Page 171 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

8. Massive sulfide clasts in polymict breccia
Dark grey, massive sulfide (S) and white barite clasts
occur together with flow-banded rhyolite, siltstone,
granite and altered volcanic lithologies. The breccia
is very poorly sorted and matrix supported, and
interpreted to be a submarine debris-flow deposit
(Binney, 1987). More than 50% of the massive
sulfide ore produced at Buchans conies from clasts
within mass-flow deposits.

Hangingwall volcanic breccia, Buchans VHMS
deposit, Ordovician; Newfoundland, Canada.
Plate 38 — Traction current structures in volcanogenic sediments and pyroclastic surge deposits
1. Reworked accretionary lapilli in volcanogenic
sediments

Whole and broken accretionary lapilli (A) occur in planar
and cross-laminated (arrow), pumiceous volcanogenic
fluviolacustrine sediments generated by reworking of non-
welded rhyolitic pyroclastic deposits. Rounding of pumice
and abrasion of the outer fine grained rims of some
accretionary lapilli, together with the bedforms, are features
that indicate fractional transport and reworking.

Modern fluviolacustrine volcanogenic sediments,
Taupo Volcanic Centre, Tongariro River Bridge,
New Zealand.
2. Cross stratification in fluvial volcanogenic
sediments
These volcanogenic sediments were generated by fluvial
erosion and reworking of primary, mostly pyroclastic
deposits. Granule and pebble components are mainly well
rounded, rhyolitic pumice and volcanic lithic clasts. Finer
components (sand and silt) consist of quartz and feldspar
crystals, pumice and glass shards. The sediments are poorly
size sorted: sand- and granule-size lithic and crystal
particles occur together with pebble-size pumice. Hydraulic
sorting during transport and deposition strongly reflects
particle density. Poor size sorting but good density sorting
is a common feature of pyroclast-rich sediments which
consist of particles that range widely in density.
Hinuera Formation, Late Pleistocene; Daltons Sand
Pit, Hauraki Rift, New Zealand.

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