The gold mineralization of the Hutti Mine is hosted by nine parallel, N
- S trending, steeply dipping, 2 - 10 m wide shear zones, that transect
Archaean amphibolites. The shear zones were formed after peak
metamorphism during retrograde ductile D, shearing in the lower
amphibolite facies. They were reactivated in the lower to mid
greenschist facies by brittle-ductile D-3 shearing and intense quartz
veining. The development of a S-2-S-3 crenulation cleavage facilitates
the discrimination between the two deformation events and
contemporaneous alteration and gold mineralization. Ductile D, shearing
is associated with a pervasively developed distal chlorite - sed cite
alteration assemblage in the outer parts of the shear zones and the
proximal biotite-plagioclase alteration in the center of the shear
zones. D3 is characterized by development of the inner
chlorite-K-feldspar alteration, which forms a centimeter-scale
alteration halo surrounding the laminated quartz veins and replaces
earlier biotite along S-3. The average size of the laminated vein
systems is 30-50 m along strike as well as down-dip and 2-6 m in width.
Mass balance calculations suggest strong metasomatic changes for the
proximal biotite-plagioclase alteration yielding mass and volume
increase of ca. 16% and 12%, respectively. The calculated mass and
volume changes of the distal chlorite-sericite alteration (ca. 11%, ca.
8%) are lower. The decrease in 6180 values of the whole rock from
around 7.5 parts per thousand for the host rocks to 6-7 parts per
thousand for the distal chlorite-sericite and the proximal
biotite-plagioclase alteration and around 5 parts per thousand for the
inner chlorite-K-feldspar alteration suggests hydrothermal alteration
during two-stage deformation and fluid flow.
The ductile D-2 deformation in the lower amphibolite facies has provided
grain scale porosities by microfracturing. The pervasive, steady-state
fluid flow resulted in a disseminated style of gold-sulfide
mineralization and a penetrative alteration of the host rocks.
Alternating ductile and brittle D3 deformation during lower to mid
greenschist facies conditions followed the fault-valve process. Ductile
creep in the shear zones resulted in a low permeability environment
leading to fluid pressure build-up. Strongly episodic fluid advection
and mass transfer was controlled by repeated seismic fracturing during
the formation of laminated quartz(-gold) veins. The limitation of quartz
veins to the extent of earlier shear zones indicate the importance of
preexisting anisotropies for fault-valve action and economic gold
mineralization. (C) 2003 Elsevier B.V. All rights reserved.