Page 170 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. The massive sulfide clasts
The lithic breccia featured in 37.1 and 37.3 contains at least
11 clasts of massive sulfide (C). They are purple-grey,
angular to subrounded, and dominated by sphalerite and
galena with only minor chalcopyrite and pyrite. Results of
lead isotope analyses of galena from one of the clasts
illustrated, and four others, fall within the field defined by
Cambrian volcanic-hosted massive sulfide deposits in the
Mount Read Volcanics (Gibson, 1991). The massive
sulfide clasts may have come from the same source as the
dacite lava clasts that dominate the lithic breccia (Fig. 53),
or else from a separate source traversed en route by the
mass flow.
Mount Read Volcanics, Cambrian; Newton Dam
Spillway, western Tasmania.

5. Massive sulfide clasts in dacite breccia
The dacite lava breccia shown here occurs at the
margin of a submarine dacite lava dome and contains
dark grey massive sulfide clasts (arrows). The host
breccia is altered (fuchsite) and weathered, but
probably originated by resedimentation of dome-
derived hyaloclastite. The source of the sulfide clasts
may have been a sulfide deposit on the flanks of the
dome.

Mount Read Volcanics, Cambrian; Hellyer mine
haulage road, western Tasmania.

6. Sulfide clasts in polymict, volcanic breccia
Polymict, volcanic lithic breccia in this outcrop forms
the base of a very thick (> 10m) volcaniclastic
megaturbidite. Most of the clasts are subrounded
quartz- or feldspar-phyric lava and the matrix is
crystal-rich sandstone with minor relict pumice
shreds. The clast of massive sulfide that occurs
beside the hammer head was probably collected en
route from a sulfide deposit traversed by the parent
submarine mass flow.

White Spur Formation, Mount Read Volcanics,
Cambrian; Howards Road, western Tasmania.

7. Massive sulfide clasts in breccia in drill core

This drill core section of volcanic lithic breccia is
texturally similar to that shown in outcrop in 37.6,
and also contains massive pyrite (P) and massive
sphalerite (S) clasts. The breccia forms the lithic-
rich base of a mass-flow unit and consists of pale
quartz- and feldspar-phyric lava clasts and
pumiceous, crystal-rich sandstone matrix.

White Spur Formation, Mount Read Volcanics,
Cambrian; near Howards Road, DDH MR1 (41 m),
western Tasmania.

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