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.

Abstract We examined the copper isotope ratio of primary and secondary copper mineralization of porphyry copper deposits. Distinct Cu isotope reservoirs exist for high-temperature hypogene, enrichment, and leach cap minerals. Chalcopyrite from high-temperature primary mineralization forms a relatively tight cluster of δ 65 Cu values of +1 to –1 per mil, whereas secondary minerals formed by low-temperature reveal a range of δ 65 Cu values from –16.96 to +9.98 per mil. Secondary chalcocite is relatively heavy, with δ 65 Cu varying from –0.3 to +6.5 per mil. Leach cap minerals dominated by Fe oxides (jarosite, hematite, and goethite) are relatively light, ranging from –9.9 to +0.14 per mil. A distinct pattern of heavier copper isotopes in supergene samples and a lighter isotopic signature exists in the leach cap and oxidation zone minerals. The pattern presents an excellent tool for using Cu isotopes for exploration through providing the following information: (1) identification of highly fractionated copper isotope ratios in copper sulfide and Fe oxide samples that indicate supergene processes and the extent of leaching and enrichment of copper, and (2) identification of highly fractionated copper isotope ratios in surface and/or groundwaters that indicate the active weathering of copper sulfides that experienced significant enrichment.

Abstract Combined isotopic dating indicates five episodes of felsic intrusion within the El Teniente orebody: (1) Sewell stock and other quartz diorite-tonalite intrusions of the eastern part crystallized from 6.46 ± 0.11 to 6.11 ± 0.13 Ma (zircon U-Pb); (2) quartz diorite-tonalite, immediately southeast of the orebody, with biotite 40 Ar/ 39 Ar plateau ages of 5.63 ± 0.12 and 5.47 ± 0.12 Ma—these ages agree with a hydrothermal overprint on zircons from the intrusions of the previous episode at 5.67 ± 0.19 to 5.48 ± 0.19 Ma (U-Pb); (3) Teniente dacite porphyry crystallized at 5.28 ± 0.10 Ma (zircon U-Pb); (4) a dacite ring dike encircling the Braden pipe crystallized at 4.82 ± 0.09 Ma (zircon U-Pb); and (5) minor dacite intrusions and dikes yielded a biotite 40 Ar/ 39 Ar plateau age of 4.58 ± 0.10 Ma, and sericite 40 Ar/ 39 Ar plateau ages of 4.56 ± 0.12 to 4.46 ± 0.10 Ma. All these felsic intrusions were emplaced within country rocks of late Miocene according to an apatite fission-track age of 8.9 ± 2.8 Ma for a mafic sill, in accord with previous K-Ar ages of 12.0 ± 0.7 to 6.6 ± 0.4 Ma for volcanic rocks from the district. Molybdenite Re-Os dating at El Teniente revealed ore deposition at 6.30 ± 0.03, 5.60 ± 0.02, 5.01 to 4.96, 4.89 ± 0.08 to 4.78 ± 0.03, and 4.42 ± 0.02 Ma, concurrent with the five intrusive episodes. The Re-Os system for molybdenite was unaffected by the various hydrothermal episodes. In contrast, the 40 Ar/ 39 Ar system of micas was reset by high-temperature (>350°C) fluid circulation and provides only a partial record of the latest history of development of this supergiant ore-forming system; biotite, sericite, and altered whole-rock samples collected throughout the orebody yielded 40 40 Ar/ 39 Ar plateau ages ranging from 5.06 ± 0.12 to 4.37 ± 0.10 Ma. These ages reveal a period of hydrothermal activity, which extended either continuously or episodically, for at least 0.69 ± 0.22 m.y. (±2σ) and that comprises a succession of three episodes of ore deposition. Separate hydrothermal episodes are thus interpreted to have lasted <0.69 ± 0.22 m.y. The Braden breccia pipe in the center of the deposit was formed as a single synmineralization event, probably related in time to the injection of the dacite ring dikes at 4.82 ± 0.09 Ma (zircon U-Pb). It was followed by quartzsericite alteration within and peripheral to, the pipe from 4.81 ± 0.12 to 4.37 ± 0.10 Ma (sericite 40 Ar/ 39 Ar). The successive intrusions of felsic bodies and their respective crystallization processes were immediately followed by genetically related, short-lived episodes of ore deposition, each associated with hydrothermal alteration. This multistage evolution, inferred from systematic dating, was not apparent from previous geochronologic data and is inferred to have contributed to the enormous volume and richness of the El Teniente. Thermal modeling of apatite fission-track data suggests that the porphyry system cooled very rapidly to temperatures below 105° ± 20°C, most likely before the intrusion of a postore hornblende-rich andesitic dike at 3.85 ± 0.18 Ma (hornblende 40 Ar/ 39 Ar). This dike cuts the southern part of the El Teniente deposit and marks the end of igneous activity in the orebody.

Abstract Re-Os isotope studies of molybdenite and other more common sulfides have yielded new geochronological and tracer information on the origin of the ore-forming metals for base metal porphyry mineralization. Because the new geochronological data are on the sulfides and not the associated alteration minerals or magmatic rocks, it is possible to constrain the mineralizing events within the context of the magmatic history of the mineralized district or region. Re-Os studies of base metal porphyry mineralization in Chile and Arizona, the two premier base metal porphyry regions in the world, indicate that the ore formation is episodic within the overall life of the magmatic arc. In Arizona, mineralization occurs in two distinct periods: ca. 75 Ma in the northwest part of the state, and ca. 55 Ma in the southeast. In Chile, the new Re-Os data on sulfides support previous K-Ar work that date mineralization at ca. 35 to 40 Ma in the northern part of the country and ca. 3 to 5 Ma in the south. The age data on sulfides typically place mineralization in the later stages of the magmatic history of the studied districts. Pyrite, chalcopyrite, and bornite from the base metal porphyry deposits have Re concentrations between 0.01 and 4,500 ppt and Os concentrations from 0.01 to 200 ppt. There is no obvious correlation between geographical setting or age and the concentration of Re and Os. There is also no obvious correlation between the Re content of the ore and its Pb isotope signature.