Page 227 - Caldera Volcanism Analysis, Modelling and Response

202 W.U. Mueller et al.

5. Normetal Caldera

The Normetal volcanic complex in the NVZ (Figure 1) is a 4 km-thick
volcano-sedimentary sequence, here redefined as the Normetal caldera (NC). The
ca. 2,728 Ma NC is part of the north Normetal block (Figure 6A, B), which
includes underlying basalt and iron-formation of the Gale Group (Latulippe, 1976).
The south Normetal block contains basalt, andesite, iron formation and gabbro
(Pe ´loquin, 1994). The Patten fault separates the northern NC from Chicobi Group
sedimentary rocks of the south Normetal block. Lafrance et al. (2000) divided the
4 km-thick NC (Figure 6B) into five volcanic phases and one volcaniclastic event
(Table 2). Phase 1 represents a basaltic subaqueous shield volcano, in which the
incipient ring fractures were established. Mafic and felsic volcanic–volcaniclastic
rocks of phases 2, 3 and 4 are indicative of various caldera-forming stages. The
20–70 m-thick Normetal volcaniclastic units and shale constitutes the Normetal
marker horizon (Table 2) and represents a pause in volcanism. Phase 5 is the
youngest volcanic succession (Lafrance et al., 2000), which hosts the Normetal
mine (11 Mt grading 5.12% Zn, 2.15% Cu, 0.549 g/t Au and 45.25 g/t Ag;
Teasdale, 1993) and the satellite Normetmar deposit (160,000 t at 12.6% Zn). The
overall NC geometry was based on lithofacies organisation, structural analysis, and
the recognition of synvolcanic faults, and permitted the separation into western,
central and eastern caldera segments (Figure 6A).

5.1. Normetal caldera phases 1 and 2

Phase 1 is the 1–2 km-thick precaldera mafic shield-building stage which is
composed of basaltic andesite, andesite and minor dacite. Massive, pillowed
and pillow breccia flows represent a typical subaqueous flow arrangement (Dimroth
et al., 1978; Cousineau and Dimroth, 1982). The 4–20 m thick pillowed flows have
0.5–2 m-wide pillows that display local radial cooling joints. Pillow breccias contain
10–30 cm amoeboid-shaped pillow fragments suggesting low-viscosity. Interstrati-
fied felsic flows of dacitic composition are massive and aphanitic.
Phase 2 (Table 2) constitutes the principal constructive and complex caldera-
forming phase of the NC. The 0.8–2.2 km-thick andesite–dacite and rhyolite units,
are divided into (i) a basal andesite unit (phase 2a), (ii) a medial felsic volcaniclastic
unit (phase 2b) and (iii) an uppermost andesite–dacite and rhyolite unit (phase 2c),
which is distributed in the western (Figure 7A, B), central (Figures 7C–D and 8a–b)
and eastern caldera segments (Figure 8C, D). The andesitic phase 2a in the western
and central segments is a 30–640 m-thick lava flow unit displaying a lateral change
from massive to pillowed flows to pillow breccia passing up-section and laterally
into lapilli tuff over 2–3 km (Figure 7A, B). A subaqueous setting dominated by
effusive volcanism is interpreted (Dimroth et al., 1978).
Phase 2b, represented by 5–50 m-thick felsic volcaniclastic deposits of graded
bedded (GB) tuff–lapilli tuff, matrix-supported lapilli tuff breccia, and clast-
supported lapilli tuff breccia, is restricted to the western and central segments. The
tuff–lapilli tuff is considered a product of high- to low-concentration turbidity