Abstract The supergiant Chuquicamata porphyry Cu-Mo deposit in northern Chile is truncated on its west side by a N-S-trending regional fault (the West fault), juxtaposing its ore to a relatively barren granodiorite (Fortuna Igneous Complex). There has been much speculation about the fate of, and extensive exploration for, the “missing half” of the deposit. It has been proposed that the west side of the fault hides the ore at depth, or that it was uplifted and the ore eroded; however, regional geologic mapping suggests that the West fault had a postore left-lateral strike-slip displacement of ca. 35 km. Accordingly, exploration, so far unsuccessful, has been focused in an area 35 km south near the Loa River and the city of Calama. In 1989, the Mina Ministro Hales (MMH) deposit was unexpectedly discovered west of the fault, under thick gravels, only 7 km south of the main mine. A previous study at MMH had suggested that mineralization was as old as 39 Ma, hence its ores were correlated with deposits of that age near Calama. Our recent U-Pb and Re-Os dating indicates that MMH mineralization was formed between 35 and 31 Ma, thus concurrently with Chuqui. The geochemistry of host Triassic and Eocene porphyry intrusions, ore mineralogy, and common Pb isotope ratios of hypogene sulfides at MMH and Chuqui proper are indistinguishable. Fluid inclusion data for paragenetically early porphyry assemblages at MMH closely mimic T h -salinity data from earlier studies at Chuqui, showing little or no evidence of boiling but indicating widely fluctuating confining pressures, compatible with hydraulic fracturing and fault movement during and after mineralization at a minimum initial lithostatic constraint of 5- to 8-km depth. We propose that MMH is a sheared-off portion of Chuqui, wedged in a fault cymoid loop and spared the full 35-km displacement of the West fault.

Abstract Recent detrital zircon results in both the central Appalachians and New England demonstrate that middle Ordovician, ‘Taconic’ island arcs, long considered to be peri-Laurentian, are built upon or associated with rock of Gondwanan affinity. This trip will visit granulite-facies orthogneiss of the Wilmington Complex, a 475–480 Ma magmatic arc, and the adjacent Wissahickon Formation. The Wissahickon Formation is intruded by and interlayered with meta-igneous rocks with arc affinity and contains detrital zircon populations characteristic of both Gondwanan and Laurentian sources. The Chester Park Gneiss, now known to have detrital zircon age spectra which match the Gondwana-derived Moretown Terrane in New England, is also featured. The trip will examine contact relationships between arc and Laurentian rocks and a newly discovered location where metapelitic rock contains garnet with crystallographically oriented rutile inclusions, possibly indicative of ultrahigh-temperature or ultrahigh-pressure metamorphism. We will discuss similarities between rocks of the central and northern Appalachians and evaluate a new model wherein the central Appalachian rocks were originally part of the Taconic arc in New England and were translated by strike-slip deformation to their present position in the orogen.