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

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.