Broken Hill Region Research Articles

Innovative imagery, data quality controls, state-wide grid merges

The NSW Department of Primary Industries recently acquired over 200,000 line kilometres of high-resolution airborne geophysical data. As part of this process, significant resources were allocated in monitoring the data quality, which resulted in contractors delivering data to a very high standard. Upon release of the new data, it was timely to update the state-wide grids for magnetic, radioelement, gravity and digital elevation data. After a mammoth effort in overcoming problems in merging aerially extensive data sets, merged state-wide grids of geophysical data is now available as a set of four DVDs. Novel approaches in imaging aeromagnetic data in the Broken Hill region are assisting in the interpretation of the geology and structure in poorly outcropping areas.

Timing of Proterozoic metamorphism in the southern Curnamona Province: implications for tectonic models and continental reconstructions*

Chemical U – Th – Pb monazite ages from metasedimentary and meta-igneous units of the Willyama Supergroup have confirmed initial SHRIMP U – Pb metamorphic zircon ages constraining the onset of the earliest tectonometamorphic event (the Olarian Orogeny) at ca 1610 Ma in the southern Curnamona Province. An additional episode of high-grade metamorphism and heterogeneously distributed retrograde metamorphism and monazite recrystallisation occurred between ca 1570 and 1550 Ma. On the basis of monazite chemical U – Th – Pb ages from across the southern Curnamona Province, tectonometamorphic models based on ca 1690 Ma low-P, high-T metamorphism in the southern Curnamona Province are not supported. Furthermore, tectonic reconstructions that rely on the correlation of ca 1690 Ma deformation and metamorphism in the Broken Hill region with a similar aged event in the Mojave tarrane of southwestern Laurentia (AUSWUS) are not supported.

Variations of stable isotopes with depth in regolith calcite cements in the Broken Hill region, Australia: Palaeoclimate evolution signal?

Twenty two samples of calcretes from seven depth-profiles in the Menindee catchment, Broken Hill region, Australia were analysed for their inorganic and organic carbon contents and inorganic carbon and oxygen isotopes. The organic carbon content is very low (from 0.06 to 0.31 wt.%) while inorganic carbon (carbonate) is up to 3.9 wt.%. Both δ 13C and δ 18O become more positive closer to the surface. Carbon isotopes vary from − 8.5‰ to −5.5‰ PDB. Oxygen isotopes vary from − 6‰ to − 1.8‰ V-PDB. Depth-related δ 13C and δ 18O variations correlate over at least 15 km and show no significant variation along the flow path. δ 13C values increase by 3‰ and δ 18O values increase by 4‰ with decreasing depth in a 1.40 m thick soil profile. The variation is interpreted to indicate an increasingly elevated air temperature, greater water stress and subsequently an aridification of the area through time. The Broken Hill calcrete data confirm that climatic evolution can be deduced from isotopic series and be applied successfully to the Broken Hill region.

Groundwater in the Broken Hill region, Australia: recognising interaction with bedrock and mineralisation using S, Sr and Pb isotopes

The supergiant Pb–Zn–Ag Broken Hill orebody and numerous other minor mineral deposits occur within the limited outcrop of the Proterozoic Curnamona Province of Australia. The vast majority of this Province is concealed by up to 200 m of transported regolith, hampering conventional exploration strategies. Approximately 300 groundwater samples were collected over the southern Curnamona Province to test whether this medium could be helpful in the search for hidden mineral deposits. Sulphur, Sr and Pb isotope composition of the groundwaters were determined and S excess (S XS), i.e., the amount of S that can be ascribed neither to evaporation nor to mixing, was calculated. Many samples were recognised to have undergone an addition of 34S-depleted S, which can be attributed to oxidation of sulfides with a Broken Hill type δ 34S signature (average ∼0‰ V-CDT). Furthermore, Sr isotopes identify the broad types of bedrock that the groundwater has been interacting with, from the less radiogenic Adelaidean rocks (and minerals) in the west (groundwater 87Sr/ 86Sr ratio as low as 0.708) to the highly radiogenic Willyama Supergroup in the east ( 87Sr/ 86Sr ratio up to 0.737). The groundwaters have 207Pb/ 204Pb and 206Pb/ 204Pb ratios comparable to, or intermediate between, various mineralisation types recognised in the area (Broken Hill, Rupee, Thackaringa, etc., types). The few samples taken in the vicinity of known mineralisation yield positive indicators (positive S XS, low δ 34S, 87Sr/ 86Sr signature of bedrock type and Pb isotope fingerprinting of mineralisation type). This study also highlights several new locations under sedimentary cover where these indicators suggest interaction with mineralisation.

Estimating the pre-mining gravity and gravity gradient response of the Broken Hill Ag-Pb-Zn Deposit

Mining of the Broken Hill Ag-Pb-Zn deposit has substantially modified what was originally a positive anomalous mass. When an airborne gravity gradiometer (AGG) survey was flown over the Broken Hill region in early 2003, the measured response reflected the modified mass distribution. To answer the questions ``What was the original response of the orebody?' and ``Would this response have been detected had the survey been flown prior to mining?', an estimate of the changes in response brought about by mining activities was made and added to the survey data to produce an image of the pre-mining gravity response. To estimate the change in response, a 3D model of the mined portion of the deposit was built. An estimate of the change in mass due to mining activities was made and this mass was distributed with uniform density throughout the model. The gravity and vertical gravity gradient response of the orebody model was then calculated, filtered to match the characteristics of the AGG data and added to the observed survey data. The ?corrected? data show distinct gravity and gravity gradient highs over the northern and southern parts of the orebody which hosted the bulk of the reserves. Although the anomalous response is close to the noise levels of the survey data, we can conclude that the AGG survey would have detected an anomalous response from the Broken Hill orebody had the survey been flown prior to mining. However, there are other geological features in the survey area that produce similar anomalies, notably a number of amphibolite units.

Geochemical signature of mineralization in weathered sediments and bedrock, Thunderdome prospect, Broken Hill region, westernNew South Wales, Australia: implications for mineral explorationunder cover

The stratiform Pb–Ag–Zn and stratabound Cu–Au mineralization of the Thunderdome prospect, located in the Mundi Mundi Plain 50 km NW of Broken Hill, NSW, is covered by c. 200 m of sediments. A downhole mineralogical and geochemical dispersion study was carried out on 245 regolith samples from six exploration drillholes to establish if geochemical dispersion from the mineralization could be identified in the overlying cover. This was achieved using a combination of qualitative and quantitative XRD, Portable Infrared Mineral Analyzer (PIMA) and XRF techniques. The stratigraphy is defined by a variable thickness of diagenetically modified and weathered Cenozoic sediments (and soil) overlying the weathered Proterozoic basement, which hosts the mineralization. The composition of the sediments and saprolite appears to have been influenced by chemical dispersion from underlying mineralization, as shown by anomalous concentrations of Cu (up to c. 13 times above background), Ni ( c. 9 times), As, Zn, Pb and Co (all c. 3 times) among others. Some of these anomalies are widespread and may be either clastic or related to adsorption on clay minerals, Fe-oxides and neoformed, poorly crystalline mineral phases. The precise transport mechanisms from the underlying mineralization need to be tested in order to gain further insights into geochemical dispersion processes under cover.

The application of the stable isotopes of sulfur and oxygen in groundwater sulfate to mineral exploration in the Broken Hill region of Australia

The oxygen and sulfur isotopic composition of sulfate dissolved in groundwater from the Broken Hill region can be used to recognize oxidation of sulfides along a flow path. This oxidation can be shown to occur either within the unsaturated zone or below the water table. This is important because it shows that hydrogeochemistry can provide a vector to blind mineralization in areas of significant regolith cover, in the Broken Hill region and, by extension, in other sulfide-rich mineral settings.

Regional structure and distribution of magnetite: implications for the interpretation of aeromagnetic data in the Broken Hill region, New South Wales

Magnetic anomalies in the Palaeoproterozoic Willyama Supergroup in the Broken Hill region have a number of different sources, including stratiform magnetite, structurally controlled magnetite and magnetite in igneous rocks. Magnetic sedimentary units are relatively uncommon; most metasedimentary rocks are chemically reduced and host few stratiform anomalies. Exceptions include units within the Paragon Group and volumetrically minor banded iron formation. Structurally controlled anomalies are common across the region and are caused by magnetite in hightemperature shear zones, tectonic fabrics, or in high-strain zones developed along the contact between rocks of contrasting competency. Magnetite formation occurred more than once during the deformational history, during both high-grade and lower-grade metamorphism. Many of the observed linear anomalies coincide with the regional S3 fabric, which formed during upper greenschist to amphibolite-facies metamorphism. Retrograde shear zones are generally magnetite-destructive and form linear zones of low magnetic intensity. Amphibolite has a generally low magnetic susceptibility, although certain units have a high susceptibility, possibly due to alteration as a result of fluid flow. The abundance of structurally controlled anomalies and the relative paucity of stratigraphic anomalies hamper the detailed extrapolation of lithological units beneath cover and means that a ‘magnetic stratigraphy’ is difficult to construct for the Willyama Supergroup. Rather, aeromagnetic data are more useful in the delineation of high-strain zones that have acted as channels for fluid flow, resulting in the formation or destruction of magnetite. Tracing these zones of fluid flow has potential benefits for the reconstruction of mineralising systems and magnetite formed in these zones may have acted as a chemical trap for metals such as gold.

Geological interpretation of high-resolution airborne geophysical data in the Broken Hill Region

New high-resolution airborne geophysical data acquired in the Broken Hill Region are being used to assess the deformational history and define the regional structural regime. The interpretation of these data may enable new structural and stratigraphic models to be postulated that will enhance and revitalise the exploration thrust within the Broken Hill Region.

Exploration rock geochemistry for Pinnacles-type mineralization at Broken Hill, N.S.W., Australia

Abstract Geochemical responses in weathered and oxidized surface metasedimentary rocks associated with stratiform lead-zinc mineralization at Stirling Hill (6 km west of Broken Hill) are compared with the geochemical responses in fresh drill core from an equivalent lithostratigraphic section with stratiform lead-zinc mineralization at the Pinnacles Mine (8 km south of Stirling Hill). Mineralization is interpreted as being volcanic exhalative and it lies within highly metamorphosed (sillimanite grade) rocks of the Willyama Supergroup. Surface rocks were classified into groups by discriminant analysis using drill core data from the Pinnacles Mine as the initial training set. The behaviour of elements in surface rocks varies with the rock group but Zn, Pb, Mn, Fe, and Co are leached from all surface rocks relative to fresh drill core. Nothwithstanding the leaching effects of weathering, common geochemical responses to mineralization have been identified in drill core and surface rocks. Coincident positive anomalies for Zn/Ba and Fe/(Na × Ba) ratios and negative anomalies for Na/(Mn × Ca) ratios uniquely define mineralization in both weathered surface rocks and in fresh drill core. The results demonstrate that the pattern of geochemical responses to Pinnacles-type stratiform volcanic-exhalative mineralization in surface rocks has survived the intensive weathering regime in the Broken Hill region.

Sulfide petrology of the Broken Hill region, New South Wales

The Proterozoic Willyama Complex hosts metamorphosed massive lead-zinc-silver sulfide deposits (including the main Broken Hill lode) distributed across three prograde regional metamorphic zones ranging from middle amphibolite to granulite facies. FeS contents of sphalerites coexisting with pyrrhotite are consistent within a single sample but range widely within a deposit and across the complex showing no correlation with metamorphic grade. Primary (metamorphic) pyrrhotite was iron rich and now consists of hexagonal pyrrhotite with exsolved troilite. Although most primary pyrrhotites were altered to monoclinic pyrrhotite, pyrite, marcasite, or mixtures of these minerals during retrograde or postmetamorphic events, sphalerites were not affected significantly. Closely spaced samples from No. 3 Lens at North mine in the main Broken Hill lode indicate that the domains of sulfide equilibria are less than 30 cm and may not be much larger than the dimension of a polished section.The partitioning of FeS between sphalerite and hexagonal pyrrhotite is a function of P, T, and a FeS . The determination of a FeS for the bulk compositions of pyrrhotites together with analyses of coexisting sphalerites permits an estimation of pressure during regional metamorphism provided that the pyrrhotites have not been altered and the temperature of sulfide equilibration is known. Four samples from the main Broken Hill lode in granulite rocks give mean pressures of 8.1 + or - 0.6 kb for an assumed temperature of final sphalerite equilibration of 650 degrees C, as suggested from sulfur isotopic data, or 9.9 + or - 0.7 kb at 800delta C which is the estimated peak metamorphic temperature.

Potassium-Argon Age Determinations on Some Rocks from the Broken Hill Region of New South Wales

AGE measurements have been made on a number of rocks from Broken Hill in Australia. The significance of these will be discussed in a forthcoming publication1. The following is a brief description of their geological settings together with those age measurements which were made using the isotope dilution technique of potassium–argon age determination.

Heat Flow at Broken Hill, New South Wales

Summary The heat flow in the Broken Hill region has been determined from temperatures measured in 18 deep depressed drill holes covering an area of 200 square miles and in 32 horizontal drill holes in mine walls. 118 thermal conductivity determinations on rock specimens representative of the area have been used. The values of heat flux range from 1.81 to 2.07 μcal/cm2 s with a mean of 1.93. The standard error of a single determination is about 5 per cent.