Page 235 - Caldera Volcanism Analysis, Modelling and Response

210 W.U. Mueller et al.

5.3. Normetal volcaniclastic deposits (normetal marker horizon)
The 20–70 m-thick volcaniclastic rocks, traceable for ca. 35 km, represent the
marker horizon from which Normetal caldera geometry can be deciphered. The
volcaniclastic rocks are composed of massive and normal graded lapilli tuff and tuffs
with up to 4 m-thick massive to finely laminated fine-grained tuffs and locally
1 m-thick laminated mudstone capping coarser sequences. The 2–10 m-thick fining
and thinning upward sequences are locally developed. Generally, 10–40 cm-thick
lapilli tuff and tuff beds contain 10–20% quartz crystals, whereas fine-grained tuff
and mudstone forms 2–25 cm and 0.3–5 cm-thick beds, respectively. The broken
and euhedral quartz crystals favour a pyroclastic origin (Stix, 1991), whereas the
massive and normal graded lapilli tuff and tuffs are interpreted as sediment gravity
flows that represent rapid fallout from high-concentration flows (Bouma T a or
S 3 -beds; Lowe, 1982; Chough and Sohn, 1990). The laminated tuff and mudstone
represent Bouma T de divisions, in which fines were deposited from volcanic fallout
and background sedimentation, respectively. Suspension sedimentation associated
with turbidites suggests a depth below storm wave base (W200 m depth).

5.4. Normetal caldera mine sequence phase 5
The 100–400 m-thick Mine Sequence hosts the Normetmar and Normetal
deposits. Phase 5 is a combination of massive mafic and felsic volcanic flows and
volcaniclastic deposits exposed 30 km along strike discontinuously. The volcanic
flows are similar to the flows of the other caldera phases. The polymetallic deposits
are hosted in the fragmental rocks. The andesitic to felsic lapilli tuff breccia, lapilli
tuff and tuff are best exposed at the Normetmar showing (Figure 9E, F). The
volcaniclastic units contain 2–5 m-thick fining-upward sequences, and display a
prominent patchy carbonate alteration (Figure 9E, F) and a subtle quartz-sericite-
chloritoid alteration assemblage. The basal, 1–3 m-thick lapilli tuff breccia contains
lithic, feldspar-phyric and vesicular pyroclasts (Figure 9G). The massive to GB,
2–50 cm-thick tuff and lapilli tuff beds locally contain ripples and crossbeds
(Figure 9H). Numerous lapilli tuff beds have an internal stratification with upper
portions of graded beds locally laminated. Outsized volcanic blocks and rip-up
clasts were observed in some beds (Figure 9I). The componentry and fining- and
thinning-upward sequences of the massive to graded tuff–lapilli tuff support a
volcanic and possibly explosive origin (Mueller et al., 1994). Massive to graded
tuff–lapilli tuff represents deposition from subaqueous density currents (Bouma T a
or S 3 -beds; Lowe, 1982), and laminated portions of graded beds (Bouma T b ) are
consistent with reduced fallout rates and transport velocity during the same
depositional event (Smellie and Hole, 1997). The stratification in planar beds and
ripples/crossbeds are S 1 -beds of Lowe (1982) and Bouma-T c subunits that indicate
bedload transport and flow unsteadiness. This is common in pyroclastic deposits due
to turbulence and rapidly changing particle concentrations with highly variable
particle densities. The massive and in part poorly graded lapilli tuff breccia is
considered a high concentration particulate density flow, possibly the equivalent of
R 3 -beds (c.f. Lowe, 1982).