Archean Au-Ag-(W) quartz vein/disseminated ... - Geology Ontario

(iii) modern earthquake ruptures along large through-going faults (e.g. San Andreas Fault)
nucleate at the base of the seismogenic zone (~ 10-15 km depth) and propagate up-dip. Some
ruptures may also penetrate downwards. Discharges of meteoric water have been observed
from fault zones following earthquakes;
(iv) pre-failure, the relative impermeability of the seismogenic zone allows metamorphic
fluid pressure to build up with the incremental opening of flat veins;
(v) seismogenic fault failure creates fracture permeability within the rupture zone allowing
sudden draining of the geo-pressurised fluid reservoir at depth; the fault thus acts as a valve;
(vi) hydrothermal deposition and self sealing leads to a re-accumulation of fluid pressure
and repetition of the cycle.
The model thus predicts large cyclic fluctuations in fluid pressure (of several hundred bars)
between lithostatic and hydrostatic, and the common occurrence of flat veins particularly
near the bottoms of Au systems.
Several lines of evidence argue against the applicability of the fault valve model (e.g. Sibson
et al., 1988, Sibson 1990) to the genesis of the Kerr Addison - Chesterville Archean Au
system, however. These are as follows:
(i) There was no major, through-going controlling "fault". The controlling structures in the
green carbonate ore are the relatively small "cherty siliceous breaks" which branch laterally
to form a framework and die out, the main stage "siliceous break'VAu-quartz vein system,
and the ductile Break deformation characterised principally by ~5:5: l flattening, not relative
displacement.
(ii) By analogy with the blind Chesterville East Zone, and based on the longitudinal contour
diagrams (Figures 51 to 55) which show that the Au system starts to close off near surface,
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