Page 180 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Part 5. Alteration: An integral part of textural evolution

After eruption, volcanic deposits are inevitably subject and syn-mineralization alteration that is overprinted by
to a sequence of processes: devitrification, hydration, the main regional cleavage and shear zones; (2) lower
diagenetic and hydrothermal alteration, diagenetic greenschist grade regional metamorphism; (3) localized
compaction, metamorphism and tectonic deformation. syn-tectonic alteration associated with strong cleavage
Each process is influenced by the existing deposit development and shear zones; and (4) localized to
texture but also overprints and modifies this texture. widespread post-cleavage alteration zones around
Consequently, as these post-eruptive processes take Devonian granites. In some areas, such as the
place, the texture evolves along a complex (but Rosebery—Hercules mining area, the volcanics have
predictable) path. We stress the concept that textures in been affected by all the main alteration events listed
volcanic deposits evolve and should not be considered above.
immutable features once created during eruption,
fragmentation and/or flowage, and final emplacement. Syn-volcanic alteration in the Mount Read Volcanics
comprises three main styles: (1) regional diagenetic
Alteration is defined here as a change in the mineralogy alteration with various assemblages of white mica,
and texture of a deposit, facilitated by the action of hot chlorite, plagioclase, quartz, epidote and K-feldspar; (2)
or cold aqueous solutions or gases. Alteration is an localized, zoned hydrothermal alteration, directly
integral part of textural evolution and can accompany all associated with massive sulfide mineralization and
the post-eruptive processes listed above. Mineralogical composed mainly of assemblages of quartz, chlorite,
and textural changes that accompany devitrification and white mica and carbonate; and (3) K-feldspar and
hydration of volcanic glass are illustrated in Parts 2-4. chlorite-rich hydrothermal alteration spatially associated
In this part, we focus on the style and textural effects of with Cambrian granites that intrude the volcanic pile.
diagenetic and hydrothermal alteration, and overprinting The mineral assemblages associated with each style are
diagenetic compaction and tectonic deformation, using the lower greenschist grade metamorphosed and foliated
examples from the Mount Read Volcanics. Emphasis is equivalents of syn-volcanic assemblages which, by
placed on features relevant to the interpretation of analogy with young altered volcanic terranes, probably
textures in volcanic rocks. We do not aim to present a originally comprised combinations of clays, zeolites,
comprehensive account of alteration in volcanic terranes micas, feldspars, quartz and carbonates.
or alteration associated with massive sulfide
mineralization. The textural effects of diagenetic and hydrothermal
alteration in the Mount Read Volcanics are illustrated
In many volcanic terranes, diagenetic and hydrothermal using lavas and shallow intrusions as examples of
alteration are intertwined and inseparable processes. competent, relatively poorly porous rocks, and
They both involve dissolution, replacement and pumiceous units as examples of incompetent, very
precipitation of minerals along fluid pathways. porous deposits. These are also the most abundant
Consequently, the distribution of alteration and the volcanic rock types in the Mount Read Volcanics and in
textures produced are strongly related to the initial many other host sequences to massive sulfide deposits.
patterns of permeability and compositional contrast
formed in volcanic deposits by eruption, fragmentation, Alteration of lavas, shallow intrusions and
devitrification and hydration processes. Some of the related autoclastic breccias (42-44)
results of diagenetic and hydrothermal alteration, such
as the formation of mechanically weak phyllosilicate Original texture
mineral assemblages and mechanically strong quartz-
feldspar mineral assemblages, in turn influence the Lavas and shallow intrusions are characterized by a
effects of subsequent diagenetic compaction, tectonic porphyritic texture of euhedral or partly resorbed
deformation and metamorphism. Therefore, the effects phenocrysts set in a fine-grained or glassy groundmass.
of diagenetic and hydrothermal alteration on textures Phenocryst size and distribution are almost uniform
cannot be understood without unraveling the textural within each emplacement unit. However, relict primary
effects of all these subsequent processes and groundmass textures vary greatly, and indicate that,
considering all the likely earlier processes that prior to diagenetic and hydrothermal alteration, the
generated the pre-alteration texture. margins of silicic units had glassy groundmasses
(commonly perlitic in rhyolitic to andesitic units). These
Alteration events in the Mount Read margins graded inwards to cores of: (1) similar glassy
texture; (2) glass with scattered spherulites; or (3)
Volcanics mainly crystalline texture (intensely spherulitic,

The main alteration events that have affected the Mount microlitic or granophyric). Most mafic units had thin,
Read Volcanics comprise: (1) Cambrian, syn-volcanic sparsely microlitic, glassy margins surrounding
intensely microlitic cores. The originally glassy margins
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