The Moonta-Wallaroo mining district, South Australia - CSIRO Publishing

The Moonta-Wallaroo mining district, South Australia: A geophysical perspective Mike Dentith1 & Duncan Cowan2 Keywords: Copper, electromagnetics, gold, gravity, induced polarisation, magnetics, Maitland SI53-12, massive sulphides, metasomatism, resistivity, skarn, Whyalla SI53-8

ABSTRACT Located on the Yorke Peninsula in South Australia, the MoontaWallaroo area is an important area of historical copper mining. Hydrothermal copper-gold-silver sulphide mineralisation occurs in Palaeoproterozoic and Mesoproterozoic metamorphic and igneous rocks, but there is no outcrop of these basement rocks due to subdued relief and extensive sedimentary cover. The lack of outcrop, plus the complexity of the local geology, means that basement geological maps of the area are largely speculative. An interpretation of semi-regional aeromagnetic data from the area suggests the basement comprises meta-igneous rocks overlain to the east by an increasing thickness of sedimentary and/or metasedimentary cover. Anomalies interpreted to be of stratigraphic origin are quite well developed in areas of meta-sedimentary rocks, and regional strike is interpreted as trending northeastsouthwest. Linear anomalies to the north and west of the Wallaroo Mines are interpreted as resulting from magnetite deposited in faults during a regional iron-rich metasomatic event. Based on this interpretation, a major east-west/northeast-southwest trending deformation zone is recognised, interpreted as a locus of reverse and dextral faulting. The entire study area is cross-cut by late brittle faults that trend mainly northwest-southeast. This interpretation/structural scenario is consistent with mine-scale structural studies in both the Moonta and Wallaroo Mines, although, the resolution of the aeromagnetic data is insufficient to map deposit-scale structures. Ground-based geophysical exploration in the Moonta-Wallaroo area is hindered by the generally electrically conductive environment and the presence of cultural features, such as wire fences. However, surface and downhole time-domain EM surveys have successfully located massive-sulphide mineralisation comprising the Poona and Wheal Hughes deposits. The Poona deposit has also been shown to give rise to an IP response, and there is evidence that the lodes elsewhere in the Moonta area are also associated with IP anomalies. INTRODUCTION The Moonta and Wallaroo copper-gold-silver mining district, located on the Yorke Peninsula in South Australia, is historically one of the most important copper mining centres in Australia. The

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Centre for Global Metallogeny School of Earth & Geographical Sciences The University of Western Australia Crawley Western Australia 6009 Australia

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Cowan Geodata Services 12 Edna Road Dalkeith Western Australia 6009 Australia

majority of mining occurred between 1860 and 1923, with an overall production of 9.6 Mt of ore, with average recovered grades of 3.7% Cu and 0.4 g/t Au. Between 1988 and 1993, mining of the newly discovered Poona and Wheal Hughes deposits produced 476,000 t of ore, averaging 3.4% Cu and 0.6 g/t Au (Newton, 1996). This paper comprises two sections. The first deals with the regional structural setting of the mineralisation, as deduced for an interpretation of aeromagnetic data undertaken by the authors. As there is virtually no outcrop of the prospective stratigraphy, due to weathering and the presence of overlying younger strata, basement geological maps of the region are based on extremely limited information. It is generally accepted that the mineralisation in the Moonta-Wallaroo area is of hydrothermal origin, and structure is known to be an important control at the mine-scale. However, in the absence of reliable basement geological maps, attempts to provide a regional structural/stratigraphic setting for the mineralisation are necessarily speculative. The acquisition of extensive aeromagnetic data from the region, as part of the Targeted Exploration Initiative South Australia (TEISA), provides an opportunity to map the local geology in a consistent fashion, and hence provide a regional geological context for the mineralisation. The second part of the paper is a summary of work undertaken, by various personnel, on behalf of mining companies that were exploring in the Moonta-Wallaroo area. A thorough exploration of the area was carried out over an almost thirty year period, beginning in the 1960s, primarily by Western Mining Corporation Ltd, North Broken Hill Ltd and Broken Hill South Ltd. This culminated in the discovery of the Poona and Wheal Hughes deposits in 1985. The basis for this summary is reports to the South Australian Mines Department by these three companies. REGIONAL GEOLOGY The Moonta-Wallaroo district lies within the Moonta Subdomain, on the eastern margin of the Gawler Craton (Parker, 1993; Conor, 1995). The sub-domain is mainly composed of Palaeoproterozoic meta-volcanic and meta-sedimentary rocks (greenschist to amphibolite facies). Conor (1993) interprets the rocks as a volcano-sedimentary sequence deposited in an intracratonic basinal setting. The Palaeoproterozoic succession has been intruded by granites of the Mesoproterozoic Hiltaba Suite, associated with which was a period of high-temperature, magnetite (haematite)-generating metasomatism. Overlying the Proterozoic basement is a succession of Neoproterozoic, Cambrian and Tertiary sediments, and surficial cover comprising aeolianite, soils and thin calcrete. The cover rocks thicken to the east, but since they are essentially non-magnetic, have no appreciable affect on magnetic data. The stratigraphic nomenclature of the Moonta-Wallaroo area has recently been revised, with the stratigraphy of Parker (1993) amended by Conor (1995). Here we use Conor's scheme, however, it should be realised that the older scheme was used during some of the exploration in the region, and as such, forms the geological basis for some published descriptions of geophysical exploration

Geophysical Signatures of South Australian Mineral Deposits

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Fig.1. Schematic illustration of proposed relationships between the main geological units in the Moonta-Wallaroo area. a) Scheme of Parker (1993), b) scheme of Conor (1995). 1 – Moonta Mines, Poona, Wheal Hughes, 2 – Doora Mine, 3 – Wallaroo Mines, 4 – New Cornwall, Wandilta Mines, P – pegmatite.

in the Moonta-Wallaroo area, for example, Williams and Fraser (1992). Figure 1 comprises schematic rock-relationship diagrams for the two stratigraphic schemes. Clearly, the fundamental difference between them is that Parker (1993) has two metasedimentary packages (Doora Schist and Wandearah Metasiltstone), separated by an unconformity, whilst Conor (1995) recognises only one unit, the Wallaroo Group, which contains all the metamorphosed igneous and sedimentary rocks in the area. Figure 2 is a basement geological map of the Moonta-Wallaroo area, based on the work of Conor (1995). From necessity, this map is a significant simplification, because many of the different geological units are interleaved by intrusive and tectonic processes. The locations of the major mineral deposits in the area are also shown. Note that confusingly, the "Wallaroo Mines" (discussed below), are actually closer to the town of Kadina; Wallaroo being a port about 10 km to the northwest. The major components of the local geology are described below. Meta-sedimentary rock types in the Moonta-Wallaroo area include fine-grained psammites and pelites (locally graphitic), impure carbonates, calcsilicates, laminated alkali-feldsparites and iron formations and pelites. Alkali-feldsparite is a fine-grained lithology composed principally of alkali-feldspar, either albite or microcline, with minor accessory minerals such as magnetite, calcite, amphibole or biotite. Its origin is problematical, with some evidence indicating replacement, but a syngenetic or diagenetic origin remain possibilities. Conor (1995) suggests that the abrupt variations in metamorphic grade observed in rocks of the Wallaroo Group are due to faulting, a high degree of strain partitioning and metasomatism. It is phenomena such as these that have hindered the geological understanding of the region. The most important meta-igneous unit in the area is the Moonta Porphyry, which is host to the mineralisation in the Moonta area, 78

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Fig.2. Map of the basement geology in the Moonta-Wallaroo area showing the locations of various deposits and prospects. Modified from Conor (1995).

including the Poona and Wheal Hughes deposits (Fig.2). This unit does not outcrop and is known only from drill holes and mining, although Parker (1993) describes its extent as "reasonably well established". At Moonta, the Moonta Porphyry is a fine-grained, foliated, porphyritic rhyolite, but in other areas it is more gneissic. Quartz and chlorite veining is common, and is both concordant and discordant. Feldspathic-haematitic alteration, of probable hydrothermal origin, is common in the Moonta area. The unit is interpreted by Parker (1993) as a sill or ignimbrite, with its present-day form the result of subsequent deformation. Alternatively it might be a crypto-dome (C. Conor, pers. comm.) Near Bute, to the east of the study area, interbedded basalt flows up to 10 m thick occur within the local meta-sedimentary succession, referred to as the Willamulka Volcanics. Both basalt and meta-siltstone are intruded by an amphibolite/dolerite intrusion approximately 40 m thick. The lithological variability of the Wallaroo Group is expected to be one source of magnetic anomalies. Positive magnetic responses are to be expected from iron formations, mafic units and to a lesser extent alkali-feldsparites. Gow et al. (1993) assign magnetic susceptibilities of 424.7 x 10-3 SI to magnetite/haematiterich sedimentary rocks, and 117.4 x 10-3 to haematitic metasiltstones, which are probably equivalents of the rocks in the Moonta-Wallaroo area. Very much lower values are assigned to calc-silicate and arkosoic rocks. Large felsic volcanic bodies, like the Moonta Porphyry, are not expected give rise to large magnetic responses, however, internal detail may be resolvable in detailed magnetic datasets. The youngest crystalline rocks in the Moonta-Wallaroo area are intrusions of the Hiltaba Suite, for example the Tickera Granite which intrudes the meta-sedimentary rocks in the north of the study area (Fig.2). Another Hiltaba Suite granitic intrusion, the Arthurton Granite, occurs in the extreme south of the study area. Both 'granites' are in fact granitic complexes and show evidence of varying amounts of deformation. Locally intense metasomatism

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Fig.3. Geological structure in the Moonta Mines area. Re-drawn from Dickinson (1942).

was induced by intrusion of the Hiltaba Suite, resulting in some plutons being partly rimmed by skarn-like aureoles. In some cases alteration has extended back into the intrusives, i.e. endoskarn. The products of this high-temperature alteration event have been grouped together under the stratigraphic name "Oorlano Metasomatite" (Fig.2). Since a principal characteristic of the high temperature skarn-forming event was the formation of magnetite, and to a lesser extent haematite, these rocks are expected to be important sources of magnetic anomalies. Over printing skarn-like alteration along the southern margin of the Tickera granite complex is a zone characterised by intense alteration to kaolinite; this zone crops out on the beach north of Wallaroo. The zone has been extensively explored by drilling and locally hosts significant bodies of disseminated pyrite that contain chalcocite mineralisation, e.g. the Alford and North Kadina prospects (see below). The alteration zone lacks magnetite, and corresponds with a ribbon-like magnetic low on aeromagnetic data. The geological structure of the basement in the MoontaWallaroo area is complicated. Two main deformational events are postulated: an ~1600 Ma event syn- and/or shortly post-Hiltaba Suite intrusion, and the 1740-1700 Ma Kimban Orogeny (Ferris et al., 2002). Field observations indicate that sedimentary layering of the Wallaroo Group is commonly near vertical and is tightly folded. Occasionally fold interference is visible, indicating that refolding has happened at least at the local scale. Lithologies are variably foliated and locally show signs of differentiation, thus indicating polyphase deformation and metamorphism. Unfortunately the lack of exposure has prevented timing relationships from being established. Moonta Area At Moonta Mines, mineralisation occurs in the Moonta Porphyry as narrow, sheet-like, lodes a few metres in thickness. These define an arcuate trend, consisting of three main zones, and bearing northnortheast to northeast (Fig.3). The lodes dip to the northwest. The mineralisation consists of sulphides, particularly pyrite and chalcopyrite, in a quartz-tourmaline-feldspar-chlorite gangue.

Dickinson (1942, 1953) recognises five classes of fault at Moonta. The first three categories are the "main lode shears", the "west lode shears" and "strike faults". The former contain the majority of mineralisation. In all cases their trend is northeasterly and their dip is towards the northwest, at around 60º. However, individual shears show significant fluctuations in dip and strike. Based on slickensides, which have steep southerly or vertical pitches, and observed displacements, these faults are considered to be mainly dextral reverse faults. The northeasterly trending faults are cross-cut by faults with east-west and northwest-southeast trends. The latter dip steeply to the southwest, or are vertical. Dickinson (1942) suggests they are tensional structures. The east-west faults are rare. They have steep to vertical dips and are associated with significant lateral displacements. Dickinson (1942) interprets the overall fault pattern at Moonta Mines in terms of a pure-shear model with the maximum compression oriented northwest-southeast, but recognised that dextral wrenching was the main type of strain. More recent models of faulting in wrench zones also lead to this interpretation. For example, the various faults of northeasterly trend probably comprise the kind of anastomosing fault complex typical of strikeslip environments, due to the presence of Riedel shears (Woodcock and Fischer, 1986). Wallaroo Area The geology of the Wallaroo Mines is quite different from that at Moonta. At Wallaroo the mineralisation occurs within deformed clastic and chemical meta-sediments and meta-volcanics. The lodes define a northeast-trending en-echelon array, are steeply dipping to vertical, and individual examples trend eastsoutheast (Fig.4). The main ore bodies are composed of chalcopyrite, pyrite and pyrrhotite, and are confined to east-west shears. Dickinson (1942, 1953) recognises three types of fault in the Wallaroo Mines area. The first class, termed the "Wallaroo fissures", contain the mineralisation and strike east-west to eastsoutheast-westnorthwest. They have dips generally greater than 75º, and may change dip direction with depth (Fig.4). Slickensides and grooving have steep southerly pitches of about 70º. There is also mention of horizontal striae, and sinistral movements are inferred. It is possible there was an early phase of reverse movements, prior to the lateral movements. However, we


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note that the thickness variations in the lodes (Fig.4) actually suggest formation could have been during dextral faulting, assuming focusing of mineralising fluids into dilatant zones associated with changes in strike direction. Dickinson (1942) describes the faults as high-angle sinistral reverse faults. It may be significant that the cross-section in Figure 4 bears a striking resemblance to a flower structure. The second class of faults at Wallaroo Mines strike northeastsouthwest, dip to the west at about 70º, and are essentially unmineralised. They cross-cut the Wallaroo lode at two locations and dextral reverse displacements are observed. Faults of the final set trend northwest-southeast and are only well developed in the area to the northeast of the main Wallaroo lode (Wandilta-New Cornwall trend) and in the Doora Mine. They are interpreted as tensional features. Dickinson (1942) interprets the Wallaroo fissures as forming before the other two faults sets, based on relative displacements. The northeast-southwest faults are envisaged as the principal conduits for mineralising fluids, with the intersecting east-west faults representing favourable sites for ore formation. However, it appears that the region has seen multiple phases of faulting, and this interpretation may be overly simplistic. INTERPRETATION OF SEMI-REGIONAL AEROMAGNETIC DATA The TEISA aeromagnetic data from the Moonta-Wallaroo area were flown at a nominal height of 80 m, with 400 m-spaced flight lines. Thus, they must be considered of moderate resolution by modern standards, and certainly insufficient to guarantee detection of structures of the scale of those controlling the mineralisation at Moonta or Wallaroo Mines. However, the data are more than adequate to map the major structural and stratigraphic features in the area, provided of course they are associated with contrasts in magnetisation. The interpretation of the TEISA aeromagnetic data is based primarily on the four images in Figure 5. The filtering operations used to create the various enhanced versions of the TMI data are described in more detail in the Appendix. These images were interpreted in a conventional fashion, with common amplitude/wavelength/textural characteristics used to infer similar geological entities, and linear anomalies or truncations of anomalies variously interpreted as 'planar' geological surfaces, such as shear zones and joints. In considering these data, however, it is essential to remember that the measured magnetic response comes not just from the lithologies nearest the surface, but also magnetised materials at depth. The interference of signals from different depths may create the appearance of cross-cutting features, when in fact they are vertically juxtaposed, but do not intersect. An example of this scenario would be anomalies associated with bedding in units that overlie an igneous intrusion. It should also be remembered that the response is due to a combination of both depositional (primary) magnetite and metamorphic (secondary) magnetite, and that there are likely to have been multiple magnetite-forming events during the geological evolution of the area. Our interpretation of the aeromagnetic data from the MoontaWallaroo area is influenced by the work of Gow et al. (1993), who worked with equivalent data from the Stuart Shelf, some 300 km to the northwest (see Williams et al. this volume and Dentith, this volume). Gow et al.'s interpretation of the basement geology, which like the rocks at Moonta-Wallaroo are part of the Gawler Craton, is based on the recognition that a major phase of ironmetasomatism has produced large volumes of secondary iron 80



oxides, whose high density and magnetic susceptibility cause them to dominate the geophysical response, more so than primary lithological variations. To quote these authors; "Although this makes stratigraphic mapping difficult, it allows the delineation of structural features on the premise that fluid flow and iron-oxide deposition … was primarily structurally controlled". As at Moonta-Wallaroo, the iron metasomatic event in question is correlated with the Mesoproterozoic Hiltaba Suite volcanoplutonic event. On the Stuart Shelf, sub-circular anomalies are interpreted as igneous intrusives. Some of these plutons have distinctive haloes of positive magnetic anomalies, which drilling has shown to be caused by magnetite-rich skarn assemblages. Anomalies with similar characteristics occur at Moonta-Wallaroo (see below). Our discussion of the aeromagnetic data is based on the careful separation of observation and interpretation, with a qualitative indication of our confidence in the latter. We begin with basic geological and geophysical observations and interpretations, and progress to a speculative tectonic model. The known occurrences of mineralisation are then considered within the context of the interpretation/model. To assist the reader in coming to their own conclusions with respect to the interpretation of the aeromagnetic data, we have labelled specific features only on the interpretational summary (Fig.5a). The magnetic images themselves (Fig.5b-e) have only deposit locations and the extent of prospects indicated. This facilitates recognition of the features in the images that are the basis of our interpretation, without obscuring the images themselves. Local-Scale Observations and Interpretations We observe that the magnetic response in the eastern third of the study area is subdued when compared with the central and western areas. There are, however, easily discernible linear anomalies trending roughly northeast-southwest, labelled "A". With reasonable confidence, we interpret these responses as originating from within metasedimentary rocks, with the linear features due to magnetite-rich sedimentary horizons, or beddingparallel bodies of igneous rocks. That is, the anomalies are indicative of stratigraphic trends. In the north and northwestern part of the region of subdued magnetic relief, we observe curvilinear features, which also trend generally northeast-southwest (labelled "B"). However, they contrast with the linear features to the south (those labelled "A"), because they are of higher amplitude and slightly shorter wavelength, but most importantly they are more curved and of shorter length. The anomalies increase in amplitude to the southwest, until in the Kadina-Smithams area they reach a maximum before their abrupt termination at "C", a northwesttrending linear magnetic feature (interpreted as a late fault, see below). The anomalies cross-cut each other in places, for example at "D", but are locally sub-parallel, with four or five closely spaced anomalies forming arcuate features (e.g. at "E"). The overall trend of these anomalies becomes more northerly towards the northeastern limit of the study area, where they are also more closely spaced. There is a change in anomaly trend across an eastwest trending feature (labelled "F") that passes to the north of the Black Oak Plain prospect. With moderate confidence we interpret the linear anomalies as indicative of stratigraphy (depositional and/or intrusive) within the predominantly meta-sedimentary units in the study area. The contrasting characteristics with the anomalies further south we explain in terms of structural influences, predominantly reverse faulting. This explains the closer proximity of the anomalies to each other (structural telescoping), cross-cutting anomalies and the local changes in strikes. We speculate that this part of the study area contains

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Fig.5. Aeromagnetic images and interpretation of the Moonta-Wallaroo district showing locations of mines, deposits and prospects. Imaging by Cowan Geodata Services, see Appendix for details. Co-ordinates are AMG zone 54. a) Interpretation, b) TMI.


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Fig.5. (cont) c) TMI with extended separation filter, d) TMI with terracing operator and gradient maxima.

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Fig.5. (cont) e) TMI with standard separation filter.

multiple, comparatively small, thrust faults; the large number of faults being a reflection of a detached structural style resulting from the bedded nature of the local rocks. In the southwest of the study area we observe a distinctive magnetic character, with many arcuate features, short-wavelength sub-circular zones and a rugose texture. This is typical of an igneous terrain composed of a suite of intrusive bodies, which is consistent with the known geology, and consequently we interpret it as such with a high degree of confidence. Particularly in the filtered magnetic images, bodies with similar characteristics are seen further to the east, and these are interpreted as the same rocks under an increased thickness of meta-sediments and cover (Neoproterozoic, Cambrian etc.). The depth to the intrusions, i.e. the thickness of overlying rocks, increases to the east, until in the extreme east (labelled "G"), the short-wavelength content of the associated magnetic anomalies is entirely lost. The discordant contact between the linear anomalies in the meta-sediments (stratigraphy) and the circular anomalies (intrusives) is clearly seen at "H". Linear magnetic lows are associated with many of the interpreted intrusions, and are particularly clear in the extendedseparation-filtered image (Fig.5c), and also the terraced image (Fig.5d). These features are interpreted as joints, or more specifically weathering and/or alteration along joint surfaces causing magnetite destruction. Some well-developed examples occur at location "I". Referring particularly to the TMI (Fig.5b) and terraced (Fig.5d) images, in the areas where intrusions are interpreted at intermediate depths, i.e. the south-central part of the study area, there are a number of localised high-amplitude anomalies. The

larger regions of this type are shown in red in Figure 5a. These may be located close to the inferred margins of the intrusions, for example the anomalies labelled "J". Alternatively, they may be within the inferred extent of the interpreted intrusions (see location "K"). In both cases the sources of the anomalies are assumed to be associated with the intrusions. Those anomalies that appear to be 'within' the intrusions are interpreted as being in the overlying roof material. With a fair degree of confidence, we interpret the source of these anomalies as iron-rich skarns and/or magnetic minerals formed in the country rocks by contact metamorphism. In a few areas the anomalies are locally linear. One such area is where the joint-related anomalies occur (location "I"), and jointing within the intrusion is interpreted as having controlled the flow of metamorphosing/hydrothermal fluids. In the north-central part of the study area there are regions with the igneous characteristics listed above, albeit without as many joint-related anomalies (labelled "L"). The larger of these corresponds with outcrops of the Tickera Granite (Fig.1), and hence we interpret a granitoid in this area with a high degree of confidence. Cutting across the northern part of the study area are a series of high-amplitude curvi-linear anomalies, which vary in trend from east-west to northeast-southwest. Various examples are labelled "M". These are observed to truncate other anomalies of similar character, e.g. at "N", and also other anomalies, for example northwest-trending, short-wavelength, linear anomalies in the Wallaroo Mines area (labelled "O"). From the cross-cutting relationships and their overall geometry, these anomalies are interpreted as due to magnetite within a fault complex. If this is correct, it is logical to assume this magnetite formed during the


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Fig.6. Regional tectonic model of the Moonta-Wallaroo area, based on the interpretation in Figure 5a.

iron-rich metasomatic event described above. This interpretation is consistent with the geology of the various prospects in the vicinity (see below). We observe that the zone of linear anomalies is sigmoidal in shape. The anomalies trend roughly east-west at the eastern and western ends of the sigmoidal zone, with northeasterly trends dominant in its central part. The significance of this observation is discussed further below. The linear magnetic low associated with kaolinitic alteration at, for example, the Alford prospect, lies on the northern side of the interpreted fault complex. Associated with both the east-west trending parts of the inferred magnetite-rich fault complex, we observe roughly parallel linear magnetic lows. The southern example (labelled "P") marks the southern limits of the interpreted zone of magnetite-bearing faults and continues eastwards, passing through the Moonta Mines area, before apparently dying out (although this could be a function of the low levels of magnetism in the rocks in the east of the study area). The northern example (labelled "F") passes to the north of the Black Oak Plain prospect, and as has already been observed, is associated with a change in orientation of anomalies interpreted as due to stratigraphy in the meta-sedimentary units in that part of the study area. Both the east-west trending anomalies are associated with a decrease in TMI, implying local destruction of magnetite. A logical explanation for this observation is magnetite-destructive alteration by fluids channelled within what are most reasonably interpreted as steeply dipping fault zones. The evidence for the nature of the faulting associated with the east-west features is as followings. Firstly, the change in trend of the stratigraphic (interpreted) anomalies as they cross the northern example, implies dextral strike-slip movements. Secondly, in the TMI (Fig.5b) and terraced (Fig.5d) images, there is a decrease in the amplitude of the long wavelength component of the magnetic field to the south of this feature. This implies the magnetic basement is at greater depth, i.e. a larger thickness of the less magnetised meta-sedimentary rocks to the south, and hence suggests a south-down vertical displacement. However, the shorter wavelength stratigraphic anomalies show effectively no change in amplitude and wavelength across the feature, implying little vertical displacement. An admittedly speculate explanation of these observations is that post-depositional vertical displacements are minor, but the feature may have controlled the original thickness/distribution of the now metamorphosed sedimentary succession. There is no convincing evidence for lateral displacements associated with the southern east-west feature, although this may be a due to a lack of marker features from which to establish offsets. However, it is informative to compare the magnetic 84


character in the igneous terrain on either side. Comparing the areas labelled "Q" and "R", particularly as seen in Figure 5c, the magnetic response is seen to be more subdued to the south of the feature. A plausible structural interpretation of this observation is southside-down vertical displacement. However, other possibilities remain. For example, this area is within the extent of the Moonta Porphyry and could be a location where this weakly magnetised intrusion is thicker, or more deeply weathered. Alternatively, it may step to a different depth/stratigraphic level, which implies the presence of an pre-existing fault (as suggested for the northern feature). In support of the first (and simpler) explanation, we note the consistent magnetic characteristics of the TMI image (Fig.5b) in the area to the west of the Moonta Porphyry (labelled "S" and "T"). This is reasonable evidence against significant post-depositional vertical movements on the east-west feature. We observe linear magnetic anomalies of comparatively short wavelength and moderate amplitude in the Doora area, extending southeastwards as far as the Vulcan prospect (labelled "U"). These anomalies are sub-parallel and trend roughly northwest-southeast. They are particularly clearly seen in Figure 5c. Immediately north of the Bert Allen prospect, an anomaly, which is essentially identical except for its westnorthwest-eastsoutheast trend, cuts across these anomalies (labelled "V"). A few kilometres to the north, extending westwards from near the Wallaroo Mines, is another similar anomaly (labelled "W"), except for its east-west trend (although these are no intersections with other features in this case). Similar, apparently cross-cutting, east-west trending anomalies are also seen a few kilometres to the east, in the vicinity of the Wandilta Mines (labelled "X"). With only moderate confidence, we interpret the northwest-trending anomalies in the Doora-Vulcan area as reflecting stratigraphy, partly because this is the most common trend, with the other two trends interpreted as reflecting discordant intrusions (which could be coincident with pre- or post-intrusion faults. We note that magnetite-rich intrusions are known in the area. We also note that east-west is the trend of the Wallaroo fissures. The Wallaroo-Vulcan-Doora area is the only place where northeast-trending lineaments are observed. These features are defined by the truncation and displacement of other anomalies; they are not anomalies themselves. Minor lateral displacements of the northwest-trending anomalies are observed, and based on Dickinson's (1942, 1945) description of the geology of the Wallaroo Mines, and their magnetic characteristics, the linears are confidently interpreted as faults. In fact, the northeasttrending shear zone, along which the lodes in the western part of Figure 4 occur, is clearly seen, for example in Figure 5c. The preferred interpretation has the northeast-trending features truncated by the east-west trending linear magnetic low ("P"). It is tempting to continue the former further southwest, since this is the area of the Moonta Mines, and this is the dominant fault trend in that area. Such as interpretation could be accommodated by the aeromagnetic data, however, it is our opinion that there is insufficient evidence to justify this interpretation based on the data available to us. It might be necessary to modify our opinion if higher resolution data become available. Across the entire study area, but especially the north-central part, we observe lineations trending mainly northwesterly, but also northnorthwest and even north-south. Minor displacements of anomalies are associated with these lineaments, which are interpreted with confidence as late-stage brittle faults. Faults of this trend occur in the Wandilta-New Cornwall area (see Figure 4), and apparently equivalent lineaments are seen in the magnetic data here ("C"). Faults of this trend seem to be particularly

Dentith & Cowan


concentrated in a corridor that passes through the Doora-WallarooVulcan-Wandilta area. We note that the coastline immediately south of Point Riley (Fig.5a) is apparently controlled by this faulting. We also note the different structural and stratigraphic trends on either side, both interpreted from the magnetic data and recognised in geological studies (Dickinson, 1942, 1945).

descriptions of these in Conor (1995) in the context of the interpretation of the aeromagnetic data. As noted above, the description of faulting in the Moonta Mines by Dickinson (1942, 1953) supports our interpretation of the magnetic data, whilst his observations from the Wallaroo Mines are consistent with the interpretation.

Regional Structural Model

The North Kadina or (West) Alford prospects are located in the north of the study area. Conor (1995) describes copper, molybdenum and uranium mineralisation in mixed feldspar-ironoxide-calcsilicate metasomatites. The prospects lie immediately to the north of major linear anomalies interpreted as due to metasomatic magnetite in fault zones, within a magnetic low. This is consistent with the geological description of the prospects, which are in a zone of kaolinitic alteration.

In the absence of a published regional structural-stratigraphic interpretation for the Moonta-Wallaroo area, we propose such a model. We recognise that geological studies currently underway may lead to its abandonment or modification, but justify our actions in terms of the need for a model of some kind to be established as a starting point for more refined interpretations. We also acknowledge that mine- and prospect-scale studies have identified multiple deformation/alteration events, although no chronology has been established. Of course aeromagnetic data cannot unravel such complexities, only allowing mapping of the major features of the crustal architecture as they exist after the most recent phase of tectonism. The preferred regional geological model is shown in Figure 6. The dominant structural feature is the fault complex that cuts across the northern half of the study area. The east-west trending zones of magnetite destruction are interpreted as dextral strike-slip faults, responsible, for example, for the change in the trend of stratigraphic anomalies to the north of the Black Oak Plain prospect. Based primarily on its sigmoidal shape, we interpret the main fault zone as being a large-scale restraining bend, which has encouraged reverse faulting. In the meta-igneous terrain to the west of Wallaroo, a comparatively small number of large faults are interpreted. To the east, where the rocks are layered metasediments, a larger number of smaller structures are interpreted. The northeast-trending faults in the Moonta Mines are interpreted as being a part of the same fault system. The most perplexing aspect of the magnetic data is the northwest-southeast orientation of 'stratigraphic' trends in the Wallaroo-Vulcan-Doora area. Anomalies interpreted as being of stratigraphic origin are interpreted to the east and northeast of this area, and define a reasonably consistent regional northeasterly strike orientation. We offer two explanations for the localised anomalous strike direction, in what is the most speculative aspect of our tectonic model. Firstly, and preferred, is that convergence at the restraining bend has led to detachment and independent rotation of the near-surface rocks in the Wallaroo-Vulcan-Doora area. It would probably be incorrect to describe the area as allochthonous, since the movements are predominantly rotational, rather than translational. The anomalous strike-direction could also be related to the underlying Moonta Porphyry, which perhaps acted as a rigid body during deformation resulting in different styles/degrees of deformation compared with the metasedimentary dominated regions to the east. A second, and less preferred, interpretation is that the different stratigraphic trend is due to upright folds with originally roughly east-west trending axes, which have been folded into a large-scale S-fold. The central segment of this fold is missing, perhaps due to faulting in the main northwest-trending corridor. The WallarooVulcan-Doora area would comprise the bottom limb of the S-fold, the more northeasterly trends in the Wandilta-Smithams area would comprise the other limb. Geological Setting of Mineralisation The study area contains a number of prospects, mines and deposits (Figs.2 and 5). It is interesting to consider the

The Willamulka prospect is described by Conor (1995) as an area of copper anomalism. The magnetic interpretation places the prospect in an area of thrust faulting within the meta-sedimentary sequence. Anomalous copper and nickel values at Correll's prospect are associated with intermediate intrusives. This is an area of linear magnetic anomalies, but it seems unlikely that the intrusives are their source. The North Britain Mine is described as having copper mineralisation associated with feldspar-iron oxide-calcsilicate skarn. This is consistent with the magnetic interpretation, which places the mine close to the faults inferred to contain metasomatic magnetite. Mineralisation at Black Oak Plain and Smithams/East Kadina is similar (Conor, 1995). The mineralisation occurs in graphitic rocks associated with albitite taconites and scapolitic alteration. These rocks will not give rise to magnetic responses, and the magnetic data contribute little to the understanding of the mineralisation, although the magnetic interpretation places these locales in one of the most structurally complex parts of the study area. The West Doora-Doora-Bert Allen-Vulcan trend lies to the south of the Wallaroo Mines, and Conor (1995) reports that the geology and mineralisation in the two areas are similar. There is broad zonation of mineralisation, with chalcopyrite-pyrite in the west passing into chalcopyrite-pyrrhotite-pyrite-sphalerite-galena in the east. Lithologies include meta-volcanic and metasedimentary rocks that have undergone polyphase deformation. Exploration has defined an indicated resource of 2.7 Mt at 2.1% Cu to 300 m depth. The magnetic characteristics of the two areas are similar. If our speculation that this region is detached is correct, the likelihood of associated structural complexity makes this area particularly prospective for hydrothermal mineralisation. Finally, the Penang Mine, in the extreme south of the study area, is described by Conor (1995) as comprising weakly mineralised scapolite-iron metasomatite within a coarse-grained granite host (Arthurton Granite). This is entirely consistent with the magnetic interpretation, which places the mine within a zone of increased TMI, interpreted as due to iron-rich skarn or contact metamorphosed country rock, on the margins of an intrusion. The description of the mine geology suggests the former is correct. DEPOSIT-SCALE GEOPHYSICAL SURVEYS Early geophysical work (1930s to 1950s) was carried out in the Moonta-Wallaroo region by the South Australian and Commonwealth Governments. EM, SP, magnetics, scintillometer and electrical potential surveys were all tried, with EM giving encouraging results. Anomalies were recognised over known


85

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NW (a)

41300E

IP data Fig.15 EM data Fig.11

Area of Fig.13

SE

VMI (nT)

Treuer's Elder’s lode lode

2500

EM data Fig.12

Poona

2000 1750 36W

Area of Fig.17

(b)

Large RVR loop

33W

Wheal Hughes

30W

31

17

17

N=2

25

10

18W

21W

6 8

19

10 8

13

10

15W

17

9W

12W

15 17

19

15

17

N=4

Moonta town site

24W

15

13

N=3

41200E

27W

Apparent Resistivity (Ω.m) N=1

8

33

17

17

Percentage Frequency Effect 0.5

N=1

0.0

N=3 0

36W

0

30

5E

1.0 2.0

1.26

Noise

N=4

Moonta Mines

1.5

3.3

N=2

1.0 1.4

2.5

1.0 1.2

1.2 2.5

1.0 0.5

1.2

1.0 1.5

1

96-0317 MESA

KILOMETRE 9W

IP data Fig.8

Fig.8. IP/resistivity data and VMI data across the Moonta lodes. Source frequencies were 2.5 and 0.25 Hz. See Figure 7 for location.

Copper lode

NW

96-0307 MESA

Fig.7. Location map showing position of Moonta, Paramatta, Poona and Wheal Hughes deposits and selected geophysical surveys. Adapted from Williams and Fraser (1992).

0

35E

lodes, apparently caused by the structures hosting the mineralisation, rather than the mineralisation itself. However, drilling of anomalies in adjoining areas located no new mineralisation, and the anomalies were therefore thought to have originated in the conductive near-surface material. In the 1960s, IP was the principal geophysical exploration method. Trial IP and resistivity surveys at Moonta during the early 1960s and 1970s showed that a characteristic resistivity range was associated with the Moonta Porphyry, and that the chloritic channels in which most lodes occur have coincident resistivity lows. IP anomalies, albeit not especially strong, were also found to be associated with the Moonta (Figs 7, 8 and 9) and Wallaroo lodes. A programme of reconnaissance frequency-domain IP/resistivity surveying was initiated, using dipole-dipole arrays and usually 91 m (300 ft) dipoles. Anomalies were followed up with similar surveys with 61 m (200 ft) dipoles. Magnetic surveys were carried out along the IP lines, and elsewhere, with the data being useful for mapping purposes. IP anomalies were detected in the East Kadina, Doora and Smithams areas (Fig.2), as well as around Moonta. However, geochemical data were also required to select the most prospective areas. SIROTEM came into its own in the 1980s, and led to the discovery of the Poona and Wheal Hughes deposits (Williams and Fraser, 1992). Interpretation of these data was complicated by the conductive overburden and the presence of cultural anomalies due to such features as cables, pipes and fences. Gravity and magnetics were successfully used in a mapping role. An INPUT survey, flown with north-south lines spaced at 305 m and an altitude of 122 m, detected numerous conductors, but the responses were probably predominantly from the overburden. Despite the technical difficulties, several previously unknown areas of copper mineralisation were discovered, including West Doora, Smitham's, Alford and Pridhams, as well as Poona and Wheal Hughes (Fig.1).


0

Green’s Beddome's Musgrave's Lode Lode Lode

Fergusson's Lode 65E

95E

125E

155E

185E

SE

100 METRES

215E

245E

275E

305E

Apparent Resistivity (Ω.m) 130

N=2 215

N=3

86

100 METRES

2250

Paramatta

IP data Fig.9

0

192

N=4 N=5

146

96 119

176 140

109 111

157 239

80 132

138 193

88 134

201

178 113

121

193

143 142

109 71

180 146

75

56 98

82 65

27 48

170

31

Percentage Frequency Effect 1.6

N=2 1.0

N=3 0.8

N=4 N=5

1.8

0.5 1.6

1.1 1.1

0.8 0.7

1.5 1.3

1.6 2.7

2.0 1.8

2.0 2.4

2.0 1.5

1.8 2.5

2.6

1.6 2.3

2.8 2.8

1.2 2.3

2.6 3.0

1.1 1.1

1.3 1.3

0.1 1.0

0.7

Fig.9. IP/resistivity data across the Moonta lodes. Source frequencies were 3 and 0.3 Hz. See Figure 7 for location.

Moonta Mines The IP/resistivity data from the Moonta Mines are have been reinterpreted by the authors using the DCIP2D package (see Appendix). As was standard practice at the time, these data were interpreted based on metal factor (MF), rather than PFE. For the purpose of re-interpretation, PFE has been re-calculated from the available data. However, since apparent resistivity is presented on the original pseudosections as apparent resistivity/2π, to one decimal place, the accuracy of the re-calculated values is degraded. Also, the data contain extreme variations in apparent resistivity, most likely associated with near-surface material. Nevertheless geologically reasonable sub-surface conductivity and chargeability distributions are obtained, with resistivity and IP anomalies in the expected locations of mineralisation (Fig.10). Poona Drilling at Poona defined a small high-grade ore body that resembles the lodes originally mined at Moonta. The lode is a planar body, 3-4 m in thickness, which dips to the north at about 50°. Ore occurs at between 15 and 100 m depth. Primary

Dentith & Cowan


NW

SE Fergusson's lode

Green's Beddome's Musgrave's lode lode lode

0

200

Treuer's Elder's lode lode

METRES

Conductivity (S/m)

0

200 300

Apparent Chargeability

Depth (m)

0

245E

305E

30W

24W

18W

12W

24W

18W

12W

0 0.062 0.033 0.003

100

0.69

200

0.39

300 0

100

0.050

200

0.025

300

185E

Depth (m)

100

125E

65E

125E

185E

245E

0.01

30W

305E

0

Depth (m)

Depth (m)

0

65E

0.000

100

0.045 0.030

200 300

0.015 0.000

Fig.10. Apparent resistivity and chargeability cross-sections derived from the IP/resistivity data in Figures 8 and 9. Amplitude (µV/A)

Amplitude (µV/A)

3000

200

3000

150

100

50

0

Channel

Wheal Hughes

Poona 2500

2500

9

Channel 1

10 11 12 13 14

28000N

28500N

Tx wire

29000N

29500N

Tx wire

30000N

2000

2000

1500

1500

30500N 96-0308 MESA

Fig.11. Large loop (1000 m) SIROTEM RVR data across the Poona and Wheal Hughes deposits along 41,200E. See Figure 7 for location. The anomalies at ends of the line are cultural in origin. The plateau of elevated response extending north of the Poona anomaly is probably a weathering or alteration effect.

mineralisation is pyrite and chalcopyrite, in a quartz-tourmalinechlorite gangue. Secondary copper mineralisation occurs in the upper half of the ore body. The lode is notable for its higher gold content than is normal for the area. During exploration of the Moonta area, SIROTEM surveys, using a 100 m coincident loop, were carried out in the vicinity of the Moonta Mines (Fig.7). Amongst many cultural conductors, a high-amplitude response was recorded slightly south of the old Paramatta Mines. Drilling showed the source to be sulphides, but with no significant quantities of copper and gold. However, the discovery provided an opportunity to test the sensitivity of largeloop TEM surveys for detecting relatively small conductive targets. The test survey used a 1000 m square transmitter loop, with 20 m stations along six north-south lines (Fig.7). The equipment used was a Zonge GGT20 transmitter with a Zonge GDP12 receiver, and also a SIROTEM and a SIROTEM RVR coil, measuring the vertical component of the TEM field. Apart from the Paramatta response, anomalies were defined that were subsequently found to be due to Poona and Wheal Hughes (Fig.11). Follow up of the Poona anomaly consisted of lines of 100 m coincident-loop SIROTEM surveys with 25 m moves, and alongstrike reconnaissance surveys, again using 100 m coincident loops. Also, further RVR measurements, at a 50 m line spacing, were

2

1000

1000

3 4

500

500

5 6 7 0 29900N

30000N

30100N

30200N

30300N

MP799 100

0 METRES

Depth of weathering

5.35m @ 4.35% Cu 2.05 g/t Au Lode

96-0309 MESA

Fig.12. Coincident-loop SIROTEM data across the Poona deposit along 41,300E. See Figure 7 for location.

carried out using the original large transmitter loop. Both vertical and horizontal component data were obtained. The detailed coincident-loop SIROTEM survey mapped a response due to a dipping body. A drill hole (MP799) sited on line 41300E, based on current filament modelling of the anomaly, intersected oregrade material (Fig.12). Note the asymmetrical twin peaks indicating a dipping plate-like conductor. To further define the geometry of the source body, an RVR survey with a 200 m square transmitter loop was undertaken. Modelling of the results was consistent with those from drilling. A


87

Dentith & Cowan


41400E

41500E

Loop 1

(a)

Loop 1

Loop 2

Loop 2

Loop 1

Coincident loop traverse

41300E

0

30300N

S

29630N

29830N

0

50

MP799

175 150 -40

Surface projection of ore body

30100N

30030N

N=1 N=2 N=3 N=4

46 127 145

39 82

187 203

17.4 30

55 115

0

50

140

6.4

96-0310 MESA

30630N

9.9

0.06

7.8

8.4 6.6

9.1 7.5

30830N

5.3 6.7

6.3 6.2

(b)

5.2

1.9 4.0

5.4 4.2

3.9

3.6 5.4

6.6

4.2

4.7

0

0.9 3.6

3.8

3.9

122

2.3

2.9

105 116

153

4.4

2.9

5.2

148

3.9

26 104

113

177

2.9 2.2

5.6 6.7

121 151

0.7

3.7

51 76

89

121

2.1 3.8

7.2

7.2

87

0.6

6.2

78 103

126

1.4

40 52

78

101

7.2

31 54

71

115

Poona

5.7

200 METRES

29830N

30030N

30230N

30430N

29830N

30030N

30230N

30430N

30630N

30830N

0.25

100

Fig.13. SIROTEM RVR data across the Poona deposit. Contours are µV/A) for loop 1. Also shown is the of response on channel 10 (µ interpreted position of conductor responsible for the observed anomaly. See Figure 7 for location.

98 130

9.1

7.3

9.8

85

101

39 61

59

74

161

10.2

29630N 0

METRES

Stations

30430N

36 45

59

85

222

Phase (mrad) 1.2 2.2 N=1 2.9 2.7 N=2 N=3 3.2 4.0 8.6 N=4 4.1 10.3

31 40

57

102

Loop 1 Interpreted conductor

20 36

58

125

N=5 N=6

Loop 2

30230N

Apparent Resistivity (Ω.m)

N=5 N=6

10

N

Poona

0.15 0.05

200 Depth (m)

Apparent Conductivity (S/m)

29630N 0

30630N

30830N

18

100

Channel

10000

100

Amplitude (µV/A)

2

200

Phase (mrad)

Depth (m)

2 3 5

1000

10

Fig.15. IP/resistivity data (100 m dipoles) across the Poona deposit along 41,250E. See Figure 7 for location. a) Pseudosections, b) apparent resistivity and chargeability cross-sections derived from a).

10 12

10

15

1

150

60500

125

60450

100

60400

-1

200 m

200 m

MP799

-100

TMI (nT)

-10

20

30

40

50

60

60350

6

Fig.14. SIROTEM DHEM data from drill hole MP799, Poona deposit. See Figure 7 for location.

75

l

96–0311 MESA

5

Depth (m)

ne

10

an

0

Ch

-10000

Amplitude (µV/A)

6 m @ 4.35% Cu 2.05 g/t Au

-1000

7 50

8

contour plot of channel 10 responses is shown in Figure 13, and was thought to provide evidence for a plunge to the northeast. DHEM showed that the drill hole had intersected the source of the anomalies seen at the surface. Figure 14 shows the data from MP799 (the discovery hole), which has a clear in-hole response at about 30-40 m. Ground magnetic data across the prospect were collected along a series of north-south traverses. The observed anomalies were assigned to variations in magnetite content of the Moonta Porphyry. A single line of gravity data showed no significant response from the Poona mineralisation. This was not unexpected, given that an anomaly of only 0.06 mGal was predicted directly over the mineralisation. 88


25

9

10

11 12 0 28200N

Depth of weathering 4m @ 3.81% Cu 1.88g/t Au

28400N

28600N

28800N

MP800 MP801 22m @ 2.07% Cu 0.37g/t Au

0

100 METRES

Intense kaolinitic alteration

Fig.16. Coincident loop SIROTEM and TMI data across the Wheal Hughes deposits along 41,135E. See Figures 7 and 17 for location.

Dentith & Cowan


(b)

41200 E

Loop A

Loop - RVR survey

28500 N

Loop B Interpreted conductor

H

MP801

60 40 20

80

MP800

L

0

28500 N

SIROTEM RVR profile

28700 N 41000 E

SIROTEM and magnetic profile

28700 N

(a)

28300 N

28300 N

100

0 METRES 41000 E

41200 E

96-0314a MESA

96-0314b MESA

28700 N

28700 N

(c)

(d)

L

72

100

H

H 140

28500 N

28500 N

MP844

MP844 H

1.0 0.9 .8 0

100

72 MP801 52 37 L

MP801 MP800

H 0 1. .9 .8 0 0 .7 0 .6 5 0 0.

MP800

28300 N

28300 N

41000 E

41200 E

96-0314c MESA

41000 E

41200 E

96-0314d MESA

Fig.17. Geophysical maps of the Wheal Hughes deposit. a) Location of selected surveys, b) SIROTEM RVR surveys, contours of channel 8 (vertical component), contours in µV/A. Stations north of 28,300N used loop A, remaining stations used loop B, c) gradient-array apparent resistivity data, contours in Ω.m, d) gradient-array IP phase data, contours in mrad.

A single line of dipole-dipole IP/resistivity data was acquired along the same line as the gravity data (Fig.15a). The survey used Zonge GDP12 equipment. There is some evidence for a correlation between the Poona mineralisation and low resistivity at shallow levels. An anomaly in the phase data is consistent with a north-dipping source, an interpretation confirmed by modelling by the authors using the DCIP2D software (Fig.15b). Wheal Hughes The Wheal Hughes deposit occurs about 1.5 km south of Poona (Fig.7), and is associated with a thick sheet of strongly kaolinised

Moonta Porphyry. Quartz-tourmaline lodes occur within the kaolinised area, striking northeast and dipping to the northwest. Again, gold occurs in association with the copper. Better definition of the large-loop TEM anomaly due to the Wheal Hughes deposit followed a similar approach to that at Poona. Results from a 100 m coincident loop survey are shown in Figure 16. Note that the survey lines for the moving-loop surveys were not perpendicular to the strike of the body, due to the need to avoid interference from an adjacent railway line. Two subsequent drill holes, MP800 and MP801 (Figs.16 and 17), intersected sulphides within an unusual zone of highly bleached and altered


89

Dentith & Cowan


10000

Channel 2 3 4 5

1000 100

Amplitude (µV/A)

10

10 12

1 0 -1

200 m

MP800

-100

200 m

-10

-1000 -10000

4 m @ 3.51% Cu 1.88 g/t Au 0

10

20

30

40

Depth (m)

50

60

70

96–0315 MESA

Fig.18. SIROTEM DHEM data from drill hole MP800, Wheal Hughes deposit. See Figure 17a for location.

kaolin. DHEM logging of these holes confirmed the intersection of the conductor (Fig.18). A small-scale RVR survey was also undertaken to provide better constraints on the geometry of the conductor. Two 300 x 300 m transmitter loops were used and contours of channel 8 (vertical component) are shown in Figure 17b. Ground magnetic data across the prospect showed a magnetic low correlating with the subcrop of the Wheal Hughes conductor, and extending above its down-dip extension (Fig.16). The source of the anomaly remains uncertain, but could be the result of kaolinisation destroying magnetite, or just variations in the magnetite content of the Moonta Porphyry as postulated at Poona. Gradient-array IP/resistivity data from a single set up were also acquired (Fig.17c and d). An eastnortheast-trending phase anomaly was mapped in the Wheal Hughes area, extending for about 200 m. Resistivity data showed a similar zone of low values, but this anomaly tends to diverge from the phase anomaly towards the southeast. These anomalies are probably caused by alteration of the host rock, and it was thought they might coincide with a less massive extension of the sulphides found at Wheal Hughes. However, drilling found only fresh porphyry and the source of the anomaly remains unknown. Other Deposits At Pridhams (Fig.1), principally pyrite-chalcopyrite mineralisation occurs in a dominantly pelitic sequence. Other lithologies include mafic volcanics and meta-siltstone with intense metasomatic alteration and quartz and carbonate veining. In this area the usefulness of IP was reduced by the presence of pipes, cables and fences, and also the low resistivity of the environment. However, drilling of anomalies did intersect copper mineralisation. Density measurements showed there to be a contrast of 0.4 g/cm3 between carbonate-sulphide-rich ore and the surrounding metasediments. Intermediate igneous rocks did not contrast with surrounding meta-siltstone. A density contrast of 0.1 g/cm3 occurs between the basement meta-sediments and the Neoproterozoic overburden. Gravity surveys were partly successful for mapping the geology, but the effects of the basement-cover interface confused interpretation. Resistivity soundings were used to identify whether particular anomalies originated in this manner. However, the effects of the density contrast between basement and 90


Fig.19. Spectral IP data across the West Doora mineralisation.

cover were sufficient to mask any gravity response from mineralisation. There is little deposit-scale geophysical data publicly available from the Wallaroo Mines area, although there is limited information from the geologically similar West Doora area. This area was selected for a 1.5 km trial IP/resistivity traverse, over weak mineralisation, using a Zonge GDP-12 spectral IP receiver. This method seems to have been successful in locating the main West Doora mineralisation, and also shallower areas of mineralisation to the southwest, where phase anomalies are present (Fig.19). However, the resistivity response is only weakly anomalous. CONCLUSIONS The fact that the geological environment in the MoontaWallaroo district contains abundant primary and secondary (metasomatic) magnetite, means that geological features can be readily mapped using aeromagnetic data. Even the relatively widely spaced aeromagnetic data (400 m flight lines) used here allow identification and mapping of major geological structures that are apparently previously unrecognised (at least in readily available published descriptions). A tectonic model involving reverse movements along northeasterly trending faults, with associated dextral strike-slip along east-west trending structures has been proposed. The almost total lack of outcrop in the Moonta-Wallaroo area suggests that more detailed magnetic surveys, some such data having already been acquired for exploration purposes, will make a crucial contribution to the understanding of the geological and metallogenic history of the area. Although the Moonta-Wallaroo area is a challenging environment for electrical and electromagnetic surveys, these

Dentith & Cowan


techniques have been demonstrated to be able to detect responses from the sulphide mineralisation present in the region, and have led directly to the discovery of the only two deposits that have been mined in recent times. ACKNOWLEDGEMENTS Peter Williams is thanked for his advice on the geophysics of Poona and Wheal Hughes. Particular thanks must go to Colin Conor for his tolerance, advice and helpful suggestions regarding the geology of the Moonta-Wallaroo area. Nevertheless, responsibility for the magnetic interpretation and the resulting regional geological model rests solely with the authors. All or part of Figures 1, 7-9 and 11-19 were drafted by PIRSA. Jayson Meyers and Phil Muir are thanked for acting as referees.

REFERENCES Conor, C.H.H., 1995. Moonta-Wallaroo Region. An Interpretation of the Geology of the Maitland and Wallaroo 1:100,000 Sheet Areas. Mines and Energy South Australia, Open File Envelope 8886. Daly, S.J., Fanning, C.M. and Fairclough, M.C., 1998. Tectonic evolution and exploration potential of the Gawler Craton, South Australia. AGSO Journal of Australian Geology and Geophysics, 17, 145-168. Dickinson, S.B., 1942. The Structural Control of Ore Deposition in Some South Australian Copper Fields. Geological Survey of South Australia, Bulletin, 20. Dickinson, S.B., 1953. The Moonta and Wallaroo copper mines. In, Edwards, A.B., (Ed), Geology of Australian Ore Deposits. Australasian Institute of Mining and Metallurgy, 487-504. Ferris, G.M., Schwarz, M.P. and Heithersay, P., 2002. The geological framework, distribution and controls of Fe-oxide and related alteration, and Cu-Au mineralisation in the Gawler Craton, South Australia: Part 1. Geological and tectonic framework. In, Porter, T.M., (Ed), Hydrothermal Iron Oxide CopperGold and Related Deposits; A Global Perspective, Volume 2. PGC publishing. Gow, P.A., Wall, V.J. and Valenta, R.K., 1993. The regional geophysical response of the Stuart Shelf, South Australia. Exploration Geophysics, 24, 513-520. Newton, A.W., 1996. Mineral Exploration and Development in South Australia. South Australia Department of Mines and Energy, Report Book, 96/1. Parker, A.J., 1993. Palaeoproterozoic. In, Drexel, J.F., Preiss, W.V. and Parker, A.J., (Eds), The Geology of South Australia, Volume 1 The Precambrian. South Australia Geological Survey, Bulletin 54, 51-105. Williams, P.K. and Fraser, G.A., 1992. The use of transient electromagnetics in exploration for conductive sulphide associated Cu-Au mineralisation at Moonta, South Australia. Exploration Geophysics, 23, 507-514. Woodcock, N.H. and Fischer, M., 1986. Strike-slip duplexes. Journal of Structural Geology, 8, 725-735.


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