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Research ArticleArticles

Petrogenesis of the Rambler Rhyolite Formation: Controls on the Ming VMS Deposit and geodynamic implications for The Taconic Seaway, Newfoundland Appalachians, Canada

Jean-Luc Pilote and Stephen J. Piercey
American Journal of Science June 2018, 318 (6) 640-683; DOI: https://doi.org/10.2475/06.2018.02
Jean-Luc Pilote
* Department of Earth Sciences, Memorial University of Newfoundland, 300 Prince Philip Drive, St. John's, Newfoundland and Labrador A1B 3X5, Canada
** Present address: Geological Survey of Canada, 490 rue de la Couronne, Québec, Québec G1K 9A9, Canada
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  • For correspondence: jean-luc.pilote@canada.ca
Stephen J. Piercey
* Department of Earth Sciences, Memorial University of Newfoundland, 300 Prince Philip Drive, St. John's, Newfoundland and Labrador A1B 3X5, Canada
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Abstract

The Ming Cu-Zn-Ag-Au volcanogenic massive sulfide (VMS) deposit is hosted by the Upper Cambrian-Lower Ordovician Rambler Rhyolite formation, which consists of a folded northeast plunging felsic dome complex, developed in the uppermost segment of the obducted supra-subduction Baie Verte oceanic tract, in the north central Newfoundland Appalachians. The deposit is overlain by interstratified mafic volcanic flows and volcaniclastic rocks of the Snooks Arm Group. The upper 1 km of the Rambler Rhyolite formation consists of a coherent felsic lithofacies at its base, overlain by a quartz-bearing volcaniclastic-dominated lithofacies (units 1.2 and 1.3). Geochemically, these rocks are light rare earth element (LREE)- and large ion lithophile element (LILE)-enriched ([La/Yb]pn = 4–20; Th = 2.58–4.05 ppm), and high field strength element (HFSE)-depleted (Y = 5.64–6.59 ppm) rhyodacite with calk-alkalic affinities (Th/Yb > 2; Zr/Y > 7). The last pulse of felsic volcanism (unit 1.3) is intimately associated with the massive sulfide lenses and has FI- and FII-type rhyolite signatures (high La/Yb ratios), commonly attributed to deep (≥ 30 km) crustal melting. The εNd(t) values of the rhyolites range from −2.5 to −1.3, indicating the rocks were influenced by crustal material. A 10 percent batch partial melt of a garnet-amphibole metamorphosed normal mid-oceanic ridge basalt (N-MORB) and island arc tholeiite (IAT) crustal material can generate a melt similar to the felsic rocks hosting the Ming deposit; however, the IAT reproduces much better the absolute abundances of LILE (Th, Sr), HFSE (Nb, Zr, Y), and middle rare earth elements (MREE) of the felsic rocks. It is interpreted that the felsic melt came from melting of a subducted slab with IAT affinity, thus explaining the depth of melt generation and FI-FII signatures of the felsic rocks. Moreover, the slab-derived siliceous melt may also have contributed magmatophile elements (for example, Ag, Au, Se, Te, Sn, Sb, Hg) into the hydrothermal system once it had reached near-surface depths, and may explain the epithermal suite element-enrichment in the Ming deposit. This study shows that strongly fractionated felsic volcanic rocks associated with boninites, such as those associated with the Ming VMS deposit, can be important hosts for precious metal-enriched VMS and may be as prospective as tholeiitic felsic volcanic sequences (for example, FIII-FIV-type rhyolite – low La/Yb ratios).

The base of the Snooks Arm Group is comprised of spatially restricted sulfide-bearing mafic breccia, overlain by a regionally extensive sedimentary sequence (εNd(t) = +3.1 to +5.5), which is in turn overlain by interstratified high-Mg basalt (εNd(t) = +1.6), Th-enriched back-arc basin basalt (BABB), enriched mid-oceanic ridge basalt (E-MORB; εNd(t) = +6.6), and LREE-enriched/low-Ti tholeiitic tuffs (εNd(t) = −0.5). Four generations of mafic to intermediate dikes and sills cross-cut the Rambler Rhyolite formation and share similar geochemical characteristics to the extrusive rocks of the Snooks Arm Group, suggesting comagmatic relationships. The combination of variations in Nb/Yb, Th /Yb, and εNd(t) values within and between units suggest melts derived from depleted to enriched mantle material with melts synchronously generated by both slab-metasomatized mantle wedge and upwelling back-arc asthenosphere.

  • petrogenesis
  • Taconic seaway
  • volcanogenic massive sulfide
  • FI-type felsic volcanic rocks
  • Appalachians
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American Journal of Science: 318 (6)
American Journal of Science
Vol. 318, Issue 6
1 Jun 2018
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Petrogenesis of the Rambler Rhyolite Formation: Controls on the Ming VMS Deposit and geodynamic implications for The Taconic Seaway, Newfoundland Appalachians, Canada
Jean-Luc Pilote, Stephen J. Piercey
American Journal of Science Jun 2018, 318 (6) 640-683; DOI: 10.2475/06.2018.02

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Petrogenesis of the Rambler Rhyolite Formation: Controls on the Ming VMS Deposit and geodynamic implications for The Taconic Seaway, Newfoundland Appalachians, Canada
Jean-Luc Pilote, Stephen J. Piercey
American Journal of Science Jun 2018, 318 (6) 640-683; DOI: 10.2475/06.2018.02
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    • Abstract
    • INTRODUCTION
    • GEOLOGIC SETTING
    • LITHOGEOCHEMISTRY AND WHOLE-ROCK Sm-Nd ISOTOPE COMPOSITIONS OF THE PACQUET COMPLEX AND COVER SEQUENCE
    • DISCUSSION
    • CONCLUSIONS
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Keywords

  • Petrogenesis
  • Taconic seaway
  • volcanogenic massive sulfide
  • FI-type felsic volcanic rocks
  • Appalachians

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