The Boundary volcanogenic massive sulfide (VMS) deposit (~ 0.5 Mt @ 3.5% Cu, 4% Zn, 1% Pb, 34.0 g/t Ag) in the Tally Pond Group, central Newfoundland, represents one of the best preserved, subseafloor-replacement style VMS deposits in the northern Appalachian orogen. The deposit is hosted within a Late Cambrian (~510 Ma) volcanic sequence consisting predominantly of rhyolitic flows and associated volcaniclastic rocks. Footwall strata are dominated by rhyolitic lapilli tuff, tuffs, lesser rhyolite flows, and in situ rhyolite breccias. The hanging wall consists of massive, quartz-bearing, flow-banded lobe and breccia facies rhyolite. The deposit occurs at and below the contact between these two units, and comprises pyrite, chalcopyrite, and lesser sphalerite (and other minerals). Massive mineralization contains abundant clasts of the surrounding host rocks, including chlorite-sericite-quartz–altered rhyolite lapilli and ash. Hydrothermally altered rocks consist of variably intense chlorite with lesser sericite and quartz. Chlorite alteration occurs in a discordant geometry, likely representing hydrothermal upflow zones, and chlorite-sericite-quartz occur as blankets that are parallel to the volcanic stratigraphy, likely representing alteration associated with replacement. The hanging-wall rhyolite flows also contain moderate to intense, pervasive quartz and sericite alteration. Both the hanging wall and footwall are characterized by strong Na2O-Sr depletions, K2O-MgO-Fe2O3-Ba enrichments, high alteration index values (e.g., Ba/Sr, chlorite-carbonate-pyrite index (CCPI), alteration index (AI)), and enrichments in base metals and volatile metals (e.g., Cu, Zn, Pb, Hg).

The presence of abundant remnant wall-rock and host-rock clasts within the ore, intricate sulfide replacement of laminated porous tuff and sand dikes, replacement fronts in host lithofacies, and intense alteration in both the footwall and hanging wall (where the hanging wall is preserved) are all features consistent with formation of the bulk of the Boundary deposit via subseafloor replacement. The deposit likely formed as a result of cooling of metal-bearing hydrothermal fluids, mixing with ambient seawater and pore water-entrained seawater within the volcanic rocks at a permeability interface between young, unlithified, highly permeable footwall volcaniclastic rocks and relatively impermeable hanging-wall rhyolitic flows. In our model, this permeability boundary was an important feature that promoted subseafloor replacement within the deposit.

Immobile element systematics of rhyolitic rocks from the Boundary deposit lack major differences in primary petrochemistry between hanging-wall and footwall strata. All rocks from the deposit area are subalkalic with transitional Zr/Y ratios (2.8–4.5), La/Sm ratios <1 (normalized to upper crust), and primitive mantle normalized signatures with slightly light rare earth element (LREE)-enriched patterns with flat heavy REE (HREE), and negative Nb, Ti, and Eu anomalies. These geochemical features, coupled with existing Nd-Pb isotope data, zircon inheritance patterns, and geologic information, are consistent with rhyolitic rocks at the Boundary deposit having formed by re-melting of arc basement, with continental crust (or recycled continental crust) present in the source region. It is likely that the deposit and its associated rhyolitic host rocks formed within a Cambrian continental(?) or pericontinental rifted arc along the margin of Ganderia, within the Iapetus Ocean.

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