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Predictive targeting outcomes:
August 2007
pmdCRCb003-07
More information:
Jamie Robinson: Structural Geologist, CSIRO, @csiro.aujamie.robinson
Warren Potma: Modelling Project Leader, CSIRO, [email protected]
CSIRO report P2007/586Authors: Jamie Robinson and Warren Potma, CSIRO Exploration and Mininng/pmd*CRC
The predictive targeting outcomes presented in this report result from numerical modeling/simulation of complex mechanical/fluid flow/chemical/thermal systems. The modeling process utilizes both empirical data and geological interpretations as a basis for
model construction and some intrinsic assumptions are required by the process. Every effort has been made to simulate these processes as accurately as possible based on the available geological interpretation and data, however, it must be noted that changes to
numerical inputs following further data acquisition or variations in geological interpretation may result in different modeling outcomes.
Kink bands containing the
mineralised tension veins may dip
to the southeast, or east-southeast
and control with the plunge
direction of the ore shoots within
area 223.
https://pmd-twiki.arrc.csiro.au/twiki/bin/view/PIRSA2MinotaurAu/WebHome
The most likely location of tension
veins associated with high gold
grades is within dilational sites
related to the kinking of the
pre-existing foliation. Greater
dilation will be produced with
greater angles between the
northeast and southwest-dipping
sections of the fabric. However,
significant dilation (in the form of
steep-dipping tension veins) is also
found where the fabric is near
vertical or steeply-dipping
suggesting that while the greater
angle is optimal, dilation is
produced when the change in
fabric dip is as little as 10 to 20
degrees.
Proximity to the major faults will
always be advantageous as the
faults act as major fluid pathways
through the area. Dilational sites,
such as within the kink zones,
draw fluid from the faults but it is
the downstream terminations that
trap fluids. Results suggest that
dilational sites in proximity to the
footwall will be the best focus for
mineralising fluid, consistent with
the know distribution of gold in
area 223.
Aim of the fluid-flow modelling:
To examine how subtle variations in the dip and strike of the pre-existing shear fabric may influence the localisation of strain and dilation.
To identify what fabric orientations are best for producing dilation with stress fields known to be associated with mineralisation and, therefore, determine the best targets for high-grade mineralisation.
Model design and setup:
The basic geometry consists of a steep west-dipping unit 100m in width representing the Central Alteration Zone (CAZ). Thin planar zones representing faults have been placed on the eastern & western margins of the CAZ around which is a matrix representing Tunkillia Augen Gneiss (TAG).
The CAZ is divided into a number of individual regions in the Z (vertical) and Y (north-south) direction so as to allow different oriented planes of weakness to be applied to the different regions of the CAZ using the ubiquitous joint constitutive model (Figure 1).
The ubiquitous joint model allows an anisotropy, or fabric/plane of weakness with its own strength properties and orientation, to be applied throughout particular zones.
Changing the thickness of the kink bands influences the amount of dilation and shear strain. Having thicker kink bands produces lower but more widespread dilation and lower shear strain in the regions of southwest-dipping fabric. When the kinks are thin, localised regions of high dilation are produced while higher, more evenly distributed shear strain is produced in the regions of southwest-dipping fabric.
Plots of integrated fluid flux, for the best of the model results, show that most fluid over the life of the model passes up the footwall fault AND flows into the dilational sites within the kinks adjacent to the faults. The results of this plot are consistent with the distribution of mineralisation within area 223 (Figure 4).
Tunkillia Deposit Scale Modelling: Predictive Targeting Outcomes
Mineralised tension veins are likely
to be upright to steeply dipping
and roughly north-south striking.
The strength of the fabric was calibrated to be slightly weaker than the host rock. The orientation of the fabric is the main variable of these models.
Dip of the dominant shear fabric tested: o o
- 70, 80 or 90 northwest/ 70, 80 southwest.
Strike of the dominant shear fabric tested: - 320, 330, 340 or 350 degrees.
N
Fault
Central Alteration Zone
Ubiquitous joint region A
Ubiquitous joint region B (Kink)
100 m
Mafic Dyke (MAD)
Dacite Dyke (DAD)
Central Alteration Zone (CAZ)
Drill hole
Au intercept 1 to 2 g/t
Au intercept 2 to 5 g/t
Au intercept 5 g/t +
Kink Band ?
BOPO
BOCO
Concentration of higher Au grades towards footwall
Increased fluid flux towards footwall
Figure 4.
predictive mineral discovery CRC - Update #2
PIRSA 2 - Minotaur Numerical Modelling Project
Figure 1.
Results:
The results indicate that variations in both strike and dip of the fabric within the Central Alteration Zone can potentially produce localised dilation and high shear strain (Figure 2).
Figure 2.
oA change in strike or dip of the fabric of 10 or less is sufficient to affect shear strain and dilation. Variations in the orientation of the planar fabric within the Central Alteration Zone are important!
o oIn the transpression stress field, with 140 compression direction, a fabric strike of 130 produced the best localisation
oof dilation, while 140 strikes produce the highest shear strain within the CAZ.
o oIn the transpression stress field, with 160 compression direction, a fabric strike of 140 produced the best localisation of dilation and high shear strain within the CAZ.
o oDips of 80 southwest produced an even distribution of shear strain while dips of 70 produced more localised zones of high shear strain.
The greater the angle between the southwest-dipping fabric and the northeast-dipping fabric the greater the potential for dilation in the kink (Figure 3). However, minor changes in the dip and strike of the fabric also produces some dilation.
Figure 3.
N
High shear strain
High shear strain
High shear strain
High shear & volume strain
High shear strain
Highvolumestrain
1
23
1
23
Schistosity Shear veinsTension veins
Kink and vein plungeKink and vein plunge
oStrike 130
Dip A = 80SW
Dip B = 70NE(Kink)
Dip A = 80SW
Dip B = 80NE(Kink)
Dip A = 80SW
N
(a)
(b)
(c)
Dip B = 90NE(Kink)
predictive mineral discovery CRC - Update #2
PIRSA 2 - Minotaur Numerical Modelling Project
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