Download Kolb, J

Kolb, J., 2008, The role of fluids in partitioning brittle deformation and ductile creep
in auriferous shear zones between 500 and 700°C. Tectonophysics, 446, 1-15.
Introduction
•Comparison of data from four gold mines associated with auriferous shear zones and/or
flexural slip folds (Renco, Zimbabwe; Navachab, Namibia; Hutti, India; Kochkar, Russia) all of
which formed in the 500-700°C range
•Attempt is to develop the concept that hydrothermal fluid flow controlled the formation
of cracks, breccias, and veins as well as the ability of the host rocks to be altered during the
course of further deformation
Background/Geologic setting(s)
•The background is broken down into three somewhat disorganized sections the first of
which focuses on the Renco mine thrusts associated with the boundary between the Limpopo
Belt and the Zimbabwe craton
-Gold deposits are characterized by a series of 1-3m wide individual qtz-kfs-plagbt-hbl mylonitic shear zones formed during 600-700°C retrogression of tonalitic gneisses, within
shear zones are local breccia zones that host the majority of hydrothermal gold
-core-mantle textures in feldspars are interpreted to have formed during
subgrain rotation and myrmekitic textures of feldspar in quartz are considered characteristic of
high temperature deformation ~ I find these assumptions a bit troubling considering both of
these features can occur in lower, middle, and shallow crustal plutonic rocks as primary features
without being induced by metamorphism or deformation
-this first section also illustrates the drastic contrast in 1) mass and volume gain,
2) amount of metasomatism, and 3) style of deformation between mylonitic portions of these
shear zones and breccia zones: breccia zones are manifested locally due to variable fluid
pressures that created a brittle regime in an otherwise ductile setting, concentration of
hydrothermal fluids in cracks/veins within breccia zones allowed for enrichment of iron and
sulfide bearing minerals, δ18O values suggest that metasomatism was played a much more
significant role in breccia zones than in the host mylonites ~ δ18O are normalized to SMOW
(standard mean ocean water) but the differences between what the author considers to be little
metasomatic change and major metasomatic alteration are only 0.1-0.5‰ and 1.5‰
respectively
•The second background section focuses on auriferous shear zones in amphibolites and
gneisses focusing on Kochkar, Russia and Hutti, India
-Gold deposits are characterized by multiple 1-10m wide chl-bt schists to
amphibolite gneisses and mylonites
-the presence of alteration minerals increase in concentration toward the center
of shear zones associated with higher deformation conditions and δ18O values that suggest a
more significant role for metasomatism toward the center of the shear zones though chl-bt
schists are much more strongly altered than amphibolite gneisses
-in these systems, gold bearing veins are not associated with the shear zones
themselves but in curvilinear en echelon extension veins that extend away from the main body of
the shear zone and into the wall rock
•The third background section is focused on auriferous flexural-slip folds in cataclastic
rocks in Navachab, Namibia that are part of a NW-verging, non-cylindrical anticline that formed
under amphibolite conditions
-in this setting, gold deposits are associated with ~40m wide by ~800m long
massive sulfide lenses within marbles and biotite schists from the steepest limb of the structure
as well as 2 distinct sets of veins, one of which is interpreted to have formed during the
formation of the original antiform, the other of which formed during compression that folded the
original structure
-generation of the massive sulfide deposit is interpreted to be related to hightemperature hydrothermal alteration that generated a gt+bt assemblage and a gt+cpx+kfs
assemblage that are restricted to brecciated layers generated by layer parallel shearing and
flexural slip
Discussion
•Suggest that there is a direct relationship between hydrothermal fluid flow and brittle
deformation in the lower crust
•In the case of Renco, Zimbabwe; a “heterogeneous permeability structure”
characterizes the shear zones such that ductile deformation in the main body of the shear zone
leads to grain refining and the ability for micro-cracks to heal quickly such that porosity is easily
destroyed; however, the author suggests that compaction during ductile deformation may create
isolated portions of the zone where fluid pressure can reach to lithostatic or higher and manifest
in isolated breccia zones
•Likewise in Navachab elevated fluid pressures were achieved in “sealed fluid
compartments” due to higher rates of flexural slip along specific marble layers
•In Hira and Kochkar, the presence or absence of iron sulfide deposits is related to the
ability of the parent unit to become permeable during ductile deformation. In each case, the
interconnected quartz+muscovite matrix of granitic units is largely not permeable; however,
amphibolites act in a more brittle manner when deformed increasing the total porosity of the
shear zone such that the entire shear zone is characterized by a higher overall reactivity between
both primary and retrograde minerals in hydrothermal fluids
•The conclusions summarize the above points toward suggesting that lower crustal shear
zones are capable of 1) isolating zones where elevated fluid pressures can generate breccias via
hydraulic fracturing, 2) composition of the host unit plays a direct role on the generation of
permeability based on fluid-mineral reactions, and 3) strain incompatibility can generate fluid
pressures that generate brittle deformation along lithologic contacts
Overall, I found much of the discussion in this paper interesting; however, I do have some
concerns about the paper overall as well as specific topics discussed by the author.
-Generally I would agree with the overall statement made by the author that
metasomatic alteration should be spatially associated with mineral deformation and
recrystalization; however, I do not think that the discussion of hydrothermal alteration
assemblages was all that complete or compelling. Some additional petrography of these rocks to
better establish the most important metasomatic break-down reactions would have been quite
helpful.
-Though the author does cite permeability measurements for metamorphic rocks
measured in a laboratory setting as well as in an exhumed shear zones no attempt is made to
quantify permeability in any of the settings described in this paper. The general observations
made by the author are intriguing, but are really not substantiated by the work presented.
-The idea of compartmentalized sections of increased fluid pressure within a shear zone
is very interesting, but there is only a very brief discussion of why such a zone of internal
brecciation should ever be generated and I am unclear as to how such a feature could be created
without drastic variation in the availability of fluids with space and time that seems largely
unconstrained.
-The idea of low magnitude earthquakes in the lower crust is presented early in the
paper as a possible challenge to the “jelly-sandwich” lithospheric strength model or simply a
reflection of localized “transient instabilities” of lower crustal shear zones. Though the author
indirectly comes down on the side of the later of these two ideas there is little discussion or
development of this aspect of the work, which I found rather frustrating.