Page 183 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

appears to be a broad spatial association between known section.
massive sulfide mineralization, moderate to strong In areas of strong tectonic deformation, tube pumice
alteration, and increased polyphase alteration style and, structure, shards and phyllosilicate patches have been
hence, pseudoclastic textures. Therefore, recognition of flattened, stretched and partly transposed into the
the style of textural modification caused by polyphase cleavage and lineation, resulting in a foliated fabric that
alteration in glassy and crystalline volcanic rocks can be mimics eutaxitic texture in welded primary pyroclastic
a promising sign in exploration for massive sulfide deposits.
mineralization.
Two-phase feldspar and phyllosilicate alteration
Alteration of pumiceous deposits (45, 46)
A second common alteration style in pumiceous
Originally glassy and permeable deposits deposits in the Mount Read Volcanics, and well
illustrated by the footwall and hangingwall sequences in
Pumiceous deposits in the Mount Read Volcanics are the Rosebery-Hercules district, is widespread secondary
mainly rhyolitic to dacitic in composition and comprise feldspar alteration (K-feldspar and albite — Allen and
non-welded subaqueous mass-flow units ranging up to Cas, 1990). Secondary feldspar nucleated around
at least 150 m thick (45.1-2). Relict primary textures feldspar phenocrysts and extended outward, filling pore
indicate that most units are matrix- to clast-supported space (mainly vesicles) and replacing the glassy bubble
pumice breccias consisting of abundant non-welded walls of pumice and shards. The feldspar alteration was
pumice clasts, subordinate lithic clasts and a matrix of incomplete and left isolated, diffuse or well-defined,
finer non-welded pumice shreds and sub-millimeter less altered patches, which were subsequently altered to
shards. Consequently, the deposits were originally very more phyllosilicate-rich compositions (45.1, 45.6-7).
porous and permeable aggregates of glassy, incompetent Many phyllosilicate-rich patches were flattened during
(structurally weak) clasts (pumice and shards). These diagenetic compaction and tectonic deformation, with
primary textural characteristics greatly influenced the the result that the textures closely resemble eutaxitic
alteration style. texture in welded ignimbrites. However, in the feldspar-
rich domains relics of uncompacted, delicate tube-
The originally highly porous, permeable and glassy vesicle and round-vesicle pumice and shard textures
nature of the deposits allowed rapid, pervasive alteration (6.4, 46.3-5) indicate that the deposits were originally
at the syn-volcanic diagenetic stage. Vast volumes of non-welded and that the feldspar alteration occurred
pumice breccia in the Mount Read Volcanics have been very early, prior to tectonic foliation, and prior to or
altered without producing distinct hydrothermal veins. during the onset of diagenetic compaction (Allen, 1990;
Hydrothermal veins are restricted to areas very close to Allen and Cas, 1990).
the ore deposits, such as around the Rosebery massive
sulfide deposit, and appear to be related to second- and The distribution of secondary feldspar alteration does
third-order alteration stages that overprint earlier, more not have a clear stratigraphic control, and it is uncertain
pervasive alteration. whether feldspar was the first alteration mineral or,
instead, replaces even earlier minerals such as zeolites.
Phyllosilicate alteration Components (K, Na, Al, SiO 2) necessary to form the
enormous volume of secondary feldspar, may have been
In areas of weak to strong phyllosilicate alteration,
pumice and shards were replaced by mechanically derived from the dissolution of volcanic glass at clast
weak, phyllosilicate-rich mineral assemblages and, contacts during diagenetic compaction, and from the
consequently, were strongly flattened by diagenetic upward migration of components (K, Na, SiO 2)
compaction. The phyllosilicate-altered rocks either liberated by diagenetic or hydrothermal leaching of
comprise very phyllosilicate-rich flattened pumice glass deeper in the volcanic pile. Important textural
clasts, set in a matrix of less phyllosilicate-rich and less evidence that supports early leaching and dissolution of
compacted pumice clasts (45.2-3), or more uniformly glassy clasts in the volcanic pile is the widespread
altered and compacted pumice clasts in which separate occurrence of a bedding-parallel, spaced, stylolitic
clasts cannot be distinguished (45.4-6). The former have dissolution foliation in many pumiceous rocks in the
fiamme textures that closely resemble eutaxitic textures Mount Read Volcanics (46.1). The bedding-parallel
in welded ignimbrites. The latter can display a foliation is composed of phyllosilicate and opaque
moderately even distribution of euhedral to subhedral minerals and pre-dates regional cleavage. The foliation
phenocrysts and closely resemble coherent, massive partly overprints syn-volcanic feldspar and
altered lavas and shallow intrusions, especially where phyllosilicate alteration. However, phyllosilicate-altered
they are poor in lithic clasts. However, these non- fiamme are commonly aligned along this foliation,
welded pumice breccias can be distinguished from indicating a spatial and possibly temporal association
genuine welded pyroclastic deposits and from lavas or between formation of the fiamme and the stylolitic
shallow intrusions by their lithofacies characteristics, foliation. The bedding-parallel foliation is interpreted as
especially the normally graded bedforms, by the a diagenetic compaction fabric, or a combination of a
compacted nature of pumice clasts, even within the fine- diagenetic compaction fabric and very early bedding-
grained graded tops and interbedded suspension-settled parallel tectonic fabric (Allen, 1990; Allen and Cas,
shard-rich layers, and by preserved relicts of non- 1990).
welded pumice and shard textures in outcrop and thin-
168