False pyroclastic textures in altered silicic lavas, with implications for volcanic-associated mineralization

Abstract

There is strong emphasis in recent literature that Zn-Cu-Pb massive sulfide deposits associated with felsic volcanism (the kuroko-type deposits) are directly related to submarine calderas formed by large submarine pyroclastic eruptions. Altered silicic volcanic rocks with the appearance of welded and nonwelded pyroclastic flow deposits, volcaniclastic debris flows, and massive- to thin-bedded tuffaceous rocks form a major component of a Silurian succession that hosts Zn-Cu-Pb massive sulfide deposits at Benambra, southeastern Australia. However, critical evaluation of rock textures indicates that these silicic volcanic rocks are mainly lavas and associated autoclastic facies with remarkably deceptive false pyroclastic and volcaniclastic textures, including apparent pumice fiamme and glass shards. Intercalated sediments indicate that most of the silicic volcanics were emplaced in subaqueous settings ranging from relatively shallow water to deep marine.The false textures were produced mainly in originally glassy lava by the combined effects of devitrification, perlitic fracture, and pervasive hydrothermal alteration. These processes are related to the emplacement and cooling history of individual lavas and to a regional, essentially synvolcanic, hydrothermal system. Tectonic foliation of the more altered and mechanically weakened rocks has led to partial dismembering of phenocrysts and groundmass fabrics, thereby further enhancing clastic appearance.Subsequent to recognition of widespread false pyroclastic textures at Benambra, similar features have been identified in three other Australian massive sulfide districts. These results suggest that a reinvestigation of host rocks is warranted at other deposits where submarine pyroclastics are documented. Noncaldera settings combining subsidence in an extensional volcano-tectonic basin, passive submarine lava effusion within the basin, and ambient sedimentation derived from basin margins (including resedimented pyroclastic debris) may be more relevant models in a number of massive sulfide districts.