Margarite, corundum, gahnite and zincohögbomite in a blackwall, Raleigh Terrane, Eastern Piedmont Province, USA

Abstract

We report an unusual occurrence of margarite, corundum, gahnite and zincohö gbomite from the Raleigh terrane in the Piedmont Province of Virginia. The assemblage occurs in a chlorite-rich blackwall associated with a small metamorphosed ultramafic rock body. The blackwall is dominated by chlorite, but also contains distinctive clusters (1À4 mm across) that typically consist of ragged Zn-rich spinel grains surrounded by masses of randomly oriented margarite. Also spatially associated with spinel are smaller grains of corundum and hö gbomite. In most cases, spinel and högbomite are sufficiently enriched in Zn to be called gahnite and zincohö gbomite, respectively. Some högbomite grains are distinctly banded in back-scattered electron images, primarily reflecting variations in Zncontent. Textures suggest that högbomite formed at the expense of spinel (although locally the reverse relationship holds), and hö gbomite compositions mimic those of spinel. Margarite appears to be a later phase, and textures imply formation via reactions with spinel, corundum and, possibly, chlorite. This occurrence of Zn-rich spinel and högbomite is clearly related to the bulk composition of the blackwall rock, which contains >2500 ppm Zn. The origin of this Zn-rich composition is unclear, but could be due to a small amount of sphalerite or zincian-staurolite in the protolith.

The genesis of metamorphosed Paleoproterozoic massive sulphide occurrences in central Colorado: geological, mineralogical and sulphur isotope constraints

“…Al-rich granulites. The högbomite compositions are shown for aluminous metasediments, ultramafic rocks, calcic skarns, Fe-Ti deposits and calc-silicate granulites reported by Spry & Petersen (1989), quartzofeldspathic gneisses (Grew et al 1990), metabauxites (Yalçin et al 1993; Feenstra 1997; Ockenga et al 1998), cordierite gneisses (Heimann et al 2006), pelites (Rakotonandrasana et al 2010) dolomitic marble (Armbruster et al 1998) and blackwall (Owens et al 2013). …”

“…They occur in various rock types metamorphosed to the upper amphibolite-granulite facies, including aluminous metasediments, ultramafic rocks Fe-Ti deposits, skarns, Fe ores and metamorphosed massive sulphide deposits (e.g. Teale, 1980; Spry & Petersen, 1989; Feenstra 1997; Rakotonandrasana et al 2010; Owens et al 2013). In Colorado, Gable and Sims (1969) reported högbomite in cordierite-bearing gneisses in the central Front Range, where it occurs along the contact between hercynite and magnetite or ilmenite.…”