THE COMPOSITION OF ZINC-RICH SPINELS ASSOCIATED WITH METAMORPHOSED MASSIVE SULFIDE DEPOSITS: A REEXAMINATION

Zincian spinels [(Zn,Fe,Mg)Al₂O₄] are accessory minerals in a variety of rock types, in particular, metamorphosed massive sulfide (mms) deposits, pegmatites, aluminous metasediments, and skarns. Previous studies suggest that spinels with 55-90 mole % ZnAl₂O₄, 10-40 mole % FeAl₂O₄, and 5-20 mole % MgAl₂O₄ are spatially associated with mms deposits and are guides to ore. Such deposits commonly contain gahnite, sphalerite, pyrite (py) and pyrrhotite (po). Thermodynamic calculations show that py-po buffers fS₂, and fixes the sphalerite composition and the gahnite:hercynite ratio of the zincian spinel. The buffering capacity of Zn-Fe sulfides is the main reason why gahnite compositions usually have a narrow range in a given mms deposit. Textural and compositional studies of spinels in twelve Proterozoic massive sulfide deposits metamorphosed to amphibolite and granulite facies in Colorado, many of which are spatially associated with Mg-rich alteration zones, shed new light on the use of spinel composition as a guide to sulfide ores. Spinels from the Independence, Bon Ton, and Sedalia deposits and some from the Green Mountain (GM), Betty (BT), Caprock (CR), Cinderella (CD) and Cotopaxi (CX) deposits show compositions that correspond to zincian spinels associated with mms deposits elsewhere in the world. However, other spinels from these last five deposits are depleted in Zn and enriched in Mg and Fe. Spinels from GM are spatially associated with gedrite and phlogopite. Similarly, Mg-rich zincian spinels also occur in rocks in or adjacent to mms dominated by Mg-rich assemblages such as phlogopite-clinochlore-anthophyllite (Ace High), phlogopite-tremolite-clinochlore (Cinderella), clinochlore-humite-phlogopite (BT), forsterite-cummingtonite-actinolite-garnet (CR), forsterite-actinolite-clinochlore-phlogopite (CX), and sapphirine, forsterite, phlogopite, anthophyllite, cordierite, diopside, and clinohumite-rich rocks (Marion, Amethyst, and an unnamed prospect). This study shows that the composition of spinels in mms deposits is influenced by reactions involving Zn- and Fe-sulfides as well as by bulk composition, which in the case of many mms deposits in Colorado causes a marked enrichment in the spinel (sensu stricto) component of zincian spinel.