Maramungee; a Proterozoic Zn skarn in the Cloncurry District, Mount Isa Inlier, Queensland, Australia

Abstract

The significant but subeconomic (1.8 Mt4.4% 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 rocks of the Maronan Supergroup that also hosts the large Pb-Zn-Ag deposits at Cannington and Pegmont. The Maramungee deposit is a product of 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 of the late to post-tectonic granites of the Williams batholith.Maramungee is hosted by upper amphibolite (sillimanite-K feldspar zone) gneisses, exceptionally iron-rich metabasites, and foliated granite. Mineralization was restricted to an elongate embayment in the margin of the Maramungee Granite pluton created by the antiformal part of a steeply inclined parasitic F 2 fold. A major photolinear feature lies immediately west of the hinge in both the granite and its host rocks. This structure was initiated by fabric intensification during attenuation of the adjacent fold limb. Mineralization was further localized by a heterogeneous altered rock package including (1) potassic rocks (generally foliated) 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 + or - clinopyroxene-bearing metabasites, and (3) a range of compositionally intermediate assemblages distinguished by the coexistence of Fe-Mn, Ca-, and K-bearing metamorphic silicates. Graphite commonly occurs at levels of several modal percent in the altered rocks but magnetite and zincian spinel are absent. The most strongly altered rocks have a chemical component derived from the metasedimentary gneisses and possibly reflect alteration of a unit which previously contained carbonates. This alteration would have involved reduction producing graphite coupled with differential addition of K, Fe, Mn, P, and F.The potassic and skarnoid rocks are distinctive components of a regional pattern of metasomatism that exploited steeply inclined D 2 structures. Early metasomatic fluids in equilibrium with graphite at Maramungee are unlikely to have been evolved from the magnetite-bearing pluton. Sulfides (mainly sphalerite, pyrrhotite, and pyrite) occur in veinlets and microbreccia matrices that overprint all the earlier rock types and alteration assemblages. They are associated with the development of retrograde (hydrous) and comparatively oxidized secondary minerals including Cl- and Fe-rich amphiboles, chlorite, scapolite, calcite, albite, epidote, and muscovite. Hematite is present in the youngest alteration assemblages. Retrograde phase relationships suggest that sulfide deposition was induced by reduction of the infiltrating fluid, initially occurred at temperatures 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. Molybdenite 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 elements. There is a crude zoning toward higher Cu/Zn ratios in the southern part of the prospect and there are sporadic occurrences of Cu-Fe sulfide mineralization that have no clear relationship to the main Zn zone associated with silicification, muscovite-chlorite alteration, and variably anomalous Au, Ag, Se, Co, REE, and U.