Three-Dimensional Imaging of the Gawler Craton

Chapter 6

Three-dimensional imaging of the Gawler Craton

6.1 Introduction

The late Archaean to Palaeoproterozoic Gawler Craton spans an area of roughly 530000 km2 across much of South Australia. While the 2-D survey, shown in Chapter 5, Figure 5.1, offered some insights into the Fowler and Nuyts Domain through close site spacing, it obviously only covers a small part of the craton. In order to better understand the larger-scale tectonic framework of the Gawler Craton, a much larger area needs to be covered with MT sites. This can be achieved through the deployment of a number of 2-D profiles starting from the central part of the Gawler Craton and crossing the margins. 2-D inversion of such data sets has become standard among the MT community and results can be obtained quickly. However, 2-D inversion routines are still reliant on the underlying assumption of 2-D geology, and in case of more complex 3-D structures, 2-D inversion should be carried out with caution. Often, subsets of data show 2-D behaviour (see Chapter 4), which can be inverted without neglecting information stored in the diagonal components of the impedance tensor. In the present case, successful 2-D inversions would still pose the problem of interpolation between the profiles, which can be doubtful in case of 3-D bodies. Recent developments in the field of 3-D inversion (Siripunvaraporn et al., 2005a) have moti- vated a 3-D modelling approach for the resistivity distribution of the Gawler Craton. A smaller 2-D array of eight sites across the Gawler Range Volcanics was deployed in 2005 (Maier et al., 2007) and Heinson et al. (2006) conducted a 2-D MT profile across the IOCG Olympic Dam deposit in the north-eastern part of the craton. These surveys, together with the data presented in Chapter 5, already cover a large portion of the Gawler Craton. In 2006, another 15 sites were deployed across the north-western part of the craton to create a rectangular grid of sites

78 6.2. Geology 79

130˚E 140˚E 120˚E 150˚E 110˚E

10˚S 10˚S

Kimberley 180

Arunta 20˚S 20˚S Pilbara 130

Musgrave 80 g r a 30 v Yilgarn i 30˚S t 30˚S −20 y Gawler u −70 n i t −120 s 40˚S 40˚S −170

130˚E 140˚E −220 120˚E 150˚E 110˚E Figure 6.1: Location of MT stations of the 3-D survey (marked as black triangles) on top of the Australian gravity map with a resolution of 0° 5 ′. Areas bounded by dashed lines show Archaean and Proterozoic terraines.

2 approximately 100 km apart. The ﬁnal grid spans an area of around 400000 km or about 1/20th of the Australian continent (Figure 6.1).

6.2 Geology

The oldest rocks in the Gawler Craton are the contemporaneous Archaean Sleaford and Mul- gathing Complexes, with the Mulgathing Complex residing in the Christie Domain (Figure 6.2). Swain et al. (2005) report emplacement ages of 2850-2510 Ma from U-Pb zircon ages. The 2480 2420 Ma Sleafordian Orogeny ended the emplacement of those complexes and led to − granulite facies metamorphism (Teasdale, 1997; Tomkins and Mavrogenes, 2002). The rem- nants of the Sleafordian Orogeny have been extensively reworked during later events. After a period of 400 million years of no tectonic activity, Proterozoic events between 2000 and 1500 Ma have largely shaped the Gawler Craton as seen today. In this time span, sedimentary processes appear to dominate in the 2000 1690 Ma interval and are followed by dominantly magmatic − processes in 1690 1500 Ma (Hand et al., 2008). − During the 2000 1690 Ma interval a number of large rift-basins developed, i.e. the now de- − formed Hutchinson Group along the eastern margin of the Gawler Craton (Parker and Lemon, 6.2. Geology 80

130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E 28˚S 28˚S

gvd09 20˚S Pilbara gvd08 gvd11 Gawler gvd12 gvd13 Tor OFFICER gvd10 Yilgarn 40˚S BASIN ren cpd02 s 120˚E 140˚E gvd07 ari FZ CHRISITIE rox39 Kar 30˚S Moondrah gvd16 gvd15 gvd14 cpd01 30˚S Gneiss Hinge gvd06 WILGENA gvd03 rox16 ie SZ lacootra SZ rab Tal gvd01 EUCLA Coo Yerda SZ RGLE ? RPIM roxe11 BASIN ? FOWLER SZ fow25 nibba FZ fow18 NUYTS Koo GAWLER RANGE

lbrinda Zon

fow11 Yar RMAH RKOB

32˚S VOLCANICS e 32˚S fow06 fow01 RMTI

RUNO

0 100 200 300 34˚S 34˚S 130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E

Station locations Hiltabe Suite Mount Woods Inlier Bathymetry intrusives gravity units Munjeela Granite Nawa Domain Mable Creek Ridge −60 −550 −50 −45 −40 −35 −30 −25 −20 −15 −10 St. Peters Suite Hutchinson Group −50 −900 −750 −600 −450 −300 −150 0 150 300 450 Peake Metamorphics Granitoids and equivalents 00 00 00 00 00 00 00 00 00 00 0 0 0 Sleaford and Fowler Domain Donington Suite and equivalents Mulgathing Complex m Coober Pedy Ridge Wallaroo Group Moondrah Gneiss paragneisses

Tunkilia Suite Figure 6.2: Left: MT stations on interpreted geology map of the Gawler Craton. Right: The gravity image of the same geographical coordinates. Many of the domain boundaries were derived from potential ﬁeld data sets, such as gravity and Total Magnetic Intensity information. Blue triangles – MT sites collected in 2005 (see Chapter 5); red triangles – MT sites collected in 2006; green triangles – 3-D survey conducted by Maier et al. (2007); brown triangles – subset of MT sites collected by Heinson et al. (2006); black triangles – collected by Matthew Scroggs in 2004 and re-processed by Selway (2006).

1982). At 1850 Ma, short-term compression during the Cornian Orogeny (Reid et al., in press) led to the emplacement of the Donington Suite granitoids to the east of the Hutchinson Group (Mortimer et al., 1988). Following the Cornian Orogeny was another extended period of rifting, leading to the deposition of several sediment packages along the eastern to northern margin, among which are the Wallaroo Group, the sediments within the Mt. Woods Inlier and Peake Metamorphics (Fanning et al., 1988, Figure 6.2). The deposited sedimentary basins have bi- modal magmatic suites associated with them. The Fowler Domain along the western margin of the Gawler Craton also contains both pelitic metasediments (Daly et al., 1998), and 1726 9 Ma ± old mafic metagabbros (Fanning et al., 2007). In the north-west of the Gawler Craton, metasediments in the Nawa Domain are thought to originate from a different source than the Archaean Gawler Craton, i.e. the Arunta Block to the north of the Craton (Payne et al., 2006). The 1730 1690 Ma Kimban Orogeny had a profound metamorphic influence on deposited metased- − iments. The Kalinjala Shear Zone is a major remnant of the deformation processes and is situated between the Donington Suite and Hutchinson Group in the south-eastern part of the Gawler Craton (Vassallo and Wilson, 2002; Thiel et al., 2005). Deformation of sedimentary sequences were reported in the Mt Woods Inlier and the Peake Metamorphics in the northern part of the Gawer Craton (Betts et al., 2003). To the west of the Craton, metamorphism reached 6.3. Two-dimensional magnetotelluric array across the Gawler Craton 81

amphibolite facies in the Fowler Domain (Teasdale, 1997). During the 1690 1500 Ma interval, igneous events were dominant in the Gawler Craton, − much more than sedimentary processes. The oldest intrusion within this period is the 1690 − 1670 Ma Tunkilia Suite in the central Gawler Craton, which forms an arcuate belt around the Nuyts Domain, while smaller discrete intrusions have also been reported in the Fowler Domain (Ferris and Schwarz, 2003). At 1630 Ma the alkaline, porphyritic rhyodacite of the Nuyts Volcanics erupted and were subsequently intruded by the 1620 1610 Ma St. Peter Suite − (Flint et al., 1990). Between 1595 and 1575Ma a large-scale magmatic event formed the Hiltaba Suite and the Gawler Range Volcanics, and form one of the largest felsic volcanic systems in the world (Daly et al., 1998). The Gawler Range Volcanics have a maximum thickness of about 1.5 km. The Hiltaba Suite comprises strongly fractionated granites to granodiorites

(>70 wt % SiO2), which consist of more mantle-derived material than the host rock in which they reside (Stewart and Foden, 2003). Several major shear zones, such as the N-S trending Yarlbrinda Shear Zone and the E-W trending Yerda Shear Zone (Figure 6.2), are believed to have been reactivated during the emplacement of the Hiltaba Suite granites and a general NW- SE shortening at 1590 1570 Ma. At 1585 Ma the St. Peter Suite was subsequently intruded − by the unfractionated Munjeela Granite, as indicated by potential ﬁeld data (see Figure 5.2). The Karari Fault Zone in the north-western part of the Gawler Craton was formed during the Karari Orogeny (1570 1540 Ma) and has reworked parts of the Coober Pedy Ridge and acted − within a shear-zone bounded domain within a transpressional belt of anastomosing shear zones (Teasdale, 1997). During the active time span 2000 1500 Ma, other events have been reported, at times based − on only a few samples. This shows the diﬃculty of obtaining a coherent picture of the processes in that time frame. A more detailed analysis of the events is described in Hand et al. (2008), however in the context of a large-scale MT survey presented in this chapter, those events are not discussed in detail.

6.3 Two-dimensional magnetotelluric array across the Gawler Craton

6.3.1 Survey details

Figure 6.2 shows the distribution of twenty-nine stations, marked as triangles, used in the 2-D array across the Gawler Craton. Color shading of the triangles refers to different stages of the data collection, which has taken place between 2003 and 2006. MT instruments developed by Adelaide University were used for this 3-D survey and usually recorded horizontal electric field components and three magnetic field components at 10 Hz or 20 Hz. The magnetic field 6.3. Two-dimensional magnetotelluric array across the Gawler Craton 82

was recorded using fluxgate magnetometers, which are sensitive in the period range between 10 104 s. Average site spacing is around 100 km between sites, yielding a total areal coverageof − 800 500 km for the entire 3-D survey. × Chapter 5 has highlighted details and results of the 2-D survey across the south-western part of the Gawler Craton. A subset of five stations has been chosen to increase the coverage of the 3-D survey to its south-western extent (see blue triangles in Figure 6.2). Stations marked with green triangles denote long-period MT stations established by Maier et al. (2007) in 2005 across the Gawler Range Volcanics. Individual sites were left in the field from between 14 to 20 days yielding MT responses of 10 104 s. To the north-east of the 3-D survey, stations roxe11, − rox16 and rox39 (bordeaux-red triangles in Figure 6.2) were selected from a MT survey across the iron oxide copper-gold Olympic Dam deposit (Heinson et al., 2006). Station rox16 is in the close vicinity to the ore deposit. Stations cpd01 and cpd02 (black triangles) are taken from a 2-D MT profile along the Stuart Highway, and were originally collected by Matthew Scroggs and reprocessed by Kate Selway. In 2006, I collected another 15 MT stations across the north-western part of the Gawler Craton, an area without previous coverage with MT sites due to access difficulties. The distribution of the 15 MT stations is limited to the availability of roads in the area, making inter-site spacing smaller than 100 km hardly feasible. In this survey, stations are distributed along off- road tracks, far away from towns. The southern part of this survey runs parallel to the railway line from Adelaide to Perth. Northernmost sites follow the Anne Beadell Highway.

6.3.2 MT data and responses

The measured time series of the sites used in the array were Fourier transformed using robust remote reference codes (Chave and Thomson, 1989), yielding MT impedances and magnetic transfer functions in the frequency domain. Figure 6.3 shows the MT impedance responses

for the oﬀ-diagonal components Zxy and Zyx, expressed as apparent resistivity and phase (for

responses for the diagonal elements see Appendix B on page 109). The plots show eight ρa and φ responses evenly distributed in log space between 32 and 4096 s and those responses were later used in the inversion process. Given the large site spacing of 100 km, the responses are not expected to be as similar between sites as in small-scale surveys (see Chapter 4 and Chapter 5). However, the phase characteristics of some stations are similar despite the large distances apart and can be attributed to geological domains. For example gvd06-10 show small phase values (< 25°) for periods up to 200 s, and increase for longer periods. The corresponding apparent resistivities are also small (usually < 50 Ω m) and increase for periods longer than 100 s. This behaviour is indicative for a sedimentary basin environment (see location of the Eucla and Oﬃcer Basin in Figure 6.2). Stations RGLE, RPIM, RMAH, RKOB, and roxe11 are situated on top of the Gawler Range 6.3. Two-dimensional magnetotelluric array across the Gawler Craton 83

cpd01 cpd02 fow01 fow06 fow11 101 102 103 101 102 103 101 102 103 101 102 103 101 102 103 104

103

102

101 App. resistivity

100 90

Phase 30

0 fow18 fow25 gvd01 gvd03 gvd06 101 102 103 101 102 103 101 102 103 101 102 103 101 102 103 104

103

102

101 App. resistivity

100 90

Phase 30

0 gvd07 gvd08 gvd09 gvd10 gvd11 101 102 103 101 102 103 101 102 103 101 102 103 101 102 103 104

103

102

101 App. resistivity

100 90

Phase 30

0 gvd12 gvd13 gvd14 gvd15 gvd16 101 102 103 101 102 103 101 102 103 101 102 103 101 102 103 104

103

102

101 App. resistivity

100 90

Phase 30

Figure 6.3: Apparent resistivity ρa(T ) in [Ωm] and phase φ(T ) in [°] plots of observed data of the 3-D survey. Shaded triangles and squares denote ρa and φ of the Zxy and Zyx components of the impedance tensor, respectively. The responses shown here were static shift corrected (see S6.4.1). Solid and dashed lines denote the Zxy and Zyx 3-D model responses, respectively. 6.3. Two-dimensional magnetotelluric array across the Gawler Craton 84

RGLE RKOB RMAH RMTI RPIM 101 102 103 101 102 103 101 102 103 101 102 103 101 102 103 104

103

102

101 App. resistivity

100 90

Phase 30

0 RUNO rox16 rox39 roxe11 101 102 103 101 102 103 101 102 103 101 102 103 104

103

102

101 App. resistivity

100 90

Phase 30

0 Figure 6.3: (cont’d) Apparent resistivity and phase plots of remaining stations of the 3-D survey. For detailed description see previous page.

Volcanics and also show similar behaviour in the phases. The phases of the Zyx-component start out at 30° for short periods but increase to about 70° at periods longer than 1000 s. The phases for the orthogonal component are more subdued, with values of 30° for short periods, which level at about 45°, indicating a 1-D distribution for the xy-mode. As highlighted in Chapter 5.3, station fow06 and fow11 have similar responses and are both situated on top of the St. Peter Suite granitoids. Figure 6.4 shows phase tensor ellipses on top of a Total Magnetic Intensity (TMI) image of the Gawler Craton. A TMI image is useful for delineating fault boundaries, since minerals with a high magnetic susceptibility (e.g. Magnetite, Ilmenite, Pyrrhotite) are often enriched in fault and shear zones. Figure 6.4 highlights major shear zones such as the Karari Fault Zone, in the north-west of the Gawler Craton and the anastomosing shear zones of the Fowler Domain (compare high-resolution TMI image in Figure 5.2 on page 56). The juxtaposed phase tensor ellipses are shaded according to the minimum phase as deﬁned in Equation (A.17) on page 108 (Caldwell et al., 2004). For short periods (42 and 85 s) the corresponding minimum 6.3. Two-dimensional magnetotelluric array across the Gawler Craton 85

Minimum phase nT 0 10 20 30 40 50 1000 1500 2000 2500 3000 3500 4000 4500

130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E 28˚S 28˚S

30˚S Karari FZ 30˚S

32˚S 32˚S

42 s 85 s

34˚S 34˚S 28˚S 28˚S

30˚S 30˚S

32˚S 32˚S

341 s 684 s

34˚S 34˚S 28˚S 28˚S

30˚S 30˚S

32˚S 32˚S

1024 s 2730 s

34˚S 34˚S 130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E Figure 6.4: Phase tensor ellipses on top of Total Magnetic Intensity map of the Gawler Craton. Ellipses are shaded according to the minimum phase values and have higher phases across the central part of the Gawler Craton. 6.3. Two-dimensional magnetotelluric array across the Gawler Craton 86 phases are small for stations west of the Karari Fault Zone and across the Gawler Range Volcanics (GRV) in the eastern part of the array. Minimum phases are larger for stations in the middle of the array, roughly between longitude 133°E and 135°E. Current flow, as defined by the orientation of the major axes, is primarily NNE-SSW-oriented, with exceptions across the GRV, where current flow is NNW-SSE. However, the resisivity distribution is more 1-D for those sites, hence no direction is preferred for current flow for shallow depths across the GRV. An exception to the overall NNE-SSW current flow are currents near sites gvd03 and gvd16, which have a WNW-ESE orientation. A likely cause are adjacent major shear zones, e.g. the Karari Fault Zone near gvd16 and the Tallacootra Shear Zone near gvd03. For intermediate periods between 100 and 1000 s the overall orientation of the phase tensor ellipses becomes more consistent in accordance with the fact that for these periods the subsurface information comes from a larger half-sphere underneath each station. Therefore the inductive responses become similar. In this period range the minimum phase values clearly show a different response for the central Gawler Craton with Φmin > 40° and stations west and north of the Karari Fault Zone and those along the eastern margin of the Gawler Craton with Φmin < 30°. Phase tensor ellipses are almost circular in the north-western part of the array, indicating a 1-D structure. For periods longer than 1000 s the discrepancy between the minimum and maximum phase enlarges due to the ocean effect, where the conductive ocean to the south of the array has a large influence on the current flow distribution, favouring the NNE-SSW-trending component. In conclusion, the phase tensor analysis indicates three major areas of induction, namely the conductive sedimentary basins west and north of the Karari Fault Zone, the “core” of the Gawler Craton between 133°E and 135°E and the consistent response of stations on top of the GRV for short and intermediate periods. Real and imaginary induction arrows in the Parkinson convention (see Equation (1.36) on page 11) are shown in Figure 6.5 for six different periods between 21 and 2730 s. For short periods, the induction arrows are dominated by resistivity structures close to the site locations and are not coherent between sites, except across the sedimentary basins to the west and the GRV, where arrows are small. Furthermore, the real and imaginary arrows are parallel, indicating 1-D or 2-D structure (1D if arrows are very small). Arrows are larger across the central part of the array and real and imaginary arrows are no longer parallel, in particular near major boundaries. For periods between 100 and 1000 s the arrows point away from the center of the craton. Most of the arrows show 2-D behaviour (real and imaginary arrows parallel), except for a few sites near the Karari Fault Zone. The inductive ocean effect causes the arrows to rotate towards the ocean for long periods, without much affecting the orientation or magnitude of the arrows across the Officer Basin in the north-west. The induction arrow responses show the same large-scale features as the phase tensor ellipses, i.e. the conductive sediments to the west, the resistive central core of the Craton, and a coherent behaviour across 6.3. Two-dimensional magnetotelluric array across the Gawler Craton 87

the GRV.

6.3.3 Inﬂuence of sedimentary basins

The Gawler Craton is surrounded by thick sedimentary sequences of the Eocene-Miocene Eucla Basin (< 500 m) to the west, the Oﬃcer Basin to the north-west (> 2500 m), and the Tertiary Torrens Basin (> 5 km) and Miocene-Pliocene Billa Kalina Basin (> 2000 m) to the east (see depth to Proterozoic basement map in Figure 6.5) (Borissova and Symonds, 1997). The Den- man Basin is part of the Eucla Basin and represents a north-south oriented thicker sequence of sediments of an old river system with thicknesses exceeding 2000 m. The Polda Basin lies about 150 km south of the 3-D survey area and is an east-west oriented trough with sediments between 2 and 4 km thick. Electrolytic conduction plays an important role in the contribution to the overall conductance of sediments. Electronic conduction is negligible as it is usually very small due to the high resistivity of the mineral constituents of rocks. The mechanism of dielectric conduction is due to displacement currents, which are essentially non-existent in the period-range used in MT (S1.3). Electrolytic conduction is dependent on the porosity and connectivity of the pores of the medium. The pore spaces are usually ﬁlled with conductive water, increasing the bulk conductance of the medium. These quantities are related by an empirical formula, also known as Archie’s Law (Archie, 1942):

m n ρs = aφ− s− ρw, (6.1)

where ρw is the conductivity of water, φ the porosity of the medium, s the fraction of pores containing water. The constants a,m,n take values of 0.5 a 2.5, 1.3 m 2.5 and n 2. ≤ ≤ ≤ ≤ ≈

If we assume a sediment with a porosity of 30 % and 50 % fluid-filled pore spaces, a pore- fluid conductivity of 5 S/m and use s = 5, a = 1.5 and m = 2, the bulk resistivity of the

sediment would be ρs = 13 Ω m. Low resistivities of around 10 Ω m for sediments are therefore not uncommon. Figure 6.6 shows four conductance maps derived from sediment thicknesses obtained from sediment depths to Neoproterozoic (Adelaidean) basement maps for sediment resistivities of 5, 10, 20 and 50 Ω m. In addition to the sediments, the conductance of the sea water was added. Therefore, the resulting conductance maps are a way of displaying inductive eﬀects of features not related to solid rock geology, but of the regolith and the conductive sea water. In general, the conductance of the sea water and the underlying marine sediments exceed 1000 S across the abyssal plains approximately 200 km south of the 3-D survey area. Across the continental shelf, sediments thicknesses decrease and the water depth does not exceed 200 m resulting in conductances of 300 400 S. For sediment conductivities of 5 10 Ω m, the − − 6.3. Two-dimensional magnetotelluric array across the Gawler Craton 88

Depth to Proterozoic basement [m] (above sea level) −4500 −3500 −2500 −1500 −500 500

130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E 28˚S 28˚S 21 s Lake Eyre Basin 85 s

Officer Basin

Billa Kalina 30˚S Basin 30˚S

Eucla Basin Tor ren

32˚S s Basin 32˚S Denman Basin

0.5 Polda Basin 0.5 Great Australian Bight Basin 34˚S 34˚S 28˚S 28˚S 170 s 341 s

30˚S 30˚S

32˚S 32˚S

0.5 0.5

34˚S 34˚S 28˚S 28˚S 1024 s 2730 s

30˚S 30˚S

32˚S 32˚S

0.5 0.5

34˚S 34˚S Figure 6.5: Real (black) and imaginary (red) Parkinson induction arrows for six periods on top of Depth to Proterozoic basement maps. Top left image illustrates positions of sedimentary basins throughout the Gawler Craton. 6.3. Two-dimensional magnetotelluric array across the Gawler Craton 89

Conductance map [S] 0 100 200 300 400 500 600 700 800 900 1000

130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E

28˚S 28˚S

30˚S 30˚S

32˚S 32˚S 5 Ohm.m 10 Ohm.m 0.5

34˚S 34˚S a b 36˚S 36˚S

28˚S 28˚S

30˚S 30˚S

32˚S 32˚S 20 Ohm.m 50 Ohm.m

34˚S 34˚S c d 36˚S 36˚S 130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E

0 10 20 30 40 50 Minimum phase Figure 6.6: Conductance maps derived from depth to Adelaidean basement (a,b,d) and from depth to Proterozoic (c) sediment thicknesses for 5 (a), 10 (b), 20 (c) and 50 Ω m (d). The depth to Adelaidean basement diﬀers from the depth to Proterozoic only in the thick sediment packages across the Torrens Basin east of the Gawler Craton (compare Figure 6.5). The conductance of the sea water was added in sub-ﬁgure a,b and d. Phase tensor ellipses and induction arrows at a period of 500 s were added for comparison. 6.4. 3D inversion of the Gawler dataset 90

conductance of the shelf area can reach up to 1000 S in the Polda Basin and the Denman Basin near site fow25. The Officer Basin to the north-west has a conductance of 1000 S or more for ρ = 5 10 Ω m. For higher resistivities ρ , the conductance of the sedimentary basin lies at s − s about 100 to 500 S. The Lake Eyre Basin in the north-east and the Torrens Basin to the east of the Gawler Craton have conductances of more than 500 S. If the Adelaidean sediments in the Torrens Basin are also taken into account (see Figure 6.6c), the conductances far exceed 1000 S and have a significant inductive effect on stations in the eastern part of the array. Comparison of the conductance maps with phase tensor (Figure 6.6a) and induction arrow responses (Figure 6.6b) suggest that the sediment resistivities are well represented by resistivities between 10 and 20 Ω m. The induction arrows show a response to the inductive effect of the

sedimentary basins. High resistivities ρs = 50 Ω m can hardly explain the behaviour of induction arrows for intermediate periods (100 500 s). Induction arrows are small over the Oﬃcer − Basin, but increase in size over the core of the Gawler Craton and point towards the conductive sedimentary basins for periods of 100 1000 s. The inﬂuence of conductive sedimentary basins − has to be taken into account when interpreting inversion results.

6.4 3D inversion of the Gawler dataset

The impedance responses of the twenty-nine MT stations were inverted for 3-D structure (Siripunvaraporn et al., 2005a). The code requires the input of the same period range for all stations, which means that period band limitations in one station effects the entire array unless a larger error floor for that particular station is chosen. Finding a compromise between the widest possible period band-width and avoiding too many large error bars (e.g. more than 100 %), led to the inversion of impedance tensor responses between 32 and 4096 s. Periods smaller than 32 s are not crucial for the site spacing of about 100 km for the 3-D survey. Us- ing the skin-depth relationship (Equation (1.15)), a 100 Ω m half-space yields a skin-depth of only 28 km for a period of 32 s. In sedimentary basins, the skin-depth would be even smaller. This shows that the inverse model has likely no multi-site constraints for short periods and is reliant on single station responses. The longest periods are more important for a model of this dimension, e.g. in a 10 Ω m halfspace responses of a period of 4096 s already yield a skin-depth of 100 km. However, the equivalent half-space resistivity of the Gawler Craton will be larger than 10 Ω m (Maier et al., 2007), which leads to a significant increase in skin-depth. We have inverted all four components of the impedance tensor to be able to address three-dimensionality in the data set. Figure 6.7 shows the grid used in the inverse process. It measures 40 54 26 cells in x,y,z × × { } direction totalling to a model size of 7000 7000 4500 km. Horizontal cell dimensions in the × × vicinity of station locations is 20 20 km, in order to allow smooth changes in resistivity × 6.4. 3D inversion of the Gawler dataset 91

Layer 1 at depth 0.00 to −0.30 kilometers

3000 3

2000 2.5

1000 2 north) to 0 1.5 x in km (south

−1000 1

−2000 0.5

−3000 0

−3000 −2000 −1000 0 1000 2000 3000 y in km (east to west) Layer 28 between 0.00 to 20.00 kilometers 0

−500

−1000

−1500

−2000

z in km −2500

−3000

−3500

−4000

−4500

−3000 −2000 −1000 0 1000 2000 3000 x in km (south to north) Figure 6.7: Grid used in the 3-D inversion of the 3-D survey with 40 54 26 cells in x,y,z × × { } direction totalling to a model size of 7000 7000 4500 km. The ocean has been included as × × a fixed structure with a resistivity of 0.3 Ω m (top figure), while the other cells were free to change starting from a 100 Ω m half-space. Bottom figure shows a N-S cross-section through the starting model. 6.4. 3D inversion of the Gawler dataset 92

MT IMPEDANCES

2D OCCAM Inversion 3D Inversion w/o +inverting for static shift static shift

Static Shift Factors Assessment of Residual of apparent resistivities 3D Inversion with and phases static shift Static Shift Factors

3D Inversion with static shift

FINAL MODEL

Figure 6.8: Flow chart for obtaining static shift factors. between stations and improve fit to the data. The grid was padded with 9 cells in all horizontal directions increasing by a factor of 1.6 towards the edges. The padding of cells is a trade- off between minimising the number of cells in the model, thus reducing computational time, and ensuring a sufficiently large model so that the inverse process can accommodate structure well within its boundaries. In the vertical direction, the cell size increases progressively by a factor of 1.4 starting from a 300 m thick surface layer. Starting model of a simple half- space has a conductivity of 100 Ω m with the ocean as a fixed structure and a resistivity of 0.3 Ω m. Seafloor topography was approximated from Smith and Sandwell (1997). Layered Earth starting models were also tried but the 3-D code tends to keep the horizontal boundaries even after a few iterations and is appears strongly affected by the a-priori layering. A uniform half-space is therefore more appropriate. The error floor on the impedance tensor components was set to 10 % and 15 % for the off- diagonal and diagonal elements, respectively. The 3-D code allows to set the model length scales δ individually for all directions. The model length scales control the smoothness in each direction. These parameters, together with the smoothing parameter τ control the model covariance Cm. During the first inversion run the values were set to τ = 5 and δx,y,z = 0.3, achieving a first-pass smooth model which displays the main features in the model. The model with the best fit to the data was subsequently used as the starting model into a second inversion, with the parameters set to τ = 5 and δx,y,z = 0.1. Once the target rms value is reached the program tries to reduce the norm while keeping the rms misfit at the desired level. 6.4. 3D inversion of the Gawler dataset 93

6.4.1 Static shift correction

An inherent problem in MT data analysis is the static shift of the apparent resistivities due to small-scale inhomogeneities near the surface. Chapter 2.1 introduced three ways to overcome this problem by using constraints from other methods, spatial filtering and inverting for static shift. Other geophysical datasets covering the array are magnetic and gravity data, which are unusuable for static shift corrections. Spatial filtering is not feasible due to the size of the array and the 3-D code does not accommodate automatic static shift correction. I have therefore extended the original Fortran code by Siripunvaraporn et al. (2005a) to accommodate static shift corrections obtained elsewhere. Given the large inter-site spacing and nonexisting knowledge of the small-scale structures acting as galvanic distorters I have tried two other ways to obtain static shift factors. The first possibility is to run 2-D inversion of lines across the array, given the station grid is regular (Figure 6.9). The 2-D Occam inversion allows the output of static shift factors after the inversion process (deGroot Hedlin and Constable, 1990). The Occam code freely assigns the static shift values when deemed necessary. The static shift factors sxy and syx (Figure 6.9), for the xy- and yx-component of the impedance tensor Z, respectively, can then be multiplied with the Z components as input into the 3D inversion. A disadvantage of this method is twofold:

i) In most cases the number of stations along each of the 2D lines crossing the array will only be limited, resulting in larger error of the static shift factor; ii) The static shift factors are strictly only obtained from the oﬀ-diagonal components of the impedance tensor.

Here, I make the assumption that static shift is due to charge accumulation and therefore only aﬀects the electric ﬁeld in x, y . Given this notion, the Z components are multiplied as follows: { }

Z Z = s Z Z Z Z = s Z Z . (6.2) xx′ xy′ xy · xx xy yx′ yy′ yx · yx yy h i h i h i h i Z′ denotes the static shift adjusted impedance tensor.

The second approach to adjust for static shift is to assess the residual distribution of ρa and φ for all Z components. If the model phase responses are randomly distributed around the observation responses, the ρa are expected to be randomly distributed, too. In cases where frequency-independent under- or overﬁtting occurs, the impedance tensor will be adjusted accordingly and the 3D inversion repeated with Z′. Another advantage of the second approach is that all four impedance tensor components can be examined, while the 2-D Occam approach was restricted to the examination of only oﬀ-diagonal component static shifts.

Residual analysis indicate a frequency-independent underﬁtting for some stations in the ρa model responses of the inversion without static shift (Figure 6.10). The ρa-residuals r have 6.4. 3D inversion of the Gawler dataset 94

2 2 132˚E 136˚E 128˚E 140˚E Line 1 3000 xy−component 1 Line 2 1 km 2500 tor) Line 3 tor) 2000 28˚S 28˚S t fac 0 Line 4 0 t fac gvd13 0 100 200 300 1500 shif shif

line1 1000 ic −1 −1 ic gvd07 gvd16 line2 500 30˚S 30˚S rox016 0 log(stat −2 −2 log(stat line3 gvd01 −500 line4 −1000 −3 −3 0 fow11 RMAH 0 100 200 300 400 500 600 700 −1500 m 32˚S 32˚S Distance [km] 0 −2000 RUNO 2 2 −2500 Line 1 yx−component 000 −3000 −1 1 Line 2 1 −2000 34˚S −3000 34˚S −3500 Line 3 −4000 0 Line 4 0 −4500

0 −5000 tic shift factor) −1 −1 tic shift factor) 36˚S 36˚S −5500 sta sta

log( −2 −2 log( −6000 136˚E 128˚E 132˚E 140˚E −3 −3 0 100 200 300 400 500 600 700 Distance [km] Figure 6.9: Left: location of the 2-D lines used to invert for static shift factors using the Occam 2-D code (deGroot Hedlin and Constable, 1990). Right: static shift factors for the xy- and yx components of the impedance tensor, obtained from the 2D Occam inversion. Highest negative static shifts are observed between 200-400km away from the eastern edge of the MT array. been defined as follows: log (ρ ) log (ρ ) r = 10 amod − 10 aobs , (6.3) log10 (σρa ) with σρa denoting the error of ρa. Visual inspection and assessing the overall misfit of 3.34 of the inversion results indicate worst fit for the inversion without static shift correction. The rms misfit for the static shift adjusted inversion approaches are better(rms = 2.72 and rms = 1.97). The 2D Occam approach finds static shift factors that achieve a better overall misfit (rms =

2.72) of Z, but some stations still exhibit frequency-independent underfitting of ρa responses Figure 6.10. While this approach should in theory achieve good results, in practice the lack of a sufficient number of stations along each of the 2D lines will lead to inexact static shift factors. The second approach of manually adjusting impedances of stations with a very high ρa residual has achieved a much better overall misfit than the 2-D Occam approach (rms = 1.97). Table B.1 shows the static shift factors obtained from the residual analysis. Figure 6.10 shows that the ρa residuals have been greatly reduced without affecting the φ residuals much. Extreme outliers in the ρa residuals across the stations are least troublesome in this approach.

6.4.2 Inversion results

Figure 6.11 shows the 3-D inversion model obtained from the MT data of 29 sites in a top view for every second layer. The minimum period used is 32 s limiting the resolution of near- surface structures. The limit strongly depends on the resistivity of the surface sediments/rocks. If the resisitvity in the vicinity of a MT site has a resistivity of 1 Ω m, the skin-depth for a period of 32 s is 2.8 km. With this in mind it is possible to detect the inﬂuence of sedimentary basins, especially if the thickness exceeds a few kilometers, as is the case along the margins of 6.4. 3D inversion of the Gawler dataset 95

Resisti vity residuals in xy Phase residuals in xy Resistivity residuals in yx Phase residuals in yx 5

without static shift correction 4

3 20 20 20 20 a a φ ρ φ ρ σ σ σ σ 2 / / / /

obs 10 10 obs 10 10 obs obs a a φ φ ρ ρ

− 1 − − − mod 0 mod 0 mod 0 0 mod a a φ φ ρ ρ 0

−10 −10 −10 −10 dual dual dual dual −1 Resi Resi Resi −20 −20 Resi −20 −20 −2

4 4 4 4 −3 30 30 30 30 3 3 3 3 20 20 20 20 −4 2 2 2 2 10 10 10 10 −5 1 1 0 1 1 log10(T) 0 Station number log10(T) Station number log10(T) 0 Station number log10(T) 0 Station number Resistivity residuals in xy Phase residuals in xy Resistivity residuals in yx Phase residuals in yx 5 static shift correction from 2-D OCCAM 4

3 20 20 20 20 a a φ φ ρ ρ σ σ σ σ 2 / / / /

obs 10 10 obs 10 10 obs obs a a φ φ ρ ρ

− − 1 − −

mod 0 0 mod 0 0 mod mod a a φ φ ρ ρ

−10 −10 −10 −10 dual dual dual dual −1 Resi Resi Resi −20 −20 Resi −20 −20 −2 4 4 4 4 −3 30 30 30 30 3 3 3 3 20 20 20 20 −4 2 2 2 2 10 10 10 10 −5 1 1 1 1 log10(T) 0 Station number log10(T) 0 Station number log10(T) 0 Station number log10(T) 0 Station number

Resistivity residuals in xy Phase residuals in xy Resistivity residuals in yx Phase residuals in yx 5

static shift correction from residual analysis 4

3 20 20 20 20 a a φ φ ρ ρ σ σ

σ σ 2 / / / /

obs 10 10 obs 10 10 obs obs a a φ φ ρ ρ 1 − − − −

mod 0 0 mod 0 0 mod mod a a φ φ

ρ ρ 0

−10 −10 −10 −10 dual dual dual dual −1 Resi Resi Resi Resi −20 −20 −20 −20 −2

4 4 4 4 −3 30 30 30 30 3 3 3 3 20 20 20 20 −4 2 2 2 2 10 10 10 10 −5 1 1 1 1 log10(T) 0 Station number log10(T) 0 Station number log10(T) 0 Station number log10(T) 0 Station number

Figure 6.10: Residuals of ρa and φ for the off-diagonal components of the impedance tensor. The final rms misfits of the different inversion schemes are: 3.34, 2.72 and 1.97, from top to bottom. the Gawler Craton. The western-most stations constrain a conductor (1 10 Ω m) at shallow − depths between 1 km and about 5 km spatially coinciding with the Eucla and Officer Basin. A shallow conductor (10 Ω m) to the north-east of the array can be attributed to Neoproterozoic meta-sediments in the Billa Kalina Basin (Figure 6.5). A moderately conductive region is located in the central part of the array with resistivities ranging from 10 50 Ω m. These − zones do not correlate with sedimentary sequences and must have different origin. Similarily, a localised conductor near stations fow01 and fow 06 is also not related to sediments. However, both localities coinicide with the highly fractionated 1590 1570 Ma Hiltaba Suite intrusives, − which had significant mineralisations and fault zone reactivation associated with them. The upper crust is very resistive with > 1000 Ω m to the south and north of the array. In the southern part, the high resistivities can be spatially attributed to the St. Peter Suite and the Gawler Range Volcanics. The middle crust beneath the array between 10 to 30 km is uniformly resistive (100 − 10000 Ω m), except towards the southern part of the array, where resistivities are lower. With 6.4. 3D inversion of the Gawler dataset 96

West East West East West East 300 300 300 4 Layer 1 at depth 0.00 to −0.30 kilometer Layer 10 at depth −14.75 to −20.94 kilometer Layer 18 at depth −227.94 to −319.41 kilometer

200 200 200 3

100 100 100 north) 2 to 0 0 0 1 −100 −100 −100 x in km (south 0 −200 −200 −200

−300 −300 −300 −1 −400 −300 −200 −100 0 100 200 300 400 −400 −300 −200 −100 0 100 200 300 400 −400 −300 −200 −100 0 100 200 300 400 300 300 300 4 Layer 3 at depth −0.72 to −1.31 kilometer Layer 11 at depth −20.94 to −29.62 kilometer Layer 20 at depth −447.51 to −626.81 kilometer

200 200 200 3

100 100 100 north) 2 to 0 0 0 1 −100 −100 −100 x in km (south 0 −200 −200 −200

−300 −300 −300 −1 −400 −300 −200 −100 0 100 200 300 400 −400 −300 −200 −100 0 100 200 300 400 −400 −300 −200 −100 0 100 200 300 400 300 300 300 4 Layer 5 at depth −2.13 to −3.28 kilometer Layer 12 at depth −29.62 to −41.77 kilometer Layer 22 at depth −877.81 to −1229.21 kilometer

200 200 200 A 3 B rth) 100 100 100 2 o no

th t 0 0 0 D C 1 −100 −100 −100 x in km (sou 0 −200 −200 E −200

−300 −300 −300 −1 −400 −300 −200 −100 0 100 200 300 400 −400 −300 −200 −100 0 100 200 300 400 −400 −300 −200 −100 0 100 200 300 400 300 300 300 4 Layer 7 at depth −4.90 to −7.16 kilometer Layer 14 at depth −58.78 to −82.59 kilometer Layer 24 at depth −1721.21 to −2410.01 kilometer

200 200 200 A 3 B rth) 100 100 100 2 o no

th t 0 0 0 D C 1 −100 −100 −100

x in km (sou 0 −200 −200 E −200

−300 −300 −300 −1 −400 −300 −200 −100 0 100 200 300 400 −400 −300 −200 −100 0 100 200 300 400 −400 −300 −200 −100 0 100 200 300 400 300 300 300 4 Layer 9 at depth −10.32 to −14.75 kilometer Layer 16 at depth −115.93 to −162.60 kilometer Layer 26 at depth −3374.31 to −4724.31 kilometer

200 200 200 A 3 B rth) 100 100 100 2 o no

th t 0 0 0 D C 1 −100 −100 −100 x in km (sou 0 −200 −200 E −200

−300 −300 −300 −1 −400 −300 −200 −100 0 100 200 300 400 −400 −300 −200 −100 0 100 200 300 400 −400 −300 −200 −100 0 100 200 300 400 y in km (east to west) y in km (east to west) y in km (east to west) Figure 6.11: View of every second horizontal slice through the 3-D resistivity model. Color scale is in log10 Ω m. The model is characterised by a conductive cover due to sediments and likely Hiltaba Suite related mineralisation systems, underlain by a resistive crust between 10 and 30 km. At depths below 30 km ﬁve distinct conductive zones (labelled A to E) become apparent. Features B and C are situated underneath the IOCG Olympic Dam and the Au- dominated mineralisation systems, respectively. 6.4. 3D inversion of the Gawler dataset 97

increasing depth a number of deep crustal conductors emerge in the model, e.g. near site gvd01 in the center of the array (labelled C in Figure 6.11,B.6,B.7) and near fow25 to the south-west (labelled D). Another very conductive (1 10 Ω m) region labelled A in the central northern − part of the Gawler Craton is located about 20 km beneath the surface and extends deeper into the earth. At depths of about 30 to 50 km those features are clearly visible together with zones of enhanced conductivity to the south-east (labelled E) near site RMTI and north of site rox16 (adjacent to the IOCG Olympic Dam deposit and labelled B). Feature C is connected to a number of near-surface conductors in the central part of the array (see dashed lines in Figure B.6 and B.7). A similar behaviour was reported underneath the Olympic Dam deposit, where a low-resistivity region is situated underneath the deposit and is attributed to upward

movement of CO2-bearing fluids and associated precipitation of graphite along grain boundaries (Heinson et al., 2006). The 3-D model presented here does also show the conductor (Figure B.6 and B.7), however with much less resolution due to fewer sites. The sensitivity of the model decreases significantly for depths larger than 100 km due to the skin-depth relation, resulting in a decline in model resolution from about horizontal layer 16. Furthermore, the resistivity of the mantle is expected to be more uniform than the crustal lithosphere. Features displayed in Figure 6.11 at layer 16 smear out at greater depths and no detail is added to the model due to the skin-depth relationship and the maximum period of 4096 s used. The deepest layers of the model converge to the 100 Ω m half-space of the starting model, as would be expected from the very low sensitivity at depths greater than 500 km. Figure 6.3 shows the model responses in comparison to the observed data for the off-diagonal components of the impedance tensor. The misfits are usually very good, with a few exception at site RKOB, gvd11 and gvd14. Even though static shift corrections have been undertaken, there is still an ambiquity in the apparent resistivity responses of the model. Therefore it is worth comparing phase information and Figure 6.12 illustrates phase tensor ellipses for the observed and modelled data set for four evenly distributed periods. The main orientation α of the ellipses is a function of all four phase tensor components (Appendix A.3) and is therefore not only a measure of good fit to each individual phase component but also a measure of the components in relation to each other. A similar orientation of the ellipses increases confidence in the structure found. It should be noted that the ellipse orientations are better replicated in the inversion with static shift correction, compared to the modelled phase tensor ellipses of the inversion without static shift (Figure 6.12). The same improvement can be observed in the ratio of the maximum and minimum phase for longer periods above 500 s (i.e. the ellipticity of the ellipses), with the static-shift corrected inversion achieving higher ratios (more elliptical ellipses) that are more similar to the observed data ellipses. 6.4. 3D inversion of the Gawler dataset 98

Modelled data response Modelled data response Observed data response with static shift correction without static shift correction 130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E 28˚S 28˚S

30˚S 30˚S

32˚S 32˚S

32s 32s 32s 34˚S 34˚S 28˚S 28˚S

30˚S 30˚S

32˚S 32˚S

128s 128s 128s 34˚S 34˚S 28˚S 28˚S

30˚S 30˚S

32˚S 32˚S

512s 512s 512s 34˚S 34˚S 28˚S 28˚S

30˚S 30˚S

32˚S 32˚S

2048s 2048s 2048s 34˚S 34˚S 130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E 130˚E 132˚E 134˚E 136˚E 138˚E

0 5 10 15 20 25 30 35 40 45 50 55 Minimum phase Figure 6.12: Comparison of phase tensors calculated from observed (left column) and modelled (middle and right column) data. Middle column denotes responses from the static shift corrected 3-D model using residual analysis, whereas the right column shows phase tensors calculated from the 3-D inversion without static shift correction. 6.5. Discussion 99

Depth to Proterozoic basement [m] (above sea level) −4500 −3500 −2500 −1500 −500 500

Layer 4 at depth −1.31 to −2.13 kilometers log(rho_a) 130˚E 132˚E 134˚E 136˚E 138˚E 300 4 28˚S 85 s 200 3

100 30˚S north) 2 to 0 1 −100 32˚S x in km (south 0 −200 0.5

−300 −1 34˚S −400 −300 −200 −100 0 100 200 300 400 y in km (east to west) Figure 6.13: Comparison of the horizontal 3-D model slice at 1.3 km to 2.1 km depth (left) and the Depth to Proterozoic basement map (right). High conductivities along the eastern and western margin of the array can be assigned to high conductances of sedimentary basins (Chapter 6.3.3). However, low resistivities in the central part of the array are not related to thick sediments and are likely due to mineralisation associated with the emplacement of the Hiltaba Suite.

6.5 Discussion

The final resistivity model of the Gawler Craton can be divided into three distinct zones with depth. The top 10 km are dominated by conductive sediment cover to the east and west of the array (Figure 6.13). The central part of the Gawler Craton also shows enhanced conductivities of the order 5 100 Ω m and spatially coincides with surace expressions of the 1590 1570 Ma − − Hiltaba Suite (Figure 6.14). Between depths of 10 30 km the middle crust is generally resistive − with resistivities exceeding 100 Ω m over an area of more than 200000 km2. These resistivity values are typical for Archaean crust (Gough, 1986) and have been reported amongst others in the North American Slave Craton (Jones et al., 2001). At depths larger than 30 km, five distinct zones of high conductivity appear in the model (labelled A to E in Figure 6.11,B.6,B.7). The resistivities usually range between 1 and 10 Ω m. Figure B.6 and B.7 show that the deep conductors are connected to the shallow zones of high conductances. The geometry and the spatial distribution of the zones of enhanced conductivity near the surface and the connection to their deeper counterparts suggests that the 3-D model shows processes related to the emplacement of the Hiltaba Suite. The Hiltaba Suite has high-T fractionated felsic rocks and coeval mafic magmatism, suggesting a mantle involvement in the petrogen- esis. Even though the Hiltaba Suite was emplaced in a narrow time-frame, the spatial distribution of the Hiltaba Suite and their associated mineralisation systems (Daly et al., 1998) divide it into two terraines, one of which is the IOCG Olympic Dam province (Skirrow et al., 2002) (near 6.5. Discussion 100

130˚E 132˚E 134˚E 136˚E 138˚E Layer 5 at depth −2.13 to −3.28 kilometers 4 28˚S 28˚S 300

gvd09 20˚S Pilbara gvd08 200 gvd11 Gawler 3 gvd12 gvd13 Tor OFFICER gvd10 A Yilgarn ren 40˚S BASIN orth) 100 cpd02 s 120˚E 140˚E 2 o n gvd07 ari FZ CHRISITIE rox39 Kar B th t 0 30˚S Moondrah gvd16 gvd15 gvd14 cpd01 30˚S

Gneiss (sou 1 Hinge gvd06 WILGENA gvd03 rox16 −100 ie SZ lacootra SZ rab Tal gvd01 EUCLA Coo RGLE km x in ? C RPIM roxe11 0 BASIN ? −200 FOWLER SZ fow25 D nibba FZ fow18 NUYTS Koo GAWLER RANGE

lbrinda Zon

fow11 Yar RMAH −300 −1 RKOB −400 −300 −200 −100 0 100 200 300 400

32˚S VOLCANICS e 32˚S fow06 fow01 Layer 13 at depth −41.77 to −58.78 kilometers RMTI E 300 4 RUNO

200 A km B 3 0 100 200 300 rth) 100

34˚S 34˚S no 2

130˚E 132˚E 134˚E 136˚E 138˚E h to 0 Station locations Hiltabe Suite Bathymetry Mount Woods Inlier sout intrusives D C 1 Munjeela Granite Nawa Domain Mable Creek Ridge m ( −100 −60 −550 −50 −45 −40 −35 −30 −25 −20 −15 −10 n k St. Peters Suite Hutchinson Group −50 Peake Metamorphics x i Granitoids and equivalents 00 00 00 00 00 00 00 00 00 00 0 0 0 Sleaford and 0 Fowler Domain Donington Suite −200 E and equivalents Mulgathing Complex m Coober Pedy Ridge Wallaroo Group Moondrah Gneiss paragneisses −300 −1 Tunkilia Suite −400 −300 −200 −100 0 100 200 300 400 y in km (east to west) Figure 6.14: Comparison of interpreted geology of the Gawler Craton and horizontal 3-D model slices at 2 km and 40 60 km depth. Low resistivities near the surface (top right) in the − center of the array spatially coincide with Hiltaba Suite intrusives surrounding the St. Peter Suite. Hiltaba Suite intrusives coincide with a major mineralisation interval responsible for the formation of the IOCG Olympic Dam provenance to the north-east and the Au-dominated mineral systems in the central Gawler Craton. Features labelled B and C in the 40 60 km model − slice are believed to be linked to the near-surface magmatic intrusions and their associated mineralisations.

site rox16) and the Au-dominated province in the Central Gawler Craton (Ferris and Schwarz, 2003) (near site gvd01). Stewart and Foden (2003) have shown that the Hiltaba Suite is always slightly more juvenile than the intruded host rock suggesting a mantle source, which has subsequently been contaminated. Betts et al. (2002) and Betts and Giles (2006) introduced the idea that the Hiltaba Suite has to be seen in the context of a back-arc regime. Arc-like rocks were identified in the Musgrave Province to the north, further supporting the notion of a back-arc regime and suggesting a south-oriented subduction zone (Wade et al., 2006). The origin of the deep crustal conductivity is crucial for the understanding of the system proposed here. Deep crustal rocks show low resistivities in laboratory measurements (Kariya and Shankland, 1983), but high conductivities have nevertheless been reported in field measurements (Hyndman and Shearer, 1989). Aqueous fluids as an explanation are tenable in active tectonic regimes. The dehydration of serpentinite to peridotite may liberate aqueous fluids in subducting oceanic crust, during amphibolite to granulite metamorphism in continental crust, in accretion of oceanic crust or in the forearc mantle (Hyndman and Peacock, 2003; Glover and Vine, 1995). However, within stable crust such as the Proterozoic Gawler Craton, 6.5. Discussion 101 the mechanism required to maintain higher amounts of fluids over a large time span are not feasible (Hyndman and Shearer, 1989). Bailey (1990) suggests that fluids can remain up to 100 Ma in the lower crust and retrograde mineral reactions are believed to consume any pore fluids remaining from peak metamorphic recrystallisation (Yardley and Valley, 1997, 2000). A more likey candidate in the present case of Palaeoproterozoic crust is graphite. A corre- lation between deep crustal resistivity anomalies and former clastic foreland basins containing biotic material has been reported in Proterozoic accretionary terraines in Laurentian North America (Boerner et al., 1996). The involved tectonic collisional processes in the Proterozoic may have increased the input of carbon into the deep crust. The carbon may be remobilised over the course of time and hydrothermal C-O-H fluids could form interconnected graphite net- works and enhance conductivity (Wannamaker, 2000; Nover et al., 2005). A similar mechanism is proposed in the case of the Gawler Craton. Luque et al. (1998) have also reported on fluid- deposited graphite in volcanic terraines, where it precipitated from hot (> 600°C) magmatic fluids. Glover (1996) states that another possible source of carbon may be from mantle degassing, a scenario which is also plausible for the Proterozoic Gawler Craton, given the slightly more juvenile rocks associated with the Hiltaba Suite. The deep conductors imaged beneath the Gawler Craton may have been the source for CO2-bearing fluids which have propagated through the crust to the surface. Vielreicher et al. (2000) have shown that the fluids can contain significant amounts of rare earth elements and gold on the example of the Phalaborwa deposit in South Africa. These could have been precipitated contemporaneously with the genesis of the Hiltaba Suite along fluid pathways, present during the reactivation of major framework shear zones (e.g. Yerda and Yarlbrinda shear zone). In conclusion, the deep crust underneath the Gawler Craton shows zones of enhanced conductivity probably due to liberated graphite films along grain-boundaries. The origin of these zones is likely linked to mantle degassing and carbon introduced from a subducting plate north of the Gawler Craton. These deep crustal conductors are connected to surface expression of Hiltaba Suite related mineralisation zones. Mineralisation systems related to the Hiltaba Suite have different geochemical signatures in the IOCG Olympic Dam province to the north-east and the Au-dominated gold province in the central Gawler Craton, which is supported by the spatial separation of the deep crustal conductors labelled B and C in Figure 6.11,B.6,B.7. Chapter 7

Summary

The thesis demonstrated the use MT analysis and modelling tools in obducted (Oman) and sub- ducted (Gawler Craton) terranes. It was demonstrated that the use of analysis tools, such as the phase tensor approach of Caldwell et al. (2004), allow the analysis of MT data even in terranes with small-scale inhomogeneities, which give rise to galvanic distortion of the impedance tensor. Furthermore, a recently available 3-D inversion code developed by Siripunvaraporn et al. (2005a) was extended to incorporate static shift corrections and applied to real data sets. The survey areas and the outcomes of the modelling across the areas are as follows:

Oman ophiolite mountains A 2-D survey was conducted across the Samail ophiolite mountains, south-east of Muscat, Oman. The aim of this survey was to investigate the emplacement processes of the Samail ophiolite by delineating major faults and geological boundaries on a crustal scale. Phase tensor analysis of measured impedances and 3-D forward modelling showed that electrical currents are drawn into valleys and lead to a static shift in the electric field component parallel to the valley. 2-D modelling (Rodi and Mackie, 2001) was undertaken using the magnetic transfer functions, the TM mode and the TE-mode phases. Using different starting models and different subsets of data achieved similar inversion models. A common feature to all models is a south-west dipping conductive zone in the top 20 km which spatially coincides with outcrops of the upper plate – lower plate contact. The lithosphere beneath the Samail ophiolite in the southern part of the profile is more resistive. The south-east dipping nature of the interface is suggestive of a subduction scenario involving early subduction towards the Arabian margin. Fowler and Nuyts Domain, South Australia A 2-D survey was conducted in 2005 crossing the Fowler and Nuyts Domain in South Australia. The Fowler Domain is situated at the western margin of the Gawler Craton. There is a substantial economic interest in the area due to large mineral deposits at the eastern (IOCG Olympic Dam deposits (Skirrow et al., 2002)) and northern margin (Au-domninated province in the Cen-

102 Summary 103

tral Gawler Craton (Ferris and Schwarz, 2003)) of the Gawler Craton. Limited outcrop impedes a coherent geological interpretation of the area and fault zones are often only delineated from analysis of potential field data. 2-D and 3-D inversions of the data set were performed using three different inversion routines (deGroot Hedlin and Constable, 1990; Rodi and Mackie, 2001; Siripunvaraporn et al., 2005a). The results were analysed in the context of constrained features such as the seafloor topography of the ocean and knowledge of sedimentary basins. Main features are a moderately resistive shallow layer corresponding to the location of the St. Peter Suite underlain by a resistive crust. The upper crust beneath the Fowler Domain has a resistivity of around 100 Ω m. To the west of the Fowler Domain, the crust is moderately resistive down to depths of 80 km. The nature for the moderately resistive crustal feature east of the Fowler Domain is not quite clear. It appears to be a superposition of conductive effects of the north-south extending sedimentary basin and the ocean effect. However, a general lower resistivity of the crust cannot be ruled out. 3-D resistivity model of the Gawler Craton A 2-D array of 29 sites were deployed across the Gawler Craton, South Australia, covering an area of 400000 km2 with an approximate site spacing of 100 km. The data were collected in between 2003 and 2006 and comprise MT measurements from 3-D surveys (Thiel and Heinson, 2006; Maier et al., 2007) and from 2-D surveys (Heinson et al., 2006; Selway, 2006, and Chapter 5 in this thesis). The aim of this work was to create an image of the lithospheric resistivity distribution of the Gawler Craton and have the opportunity to combine the information with other geophysical and geological data. Induction arrows and phase tensor ellipses were compared with potential field data and sediment to basement maps. It could be shown that the sedimentary basins surrounding the central Gawler Craton have an inductive influence at shorter periods. For the 3-D inversion of the data set, the 3-D code developed by Siripunvaraporn et al. (2005a) was extended to incorporate static shift corrections. Two ways of obtaining these static shift factors are introduced; firstly, 2-D Occam inversions of profiles crossing the array can be used to obtain static shifts. Secondly, a residual analysis of apparent resistivity and phases indicates sites with strong static shift when the apparent resistivities of the model show period-independent over- or underestimation compared to observed resistivities, while the phases are unaffected. The latter method produces the best static shift corrections for this 3-D survey. However, a closer site spacing and more sites would likely favour the former method of performing 2-D inversions in order to obtain static shift corrections. The inverse model of the Gawler Craton shows conductive areas at the location of the sedimentary basins surrounding the craton and near outcrops of the Hiltaba Suite. The middle crust between 10 and 30 km is resistive. At greater depths five zones of enhanced conductivities are visible. The deep conductors 7.1. Future work 104

are connected to the conductive features near the surface. It is possible that the deep

conductors may be the source of CO2-bearing ﬂuids, which have propagated through the crust to the surface. These ﬂuids could have deposited carbon in form of graphite along

grain-boundaries. Vielreicher et al. (2000) have also reported that CO2-bearing ﬂuids can contain signiﬁcant amounts of rare earth elements and gold. In the case of the Gawler Craton, these minerals could have been deposited during the genesis of the Hiltaba Suite, a magmatic event which is related to the IOCG Olympic Dam and Au-dominated mineralisation systems of the Gawler Craton.

7.1 Future work

The collection of the Oman survey was planned in 2004 and conducted in January 2005, before the 3-D code by Siripunvaraporn et al. (2005a) became widely available. Today, with the knowledge of the complexity of the MT data collected along a profile across the Samail ophiolite and the availability of 3-D modelling codes, it would be most beneficial to have more sites across the Oman ophiolite mountains. A 2-D areal array would most likely be the best option, however, one may be restricted to measurements along valleys due to the topography of the area. Topography effects could be modelled new 3-D modelling codes, which can easily incorporate topography (Franke et al., 2007b). Such a survey could further improve the understanding of the ophiolite emplacement The 3-D survey across the Gawler Craton has shown that it is possible to model a large area of continental lithosphere. However, using 29 MT sites and a grid size of 40 54 26 cells × × with a total model dimension of 7000 7000 4500 km results in significant modelling time × × (usually 2 weeks for about 10 iterations on a 2.4 Ghz Intel Duo Core PC with 4Gb RAM). It is nevertheless desirable to have more sites to achieve a closer site spacing. This would increase the resolution of the model and put more constraint on model features. Another option is to collect smaller arrays over areas of interest. The study here has shown that the mineralisation systems likely have an electrical response. However, coarse site spacing impedes interpretation of upper crustal features unless closer site spacing and a wider period band-width can be achieved. It would be beneficial to have small-scale arrays across some of the major known mineral deposits to further contribute to the understanding of their genesis.