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ABSTRACT

Blocks of variably serpentinized oceanic mantle peridotite (harzburgite, olivine orthopyroxenite, and dunite) are entrained within a latest Cretaceous (“Laramide”) low-angle subduction channel, the Orocopia Schist, exposed at Cemetery Ridge, southwest Arizona. Oceanic peridotite, serpentinized by seawater, is strikingly out of place in this region of Paleoproterozoic to Jurassic continental crust. Correspondingly, the Cemetery Ridge peridotite contains four serpentinization-related sulfide or intermetallic minerals quite unusual for an inland, continental tectonic setting: pentlandite, cobalt pentlandite, heazlewoodite, and awaruite. The peridotite also contains two Ni-arsenide minerals, orcélite and maucherite; and, less commonly, the sulfides pyrrhotite, bismuthinite, bornite, and parkerite(?). These minerals form tiny to small (~3–100 μm) grains sparsely scattered amongst the profusion of serpentine and magnetite produced by serpentinization of olivine; many grains are enclosed in magnetite.

The three principal sulfide minerals at Cemetery Ridge are pentlandite [(Ni,Fe)8.93S8] in all samples studied, and cobalt pentlandite [(Co,Ni,Fe)9.01S8] and heazlewoodite (Ni2.98S2) in many or most. Pentlandite and cobalt pentlandite form a discontinuous series to 74 molar percent of the Co end member. High-Co pentlandite has systematically elevated Ni/Fe. Pyrrhotite (Fe7.88S8) occurs only in one high-S sample. Although orcélite (Ni4.71As2) and maucherite (Ni11.06As8) are volumetrically rare, one or both are found in most samples. Awaruite (Ni5Fe) is extremely rare, a few minute blebs in two samples.

Sulfide assemblages at Cemetery Ridge indicate the highly reduced pore fluid typical of serpentinization. Sulfur was introduced into Cemetery Ridge peridotite during the early stages of serpentinization, as one aspect of a general enrichment in fluid-mobile elements in a suprasubduction (mantle-wedge) environment. Further serpentinization was accompanied by progressive desulfurization, with concomitant transformation from minerals of higher to lower sulfidation, and partial transfer of first Fe then Ni from sulfide to oxide phases.

Five Ni or Ni-Co sulfide or arsenide minerals at Cemetery Ridge are new and unexpected for Arizona, where serpentinite is quite rare and Ni or Co deposits unknown. Sulfide minerals and assemblages at Cemetery Ridge are comparable to those of serpentinites in such places as the California and Oregon Coast Ranges and Mid-Atlantic Ridge, emphasizing the uniqueness of the tectonic setting of the subducted oceanic peridotite at Cemetery Ridge, and enhancing the status of the Orocopia and related schists as the world’s type (first-recognized, best-known) low-angle paleosubduction complex.

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