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

Available evidence indicates that the #21 Fe-tholeiite flows may have acted as a more
competent structural unit within the softer ultramafic and transitional host rocks. Wide
quartz veins and intense silification occur at the north and south margins of the #21 orebody,
connected to quartz veining in the interior. The controls on flow ore distribution are thus:
(i) proximity to fluid supply within the green carbonate alteration envelope in the adjacent
ultramafic rocks; (ii) primary lithological, due to the geometry of the host Fe-tholeiite unit
(possibly that of an original pillowed volcanic pile) modified by a structural overprint
(boudinage, folding, flexures); (iii) structural: competency contrast during ductile defor
mation which resulted in increased dilatancy, veining and fluid flow particularly at the
margins of the #21 orebody.
Flow Ore at Depth: Isoclinal Folding and "Root Zone" Pinch Out
With increasing depth below the 3250 ft level, the #21 flow lithology recedes and shortens
from both ends, and develops a prominent, curved "hook" at its west end. Between the 3700
ft and 4000 ft levels, the #21 flow lithology is isoclinally folded and thickened, and a separate
#21 "S" orebody appears to the south. Below this, the favourable flow lithology tapers down
into two zones which pinch out completely just above the 4600 ft level. Thus the "mining
root" (Kerr Addison flow oic; ~4600 ft) does not correspond to the "geological root" of the
system; the source of the hydrothermal ore fluids still lies at depth to the east within the green
carbonate alteration envelope. Flow ore grade is highest (~20 g/t Au) at the ~3850 ft level,
due to intense wallrock sulphidation and initial precipitation of Au from hydrothermal
solutions where the fluids first hit and interacted with the Fe-rich flow lithology. That this
point of highest grade flow ore is some ~700 ft above its pinch out at the 4600 ft level may
indicate that the fluids did not come up the Fe-tholeiite root exactly, but instead entered the
flow ore lithologies laterally from the east at some point above the Fe-tholeiite root. This
suggestion is in agreement with the observational evidence cited above.