Metavolcanic rocks of the Winterville Formation from the prehnite-analcime subfacies of the prehnite-pumpellyite facies in north-central Aroostook County, Maine, contain an alteration assemblage including chlorite, chlorite/smectite (C/S), analcime, prehnite, and calcite. Field and laboratory study has identified areas where hydrothermal alteration has been pervasive in and around pillows. Compositional, crystal chemical, and structural variations in chlorite appear to be related to distance from this hydrothermal alteration. Samples were studied by whole-rock chemical analysis, electron microprobe analysis of individual mineral grains, X-ray powder diffraction of the clay fraction, and by computer modeling of diffraction patterns to determine the percentage of chlorite in interstratified C/S and to estimate the distribution of Fe and the size of coherent diffracting domains in pure chlorites. Whole-rock and pyroxene compositions suggest that the rocks have undergone Mg metasomatism. Modeling of X-ray diffraction data indicates that the percentage of chlorite in C/S increases to 100%, that Fe atoms become more equally distributed between octahedral sites in chlorite as it becomes more Fe-rich, and that diffracting domains grow larger with proximity to areas of more intense hydrothermal alteration. Analcime also increases near areas of hydrothermal alteration. The areal distribution of hydrothermal effects suggests that the alteration occurred as two separate events, or that two different thermal regimes were active concurrently.

Consideration of stratigraphic sections from five areas of northeastern Maine suggests that marked lateral lithofacies changes occur within the Ordovician and Silurian systems over very short distances. Three regionally extensive Ordovician (Caradocian-Ashgillian) lithofacies are present in the area of this study: (1) a western volcanic-graywacke-slate facies (Winterville Formation), (2) a medial slate-graywacke facies (Madawaska Lake Formation), and (3) an eastern slate-limestone-graywacke facies (lower Carys Mills Formation). The zones of gradual transition between these Ordovician lithofacies are subparallel to the trends of major structural elements. All of the lithofacies appear to have formed in relatively deep water and relatively far from cratonal source areas. The transition from Ordovician to Silurian is conformable and gradual in the eastern part of the area in contrast to abrupt and unconformable transitions in the central and western portions. Mild Taconian deformation during the latest Ordovician and earliest Silurian created western emergent areas that shed clastic debris into a continuing Silurian sedimentary basin in the east. In the eastern part of the area, continuous and fossiliferous stratigraphic successions representing almost the entire Silurian are present. The Early Silurian (Llandoverian-early Wenlockian) is represented by three principal lithofacies: (1) a western sandstone-conglomerate-slate facies (Frenchville Formation), (2) a central phyllitic slate-limestone-ironsione facies (New Sweden Formation), and (3) an eastern calcareous mudstone-limestone facies (Spragueville Formation). The Frenchville and New Sweden Formations contain material eroded from exposed Winterville and Madawaska Lake rocks in the Taconian land area(s) to the west. The Lower Silurian units are overlain conformably by thin-bedded flysch (Jemtland Formation) of late Wenlockian-early Ludlovian age that was also derived for the most part from western source areas. The youngest eastern Silurian unit is the Fogelin Hill Formation which overlies the Jemtland Formation conformably and consists of thinly interlayered red slate, green slate, and fine-grained, laminated, calcareous graywacke. In the western part of the region, the Lower and Middle Silurian are not present, and Upper Silurian sandstone, conglomerate, limestone, shale, and volcanic rocks unconformably overlie the Winterville Formation. The Upper Silurian sedimentary rocks, derived largely from the Winterville Formation, formed along irregular and possibly narrow shelves around emergent areas of the western Taconian land mass that had become largely submerged by Late Silurian time. No melange units have been found in the Ordovician and Silurian rocks of northeastern Maine to suggest the presence of a subduction zone as proposed by Bird and Dewey (1970). The Winterville volcanic rocks may be interpreted to be part of a complex island arc with a trench located outside the area to the southeast or northwest.

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 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 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 Two volcanogenic massive sulfide deposits in northern Maine were dated by zircon U-Pb SHRIMP-RG geochronology. Most zircons from the Bald Mountain and Mount Chase deposits are light brown, equant, euhedral to subhedral, multifaceted, and contain multiple, euhedral growth zones; cores or inclusions indicative of inherited domains are absent. For Bald Mountain the best age estimate is 467 ± 4 Ma, and for the Mount Chase deposit, 60 km to the south, the best age is 467 ± 5 Ma. 206 Pb/ 238 U ages (49 zircons) at Bald Mountain range from 446 to 1335 Ma, with a major peak from 446 to 490 Ma. Zircons younger than 467 Ma were affected by Pb loss; four zircons have 206 Pb/ 238 U ages older than 490 Ma due to inheritance of older zircons. At Mount Chase, 206 Pb/ 238 U ages (40 zircons) range from 418 to 1384 Ma with a major peak from 450 to 485 Ma. Again, zircons younger than 467 Ma were affected by Pb loss and eight zircons having 206 Pb/ 238 U ages older than 548 Ma, up to 1384 Ma, are due to inheritance. Pb isotope compositions of sulfide minerals (galena, pyrite, chalcopyrite, pyrrhotite) at Bald Mountain representing all major hypogene paragenetic stages of mineralization range in 206 Pb/ 204 Pb from 18.048 to 18.264, 207 Pb/ 204 Pb from 15.535 to 15.655, and 208 Pb/ 204 Pb from 37.803 to 38.160. Galena from the earliest exhalative stage of mineralization is less radiogenic than chalcopyrite, pyrrhotite, and pyrite, which were deposited by later replacement. Hydrothermal siderite and calcite from late parageneses range in 206 Pb/ 204 Pb from 18.131 to 22.519, 207 Pb/ 204 Pb from 15.555 to 15.856, and 208 Pb/ 204 Pb from 37.831 to 37.957. Pb isotope compositions of the sulfides plot along a narrow band on standard uranogenic and thorogenic plots. Calculated values of μ representing the sources contributing to the sulfides (9.5–10) are mostly higher than the average crustal Pb evolution curve and attest to a crustal influence on the Pb isotope compositions. Sulfides reflecting μ values lower than the average crustal curve point to the contribution of mantle isotopic compositions. The galenas are more radiogenic than hydrothermal sulfides in sediment-free modern environments, such as primitive arcs and sulfides associated with rifted arc basins, but resemble sulfides from sedimented ridges. The intrinsically high 207 Pb/ 204 Pb values of many of the Bald Mountain sulfides reflect radiogenic Pb derived from continental crustal basement or deep footwall sedimentary rocks. We conclude that Bald Mountain formed during rifting of a continental block (peri-Gondwanan) or was formed near crustal rocks characterized by high U/Pb values that provided the radiogenic Pb source for the sulfides. Sulfides from the Bald Mountain deposit are less radiogenic than the massive sulfide ores of the world-class Bathurst Mining Camp in New Brunswick and the Mount Chase deposit in the Central Maine terrane. Sulfides from the Bald Mountain deposit have a range of Pb isotope compositions identical to those of massive sulfides from Early to Middle Ordovician island-arc sequences from the Exploits subzone in the Central mobile belt of Newfoundland and some deposits from coastal Maine. Massive sulfides from the Notre Dame subzone in the Central mobile belt of Newfoundland, from the Eastern Townships of Quebec, and from the Vermont Cu belt are less radiogenic than the Bald Mountain deposit.