Page 161 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

(60.2 m), Hellyer mine, western Tasmania.
5. Altered pumiceous sandstone

A. This grey, non-descript, massive to diffusely layered
outcrop occurs at the top of very thick (several tens of
metres), feldspar-phyric tube pumice breccia units in the
footwall to the Hercules massive sulfide deposit. The
pumice clasts are considered to be pyroclasts produced by
an explosive silicic eruption in the vicinity, and redeposited
by syn-eruptive, submarine, volcaniclastic mass flows
(Allen and Hunns, 1990). Millimetre pumice wisps are
visible with a hand lens but require a careful search.

Mount Read Volcanics, Cambrian; 4-Level Road,
Hercules mine, western Tasmania.

B. Samples from the outcrop in 33.5A are composed of
millimetre-sized tube pumice wisps (T), together with
minor feldspar crystals. The pumice wisps are variably
compacted but are non-welded and have well preserved,
undeformed vesicular microtextures. The vesicles (arrow)
are now filled with albite(?) and vesicle walls have been
replaced by sericite. Plane polarised light.

Mount Read Volcanics, Cambrian; specimen 41971, 4-
Level Road, Hercules mine, western Tasmania.
6. Polymict volcaniclastic lithic breccia
This very poorly sorted breccia consists mainly of dacite
(D) fragments with minor chert (C) pebbles and mudstone
intraclasts. The dacite clasts are feldspar-phyric, flow
banded (F), spherulitic and blocky with highly irregular
margins. The matrix comprises volcanic lithic granules in
dark grey mud and is locally pyritic (P). The breccia is part
of a below-wave-base, relatively deep submarine sequence.
The dacite clasts probably came from a local, intrabasinal
source and may have been generated by quench
fragmentation. They have been mixed with other clast types
and with the mud matrix during subsequent mass-flow
resedimentation.
Mount Read Volcanics, Cambrian; specimen HL-2,
Hellyer mine, western Tasmania

7. Mixed provenance submarine mass-flow deposits
Rounded non-volcanic clasts of polycrystalline quartz,
chert, mudstone and metapelite have been reworked in a
subaerial or shoreline environment prior to
resedimentation. The pale volcanic clasts probably came
from a submarine source.
Mount Read Volcanics, Cambrian; DDHMAC20
(275.5 m), Hellyer mine, western Tasmania.

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