Stratabound base-metal sulfide deposits and occurrences are present in metasedimentary rocks of the Neoproterozoic and Paleozoic Nome Complex on south-central Seward Peninsula, Alaska. Stratabound and locally stratiform deposits including Aurora Creek (Zn-Au-Ba-F), Wheeler North (Pb-Zn-Ag-Au-F), and Nelson (Zn-Pb- Cu-Ag), consist of lenses typically 0.5–2.0 m thick containing disseminated to semimassive sulfides. Host strata of the Aurora Creek and Wheeler North deposits are variably calcareous and graphitic siliciclastic metasedimentary rocks of Middle Devonian or younger age based on detrital zircon geochronology; the Nelson deposit is within Ordovician–Devonian marble (Till et al., this volume, Chapter 4). Deformed veins such as Quarry (Zn-Pb-Ag-Ba-F) and Galena (Pb-Zn-Ag-F) occur in a unit composed mainly of marble and schist; fossil and detrital zircon data indicate that this unit contains rocks of Ordovician, Silurian, and Devonian age. None of these Zn- and Pbrich deposits or occurrences has spatially associated metavolcanic or intrusive rocks. All were deformed and metamorphosed to blueschist facies and then retrograded to greenschist facies during the Jurassic and Early Cretaceous Brookian orogeny. Disseminated Cu-rich deposits including Copper King (Cu-Bi-Sb-Pb-Ag-Au) and Wheeler South (Cu-Ag-Au) occur in silicified carbonate rocks and have textures that indicate a pre- to syn-metamorphic origin. The Zn- and Pb-rich sulfide deposits and occurrences consist mainly of pyrite, sphalerite, and/or galena in a gangue of quartz and carbonate. Minor minerals include arsenopyrite, chalcopyrite, magnetite, pyrrhotite, tetrahedrite, barite, fluorite, and chlorite; gold and electrum are trace to minor constituents locally. Sphalerite is uniformly unzoned and commonly aligned in the dominant foliation. These textures, together with the presence of folded layers of barite at Aurora Creek and folded sulfi de layers at Wheeler North, indicate that mineralization in the stratabound deposits predated deformation and metamorphism. Electron microprobe (EMP) analyses of the carbonate gangue show three major compositions comprising siderite, ankerite, and lesser dolomite. The Cu-rich deposits differ in containing chalcopyrite and bornite in a quartzose matrix. Altered wall rocks surrounding the Zn- and Pb-rich deposits and occurrences have aluminous assemblages composed of muscovite + chloritoid + siderite + chlorite + quartz ± tourmaline ± ilmenite ± apatite ± monazite. Muscovite within these assemblages and in sulfide-rich samples is phengitic and locally enriched in barium; chloritoid at Aurora Creek is enriched in zinc. Minor minerals including pyrite, sphalerite, galena, chalcopyrite, barite, and hyalophane occur as fine-grained disseminations. These altered rocks vary from small lenses a few meters thick to large zones tens of meters in thickness that extend along strike, discontinuously, for 4 km or more. Whole-rock geochemical analyses of the altered rocks from deposit-proximal and deposit-distal settings reveal generally lower SiO 2 /Al 2 O 3 ratios and higher Fe 2 O 3 T /MgO ratios compared to those of unaltered clastic metasedimentary rocks of the Nome Complex and of average shale or graywacke. The deposit-proximal samples are also characterized by anomalously high Zn, Pb, Hg, and Sb, relative to the unaltered metasediments. These data, together with mass change calculations, suggest that the aluminous rocks formed as replacements of permeable graywacke in semi-conformable alteration zones, beneath the seafloor contemporaneously with Zn-and/or Pb-rich sulfide mineralization. Exposures of all three stratabound Zn-Pb deposits show evidence of deformation and recrystallization that occurred in a largely brittle deformational regime. This evidence includes small faults and veins that cut foliation and localized zones of breccia. Sulfide minerals, fluorite, quartz, chlorite, and carbonate minerals crystallized within these structures, which probably formed during Cretaceous deformation of the Nome Complex. Previous studies of the Zn-Pb(-Ag-Au-Ba-F) deposits and occurrences have invoked models of epigenetic veins, volcanogenic massive sulfides (VMS), or carbonate- replacement deposits (CRD). In contrast, our field and laboratory data (including sulfur isotopes; Shanks et al., this volume) suggest that these Zn- and/or Pb-rich deposits represent different levels of sediment-hosted, seafloor-hydrothermal systems, with stratabound and locally stratiform deposits such as Aurora Creek and Wheeler North having formed on the seafloor and/or in the shallow subsurface like many sedimentary-exhalative (SEDEX) deposits worldwide. The deformed veins such as Quarry and Galena are interpreted to have formed deep in the subsurface, possibly as feeders to overlying SEDEX deposits such as Aurora Creek. Formation of all of the Zn- and Pb-rich deposits and occurrences took place during episodic rifting of the continental margin between the Ordovician and Mississippian(?). Regional relationships are consistent with at least some of the deposits having formed in Late Devonian–Mississippian(?) time.

The 1766-m-deep Eyreville B core from the late Eocene Chesapeake Bay impact structure includes, in ascending order, a lower basement-derived section of schist and pegmatitic granite with impact breccia dikes, polymict impact breccias, and cataclas tic gneiss blocks overlain by suevites and clast-rich impact melt rocks, sand with an amphibolite block and lithic boulders, and a 275-m-thick granite slab overlain by crater-fill sediments and postimpact strata. Graphite-rich cataclasite marks a detachment fault atop the lower basement-derived section. Overlying impactites consist mainly of basement-derived clasts and impact melt particles, and coastal-plain sediment clasts are underrepresented. Shocked quartz is common, and coesite and reidite are confirmed by Raman spectra. Silicate glasses have textures indicating immiscible melts at quench, and they are partly altered to smectite. Chrome spinel, baddeleyite, and corundum in silicate glass indicate high-temperature crystallization under silica undersaturation. Clast-rich impact melt rocks contain α-cristobalite and monoclinic tridymite. The impactites record an upward transition from slumped ground surge to melt-rich fallback from the ejecta plume. Basement-derived rocks include amphibolite-facies schists, greenschist(?)-facies quartz-feldspar gneiss blocks and subgreenschist-facies shale and siltstone clasts in polymict impact breccias, the amphibolite block, and the granite slab. The granite slab, underlying sand, and amphibolite block represent rock avalanches from inward collapse of unshocked bedrock around the transient crater rim. Gneissic and massive granites in the slab yield U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon dates of 615 ± 7 Ma and 254 ± 3 Ma, respectively. Postimpact heating was <~350 °C in the lower basement-derived section based on undisturbed 40 Ar/ 39 Ar plateau ages of muscovite and <~150 °C in sand above the suevite based on 40 Ar/ 39 Ar age spectra of detrital microcline.

Optical and electron-beam petrography of melt-rich suevite and melt-rock clasts from selected samples from the Eyreville B core, Chesapeake Bay impact structure, reveal a variety of silicate glasses and coexisting sulfur-rich melts, now quenched to various sulfide minerals (±iron). The glasses show a wide variety of textures, flow banding, compositions, devitrification, and hydration states. Electron-microprobe analyses yield a compositional range of glasses from high SiO 2 (>90 wt%) through a range of lower SiO 2 (55–75 wt%) with no relationship to depth of sample. Some samples show spherical globules of different composition with sharp menisci,suggesting immiscibility at the time of quenching. Isotropic globules of higher interfacial tension glass (64 wt% SiO 2 ) are in sharp contact with lower-surface-tension, high-silica glass (95 wt% SiO 2 ). Immiscible glass-pair composition relationships show that the immiscibility is not stable and probably represents incomplete mixing. Devitrification varies and some low-silica, high-iron glasses appear to have formed Fe-rich smectite; other glass compositions have formed rapid quench textures of corundum, orthopyroxene, clinopy-roxene, magnetite, K-feldspar, plagioclase, chrome-spinel, and hercynite. Hydration (H 2 O by difference) varies from ~10 wt% to essentially anhydrous; high-SiO 2 glasses tend to contain less H 2 O. Petrographic relationships show decomposition of pyrite and melting of pyrrhotite through the transformation series; pyrite→pyrrhotite→troilite→iron. Spheres (~1 to ~50 μm) of quenched immiscible sulfide melt in silicate glass show a range of compositions and include phases such as pentlandite, chalcopyrite, Ni-As, monosulfide solid solution, troilite, and rare Ni-Fe. Other sulfide spheres contain small blebs of pure iron and exhibit a continuum with increasing iron content to spheres that consist of pure iron with small, remnant blebs of Fe-sulfide. The Ni-rich sulfide phases can be explained by melting and/or concentrating target-derived Ni without requiring an asteroid impactor source component. The presence of locally unaltered glasses in these rocks suggests that in some rock volumes, isolation from postimpact hydrothermal systems was sufficient for glass preservation. Pressure and temperature indicators suggest that, on a thin-section scale, the suevites record rapid mixing and accumulation of particles that sustained widely different peak temperatures, from clasts that never exceeded 300 ± 50 °C, to the bulk of the glasses where melted sulfide and unmelted monazite suggest temperatures of 1500 ± 200 °C. The presence of coesite in some glass-bearing samples suggests that pressures exceeded ~3 GPa.

Abstract Textural and mineralogical studies of the Bald Mountain Cu-Zn-Au-Ag massive sulfide deposit, northern Maine, document a well-preserved premetamorphic hydrothermal evolution involving both exhalative and subsea-floor replacement processes. The 30-million metric tonne (Mt) Bald Mountain deposit forms a thick bowl-shaped accumulation of sulfides up to 215 m thick within a synvolcanic sea-floor graben of Early Ordovician age. Five principal stages (facies) of mineralization are recognized. Stage I mainly developed Fe sulfide mounds composed of fine-grained pyrite (As- and Sb-rich) and probably marcasite, with locally abundant sphalerite, sparse galena, and silica. Framboidal pyrite and colloform pyrite ± sphalerite ± galena are present locally, near the stratigraphic top of the deposit. During late stage I mineralization partial collapse of the sulfide mounds took place, probably due to dissolution of matrix anhydrite, producing thin to very thick (up to 20 m) accumulations of pyrite-quartz breccias. Following this mound collapse, stage I resumed with exhalative mineralization that filled the graben to its rim. Related mineralization formed volumetrically minor replacements of rhyolite ignimbrite. Stage I massive sulfides have a geochemical signature marked by generally high contents of Zn, Pb, As, Sb, Ag, Au, Hg, and Tl. This mineralization was succeeded, mostly within the graben structure, by the precipitation of low-temperature deposits of silica and Fe oxyhydroxides (now hematitic chert) that cover stage I deposits to depths of up to 28 m, as a hydrothermal cap to the massive sulfides below. Stage II formed mainly pyrrhotite-chalcopyrite replacements of stage I sulfides in the deep subsurface at temperatures of ca. 340° to 400°C based on arsenopyrite geothermometry. Stage II developed mainly after precipitation of the exhalative ferruginous silica cap, which may have sealed in the system thermally and chemically against shallow seawater entrainment. In addition to high Cu, stage II deposits contain abundant Co and Se. During this and subsequent stages of mineralization, older stage I sulfides underwent extensive recrystallization and zone refining at ∼250° to 325°C, accompanied by the replacement of pyrite by sphalerite ± galena, formation of euhedral quartz, arsenopyrite, and pyrite ± electrum, and remobilization of galena into small veins. Geometric relationships involving mineral assemblages and whole-rock (massive sulfide) geochemical data, together with textural information, suggest that Zn, Pb, As, Sb, Hg, and Tl in stage I deposits were dissolved and transported both upward and laterally by the zone refining for at least 150 m, resulting in very low contents of these elements within underlying stage II deposits. Gold was also remobilized and locally concentrated by the zone refining. Stage III deposits consist of wavy quartz ± chalcopyrite veins and replacements, reflecting their emplacement into unlithified massive sulfide mounds. Coeval to younger stage IV mineralization produced a complex assemblage of coarse pyrite with major amounts of chalcopyrite, magnetite, and greenalite; siderite, quartz, minnesotaite, ferropyrosmalite, and sphalerite are generally minor. Like stage II deposits, those of stage IV are significantly enriched in Co and Se, relative to stage I deposits. Stage IV also developed by subsea-floor zone refining, superimposed on older stages as veins and replacements, preferentially in the lower part of the deposit. Epigenetic hematization and silicification of fine-grained hanging-wall sediments (now argillites) between andesite flows, and of overlying fine-grained rhyolite ignimbrites, may have occurred when stage IV fluids breeched the ferruginous silica cap. Later, stage V mineralization formed siderite-rich veins with variable amounts of quartz, pyrite, marcasite, pyrrhotite, sphalerite, greenalite, magnetite, hematite, and calcite, both in the massive sulfide body and the stringer zone. Occurrences of very Fe rich silicates in stages II, IV, and V, and of siderite in stages IV and V, contrast with the absence of Mg-bearing silicates and carbonates throughout the deposit (excluding the footwall stringer zone). These compositions record the involvement of end-member Fe-rich hydrothermal fluids during formation of late sulfide veins and replacements, without appreciable amounts of shallow entrained (unreacted) seawater. Stage IV and V deposits precipitated from CO 2 -rich fluids based on their abundant siderite gangue. Stage IV formed mainly by the oxidation of stage II pyrrhotite, producing assemblages of pyrite ± magnetite (and rare magnetite without pyrite) together with locally prominent Fe 3+ -bearing greenalite; this relatively high f O2 environment continued during stage V mineralization, forming greenalite and hematite. Oxidation may have resulted from fluid boiling due to breaching of the ferruginous silica cap and consequent lowering of pressure in the hydrothermal system.

The Middle Pennsylvanian Fire Clay tonstein, mostly kaolinite and minor accessory minerals, is an altered and lithified volcanic ash preserved as a thin, isochronous layer associated with the Fire Clay coal bed. Seven samples of the tonstein, taken along a 300-km traverse of the central Appalachian basin, contain cogenetic phenocrysts and trapped silicate-melt inclusions of a rhyolitic magma. The phenocrysts include beta-form quartz, apatite, zircon, sanidine, pyroxene, amphibole, monazite, garnet, biotite, and various sulfides. An inherited component of the zircons (determined from U-Pb isotope analyses) provides evidence that the source of the Fire Clay ash was Middle Proterozoic (Grenvillian) continental crust inboard of the active North American margin. 40 Ar/ 39 Ar plateau ages of seven sanidine samples from the tonstein have a mean age of 310.9 ± 0.8 Ma, which suggests that it is the product of a single, large-volume, high-silica, rhyolitic eruption possibly associated with one of the Hercynian granitic plutons in the Piedmont. Biostratigraphic analyses correlate the Fire Clay coal bed with a position just below the top of the Trace Creek Member of the Atoka Formation in the North American Midcontinent and near the Westphalian B-C boundary in western Europe.