walker Creek intrusions, a suite of peraluminous tonalite, trondjhemite, and granodiorite dikes that intruded the Complex at ca. 51 Ma and produced a similar metamorphic aureole. Both intrusive suites have

Pressure, temperature and fluid conditions during emerald precipitation, southeastern Yukon, Canada: fluid inclusion and stable isotope evidence

Abstract 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 H 2 O–CO 2 –CH 4 (±N 2 ±H 2 S) bearing saline brines. The estimated fluid composition is approximately 0.9391 mol% H 2 O, 0.0473 mol% CO 2 , 0.0077 mol% CH 4 and 0.0059 mol% NaCl (∼2 wt.% NaCl eq.). Fluid inclusion and stable isotope studies are consistent with vein formation in the temperature range 365–498 °C, 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.

The Cobalt Mining District: Silver Sources, Transport and Deposition

Cobalt, Ontario, is renowned for the 12.6 billion grams (445 million ounces) of silver produced from the area since discovery in 1903 by workers of the Timiskaming and Northern Ontario Railway. Native silver generally occurs with cobalt arsenides and sulfosalts in near-vertical carbonate veins cutting the Huronian sedimentary rocks of the Gowganda Formation, the Archean metavolcanics and/or the Nipissing diabase. All major deposits have been found within a few hundred meters of the unconformity between the Archean and Huronian rocks in general proximity to the Nipissing diabase and volcanogenic sulfide mounds within the Archean meta-volcanics. Silver has been mobilized from one or more of the local country rocks by hyper-saline brines and deposited in or near zones of mixing where the saline brines encounter paleometeoric water transported to depth along the unconformity or local structures. Previous work has shown that chloride complexes are the dominant ligands responsible for silver transport. These hypersaline brines, represented as halite-bearing fluid inclusions at room temperature, have been trapped as primary fluid inclusions within vein minerals. Pressure-temperature conditions of vein formation have been derived from mineral equilibria, maximum lithostat and fluid-inclusion studies. These data are consistent with vein formation occurring over the temperature range 300°C to 350°C, with pressures constrained between 60 Mpa and 136 Mpa (600 bars and 1360 bars).

Fluid inclusion, stable isotope and Ar-Ar evidence for the age and origin of gold-bearing quartz veins at Mont Chemin, Switzerland

SummaryA new Swiss gold occurrence at Mont Chemin, comprising gold-bearing quartz veins, displays many characteristics that are typical of mesothermal gold deposits within the Alps and globally. The most notable of these features are: i) the presence of NaCl-H2O-CO2-bearing fluid with an XCO2 of approximately 0.016 and NaCl equivalents in the range 4.6 to 10.6 weight percent, ii) greenschist formational temperatures and pressures in the range 265-285 °C and 700-1400 bars; and iii) the proximity of the occurrence to the Rhone-Simplon Line, a deep crustal structure in the Swiss Alps.Corrected Ar-Ar data for hydrothermal adularia, considered to be contemporaneous with mineral deposition from the gold-bearing fluid, yields an age of 9.9 ±1.0 Ma. Geothermal gradients and uplift rates derived from the Ar-Ar age data and the geothermometry are in agreement with existing data for this region, and indicate that the hydrothermal activity at the Mont Chemin gold occurrence records one of the last Alpine metamorphic events in the northeastern Mont Blanc massif.Temperature estimates from fluid-muscovite-quartz-feldspar equilibrium and oxygen isotope thermometry of coexisting adularia and quartz are combined with the fluid inclusion isochores to derive depositional pressures. These data yield geothermal gradients on the order of 50 °C/km and uplift rates of 0.44 mm/a for the NE portion of the Mont Blanc massif.ZusammenfassungEin neues Schweizer Goldvorkommen am Mont Chernin, es handelt sich um Goldführende Quarzgänge, zeigt viele Charakteristika, die für mesothermale Goldlagerstätten der Alpen und weltweit typisch sind: i) Die Anwesenheit von NaCl-H2O-CO2 Fluiden mit einem XCO2 von ca. 0.016 und NaCl zwischen 4.6 und 10.6 Gew. % Äquiv.ii) Grünschieferfazielle Bildungstemperaturen und -drucke von 265-285°C bzw. 7001400bar. iii) Die Nähe der Vorkommen zur Rhone-Simplon Linie, einer tiefgreifenden Struktur in der Kruste der Schweizer Alpen.Korrigierte Ar-Ar Daten von hydrothermalem Adular, der als zeitgleich mit den Minerallagerstätten gebildet, angesehen wird, ergaben ein Alter von 9.9 ± 1.0 Ma. Die aus aus den Ar-Ar Daten bestimmten geothermalen Gradienten und Hebungsraten und die Ergebnisse der Geothermometrie stimmen mit bisher existierenden Daten aus dieser Region überein und zeigen, daß die hydrothermale Aktivität in den Goldvorkommen des Monte Chemin eines der letzten alpidischen metamorphen Ereignisse im nordöstlichen Mont Blanc Massiv darstellt.Temperaturabschätzungen aus Fluid-Muscovit-Quarz-Feldspat Gleichgewichten und Sauerstoffisotopen-Thermometrie an koexistierendem Quarz und Adular werden mit den Isochoren der Flüssigkeitseinschlüsse kombiniert, um die Bildungsdrucke abzuleiten. Diese Daten ergeben geothermische Gradienten in der Größenordnung von ca. 50 °C/km und Hebungsraten von 0.44 mm/Jahr für den Nordostteil des Mont Blanc Massives.

Evaluation of the fluid-inclusion crushing-stage as an aid in exploration for mesothermal gold-quartz deposits

Abstract Case studies in the Wabigoon and Uchi Archaean greenstone belts of Canada demonstrate that the liquid CO 2 -bearing fluid inclusions, which are diagnostic of mesothermal gold-quartz-carbonate-sulphide deposits, may be reliably and reproducibly detected in the field by crushing-stage analysis. The technique alone cannot distinguish veins of different ore grade in a given exploration terrain, but after preliminary crushing tests to assess the composition of local metamorphic fluids, samples from potentially auriferous mesothermal deposits may be inexpensively and unequivocally identified in the field. The most powerful application of the crushing-stage is in geochemical surveys of sediments in which diagnostic fluid inclusions in quartz may offer a much wider exploration target than the related gold anomaly. Unmistakable haloes of liquid CO 2 -bearing fluid inclusions are rapidly detectable in the wall rocks surrounding mesothermal veins. The haloes are spatially as extensive as the alteration mineral assemblages, and thus in field situations where outcrop is sparse or highly weathered, crushing analysis may aid in targeting areas for more detailed exploration.

F-OH substitution in natural tremolite, talc, and phlogopite

AbstractThe distribution of F between tremolite and talc has been determined in metamorphosed siliceous carbonates from the Grenville Province, Ontario. Wavelength dispersive electron microprobe analyses of contiguous, texturally compatible tremolite-talc pairs indicate that the substitution of F for OH is the most significant deviation from end-member stoichiometry in the samples studied. Mixing of F and OH components has been represented by an ideal solution model for F in tremolite and an asymmetric model for F in talc. Both linear and nonlinear regression techniques have been used to derive activity coefficients for the exchange of one equivalent of OH and F components in talc. The following expressions are the result of nonlinear regression of 32 analyses from coexisting mineral pairs: