David R. Lentz

Carbonatite genesis: A reexamination of the role of intrusion-related pneumatolytic skarn processes in limestone melting

The unusual elemental enrichment typical of many carbonatites and their stable and radiogenic isotope signatures—which are unlike those of sedimentary limestones—forced researchers to abandon limestone melting theories in the early 1960s and to support mantle-related models of carbonatite genesis. However, the fluid compositions [CO2/(H2O + CO2) = 0.05] required to melt limestone near its eutectic in the CaO-MgO-CO2-H2O system (600–675 °C) are virtually identical to those found in infiltrative magmatic-hydrothermal, skarn-forming systems; therefore, carbonates within such systems would melt via volatile fluxing. Skarn-related decarbonation reactions produce the CO2 required to form the carbonic acid (H2CO3) in the infiltrative H2O-rich fluid essential to carbonate melting. In addition to H2O, other fluxes (HF, HCl, H3PO4) and related salts derived from fluid-phase saturation of silicate intrusions could further depress the carbonate-melting eutectic temperatures, as well as enhance mass transfer of mineralizing elements into a forming skarn system and any low-viscosity carbonate melts produced within the skarn. The isotopic signatures of the resultant carbonate melts should reflect the elemental mass transfer of constituents from the intrusion, as well as Rayleigh decarbonation and elemental mixing processes typical of contact-metasomatic (pneumatolytic) processes. Many intrusions exsolving volatiles into limestone during final stages of solidification should produce some carbonate melt. Only carbonatites with enrichments in F, P, Sr, Nb, U, Th, and rare-earth-elements, have been considered intrusive melts, whereas the solidified products of other melts may have been erroneously considered hydrothermal veins.

U, Mo, and REE mineralization in late-tectonic granitic pegmatites, southwestern Grenville Province, Canada

Abstract Late-tectonic granitic pegmatites occur throughout most of the southwestern Grenville Province of Ontario and Quebec. These vary from mineralogically simple (3–4 major phases) to complex (> 4 major phases) pegmatites; the latter are variably enriched in U, Th, Mo, and rare-earth elements (REE) and Zr and Nb. The mineralogical and whole-rock geochemical data obtained from simple pegmatites suggest crystallization from low-temperature, H 2 O-undersaturated partial melts generated in amphibolite- to granulite-grade felsic paragneisses and orthogneisses that have undergone an earlier stage of melting. These magmas coalesced and fractionated during ascent to mid-crustal levels. The mineralogical complexity and increase in abundance of ferromagnesian silicates in individual pegmatites arises from interaction with the host rocks (hybridization/skarnification). Previous research has shown that there is an empirical relationship between ferromagnesian silicate content and U, Th, and REE enrichment, although this phenomena has not been investigated petrogenetically. Interaction with the host rocks (hybridization) produced compositional changes in the pegmatites through bimetasomatic (two way) exchange processes that modified normal fractional crystallization processes. High concentrations of Ca, Fe, Mg, and possibly Ti, depending on the host rock, are interpreted to indicate bimetasomatic exchange resulting in a mineralogical modification of the pegmatitic magma. For instance, the introduction of Ca into the pegmatite magma may lead to local saturation of titanite and/or allanite. High concentrations of U, Th, REE, and Nb in these titanites suggests that elemental distribution coefficients are affected within the pegmatite melts by the appearance of hybrid titanite. The role of a fluid phase in the hybridization/skarnification process is two fold: (1) the fluid phase will enhance bimetasomatic diffusive exchange of constituents between the host rocks (e.g., Ca, Fe, Mg, Ti) and the pegmatite (Si, Al, K, Na, Fe, U, Th, REE, Nb, Zr, H 2 O, F, Cl) over that of normal melt-solid diffusivities; and (2) the fluid may be enriched in certain constituents that become saturated during metasomatic interaction between the pegmatite and host rocks.

Petrogenesis and Geochemical Composition of Biotites in Rare-Element Granitic Pegmatites in the Southwestern Grenville Province, Canada

SummaryField, mineralogical, and chemical determinations of biotite from late-tectonic rare-element (U, Th, Mo, Nb, REE) Grenville pegmatites are used to characterize and evaluate their petrogenesis in part of the southwestern Grenville Province. These pegmatites occur within middle to upper amphibolite facies rocks along and adjacent to shear zones and have hybridized margins because of interaction with their host rocks. Endo- and exomorphic biotite forms by the mechanical incorporation or hydrothermal replacement of pre-existing biotite, hornblende, Ca pyroxene and/or feldspar; accompanied by chemical re-equilibration, an increase in grain size, and inherit some of the chemical characteristics of the pegmatite. In general, the Fe/(Fe + Mg) ratio ranges between 0.22 and 0.86. The most highly fractionated biotites have high Fe/(Fe + Mg), Al, Mn, Rb, Nb, and Zn and low Ba. The chemical compositions of biotite from unzoned, partially-zoned, and zoned pegmatites indicate a trend of increasing chemical fractionation based on LIL enrichment.Overlap in calculated log

Halogen signatures of biotites from the Maher-Abad porphyry copper deposit, Iran: characterization of volatiles in syn- to post-magmatic hydrothermal fluids

(f_{H_2 O} /f_{HF} )

Petrology, geochemistry, and U-Pb (zircon) age of the quartz-feldspar porphyry dyke at the Lake George antimony mine, New Brunswick: implications for origin, emplacement process, and mineralization

(3.2 to 4.7) and log

Effects of incidence angles on mapping accuracy of surficial materials in the Umiujalik Lake area, Nunavut, using RADARSAT-2 polarimetric SAR images. Part 2. Polarimetric analysis

(f_{H_2 O} /f_{HCl} )