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Strain and displacement in the Middle Vale Reef at Telfer, Western Australia
Ore Geology Reviews, 8 (1993) 189-202 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
189
Strain and displacement in the Middle Vale Reef at Telfer, Western Australia Julian R. Vearncombe and Anthea P. Hill* Key Centrefor Strategic Mineral Deposits, The University of WesternAustralia, Nedlands, WA 6009, Australia (Received October 20, 1991; revised version accepted October 21, 1992)
ABSTRACT Vearncombe, J.R. and Hill, A.P., 1993. Strain and displacement in the Middle Vale Reef at Telfer, Western Australia. In: D.I. Groves and J.M. Bennett (Editors), Structural Setting and Controls on Mineral Deposits. Ore Geol. Rev., 8: 189202. Mineralisation in the Middle Vale Reef at Telfer is coincident with the most deformed rocks in a strata-parallel shear zone in an argillaceous and calcareous horizon. Cleavage to bedding angle and three-dimensional strain calculated by Rf~ analysis were determined in a profile across the Middle Vale Reef. They show significant volume loss (about 40%), but shear strain was a minor component of the deformation (7 < 1.5 ). From a balanced section, bedding-plane slip along the Middle Vale Reef appears to have been less than 10 m and the deformation was dominated by material removal resulting in the loss of volume in the host siltstones. The same zone, which was a conduit for fluid during volume loss, reactivated with the addition of quartz-sulphide veins, resulting in a net volume gain. Reactivation of surfaces with a high permeability, such as the Middle Vale Reef, is confirmed as a major control on shear zone-hosted mineralisation.
Introduction
Strata-parallel mineralisation is controversial with both syngenetic and epigenetic origins argued for the deposits. Critical to the models for an epigenetic origin are the localisation of deformation and channelised fluid flow in specific horizons within the stratigraphy. Bedding-plane slip and dilation related to buckling of the stratigraphy are frequently invoked as the controls on epigenetic strata-parallel mineralisation, but the structural elements of the zone are only rarely documented. It is the purpose of this paper to examine the strain and displacement in a mineralised horizon at Tel*Present address: CODES, Department of Geology, University of Tasmania, Hobart, Tasmania, Australia. Correspondence to: J.R. Vearncombe, Key Centre for Strategic Mineral Deposits, The University of Western Australia, Nedlands 6009, Australia.
fer to determine the structural controls on strata-parallel mineralisation there. Telfer lies in the northern part of the Paterson Province in the Upper Proterozoic Yeneena Group which unconformably overlies the Lower to Middle Proterozoic Rudall Metamorphic Complex (Williams and Myers, 1990). The major gold deposits occur within the Telfer Dome, one of several NW-trending, en echelon, asymmetric anticlines (Fig. 1 ). High-grade gold-copper mineralisation is associated with discordant veins and continuous stratiform to strata-bound reefs within compositionally favourable argillaceous and carbonaceous horizons, including the Middle Vale Reef (MVR). Recently, epigenetic models have been presented by GoeUnicht et al. (1989), and Dimo (1990) which emphasise structural controls including bedding-plane slip, and relate the mineralisation to late defor-
0169-1368/93/$6.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
190
J.R. VEARNCOMBEAND A.P.HILL
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~oot~a,I s~ndsto~e Malu Ouarlz,~e
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Fig. 1 (a). The regional geology of the Telfer area in the Paterson Province. Fig. 1 (b). Geology and economic gold mineralisation of the Telfer dome, sample locality indicated by the arrow and cross-section (Fig. 2) shown by the line x - x " . Insert: the Telfer Formation (mine sequence) stratigraphy (adapted from an unpublished Newmont Australia Ltd. report). (Maps of Telfer area from Goellnicht et al., 1989; and McNaughton and Goellnicht, 1990).
STRAIN AND DISPLACEMENT IN THE MIDDLE VALE REEF AT TELFER, WESTERN AUSTRALIA
mation and granitoid emplacement. The MVR is a shear zone, in the sense of Ramsay and Huber ( 1987, p. 595 ), in being a zone of high deformation which is long relative to its width and surrounded by rocks showing a lower state of finite strain. The rocks of the MVR display strain markers which enable detailed study of the tectonic strain, from which implications for the progressive deformation, displacement and volume changes can be drawn for this bedding-parallel shear zone. Strain markers are rare in mineralised shear zones and the displacement on the shear zone can not usually be calculated. Thus the MVR provides an excellent opportunity to quantify deformation and test some of the common assumptions regarding the geometry of mineralised bedding-parallel deformation zones.
The Telfer Gold deposits Gold mineralisation at Telfer was discovered in 1971, and the mine began production in 1977. Telfer has produced more than 2.6 million ounces of gold, with proved and probable reserves of about 32.6 million tonnes at 2.1g/t (2.2 million ounces Au) (Newmont Australia Ltd., 1991 ) making it one of Australia's largest gold producers. Mineralisation at Telfer occurs within subgreenschist facies sandstones and siltstones of the Telfer Dome, comprising two subsidiary structures, Main Dome and West Dome which are asymmetric to the NE. Mineralisation is stratiform to stratabound in sulphide/oxide quartz veins hosted in calcareous and argillaceous horizons, and the reefs are termed the Middle Vale Reef (MVR) and the E-reefs. The MVR is a quartz-pyrite-chalcopyrite-bearing horizon near the base of the Middle Vale Siltstone Member. The MVR is up to three metres wide (0.5 m wide in the profile studied in this paper) and extends at least 10 km 2 (Goellnicht, 1987; Dimo, 1990). Stockwork and sheeted vein systems in the footwall and hanging wall of the reefs are also mineralised.
191
The Middle Vale Siltstone Member is a finegrained and thin-bedded argillaceous siltstone, with minor carbonaceous limestone and calcareous sandstone. PetrographicaUy these rocks contain highly variable amounts of kaolinite and quartz, with minor tourmaline, carbonate, haematite, sericite and hydrothermal biotite. The kaolinite has a presumed origin related to supergene (Tertiary or Recent) weathering. The rock contains numerous elliptical spots up to 5 mm in diameter, these spots were originally carbonate concretions and are replaced by quartz and in some examples by the supergene kaolinite. The MVR carries a well developed cleavage and bedding-plane deformation will be shown to be an important factor in the formation of the ore deposit. Goellnicht (1987) and Dimo (1990) suggest that dilation and flexural-slip during the doming event could explain the extensive and concordant nature of the MVR and local mineralisation of other stratigraphic horizons. The MVR is dominated by vein material which lacks evidence of superposed strain. Goellnicht et al. (1989) interpreted the veining to be late in the deformation history of the Telfer Dome. Laminated, crudely banded or massive sulphides and interstitial quartz form the upper part of the MVR; the lower part consists of fractured, milky white, very coarsegrained, vein quartz with interstitial oxides after sulphides (Goellnicht, 1987 ).
Structure of Main Dome Main Dome and West Dome are similar, both are doubly-plunging anticlines and are asymmetric to the NE; they both have faulted fold closures. Concordant and discordant veins are common in both domes. Bedding dip on the NE limb of Main Dome steepens from about 30 ° at surface to about 55 ° at depths of 700 m (Turner, 1982 ). The axial plane strikes NW-SE and dips steeply to the SW. Faults in Main Dome are imbricate thrust faults, best developed in the axial region of the
192
J.R. VEARNCOMBEAND A.P. HILL
lel to the movement direction as defined by quartz fibres. Thrust faults have displacements of less than 10 m. A Triangle Zone (terminology after Butler, 1982 ) in the SE closure of Main Dome is formed by a NE-dipping backthrust in combination with a SW-dipping thrust. Movement along both thrusts that define the Triangle Zone appears to be taken up by bedding-plane slip along the MVR. Other Sor SW-dipping faults in Main Dome do not intersect the MVR, but curve into bedding planes along stratigraphically higher horizons. Bedding-plane slip is important in Main Dome, as indicated by quartz fibres along bedding planes and locally developed cleavages oblique to bedding, and is supported by measurement on the balanced cross-section. Line B, perpendicular to bedding in the deformed section, does not restore as a straight line in the basal layers. To make the curved line perpendicular to bedding in the deformed section, bedding-plane slip in the basal layers must be about 5 m, SW over NE, as indicated by the
dome, and cross-cutting, subvertical faults. The subvertical faults have minor offsets and are N- to NNE-trending and cross-cut all structures. Movement on these faults is dominantly dextral strike-slip, and in section the apparent separation geometry is both normal and reverse. Two major late, subvertical faults, together named the Graben Faults, offset the MVR by up to 10 m. These structures post-date the thrust faults and mineralisation. They are absent along the section line (Fig. 2 ). The thrust faults are SSW- or SW-dipping, listric and curve toward parallelism with the bedding planes with depth; some develop folds in their hanging walls. Fault geometry combined with available mineral lineation data from throughout the Telfer Dome, suggest movement was mostly from the SW to the NE (Hill, 1990). An incomplete section, due to the limits of mining, across the SE end of Main Dome has been balanced and restored (Fig. 2; also see Appendix for comments on the method of balancing sections). It is constructed paralN.E.
S.W.
A RJ.~m) 510 R.L (m) 600~
~500
490 - -
~
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~480
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490
REFERENCE MVR
~
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I
20
30
I
J
40metwes
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Sandstone
Rovorso tho'~t, w#~ f l ~ v e ~ o n t dk'ocllon
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Fig. 2. Balanced and restored cross-sections across the SE closure of Main Dome. The Triangle Zone defined by both opposed m o t i o n a n d dip on reverse faults a n d making a triangular structure is on the left o f the section. For location o f section see the line x - x " in Fig. lb.
STRAIN AND DISPLACEMENT IN THE MIDDLE VALE REEF AT TELFER, WESTERN AUSTRALIA
arrows on the restored template. This value compares with 21 m of bedding-plane slip determined from a cross-section through the SE end of West D o m e (Hill, 1990). The absolute shortening across a balanced section can be used to calculate the depth to d~collement (Laubscher, 1962; Hossack, 1979), but to calculate the absolute shortening, the original predeformation height of a regional datum must be known. This information is unavailable and hence only a m i n i m u m value of shortening and hence a m i n i m u m depth to d~collement can be calculated. For the section in Fig. 2 these calculate as a shortening of 17 m or 9% and a m i n i m u m excess area (area on the cross-section between undeformed and deformed d a t u m horizon) of 1312 m 2 giving a m i n i m u m depth to d~collement of 76 m. A similar calculation (Hill, 1990) on a cross-section through West D o m e gave a depth to d~collement 140 m deeper than our estimate. The assumptions in generating these resuits across a very small section line are such that it is unlikely that the results are accurate enough to predict the depth to a d~collement, but they do clearly show that if there is a d~collement it is well below the MVR. The results from the balanced section suggesting only a few metres of slip on the bedding planes, and showing that any d~collement is well below the MVR, raise the important question: what was special about the mineralised horizons such as the MVR? To answer this we need to know more about the deformation in the MVR; what are the fabrics in the MVR and under what shear rkgime did they form? What was the role of volume change? (Volume loss usually accompanies low-grade metamorphism and cleavage development, and volume gain in the form o f m e t a s o m a t i c alteration and veining can accompany mineralisation; Barley and Groves, 1989.) How much slip is indicated by the cleavage oblique to bedding and does this agree with the slip determined from the balanced cross-section? These problems can be solved by knowledge of the finite strain in
193
the MVR and are addressed in the following strain study. Strain in the MVR
In the MVR a cleavage defined by the alignment of micaceous minerals parallels a shape fabric defined by the ellipsoidal kaolinite and quartz spots replacing carbonate concretions in the siltstone. The carbonate nodules probably developed during diagenesis, they were partly replaced by quartz during either continued diagenesis or low-grade metamorphism, and cleavage affects the markers and matrix equally. Thus the markers record the tectonic strain in the siltstones. Kaolinite replacement is related to more recent supergene processes, and appears to preserve the deformational shapes. The cleavage and shape fabric are at a moderate angle to bedding in the hanging and footwalls, and at an angle close to bedding in the MVR (Fig. 3). Similarly, the spots are variable but approach spherical in the wallrock, and progressively change shape into the MVR where they have strongly flattened crosssections (Fig. 3 ). Data collected from a study section through the MVR on a haul road between Pits 4 and 5 (for sample locality see Fig. l b) are shown in Table 1, and Figs. 4 and 5. The strained spots are oriented such that on average they lie in the plane of the cleavage. The angle the cleavage (and shape fabric of the spots) makes to bedding decreases from between 30 ° and 45 ° in the wallrock to about 12 ° in the MVR (Fig. 5c). This curvature is coincident with an increase in strain (Fig. 5a) into the MVR and is typical of the curvature of foliation into a shear zone. If cleavage curvature is assumed to be a product of simple shear alone, values of shear strain ~,can be calculated (Ramsay and Graham, 1970) and a displacement along the MVR of about 18 m can be calculated by integrating the shear strain across the profile (e.g., the area under the curve "shear, no volume loss" in Fig. 5d). If deformation was by simple shear the shape fabrics
194
SW
J.R. VEARNCOMBEANDA.P.HILL
/ J
O O
J
O
NE 12
10
MVR
J
J
7.5
O O
J
O
cleavage
strain
6
4 real size of strain markers about 5mm
2 metres above datum
Fig. 3. Schematic profile through the MVR in the area between open pits 3 and 4 on the SW flank of the Telfer Dome. For location see Fig. 1b. The profile shows the variation in cleavage angle to bedding as defined by the lines bounding the MVR and the variation in strain in the X Z section in strain markers in the host rocks to the MVR in a vertical section of just over 12 m.
in the MVR would be a product of plane strain and have a k value (shape of the strain ellipse ) of 1. This is tested by three-dimensional analysis of the strain. Three-dimensional strains were determined on the spots within the host rock to the MVR, and in adjacent hanging wall and footwall siltstones in the study section over a stratigraphic
distance of about 12.5 m with the MVR at the interval 7.5 to 8.0 m. The strained spots show sufficient variation in shape to warrant strain analysis by the Rf/(~ technique (Ramsay, 1967; Lisle, 1985; see appendix). The orientation of the long axis of the spots and the ratio of the long to the short axes of the strained spots were determined in both the X Z and Y Z sections of
STRAIN AND DISPLACEMENTIN THE MIDDLEVALEREEFAT TELFER, WESTERNAUSTRALIA
195
TABLE1
Straindataacrossthe M i d d l e V a l e R e e L T e l ~ r ( s t r a i n s d e t e r m i n e d b y t h e m e t h o d d e s c r i b e d b y L i s l e , Sample
Distance
X/Z
L M N O 02 P Q R S
2.6 2.9 3.2 6.9 6.9 7 7.1 7.1 7.45 7.5 7.55 7.7 8 8.3 8.5 8.5 9.7 10.4 11.5 12.5
R:XZ
R:YZ
R:XY
~ymm
X2
n
H
~ymm
~2
52 1.58 0.96 55 1.85 0.87 52 2.32 0.92 52 1.68 0.85 52 1.66 0.77 52 2.43 0.92 57 2.03 0.84 52 2.29 0.77 52 3.08 0.89 Base of Middle Vale Reef 52 3.37 0.85 51 2.34 0.90 Top of Middle Vale Reef 51 1.94 0.90 52 1.76 0.92 51 1.82 0.75 31 1.89 0.65 51 1.70 0.63 52 1.56 0.92 52 1.54 0.85
17 40 7 21 56 4 6 10 17
52 62 51 51 52 52 52 52 35
1.77 1.73 1.81 1.77 1.67 1.89 1.9 2.04 2.13
0.92 0.87 0.90 0.94 0.73 0.96 0.89 0.89 0.78
33 12 34 21 28 22 10 21 4
1.6 1.9 2.4 1.8 1.9 2.3 2 2.3 3.2
1.6 1.9 1.8 1.8 1.9 1.8 1.9 2.1 2.1
1.00 1.00 1.33 1.00 1.00 1.28 1.05 1.10 1.52
5 12
52 33
2.88 2.24
0.89 0.85
6 30
3.2 2.4
2.85 2.3
1.12 1.04
21 38 55 21 32 29 38
32 51 51 51 52 50 51
1.84 1.57 1.61 1.66 1.56 1.39 1.69
0.75 0.82 0.78 0.71 0.73 0.80 0.90
26 19 25 45 40 25 26
1.9 1.9 1.8 1.9 1.7 1.7 1.6
1.9 1.5 1.5 1.8 1.5 1.5 1.6
1.00 1.27 1.20 1.06 1.13 1.13 1.00
n 44-59A B C F G H 1 J K
Y/Z
1985)
H
distance = metres from base of section. X/Z=data related to the ratio of the long (X) to the short axis (Z). Y/Z=data related to the ratio of the intermediate (Y) to the short axis (Z). n = number of readings. H = harmonic mean. Isyram = index of symmetry for a sample of size about 50, an Isymrn greater than 0.60 indicates a highly symmetrical pattern of the data points at the 95% confidence level. X2= statistical function to determine the distribution of the data points values, less than 14.0 indicate an even distribution. (Note: even distributions are very difficult to obtain at very low strains). R:XZ=strain ratio in the X Z section. R: YZ= strain ratio in the YZ section. R:XY= strain ratio in the X Y section calculated from R:XZ and R: YZ.
all the samples. Where possible at least 50 readings on each section were made. Data were treated by the statistical tests in Lisle (1985) and are shown in Table 1. At the 95% confidence level, all data have an index of symmetry greater than 0.6 indicating a highly symmetrical pattern after straining and suggesting that the markers had random orientations before straining (Lisle, 1985). Most of the sections show a value of more than 14 for the X2 test suggesting an uneven distribution of data points on the standard strain ratio curves as published by Lisle (1985 ). An even distribu-
tion of points is preferred for Rf/O studies and is achieved in the more highly strained samples. However, the majority of our samples show very low strains where this test is of less relevance. We justify the quality of our data by two sets of test samples. Samples 44-59F and 44-59G are separate samples of rock from the same height in the profile, and samples 44-590 and 44-5902 are the same sample specimen split in two and measured independently. The variation in the measured strain ratio within these pairs of test samples is 0.1 or less. Our
J.R.VEARNCOMBEANDA.P.HILL
196
.41 2.2
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1.8 ] 1.6 t 1.2~
.
~
flattening
~ ~ J
""-
I -
1.2
1.4
1.6
1.'~ "
i
2.2
2.4
2.;
2.8
R:YZ Fig. 4. Flinn deformation plot of strain data from the section through the Middle Vale Reef at Telfer. Arrows indicate the effect of increments of simple shear or volume loss.
strain data appear to be valid and are consistent (see Figs. 4 and 5 ). The results of these strain determinations are shown in Table 1, and Table 2 shows parameters determined from the strain values. Strain data can be represented in a variety of ways (see Hobbs et al., 1976; Ramsay and Huber, 1983 ). A commonly used graphical representations is shown in the conventional Flinn plot (Fig. 4 ). But to simplify the presentation of the results and make clear for geologists unfamiliar with strain data we use complimentary plots (Fig. 5) and established equations and practice to resolve the strain data into its components for ease of interpretation and presentation. The absolute magnitude of strain, with no allowance for the shape of the ellipsoid can be represented by the d-value (see appendix) which shows a progressive increase from a low value of about 0.6 in the wall rock to a moderate value about 1.8 in the MVR (Fig. 5a). These results clearly show that the mineralised unit of the MVR coincides with the most deformed part of the section. The shape of the strain ellipsoid is repre-
sented by the k-value (see appendix ). The siltstones have k-values between 0 and 0.55 (Fig. 5c) indicating strong flattening. Deformation was not plane strain as would be expected if the deformation were by simple shear alone. The origin of the flattening component could be a component of pure shear, or by the loss of volume, or a combination of both. Here, we use published arguments on the origin of flattening in discrete shear zones of this type as being dominantly volume loss (Ramsay and Graham, 1970). In contrast, pure shear demands complex accommodation strains; strains which are not observed at Telfer (Ramsay and Huber, 1987 ). The expected two-dimensional geometry for a continuous pure shear system in a shear zone is shown in Fig. 6. Variable pure shear components across a zone would produce a displacement which must be accommodated by discontinuities between adjacent elements (Fig. 6b) or produce a shear strain component with an increasing value away from a fixed reference or pin line (Fig. 6c). Shear strain would vary as a function of distance and as a function of the gradient of the pure strain component across the zone. For pure shear with
STRAINAND DISPLACEMENTIN THE MIDDLEVALEREEFAT TELFER,WESTERNAUSTRALIA
a
197
b
2
k value
1 :
dvalue 16 1.2 1 :
.8 4
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distance, metres
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6
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vol, lose
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6
8
10
12
14
distance, metres
, 4
4
6
- L_ u-,-u-~8 10 12 4 distance, metres
Fig. 5. Graphs of distance versus structural parameter through the Middle Vale R e e f ( M V R ) , at Telfer. The M V R is in the position 7.5 to 8 m above the datum pit floor on the SW wall of Pit 4 on the SW limb of the Telfer dome. (a). The d value, a measure of the intensity of deformation. Deformation increases into the zone and is at a m a x i m u m in the MVR. (b). The k value, a measure of the shape of the strain ellipsoid, with k = 0 for pure flattening, k = 1 for plane strain (simple shear) and k = infinity for constrictional strain. There is no consistent pattern of ellipsoid shapes although all are less than k = 0 . 5 5 indicating flattening strains. (c). The decrease in the angle of cleavage to bedding, from about 45 ° on the margin of the profile to about 12 ° in the centre of the zone. (d). The shear strain estimated from the curvature of cleavage assuming plane strain (an erroneous assumption given the flattening strains) and the shear strain calculated assuming volume loss before and after volume loss. (e). Volume loss calculations show that volume loss was at a maxim u m in the M V R and was as high as 50%.
J.R. VEARNCOMBEAND A.P. HILL
198 TABLE 2 Parameters describing the strain across the Middle Vale Reef, Teller Sample
Distance
Strain results
Cleavage
to bedding*
calc. shear
R:XZ calc. shear
R:XZ actual
k
Volume loss and shear strain
d
volume loss
shear before
shear after
(%) 44-59A B C F G H 1 J K L M N O 02 P Q R S
2.6 2.9 3.2 6.9 6.9 7 7.1 7.1 7.45 7.5 7.55 7.7 8 8.3 8.5 8.5 9.7 10.4 11.5 12.5
31 1.06 32 0.98 25 1.68 33 0.89 27 1.45 16 3.20 17 2.97 19 2.56 Base of Middle Vale Reef 15 3.46 17 2.97 Top of Middle Vale Reef 12 4.49 15 3.46 17 2.97 28 1.35 39 0.43 37 0.57 44 0.07
3 3 5 2 4 12 9 7
1.6 1.9 2.4 1.8 1.9 2.3 2 2.3 3.2
0.00 0.00 0.42 0.00 0.00 0.35 0.06 0.09 0.48
0.60 0.90 0.87 0.80 0.90 0.85 0.90 1.10 1.22
10 0 21 5 15 38 40 37
0.65 0.85 0.45 0.65 0,77 0.40 0.55 0,90
0.60 0.85 0.70 0.70 0.90 0.65 0.85 1.40
16 11
3.2 2.4
0.07 0.03
1.85 1.30
50 43
0.75 0.80
1.40 0.90
24 14 11 4 2 2 1
1.9 1.9 1.8 1.9 1.7 1.7 1.6
0.00 0.53 0.40 0.07 0.27 0.27 0.00
0.90 0.57 0.54 0.80 0.52 0.52 0.60
41 38 34 16 0 0 0
0.30 0.35 0.35 0.60 0.50 0.50 0.45
0.50 0.55 0.55 0.65 0.60 0.60 0.40
*to bedding = angle made by cleavage to bedding. calc. shear = shear strain calculated from the angle of bedding to cleavage (N.b., this assumes that deformation was plane strain and there was no volume loss and gives erroneous results). R:XZcaj¢. shear= strain ratio in the X Z section calculated from calc. shear. (N.b., these calculated values differ significantly from the actual values and shows that deformation involved more than simple shear). R:XZact,at=measured values of strain in the X Z section from Table l, k = k value, a measure of the shape of the strain ellipsoid as defined by Ramsay and Huber ( 1983, p. 200). Prolate strains tend towards infinity and oblate strains towards 0, plane strain = I. d = d value measure of the amount of deformation as defined by Ramsay and Huber (1983, p. 202). Larger values indicate increasing deformation. vol. loss% = calculated volume loss. shear before = shear strain assuming simple shear before volume loss. shear after= shear strain assuming simple shear after volume loss
continuous displacements the high strain zone would be enveloped by two zones of non-coaxial strain with opposite movement sense (Ramsay and Huber, 1987, p. 608-613 ). This "tooth paste" or "cream cake effect" is not observed a Telfer. From the evidence available to us it is not possible to rule out small components of pure shear, but a heterogeneous pure shear component with axes parallel to the shear zone walls cannot exist by itself.
Volume loss is likely during compaction of sediments, cleavage development, and lowgrade metamorphism; all three processes being accompanied by the expulsion of water. The lateral stratigraphic equivalent of the MVR is a carbonate or chert and carbonate unit (GoeUnicht et al., 1989); at Telfer diagenetic carbonate was partly replaced by quartz, and the clays were metamorphosed to micas. Extensive kaolinite alteration due to supergene
STRAINAND DISPLACEMENTIN THE MIDDLEVALEREEFAT TELFER,WESTERNAUSTRALIA
(a) Original Grid
0 0 0 0 0 0 (b)
Pure
shear
I II I
I
III (c) P u r e
displacements
- discontinuous
shear
- continuous
Ilol I
Io
Ilol displacements
I I I I I 1ol
I 1 I 1 I Iol Fig. 6. Pure shear with principal strains parallel and perpendicular to the shear zone walls. In (b) and (c) the fixed vertical pin line for shear zone parallel displacement is in the centre of the illustration. This illustrates the "cream cake" effect of pure shear and a geometry not seen at Telfer. ( F r o m fig. 26.24 of Ramsay and Huber, 1987).
processes dominates the rock and hence detailed textural evidence for volume loss is hidden. However, geometrical considerations suggest that volume loss during cleavage development was the dominant process, although we can not rule out small components of pure shear. In the following analysis, we assume that all the flattening is due to volume loss, and the influence of pure shear was negligible. Volume loss and shear strain can be quantified using the method of Ramsay and Huber (1987) (also see Coward and White, 1988) from the measured R : X Z strain ratios and cleavage/bedding angle for each sample. The value of the simple shear strain y is dependent on the order of the addition of shear strain:
199
either before or after volume loss. But, volume loss is independent of the order in which the deformation components are added giving the same result whether volume loss occurred before or after simple shear. The results of these calculations are shown in Figs. 5d and 5e. Shear strain before volume loss varied between 7= 0.3 and 0.9, and shear strain after volume loss varies between 7=0.4 and 1.4. These differences are minor compared to the shear strain calculated on the erroneous assumption that the curvature in the cleavage into the deformation zone was all due to simple shear, where the variation in y is between 0.07 and 4.5. Strain data on the Flinn plot (Fig. 4) are mostly along the abscissa (oblate field) making excursions into less oblate space suggesting local zones of simple shear superposed on existing flattening fabrics. For this reason volume loss is interpreted to predate simple shear. Volume loss varies from zero in the hanging wall and footwall to about 40% in the host rock to the MVR, and as high as 500/0 (Fig. 5e). Displacement along the MVR calculated from the shear strain (see appendix) is about 6 m. This measure of the slip is that generated in the MVR and host rocks by the ductile strain, and does not include slip on discrete bedding-parallel fault planes without strain in the host rock. However, balanced cross-sections suggest that fault displacements including bed-parallel slip are mostly less than 10 m in the open pits (Hill, 1990). Discussion The above results show that mineralisation in the MVR is coincident with the most deformed rocks, but the shear strain in the MVR is low (7 < 1.5 ) and displacements were likewise low (probably less than 10 m). Assuming that the pure shear component was minor, volume loss calculations on the matrix of the host rock indicate a loss of about 40% of material in the horizon. But these do not include the volume addition by quartz veins which clearly
200
cut the cleavage and shape fabric; quartz veining is highly variable but visually estimated at up to 70% over 0.5 m intervals. Volume loss in the siltstones of the MVR contrasts with the volume gain by veining along the same horizon. Given the opposite nature of these two processes it is improbable that they operated synchronously within the same finite-element of rock, although volume redistribution is a possibility. As veins clearly crosscut the strained siltstones, volume loss logically preceded volume addition by veining. Along the MVR, volume loss involved the expulsion of a fluid phase (mostly water) and may have been accompanied by dissolution, but textural evidence for this is masked by the kaolinite overprint. The fluid expulsion represents an increase in permeability along the MVR relative to the far-field host-rock permeability. Subsequently, the same porous plane became the focus for mineralising fluids. It appears to have had a low cohesion and reactivated with the addition of quartz-sulphide veins. Our results from the strain studies confirm the results from balanced cross-sections which show that the MVR is not a dbcollement, and any dbcollementis probably hundreds of metres below the surface. Mineralisation is in the crestal region of asymmetric anticlinal domes, developed in the tip region of blind thrusts or steep reverse faults which could have acted as conduits for fluids below the present level of exposure (Hill, 1990). The stockwork vein sets and bedding-parallel reefs, such as the MVR, are interpreted to have formed from an overpressured fluid system (Hill, 1990).
Conclusions Mineralisation in the MVR is coincident with the most deformed rocks which show significant volume loss: shear strain was a minor component of the deformation (~, < 1.5 ). Volume loss in the siltstone host to the MVR was up to 50% and contrasts with the volume gain
J.R. VEARNCOMBE AND A.P. HILL
by veining along the same horizon. As veins clearly cross-cut the strained siltstones, volume loss probably preceded volume addition by veining. The same plane which was a conduit for material removal, has a high permeability and reactivated with the addition of quartz-sulphide veins. Implications are that strata-parallel mineralised zones with curving deformation fabrics and local high strains are not necessarily the result of bedding-plane slip alone, but may include significant flattening accommodated through volume loss. Importantly, a simplistic examination of the shear zone kinematics and vein geometry is not an adequate treatment of the deformation. It is unlikely that strain studies will form part of an exploration program, but exploration and mine geologists working with shear zones need to be aware of of the different displacements and strains which comprise a shear zone. In particular, volume loss as well as simple shear can be important at low metamorphic grades. Reactivation of surfaces with a low cohesion, such as the MVR, is confirmed as a major control on shear zone-hosted mineralisation.
Acknowledgements Telfer Gold Mine supported two visits to Telfer by JRV and kindly funded research for APH's Honours project. Joanne Cannon and Cecilia D'Ercole helped with the strain determinations, Nicola Netherway (n~e Goellnicht) assisted with sample collection. Nicola Netherway and John Ridley commented on an earlier draft of this manuscript. Telfer Gold Mine is thanked for permission to publish this work.
Appendix This paper refers to a number of structural parameters deduced by a variety of well established techniques published in the structural geology literature. The following are a brief de-
201
STRAIN AND DISPLACEMENTIN THE MIDDLE VALEREEF AT TELFER, WESTERN AUSTRALIA
scription of the parameters and techniques, but the reader should examine the given references for a through treatment of the subjects. A cross-section drawn in the movement direction is said to balance when the area of the deformed section is equal to the area of the restored (undeformed) section (Dahlstrom, 1969). Thus, a balanced section is a geometrically possible section. At Telfer restoring the Triangle Zone is difficult. If pinned in undeformed beds to the NE, space problems are created on the restored section; that is gaps and unrealistic structures appear on the undeformed template. However, by using a pin line through the lower beds in the Triangle Zone the structure balances (Fig. 2 ). In this paper strains were determined according to the Rf/O method (Ramsay, 1967; Lisle, 1985). Rf/(J is a commonly used technique to determine finite strain on a surface containing elliptical markers each with long to short axis ratio of R f the shape of the final ellipsoid. ¢ is the acute angle made by the long axis of the ellipse to the average orientation of the long axes of all the ellipses. At Telfer, this average parallels the cleavage in the sample. Each strain marker or spot represents a point on a graph of Rfagainst q~. A statistical test of the index of symmetry Isymm is used to determine the symmetry of the data set. A symmetrical data set suggests that although the strain markers were not necessarily spherical, they had a random orientation before straining. Isymm is tested at the 95% confidence level according to the method of Lisle ( 1985 ). The g 2 is a test of the distribution of data on the Rf i e plot. An even distribution of data points is preferred, but are very difficult to obtain on samples at low strains with sample sizes of about 50. The final result, a measure of the strain in the measured section is determined from sets of standard curves on a file of standard graphs when compared with the data on the Rf/q~ plot. These standard curves and graphs are reproduced in Lisle (1985) and the results given here are the best solutions for the sections in
the XZ and YZ sections giving the average strain ratios R:XZ and R: YZ, respectively. A Flinn plot is a graph of the R:XY versus R: YZ. It is a popular portrayal of three-dimensional strain data, but suffers from the disadvantage of plotting ratios, and hence many geologists find it difficult to conceptualise the data. However, the presentation of data is made simpler in this paper by using two alternative parameters to describe the position of data on the Flinn plot. These are the k and d values. The d value (Ramsay and Huber, 1983, p. 202) represents the amount of deformation equivalent to the distance of the point from the origin on a Flinn plot where:
d= [ ( R : X Y - 1)2+ ( R : Y Z - 1 ) 2 ]
0.5
and k value represents the shape of the strain ellipsoid where:
k= ( R : X Y - 1 ) / (R: Y Z - 1) with oblate strains plotting along the abscissa where k = 0, plane strain where the intermediate Y axis remains unstrained (without volume loss) has a value k = 1 and, prolate strain k = oo plotting along the ordinate. The above strain parameters are usually determined assuming that the rock has not been subject to volume loss. However, a loss of volume in the rock is common at low metamorphic grade as dewatering, compaction and cleavage development occur. Absolute values for volume loss are often difficult to determine. However, in shear zones lacking volume change (simple-shear shear zones) the finitestrain fabric and its orientation are directly related (Ramsay and Graham, 1970) . Deviations from this precise relationship can be determined from finite-strain measurements and cleavage orientation relative to the movement vector of the shear zone, and are used to calculated volume loss in the shear zone. The mathematics involved are complex, but are determined using data in the XZ section and are detailed in Ramsay and Huber (1987, session 26) and Coward and White (1988). Coward
202
and White graph the strain ratio versus cleavage angle and contour the plot for amounts of volume loss and shear strain ~,. Volume loss estimates and shear strain ~, used in this paper are deduced by this method. Where the shear strain ~, varies across a shear zone displacement on the shear zone can be determined by the method of Ramsay and Graham (1970) from integrating the shear strain across the width of the shear zone:
D=f ~,dy where D is the total displacement and y is the distance across the zone. This applies to a displacement involving shear alone, or shear followed by volume loss. The technique is equivalent to calculating the area under a curve on a graph of shear strain against distance (e.g., Fig. 5d). Assuming volume loss followed by shear, the same displacement is given by: D = ~ 7ad dy where ~'ais the shear strain following volume loss, and d is the volume loss. References Barley, M.E. and Groves, D.I., 1989. Exploration significance of regional and local scale hydrothermal alteration patterns in greenstone belts. Miner. Energy Res. Inst. West. Aust., Rep. 65, 83 pp. Butler, R.W.H., 1982. The terminology of structures in thrust belts. J. Struct. Geol., 4: 239-245. Coward, M.P. and White, S.H., 1988. Deformation and Mineralisation. Short Course Notes, Tectonics Div., Geol. Soc. S. Afr., 453 pp. Dahlstrom, C.D.A., 1970. Structural geology in the eastern margin of the Canadian Rocky Mountains. Bull. Can. Pet. Geol., 18: 332-406. Dimo, G., 1990. Telfer Gold Deposits. In: F.E. Hughes
J.R. VEARNCOMBE AND A.P. HILL
(Editor), Geology of the Mineral Deposits of Australia and Papua New Guinea. Australas. Inst. Min. Metall. Monogr., 14:643-651. Goellnicht, N.M., 1987. Constraints on the timing and source of gold mineralization at Main Dome, Telfer, Western Australia. B.Sc. Honours thesis, The University of Western Australia (unpubl.), 78 pp. Goellnicht, N.M., Groves, D.I., McNaughton, N.J. and Dimo, G., 1989. An epigenetic origin for the Telfer gold deposits, Western Australia. Econ. Geol. Monogr., 6: 151-167. Hill, A.P., 1990. Structure of West Dome, Telfer, Western Australia and its significance to mineralisation and regional tectonics. B.Sc. Honours thesis, The University of Western Australia (unpubl.), 58 pp. Hobbs, B.E., Means, W.D. and Williams, P.F., 1976. An Outline of Structural Geology. Wiley, Chicester, 571 PP. Hossack, J.R., 1979. The use of balanced cross-sections in the calculation of orogenic contraction: a review. J. Geol. Soc. London, 136:705-711. Lisle, R.J., 1985. Geological Strain Analysis, A Manual for6 the Rf/(~ Method. Pergamon Press, Oxford, 98 pp. Laubscher, H.P., 1962. Die Zweiphase Hypothese der Jurafaltung. Eclogae Geol. Helv., 55: 161-166. McNaughton, N.J. and Goellnicht, N.M., 1990. The age and radiothermal properties of the Mount Crofton Granite, Telfer area, Western Australia. Aust. J. Earth Sci., 37: 103-106. Newmont Australia Ltd., 1991. Annual Report 1990. 64 pP. Ramsay, J.G., 1967. Folding and Fracturing in Rocks. McGraw-Hill, New York, N.Y., 568 pp. Ramsay, J.G. and Graham, R.H., 1970. Strain variation in shear belts. Can. J. Earth Sci., 7: 786-813. Ramsay, J.G. and Huber, M.I., 1983. The Techniques of Modern Structural Geology, Vol. 1: Strain Analysis. Academic press, London, pp. 1-308. Ramsay, J.G. and Huber, M.I., 1987. The Techniques of Modern Structural Geology, Vol. 2: Folds and Fractures. Academic press, London, pp. 309-700. Turner, C.C., 1982. The Telfer Gold Deposits, Western Australia: Stratigraphy, sedimentology and gold mineralization of the Proterozoic Yeneena Group. Unpubl. Ph.D. thesis, Univ. New England Armidale, N.S.W., 276 pp. Williams, I.R. and Myers, J.S., 1990. Paterson Orogen. Geol. Surv. W. Aust. Mem., 3: 274-286.
189
Strain and displacement in the Middle Vale Reef at Telfer, Western Australia Julian R. Vearncombe and Anthea P. Hill* Key Centrefor Strategic Mineral Deposits, The University of WesternAustralia, Nedlands, WA 6009, Australia (Received October 20, 1991; revised version accepted October 21, 1992)
ABSTRACT Vearncombe, J.R. and Hill, A.P., 1993. Strain and displacement in the Middle Vale Reef at Telfer, Western Australia. In: D.I. Groves and J.M. Bennett (Editors), Structural Setting and Controls on Mineral Deposits. Ore Geol. Rev., 8: 189202. Mineralisation in the Middle Vale Reef at Telfer is coincident with the most deformed rocks in a strata-parallel shear zone in an argillaceous and calcareous horizon. Cleavage to bedding angle and three-dimensional strain calculated by Rf~ analysis were determined in a profile across the Middle Vale Reef. They show significant volume loss (about 40%), but shear strain was a minor component of the deformation (7 < 1.5 ). From a balanced section, bedding-plane slip along the Middle Vale Reef appears to have been less than 10 m and the deformation was dominated by material removal resulting in the loss of volume in the host siltstones. The same zone, which was a conduit for fluid during volume loss, reactivated with the addition of quartz-sulphide veins, resulting in a net volume gain. Reactivation of surfaces with a high permeability, such as the Middle Vale Reef, is confirmed as a major control on shear zone-hosted mineralisation.
Introduction
Strata-parallel mineralisation is controversial with both syngenetic and epigenetic origins argued for the deposits. Critical to the models for an epigenetic origin are the localisation of deformation and channelised fluid flow in specific horizons within the stratigraphy. Bedding-plane slip and dilation related to buckling of the stratigraphy are frequently invoked as the controls on epigenetic strata-parallel mineralisation, but the structural elements of the zone are only rarely documented. It is the purpose of this paper to examine the strain and displacement in a mineralised horizon at Tel*Present address: CODES, Department of Geology, University of Tasmania, Hobart, Tasmania, Australia. Correspondence to: J.R. Vearncombe, Key Centre for Strategic Mineral Deposits, The University of Western Australia, Nedlands 6009, Australia.
fer to determine the structural controls on strata-parallel mineralisation there. Telfer lies in the northern part of the Paterson Province in the Upper Proterozoic Yeneena Group which unconformably overlies the Lower to Middle Proterozoic Rudall Metamorphic Complex (Williams and Myers, 1990). The major gold deposits occur within the Telfer Dome, one of several NW-trending, en echelon, asymmetric anticlines (Fig. 1 ). High-grade gold-copper mineralisation is associated with discordant veins and continuous stratiform to strata-bound reefs within compositionally favourable argillaceous and carbonaceous horizons, including the Middle Vale Reef (MVR). Recently, epigenetic models have been presented by GoeUnicht et al. (1989), and Dimo (1990) which emphasise structural controls including bedding-plane slip, and relate the mineralisation to late defor-
0169-1368/93/$6.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
190
J.R. VEARNCOMBEAND A.P.HILL
. . . . .
j. . . . . . . . . . . . . . .
°Minor
--~ Syncline
22°00'S
I
lb sw
Gonorallzed ~octio~ ~f NE limbol Main Dome
upper vale Si~stone
E
IA
NE
E 2
Oule, s~llstoo~ Rim S ~ . d ~ o ~ e ~ea,an S ~ s I o n e
~oot~a,I s~ndsto~e Malu Ouarlz,~e
M,~era~zat,o~
Pb ,S~loDesan,p=~
01
\
Fig. 1 (a). The regional geology of the Telfer area in the Paterson Province. Fig. 1 (b). Geology and economic gold mineralisation of the Telfer dome, sample locality indicated by the arrow and cross-section (Fig. 2) shown by the line x - x " . Insert: the Telfer Formation (mine sequence) stratigraphy (adapted from an unpublished Newmont Australia Ltd. report). (Maps of Telfer area from Goellnicht et al., 1989; and McNaughton and Goellnicht, 1990).
STRAIN AND DISPLACEMENT IN THE MIDDLE VALE REEF AT TELFER, WESTERN AUSTRALIA
mation and granitoid emplacement. The MVR is a shear zone, in the sense of Ramsay and Huber ( 1987, p. 595 ), in being a zone of high deformation which is long relative to its width and surrounded by rocks showing a lower state of finite strain. The rocks of the MVR display strain markers which enable detailed study of the tectonic strain, from which implications for the progressive deformation, displacement and volume changes can be drawn for this bedding-parallel shear zone. Strain markers are rare in mineralised shear zones and the displacement on the shear zone can not usually be calculated. Thus the MVR provides an excellent opportunity to quantify deformation and test some of the common assumptions regarding the geometry of mineralised bedding-parallel deformation zones.
The Telfer Gold deposits Gold mineralisation at Telfer was discovered in 1971, and the mine began production in 1977. Telfer has produced more than 2.6 million ounces of gold, with proved and probable reserves of about 32.6 million tonnes at 2.1g/t (2.2 million ounces Au) (Newmont Australia Ltd., 1991 ) making it one of Australia's largest gold producers. Mineralisation at Telfer occurs within subgreenschist facies sandstones and siltstones of the Telfer Dome, comprising two subsidiary structures, Main Dome and West Dome which are asymmetric to the NE. Mineralisation is stratiform to stratabound in sulphide/oxide quartz veins hosted in calcareous and argillaceous horizons, and the reefs are termed the Middle Vale Reef (MVR) and the E-reefs. The MVR is a quartz-pyrite-chalcopyrite-bearing horizon near the base of the Middle Vale Siltstone Member. The MVR is up to three metres wide (0.5 m wide in the profile studied in this paper) and extends at least 10 km 2 (Goellnicht, 1987; Dimo, 1990). Stockwork and sheeted vein systems in the footwall and hanging wall of the reefs are also mineralised.
191
The Middle Vale Siltstone Member is a finegrained and thin-bedded argillaceous siltstone, with minor carbonaceous limestone and calcareous sandstone. PetrographicaUy these rocks contain highly variable amounts of kaolinite and quartz, with minor tourmaline, carbonate, haematite, sericite and hydrothermal biotite. The kaolinite has a presumed origin related to supergene (Tertiary or Recent) weathering. The rock contains numerous elliptical spots up to 5 mm in diameter, these spots were originally carbonate concretions and are replaced by quartz and in some examples by the supergene kaolinite. The MVR carries a well developed cleavage and bedding-plane deformation will be shown to be an important factor in the formation of the ore deposit. Goellnicht (1987) and Dimo (1990) suggest that dilation and flexural-slip during the doming event could explain the extensive and concordant nature of the MVR and local mineralisation of other stratigraphic horizons. The MVR is dominated by vein material which lacks evidence of superposed strain. Goellnicht et al. (1989) interpreted the veining to be late in the deformation history of the Telfer Dome. Laminated, crudely banded or massive sulphides and interstitial quartz form the upper part of the MVR; the lower part consists of fractured, milky white, very coarsegrained, vein quartz with interstitial oxides after sulphides (Goellnicht, 1987 ).
Structure of Main Dome Main Dome and West Dome are similar, both are doubly-plunging anticlines and are asymmetric to the NE; they both have faulted fold closures. Concordant and discordant veins are common in both domes. Bedding dip on the NE limb of Main Dome steepens from about 30 ° at surface to about 55 ° at depths of 700 m (Turner, 1982 ). The axial plane strikes NW-SE and dips steeply to the SW. Faults in Main Dome are imbricate thrust faults, best developed in the axial region of the
192
J.R. VEARNCOMBEAND A.P. HILL
lel to the movement direction as defined by quartz fibres. Thrust faults have displacements of less than 10 m. A Triangle Zone (terminology after Butler, 1982 ) in the SE closure of Main Dome is formed by a NE-dipping backthrust in combination with a SW-dipping thrust. Movement along both thrusts that define the Triangle Zone appears to be taken up by bedding-plane slip along the MVR. Other Sor SW-dipping faults in Main Dome do not intersect the MVR, but curve into bedding planes along stratigraphically higher horizons. Bedding-plane slip is important in Main Dome, as indicated by quartz fibres along bedding planes and locally developed cleavages oblique to bedding, and is supported by measurement on the balanced cross-section. Line B, perpendicular to bedding in the deformed section, does not restore as a straight line in the basal layers. To make the curved line perpendicular to bedding in the deformed section, bedding-plane slip in the basal layers must be about 5 m, SW over NE, as indicated by the
dome, and cross-cutting, subvertical faults. The subvertical faults have minor offsets and are N- to NNE-trending and cross-cut all structures. Movement on these faults is dominantly dextral strike-slip, and in section the apparent separation geometry is both normal and reverse. Two major late, subvertical faults, together named the Graben Faults, offset the MVR by up to 10 m. These structures post-date the thrust faults and mineralisation. They are absent along the section line (Fig. 2 ). The thrust faults are SSW- or SW-dipping, listric and curve toward parallelism with the bedding planes with depth; some develop folds in their hanging walls. Fault geometry combined with available mineral lineation data from throughout the Telfer Dome, suggest movement was mostly from the SW to the NE (Hill, 1990). An incomplete section, due to the limits of mining, across the SE end of Main Dome has been balanced and restored (Fig. 2; also see Appendix for comments on the method of balancing sections). It is constructed paralN.E.
S.W.
A RJ.~m) 510 R.L (m) 600~
~500
490 - -
~
480--
~480
47~ - - m
490
REFERENCE MVR
~
0
5 10
I
i
I
20
30
I
J
40metwes
J
Sandstone
Rovorso tho'~t, w#~ f l ~ v e ~ o n t dk'ocllon
mn-am
Fig. 2. Balanced and restored cross-sections across the SE closure of Main Dome. The Triangle Zone defined by both opposed m o t i o n a n d dip on reverse faults a n d making a triangular structure is on the left o f the section. For location o f section see the line x - x " in Fig. lb.
STRAIN AND DISPLACEMENT IN THE MIDDLE VALE REEF AT TELFER, WESTERN AUSTRALIA
arrows on the restored template. This value compares with 21 m of bedding-plane slip determined from a cross-section through the SE end of West D o m e (Hill, 1990). The absolute shortening across a balanced section can be used to calculate the depth to d~collement (Laubscher, 1962; Hossack, 1979), but to calculate the absolute shortening, the original predeformation height of a regional datum must be known. This information is unavailable and hence only a m i n i m u m value of shortening and hence a m i n i m u m depth to d~collement can be calculated. For the section in Fig. 2 these calculate as a shortening of 17 m or 9% and a m i n i m u m excess area (area on the cross-section between undeformed and deformed d a t u m horizon) of 1312 m 2 giving a m i n i m u m depth to d~collement of 76 m. A similar calculation (Hill, 1990) on a cross-section through West D o m e gave a depth to d~collement 140 m deeper than our estimate. The assumptions in generating these resuits across a very small section line are such that it is unlikely that the results are accurate enough to predict the depth to a d~collement, but they do clearly show that if there is a d~collement it is well below the MVR. The results from the balanced section suggesting only a few metres of slip on the bedding planes, and showing that any d~collement is well below the MVR, raise the important question: what was special about the mineralised horizons such as the MVR? To answer this we need to know more about the deformation in the MVR; what are the fabrics in the MVR and under what shear rkgime did they form? What was the role of volume change? (Volume loss usually accompanies low-grade metamorphism and cleavage development, and volume gain in the form o f m e t a s o m a t i c alteration and veining can accompany mineralisation; Barley and Groves, 1989.) How much slip is indicated by the cleavage oblique to bedding and does this agree with the slip determined from the balanced cross-section? These problems can be solved by knowledge of the finite strain in
193
the MVR and are addressed in the following strain study. Strain in the MVR
In the MVR a cleavage defined by the alignment of micaceous minerals parallels a shape fabric defined by the ellipsoidal kaolinite and quartz spots replacing carbonate concretions in the siltstone. The carbonate nodules probably developed during diagenesis, they were partly replaced by quartz during either continued diagenesis or low-grade metamorphism, and cleavage affects the markers and matrix equally. Thus the markers record the tectonic strain in the siltstones. Kaolinite replacement is related to more recent supergene processes, and appears to preserve the deformational shapes. The cleavage and shape fabric are at a moderate angle to bedding in the hanging and footwalls, and at an angle close to bedding in the MVR (Fig. 3). Similarly, the spots are variable but approach spherical in the wallrock, and progressively change shape into the MVR where they have strongly flattened crosssections (Fig. 3 ). Data collected from a study section through the MVR on a haul road between Pits 4 and 5 (for sample locality see Fig. l b) are shown in Table 1, and Figs. 4 and 5. The strained spots are oriented such that on average they lie in the plane of the cleavage. The angle the cleavage (and shape fabric of the spots) makes to bedding decreases from between 30 ° and 45 ° in the wallrock to about 12 ° in the MVR (Fig. 5c). This curvature is coincident with an increase in strain (Fig. 5a) into the MVR and is typical of the curvature of foliation into a shear zone. If cleavage curvature is assumed to be a product of simple shear alone, values of shear strain ~,can be calculated (Ramsay and Graham, 1970) and a displacement along the MVR of about 18 m can be calculated by integrating the shear strain across the profile (e.g., the area under the curve "shear, no volume loss" in Fig. 5d). If deformation was by simple shear the shape fabrics
194
SW
J.R. VEARNCOMBEANDA.P.HILL
/ J
O O
J
O
NE 12
10
MVR
J
J
7.5
O O
J
O
cleavage
strain
6
4 real size of strain markers about 5mm
2 metres above datum
Fig. 3. Schematic profile through the MVR in the area between open pits 3 and 4 on the SW flank of the Telfer Dome. For location see Fig. 1b. The profile shows the variation in cleavage angle to bedding as defined by the lines bounding the MVR and the variation in strain in the X Z section in strain markers in the host rocks to the MVR in a vertical section of just over 12 m.
in the MVR would be a product of plane strain and have a k value (shape of the strain ellipse ) of 1. This is tested by three-dimensional analysis of the strain. Three-dimensional strains were determined on the spots within the host rock to the MVR, and in adjacent hanging wall and footwall siltstones in the study section over a stratigraphic
distance of about 12.5 m with the MVR at the interval 7.5 to 8.0 m. The strained spots show sufficient variation in shape to warrant strain analysis by the Rf/(~ technique (Ramsay, 1967; Lisle, 1985; see appendix). The orientation of the long axis of the spots and the ratio of the long to the short axes of the strained spots were determined in both the X Z and Y Z sections of
STRAIN AND DISPLACEMENTIN THE MIDDLEVALEREEFAT TELFER, WESTERNAUSTRALIA
195
TABLE1
Straindataacrossthe M i d d l e V a l e R e e L T e l ~ r ( s t r a i n s d e t e r m i n e d b y t h e m e t h o d d e s c r i b e d b y L i s l e , Sample
Distance
X/Z
L M N O 02 P Q R S
2.6 2.9 3.2 6.9 6.9 7 7.1 7.1 7.45 7.5 7.55 7.7 8 8.3 8.5 8.5 9.7 10.4 11.5 12.5
R:XZ
R:YZ
R:XY
~ymm
X2
n
H
~ymm
~2
52 1.58 0.96 55 1.85 0.87 52 2.32 0.92 52 1.68 0.85 52 1.66 0.77 52 2.43 0.92 57 2.03 0.84 52 2.29 0.77 52 3.08 0.89 Base of Middle Vale Reef 52 3.37 0.85 51 2.34 0.90 Top of Middle Vale Reef 51 1.94 0.90 52 1.76 0.92 51 1.82 0.75 31 1.89 0.65 51 1.70 0.63 52 1.56 0.92 52 1.54 0.85
17 40 7 21 56 4 6 10 17
52 62 51 51 52 52 52 52 35
1.77 1.73 1.81 1.77 1.67 1.89 1.9 2.04 2.13
0.92 0.87 0.90 0.94 0.73 0.96 0.89 0.89 0.78
33 12 34 21 28 22 10 21 4
1.6 1.9 2.4 1.8 1.9 2.3 2 2.3 3.2
1.6 1.9 1.8 1.8 1.9 1.8 1.9 2.1 2.1
1.00 1.00 1.33 1.00 1.00 1.28 1.05 1.10 1.52
5 12
52 33
2.88 2.24
0.89 0.85
6 30
3.2 2.4
2.85 2.3
1.12 1.04
21 38 55 21 32 29 38
32 51 51 51 52 50 51
1.84 1.57 1.61 1.66 1.56 1.39 1.69
0.75 0.82 0.78 0.71 0.73 0.80 0.90
26 19 25 45 40 25 26
1.9 1.9 1.8 1.9 1.7 1.7 1.6
1.9 1.5 1.5 1.8 1.5 1.5 1.6
1.00 1.27 1.20 1.06 1.13 1.13 1.00
n 44-59A B C F G H 1 J K
Y/Z
1985)
H
distance = metres from base of section. X/Z=data related to the ratio of the long (X) to the short axis (Z). Y/Z=data related to the ratio of the intermediate (Y) to the short axis (Z). n = number of readings. H = harmonic mean. Isyram = index of symmetry for a sample of size about 50, an Isymrn greater than 0.60 indicates a highly symmetrical pattern of the data points at the 95% confidence level. X2= statistical function to determine the distribution of the data points values, less than 14.0 indicate an even distribution. (Note: even distributions are very difficult to obtain at very low strains). R:XZ=strain ratio in the X Z section. R: YZ= strain ratio in the YZ section. R:XY= strain ratio in the X Y section calculated from R:XZ and R: YZ.
all the samples. Where possible at least 50 readings on each section were made. Data were treated by the statistical tests in Lisle (1985) and are shown in Table 1. At the 95% confidence level, all data have an index of symmetry greater than 0.6 indicating a highly symmetrical pattern after straining and suggesting that the markers had random orientations before straining (Lisle, 1985). Most of the sections show a value of more than 14 for the X2 test suggesting an uneven distribution of data points on the standard strain ratio curves as published by Lisle (1985 ). An even distribu-
tion of points is preferred for Rf/O studies and is achieved in the more highly strained samples. However, the majority of our samples show very low strains where this test is of less relevance. We justify the quality of our data by two sets of test samples. Samples 44-59F and 44-59G are separate samples of rock from the same height in the profile, and samples 44-590 and 44-5902 are the same sample specimen split in two and measured independently. The variation in the measured strain ratio within these pairs of test samples is 0.1 or less. Our
J.R.VEARNCOMBEANDA.P.HILL
196
.41 2.2
"~
1.8 ] 1.6 t 1.2~
.
~
flattening
~ ~ J
""-
I -
1.2
1.4
1.6
1.'~ "
i
2.2
2.4
2.;
2.8
R:YZ Fig. 4. Flinn deformation plot of strain data from the section through the Middle Vale Reef at Telfer. Arrows indicate the effect of increments of simple shear or volume loss.
strain data appear to be valid and are consistent (see Figs. 4 and 5 ). The results of these strain determinations are shown in Table 1, and Table 2 shows parameters determined from the strain values. Strain data can be represented in a variety of ways (see Hobbs et al., 1976; Ramsay and Huber, 1983 ). A commonly used graphical representations is shown in the conventional Flinn plot (Fig. 4 ). But to simplify the presentation of the results and make clear for geologists unfamiliar with strain data we use complimentary plots (Fig. 5) and established equations and practice to resolve the strain data into its components for ease of interpretation and presentation. The absolute magnitude of strain, with no allowance for the shape of the ellipsoid can be represented by the d-value (see appendix) which shows a progressive increase from a low value of about 0.6 in the wall rock to a moderate value about 1.8 in the MVR (Fig. 5a). These results clearly show that the mineralised unit of the MVR coincides with the most deformed part of the section. The shape of the strain ellipsoid is repre-
sented by the k-value (see appendix ). The siltstones have k-values between 0 and 0.55 (Fig. 5c) indicating strong flattening. Deformation was not plane strain as would be expected if the deformation were by simple shear alone. The origin of the flattening component could be a component of pure shear, or by the loss of volume, or a combination of both. Here, we use published arguments on the origin of flattening in discrete shear zones of this type as being dominantly volume loss (Ramsay and Graham, 1970). In contrast, pure shear demands complex accommodation strains; strains which are not observed at Telfer (Ramsay and Huber, 1987 ). The expected two-dimensional geometry for a continuous pure shear system in a shear zone is shown in Fig. 6. Variable pure shear components across a zone would produce a displacement which must be accommodated by discontinuities between adjacent elements (Fig. 6b) or produce a shear strain component with an increasing value away from a fixed reference or pin line (Fig. 6c). Shear strain would vary as a function of distance and as a function of the gradient of the pure strain component across the zone. For pure shear with
STRAINAND DISPLACEMENTIN THE MIDDLEVALEREEFAT TELFER,WESTERNAUSTRALIA
a
197
b
2
k value
1 :
dvalue 16 1.2 1 :
.8 4
4~MVR
o
~- :~ ~
8-
io
i2
distance, metres
14 .ol o
2
4
6-
8-
to
1£"
4
distance, metres
d-- . . . . .
C 45 ~ cleavage
-
40
T
5
55 30 25 20
shear strain
I~ ip/l
I I
,sheor,novo, llld,
I
[] smearafter
l
,oe
15 10
L
5 0 0
2
4
8 I0 12 14 distance, metres
6
e
\
I
\
/
I
0
6O
2
volume
0
I
vol, lose
/
, 2
6
8
10
12
14
distance, metres
, 4
4
6
- L_ u-,-u-~8 10 12 4 distance, metres
Fig. 5. Graphs of distance versus structural parameter through the Middle Vale R e e f ( M V R ) , at Telfer. The M V R is in the position 7.5 to 8 m above the datum pit floor on the SW wall of Pit 4 on the SW limb of the Telfer dome. (a). The d value, a measure of the intensity of deformation. Deformation increases into the zone and is at a m a x i m u m in the MVR. (b). The k value, a measure of the shape of the strain ellipsoid, with k = 0 for pure flattening, k = 1 for plane strain (simple shear) and k = infinity for constrictional strain. There is no consistent pattern of ellipsoid shapes although all are less than k = 0 . 5 5 indicating flattening strains. (c). The decrease in the angle of cleavage to bedding, from about 45 ° on the margin of the profile to about 12 ° in the centre of the zone. (d). The shear strain estimated from the curvature of cleavage assuming plane strain (an erroneous assumption given the flattening strains) and the shear strain calculated assuming volume loss before and after volume loss. (e). Volume loss calculations show that volume loss was at a maxim u m in the M V R and was as high as 50%.
J.R. VEARNCOMBEAND A.P. HILL
198 TABLE 2 Parameters describing the strain across the Middle Vale Reef, Teller Sample
Distance
Strain results
Cleavage
to bedding*
calc. shear
R:XZ calc. shear
R:XZ actual
k
Volume loss and shear strain
d
volume loss
shear before
shear after
(%) 44-59A B C F G H 1 J K L M N O 02 P Q R S
2.6 2.9 3.2 6.9 6.9 7 7.1 7.1 7.45 7.5 7.55 7.7 8 8.3 8.5 8.5 9.7 10.4 11.5 12.5
31 1.06 32 0.98 25 1.68 33 0.89 27 1.45 16 3.20 17 2.97 19 2.56 Base of Middle Vale Reef 15 3.46 17 2.97 Top of Middle Vale Reef 12 4.49 15 3.46 17 2.97 28 1.35 39 0.43 37 0.57 44 0.07
3 3 5 2 4 12 9 7
1.6 1.9 2.4 1.8 1.9 2.3 2 2.3 3.2
0.00 0.00 0.42 0.00 0.00 0.35 0.06 0.09 0.48
0.60 0.90 0.87 0.80 0.90 0.85 0.90 1.10 1.22
10 0 21 5 15 38 40 37
0.65 0.85 0.45 0.65 0,77 0.40 0.55 0,90
0.60 0.85 0.70 0.70 0.90 0.65 0.85 1.40
16 11
3.2 2.4
0.07 0.03
1.85 1.30
50 43
0.75 0.80
1.40 0.90
24 14 11 4 2 2 1
1.9 1.9 1.8 1.9 1.7 1.7 1.6
0.00 0.53 0.40 0.07 0.27 0.27 0.00
0.90 0.57 0.54 0.80 0.52 0.52 0.60
41 38 34 16 0 0 0
0.30 0.35 0.35 0.60 0.50 0.50 0.45
0.50 0.55 0.55 0.65 0.60 0.60 0.40
*to bedding = angle made by cleavage to bedding. calc. shear = shear strain calculated from the angle of bedding to cleavage (N.b., this assumes that deformation was plane strain and there was no volume loss and gives erroneous results). R:XZcaj¢. shear= strain ratio in the X Z section calculated from calc. shear. (N.b., these calculated values differ significantly from the actual values and shows that deformation involved more than simple shear). R:XZact,at=measured values of strain in the X Z section from Table l, k = k value, a measure of the shape of the strain ellipsoid as defined by Ramsay and Huber ( 1983, p. 200). Prolate strains tend towards infinity and oblate strains towards 0, plane strain = I. d = d value measure of the amount of deformation as defined by Ramsay and Huber (1983, p. 202). Larger values indicate increasing deformation. vol. loss% = calculated volume loss. shear before = shear strain assuming simple shear before volume loss. shear after= shear strain assuming simple shear after volume loss
continuous displacements the high strain zone would be enveloped by two zones of non-coaxial strain with opposite movement sense (Ramsay and Huber, 1987, p. 608-613 ). This "tooth paste" or "cream cake effect" is not observed a Telfer. From the evidence available to us it is not possible to rule out small components of pure shear, but a heterogeneous pure shear component with axes parallel to the shear zone walls cannot exist by itself.
Volume loss is likely during compaction of sediments, cleavage development, and lowgrade metamorphism; all three processes being accompanied by the expulsion of water. The lateral stratigraphic equivalent of the MVR is a carbonate or chert and carbonate unit (GoeUnicht et al., 1989); at Telfer diagenetic carbonate was partly replaced by quartz, and the clays were metamorphosed to micas. Extensive kaolinite alteration due to supergene
STRAINAND DISPLACEMENTIN THE MIDDLEVALEREEFAT TELFER,WESTERNAUSTRALIA
(a) Original Grid
0 0 0 0 0 0 (b)
Pure
shear
I II I
I
III (c) P u r e
displacements
- discontinuous
shear
- continuous
Ilol I
Io
Ilol displacements
I I I I I 1ol
I 1 I 1 I Iol Fig. 6. Pure shear with principal strains parallel and perpendicular to the shear zone walls. In (b) and (c) the fixed vertical pin line for shear zone parallel displacement is in the centre of the illustration. This illustrates the "cream cake" effect of pure shear and a geometry not seen at Telfer. ( F r o m fig. 26.24 of Ramsay and Huber, 1987).
processes dominates the rock and hence detailed textural evidence for volume loss is hidden. However, geometrical considerations suggest that volume loss during cleavage development was the dominant process, although we can not rule out small components of pure shear. In the following analysis, we assume that all the flattening is due to volume loss, and the influence of pure shear was negligible. Volume loss and shear strain can be quantified using the method of Ramsay and Huber (1987) (also see Coward and White, 1988) from the measured R : X Z strain ratios and cleavage/bedding angle for each sample. The value of the simple shear strain y is dependent on the order of the addition of shear strain:
199
either before or after volume loss. But, volume loss is independent of the order in which the deformation components are added giving the same result whether volume loss occurred before or after simple shear. The results of these calculations are shown in Figs. 5d and 5e. Shear strain before volume loss varied between 7= 0.3 and 0.9, and shear strain after volume loss varies between 7=0.4 and 1.4. These differences are minor compared to the shear strain calculated on the erroneous assumption that the curvature in the cleavage into the deformation zone was all due to simple shear, where the variation in y is between 0.07 and 4.5. Strain data on the Flinn plot (Fig. 4) are mostly along the abscissa (oblate field) making excursions into less oblate space suggesting local zones of simple shear superposed on existing flattening fabrics. For this reason volume loss is interpreted to predate simple shear. Volume loss varies from zero in the hanging wall and footwall to about 40% in the host rock to the MVR, and as high as 500/0 (Fig. 5e). Displacement along the MVR calculated from the shear strain (see appendix) is about 6 m. This measure of the slip is that generated in the MVR and host rocks by the ductile strain, and does not include slip on discrete bedding-parallel fault planes without strain in the host rock. However, balanced cross-sections suggest that fault displacements including bed-parallel slip are mostly less than 10 m in the open pits (Hill, 1990). Discussion The above results show that mineralisation in the MVR is coincident with the most deformed rocks, but the shear strain in the MVR is low (7 < 1.5 ) and displacements were likewise low (probably less than 10 m). Assuming that the pure shear component was minor, volume loss calculations on the matrix of the host rock indicate a loss of about 40% of material in the horizon. But these do not include the volume addition by quartz veins which clearly
200
cut the cleavage and shape fabric; quartz veining is highly variable but visually estimated at up to 70% over 0.5 m intervals. Volume loss in the siltstones of the MVR contrasts with the volume gain by veining along the same horizon. Given the opposite nature of these two processes it is improbable that they operated synchronously within the same finite-element of rock, although volume redistribution is a possibility. As veins clearly crosscut the strained siltstones, volume loss logically preceded volume addition by veining. Along the MVR, volume loss involved the expulsion of a fluid phase (mostly water) and may have been accompanied by dissolution, but textural evidence for this is masked by the kaolinite overprint. The fluid expulsion represents an increase in permeability along the MVR relative to the far-field host-rock permeability. Subsequently, the same porous plane became the focus for mineralising fluids. It appears to have had a low cohesion and reactivated with the addition of quartz-sulphide veins. Our results from the strain studies confirm the results from balanced cross-sections which show that the MVR is not a dbcollement, and any dbcollementis probably hundreds of metres below the surface. Mineralisation is in the crestal region of asymmetric anticlinal domes, developed in the tip region of blind thrusts or steep reverse faults which could have acted as conduits for fluids below the present level of exposure (Hill, 1990). The stockwork vein sets and bedding-parallel reefs, such as the MVR, are interpreted to have formed from an overpressured fluid system (Hill, 1990).
Conclusions Mineralisation in the MVR is coincident with the most deformed rocks which show significant volume loss: shear strain was a minor component of the deformation (~, < 1.5 ). Volume loss in the siltstone host to the MVR was up to 50% and contrasts with the volume gain
J.R. VEARNCOMBE AND A.P. HILL
by veining along the same horizon. As veins clearly cross-cut the strained siltstones, volume loss probably preceded volume addition by veining. The same plane which was a conduit for material removal, has a high permeability and reactivated with the addition of quartz-sulphide veins. Implications are that strata-parallel mineralised zones with curving deformation fabrics and local high strains are not necessarily the result of bedding-plane slip alone, but may include significant flattening accommodated through volume loss. Importantly, a simplistic examination of the shear zone kinematics and vein geometry is not an adequate treatment of the deformation. It is unlikely that strain studies will form part of an exploration program, but exploration and mine geologists working with shear zones need to be aware of of the different displacements and strains which comprise a shear zone. In particular, volume loss as well as simple shear can be important at low metamorphic grades. Reactivation of surfaces with a low cohesion, such as the MVR, is confirmed as a major control on shear zone-hosted mineralisation.
Acknowledgements Telfer Gold Mine supported two visits to Telfer by JRV and kindly funded research for APH's Honours project. Joanne Cannon and Cecilia D'Ercole helped with the strain determinations, Nicola Netherway (n~e Goellnicht) assisted with sample collection. Nicola Netherway and John Ridley commented on an earlier draft of this manuscript. Telfer Gold Mine is thanked for permission to publish this work.
Appendix This paper refers to a number of structural parameters deduced by a variety of well established techniques published in the structural geology literature. The following are a brief de-
201
STRAIN AND DISPLACEMENTIN THE MIDDLE VALEREEF AT TELFER, WESTERN AUSTRALIA
scription of the parameters and techniques, but the reader should examine the given references for a through treatment of the subjects. A cross-section drawn in the movement direction is said to balance when the area of the deformed section is equal to the area of the restored (undeformed) section (Dahlstrom, 1969). Thus, a balanced section is a geometrically possible section. At Telfer restoring the Triangle Zone is difficult. If pinned in undeformed beds to the NE, space problems are created on the restored section; that is gaps and unrealistic structures appear on the undeformed template. However, by using a pin line through the lower beds in the Triangle Zone the structure balances (Fig. 2 ). In this paper strains were determined according to the Rf/O method (Ramsay, 1967; Lisle, 1985). Rf/(J is a commonly used technique to determine finite strain on a surface containing elliptical markers each with long to short axis ratio of R f the shape of the final ellipsoid. ¢ is the acute angle made by the long axis of the ellipse to the average orientation of the long axes of all the ellipses. At Telfer, this average parallels the cleavage in the sample. Each strain marker or spot represents a point on a graph of Rfagainst q~. A statistical test of the index of symmetry Isymm is used to determine the symmetry of the data set. A symmetrical data set suggests that although the strain markers were not necessarily spherical, they had a random orientation before straining. Isymm is tested at the 95% confidence level according to the method of Lisle ( 1985 ). The g 2 is a test of the distribution of data on the Rf i e plot. An even distribution of data points is preferred, but are very difficult to obtain on samples at low strains with sample sizes of about 50. The final result, a measure of the strain in the measured section is determined from sets of standard curves on a file of standard graphs when compared with the data on the Rf/q~ plot. These standard curves and graphs are reproduced in Lisle (1985) and the results given here are the best solutions for the sections in
the XZ and YZ sections giving the average strain ratios R:XZ and R: YZ, respectively. A Flinn plot is a graph of the R:XY versus R: YZ. It is a popular portrayal of three-dimensional strain data, but suffers from the disadvantage of plotting ratios, and hence many geologists find it difficult to conceptualise the data. However, the presentation of data is made simpler in this paper by using two alternative parameters to describe the position of data on the Flinn plot. These are the k and d values. The d value (Ramsay and Huber, 1983, p. 202) represents the amount of deformation equivalent to the distance of the point from the origin on a Flinn plot where:
d= [ ( R : X Y - 1)2+ ( R : Y Z - 1 ) 2 ]
0.5
and k value represents the shape of the strain ellipsoid where:
k= ( R : X Y - 1 ) / (R: Y Z - 1) with oblate strains plotting along the abscissa where k = 0, plane strain where the intermediate Y axis remains unstrained (without volume loss) has a value k = 1 and, prolate strain k = oo plotting along the ordinate. The above strain parameters are usually determined assuming that the rock has not been subject to volume loss. However, a loss of volume in the rock is common at low metamorphic grade as dewatering, compaction and cleavage development occur. Absolute values for volume loss are often difficult to determine. However, in shear zones lacking volume change (simple-shear shear zones) the finitestrain fabric and its orientation are directly related (Ramsay and Graham, 1970) . Deviations from this precise relationship can be determined from finite-strain measurements and cleavage orientation relative to the movement vector of the shear zone, and are used to calculated volume loss in the shear zone. The mathematics involved are complex, but are determined using data in the XZ section and are detailed in Ramsay and Huber (1987, session 26) and Coward and White (1988). Coward
202
and White graph the strain ratio versus cleavage angle and contour the plot for amounts of volume loss and shear strain ~,. Volume loss estimates and shear strain ~, used in this paper are deduced by this method. Where the shear strain ~, varies across a shear zone displacement on the shear zone can be determined by the method of Ramsay and Graham (1970) from integrating the shear strain across the width of the shear zone:
D=f ~,dy where D is the total displacement and y is the distance across the zone. This applies to a displacement involving shear alone, or shear followed by volume loss. The technique is equivalent to calculating the area under a curve on a graph of shear strain against distance (e.g., Fig. 5d). Assuming volume loss followed by shear, the same displacement is given by: D = ~ 7ad dy where ~'ais the shear strain following volume loss, and d is the volume loss. References Barley, M.E. and Groves, D.I., 1989. Exploration significance of regional and local scale hydrothermal alteration patterns in greenstone belts. Miner. Energy Res. Inst. West. Aust., Rep. 65, 83 pp. Butler, R.W.H., 1982. The terminology of structures in thrust belts. J. Struct. Geol., 4: 239-245. Coward, M.P. and White, S.H., 1988. Deformation and Mineralisation. Short Course Notes, Tectonics Div., Geol. Soc. S. Afr., 453 pp. Dahlstrom, C.D.A., 1970. Structural geology in the eastern margin of the Canadian Rocky Mountains. Bull. Can. Pet. Geol., 18: 332-406. Dimo, G., 1990. Telfer Gold Deposits. In: F.E. Hughes
J.R. VEARNCOMBE AND A.P. HILL
(Editor), Geology of the Mineral Deposits of Australia and Papua New Guinea. Australas. Inst. Min. Metall. Monogr., 14:643-651. Goellnicht, N.M., 1987. Constraints on the timing and source of gold mineralization at Main Dome, Telfer, Western Australia. B.Sc. Honours thesis, The University of Western Australia (unpubl.), 78 pp. Goellnicht, N.M., Groves, D.I., McNaughton, N.J. and Dimo, G., 1989. An epigenetic origin for the Telfer gold deposits, Western Australia. Econ. Geol. Monogr., 6: 151-167. Hill, A.P., 1990. Structure of West Dome, Telfer, Western Australia and its significance to mineralisation and regional tectonics. B.Sc. Honours thesis, The University of Western Australia (unpubl.), 58 pp. Hobbs, B.E., Means, W.D. and Williams, P.F., 1976. An Outline of Structural Geology. Wiley, Chicester, 571 PP. Hossack, J.R., 1979. The use of balanced cross-sections in the calculation of orogenic contraction: a review. J. Geol. Soc. London, 136:705-711. Lisle, R.J., 1985. Geological Strain Analysis, A Manual for6 the Rf/(~ Method. Pergamon Press, Oxford, 98 pp. Laubscher, H.P., 1962. Die Zweiphase Hypothese der Jurafaltung. Eclogae Geol. Helv., 55: 161-166. McNaughton, N.J. and Goellnicht, N.M., 1990. The age and radiothermal properties of the Mount Crofton Granite, Telfer area, Western Australia. Aust. J. Earth Sci., 37: 103-106. Newmont Australia Ltd., 1991. Annual Report 1990. 64 pP. Ramsay, J.G., 1967. Folding and Fracturing in Rocks. McGraw-Hill, New York, N.Y., 568 pp. Ramsay, J.G. and Graham, R.H., 1970. Strain variation in shear belts. Can. J. Earth Sci., 7: 786-813. Ramsay, J.G. and Huber, M.I., 1983. The Techniques of Modern Structural Geology, Vol. 1: Strain Analysis. Academic press, London, pp. 1-308. Ramsay, J.G. and Huber, M.I., 1987. The Techniques of Modern Structural Geology, Vol. 2: Folds and Fractures. Academic press, London, pp. 309-700. Turner, C.C., 1982. The Telfer Gold Deposits, Western Australia: Stratigraphy, sedimentology and gold mineralization of the Proterozoic Yeneena Group. Unpubl. Ph.D. thesis, Univ. New England Armidale, N.S.W., 276 pp. Williams, I.R. and Myers, J.S., 1990. Paterson Orogen. Geol. Surv. W. Aust. Mem., 3: 274-286.