Page 192 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

4. Diagenetically compacted, altered pumice breccia
Rhyolitic pumice clasts (pale grey) have been almost
uniformly altered to clays and subsequently strongly
compacted. In contrast, lithic clasts (dark; L) are
unaltered and uncompacted. Pumice compaction has
produced weakly defined, bedding-parallel foliation.
The samples come from a non-welded, pumice-rich,
submarine mass-flow deposit.

T 2 Tuffs, Otaki Formation, Miocene; Hokuroku
district, Japan.

5. Foliated, compacted, sericite-altered pumice
breccia
Pumice clasts in this rhyolitic pumice breccia have
been strongly altered, compacted and deformed,
forming an almost homogeneous mass of foliated
sericite. White spots are feldspar crystals (within
pumice clasts). Their apparently even distribution
imparts a porphyritic texture resembling a deformed
coherent lava or intrusion.

Mount Read Volcanics, Cambrian; Rosebery mine
footwall, western Tasmania.
6. Heterogeneously altered pumice breccia
Green and pink colour mottling here is the result of
heterogeneous alteration of rhyolitic pumice breccia,
and imparts a pseudobreccia texture overprinting the
original clastic texture. Discontinuous chlorite-
epidote alteration domains (green) resemble large
scattered clasts and the continuous feldspar-rich
domain (pink and grey) resembles a finer grained
clastic matrix of different composition. The pumice
clasts are feldspar-phyric (small white spots).

Mount Read Volcanics, Cambrian; near Howards
Road, DDH MR1 (82 m), western Tasmania.

7. Altered pumice breccia with prominent fiamme
The dark grey fiamme are chlorite-altered pumice
that has been strongly compacted parallel to bedding.
Some fiamme are single altered pumice clasts and
others comprise two or more pumice clasts. The
fiamme are separated by a pseudo-matrix (white)
comprising largely uncompacted relict pumice clasts,
indicating that the pumice breccia was originally
non-welded. Early feldspar alteration in the
pseudomatrix and sericite alteration in the fiamme
have been overprinted by silicification and chlorite
alteration respectively. The pumice breccia occurs in
the footwall alteration zone of the Hercules massive
sulfide ore body.
Mount Read Volcanics, Cambrian; Hercules mine

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