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The central and southern White Mountains constitute a well-exposed section of the middle to late Mesozoic convergent margin of western North America. New 40 Ar/ 39 Ar analyses were obtained for 8 Ca-amphibole and 13 biotite specimens from igneous and metamorphic wall rocks (8 rock samples from the Barcroft pluton, a single McAfee Creek-type aplite, 7 mafic dikes/metadikes, and two Andrews Mountain quartzites). Most of the spectra are very complicated. Except for one relatively fresh and four apparently fresh diabase dikes, all 9 granitoids, the 2 metadiabasic dikes, and both contact metamorphic quartzites partially lost (or gained) argon during heating and/or deuterichydrothermal alteration over ~100 m.y. of igneous arc evolution. New granitoid ages are similar toor younger than-published U/Pb ages for the Middle Jurassic Barcroft and the Late Cretaceous McAfee Creek plutons, and are similar to published K-Ar and 40 Ar/ 39 Ar ages of the Cottonwood/ Beer Creek and other plutons in the central and southern White Mountains. Contact metamorphic ages of wall rocks approximate nearby granitoid emplacement ages. Mafic dikes invaded the area episodically; most yield disturbed 40 Ar/ 39 Ar spectra exhibiting a range of cooling ages. Some, metamorphosed by the Barcroft pluton, must be older than 165 Ma; others give apparent ages of 144 (Independence dike swarm), 115-118, and 98 Ma. Clearly, multiple stages of mafic dike injection accompanied mid-and late-Mesozoic accretion of this sector of the North American continental margin.

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The Gibbs free energy of formation of nukundamite (Cu 3.38 Fe 0.62 S 4) was calculated from published experimental studies of the reaction 3.25 Cu 3.38 Fe 0.62 S 4 + S 2 = 11 CuS + 2 FeS 2 in order to correct an erroneous expression in the published record. The correct expression describing the Gibbs free energy of formation (kJ•mol-1) of nukundamite relative to the elements and ideal S 2 gas is ⌬ f G°n ukundamite, T(K) =-549.75 + 0.23242 T + 3.1284 T 0.5 , with an uncertainty of 0.6%. An evaluation of the phase equilibria of nukundamite with associated phases in the system Cu-Fe-S as a function of temperature and sulfur fugacity indicates that nukundamite is stable from 224 to 501°C at high sulfidation states. At its greatest extent, at 434°C, the stability field of nukundamite is only 0.4 log f(S 2) units wide, which explains its rarity. Equilibria between nukundamite and bornite, which limit the stability of both phases, involve bornite compositions that deviate significantly from stoichiometric Cu 5 FeS 4. Under equilibrium conditions in the system Cu-Fe-S, nukundamite + chalcopyrite is not a stable assemblage at any temperature.