Page 184 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

In areas of moderate to strong tectonic deformation, the mining district, where these textures have been most
bedding-parallel foliation is irregularly and coarsely closely studied, the boldness of the fiamme texture
crenulated. Bedding-parallel phyllosilicate fiamme are increases within 1 km of the ore bodies. This change is
also coarsely crenulated and have developed cusps due to increased colour contrast between the dark
which are strongly aligned along the regional cleavage fiamme and the light matrix domain, which, in turn, is
(46.1). Tube pumice and shards in the feldspar-rich due to enrichment of dark phyllosilicate in the fiamme
matrix domains are more strongly transposed into the and feldspar and/or quartz in the matrix domain. Locally
regional cleavage and lineation than are the bedding- (for example, at Rosebery) or over extensive areas (for
parallel foliation and fiamme. This strong alignment of example, around Hercules) within 50—1000 m of the
tube pumice structure and shards superficially resembles ore bodies, the sericite fiamme are replaced by chlorite
a eutaxitic welding fabric (45.1, 46.4). However, in and the matrix domain is silicified, resulting in further
addition to the relict non-welded texture of many enhancement of the fiamme texture (45.7). Within 50-
pumice clasts, this false welding fabric can be dis- 100 m of the ore bodies, strong silicification, carbonate
tinguished from true welding by its orientation: it is alteration, and sericite or chlorite alteration, form
parallel to cleavage and oblique to the bedding-parallel irregular zones with mottled to nodular to locally
foliation and fiamme; a genuine welding fabric would massive alteration textures (46.7). These alteration
be parallel to fiamme, and oblique to and crenulated by textures thoroughly overprint and obscure primary
the regional cleavage. textures and fiamme.

In areas of intense cleavage, the bedding-parallel General trends and implications
foliation and fiamme are almost completely transposed
into the regional cleavage and lineation (45.1). Intense With very careful observation, relicts of primary non-
foliation overprinting very competent feldspar- and welded pumice texture can be identified in most of the
quartz-rich alteration has produced augen schists (46.6). altered and deformed pumiceous deposits, including the
The augen are pumice clasts extensively replaced by augen schists and intensely altered rocks adjacent to the
feldspar and quartz. The anastomosing phyllosilicate ore bodies. In general, alteration has caused the non-
cleavage that wraps around the augen represents the welded pumiceous deposits to evolve in texture toward
transposed fiamme and bedding-parallel foliation. that of matrix-supported, welded pyroclastic deposits
and especially welded pyroclastic flow deposits. The
Two-phase feldspar and phyllosilicate alteration of implications of these alteration effects are similar to
pumiceous deposits and the resultant fiamme textures those for the altered lavas, shallow intrusions and
are widespread in the Mount Read Volcanics. They related breccias: all post-depositional processes must be
occur in zones up to at least 25 km in strike extent and 2 carefully considered when interpreting textures in
km in stratigraphic thickness. The fiamme are mainly volcanic rocks, and recognition of the textures produced
sericite-rich except close to mineralization, where they by polyphase alteration in pumiceous volcaniclastic
are commonly chlorite-rich. In the Rosebery— Hercules rocks can be a guide to mineralization.

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