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ZINCIAN SPINEL ASSOCIATED WITH METAMORPHOSED PROTEROZOIC BASE-METAL SULFIDE OCCURRENCES, COLORADO: A RE-EVALUATION OF GAHNITE COMPOSITION AS A GUIDE IN EXPLORATION

Adriana Heimann¹, Paul G. Spry^1, and Graham S. Teale²

¹ Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011–3212, U.S.A.
² Teale and Associates, P.O. Box 740, North Adelaide, South Australia 5006, Australia

E-mail address: pgspry{at}iastate.edu

Zincian spinel [(Zn,Fe,Mg)Al₂O₄] is an accessory mineral in a variety of rock types, in particular, metamorphosed massive-sulfide deposits, pegmatites, aluminous metasedimentary rocks, skarns, marbles, and sulfide-bearing granulites. Textural and compositional studies of zincian spinel in thirteen Proterozoic sulfide deposits metamorphosed to the amphibolite and granulite facies in Colorado, many of which are spatially associated with Mg-rich alteration zones, shed new light on the use of zincian spinel composition as a guide to sulfide ores. Zincian spinel from the Independence, Bon Ton, and Sedalia deposits and some cases from the Green Mountain, Betty, Caprock, Cinderella, and Cotopaxi deposits show compositions that correspond to zincian spinel associated with metamorphosed massive-sulfide deposits hosted by hydrothermally altered Fe–Al-rich metasedimentary and metavolcanic rock units elsewhere in the world (Gahnite_44–90Hercynite_0–45Spinel_0–25). However, other compositions of zincian spinel from these last five deposits and from three locations in the Wet Mountains (Marion and Amethyst prospects and an unnamed prospect) are depleted in Zn and enriched in Mg and Fe. These examples of Mg-rich zincian spinel occur in rocks in or adjacent to metamorphosed massive sulfides dominated by Mg-rich minerals such as phlogopite, anthophyllite, gedrite, tremolite, forsterite, cummingtonite, sapphirine, cordierite, enstatite, and clinohumite. The composition of zincian spinel in and surrounding metamorphosed massive-sulfide deposits is influenced by reactions involving Zn and Fe sulfides, as well as by bulk-rock composition, which in the case of many metamorphosed massive-sulfide deposits in Mg-rich alteration zones in Colorado, causes a marked enrichment in the spinel component of zincian spinel, to Gahnite_0–65Hercynite_0–50Spinel_25–90. Knowledge of the geological setting must be considered when using zincian spinel as a guide to metamorphosed massive-sulfide deposits.

Keywords: gahnite, exploration guide, massive sulfides, magnesium enrichment, Colorado.

This article has been cited by other articles:

				A. Heimann, A. Heimann, P. G. Spry, G. S. Teale, and C. E. Jacobson CORONAS, SYMPLECTITIC TEXTURES, AND REACTIONS INVOLVING ALUMINOUS MINERALS IN GEDRITE - CORDIERITE - GARNET GNEISS FROM EVERGREEN, FRONT RANGE, COLORADO Can Mineral, October 1, 2006; 44(5): 1025 - 1044. [Abstract] [Full Text] [PDF]