Page 246 - Caldera Volcanism Analysis, Modelling and Response

Characterisation of Archean Subaqueous Calderas in Canada 221

sulphide deposit. Although the Mattabi-type is used as the classic area for semi-
conformable carbonate alteration detailed studies have yet been conducted, so that
our study focuses on the available data. The general carbonate distribution is best
exemplified by the Normetal caldera (Figure 15A, B) as samples were collected over
30 km along strike so that a representative set of proximal and distal hydrothermal
carbonates is given (Figures 14a and 15b). Results show that close to the massive
deposits Fe–(Mg) carbonate species are prominent. The siderite–(sideroplesite)–Fe
ankerite occurs adjacent to the Normetal and Normetmar deposits whereas
the calcite–dolomite pairing is in the footwall well below the major subsurface
discharge zone. Fe–ankerite, ankerite to Fe–dolomite represents the interme-
diate stage of alteration, whereby Fe–ankerite may also be part of the proximal
carbonate assemblage, suggesting a transitional overlap. As hydrothermal carbonate
assemblages are laterally pervasive identifying distinct varieties is significant so that

Figure 15 Distribution of the carbonate alteration in the Normetal caldera. (A) Cross-section
of the Normetal caldera with western, central and eastern segments and bordering normal to
reverse faults. (B) Hydrothermal alteration pattern superposed on the caldera. Note the discor-
dant root zone along the synvolcanic faults and the km-scale proximal to distal carbonate
alteration pattern (modi¢ed with permission from Lafrance, 2003).