Ag. Galley, CHARACTERISTICS OF SEMI-CONFORMABLE ALTERATION ZONES ASSOCIATED WITH VOLCANOGENIC MASSIVE SULFIDE DISTRICTS, Journal of geochemical exploration, 48(2), 1993, pp. 175-200
Semi-comformable alteration zones are present within the footwall stra
tigraphy of base metal horizons hosted within various volcanic environ
ments. In large-scale, subaqueous, continental rifts (Bergslagen and I
berian Pyrite Belt mining base-metal districts), semi-conformable alte
ration zones may be tens of kilometres in strike length and hundreds o
f metres thick. In base-metal camps hosted within smaller tectonic dep
ressions (oceanic rifts and cauldrons) the semi-conformable alteration
zones are of smaller scale, but still affect much of the volcanic str
atigraphy within the boundaries of the down-faulted terrane. The semi-
conformable alteration systems consist of vertically stacked zones tha
t superficially resemble regional metamorphic facies. Their formation
includes chloritization, spilitization, silicification and epidotizati
on. Chemical gradients present across these zones indicate that their
formation is a result of a series of simultaneous and consecutive meta
somatic reactions between seawater and the volcanic pile that take pla
ce at progressively higher temperatures with depth in the stratigraphy
. The isotherms controlling the reactions are parallel to the sub-hori
zontal upper contact of the underlying synvolcanic intrusion. The boun
daries of these reaction zones are therefore semi-conformable with the
volcanic stratigraphy. The semi-conformable alteration zones form by
seawater-rock reactions that take place along the downwelling segment
of a hydrothermal fluid convection system. Low temperature reactions (
50-degrees to 140-degrees-C) take place in the shallow subseafloor to
form a Mg-K enriched zeolite metasomatic facies. Diffuse hydrothermal
discharge associated with this early stage of alteration results in th
e accumulation of regional-scale Fe-rich chemical sediments. Further d
ownwards circulation of a chemically evolving seawater results in Na-M
g enrichment of the rocks at moderate temperatures (140-degrees to 300
-degrees-C), followed by an Na-enriched greenschist metasomatic facies
(300-degrees to 400-degrees-C). Reactions at the greenschist-amphibol
ite metasomatic boundary include intense leaching of base-metals and t
he silicification of the rocks. The permeability reduction due to sili
cification isolates the underlying amphibolite metasomatic facies rock
s to form a Ca-Fe enriched, base-metal rich reservoir zone. Periodic b
reaching of the reservoir allows metal-rich fluids to rise to the seaf
loor to form massive sulphide deposits. The recognition of the chemica
l gradients across these semi-conformable alteration zones will allow
explorationists to more easily target and locate massive sulphide hori
zons.