R. Kelly Vance

Volcanic rock-hosted gold and base-metal mineralization, associated with Neoproterozoic-Early Paleozoic, back-arc extension in the Carolina terrane, southern Appalachian Piedmont

Volcanogenic mineral deposits in the Carolina terrane, southern Appalachian Piedmont,include Kuroko-type polymetallic massive sulfide deposits and disseminated gold-pyrite deposits associated with propylitic, silicic, argillic, and advanced argillic alteration. Host rocks are metavolcaniclastic and metaepiclastic rocks of a Neoproterozoic-Early Cambrian magmatic arc. The favorable gold horizon is the transition from a lower succession of andesitic and rhyolitic pyroclastic rocks with basal mafic lavas to an upper sequence of epiclastic sedimentary units and minor lava and ash flows. Kuroko-type deposits are associated with mafic to bimodal volcanic rocks in the upper sequence. Whole-rock oxygen isotope analyses indicate that gold mineralization is associated with a transition from hydrothermal systems dominated by isotopically relatively light (δ18O = -6‰ to -10‰) waters, typical of high-latitude subaerial systems, to seawater (δ18O = 0‰). Plots of δ18O vs. SiO2 of the host rocks show a compositional gap associated with mineralization at the subaerial to submarine transition. Values of δ18O for the hydrothermal waters, lithostratigraphic analyses, and tectonic models of the Carolina terrane demonstrate that mineralization coincided with extension in a rifted arc.

The Palynology of Upper Pleistocene and Holocene Sediments from the Eastern Shoreline and Central Depression of St. Catherines Island, Georgia, USA

Sediment samples from the Hokes Landing area of St. Catherines Island, Georgia, were analyzed for their pollen and spore contents, and were further analyzed for their clay mineralogy and time of deposition. Samples were retrieved from a core collected in July, 2013, and were recovered from a site lying near an early-twentieth-century oyster boiler on the eastern side of the island, the Hokes Landing oyster boiler. The continuous core was more than 4.5 m long, representing one of the longest continuous cores collected on the island. Palynological and radiocarbon analyses provide significant information concerning the Late Pleistocene and Holocene evolution of the island; one of the most ancient finite radiocarbon ages (31,990 ± 230 years BP) came from the Hokes Landing site. Clay analyses indicate the presence of montmorillonite, kaolinite and illite, indicating sediment contributions from marine and terrestrial environments. Palynological data show that the freshwater fern Woodwardia grew in abundance at the site nearly 32,000 years ago. Investigation of other strata from the island, and comparison with data derived from other locations on the Georgia coastal plain, were undertaken. Emphasis was placed on the timing and topographic extent of the Pleistocene sea level lowstand along the eastern seaboard of North America. The effects of the eastward migration of the Georgia coastal zone during the lowstand are preserved in sediments at Hokes Landing, and provide a convincing new source of information concerning the nature of Pleistocene lowstand terrestrial communities in that region.

Metaultramafic schists and dismembered ophiolites of the Ashe Metamorphic Suite of northwestern North Carolina, USA

ABSTRACT Metaultramafic rocks (MUR) in the Ashe Metamorphic Suite (AMS) of northwestern North Carolina include quartz ± feldspar-bearing QF-amphibolites and quartz-deficient, locally talc-, chlorite-, and/or Mg-amphibole-bearing TC-amphibolites. Some workers divide TC-amphibolites into Todd and Edmonds types, based on mineral and geochemical differences, and we provisionally add a third type – olivine ± pyroxene-rich, Rich Mountain-type rocks. Regionally, MUR bodies range from equant, Rich Mountain- to highly elongate, Todd-TC-amphibolite-type bodies. The MURs exhibit three to five mineral associations containing assemblages with olivine, anthophyllitic amphibole, Mg-hornblende, Mg-actinolite, cummingtonite, and serpentine representing decreasing eclogite to greenschist facies grades of metamorphism over time. MUR protoliths are difficult to determine. Southwestern MUR bodies have remnant olivine ± pyroxene-rich assemblages representing ultrabasic-basic, dunite-peridotite-pyroxenite protoliths. Northeastern TC-amphibolite MURs contain hornblende and actinolitic amphiboles plus chlorites – aluminous and calcic assemblages suggesting to some that metasomatism of basic, QF-amphibolites yields all TC-amphibolites. Yet MgO-CaO-Al2O3 and trace element chemistries of many TC-amphibolites resemble compositions of plagioclase peridotites. We show that a few AMS TC-amphibolites had basaltic/gabbroic protoliths, while presenting arguments opposing application of the metasomatic hypothesis to all TC-amphibolites. We establish that MUR bodies are petrologically heterolithic and that TC-amphibolites are in contact with many rock types; that those with high Cr, Ni, and Mg have olivine- or pyroxene-dominated protoliths; that most exhibit three or more metamorphic mineral associations; and that contacts thought to be metasomatic are structural. Clearly, different MUR bodies have different chemistries representing various protoliths, and have different mineral assemblages, reflecting both chemical composition and metamorphic history. Spot sampling of heterolithic MUR bodies does not reveal MUR body character or history or allow ‘type’ designations. We recommend that the subdivision of MUR bodies into ‘types’ be abandoned and that the metasomatic hypothesis be carefully applied. AMS MURs and associated metamafic rocks likely represent fragments of dismembered ophiolites from various ophiolite types.