Pb–Zn–(As) enrichments in amphibolites from Broken Hill-type ore systems, NW Queensland: products of retrograde hydrothermal dispersion

Abstract

Amphibolites typically lacking visual signs of Pb–Zn mineralization are associated with pelitic gneisses at the Cannington and Maramungee ‘Broken Hill-type’ Pb–Zn–Ag deposits in the eastern Mount Isa Inlier. Within 150 m of the sulphide-rich lodes these have consistently anomalously high chalcophile metal contents. The enrichments broadly mimic the primary distinctions between the two deposits in respect to the relative abundances of Zn, As and Pb. At Maramungee, metal concentrations of thin amphibolites broadly increase towards the mineralized rock body and comparison with data from gneisses illustrates the general superiority of basic rocks as a sample medium due to their inherently low Pb-content and comparatively homogenous primary chemistry. Most amphibolite at Cannington occurs in a single metamorphosed sill that is consistently anomalous, though with irregular chalcophile element distributions. Petrographic and SEM studies show that the metal enrichments are related to sulphides in complex retrograde parageneses which have selectively replaced metamorphic (calcic) plagioclase. Most galena occurs as sub-10 µm grains. Chalcophile elements were dispersed in a hydrous fluid and selectively enriched in the amphibolites during retrograde metamorphism under greenschist to sub-greenschist facies conditions. Routine analysis of basic rocks for Pb (and Zn) might therefore provide a simple proximity indicator for Broken Hill-type deposits, especially in poorly exposed terrains where amphibolites are the most common cause of magnetic anomalies tested by exploration drilling.