Middleback Ranges Research Articles

Episodic mafic magmatism in the Eyre Peninsula: Defining syn- and post-depositional BIF environments for iron deposits in the Middleback Ranges, South Australia

Banded iron formation (BIF)-derived iron ore deposits of the Middleback Ranges, Eyre Peninsula, South Australia, are hosted within Archean to Paleoproterozoic basement in the southeastern Gawler Craton. Mafic sills and dikes are associated with the orebodies throughout the belt and although described previously, have never been studied in detail. Two main types of mafic rocks, amphibolites and dolerites, are distinguishable from field occurrence and mineralogy. Amphibolites interbedded with BIFs from the southern Iron Magnet deposit are characterized by extensive overprinting. These amphibolites contain metamorphic assemblages of coexisting Mg-Fe- and Ca-amphiboles, garnet, corundum, biotite and relict titaniferous magnetite displaying ilmenite exsolution. Amphibolite petrography and geochemistry in the southern part of the belt indicate formation in a complex BIF depositional environment, with possible detrital input of titaniferous magnetite from igneous rocks. In contrast, NW-trending dolerite dikes occur throughout the belt and contain assemblages consisting of igneous magnesiohornblende and plagioclase. SHRIMP U–Pb zircon ages for amphibolites and dolerites yield two age groups. Amphibolite from Iron Magnet yields a 207Pb/206Pb weighted mean age of 2542 ± 15 Ma. The most robust 206Pb/238U weighted mean age of 782 ± 23 Ma for dolerites comes from the Iron Queen deposit in the central part of the Middleback Ranges. LA-ICP-MS U–Pb geochronology of U-bearing hematite from the Iron Knight South deposit yields ages of ~680 Ma. Taken together, these results suggest mafic magmatism contemporaneous with Archean BIF deposition followed by a high-grade metamorphic event (~2.47–2.41 Ga Sleafordian Orogeny). Dikes from Cape Donington, the southernmost point on the Eyre Peninsula are, instead, associated with the 1.73–1.69 Ga Kimban Orogeny, as evidenced by zircon from a pyroxene-rich dolerite, which yields a 206Pb/238U weighted mean age of 1711 ± 14 Ma. The dolerite ages from the Iron Queen deposit provide the first evidence for the presence of the ~800 Ma craton-scale Gairdner Large Igneous Province in the Middleback Ranges. The young U–Pb hematite age from Iron Knight South and post-emplacement alteration of dikes of Gairdner age from the central part of the Middleback Ranges belt provide additional support for multiple events leading to upgrading of iron ores.

Petrographic and geochronological constraints on the granitic basement to the Middleback Ranges, South Australia

The Middleback Ranges is a 60 km-long BIF-endowed iron ore belt located in the southeastern Gawler Craton and is the largest exploited iron resource in South Australia. The belt is hosted within an Archean- to Paleoproterozoic-aged granitic-gneissic basement terrane. Petrographic and geochronological constraints are given for a suite of granites associated with deposits throughout the Middleback Ranges and compared with other regional granites. Granites from the Middleback Ranges show complex mineralogical and geochemical features indicating multiple overprinting events. Sensitive high-resolution ion microprobe U-Pb zircon ages from all granites across the Middleback Ranges yield Archean ages, suggesting the existence and a more extensive distribution of Mesoarchean crust than previously recognized beneath the entire Middleback Ranges BIF sequence. Grey and pink granites from the central Middleback Ranges (Sultan prospect) produced the oldest 207Pb/206Pb weighted mean ages within the sample suite (3241 ± 6 and 3238 ± 9 Ma, respectively). Zircon in the latter granite also features overgrowths dated at ∼2.6 Ga and interpreted to be metamorphic. A second group of younger granites from the Iron Monarch deposit and a sample of Cooyerdoo Granite (northern Middleback Ranges) have a minimum crystallization age of 3165 ± 8 Ma and a 207Pb/206Pb weighted mean magmatic age of 3134 ± 18 Ma, respectively. The youngest granites analyzed here derive from the Iron Baron deposit in the central part of the belt, which display strong deformation and have a minimum crystallization age of 3089 ± 9 Ma, and from the Iron Magnet deposit in the southern part of the belt (207Pb/206Pb weighted mean age of 3023 ± 33 Ma). Petrographic and geochemical features observed within the sample suite indicate that post-Archean overprinting events led to alteration of the granitic basement and may infer that fluid-rock interaction and granite leaching were important for BIF upgrading processes and iron-oxide formation.

Iron-oxides constrain BIF evolution in terranes with protracted geological histories: The Iron Count prospect, Middleback Ranges, South Australia

When studied at appropriate scales of observation and using complementary methods, the intrinsic textural and geochemical complexity of iron-oxides can provide unparalleled insights into the evolution of iron ore resources from banded iron formation deposition to ore formation. Iron Count is one of >20 deposits and prospects within the Archean Middleback Ranges iron ore belt, South Australia, in which iron-oxides from BIFs and ores display variable textural and trace element signatures. Integrated petrography, iron-oxide laser-ablation mapping and geochemistry, and in-situ dating of BIF-derived ores was undertaken to assess whether features observed in samples from this representative prospect might be attributed to localized and superimposed overprint processes.The prospect consists of a West and East ridge, each characterized by a sequence of overprints expressed as interconversions between iron-(hydr)oxides and accompanying variation in trace element concentrations. Pseudomorphic replacement of early magnetite by hematite (martite), followed by replacement by iron-hydroxides is recognized in samples from the East Ridge. Further overprints are expressed as veining, brecciation and crystallization of (micro)platy hematite rich in granitophile elements (Sn, Mo, W, U). Martitization of early magnetite in the West Ridge is accompanied by enrichment in Zn, Co and Ni in martite. Subsequent recrystallization of martite/iron-hydroxides by granoblastic hematite is marked by enrichment in Ti, Ta, Nb, REE and granitophile elements, and co-crystallization of REE-minerals. This is followed by crystallization of (micro)platy hematite, similarly rich in these elements. Enrichment in Ti and granitophile elements in granoblastic hematite is interpreted to infer crystallization from highly saline fluids generated by interaction between evaporite-bearing sedimentary rocks and granite-derived fluids. Post-Archean Australian Shale-normalized REY fractionation trends of iron-oxides from the East Ridge are consistent with precipitation of iron-rich minerals from a mixture of anoxic seawater and hydrothermal vent fluids. Analogous trends from the West Ridge lack a seawater signature. Contrasts in REY fractionation trends between the two ridges is interpreted as a combination of the loss of primary seawater signature due to severe overprinting in the West Ridge, and to lateral variation in the marine depositional environment. Results validate the hypothesis that variation in iron-oxide trace element signatures seen throughout the Middleback Ranges iron ore belt results from overprinting due to superimposed tectono-magmatic events that have locally affected each deposit. Despite undergoing different sequences of ore-forming stages, as well as possibly distinct depositional environments, the two ridges share a common overprint, dated here at ~1790 Ma using in-situ UPb geochronology of co-existing hematite, monazite and xenotime. This age is considered to represent the timing of interaction between granite-derived fluids with ore-hosting rock. Such interpretation is supported by the geochemical signatures of the dated iron-oxides showing enrichment in granitophile elements and coincident ages for the Wertigo Granite and Myola Volcanics, which occur proximal to the prospect and were emplaced/erupted during intracontinental rifting. The approach undertaken here, combining petrography, iron-oxide geochemistry and in-situ dating of BIF-derived ores is generically applicable to other iron resources within the Middleback Ranges or analogous terranes elsewhere.

Discrimination and Variance Structure of Trace Element Signatures in Fe-Oxides: A Case Study of BIF-Mineralisation from the Middleback Ranges, South Australia

Uni- and multivariate statistical analyses of trace element laser-ablation inductively coupled plasma mass spectrometry data for Fe-oxides from banded iron formation (BIF) and BIF-hosted ores in 13 deposits/prospects of the Middleback Ranges, South Australia are presented. The obtained trace element signatures were considered within a petrographic-textural framework of iron-oxide evolution from magnetite through clean martite and porous martite to platy hematite, to evaluate changes in trace element concentrations with respect to the ore enrichment processes. Statistically valid distinctions among different hematite textures were indicated for most trace elements by linear mixed-effects models. Furthermore, the hematite data showed significant intra-class correlations between spot-analyses within individual polished blocks and correlations between polished blocks within individual deposits. The data are thus aggregated within their hierarchical levels. Two linear discriminant function analyses were performed to determine the combinations of trace elements that can distinguish hematite by textures and by location within the Middleback Ranges. Tin, a significant discriminator element in both models, reflects the regional influence of granite-affiliated hydrothermal fluids on the clean martite. This granitic signature, therefore, postdates formation of magnetite BIFs and potentially represents a supergene ore enrichment stage. The combination of Ni, Co, Ti and Nb was discovered to be uniquely attributed to discrimination of the Northern and Southern Middleback Ranges, indicating very specific local settings unrelated to hematite textures. Both local and regional settings impacting on the trace element signatures of Fe-oxides throughout iron ore formation are recognised, suggesting distinct ore enrichment conditions within various segments of the belt.

Petrography and trace element signatures of iron-oxides in deposits from the Middleback Ranges, South Australia: From banded iron formation to ore

The Middleback Ranges is a major iron ore belt in the southeastern region of the Gawler Craton, South Australia, interpreted to be of BIF origin. Iron ore deposits are hosted within ∼2550 Ma metasedimentary rocks of the Middleback Group and occur as a series of N-S trending hills, forming a ∼60 km-long magnetic anomaly. A petrographic-geochemical study of iron-oxides from BIFs and iron ores was undertaken on samples from thirteen locations spanning the strike of the belt. Iron-oxides are texturally diverse due to multiple processes accompanying and postdating ore formation. Primary magnetite features preserved in the southern segment of the belt display distinct overprinting features (e.g., increased porosity, reworked grain boundaries) and multiple generations of growth associated with deposition of trace minerals, including native gold. Northwards along strike, this overprint is expressed by the pseudomorphic replacement of magnetite by hematite (martite) and is locally associated with brecciation, the presence of rare earth element (REE)-minerals, and replacement by iron-hydroxides. Whereas evidence of microplaty hematite accompanying martitization and predating iron-hydroxides is observed throughout the belt, distinct iron-oxide generations postdating the iron-hydroxides are inferred based on compositional zoning with respect to Si and intimate relationships between iron-oxides and -hydroxides. Chondrite-normalized fractionation trends obtained from the various iron-oxides and from different BIF types generally display LREE-enrichment and distinct positive Eu- and Y-anomalies. An increasing ΣREY- and LREE-trend accompanies martitization. The presence of specific element groups, e.g., granitophile elements (U, W, Sn, Mo), or transition metals (Cr, Mn, Co, Ni, Ti, V, Nb) within the lattice of iron-oxides suggests their formation in evolving environments associated with the emplacement of felsic and mafic lithologies, respectively. The impact of local setting on iron-oxide formation is highlighted by complex trace element signatures of iron-oxides; the northern segment of the belt is relatively enriched in As and Sb, while the southern segment is enriched in granitophile elements and REY. Further complexity is shown in the local variation of Mn and Zn in iron-oxides in the southern segment of the belt, where the Iron Magnet deposit shows the strongest correlation between the two elements. Various depositional settings for BIFs are inferred based upon Post-Archean Australian Shale-normalized REY fractionation trends of iron-oxides and various water types. These settings include environments where iron-minerals precipitate from mixtures of 1) anoxic seawater and high-temperature hydrothermal fluids, 2) anoxic seawater and low-temperature hydrothermal fluids, and 3) oxygen-richer seawater and low-temperature hydrothermal fluids. A genetic model for ore formation is proposed based upon textural and compositional variations observed in iron-oxides throughout the belt and includes formation resulting from supergene fluids rich in elements leached from local granites penetrating BIF, interaction with granite-derived hydrothermal fluids, and heat generated during emplacement of younger dikes. Recognition of petrographic features linked to changes in composition demonstrates the utility of iron-oxides to trace iron ore formation in a temporal and spatial context, with implications for ore genesis and models of mineral exploration.

Some Surface Characteristics of Six Hematite Ores from the Middleback Range Area, South Australia

Surface chemical techniques were used in an attempt to characterise eight samples of hematite. The hematites studied were six samples from the Middleback Ranges (Iron Knob (2 samples), Iron Monarch, Iron Prince, Iron Princess and Iron Baron) and two high purity, synthetic sa mples. The isoelectric points of the hematite samples showed a wide variation in value (from <2 to 8) whereas the zero points of charge were in the range 5.5 to 7.5. Using a simple mathematical calculation it was possible to estimate the surface charge characteristics of these hematite samples which gave a much better picture of the true surface properties of these samples in an aqueous environment. It was hoped that this study would contribute to a better understanding of the interaction between different hematite samples and water, a mechanism that is important in many industrial applications e.g. slurry pumping, agglomeration and granulation.

Influence of 25% Goethite on the Surface Chemistry of Hematite

Surface chemical techniques were used in an attempt to characterise two samples of hematite from the Iron Duke deposit in the Middleback Ranges of South Australia. The surface chemistry of the two naturally occurring samples was compared to a high purity, synthetic sample. The presence of 25% goethite affected the isoelectric point of the hematite, but the zero points of charge for all the samples examined in this study were identical. The differences between the values of isoelectric points and zero points of charge for these samples were deemed due to the presence of fine silica in the ore samples and the presence of unidentified amorphous material in the synthetic sample.

Palaeomagnetism of Australian Hematite Ore Bodies-I The Middleback Ranges of South Australia

Summary The results of a palaeomagnetic study on high-grade hematite ore bodies of South Australia are reported. The two major ore bodies, Iron Monarch and Iron Prince, gave consistent directions of magnetization with either steep to moderate negative and positive inclinations. The positive and negative groups of Iron Monarch are not antiparallel. Because of the presumably long time interval represented by the sampling, this is interpreted as being due to polar wandering with a superimposed reversal of the Earth's field. The remanent magnetization is carried by hematite and is extremely stable against alternating field and thermal demagnetization. The intensity of the NRM is very weak compared to the TRM, which suggests it to be a CRM in origin. Lack of a detailed polar wandering curve for the Pre-Cambrian of Australia prevents precise palaeomagnetic dating of the ores, but the results clearly suggest a Pre-Cambrian age. The magnetic anisotropy of the ores is low and the distribution of the principal susceptibility axes reflects in part the structure of the host rock. The Iron Duke orebody did not yield a reliable palaeomagnetic result.

Patterns of movement in the small dasyurid (Ningaui yvonneae)

The movement behaviour of a small dasyurid (Ningaui yvonneae) was investigated using mark–recapture data collected over 28 months from a population in the Middleback Ranges, South Australia. We were particularly interested in assessing variability in patterns of movement between the sexes and across seasons, as this has potentially important implications for population composition and dynamics. The species was found to be fairly mobile, with frequent and sometimes large movements relative to the size of the animal (up to 900+ m). Average distances moved between recaptures within and between trap sessions were 84 m and 160 m respectively, with nearly half of all recorded movements being between trapping grids (i.e. &gt;80 m). Relatively high recapture rates (many separated by more than 100 days) and measures of site fidelity indicated that the species was not particularly transitory, but probably establishes large and/or drifting home ranges. However, patterns of movement varied considerably according to sex and the time of year. Female N. yvonneae were generally more sedentary than males, with few females moving between trapping grids, and most short-term movements (&lt;100 days apart) being less than 70 m. Movements of females for which the records were separated by more than 100 days tended to be larger, with an average between-capture distance &gt;200 m. In contrast, males often moved between trapping grids, even over short periods. Some short-term movements were more than 600 m in length, and the average distance moved within trapping sessions was &gt;100 m. During the breeding season, males more than doubled the average distances moved between recaptures, when they were presumably roaming in search of females. For females, movement distances were similar during the prebreeding and breeding seasons, but were significantly smaller in the postbreeding season, when they were caring for young.

Geophysical exploration for iron ore in the Middleback Ranges, South Australia

Iron has been mined in the Middleback Ranges-Katunga Hills in the northeast Eyre Peninsula since the turn of the last century. The iron ore mineralisation was formed primarily by supergene enrichment of Palaeoproterozoic iron formation and most deposits are located close to outcropping iron formation. Faults and fold axes probably acted as fluid conduits localising and focusing the fluids required for the ore-forming processes. Basic and ultrabasic intrusives probably acted as impervious barriers, further localising these fluids. The iron formations in the Middleback Ranges are both dense and highly magnetic when compared with other lithologies in the area and this has enabled gravity and magnetic data to be used successfully to map structure and stratigraphy. The iron ores are in general denser, but less magnetic, than the iron formation, although there is overlap in their range of physical properties. Most iron-ore deposits give rise to positive gravity anomalies, but the recognition of these anomalies is hindered by the presence of larger anomalies due to the iron formations. Also, similar anomalies may be caused by basic intrusives. There is no recognisable response from the ores in magnetic data. Trial electromagnetic surveys defined anomalies in some areas of mineralisation. However, petrophysical measurements show the ores are not conductive and these responses are probably caused by saline groundwater and clay minerals.

Population ecology of Ningaui yvonneae (Dasyuridae : Marsupialia) in the Middleback Ranges, Eyre Peninsula, South Australia

A population of the southern ningaui (Ningaui yvonneae) was studied in the semi-arid regions of South Australia. Data were collected on the population ecology of the species, including capture success, breeding biology and life history. Between August 1996 and December 1998, 307 captures were recorded from 11 016 trap-nights. Most individuals were caught only once and males were caught more frequently than females. Capture success varied greatly between seasons, with lowest captures being recorded in winter. Winter is a time of stress for N. yvonneae – mean weights stabilised between March and June. N. yvonneae is polyoestrous, with mating recorded from September to February. However, most matings took place in mid-October. The population was dominated by a single cohort, with the life span for N. yvonneae being approximately 14 months.

Palaeomagnetism of Australian Hematite Ore Bodies--II Western Australia

Summary Iron ore deposits of the Lake Superior type within the Archaean and Proterozoic banded iron formations of Western Australia show consistent and stable directions of remanent magnetization. The palaeomagnetic poles of the ores do not correlate with poles of Post-Cambrian Australian rocks, which suggests a Pre-Cambrian age for the ore formation. Magnetic anisotropies of the ores are low and the distribution of the principal susceptibility axes tends to reflect the magnetic fabric of the host rocks. In a previous publication (Chamalaun & Porath 1967) we reported the results of a palaeomagnetic study on the hematite ore bodies of the Middleback Ranges in South Australia. It was found that these were consistently and stably magnetized with a Pre-Cambrian direction. We therefore extended the study to the Western Australian ore bodies, the results of which are reported here. A large number of high-grade hematite ore bodies are known to occur in the Archaean and Proterozoic banded iron formations of Western Australia, but in order to obtain fresh subsurface samples we have sampled only those that are actively mined. The localities of the ore bodies sampled are shown in Fig. 1. The Mt Goldsworthy and Koolyanobbing deposits occur in Archaean banded iron formations, whilst the host rocks for the Mt Tom Price and Mt Newman ore bodies are Proterozoic iron formations. 2. Results (a) Mt Goldsworthy. The Mt Goldsworthy deposit is located at the northern extremity of the Western Australian Archaean shield. A folded and faulted banded iron formation, which consists of alternating layers of silica and hematite, forms the host rock of a number of high-grade hematite ore bodies of varying origins. Brandt (1964, 1966) makes a distinction between lode ore and crust ore deposits. Lode ores are deep lenses of massive finely crystalline hematite (average grain size about 20p). The major deposit at Mt Goldsworthy consists of lode ore and is controlled by a steep angled transcurrent fault. Pebbles of massive hematite ore occur in the basal Proterozoic conglomerate indicating that the ore formed in Pre-Proterozoic times (Brandt 1964). Brandt (1966) favours a hypogene origin of the lode ore along the lines suggested by Dorr (1965) for the Brazilian ores. Crust ores form shallow cappings of hematite and goethite on the upturned

AEROMAGNETIC PROSPECTING FOR IRON ORE ON EYRE PENINSULA, SOUTH AUSTRALIA

The analysis of anomalies on the aeromagnetic survey maps of the Middleback Ranges and other areas of Eyre Peninsula in South Australia which were flown in the search for iron‐ore deposits has provided depth estimates which are in good agreement with known and later discovered geological facts. The average error involved in these estimates is of the order of 15 percent.