Evolution of pyroxene-pyroxenoid-garnet alteration at the Cannington Ag-Pb-Zn deposit, Cloncurry District, Queensland, Australia

Abstract

The Cannington deposit (47.3 million metric tons at 10.9% Pb, 4.3% Zn, and 490 g/t Ag) is hosted by sillimanite-K feldspar zone gneisses of the 1.67 Ga Maronan supergroup in the Mount Isa Eastern fold belt (Cloncurry mining district). Distinctive Fe-rich silicate host rocks at Cannington were formed during a complex history of postpeak metamorphic alteration.Fe-(Mn-Ca)-rich alteration assemblages associated with the main economic resource in the southern part of the deposit occur as two strata-bound, discontinuous zones that form sharp contacts with host rocks. The zones have a lateral extent of 600 m and vary in thickness from 5 to 100 m. Earliest largely anhydrous and graphite stable alteration assemblages are dominated by quartz and almandine and have strong tectonite fabrics. These rocks are peripheral to and overprinted by weakly deformed, manganiferous, pyroxene-pyroxenoid, Ca-rich garnet alteration assemblages. The latter include hedenbergite, pyroxferroite-pyroxmangite, fayalite, calcic almandine, quartz, and apatite. Immobile element concentrations and mass balance considerations demonstrate that rocks with the pyroxene-pyroxenoid-garnet assemblage (excepting minor occurrences in basic rocks) contain an alkali-depleted pelitic component diluted by significant proportions of CaO, FeO, MnO, P 2 O 5 , C, and/or SiO 2 . No unaltered rock type at Cannington has these characteristics, but the metasedimentary origin and skarn-like features suggest that carbonate-bearing or calc-silicate precursors could have been present at least locally.Hydrous Fe-K-Cl-rich assemblages characterized by hornblende, biotite, pyrosmalite, and dannemorite occur in veins and as alteration of pyroxene-pyroxenoid-garnet rocks. Very high salinity, Na-K-Fe-Mn-Pb-rich primary fluid inclusions occur in quartz of this stage. Later sulfide-magnetite-fluorite mineralization was associated with further retrograde hydration that produced phyllosilicates such as greenalite. The magnetic anomaly that led to the discovery of the deposit is interpreted to be a product of the retrograde oxidation. Approximately 50 percent of the potentially economic resource occurs as vein and breccia mineralization superimposed on pyroxene-pyroxenoid-garnet-altered rocks. The distribution of distinct Ag-Pb-rich and Zn-dominated mineralization, both in association with magnetite-fluorite, is related to variations in the distribution of older pyroxene-pyroxenoid alteration assemblages, producing a pattern of mineralization that is broadly similar to that of Zn skarn deposits. Highest grades of Ag and Pb occur with pyroxenoids (Fe-Mn) in an outer envelope that encloses Zn-dominated (+ Cu, As) mineralization concentrated with hedenbergite (Ca-Fe). This implies that there was a post-metamorphic thermocheniical control on fractionation of Pb from Zn. This spatial relationship exhibited by anhydrous alteration assemblages and metals implies that the deposit was at the very least significantly reconstituted during postpeak metamorphic alteration, or that metals were transported to the deposit as a later part of the same alteration system that produced the Fe-(Mn-Ca)-rich assemblages.