MARAMUNGEE - A PROTEROZOIC ZN SKARN IN THE CLONCURRY DISTRICT, MOUNT ISA INLIER, QUEENSLAND, AUSTRALIA

The significant but subeconomic (1.8 Mt @ 4.4% Zn) Maramungee deposit is 90 km south-southeast of Cloncurry in northwest Queensland. It lies in the Eastern fold belt of the Proterozoic Mount Isa inlier within r ocks of the Maronan Supergroup that also hosts the large Pb-Zn-Ag depo sits at Cannington and Pegmont. The Marainungee deposit is a product o f a series of processes that occurred during the metamorphic evolution of the Eastern fold belt and the brittle structures that host sulfide mineralization are likely to have been active after the emplacement o f the late to post-tectonic granites of the Williams batholith. Maramu ngee is hosted by upper amphibolite (sillimanite-K feldspar zone) peis ses, exceptionally iron-rich metabasites, and foliated granite. Minera lization was restricted to an elongate embayment in the margin of the Maramungee Granite pluton created by the antiformal part of a steeply inclined parasitic F2 fold. A major photolinear feature lies immediate ly west of the hinge in both the granite and its host rocks. This stru cture was initiated by fabric intensification during attenuation of th e adjacent fold limb. Mineralization was further localized by a hetero geneous altered rock package including (1) potassic rocks (generally f oliated) with large amounts of microcline and/or biotite, (2) skarns ( partly discordant) including both manganoan hedenbergite + Fe-Mn-rich grossularite + quartz + apatite rocks and Ca-rich almandine +/- clinop yroxene-bearing metabasites, and (3) a range of compositionally interm ediate assemblages distinguished by the coexistence of Fe-Mn, Ca-, and K-bearing metamorphic silicates. Graphite commonly occurs at levels o f several modal percent in the altered rocks but magnetite and zincian spinel are absent. The most strongly altered rocks have a chemical co mponent derived from the metasedimentary gneisses and possibly reflect alteration of a unit which previously contained carbonates. This alte ration would have involved reduction producing graphite coupled with d ifferential addition of K, Fe, Mn, P, and F. The potassic and skarnoid rocks are distinctive components of a regional pattern of metasomatis m that exploited steeply inclined D, structures. Early metasomatic flu ids in equilibrium with graphite at Maramungee are unlikely to have be en evolved from the magnetite-bearing pluton. Sulfides (mainly sphaler ite, pyrrhotite, and pyrite) occur in veinlets and microbreccia matric es that overprint all the earlier rock types and alteration assemblage s. They are associated with the development of retrograde (hydrous) an d comparatively oxidized secondary minerals including Cl- and Fe-rich amphiboles, chlorite, scapolite, calcite, albite, epidote, and muscovi te. Hematite is present in the youngest alteration assemblages. Retrog rade phase relationships suggest that sulfide deposition was induced b y reduction of the infiltrating fluid, initially occurred at temperatu res of 450-degrees to 500-degrees-C, and may have continued through a significant cooling interval. Zn is the only economically significant metal present, although Pb, Ag, Cu, Cd, and Sb are commonly anomalous and there are sporadically elevated amounts of Au, As, and Mo. Molybde nite is associated with fluorite and enrichments of Rb, Ba, U, Th, and light REE in the early potassic alteration. With the exception of Zn and Cd there is little overall correlation among the chalcophile eleme nts. There is a crude zoning toward higher Cu/Zn ratios in the souther n part of the prospect and there are sporadic occurrences of Cu-Fe sul fide mineralization that have no clear relationship to the main Zn zon e associated with silicification, muscovite-chlorite alteration, and v ariably anomalous Au, Ag, Se, Co, REE, and U.