Facies architecture of mineralized submarine volcanic sequences; Cambrian Mount Read Volcanics, western Tasmania

Abstract

The Middle to Late Cambrian Mount Read Volcanics, western Tasmania, comprise compositionally and texturally diverse lavas and volcaniclastic rocks, most of which were emplaced in submarine environments below wave base. The facies architecture reflects the contrasting character and geometry of primary volcanic and volcaniclastic facies which are strongly controlled by eruption style and eraplacement processes. In the northern Mount Read Volcanics the principal volcanic facies are (1) silicic, intermediate, and mafic lavas; (2) juvenile volcaniclastic deposits, generated in association with the extrusion of lava flows, or else produced by explosive eruptions; and (3) synvolcanic intrusions involving silicic and mafic magmas emplaced into unconsolidated host sequences. Distribution and lithofacies characteristics suggest that deposits from both intrabasinal submarine and extrabasinal subaerial and/or shallow-marine volcanic centers are present. The volcanic facies are interbedded with a sedimentary facies association comprising black mudstone and/or graded bedded sandstone of mixed volcanic and Precambrian basement provenance. The predicted facies geometry of the principal volcanic facies has guided our approach to correlation in the Mount Read Volcanics. Mass-fiow-emplaced pumiceous volcaniclastic facies provide the best framework for correlation, because these are produced in large volumes, erupted infrequently, eraplaced rapidly, and are widely distributed. Distinctive units of this type that occur near Hellyer (in the Southwell Subgroup) and 40 km to the south at White Spur-Howards Road (White Spur Formation) could each be part of the same regionally extensive volcaniclastic facies association. A similar volcaniclastic facies association hosts the Hercules and Rosebery massive sulfide deposits and clearly demonstrates that such associations are prospective for this style of mineralization.