The Abra sedimentary-hosted Pb-Ag-Cu-Au deposit, Western Australia: A geophysical case study

2021 ◽  
Vol 40 (2) ◽  
pp. 129-138
Author(s):  
David Stannard ◽  
Jayson Meyers ◽  
Angelo Scopel
Keyword(s):  
Iron Oxide ◽  
Western Australia ◽  
Land Surface ◽  
Spectral Ratio ◽  
Induced Polarization ◽  
Seismic Velocities ◽  
Impedance Contrast ◽  
Passive Seismic ◽  
Host Rocks ◽  
Feeder Zone

Abra is a high-grade sedimentary-hosted Pb deposit located in the Paleoproterozoic Edmund Basin in Western Australia. Mineralization is blind, with the top of the deposit occurring 250 m beneath the land surface. The deposit consists of a stratiform apron of Pb-Ag-Ba mineralization in laminated iron-oxide- and barite-altered dolomite and siltstone, which overlies a feeder zone of chlorite-altered, brecciated, and veined carbonatic siltstone that contains Pb-Ag mineralization in the core that transitions to Pb-Cu and Cu-Au at depth. Abra is characterized by discrete geophysical anomaly responses in magnetic, gravity, time-domain electromagnetic (TDEM), and induced polarization survey data. A +450 nT magnetic anomaly is caused by magnetite in the lower stratiform zone. Dense galena, barite, dolomite, and iron-oxide mineralization in the apron and galena in the feeder zone is surrounded by lower-density sedimentary host rocks, which results in a +1 mGal gravity anomaly. Airborne, ground, and downhole TDEM surveying resolved known mineralization as weak electromagnetic conductor responses, and petrophysical testing on core samples shows that this is caused by galena. Pole-dipole-induced polarization surveying resolved a +20 ms chargeability anomaly on the southern flank of the deposit. This chargeable anomaly response is related to disseminated galena, pyrite, chalcopyrite, and alteration. Joint audiomagnetotelluric-magnetotelluric 2D inverted data sections resolved Abra as a broad weakly conductive anomaly. Weak conductor responses associated with Abra were also resolved in 2D and 3D inversion modeling of airborne Z-axis tipper electromagnetic data. 2D seismic reflection surveying resolved Abra as strong flat-lying seismic reflectors, which are bounded and offset by faults and surrounded by a seismically bland zone. The seismic reflections are related to significant density contrasts between high-density stratiform mineralization that is in contact with low-density sedimentary host rocks, as the mineralization and host rocks have similar seismic velocities. Passive seismic horizontal to vertical spectral ratio surveying resolved the top of the deposit as a subtle layer sitting below a flat impedance contrast horizon that is interpreted as weathered siltstone on top of diagenetically cemented siltstone.

scholarly journals Potentials and pitfalls of permafrost active layer monitoring using the HVSR method: A case study in Svalbard

2018 ◽  
Author(s):  
Andreas Köhler ◽  
Christian Weidle
Keyword(s):  
Active Layer ◽  
Spectral Ratio ◽  
Time Lapse ◽  
Careful Analysis ◽  
Temporal Variations ◽  
Soil Conditions ◽  
Seismic Velocities ◽  
Essential Information ◽  
Hvsr Method ◽  
Changing Noise

Abstract. Time-lapse monitoring of the sub-surface using ambient seismic noise is a popular method in environmental seismology. We assess the reliability of the Horizontal-to-Vertical Spectral Ratio (HVSR) method for monitoring seasonal permafrost active layer variability in northwest Svalbard. We observe complex HVSR variability between 1 and 50 Hz in the record of a temporary seismic deployment covering frozen and thawn soil conditions between April and August 2016. While strong variations are due to changing noise conditions, mainly affected by wind speed and degrading coupling of instruments during melt season, a seasonal trend is observed at some stations that has most likely a sub-surface structural cause. A HVSR peak emerges close to the Nyquist frequency (50 Hz) in beginning of June which is then gradually gliding down, reaching frequencies of about 15–25 Hz in the end of August. This observation is consistent with HVSR forward-modeling for a set of structural models that simulate different stages of active layer thawing. Our results reveal a number of potential pitfalls when interpreting HVSRs and suggest a careful analysis of temporal variations since HVSR seasonality is not necessarily related to changes in the sub-surface. We compile a list of recommendations for future experiments, including comments on network layouts suitable for array beamforming and waveform correlation methods that can provide essential information on noise source variability. In addition, we investigate if effects of changing noise sources on HVSRs can be avoided by utilizing a directional, narrow-band (4.5 Hz) repeating seismic tremor which is observed at the permanent seismic broadband station KBS in the study area. A significant change of the radial component HVSR shape during summer months is observed for all tremors. We show that a thawn active layer with very low seismic velocities would affect Rayleigh wave ellipticities in the tremor frequency band.

MAPPING THE MINERAL ZONATION AT THE ERNEST HENRY IRON OXIDE COPPER-GOLD DEPOSIT: VECTORING TO Cu-Au MINERALIZATION USING MODAL MINERALOGY

2022 ◽  
Vol 117 (2) ◽  
pp. 485-494
Author(s):  
Tobias U. Schlegel ◽  
Renee Birchall ◽  
Tina D. Shelton ◽  
James R. Austin
Keyword(s):  
Iron Oxide ◽  
Gamma Ray ◽  
Geochemical Data ◽  
Iocg Deposits ◽  
Mineral Mapping ◽  
Gamma Ray Spectrometer ◽  
Mineral Assemblages ◽  
Copper Gold ◽  
Host Rocks ◽  
Acid Alteration

Abstract Iron oxide copper-gold (IOCG) deposits form in spatial and genetic relation to hydrothermal iron oxide-alkali-calcic-hydrolytic alteration and thus show a mappable zonation of mineral assemblages toward the orebody. The mineral zonation of a breccia matrix-hosted orebody is efficiently mapped by regularly spaced samples analyzed by the scanning electron microscopy-integrated mineral analyzer technique. The method results in quantitative estimates of the mineralogy and allows the reliable recognition of characteristic alteration as well as mineralization-related mineral assemblages from detailed mineral maps. The Ernest Henry deposit is located in the Cloncurry district of Queensland and is one of Australia’s significant IOCG deposits. It is known for its association of K-feldspar altered clasts with iron oxides and chalcopyrite in the breccia matrix. Our mineral mapping approach shows that the hydrothermal alteration resulted in a characteristic zonation of minerals radiating outward from the pipe-shaped orebody. The mineral zonation is the result of a sequence of sodic alteration followed by potassic alteration, brecciation, and, finally, by hydrolytic (acid) alteration. The hydrolytic alteration primarily affected the breccia matrix and was related to economic mineralization. Alteration halos of individual minerals such as pyrite and apatite extend dozens to hundreds of meters beyond the limits of the orebody into the host rocks. Likewise, the Fe-Mg ratio in hydrothermal chlorites changes systematically with respect to their distance from the orebody. Geochemical data obtained from portable X-ray fluorescence (p-XRF) and petrophysical data acquired from a magnetic susceptibility meter and a gamma-ray spectrometer support the mineralogical data and help to accurately identify mineral halos in rocks surrounding the ore zone. Specifically, the combination of mineralogical data with multielement data such as P, Mn, As, P, and U obtained from p-XRF and positive U anomalies from radiometric measurements has potential to direct an exploration program toward higher Cu-Au grades.

scholarly journals Case studies on the application of passive seismic horizontal to vertical spectral ratio (HVSR) surveying for heavy mineral sand exploration

2019 ◽  
Vol 2019 (1) ◽  
pp. 1-4
Author(s):  
Nigel Cantwell ◽  
Matthew Owers ◽  
Jayson Meyers ◽  
Sharna Riley
Keyword(s):  
Case Studies ◽  
Spectral Ratio ◽  
Heavy Mineral ◽  
Passive Seismic

scholarly journals Crevasse-induced Rayleigh-wave azimuthal anisotropy on Glacier de la Plaine Morte, Switzerland

2018 ◽  
Vol 60 (79) ◽  
pp. 96-111 ◽  
Author(s):  
Fabian Lindner ◽  
Gabi Laske ◽  
Fabian Walter ◽  
Adrian K. Doran
Keyword(s):  
Rayleigh Wave ◽  
Drainage System ◽  
Fracture Network ◽  
Wide Distribution ◽  
Azimuthal Anisotropy ◽  
Seismic Velocities ◽  
Ice Shelves ◽  
Passive Seismic ◽  
Iceberg Calving ◽  
And Inversion

AbstractCrevasses and englacial fracture networks route meltwater from a glacier's surface to the subglacial drainage system and thus influence glacial hydraulics. However, rapid fracture growth may also lead to sudden and potentially hazardous structural failure of unstable glaciers and ice dams, rifting of ice shelves, or iceberg calving. Here, we use passive seismic recordings to investigate the englacial fracture network on Glacier de la Plaine Morte, Switzerland. Glacier dynamics and the drainage of an ice-marginal lake give rise to numerous icequakes, the majority of which generate dispersed, high-frequency Rayleigh waves. A wide distribution of events allows us to study azimuthal anisotropy between 10 and 30 Hz in order to extract englacial seismic velocities in regions of preferentially oriented crevasses. Beamforming applied to a 100-m-aperture array reveals azimuthal anisotropy of Rayleigh-wave phase velocities reaching a strength of 8% at high frequencies. In addition, we find that the fast direction of wave propagation coincides with the observed surface strike of the narrow crevasses. Forward modeling and inversion of dispersion curves suggest that the azimuthal anisotropy is induced by a 40-m-thick crevassed layer at the surface of the glacier with 8% anisotropy in shear-wave velocity.

Geological controls on the morphology of an overdeepened canyon investigated by seismic surface waves (Lower Aare Valley, Switzerland)

2020 ◽  
Author(s):  
Lukas Gegg ◽  
Lorenz Keller ◽  
Marius W. Buechi ◽  
Thomas Spillmann ◽  
Gaudenz Deplazes ◽  
...  
Keyword(s):  
Surface Waves ◽  
Cross Sections ◽  
Spectral Ratio ◽  
3D Models ◽  
Ambient Vibration ◽  
Northern Switzerland ◽  
Fold And Thrust Belt ◽  
Elevation Data ◽  
Seismic Surface Waves ◽  
Passive Seismic

<p><span><span>Subglacial overdeepenings are common features of past and presently glaciated landscapes. In the Northern Alpine Foreland, these troughs occur mostly within the rather soft, poorly lithified sandstones of the Molasse basin. An exceptional setting is the Lower Aare Valley in Northern Switzerland, where a narrow, finger-like overdeepening (Gebenstorf-Stilli Trough) has been incised more than 100 m below the present surface into the fold-and-thrust belt of the Jura Mountains with its diverse Mesozoic lithologies including competent limestone units. Consequently, the morphology of this overdeepening can provide valuable information on lithological and structural controls on subglacial overdeepening erosion.</span></span></p><p><span><span>We investigate the Gebenstorf-Stilli Trough with three scientific boreholes located along the South-North oriented trough axis. In addition, a set of seismic cross-sections has been acquired by a combination of active and passive seismic approaches analysing surface waves, namely passive horizontal-to-vertical spectral ratio (HVSR) measurements, active measurements applying multiple filter analysis of group velocity (MFA), and extended spatial auto correlation of ambient vibration array data (ESAC). </span></span></p><p><span><span>Preliminary results show that the base of the overdeepening can be well imaged using our methodology. In combination with borehole information, surface elevation data and 3D models of the subsurface geology, we see great potential to better constrain the morphology of the Gebenstorf-Stilli Trough, and to assess how different bedrock lithologies and structures influence subglacial overdeepening erosion – an underexplored and poorly understood issue.</span></span></p>

The influence of landforms on sandalwood (Santalum spicatum (R.Br) A.Dc.) size structure and density in the north-eastern goldfields, Western Australia

2002 ◽  
Vol 24 (2) ◽  
pp. 219 ◽  
Author(s):  
J. E. Brand ◽  
P. J. Jones
Keyword(s):  
Western Australia ◽  
Land Surface ◽  
Size Structure ◽  
The Other ◽  
Total Density ◽  
Surface Type ◽  
The North ◽  
North Eastern ◽  
Land Systems ◽  
Santalum Spicatum

The density of Santalum spicatum was compared between 'land systems' and between 'land surface types' on four sheep stations in the north-eastern Goldfields: Yakabindie, Tarmoola, Glenorn and Minara. S.A spicatum density was recorded in 4–6A ha transect plots, with a total of 14,090 ha surveyed. Within each transect plot, the S. spicatum were divided into five groups based on stem diameter at 150A mm: < 25 mm, 25–74 mm, 75–124 mm, 125–174 mm and > 174 mm. The proportion of S. spicatum in each of the five size categories was similar between land surface types and between land systems, with the majority in two groups: 75–124 mm and 125–174 mm. S. spicatum recruitment was low, with less than 1.5 % seedlings (< 25 mm) and 7.9 % saplings (25–74 mm). Total density of S. spicatum on hills and ridges (0.65 stems/ha) was significantly higher than any other land surface type. The sandplains (0.05 stems/ha) supported the least. Within land systems, Laverton and Bevon (both hills and ridges) had the highest S. spicatum density. Yakabindie supported higher densities of S. spicatum than the other stations.

Agate recrystallisation: Evidence from samples found in Archaean and Proterozoic host rocks, Western Australia

2006 ◽  
Vol 53 (2) ◽  
pp. 235-248 ◽  
Author(s):  
T. Moxon ◽  
D. R. Nelson ◽  
M. Zhang
Keyword(s):  
Western Australia ◽  
Host Rocks
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