ASEG Extended Abstracts - Geophysical Signatures of West Australian Mineral Deposits, 1994

The evolution of Western Australian terrains spans over 4000 million years. During this time, a wide variety of mineral deposits have formed. The ca 2700 to 2500 Ma granitoid-greenstone terrains of the Yilgarn Craton are the most intensely mineralised with world-class nickel and gold deposits, as well as mineralised pegmatites. The Pilbara Craton is less welt mineralised but is overlain by the Late Archaean to Palaeoproterozoic Hamersley Basin which contains world-class iron ore deposits. Proterozoic orogens and basins contain sporadic and locally important gold, copper, copper-nickel, lead-zinc, and uranium mineralisation. Diamondiferous lamproites and kimberlites have intruded within, and adjacent to, the Kimberley Craton since the Proterozoic. These include one of the world's largest diamond deposits at Argyle. The Canning Basin hosts late Paleozoic carbonate-hosted zinc-lead mineralisation. Since the Mesozoic, heavy-mineral sands have been deposited, mainly in the Perth Basin, and lateritic weathering has resulted in world-class bauxite deposits. This deep weathering has also resulted in supergene enrichment of gold, nickel, rare earth elements and uranium.

Geophysical exploration in Western Australia is hindered by a mantle of conductive and magnetic weathered rocks that covers much of the State. This has required the adaptation of most geophysical methods for successful application in Western Australian conditions, and has ted to the development and widespread use of, for instance, high-resolution aeromagnetics and time-domain electromagnetic methods. However, these difficulties have not prevented geophysics from being an integral part of exploration tor base metal, diamond, gold, iron ore, manganese, nickel and uranium deposits in Western Australia.

Mississippi Valley-type base-metal deposits are difficult geophysical targets and direct detection of the ore is not usually possible. However, gravity and magnetic data can be used to locate basement highs associated with such deposits and, on a semi-regional scale, induced polarisation surveys have been used to locate marcasite halos associated with the orebodies. Volcanic-hosted massive sulphide base-metal deposits have variable geophysical responses. Physical property contrasts with their host are highly variable and thus methods such as magnetics, induced polarisation and electromagnetics may fail to generate recognisable responses. Mise-a-la-masse surveys have proved highly successful for mapping such mineralisation on a prospect scale, once it has been intersected by drilling. The only example of a sedimentary exhalative deposit in the State for which data are available has distinct gravity, magnetic and time-domain electromagnetic anomalies.

Diamonds in Western Australia mainly occur in lamproite pipes. These pipes have variable magnetisations but can usually be detected using high-resolution aeromagnetic surveys. The pipes can also be conductive and mapped using electromagnetic techniques if the host rocks are suitably resistive.

The major geophysical method utilised in gold exploration is high-resolution aeromagnetics which is used to map favourable structures and rock types. Electrical and electromagnetic methods can also be used where gold is associated with sulphides.

Geophysics has been comparatively tittle used in exploration for iron ore. Exploration for supergene-enriched deposits mainly uses aeromagnetics, to map favourable structures and to detect magnetite destruction and replacement associated with mineralisation, and gamma-ray logging for stratigraphic correlation purposes.

The major technique used in manganese exploration is the gravity method, taking advantage of the positive density contrast between ore and host rocks.

The mineral sands industry uses aeromagnetic data to map placer deposits containing ilmenite, but the relatively low cost of drilling limits the use of geophysical exploration methods.

Nickel sulphide mineralisation can be directly detected using induced polarisation and electromagnetic techniques. Gravity and magnetic surveys are also used, but mainly in a mapping role.

Carbonatith intrusions associated with rare-earth-element mineralisation give rise to large magnetic anomalies. Radiometric and gravity anomalies can also occur.

Uranium mineralisation has been directly detected using radiometric data, but some deposits are concealed below cover. Magnetic, electromagnetic, electrical and gravity surveys can be used to locate the rocks and structures which host mineralisation.

Aeromagnetics became recognised as a valuable aid to exploration in Western Australia during the "nickel boom" of the 1960s. The now commonplace term "high-resolution aeromagnetics" evolved during the 1980s, mainly to meet demands of a resurgent gold exploration industry which required a greater understanding of geological structure and rock type than was possible with conventional field mapping. High-resolution aeromagnetic surveys are typically flown at a line spacing of 200 m, a terrain clearance of 60 m and with a magnetometer sampling intervai of 7 m. Imaging of the data provides a greater range of options for highlighting subtle structural features than conventional contours. Image maps may be interpreted using skills of photogeology and satellite-image interpretation and have been a major factor in bridging the communication gap between geophysicists and geologists.

The effectiveness of radiometric data in Western Australia, collected simultaneously on virtually all high-resolution aeromagnetic surveys, has been constrained by lack of outcrop and transported soils. In appropriate areas the method is successful at mapping different types of feisic igneous, sedimentary and metasedimentary rocks, and also in identifying hydrothermal alteration zones displaying potassic enrichment.

Problems of high surface conductivity have, in the past, limited the application of airborne time-domain electromagnetic surveys. The new generation of digital systems (e.g., QUESTEM) has improved depths of penetration through the real-time removal of unwanted noise. Airborne time-domain electromagnetics is no longer only an anomaly detector, but a three-dimensional regional conductivity mapping tool which, when integrated with magnetic data, enables the targeting of bedrock conductors to be controlled by interpreted geology.

The exploration strategy in the search for Mississippi Valley-type (MVT) mineralisation on the Lennard Shelf has been modelled on the approach commonly used in exploration for similar mineralisation in North America. Regional areas of interest are defined using a combination of geology, geochemistry and geophysics, then systematically grid-drilled. Several MVT lead-zinc deposits have been discovered on the Lennard Shelf through the application of this strategy.

Mississippi Valley-type lead-zinc deposits are difficult geophysical targets. Their geophysical characteristics have been studied on the Lennard Shelf to provide a guide for more cost-effective exploration both on the Lennard Shelf and elsewhere. On a regional scale, gravity and aeromagnetic surveys, used in conjunction with geochemistry, effectively focus exploration into favourable structural and lithological settings. Detailed gravity and seismic surveys delineate areas of the host carbonates in regions of shallow cover.

Several known Mississippi Valley-type deposits and prospects on the Lennard Shelf are associated with extensive marcasite haloes and associated induced polarisation anemalies. In areas of shallow cover, the induced polarisation method can be used to cost effectively define prospective areas by mapping mineralised systems on a semi-regional scale. These areas can then be tested using grid drilling.

The Abra base-metal deposit is a large, deeply buried, low-grade mineralised body located in the Jillawarra mineralised belt in the Bangemall Basin, Western Australia. It has no surface geological or geochemical expression. The deposit was discovered in 1981 by drill testing a 270 m deep target, based on the detailed modelling of a 400 nT bullseye magnetic anomaly which had a coincident weak residual gravity anomaly. Follow-up drilling broadly confirmed the original magnetic interpretation, and outlined an estimated 200 Mt of low-grade iron-barium-lead-silver-copper-gold mineralisation.

Both moving-loop and large fixed-loop time-domain electromagnetic surveys recorded a broad anomaly over the Abra mineralised system. The anomalous time-domain electromagnetic transients have an exponential decay with a time constant of about 1.3 ms, indicative of low conductance. Downhole time-domain electromagnetic and mise-a-la-masse surveying confirms that the whole of the mineralised system, including both the stratiform zone and the underlying stringer zone, is weakly and uniformly conductive. No specific zones of locally greater conductance, which might be indicative of higher-grade mineralisation, were detected in the surface time-domain electromagnetic work. In the downhole time-domain electromagnetic surveying, the broad response to the bulk mineralised body is complicated by the use of relatively small transmitter loops which have "selectively" energised portions of the large conductive body, resulting in both "in-hole" and "off-hole" responses being recorded, depending upon the relative transmitter loop-conductor-drillhole geometry. Several localised secondary conductor responses (both in-hole and off-hole) were also detected but, to date, this work has not successfully demonstrated the presence of discrete high-grade zones of significant dimensions within the overall system.

The Scuddles copper-zinc deposit was discovered in 1979. Before mining commenced in 1990, many exploration techniques were tested over the deposit. Of the geophysical techniques, aeromagnetics and gravity have proved the best regional mapping tools. For direct detection of the mineralisation, time-domain electromagnetics from both surface and drillhole has been the most effective. Mise-a-la-masse successfully outlined the limits of the mineralisation whereas IP responded to shallow mineralisation only.

Several airborne electromagnetic surveys have been flown over the nearby Gossan Hill deposit, which has similar characteristics to Scuddles but is considerably shallower. All have failed to produce anomalies which would warrant follow up on a regional exploration survey.

Mons Cupri is an Archaean proximal, volcanogenic massive-sulphide deposit in the west Pilbara of Western Australia. Mineralisation occurs in the Mons Cupri Volcanics of the Whim Creek Belt. Stockwork copper-sulphides in altered Mount Brown Rhyolite are overlain by shallowly dipping, massive, copper-lead-zinc sulphides in volcaniclastic sedimentary rocks.

Two airborne magnetic surveys detected a 70 nTanomaly in broad correlation with the outline of the mineralisation. The likely source of this anomaly is magnetite within a chlorite alteration pipe. One trial and two surveys with airborne electromagnetics have been conducted. None of the systems, including GEOTEM II, yielded anomalies over Mons Cupri itself.

Seven trials and one survey with ground electromagnetics were also completed. Again, none produced anomalous results attributable to the mineralisation.

Two induced polarisation surveys detected slightly lower resistivities over the stockwork mineralisation; a magnetic induced polarisation survey obtained the opposite result. However, higher percentage frequency effects, chargeabilities, and relative phase shifts clearly coincided with the stockwork mineralisation. None of the systems unequivocally detected the massive sulphides at depth.

Mons Cupri is a difficult geophysical target. Low total-sulphide content, deep massive sulphides and a lack of electrical continuity combine to produce an enigmatic response and to defeat detection by electromagnetic systems.

The Sally Malay nickel deposit has clear physical property contrasts with its host rocks and therefore lends itself to detection and mapping by geophysical methods. Conductivities as high as 30,000 S/m compared to the resistive country rock mean that electromagnetics is the best method for locating and mapping such a deposit. The magnetic susceptibility and chargeability are two orders of magnitude higher than the country rock, making magnetics and induced polarisation useful methods. Although there is a density contrast of 1 g/cm³, the gravity method was not used, largely because of the success of electromagnetics, but also because of the steep topography around the deposit.

Its short strike length means that the mineralisation does not make a good airborne target using conventional line spacings.

The limited geological outcrop within the Kambalda district, when coupled with the favourable physical properties of nickel sulphides, make geophysical methods an important tool in the exploration for Archaean nickel deposits in this area. Present exploration strategy uses detailed airborne and surface magnetics in the targeting of favourable ore environments, structures, and prospective ultramafic-mafic contacts. Surface and downhole electrical and electromagnetic techniques are then applied to optimise prospect drilling and directly detect nickel sulphides. Thick, conductive overburden, magnetic "noise" originating in near-surface laterites, "false" anomalies due to conductive sedimentary units, and the extensive blanket of lake sediments in certain areas continue to present challenges to successful exploration.

The Rocky’s Reward nickel sulphide deposit is located in the Agnew-Wiluna greenstone belt, about 2 km north of the Perseverance (Agnew) nickel mine. The belt lies within the northern portion of the Eastern Goldfields Province of the Archaean Yilgarn Craton, Western Australia. Ore-grade mineralisation was discovered at Rocky’s Reward in 1984 as a result of drill testing a geochemically anomalous gossan.

Geophysical surveys (airborne and ground magnetics, induced polarisation/resistivity) had been carried out over or in the vicinity of the deposit well before the discovery of mineralisation. However, even though a magnetic anomaly was clearly delineated over the Rocky’s Reward deposit, the target was not selected for follow up at that stage as the surface geological expression did not fit the existing conceptual geological model.

A large amount and variety of geophysical work, including airborne and surface time-domain electromagnetics, induced polarisation/resistivity, controlled source audiomagnetotellurics, gravity and downhole surveys was subsequently completed following the discovery of mineralisation at Rocky’s Reward. The object of these surveys was to map and characterise the deposit geophysically, in order to assist in the delineation of the extent and geometry of the mineralisation, and to evaluate geophysical techniques applicable to further exploration in the area.

The deposit represents an excellent target for several geophysical techniques because of its shallow depth, geometry, and physical property contrasts of the ore and its host with surrounding rocks. A combination of ground magnetics and time-domain electromagnetics proved to be the most definitive and economical for detecting and mapping the deposit.

The Telfer gold deposits are hosted by Middle Proterozoic marine sedimentary rocks of the northeastern Paterson Orogen. They occur within two en echelon, asymmetric, doubly plunging anticlines, with ore being extracted from reefs and stockworks.

Regional magnetic and gravity surveys have been undertaken to assist in mapping stratigraphy, intrusions and structures in the Telfer district. These surveys indicate the presence of intrusions close to the Telfer gold deposits, which is regarded as supporting a genetic relationship between granitoids and mineralisation. The Telfer mineralisation itself has no gravity or magnetic signature.

The narrowness of the reefs, deep oxidation and the presence of shallow, thin, electrically resistive beds make the Telfer gold deposits a difficult geophysical target. Direct current resistivity techniques were used to assist mapping of the quartz reefs. Surface and downhole electromagnetic pulse surveys undertaken at Main Dome after overburden stripping detected subtle responses coincident with the Middle Vale Reef.

The Fortnum gold deposit is a structurally controlled gold system hosted by Lower Proterozoic sedimentary and volcaniclastic rocks of the Glengarry Group in the Glengarry Basin of Western Australia. Geophysical techniques applied at Fortnum include ground and airborne magnetics, resistivity, induced polarisation and gravity. Magnetic and resistivity data enabled extrapolation of geological information to areas concealed by transported cover, and the interpretation of structures believed to have influenced gold deposition. Magnetic and gravity data delineated several features for which geological explanations remain speculative. The application of geophysics did not detect mineralisation, nor was it expected to, but has been a valuable tool in advancing our geological knowledge of the system.

The Mount Morgans Archaean gold deposit is hosted by banded iron-formation and has produced over 25 t of gold to date. The gold is associated with sulphides which have replaced magnetite.

Aeromagnetic data covering the Mount Morgans deposit show a variably depleted magnetic effect caused by the replacement of magnetite by non-magnetic minerals, chiefly pyrite. Coincident with the magnetic effect is a conductive response recorded using airborne electromagnetics. The likely source of this conductivity anomaly is the weathering profile.

The main ground geophysical surveys over the deposit are magnetics and 50 m dipole-dipole induced polarisation. The induced polarisation detected a chargeability anomaly associated with the sulphide mineralisation and a conductive response associated with the weathering of sheared rocks. Surveys of magnetometric resistivity and very low-frequency electromagnetics have been conducted over the Mount Morgans North resource, a similar but smaller gold resource located on the BIF ridge north of the main Mount Morgans deposit. The responses observed over this resource confirm the conductive and chargeable nature of the sulphide mineralisation associated with the gold.

Mineralisation at the Bounty gold deposit is in a steeply plunging zone within a sheared iron-formation in the Archaean Forrestania greenstone belt. The gold was deposited together with pyrrhotite, replacing magnetite, but later dyke intrusion has converted pyrrhotite to magnetite adjacent to the dyke. Geophysical surveys have included magnetic and transient electromagnetic measurements. Magnetic data show the Bounty mineralisation is highly magnetic. The magnetic information has mainly been used to help map stratigraphy and structure, seeking favourable sites for mineralisation. An orientation transient electromagnetic survey showed that the Bounty orebody is a good conductor. More extensive surveys delineated an anomaly over the North Bounty deposit also, and discovered several other conductive zones which, so far, have been found only to be barren sulphidic shale and chert horizons.

Tuckabianna Gold Mine is located about 25 km east of Cue, on the Mount Magnet-Meekatharra Shear Zone. The mine sequence consists largely of basalt, dolerite to gabbro, mafic schists, banded iron-formation (BIF), and intrusive quartz-feldspar porphyry dykes and sills. Gold mineralisation is hosted by the NE-striking, E-dipping BIFs with shears localising the ore zones. Laterite derived from the weathering of these BIFs may contain a significant gold resource.

Downhole logging has shown that the BIFs typically have lower relative gamma activity and higher densities, resistivities and susceptibilities compared with the mafic units and porphyry.

Petrophysical measurements of magnetic susceptibility, anisotropy of magnetic susceptibility and natural remanent magnetisation have characterised two BIF types, although no relationship between gold mineralisation and the BIF types has been conclusively demonstrated.

Tuckabianna’s regional geophysical dataset consists of Bureau of Mineral Resources (BMR) aeromagnetic and gravity data and high-resolution aeromagnetic and radiometric data. Of these, the most useful have been the high-resolution aeromagnetic data, because they clearly define the BIFs and structures which can localise gold mineralisation. Such lithological and structural detail cannot be recognised in either the BMR regional magnetic and gravity datasets or the high-resolution radiometric dataset.

The Geoscan remote-sensing system has been flown over Tuckabianna. However, the laterite cover (1-21 m thick) has blanketed much of the geology and only some rock types and major lineaments can be identified from the data.

Ground geophysical surveys have been dominated by the magnetic method, which has been extremely useful in delineating the BIFs and structural features, as has the gravity method. Electrical surveys, including magnetometric resistivity, induced polarisation (frequency and time-domain) and electromagnetic methods have also been used. These electrical techniques have not been completely successful in the direct detection of mineralised zones. However, the resistive nature of the BIF is such that these methods may be successfully used to locate the BIF and porphyry, in addition to delineating some structures.

A resource of about 1 Mt of gold at 2.7 g/t has been delineated at Mount York, 120 km south-southeast of Port Hedland in the Pilbara Region of Western Australia. The deposit is contained within a banded iron-formation.

In order to supplement ongoing geological investigations to define the resource, a geophysical programme was undertaken to assist mapping and also delineate primary sulphide zones which could have associated gold mineralisation. Aeromagnetic, spectral induced polarisation, surface and downhole electromagnetic surveys, and downhole density logging were undertaken.

The magnetic data clearly outlined the lateral extent of the banded iron-formation but were unable to delineate subtle structure which was thought to control primary mineralisation. Induced polarisation and electromagnetic surveying provided numerous targets in both the primary and oxidised zones. Drill testing of these primary-zone targets intersected sulphide mineralisation but, unfortunately, no gold mineralisation of economic width and grade. Downhole density logging of the secondary oxide zone allowed the density of the mineralisation to be better defined and hence provided a sound base for resource calculations.

Leases held by Kalgoorlie Consolidated Gold Mines Pty Ltd over the Kalgoorlie Goldfield have an approximate area of 30 by 10 km, with the main production areas of Fimiston, Mount Charlotte and Mount Percy within the central portion of this tenement block. Due to the proximity of the leases to residential areas, significant portions of land are inaccessible for exploration. Near-surface contamination, as a result of historical mining and prospecting, also presents problems, as do the deep weathering profile and associated conductive overburden, which covers most of the Kalgoorlie Goldfield.

Due to the relatively small size of the lease holdings and the constraints detailed above, the currently employed geophysical techniques mainly involve detailed ground surveys and include petrophysical studies of the three principal styles of mineralisation and the surrounding host rocks. The aims of the surveys are improved definition of geological features, and indirect detection of the three principal styles of mineralisation recognised at Fimiston (Golden Mile), Mount Charlotte and Mount Percy.

The petrophysical data indicate that gravity, magnetics and induced polarisation can be used for the delineation of rock types whereas induced polarisation has potential to identify mineralisation. The combination of gravity and ground magnetic surveys at a prospect scale permits considerable refinement of the structural and lithological features in areas of poor outcrop. Studies are ongoing evaluating the potential use ofdownhole induced polarization for detection of Mount Charlotte-style stockwork mineralisation, and the use of ground penetrating radar to detect voids for underground mining.

The Bottle Creek gold deposit lies on a north-trending structural break between two major terrains in the Ularring greenstone belt, north of Kalgoorlie. Aeromagnetic data in the vicinity of the deposit suggest several structures which appear to have controlled gold mineralisation. The mineralised horizon lies at a structurally discordant, north-northwest-trending internal junction within a thick mafic sequence in the eastern terrain. Lithological variations within this sequence have been inferred to represent distinct upper and lower groupings, and are structurally determined. Interpreted dips within the eastern terrain are very steep and can be contrasted with much shallower regional dips in the western terrain. Mineralisation within the Emu Formation host, which occupies a sheared zone, contains pyrrhotite but the observed magnetic responses along the structure appear to reflect magnetite-constructive alteration near the boundary. There are two shears; the largest separates western and eastern sequences, which meet acutely but are not mineralised along the shear. There is a lesser parallel structure further east. The structures are separated by a mafic unit which generates a chain of isolated magnetic responses along this structure. The Bottle Creek deposits are associated with sheared and altered junctions along the, apparently, lesser shear. Major deposits appear to be localised where this shear is intersected by large northeast-trending fractures.

The relationship between gold mineralisation, genetically related mineral assemblages, magnetic susceptibility and features observable in ground and airborne magnetic surveying is established at three gold mines in the Victory-Defiance gold camp at Kambalda, Western Australia. The gold mines are Orion, North Orchin and Revenge. There are significant magnetite-stable alteration haloes enveloping the gold lodes comprising these deposits.

The magnetite-stable alteration is peripheral to gold lodes in differentiated dolerites, metabasalts and metasedimentary rocks which have undergone lower to mid-greenschist fades metamorphism. The magnetite alteration is partly coincident with the well-documented chlorite and biotite alteration zones.

The magnetic susceptibilities of the magnetite-stable alteration assemblages range to 100 x 10~³ SI units. It is noted that the magnetic properties of the Kapai Slate vary considerably on a regional scale but appear to be consistently high (up to 400 x 10~³ SI units) within the Victory-Defiance gold camp.

Magnetic maxima are coincident with all three gold deposits. The amplitude of the maxima observed in low-level aeromagnetic surveys are 30, 300 and 400 nT for Orion, North Orchin and Revenge, respectively.