Page 19 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Fig. 5 Characteristics of volcanogenic sedimentary deposits (epiclastic volcanic deposits) (Part 4).
interpretations that imply eruption styles, transport and all compositions but not a common metamorphic phase;
deposition mechanisms, and depositional environments Pyrite ─ an important hydrothermal alteration phase that
(Parts 3 and 4). Lithological terms used for primary is extensively developed in the footwall of many VHMS
pyroclastic deposits (Fig. 6; Table 3) are well deposits;
established, but their use presupposes that interpretation. Carbonate ─ frequently associated with metamorphism
Note, in particular, that "tuff" is reserved for primary of dacitic, andesitic and basaltic volcanics, and also
pyroclastic deposits. "Tuffaceous" implies the presence results from hydrothermal alteration of volcanics in
of pyroclasts and is commonly applied to reworked and close proximity to VHMS deposits;
resedimented pyroclast-rich deposits. There is, at Epidote ─ uncommon as a hydrothermal alteration
present, no adequate terminology for autoclastic phase, but typical of metamorphosed andesitic and
deposits, nor for resedimented syn-eruptive pyroclastic basaltic volcanics.
and autoclastic deposits. The terms given in Table 3 are
merely those that are frequently used, with some Other, less common alteration phases that may be
modifications added here for consistency with associated with VHMS deposits are albite, K-feldspar,
established grain size classifications of pyroclastic and hematite and a variety of clay minerals.
volcanogenic sedimentary deposits.

Ancient volcanic sequences may contain non-primary
mineral assemblages, as a result of hydrothermal
alteration or metamorphism. The distinction between
these origins is critically important in mineral
exploration. For both coherent volcanic and
volcaniclastic deposits, descriptive terminology can
include alteration mineralogy and distribution (Tables
1,2). Omission of the alteration term implies that the
deposit is essentially unaltered. Alteration minerals
frequently encountered in volcanic host sequences to
massive sulfide deposits are listed below:
Chlorite ─ a particularly common metamorphic phase in
andesitic and basaltic volcanics, but also an important
hydrothermal alteration phase in silicic (rhyolitic or
dacitic) volcanics in the footwall of volcanic-hosted
massive sulfide (VHMS) deposits;
Sericite ─ results from metamorphism of silicic
volcanics, especially volcaniclastic deposits; it is also a
major, regionally extensive hydrothermal alteration
phase in the footwall of VHMS deposits and related Fig. 6 Grain size terms used for primary pyroclastic
chemical sediments; rocks (Fisher, 1966b).
Silica ─ generally typical of hydrothermal alteration of

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