SERPENTINITE-HOSTED NICKEL, IRON, AND COBALT SULFIDE, ARSENIDE, AND INTERMETALLIC MINERALS IN AN UNUSUAL TECTONIC SETTING, SOUTHWEST ARIZONA

We recently discovered blocks of oceanic mantle peridotite—harzburgite and dunite, serpentinized by seawater or derivative fluids—in the early Paleogene Orocopia Schist low-angle subduction complex exposed at Cemetery Ridge (CR), southwest Arizona. Southern Arizona is an inboard region of intermediate to silicic Paleoproterozoic continental crust heavily overprinted by Mesoproterozoic, Mesozoic, and Cenozoic continental magmatism, largely silicic. Oceanic peridotite is strikingly out-of-place there. Correspondingly, the peridotite at CR contains several serpentinization-related sulfide, arsenide, and intermetallic minerals that are rare in or new to Arizona. One (orcélite) is also rare globally.

The most common opaque mineral in CR serpentinite is abundant (~ 5 %) magnetite, intimately intergrown with mesh-textured serpentine derived from olivine. Magnetite encloses and is intergrown with far less abundant, but ubiquitous, small (~ 30–100 µm) grains of pentlandite to cobaltoan pentlandite and heazlewoodite (Ni₃S₂). A few rocks also contain sparse pyrrhotite. Five additional minerals are even less abundant, typically several tiny (~ 5–20 µm) grains per thin section: chalcopyrite, bismuthinite, awaruite (Ni₃Fe), and two of the six naturally occurring Ni arsenides: orcélite (Ni₄_.₇₅As₂) and maucherite (Ni₁₁As₈). Despite the difficulty of analyzing small grains by EMPA, most of these minerals closely match ideal and published compositions.

Textural relations imply this generalized sequence: formation of pentlandite (s.l.) and heazlewoodite, then partial replacement of pentlandite by heazlewoodite, with magnetite formation throughout and continuing after. These mineral assemblages indicate a highly reduced pore fluid, typical of serpentinization. Traces of awaruite are probably remnants of an early, most reduced stage. The observed sequences of mineral inter- and overgrowths and replacements suggest progressively less reducing conditions as serpentinization proceeded; a trend reported from other serpentinites.

Heazlewoodite, awaruite, orcélite, and maucherite apparently are new minerals for Arizona. (On-line reports of awaruite in the Canyon Diablo meteorite are apocryphal.) As our study continues, we will not be surprised to find additional unusual minerals.