Page 125 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

1984). In any one deposit, some of the hummocks consist currents of volcanic gas and steam that are produced by
of block facies, either a small number of blocks all of explosive eruptions and transport pyroclasts largely by
one lithology or groups of blocks of different traction.
lithologies, and other hummocks consist of only mixed
facies or combinations of mixed facies and debris
avalanche blocks (Glicken, 1991). Deposit thicknesses
range from several metres to more than 100 m. Volcanic
debris avalanches that include or incorporate large
volumes of water or snow or ice may transform
laterally into debris flows, and deposits from the two
flow types are commonly closely associated.

Massive sulfide clast-bearing submarine
volcaniclastic mass-flow deposits (37)
The importance of studies of the clast populations in
mass-flow deposits is clearly demonstrated by cases
where clasts derived from massive sulfide deposits are
included. These are not uncommon in submarine
volcaniclastic mass-flow deposits in host sequences to
massive sulfide mineralization, such as the Mount Read
Volcanics (37). The clasts could be derived from a
massive sulfide deposit present at the source of the mass
flow (e.g. Fig. 53) or collected from a massive sulfide
deposit exposed somewhere along the flow path. The
transport distance depends on the size of the clasts and
the character of the host mass flow. High-particle
concentration mass flows, such as high-density turbidity
currents, cohesive debris flows and density-modified
grain flows, can probably transport small clasts (up to a
few cm across) for substantial distances (kilometers).
Transport distances of large massive sulfide clasts (10
cm or more) are likely to be much less, but cannot be
precisely established without data on the dynamic
properties of the host mass flow. Some idea of the
source direction can be obtained, in favourable
circumstances, by systematically mapping the
maximum dimensions of clasts (massive sulfide and
dense lithic fragments) and the host unit thickness.
Other clast types present in the mass-flow deposit,
especially any juvenile magmatic clasts, help constrain the
setting of the massive sulfide deposit, and may include
samples of the associated alteration styles. Fig. 53 Cartoon showing one interpretation of the
origin of massive sulfide clasts in a submarine
The implications for massive sulfide exploration are volcaniclastic mass-flow deposit in the Mount Read
ambiguous: although the clasts provide clear evidence of Volcanics, western Tasmania (Newton Dam Spillway).
the presence of a massive sulfide deposit, it may have The final frame shows the outcrop where the clasts
been destroyed or dispersed by the mass-flow forming occur and a highly speculative reconstruction of the
event or since eroded and, in most cases, neither the present-day subsurface structure, including the source
direction nor the distance to the source of the clasts can massive sulfide deposit. (J. McPhie, unpubl. data.)
be accurately determined.
Tractional sedimentary structures (38, 40)
Traction transport and volcaniclastic Sustained currents of water generate a series of
traction current deposits tractional sedimentary structures in cohesionless
particulate aggregates that reflect the mean flow velocity
Traction refers to particulate transport of cohesionless and the mean grain size (Allen, 1985; Collinson and
grains entrained by fluid currents (water, air or Thompson, 1989) (Fig. 54). Tractional sedimentary
volcanic gas). Particles roll or slide along at the base of structures are also generated by currents of air (wind)
the current (bedload transport), skip or bounce along and volcanic gas (pyroclastic surges). Experiments with
(saltation), and intermittently go into suspension. sediments and water currents have shown that increasing
Traction currents involving wind and water are flow velocities produce a sequence of bedforms,
especially important in formation of volcanogenic comprising ripples, sandwaves and dunes. Internally,
sedimentary deposits. Primary pyroclastic surges are these bedforms consist of asymmetric cross

110