Page 243 - Caldera Volcanism Analysis, Modelling and Response

218 W.U. Mueller et al.

altered to carbonate. The turbiditic tuffs have cherty Bouma T b and T d divisions
with moderately (Figure 13C) to intense carbonate altered T a -divisions
(Figure 13D). Amorphous silica mounds at the seafloor interface and volcanic
sands cemented by amorphous silica are components of the modern Okinawa
Trough sites (Glasby and Notsu, 2003).
Extensive carbonate alteration is characteristic of subaqueous Archean
volcanic centres. The Archean atmosphere is inferred to have contained high
levels of CO 2 (Kasting, 1993), thus necessitating one or more significant CO 2
sinks. Silicate weathering on continents represented one important sink for CO 2 ,as
this process was further intensified by humid Archean conditions (des Marais,
1994). A second important CO 2 sink was the ocean, where dissolution
and precipitation of carbonate species occurs. Archean carbonate precipitation
has been attributed to the influence of faults and seafloor hydrothermal
activity (Nakamura and Kato, 2004). The latter applies to the present study.
The apparent increase in carbonate alteration in Archean caldera-hosted VMS
systems could be attributed to an elevated carbon flux into oceanic crust, due to
seafloor hydrothermal alteration (Nakamura and Kato, 2004). High level
synvolcanic plutons and inferred seawater temperatures of 55–851C(Knauth
and Lowe, 2003) facilitated the leaching of host rocks by percolating fluids.
Additional sources for CO 2 are magmatic, especially with VMS deposits (Glasby
and Notsu, 2003).

7.1. Results of hydrothermal carbonate species

The hydrothermal carbonate (Figure 14A, B) in the Hunter Mine and Normetal
calderas ranges in composition from calcite, dolomite, ankerite to siderite. Based on
field relationships the carbonates indicate distal to proximal settings with respect
to zones of focused, higher temperature, high Fe, hydrothermal discharge to the
massive sulphide deposit or mineralisation. Microprobe analysis of hydrothermal
carbonates in the studied felsic-dominated calderas identified several Fe- Mg- and
Ca-rich species. Samples (n-71) from the Normetal caldera show a distinct
distribution of hydrothermal carbonate with the Fe-rich varieties proximal and
Ca-rich varieties distal to the deposit or mineralisation (Figure 14A). The
organisation of carbonate species occurs in couples so that distal calcite — (Fe-)
dolomite grades into medial-proximal Fe–dolomite — ankerite, which in turn
changes into proximal siderite (sideroplesite) — Fe–ankerite. The dolomite to
ankerite trend is readily observed (Figure 14A) in a felsic-dominated sequence (this
study), whereas a pronounced siderite–magnesite distribution is favoured in
ultramafic rocks (Kidd Creek; Schandl and Wicks, 1993; Figure 14B). The studied
carbonate alteration in HMC (Figure 14B) was restricted to the massive sulphide
showing. As can be seen, of the 92 analysed carbonates Fe–ankerite and siderite
(sideroplesite) were prevalent. Even though this seems straightforward the overall
hydrothermal zonation pattern is far more complex than previously perceived as
fluids evolve and carbonate species may overprint previous lower temperature or
less focused phases (Mueller et al., 2005).