Page 25 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

intrabasinal hyaloclastite is resedimented. Both syn- eruptions and redeposited into deeper water settings;
eruptive and post-eruptive volcaniclastic mass-flow Syn-volcanic intrusions ─ largely conformable,
deposits can occur, and are usually associated with emplaced into and locally mixed with wet,
volcaniclastic mudstone and siltstone formed by settling unconsolidated host sediments, forming peperite and sill
from suspension. Studies of ancient sequences suggest complexes.
that, in general, welded pumiceous pyroclastic deposits
are uncommon in below-wave-base environments and
restricted to special circumstances that allow heat
retention and primary transport in a deep subaqueous
setting.

Non-volcanic facies are typically interbedded with the
volcanic facies, and are especially important in
constraining the depositional environment in cases
where the volcanic facies are dominated by very thick
massive lavas, intrusions and/or volcaniclastic deposits.
Non-volcanic sedimentary facies mainly comprise
turbidites and hemi-pelagic mudstone, together with
minor biogenic, biochemical and chemical sedimentary
deposits. Fossils in intercalated sedimentary facies may
also provide independent constraints on the water depth
of the depositional setting. In general, the non-volcanic
sedimentary facies lack abundant or large traction
structures, such as cross bedding, scours, or channels.

Introduction to the Mount Read Volcanics

The Mount Read Volcanics, western Tasmania, consist
of compositionally and texturally diverse, Middle to
Late Cambrian lavas and volcaniclastic rocks (Corbett,
1992). The volcanics have been affected by regional
deformation and metamorphism, and locally
hydrothermal alteration is intense. These volcanics are Fig. 10 Distribution of the principal lithostratigraphic
famous worldwide for the abundance and richness of formations and major massive sulfide deposits in the
deposits of massive sulfides that they contain (e.g. Cambrian Mount Read Volcanics of western Tasmania.
Mount Lyell, Hercules, Rosebery, Que River, Hellyer ─ Modified from Corbett (1992).
Solomon, 1989; Large, 1992) ( Fig. 10). They present a
considerable but typical challenge for mapping, textural These volcanic facies are interbedded with sedimentary
interpretation and volcanic facies analysis. The Mount facies comprising laminated or massive, black mudstone
Read Volcanics comprise the following and graded bedded sandstone turbidites of mixed
lithostratigraphic units: the Central Volcanic Complex, volcanic and metasedimentary Precambrian basement
the Western volcano-sedimentary sequences, the provenance. Middle Cambrian trilobites and other
Eastern quartz-porphyritic sequence and the Tyndall marine fossils are sparsely distributed in the
Group (Corbett, 1992) (Fig. 10). Lavas and syn-volcanic sedimentary facies (Corbett, 1992).
intrusions of the Mount Read Volcanics are
predominantly rhyolites and dacites, with locally Given this selection of facies with which to work, an
abundant andesites and basalts, that conform to calc- attempt has been made to reconstruct the Cambrian
alkaline trends on geochemical variation diagrams facies architecture of the Mount Read Volcanics (Fig.
(Crawford et al., 1992). 11). The best correlation framework for reconstructing
facies architecture is provided by volcanic facies that
The principal volcanic facies in the Mount Read are erupted in large volumes, deposited rapidly, and
Volcanics are (McPhie and Allen, 1992): widespread. Mass-flow-emplaced pumiceous volcani-
clastic facies generated by large-magnitude explosive
Silicic, intermediate, and mafic lavas ─ lavas are eruptions are the obvious first choice. Some examples in
common in the Central Volcanic Complex and occur at the Mount Read Volcanics have been traced for over 12
many localities in the Western volcano-sedimentary km along strike and reveal the presence of syn-volcanic
sequences; faults that created important sediment traps and
Syn-eruptive volcaniclastic deposits ─ two main types environments favorable to sulfide mineralization.
of subaqueous, juvenile clast-rich, volcaniclastic mass-
flow deposits occur; one is dominated by poorly- or The volcanic host sequences to two massive sulfide
non-vesicular, blocky lava clasts and related to the deposits in the Mount Read Volcanics, Hellyer and
subaqueous emplacement of lava flows and lava domes; Rosebery-Hercules, are featured in many of the plates
the other contains abundant silicic pumice clasts (Figs 12, 13). The Hellyer deposit is a typical mound-
produced by subaerial or shallow subaqueous explosive style (Kuroko type), polymetallic, sea-floor massive

13