Zincian Spinel Associated With Metamorphosed Proterozoic Base-Metal Sulfide Occurrences, Colorado: A Re-Evaluation of Gahnite Composition as a Guide in Exploration

Abstract

Zincian spinel [(Zn,Fe,Mg)Al 2 O 4 ] is an accessory mineral in a variety of rock types, in particular, metamorphosed massivesulfide 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 44-90 Hercynite 0-45 Spinel 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 0-65 Hercynite 0-50 Spinel 25-90. Knowledge of the geological setting must be considered when using zincian spinel as a guide to metamorphosed massive-sulfide deposits.

Melt evolution in felsic dikes inferred from the composition of gahnite in two new occurrences, Pala Chief and Elizabeth R composite dikes, California

“…As the formation of gahnite and sphalerite in Zn-bearing systems is controlled largely by the fugacity of S in the system (Spry and Scott 1986a, b;Heimann et al 2005), gahnite is the only Zn-bearing mineral present in S-poor pegmatites, whereas sphalerite is the main host of Zn in S-rich pegmatite systems (e.g., Černý and Hawthorne 1982). In the Pala Chief and Elizabeth R aplites gahnite is the main host of Zn.…”

“…Therefore, the presence of gahnite in the Pala Chief and Elizabeth R dikes, as well as in other sulfide-poor dikes, indicates that the melts were saturated with Zn and Al at high enough levels to form gahnite with quite high Zn content, and so poor in S as to exclude the formation of sphalerite. Other factors that have been evaluated in the formation of Zn-bearing assemblages, in particular in metamorphosed massive sulfide systems, are temperature and pressure (e.g., Spry and Scott 1986a;Heimann et al 2005), but a discussion of these is beyond the scope of this work. We note, however, that the overall stability field of spinel increases towards lower temperatures with the introduction of Zn in the system.…”

“…In addition, gahnite is a common accessory mineral in metamorphosed massive sulfide deposits and Zn-rich hydrothermal alteration zones, a rare accessory phase in marbles, metabauxites, skarns, and aluminous granulites, and even rarer in banded iron formations (e.g., Spry and Scott 1986a, b;Heimann et al 2005Heimann et al , 2006. Because of its resistant character, gahnite is also found in placers and other sediment deposits (e.g., Morris et al 1997;McClenaghan 2005).…”

“…Numbered fields modified fromNêmec (1973) and re-defined bySpry and Scott (1986b) andHeimann et al (2005) for gahnite in: 1. Marbles.…”

Indicator minerals as guides to base metal sulphide mineralisation in Betul Belt, central India

“…From a study on the gahnite compositions from base metal sulphide occurrences, Colorado, Heimann et al (2005) postulated that the composition of zincian spinel in and around metamorphosed massive sulphide deposits is influenced by bulk rock composition which in the case of Mg rich alteration zones in Colorado causes a marked enrichment in the spinel component (MgAl 2 O 4 ) of zincian spinel. Present study on compositions of zincian spinel from different prospects of the Betul Belt does not corroborate their findings and it clearly shows that zincian spinel from both the alteration zone (Mg-rich and Al-rich) have similar restricted composition (table 1, figure 4).…”