Geochemical and metallogenic contrasts in Paleozoic granitic rocks of the northern New England Appalachians

Abstract

Paleozoic granitic rocks from the Brompton-Cameron terrane, Central Maine terrane, and peri-Gondwanan terranes in New England were investigated to contrast their geochemical and metallogenic features. Modal and major element compositions for 50 plutons (>900 determinations) generally lack diagnostic features or systematic gradients that uniquely distinguish terranes. Calc-alkalic series Ordovician-Devonian granites predominate in the Brompton-Cameron terrane as well as in Devonian granites from the Central Maine terrane and Silurian-Devonian granites from peri-Gondwanan terranes. No systematic major element differences exist among granitic rocks from different terranes. Trace element contents, however, point to groups showing subtle differences, broadly consistent with their distribution within known terrane subdivisions. For example, the highest contents of Sr (~200 to 800 ppm at SiO 2 = 65 wt%) are found in Brompton-Cameron plutons and the lowest (<350 ppm) are in peri-Gondwanan plutons; Central Maine terrane plutons are intermediate. Normal mid-ocean ridge basalt (N-MORB) normalized diagrams for Silurian and Devonian granites from all the terranes show deep Nb and Ta troughs. Deep negative Eu anomalies, generally flat rare earth element (REE) chondrite-normalized patterns, and high heavy REE (>10× chondrites) contents distinguish peri-Gondwanan granites (SiO 2 = 54-70 wt%) from Brompton-Cameron granites. Granites from the Central Maine terrane have moderate negative Eu anomalies and heavy REE contents intermediate between those of Brompton-Cameron and peri-Gondwanan granites. Ordovician and Devonian granites (SiO 2 <70 wt%) in the Brompton-Cameron terrane indicate volcanic-arc affinities; the Devonian plutons also have late collisional and postcollisional affinities. Silurian granites in peri-Gondwanan terranes are related to volcanic-arc and within-plate tectonic settings, in contrast to Devonian granites, which occur in volcanic-arc, within-plate, and late collisional to postcollisional tectonic settings. Devonian granites from the Central Maine terrane also have strong affinities to volcanic-arc and within-plate tectonic settings.The granitic rocks are associated with the following types of ore deposits: porphyry Cu-Mo; porphyry Mo(?); W-Mo stockworks, skarns, and greisens, and Sn veins; and U-Mo in veins, fractures, and shear zones. Granites associated with porphyry Cu-Mo mineralization in the Brompton-Cameron terrane are the most oxidized (Fe 2 O 3 /FeO ~0.6 to >10); granites hosting Sn occurrences in peri-Gondwanan terranes are the least oxidized (Fe 2 O 3 /FeO ~0.15 to 1) and those granites associated with W and Mo mineralization have intermediate values.N-MORB normalized multielement diagrams and chondrite-normalized REE diagrams of representative, unaltered portions of Ordovician and Devonian granitic rocks

Re–Os geochronology and fluid inclusion microthermometry of molybdenite mineralisation in the late-Caledonian Omey Granite, western Ireland

“…period of granite intrusions with associated molybdenite mineralisation systems. 1,6,30,40 This study combines Re-Os molybdenite geochronology with fluid inclusion petrography and microthermometry to investigate molybdenite mineralisation associated with the Omey Granite in the Connemara sector of the Appalachian-Caledonian Orogen. The Omey Granite is one of three satellite plutons that occur to the NW of the Galway Batholith (Fig.…”

A review of molybdenite, and fluorite mineralisation in Caledonian granite basement, western Ireland, incorporating new field and fluid inclusion studies, and Re-Os and U-Pb geochronology

“…The Caledonian-Appalachian Orogen is marked by a ~50 m.y. period of granite intrusions with associated molybdenite mineralisation systems (Ayuso, 1999;Plant, 1986;Whalen, 1993;Lynch et al 2009). Furthermore, granitic basement rocks invariably contain a variety of economic and sub-economic mineral and hydrocarbon deposits that postdate the crystalline host.…”