ABSTRACT Upper Cambrian through Middle Ordovician sedimentary strata of the Marathon/Solitario Basin (west Texas), which were folded and thrust-faulted during late Paleozoic Appalachian-Ouachita orogenesis, preserve evidence of the pre-Pangean history of the central southern Laurentian margin. New detrital zircon analyses reported here are from three Marathon Basin/Solitario formations: the upper Cambrian Dagger Flat Sandstone; the Lower Ordovician Marathon Formation, including the Rodrigues Tank Sandstone Member; and the Middle Ordovician Ft. Peña Formation. The far-southwestern outcrops of those Iapetus margin strata are within the Solitario dome (Presidio and Brewster Counties, Texas). Solitario zircon U/Pb geochronological results (laser ablation–inductively coupled plasma–mass spectrometry [LA-ICP-MS], sensitive high-resolution ion microprobe [SHRIMP]) expand the record of Cryogenian rifting as the Cuyania terrane separated from Laurentia. We evaluated these new data along with earlier geochronological and geochemical results from rift-related lava clasts in Lower–Middle Ordovician sedimentary subaqueous debris-flow deposits in the northwestern Marathon Basin. Deepening of the Iapetus seaway near the Laurentian margin (late Cambrian–Middle Ordovician) stimulated headward erosion of drainages, reflected in the systematic north-northwestward shift in zircon provenance from the west Texas Grenvillian and Southern Granite-Rhyolite Provinces to Yavapai-Mazatzal and Cheyenne Belt sources. The Cuyania rifted terrane underwent subduction at the western Gondwanan margin of the Iapetus Ocean in mid-Ordovician time (486 ± 7 Ma to 463 ± 4 Ma), and the resulting volcanism in the Famatina complex (Argentina) was most intense from ca. 472 to 468 Ma. Magmatic zircons from Ft. Peña bentonitic layers have identical U/Pb (488–468 Ma) and biostratigraphic (Darriwilian) ages to those from Famatinian bentonites at Talacasto (470 ± 5 Ma) in the Precordillera of Cuyania. Geologically constrained paleomagnetic reconstructions for 470 Ma depict the proximity of the Famatina arc, the rifted Cuyania terrane, and southern Laurentia at low southern latitudes (equator to ~30°S). These first U/Pb geochronological data from the Marathon/Solitario depocenter of western Iapetus appear to be compatible with such a configuration and can serve as test data for emerging tectonic interpretations.

Abstract The giant (>20 Moz) Telfer Au-Cu deposit is located in the Paterson Province of Western Australia and is hosted by complexly deformed marine Neoproterozoic metasedimentary siltstones and quartz arenites. The Telfer district also contains magnetite- and ilmenite-series granitoids dated between ca. 645 and 600 Ma and a world-class W skarn deposit associated with the reduced, ~604 Ma O’Callaghans granite. Based on monazite and xenotime U-Pb geochronology, Telfer is estimated to be older than O’Callaghans, forming between 645 and 620 Ma. Au-Cu mineralization at Telfer is hosted in multistage, bedding-parallel quartz-dolomite-pyrite-chalcopyrite reefs and related discordant veins and stockworks of similar composition that were emplaced into two NW-striking doubly plunging anticlines or domes. Mineralization is late orogenic in timing, with hot (≤460°C), saline (<50 wt % NaCl equiv) ore fluids channeled into preexisting domes along a series of shallow, ENE-verging thrust faults and associated fault-propagated fold corridors. A combination of fault-propagated fold corridors acting as fluid conduits below the apex of the Telfer domes and the rheology and chemical contrast between interbedded siltstone and quartz arenite units within the dome are considered key parameters in the formation of the Telfer deposit. Based on the presence of the reduced Au-Cu-W-Bi-Te-Sn-Co-As assemblage, saline and carbonic, high-temperature hydrothermal fluids in Telfer ore, and widespread ilmenite-series granites locally associated with W skarn mineralization, Telfer is considered to be a distal, intrusion-related gold deposit, the high copper content of which may be explained by the predominance of highly saline, magmatic fluids in gangue assemblages cogenetic with ore.