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Lithogeochemistry and paleotectonic setting of the Bald Mountain massive sulfide deposit, northern Maine


, : Lithogeochemistry and paleotectonic setting of the Bald Mountain massive sulfide deposit, northern Maine. Economic Geology Monographs 11(Pages 79-109

The present study was undertaken to document the lithogeochemistry of the principal volcanic units hosting the Early Ordovician Bald Mountain Cu-Zn-Au-Ag massive sulfide deposit in northern Maine as well as that of volcanic units from the surrounding region. Results document several distinct petrochemical associations that reflect variations in the tectono-magmatic evolution of an intraoceanic arc to continental arc-back-arc complex along the convergent Ordovician margin of Gondwana. Footwall and immediate hanging-wall rocks at Bald Mountain are composed of tholeiitic basalt-andesite massive to pillowed lava flows and hyaloclastite breccias, and felsic aphyric, pumice- and crystal-rich ignimbrites. Mafic rocks are characterized by low incompatible element contents, flat to slightly light rare earth element-depleted patterns, and prominent negative anomalies for Nb, Ta, Zr, Hf, and Ti on chondrite-normalized trace element plots. The felsic ignimbrites are tholeiitic dacite-rhyodacite having trace element characteristic similar to that of the associated mafic rocks. The footwall and immediate hanging-wall rocks define a bimodal tholeiitic, mafic-dominated intraoceanic arc volcanic suite similar in composition to recent volcanic rocks from the Kermadec arc in the southwest Pacific. In contrast to the lower, mafic-dominated section, the upper hanging-wall section at Bald Mountain is composed of dominantly felsic volcaniclastic rocks and associated sediments with only minor andesite flows and/or sills. The volcanic rocks are transitional to calc-alkaline with the felsic rocks showing high Th ( approximately 3-18 ppm) and light REE (La approximately 11-68 ppm) contents, and negative Nb, Ta, P, and Ti anomalies. Their geochemistry and isotopic signatures indicate significant involvement of an enriched continental crust component in their source region. The upper hanging-wall rocks are similar in composition to calc-alkaline suites from mature arc and continental margin arc-back-arc settings like the Taupo Volcanic Zone of New Zealand. Volcanic rocks from the surrounding Munsungun-Pennington Mountain and Weeksboro-Lunksoos Lake anticlinoria in northern Maine include basalts having arc, continental back-arc, and within-plate non-arc (no negative Nb-Ta anomalies) petrotectonic affinities. Associated felsic volcanic rocks have enriched calc-alkaline compositions with continental crustal geochemical and isotopic signatures. The various volcanic units partially overlap in composition with some volcanic rocks from the hanging-wall portion of the Bald Mountain sequence. Volcanic rocks hosting the Mount Chase Zn-Pb-Cu-Ag-Au massive sulfide deposit in the Weeksboro-Lunksoos Lake anticlinorium, southeast of Bald Mountain, are coeval with upper hanging-wall units at Bald Mountain and have continental back-arc basin compositional affinities. The Bald Mountain sequence is interpreted to reflect the progression from an oceanic to transitional continental crustal setting as part of the evolving Popelogan Arc-Tetagouche-Exploits back-arc basin system. The setting may have been analogous to that observed along the present Kermadec-Have Trough-Taupo Volcanic Zone arc and back-arc system in the southwest Pacific. The Bald Mountain deposit most likely formed in a submarine oceanic arc volcano caldera located proximal to, but offshore from, the continental back-arc basin in which the Mount Chase deposit developed.

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