Three-dimensional gravity modelling applied to the exploration of uranium unconformity-related basement-hosted deposits: the Contact prospect case study, Kiggavik, northeast Thelon region (Nunavut, Canada)

2017 ◽  
Vol 54 (8) ◽  
pp. 869-882 ◽  
Author(s):  
Régis Roy ◽  
Antonio Benedicto ◽  
Alexis Grare ◽  
Mickaël Béhaegel ◽  
Yoann Richard ◽  
...  
Keyword(s):  
Gravity Data ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
3D Models ◽  
Low Density ◽  
Uranium Deposits ◽  
Geological Interpretation ◽  
Thelon Basin ◽  
Dimensional Gravity ◽  
Density Values

In unconformity-related uranium deposits, mineralization is associated with hydrothermal clay-rich alteration haloes that decrease the density of the host rock. In the Kiggavik uranium project, located in the eastern Thelon Basin, Nunavut (Canada), basement-hosted shallow deposits were discovered by drilling geophysical anomalies in the 1970s. In 2014, gravity data were inverted for the first time using the Geosoft VOXI Earth ModellingTM system to generate three-dimensional (3D) models to assist exploration in the Contact prospect, the most recent discovery at Kiggavik. A 3D unconstrained inversion model was calculated before drilling, and a model constrained by petrophysical data was computed after drilling. The unconstrained inversion provided a first approximation of the geometry and depth of a low-density body and helped to collar the discovery holes of the Contact mineralization. The constrained inversion was computed using density values measured on 315 core samples collected from 21 drill holes completed between 2014 and 2015. The constrained modelling highlights three shallower and smaller low-density bodies that match the geological interpretation and refines the footprint of the gravity anomalies in relation to the current understanding of the deposit. The 3D inversion of gravity data is a valuable tool to guide geologists in exploration of shallow basement-hosted uranium deposits associated with alteration haloes and to assess the deposit gravity geometry.

scholarly journals Basin Depth Mapping Beneath Wellington City Based on Residual Gravity Anomalies

2021 ◽  
Author(s):  
◽  
Alistair Stronach
Keyword(s):  
Gravity Anomaly ◽  
Sedimentary Basin ◽  
Gravity Data ◽  
Three Dimensional ◽  
Maximum Amplitude ◽  
Central Business District ◽  
Gravity Anomalies ◽  
Depth Map ◽  
Capital City ◽  
Detailed Knowledge

<p><b>New Zealand’s capital city of Wellington lies in an area of high seismic risk, which is further increased by the sedimentary basin beneath the Central Business District (CBD). Ground motion data and damage patterns from the 2013 Cook Strait and 2016 Kaikōura earthquakes indicate that two- and three-dimensional amplification effects due to the Wellington sedimentary basin may be significant. These effects are not currently accounted for in the New Zealand Building Code. In order for this to be done, three-dimensional simulations of earthquake shaking need to be undertaken, which requires detailed knowledge of basin geometry. This is currently lacking, primarily because of a dearth of deep boreholes in the CBD area, particularly in Thorndon and Pipitea where sediment depths are estimated to be greatest.</b></p> <p>A new basin depth map for the Wellington CBD has been created by conducting a gravity survey using a modern Scintrex CG-6 gravity meter. Across the study area, 519 new high precision gravity measurements were made and a residual anomaly map created, showing a maximum amplitude anomaly of -6.2 mGal with uncertainties better than ±0.1 mGal. Thirteen two-dimensional geological profiles were modelled to fit the anomalies, then combined with existing borehole constraints to construct the basin depth map. </p> <p>Results indicate on average greater depths than in existing models, particularly in Pipitea where depths are interpreted to be as great as 450 m, a difference of 250 m. Within 1 km of shore depths are interpreted to increase further, to 600 m. The recently discovered basin bounding Aotea Fault is resolved in the gravity data, where the basement is offset by up to 13 m, gravity anomaly gradients up to 8 mGal/km are observed, and possible multiple fault strands identified. A secondary strand of the Wellington Fault is also identified in the north of Pipitea, where gravity anomaly gradients up to 18 mGal/km are observed.</p>

scholarly journals THREE-DIMENSIONAL ANALYSIS OF NORMAL FAULT ZONES IN KARDIA MINE, PTOLEMAIS BASIN, NW GREECE

2016 ◽  
Vol 50 (1) ◽  
pp. 15 ◽  
Author(s):  
E. Delogkos ◽  
T Manzocchi ◽  
C. Childs ◽  
C. Sachanidis ◽  
T. Barmpas ◽  
...  
Keyword(s):  
Three Dimensional ◽  
Normal Fault ◽  
3D Models ◽  
Fault Zones ◽  
System Structure ◽  
Open Pit ◽  
Fault System ◽  
Zone Structure ◽  
Geological Interpretation ◽  
Lignite Mine

Six normal fault zones, with throws ranging from a few meters up to 50 m, were studied within an active, open pit, lignite mine in Ptolemais. Each fault was mapped 20 times over a period of five years because at intervals of ca. 3 months working faces are taken back between 20 and 50 m exposing fresh fault outcrops for mapping.Various resolutions of photographs and structural measurements were imported into a fully georeferenced 3D structural interpretation package, resulting in aseismic scale and outcrop resolution 3D fault volume with outcrop and panoramic photographs acting as the seismic sections in equivalent seismic surveys. Low resolution 3D models for the fault system structure at mine scale and higher-resolution 3D models for the fault zone structure were produced after geological interpretation and they can be used for qualitative and quantitative analysis.

Gravity ideal bodies

Geophysics ◽  
1987 ◽  
Vol 52 (9) ◽  
pp. 1265-1278 ◽  
Author(s):  
Mark E. Ander ◽  
Stephen P. Huestis
Keyword(s):  
Gravity Data ◽  
Three Dimensional ◽  
Computer Code ◽  
Solution Set ◽  
Rio Grande Rift ◽  
Positive Anomaly ◽  
Dimensional Gravity ◽  
Exploration Tool ◽  
Ideal Body ◽  
Rigorous Bounds

The interpretation of gravity anomaly data suffers from a fundamental nonuniqueness, even when the solution set is bounded by physical or geologic constraints. Therefore, constructing a single solution that fits or approximately fits the data is of limited value. Consequently, much effort has been applied in recent years to developing inverse techniques for rigorous deduction of properties common to all possible solutions. To this end, Parker developed the theory of an ideal body, which characterizes the extremal solution with the smallest possible maximum density. Gravity ideal‐body analysis is an excellent reconaissance exploration tool because it is especially well suited for handling sparse data contaminated with noise, for finding useful, rigorous bounds on the infinite solution set, and for predicting accurately what data need to be collected in order to tighten those bounds. We present a practical three‐ dimensional gravity ideal‐body computer code, IDB, that can optimize a mesh with over [Formula: see text] cells when used on a CRAY computer. Using actual gravity data, we use IDB to produce ideal‐body tradeoff curves that bound the solution set and show how to restrict the bound on the solution further by applying geologic and geophysical data to the tradeoff curves. As an example, we compare two‐dimensional and three‐dimensional ideal‐body results from a study of a positive anomaly associated with the Lucero uplift located on the western flank of the Rio Grande rift in New Mexico.

Interactive modeling and interpretation of three dimensional gravity data

1982 ◽  
Author(s):  
H.‐J. Goetze ◽  
F. Keller ◽  
B. Lahmeyer ◽  
O. Rosenbach
Keyword(s):  
Gravity Data ◽  
Three Dimensional ◽  
Interactive Modeling ◽  
Dimensional Gravity

Framework geophysical modelling of granitoid versus supracrustal basement to the northeast Thelon Basin around the Kiggavik uranium camp, Nunavut

2013 ◽  
Vol 50 (6) ◽  
pp. 667-677 ◽  
Author(s):  
V. Tschirhart ◽  
W.A. Morris ◽  
C.W. Jefferson
Keyword(s):  
Gravity Data ◽  
Quantitative Comparison ◽  
Rock Properties ◽  
Intrusive Complex ◽  
Uranium Deposits ◽  
Metasedimentary Rocks ◽  
Thelon Basin ◽  
Basement Gneiss ◽  
Geological Constraints ◽  
Depth Extent

The northeast Thelon Basin in the Kivalliq region of Nunavut is prospective for uranium deposits. Recently discovered basement-hosted, unconformity-associated prospects west of Kiggavik are restricted to deformed and metamorphosed Neoarchean psammitic enclaves of the Woodburn Lake group within 1.83 Ga Hudson granite and Martell syenite that together comprise the Shultz Lake intrusive complex (SLIC). The depth and geometry of the intrusive complex are relatively unknown as the geological constraints are poor; the drilling is sparse and of shallow depth extent as it was not targeting the basement but shallower multiply faulted and highly altered demagnetized zones. This study aims to constrain the geometry and context of the Shultz Lake intrusive complex with respect to the ore-hosting Neoarchean metasedimentary rocks and intersecting reactivated fault arrays through geophysical modelling of detailed aeromagnetic and gravity data integrated with new geological knowledge. By integrating detailed gravity, aeromagnetic, and structural geology observations measured along a series of transects with a petrophysical rock properties database, it is possible to derive constraints on the depth and thickness (200–300 m) of the SLIC. Quantitative comparison and integration of multiple hypothetical geometries favours a model wherein the SLIC, together with metasedimentary and older basement gneiss, has been structurally emplaced over the Neoarchean metasediments.

Topography of the crust–mantle interface under the Western Superior craton from gravity data

2003 ◽  
Vol 40 (10) ◽  
pp. 1307-1320 ◽  
Author(s):  
B Nitescu ◽  
A R Cruden ◽  
R C Bailey
Keyword(s):  
Gravity Data ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
Gravity Inversion ◽  
Constant Density ◽  
Inversion Algorithm ◽  
Data Set ◽  
Mantle Boundary ◽  
Refraction Seismic ◽  
Superior Craton

The Moho undulations beneath the western part of the Archean Superior Province have been investigated with a three-dimensional gravity inversion algorithm for a single interface of constant density contrast. Inversion of the complete gravity data set produces unreal effects in the solution due to the ambiguity in the possible sources of some crustal gravity anomalies. To avoid these effects a censored gravity data set was used instead. The inversion results are consistent with reflection and refraction seismic data from the region and, therefore, provide a basis for the lateral correlation of the Moho topography between parallel seismic lines. The results indicate the existence of a major linear east–west-trending rise of the Moho below the metasedimentary English River subprovince, which is paralleled by crustal roots below the granite–greenstone Uchi and Wabigoon subprovinces. This correlation between the subprovincial structure at the surface and deep Moho undulations suggests that the topography of the crust–mantle boundary is related to the tectonic evolution of the Western Superior belts. Although certain features of the crust–mantle boundary are likely inherited from the accretionary and collisional stages of the Western Superior craton, gravity-driven processes triggered by subsequent magmatism and crustal softening may have played a role in both the preservation of those features, as well as in the development of new ones.

Relationship between rock physical properties and spectral mineralogy applied to exploration for an unconformity-related uranium deposit (Saskatchewan, Canada)

2020 ◽  
Vol 57 (11) ◽  
pp. 1349-1364
Author(s):  
Arthur Menier ◽  
Régis Roy ◽  
Grant Harrison ◽  
Ryan W. Zerff ◽  
Dwayne Kinar
Keyword(s):  
Ir Spectroscopy ◽  
Physical Properties ◽  
Three Dimensional ◽  
Physical Property ◽  
High Porosity ◽  
Uranium Deposits ◽  
Athabasca Basin ◽  
Rock Types ◽  
Density Values ◽  
Alteration Processes

Infrared (IR) spectroscopy has been used to characterize clay and clay-sized minerals present in drill cores that are associated with unconformity-related uranium deposits. Physical properties have been measured on samples to gain empirical data about the rock types and associated relationships with geophysical survey data. These data can be used to build three-dimensional geological models and constrain geophysical inversions. The objective of this study is to verify whether a relationship exists between rock physical properties and IR spectral mineralogy. Physical properties were measured on 427 core samples collected from the Martin Lake project, which is located in the southeastern Athabasca Basin (Saskatchewan, Canada). Results indicate that resistivity, density, and porosity are correlated to each other, especially within basement units. A comparison of their distribution with the IR spectral mineralogy demonstrates a relationship for each altered and unaltered samples. The samples with low resistivity and density, and high porosity are characterized by the presence of a di-trioctahedral (Al–Mg) chlorite (sudoite) due to the hydrothermal alteration processes. The unaltered samples with higher resistivity and density, and low porosity contain a tri-octahedral (Fe–Mg) chlorite as a result of metamorphic processes. Eleven mineralogical classes can be established based on IR spectroscopy. A percentile-based approach has been proposed and tested to define physical property ranges for each of the classes to predict resistivity and density values downhole.

Correlation of gravity anomalies with Yellowknife Supergroup rocks, North Arm, Great Slave Lake

1980 ◽  
Vol 17 (11) ◽  
pp. 1506-1516 ◽  
Author(s):  
R. A. Gibb ◽  
M. D. Thomas
Keyword(s):  
Greenstone Belt ◽  
Gravity Data ◽  
Volcanic Rocks ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
Rock Density ◽  
The North ◽  
Great Slave Lake ◽  
Western Shore ◽  
Gravity Map

A gravity map compiled from observations made on the frozen surface of Great Slave Lake shows that the positive gravity anomaly associated with the Yellowknife greenstone belt extends offshore into the North Arm of the lake. On the western shore of Yellowknife Bay the axis of the anomaly coincides with mafic volcanic rocks of the Kam Formation. Offshore the axis continues southwards for about 10 km to the West Mirage Islands where it takes a dramatic turn to the southeast and continues for a further 60 km to the Outer Whaleback Rocks. Using the geology and rock density determinations on land for control, a three-dimensional geological model comprising a large number of prismatic blocks was derived from the gravity anomalies. In the model the simplifying assumption has been made that the greenstone belt is everywhere floored by granodiorite similar to the adjacent Western and South-east granodiorites. According to the model, mafic volcanic rocks of the Kam Formation are generally 1–3 km thick with a maximum thickness of 7 km at the mouth of Yellowknife Bay. Greywacke and mudstone of the Burwash Formation vary in thickness from 1 to 3 km. Locally these sedimentary rocks attain a thickness of 8 km but this is probably an overestimated value as they may very well be underlain by volcanic rocks of the Kam Formation. The presence of a third pluton of granodiorite flanking the belt to the southwest is also inferred from the gravity data. Previous seismic work indicated a greenstone basin with an average thickness of about 10 km. However, reexamination of the seismic records suggests that weak arrivals interpreted as originating from the base of the greenstone belt are more likely to be pulses associated with earlier arrivals.

Three‐dimensional gravity or magnetic constrained depth inversion with lateral and vertical variation of contrast

Geophysics ◽  
1990 ◽  
Vol 55 (3) ◽  
pp. 327-335 ◽  
Author(s):  
D. Chenot ◽  
N. Debeglia
Keyword(s):  
Gravity Data ◽  
Three Dimensional ◽  
Sediment Compaction ◽  
Inhomogeneous Density ◽  
Dimensional Gravity ◽  
Well Data ◽  
Starting Model ◽  
Basin Area ◽  
Local Depth

Depth‐mapping inversion of gravity or magnetic fields generally assumes that anomalies originate from a main density or magnetization contrast interface. This particular inversion takes into account inhomogeneous density or magnetization distributions reflecting sediment compaction and basement heterogeneities: above the interface, the density can be approximated by an exponential function, and below it, an intrabasement contrast map can be used. The inversion also integrates local depth constraints from wells or seismic data, as well as general constraints set on the geometry and the contrast of the interface. After field transformations, spectral analysis and constraints help to define a starting model characterized mainly by the interface mean depth and the mean parameter contrast between the two media. The depth adjustment is completed iteratively under constraints using a space‐domain formulation derived from the Bouguer‐slab approximation. The interface model effect is computed in the wavenumber domain. A model data example shows the accuracy of the inversion and illustrates the role of the constraints. In a field example of a basin area where constraints can be derived from numerous well data, successive inversions of gravity data result in an isodepth map of the basement. The compatibility of the map with local depth constraints from wells is obtained by taking into account density heterogeneities related to known lithologic variations in the basement.

Sign in / Sign up

Export Citation Format

Share Document