Abstract 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 (˜3–18 ppm) and light REE (La ˜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-Havre 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.

Abstract The Bald Mountain deposit, a medium-sized (30 Mt) volcanic-hosted massive sulfide (VHMS) deposit of Early Ordovician age in northern Maine, was selected for detailed study because it is one of the best-preserved such deposits in the world. The massive sulfide lies within a 5-km-thick stratigraphic section that forms the Bald Mountain sequence. This study focuses on the volcanic and sedimentary evolution of the Bald Mountain sequence, with the goal of understanding the controls of deep-water volcanotectonic processes on the generation of massive sulfide mineralization.

Abstract The Bald Mountain volcanogenic massive sulfide (VMS) deposit of Early Ordovician age in northern Maine contains 30 million metric tons (Mt) of Cu-Zn-Au-Ag sulfides. It is exceptionally well preserved, lacking penetrative deformation, and having experienced only prehnite-pumpellyite–grade regional metamorphism. The deposit occurs within a homoclinal west-dipping volcanic sequence that consists of, from bottom to top, basalt and basaltic andesite, crystal-poor rhyolite ignimbrite, massive sulfide and related units, crystal-rich rhyolite ignimbrite and intercalated andesite, carbonaceous argillite, and rhyolitic volcaniclastic rocks. Basalts stratigraphically below the massive sulfide are intruded by an elongate body of tonalite-plagiogranite; gabbros intrude rocks both above and below the massive sulfides. The basal contact of the host volcanic sequence is believed to be a thrust with underlying Middle Ordovician clastic sedimentary rocks; the upper contact is depositional with the Middle to Upper Ordovician Winterville Formation and, in places, with Silurian conglomerates. Ordovician synvolcanic faults that predominantly strike 025°, 050° to 060°, 325° to 335°, and 350° formed a small (320 × 275 m) synvolcanic graben in which as much as 215 m of massive sulfide accumulated. Hydrothermal solutions utilized these faults as fluid conduits, causing structurally controlled epidote and silica alteration in the deep footwall. Structurally controlled alteration is also indicated by the presence of magnetic low areas in mafic rocks up to 1 km below the deposit. Movement of zinc- and copper-rich fluids was controlled by the location of the Ordovician faults. Zinc-rich fluids were concentrated along faults that bound the northern, western, and southern sides of the small graben; copper-rich fluids moved along faults that define the eastern side of the graben. Rocks overlying the massive sulfide body show little evidence of the growth faulting that occurred within and below the deposit, indicating that most extensional deformation ceased shortly after exhalative sulfide deposition. Synvolcanic Ordovician faulting and graben formation are the principal causes for the small lateral dimensions of the Bald Mountain deposit relative to those of most VMS deposits of comparable tonnage. Postsulfide deformational events occurred in the Late Ordovician to Early Silurian when rocks hosting the Bald Mountain deposit were thrust over Ordovician clastic sedimentary rocks and in the Early Devonian when Acadian faulting and folding segmented the deposit and tilted it to the west.

Abstract Radiogenic isotope (Nd-Pb-Sr) studies of the Early Ordovician Bald Mountain and Mount Chase massive sulfide deposits characterize the volcanic host rocks, constrain the types of sources that contributed to their isotopic signatures, and provide key fingerprints for regional comparisons. The ε Nd signatures (∼6.5–3.3) of volcanic rocks in the footwall and immediate hanging wall at Bald Mountain point to predominant contributions from mantle associated with an intra-oceanic island arc. Felsic volcaniclastic rocks in the footwall have εNd values (∼5.2–4.5) that overlap those in the underlying mafic volcanic rocks (∼6.5–4.7). In contrast, felsic volcani-clastic rocks from the immediate hanging wall have higher εNd values (∼5.8–4.4) than the more mafic rocks (∼3.7–3.3). The volcanic sequence in the upper hanging wall at Bald Mountain consists of calc-alkaline rocks akin to those from areas where continental crust is undergoing rifting. These rocks are characterized by enrichment in the light rare earth elements, Th, U, and other incompatible trace elements, and by a very wide range of ε Nd values (∼+2.4 to –9.7). Initial 87Sr/86Sr ratios in the mafic volcanic rocks of the footwall at Bald Mountain are as low as ∼0.7034 and in the felsic volcaniclastic rocks of the immediate hanging wall range up to ∼0.7093. In the same rocks, 206 Pb/ 204 Pb ratios range from 18.414 to 25.946, 207 Pb/ 204 Pb from 15.554 to 15.955, and 208 Pb/ 204 Pb from 37.813 to 38.647. Volcanic rocks from the footwall at Bald Mountain display somewhat lower Pb isotope compositions than rocks from the hanging wall. The Nd, Sr, and Pb isotope signatures of the volcanic rocks broadly become more radiogenic higher in the stratigraphic section as the crustal contribution increases. Rhyolitic to intermediate volcanic and volcaniclastic rocks host the Mount Chase massive sulfide deposit. These rocks exhibit wide variations in εNd values. Arc-related volcanic rocks have distinctly lower εNd values (∼ –2.5 to –2.6) than volcanic rocks from back-arc or within-plate settings (ε Nd ∼6.0–0.8). Felsic volcanic rocks from Mount Chase, the upper hanging wall at Bald Mountain, and the Winterville Formation have a wide range of ε Nd values (∼+2.5 to –12.7). For Mount Chase, 206Pb/204Pb spans from 18.434 to 19.676, 207Pb/204Pb from 15.637 to 15.710, and 208Pb/204Pb from 39.621 to 39.664. For the Winterville Formation, the range of 206Pb/204Pb is from 19.661 to 31.858, 207Pb/204Pb from 15.633 to 16.225, and 208Pb/204Pb from 38.128 to 55.293. Sedimentary rocks near the Bald Mountain deposit have negative ε Nd and radiogenic Pb isotope values (206Pb/204Pb = 19.606–21.203, 207Pb/204Pb = 15.591–15.712, and 208Pb/204Pb = 38.097–43.536). Pb isotope compositions of the volcanic rocks in northern Maine generally link this region to source contributions characteristic of peri-Gondwanan terranes that are typically radiogenic and have relatively high 207Pb/204Pb ratios. Nd isotope signatures of the felsic volcanic rocks overlap the isotopic field characterizing the Brookville block (Avalonian) in New Brunswick. An intra-oceanic arc is implicated for the footwall and immediate hanging-wall volcanic rocks at Bald Mountain and a continental arc and back-arc setting is suggested for the volcanic rocks hosting the Mount Chase deposit and calc-alkaline rocks in the upper hanging wall of Bald Mountain. Transition from tholeiitic to calc-alkaline volcanism at Bald Mountain records the evolution from an intra-oceanic and mantle-dominated setting to an extensional and crustal-dominated setting. The compositional variations of volcanic rocks hosting the massive sulfide deposits in Maine resemble those in modern arc systems in the Pacific Ocean. The tectonic evolution of the Tetagouche-Exploits basin in New Brunswick also explains many of the geochemical features of correlative volcanic rocks hosting the Bald Mountain and the Mount Chase massive sulfide deposits in Maine.

Abstract This field trip examines the geology of rocks of the Keweenawan Supergroup (1.1 Ga) and related intrusive rocks of the Midcontinent rift system (MRS) in the western part of the northern peninsula of Michigan. The combination of stops includes all formations of the Keweenawan Supergroup in this region. Examination of all described localities requires more than a single day and participants are encouraged to use this guidebook on their own to supplement the localities that will be visited on our one-day trip. Stops are numbered in stratigraphic order, from oldest to youngest, not in the order in which they will be visited.