Richard P. Taylor

Cordilleran slab windows

The geometry and geologic implications of subducted spreading ridges are topics that have bedeviled earth scientists ever since the recognition of plate tectonics. As a consequence of subduction of the Kula-Farallon and East Pacific rises, slab windows formed and migrated beneath the North American Cordillera. The probable shape and extent of these windows, which represent the asthenosphere-filled gaps between two separating, subducting oceanic plates, are depicted from the Late Cretaceous to the present. Possible effects of the existence and migration of slab windows on the Cordillera at various times include cessation of arc volcanism and replacement by rift or plate- edge volcanism; lithospheric uplift, attenuation, and extension; and increased intensity of compressional tectonism. Eocene extensional tectonism and alkaline magmatism in southern British Columbia and the northwestern United States were facilitated by slab-window development.

Water solubility in aluminosilicate melts of haplogranite composition at 2 kbar

Abstract The compositional dependence of H 2 O solubility was investigated at 2 kbar and 800°C in haplogranite melts (system SiO 2 1bNaAlSi 3 O 8 1bKAlSi 3 O 8 or Qz1bAb1bOr). The sixteen investigated compositions contained 25, 35 or 45 wt.% normative Qz and various Ab/(Ab+Or) ratios (0.15–0.92). Starting solid materials were anhydrous bubble-free glasses to which 10 wt.% H 2 O was added. The H 2 O contents of the isobarically quenched melts (glasses) were measured by Karl-Fischer titration. The results show that H 2 O solubility in aluminosilicate melts depends significantly upon anhydrous composition. The highest solubility values are obtained for the most Ab-rich melts. At a constant normative quartz content, the solubility of water decreases from 6.49 ± 0.20 wt .% H 2 O for a composition Qz 35 Ab 60 Or 05 (normative composition expressed in wt.%) to 5.50 ± 0.15 wt .% H 2 O for a composition Qz 35 Ab 10 Or 55 . Along this join, the most significant changes are observed for Ab-rich melts whereas H 2 O solubility in Or-rich melts remains almost constant. The H 2 O solubility data imply that H 2 O is preferentially associated with the Ab component in aluminosilicate melts. Application of the results to natural granitic melts suggests that Na-rich, H 2 O-saturated melts may be significantly less viscous than H 2 O-saturated, K-rich melts. The temperature dependence of H 2 O solubility, investigated for composition Qz 28 Ab 38 Or 34 at 2 kbar, is low. Increasing temperature from 750° to 1150°C only causes a decrease in H 2 O solubility from 6.00 to 5.41 wt.% H 2 O. These data are in agreement with previous data obtained for albite melts.

Rare-earth element distributions in uraninites: Implications for ore genesis

Abstract Analysis of the rare-earth element (REE) distributions in uraninites from a variety of hydrothermal mineral deposits indicates that wide variations are to be anticipated. Although crystal structure must exert a fundamental influence upon REE substitution in uraninite, the present study suggests that other factors, in particular hydrothermal fluid chemistry, may be of equal importance. Total REE (ΣREE) contents range from > 15,000 ppm in uraninites from ultrametamorphic deposits to Eu Eu ∗ = ; LREE HREE = 0.08 – 1.10 ); they are interpreted to result from variations in the oxygen fugacity and the nature of the anionic complexes in the hydrothermal fluids responsible for ore transport. Additionally in uraniferous quartz pebble conglomerates the REE patterns of the detrital uraninites provide direct evidence regarding the provenance of these sedimentary U ores.

Pre-eruptive melt composition and constraints on degassing of a water-rich pantellerite magma, Fantale volcano, Ethiopia

To determine the pre-eruptive composition of peralkaline magma at Frantale volcano, Ethiopia, we have studied glass inclusions in phenocrysts from a lateceupting, glassy pantelleritic lava flow. Matrix glass and crystal-free glass inclusions in quartz were analyzed for all major and most minor elements by electron microprobe and for H2O and 15 lithophile trace elements by ion microprobe (SIMS). Compositions of inclusions may have been slightly modified by post-trapping quartz crystallization, the average concentrations of all constituents but silica may be artificially high by 10% relative. Glass inclusions contain extreme enrichments in H2O (mean of 4.6 to 4.9 wt%) and several lithophile trace elements, which suggest that the lava erupted from a highly evolved, water-rich fraction of magma. The pre-eruptive concentration of water was much higher than that generally considered to occur in pantellerite magmas. Trends observed for lithophile elements in whole-rock samples from pre-,syn-and post-caldera eruptive units are mimicked in glass inclusions from the studied pantellerite lava; concentrations of Rb, Y, Zr, Nb, and Ce±Cl increase with progressive differentiation. With the exception of Cl and H2O contents, the composition of matrix glass is similar to that of glass inclusions suggesting: that few constituents exsolved from magma or cooling glass; eruption and quench of the lava occurred rapidly; and the matrix glass is, largely, compositionally representative of melt. Higher average abundances of Cl and H2O in glass inclusions suggest that these volatiles exsolved after melt entrapment; degassing could have occurred as either an equilibrium or disequilibrium process.

Porphyry molybdenum mineralization in a continental collision setting at Malala, northwest Sulawesi, Indonesia

Abstract The Malala deposit in northwest Sulawesi is the only known porphyry molybdenum occurrence in Indonesia. It is typical of the fluorine-poor (quartz monzonite or differentiated monzogranite) class of molybdenum deposits and belongs to the plutonic sub-type. The mineralized system is associated with porphyritic intrusive rocks of predominantly granitic composition (Malala porphyries) which occur as late differentiates in the roof zone of a composite pluton (Dondo batholith). Major oxide, trace element, isotope, and mineralogical data indicate that the various intrusive phases of the Malala-Dondo suite had a common magma source, are magnetite-series, LILE-enriched, (Caledonian) I-type granitoids, and belong to the high-K calc-alkaline series. The intrusive suite forms part of a 600 km long belt of granites and granodiorites, which were emplaced in a continental margin (“Western Sulawesi”) in Late Mio-Pliocene time, during and following the collisions between several microcontinents and the Mesozoic-Tertiary western magmatic arc/eastern subduction complex which forms the island of Sulawesi. The granitoids have initial Sr isotope ratios of 0.71–0.72, and are interpreted to be the results of partial melting of lower crust (possibly underthrusted continental crust of Precambrian to Paleozoic age) due to lithospheric thickening in a continental collision regime. Alteration and mineralization at Malala are erratically, and in most places weakly, developed over an area of 4 km 2 , predominantly as a “shell” up to 50 m thick at the intrusive contact. Highest grades are found in the East Zone, an elongate, steeply dipping, NW trending, fault controlled mineralized zone, that has an estimated resource of 100 Mt at 0.14% MoS 2 . No well-defined zoning of discrete alteration and sulphide mineral assemblages has been recognized. Petrographic, fluid inclusion and oxygen isotope data suggest that fluids of two origins were involved in the evolution of the mineralized system at Malala. Early hypersaline (40–65 eq. wt.% NaCl), hot (400–700°C) fluids were of magmatic derivation and circulated through fractures while the host intrusion was initially still in a semi-consolidated state, producing potassic alteration, barren quartz veins (stage 1) and quartz-K-feldspar-apatite-molybdenite veins (stage II). This early event was terminated upon introduction of predominantly meteoric, less saline fluids, which flowed through newly formed fractures and older reopened veins at temperatures in the range of 200 to 400°C, resulting in wall-rock alteration to, and vein deposition of, sericite, chlorite, carbonate and base metal sulphides (stage III). Finally, carbonate and kaolinite/dickite were deposited in late fractures (stage IV). Malala shows both significant similarities with and differences from other F-poor porphyry molybdenum deposits. The similarities appear to be in the nature of the cogenetic igneous rocks (granite, quartz monzonite and granodiorite) having relatively high Sr and Ba, and low Rb and Nb contents compared to F-rich Climax-type deposits, the geochemistry of the hydrothermal system (high Cu, low F and Sn), general vein paragenesis and lack of multiple ore shells. Malala differs from most other deposits with respect to its continent-continent collision setting, the late magmatic (“deuteric”) nature of the molybdenum mineralization and the dominance of carbonate alteration.

Geology of the Two Duck Lake Intrusion and the Marathon Cu-PGE Deposit, Coldwell Complex, Northern Ontario

Detailed mapping of the Eastern Gabbro, Coldwell Complex, Ontario, revealed that the copper and platinum-group-element deposit on the Marathon property is confined to a coarse-grained to pegmatitic gabbro-diorite intrusion, the Two Duck Lake intrusion (TDLI). Exposed along the eastern boundary of the Coldwell Complex, the TDLI was emplaced as a sill whose orientation is slightly oblique to the general structure of the Eastern Gabbro. The TDLI has been subdivided into lower , middle and upper sub-units on the bases of lithological, textural, mineralogical, and geochemical criteria. Both border sub-units are sulfide-, base- and precious metal-enriched, and exhibit abundant evidence of host-rock assimilation. The primary mineral assemblage in the TDLI is plagioclase, olivine, clinopyroxene, Fe-Ti oxides, with accessory orthopyroxene, biotite, apatite; this primary mineral assemblage is essentially preserved in the core of the intrusion (i.e., the middle subunit). The Cu-PGE Marathon deposit, delineated mainly in the upper sub-unit of the intrusion, consists of disseminated Fe-Cu-Ni sulfides, and platinum-group-minerals derived by hydrothermal remobilization from primary magmatic sulfides. Enrichment in Cu and PGE resulted from partial replacement of original Fe-Ti oxides and pyrrhotite in the primary mineral assemblage by reaction with (a) residual magmatic fluid, and (b) fluids derived from the breakdown of host rock xenoliths and hydrothermal alteration along the lower contact of the TDLI.

40Ar/1b39 Ar laser microprobe study of fluids in different colour zones of a hydrothermal scheelite crystal from the Dae Hwa WMo mine, South Korea

Abstract Laser probe analyses of Ar isotopes in fluid inclusions in five neutron-irradiated zones from a single scheelite crystal indicate the presence of two fluid components: (1) a Cl-rich hydrothermal fluid containing excess 40Ar; and (2) a meteoric fluid containing dissolved Ar with a 40 Ar 36 Ar ratio similar to the atmospheric value, and low Cl content. Core to rim variations indicate increasing dilution of the hydrothermal fluid with the meteoric component; however, the expected decrease in 40 Ar 36 Ar ratio is not recorded. At present the explanation of this is not clear; however, two possibilities are that either the dilution was accompanied by a complex process of boiling and fluid mixing, or that the 40 Ar 36 Ar ratio of the hydrothermal fluid increased with time due to leaching or diffusive loss of 40Ar from country rock minerals within the hydrothermal system, without significant addition of Cl.

The Nature and Distribution of Tantalum-bearing Minerals in Newly Discovered, Rare-Element Pegmatites at the Musselwhite Mine, Northwestern Ontario

Newly discovered pegmatites distributed throughout the sub-surface environs of the Musselwhite Archean gold deposit occur as narrow dikes that intrude most of the major lithological units. The mineralogy and textures of the pegmatite dikes are characteristic of granitic pegmatites of the “Rare-Element Class”; more specifically, they closely resemble varieties of the “Complex-type” granitic pegmatites that are commonly associated with zones of tantalum mineralization. The essential minerals in the dikes comprise albite, quartz, and K-feldspar, together with lesser amounts of mica (including lepidolite), spessartine, tourmaline, and locally, spodumene. Fine-grained (30 μm to 300 μm), tantalum-bearing minerals occur in the quartzo-feldspathic matrix of the dikes as disseminations, in interstices, and in small vugs. The nature of the tantalum-bearing mineral assemblage present in individual pegmatite dikes varies from a simple mineral assemblage composed of ferrocolumbite (with around 39 wt% Ta2O5) in the most northerly dike, to more complex assemblages comprised of manganotantalite (with up to 75 wt% Ta2O5), ferrotapiolite (with up to 79 wt% Ta2O5), and microlite (with up to 76 wt% Ta2O5) that are present in dikes located several kilometers to the south and southwest. The compositional characteristics and textural relationships of the tantalum-bearing minerals in the dikes provide evidence of igneous fractionation trends that are typical of rare-element granitic pegmatites. Such trends suggest that the newly discovered pegmatites represent part of a more extensive group of cogenetic, tantalum-mineralized dikes.