Gravity Forward Modeling Research Articles

Geophysical and geochemical nature of relaminated arc‐derived lower crust underneath oceanic domain in southern Mongolia

AbstractThe Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E‐W trending Neoproterozoic cratons and Silurian‐Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high‐density material typical of lower oceanic crust but is composed of low‐ to intermediate‐density rocks resembling continental crust. The nature of this lower crust is constrained by the whole‐rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high‐K calc‐alkaline and Early Permian A‐type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic‐gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the “Khantaishir” arc (520–495 Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc‐like protoliths are thermodynamically modelled at granulite‐ and higher amphibolite‐facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ± 20 kg/m3) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.

Gravimetric monitoring of the first field‐wide steam injection in a fractured carbonate field in Oman – a feasibility study

ABSTRACTGas‐Oil Gravity Drainage is to be enhanced by steam injection in a highly fractured, low permeability carbonate field in Oman. Following a successful pilot, field‐wide steam injection is being implemented, first of this type in the world. A dedicated monitoring program has been designed to track changes in the reservoir. Various observations are to be acquired, including, surface deformation, temperature measurements, microseismic, well logs, pressure and saturation measurements to monitor the reservoir. Because significant changes in the reservoir density are expected, time‐lapse gravimetry is also being considered. In this paper we investigate the feasibility of gravimetric monitoring of the thermally enhanced gravity drainage process at the carbonate field in Oman. For this purpose, forward gravity modelling is performed. Based on field groundwater measurements, the estimates of the hydrological signal are considered and it is investigated under what conditions the groundwater influences can be minimized. Using regularized inversion of synthetic gravity data, we analyse the achievable accuracy of heat‐front position estimates. In case of large groundwater variations at the field, the gravity observations can be significantly affected and, consequently, the accuracy of heat‐front monitoring can be deteriorated. We show that, by applying gravity corrections based on local observations of groundwater, the hydrological influences can to a large extent be reduced and the accuracy of estimates can be improved. We conclude that gravimetric monitoring of the heat‐front evolution has a great potential.

A multidiscipline method of geological survey,seismic profile and gravity measurement applied to fold-and-thrust belt:a case study along the Hutubi River in the northern piedmont of Tianshan

Fold-and-thrust belts are hot topics in the research of orogens. However, the single geological or geophysical discipline often provides multisolutions on the geometry of a fold-and-thrust belt. We introduce here a multidisciplinary method combining geological and geophysical methods, take the Hutubi River section as a case to study the geometry of the fold-and-thrust belt in the northern piedmont of Tianshan. Firstly, we have carefully taken geological survey and got structural data on the surface. Secondly, petroleum seismic profile was re-interpreted based on the surface data and drilled wells. However, the seismic profile is absent in the contact zone between the mountain and the Junggar basin. We therefore carried out gravity measurements and forward modeling along this profile with the densities of the basement and sediments. The result shows that the northern flank fault of Tianshan doesn't exist along the Hutubi River profile, and the basin sediments could be continually followed from the basin to the mountain interior. This indicates that the tectonic structures in the contact zones between the mountain and the basin are variable, as the western section in the Jingou River section shows that the Tianshan basement thrusts northward on the basin sediments. Based on the balance-section technology, the restored section shows 4. 8 km of shortening. Comparing with previous results, this indicates the heterogeneous deformation along the northern piedmont of Tianshan. It also implies that this multidisciplinary method could be widely used in the fold-and-thrust belt.

典型地质模型2D与3D重力正演模拟对比研究

本文典型地质模型三维重力正演模拟研究是在二维重力正演方法总结的基础上，对地质模型作三维重力正演探究。其中所建的地质模型以球体为主。分别导出了二维与三维重力布格异常，重力异常各阶导数公式。在MATLAB环境下实现了模型的二维与三维重力异常曲线图。结合二维重力正演的分析方法，本文对模型进行了三维重力正演分析，对比讨论了三维重力正演与二维重力正演的优缺点，得到了二维重力正演只能反应地质体垂直切片的重力信息，三维重力正演可以就反映出地质体的整个重力信息，小的局部地质体可以通过高阶导数分析得出其规模形状的结论。 The study of three-dimensional gravity forward of typical geological model was based on the two- dimensional gravity forward method. The sphere is presented as the main geological model, which derives two-dimensional and three-dimensional gravitational Bouguer anomaly, the gravitational anomaly formula of every derivative respectively. The two-dimensional and three-dimensional gravity anomaly curve was achieved in the MATLAB environment. Combining with the analytical method of two-dimensional gravity forward modeling, this paper conducted a three-dimensional gravity forward modeling analysis, as well as compared and discussed the advantages and disad-vantages of two-dimensional gravity forward and three-dimensional gravity forward, and then got the results that two-dimensional gravity forward can only reflect gravity information of geological vertical slice, while three-dimensional gravity forward can reflect gravity information of the whole geological body. Small local geological body can obtain its size and shape through analyzing high- order derivative.

Repeat ridge jumps and microcontinent separation: insights from NE Arabian Sea

Microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume–ridge interactions. India separated from Seychelles at ∼64 Ma by asymmetric sea floor spreading initially when the spreading centre in the Mascarene Basin jumped towards the Indian sub-continent between magnetic chrons C29 and C28. The subsequent tectonics is difficult to comprehend since Laxmi Ridge-another microcontinent-formed during the later phase. Most of the studies considered the Laxmi Ridge as a sliver. Others considered it to be oceanic crust. High resolution, deep (∼25 km) seismic data reveals that (i) the ridge possesses > 15 km deep sea-ward dipping reflector (SDR) packages; (ii) normal faulted rift valleys devoid of syn-rift sedimentary packages; and (iii) axial magma chambers 5–7 km beneath the ridge top. Additionally, from 2D forward gravity models we deduce that the ridge most possibly comprises of high density (oceanic) crust. We conclude the Laxmi Ridge to be indeed composed of oceanic crust and a fossil spreading centre. We thus identified the ridge jumps and their relation to the Seychelles microcontinent separation.Previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating, spreading rate at the ridge, and plate ages. For repeated ridge jumps, the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating. We find that the medium spreading rates and high magmatic heating due to the Réunion plume and young plates favoured rapid and repeated ridge jumps towards the plume.

Study of the Earth׳s short-scale gravity field using the ERTM2160 gravity model

This paper describes the computation and analysis of the Earth׳s short-scale gravity field through high-resolution gravity forward modelling using the Shuttle Radar Topography Mission (SRTM) global topography model. We use the established residual terrain modelling technique along with advanced computational resources and massive parallelisation to convert the high-pass filtered SRTM topography – complemented with bathymetric information in coastal zones – to implied short-scale gravity effects. The result is the ERTM2160 model (Earth Residual Terrain Modelled-gravity field with the spatial scales equivalent to spherical-harmonic coefficients up to degree 2160 removed). ERTM2160, used successfully for the construction of the GGMplus gravity maps, approximates the short-scale (i.e., ~10km down to ~250m) gravity field in terms of gravity disturbances, quasi/geoid heights and vertical deflections at ~3 billion gridded points within ±60° latitude. ERTM2160 reaches maximum values for the quasi/geoid height of ~30cm, gravity disturbance in excess of 100mGal, and vertical deflections of ~30″ over the Himalaya mountains. Analysis of the ERTM2160 field as a function of terrain roughness shows in good approximation a linear relationship between terrain roughness and gravity effects, with values of ~1.7cm (quasi/geoid heights), ~11mGal (gravity disturbances) and 1.5″ (vertical deflections) signal strength per 100m standard deviation of the terrain. These statistics can be used to assess the magnitude of omitted gravity signals over various types of terrain when using degree-2160 gravity models such as EGM2008. Applications for ERTM2160 are outlined including its use in gravity smoothing procedures, augmentation of EGM2008, fill-in for future ultra-high resolution gravity models in spherical harmonics, or calculation of localised or global power spectra of Earth׳s short-scale gravity field. ERTM2160 is freely available via http://ddfe.curtin.edu.au/gravitymodels/ERTM2160.

Characterization of geothermally relevant structures at the top of crystalline basement in Switzerland by filters and gravity forward modelling

Some of the major geothermal anomalies in central Europe are linked to tectonic structures within the top of crystalline basement, which modify strongly the top of this basement. Their assessment is a major challenge in exploration geophysics. Gravity has been proven to be suitable for the detection of mainly large scale lithological and structural inhomogeneities. Indeed, it is well known and proven by different wells that, for example, in northern Switzerland extended negative anomalies are linked to such structures. Due to depth limitation of wells, there vertical extension is often unknown. In this study, we have investigated the potential of gravity for the geometrical characterization of such basement structures. Our approach consists in the combination of the series of Butterworth filters, geological modelling and best-fitting between observed and computed residual anomalies. In this respect, filters of variable wavelength are applied to observed and computed gravity data. The geological model is discretized into a finite element mesh. Near-surface anomalies and the effect of the sedimentary cover were eliminated using cut-off wavelength of 10km and geological and seismic information.We analysed the potential of preferential Butterworth filtering in a sensitivity study and applied the above mentioned approach to part of the Swiss molasses basin. Sensitivity analyses reveal that such sets of residual anomalies represents a pseudo-tomography revealing the distribution of different structures with depth. This finding allows for interpreting negative anomalies in terms of 3-D volumes. Best-fitting then permits determination of the most likely 3-D geometries of such basement structures. Our model fits both, geological observations and gravity: among 10 deep boreholes in the studied area, six reach the respective units and confirm our distribution of the negative (and positive) anomalies.

Structural Characteristics of the Andaman Forearc Inferred from Interpretation of Multichannel Seismic Reflection Data

AbstractThe Andaman Forearc Basin (AFB) is asymmetric in configuration and filled with a ∼6 km‐thick pile of Neogene to Recent sediments (∼4 s in two‐way travel time: TWT) with distinct zonation. It shows gradual thinning up to ∼3 km (0.8 s in TWT) towards the eastern end with a seabed gradient of 1:30. Thick deformed sediments ∼2 s (TWT) of the Outerarc are associated with intense faulting and occasional folding caused by recent tectonics. Development of a series of faults within the upwarped sedimentary column of Oligocene top to Recent is observed with a rotated fault block. These features are manifestations of Recent igneous intrusion, and reveal the presence of a mild N–S compressional regime. Its effect on the AFB resulted in further uplift of sediments, which can now be seen as the Invisible Bank. Forward gravity modelling supporting our seismic interpretation reveals that it is associated with igneous intrusion from the Moho (∼9 km depth), and also suggests that continental crust underlies the AFB. Strong Bottom Simulating Reflector (BSR)‐like features in the Miocene sediments of Outerarc and Forearc basin at a depth of 0.6 s below the seabed suggest the inferred probable occurrence of gas hydrates in the AFB.

Band‐limited topographic mass distribution generates full‐spectrum gravity field: Gravity forward modeling in the spectral and spatial domains revisited

AbstractMost studies on gravity forward modeling in the spectral domain truncate the gravitational potential spectra at a resolution commensurate with the input topographic mass model. This implicitly assumes spectral consistency between topography and implied topographic potential. Here we demonstrate that a band‐limited topographic mass distribution generates gravity signals with spectral energy at spatial scales far beyond the input topography's resolution. The spectral energy at scales shorter than the resolution of the input topography is associated with the contributions made by higher‐order integer powers of the topography to the topographic potential. The pth integer power of a topography expanded to spherical harmonic degree n is found to make contributions to the topographic potential up to harmonic degree p times n. New numerical comparisons between Newton's integral evaluated in the spatial and spectral domain show that this previously little addressed truncation effect reaches amplitudes of several mGal for topography‐implied gravity signals. Modeling the short‐scale gravity signal in the spectral domain improves the agreement between spatial and spectral domain techniques to the μGal level, or below 10−5 in terms of relative errors. Our findings have important implications for the use of gravity forward modeling in geophysics and geodesy: The topographic potential in spherical harmonics must be calculated to a much higher harmonic degree than resolved by the input topography if consistency between topography and implied potential is sought. With the improved understanding of the spectral modeling technique in this paper, theories, and computer implementations for both techniques can now be significantly better mutually validated.

Crustal structure beneath the Paleozoic Parnaíba Basin revealed by airborne gravity and magnetic data, Brazil

Abstract The Parnaiba Basin is a large Paleozoic syneclise in northeastern Brazil underlain by Precambrian crystalline basement, which comprises a complex lithostructural and tectonic framework formed during the Neoproterozoic–Eopaleozoic Brasiliano–Pan African orogenic collage. A sag basin up to 3.5 km thick and 1000 km long formed after the collage. The lithologic composition, structure, and role in the basin evolution of the underlying basement are the focus of this study. Airborne gravity and magnetic data were modeled to reveal the general crustal structure underneath the Parnaiba Basin. Results indicate that gravity and magnetic signatures delineate the main boundaries and structural trends of three cratonic areas and surrounding Neoproterozoic fold belts in the basement. Triangular-shaped basement inliers are geophysically defined in the central region of this continental-scale Neoproterozoic convergence zone. A 3-D gravity inversion constrained by seismological data reveals that basement inliers exhibit a 36–40.5 km deep crustal root, with borders defined by a high-density and thinner crust. Forward modeling of gravity and magnetic data indicates that lateral boundaries between crustal units are limited by Brasiliano shear zones, representing lithospheric sutures of the Amazonian and Sao Francisco Cratons, Tocantins Province and Parnaiba Block. In addition, coincident residual gravity, residual magnetic, and pseudo-gravity lows indicate two complex systems of Eopaleozoic rifts related to the initial phase of the sag deposition, which follow basement trends in several directions.

Ellipsoidal topographic potential: New solutions for spectral forward gravity modeling of topography with respect to a reference ellipsoid

AbstractForward gravity modeling in the spectral domain traditionally relies on spherical approximation. However, this level of approximation is insufficient for some present day high‐accuracy applications. Here we present two solutions that avoid the traditional spherical approximation in spectral forward gravity modeling. The first solution (the extended integration method) applies integration over masses from a reference sphere to the topography and applies a correction for the masses between ellipsoid and sphere. The second solution (the harmonic combination method) computes topographic potential coefficients from a combination of surface spherical harmonic coefficients of topographic heights above the ellipsoid, based on a relation among spherical harmonic functions introduced by Claessens (2005). Using a degree 2160 spherical harmonic model of the topographic masses, both methods are applied to derive the Earth's ellipsoidal topographic potential in spherical harmonics. The harmonic combination method converges fastest and—akin to the EGM2008 geopotential model—generates additional spherical harmonic coefficients in spectral band 2161 to 2190 which are found crucial for accurate evaluation of the ellipsoidal topographic potential at high degrees. Therefore, we recommend use of the harmonic combination method to model ellipticity in spectral‐domain forward modeling. The method yields ellipsoidal topographic potential coefficients which are “compatible” with global Earth geopotential models constructed in ellipsoidal approximation, such as EGM2008. It shows that the spherical approximation significantly underestimates degree correlation coefficients among geopotential and topographic potential. The topographic potential model is, for example, of immediate value for the calculation of Bouguer gravity anomalies in fully ellipsoidal approximation.

Elastic thickness structure of the Andaman subduction zone: Implications for convergence of the Ninetyeast Ridge

We use the Bouguer coherence (Morlet isostatic response function) technique to compute the spatial variation of effective elastic thickness (Te) of the Andaman subduction zone. The recovered Te map resolves regional-scale features that correlate well with known surface structures of the subducting Indian plate and the overriding Burma plate. The major structure on the India plate, the Ninetyeast Ridge (NER), exhibits a weak mechanical strength, which is consistent with the expected signature of an oceanic ridge of hotspot origin. However, a markedly low strength (0<Te<3km) in that region, where the NER is close to the Andaman trench (north of 10°N), receives our main attention in this study. The subduction geometry derived from the Bouguer gravity forward modeling suggests that the NER has indented beneath the Andaman arc. We infer that the bending stresses of the viscous plate, which were reinforced within the subducting oceanic plate as a result of the partial subduction of the NER buoyant load, have reduced the lithospheric strength. The correlation, Te<Ts (seismogenic thickness) reveals that the upper crust is actively deforming beneath the frontal arc Andaman region. The occurrence of normal-fault earthquakes in the frontal arc, low Te zone, is indicative of structural heterogeneities within the subducting plate. The fact that the NER along with its buoyant root is subducting under the Andaman region is inhibiting the subduction processes, as suggested by the changes in trench line, interrupted back-arc volcanism, variation in seismicity mechanism, slow subduction, etc. The low Te and thinned crustal structure of the Andaman back-arc basin are attributed to a thermomechanically weakened lithosphere. The present study reveals that the ongoing back-arc spreading and strike-slip motion along the West Andaman Fault coupled with the ridge subduction exerts an important control on the frequency and magnitude of seismicity in the Andaman region.

Interpretation of gravity data by the continuous wavelet transform: The case of the Chad lineament (North-Central Africa)

Abstract A slightly bended gravity high along the Chad lineament in Central North Africa is analyzed and interpreted by the continuous wavelet transform (CWT) method. We use scale normalization on the continuous wavelet transform, allowing analysis of the gravity field in order to determine the sources at different depths. By focusing on homogenous standard sources, such as sphere or cube, horizontal cylinder or prism, sheet and infinite step, we derive the relationships between the source depth and pseudo-wavenumber. Then the source depth can be recovered from tracing the maximal values of the modulus of the complex wavelet coefficients in the CWT-based scalograms that are function of the pseudo-wavenumber. The studied area includes a central gravity high up to 75 km wide, and a secondary high that occurs at the southern part of the anomaly. The interpretation of the depth slices and vertical sections of the modulus maxima of the complex wavelet coefficients allows recognition of a relatively dense terrane located at middle crustal levels (10–25 km depth). A reasonable geological model derived from the 2.5D gravity forward modelling indicates the presence of high density bodies, probably linked to a buried suture, which were thrusted up into the mid-crust during the Neo-Proterozoic terrane collisions between the Saharan metacraton and the Arabian-Nubian shield. We conclude that the Chad line delineates a first order geological boundary, missing on the geologic maps.

RTM Gravity Forward-Modeling Using Topography/Bathymetry Data to Improve High-Degree Global Geopotential Models in the Coastal Zone

We apply the residual terrain modeling (RTM) technique for gravity forward-modeling to successfully improve high-resolution global gravity fields at short spatial scales in coastal zones. The RTM scheme is combined with the concept of rock-equivalent topography, allowing to use a single uniform constant mass-density in the RTM forward-modeling, both at land and sea. SRTM30_PLUS bathymetry is merged with higher-resolution SRTM V4.1 land topography, and expanded into spherical harmonics to degree 2160, yielding a new and consistent high-degree RTM reference surface. The forward-modeling performance is demonstrated in coastal zones of Greece and Canada using ground-truth vertical deflections, gravity from land and shipborne gravimetry, and geoid heights from GPS/leveling, with improvements originating from bathymetry clearly identified. We demonstrate that the SRTM30_PLUS bathymetry carries information on gravity field structures at spatial scales less than 5 arc minutes, which can be used to augment EGM2008 in (rugged) coastal zones, both over land and marine areas. This may be of value (i) to partially reduce the signal omission error in EGM2008/GOCE-based height transfer in areas devoid of dense gravity data, (ii) to fill the gap between land gravity and shipborne gravity along rugged coastlines, and (iii) for the development of next-generation altimetric gravity fields.

TESLA GPUs versus MPI with OpenMP for the Forward Modeling of Gravity and Gravity Gradient of Large Prisms Ensemble

An implementation with the CUDA technology in a single and in several graphics processing units (GPUs) is presented for the calculation of the forward modeling of gravitational fields from a tridimensional volumetric ensemble composed by unitary prisms of constant density. We compared the performance results obtained with the GPUs against a previous version coded in OpenMP with MPI, and we analyzed the results on both platforms. Today, the use of GPUs represents a breakthrough in parallel computing, which has led to the development of several applications with various applications. Nevertheless, in some applications the decomposition of the tasks is not trivial, as can be appreciated in this paper. Unlike a trivial decomposition of the domain, we proposed to decompose the problem by sets of prisms and use different memory spaces per processing CUDA core, avoiding the performance decay as a result of the constant calls to kernels functions which would be needed in a parallelization by observations points. The design and implementation created are the main contributions of this work, because the parallelization scheme implemented is not trivial. The performance results obtained are comparable to those of a small processing cluster.

3D crustal structure and its tectonic implication for nile delta and greater cairo regions, egypt, from geophysical data

In the present study the crustal structures of Nile Delta, including Greater Cairo province, and its surroundings, were evaluated using 3D forward gravity and magnetic modeling. The interpretation is also based on the seismic reflection results, well logs data and previous density models. The present results allow to improve the understanding of both the crustal thicknesses and density distributions between the sedimentary cover and the upper mantle in the study area.The high Bouguer anomalies near the Mediterranean coast are mainly caused by deep-seated structures. The crust beneath the Mediterranean coastal region is typically continental, with a thickness of 24 km beneath Rositta and Damitta branches, which increases toward the south. The Greater Cairo province has been modeled with maximum crustal thickness of ca. 34 km. The negative gravity anomalies with minimum values are due to the effect of sedimentary cover and/or basement relief geometry.The main results of this work suggest that the study area...

Displacement along the Red River Fault constrained by extension estimates and plate reconstructions

Significant E‐W extension and/or compression must have been generated by displacements along the Red River Fault (RRF) since its curvature does not match a small circle centered at the Euler pole for the Indochina–south China plate pair. The amount of extension perpendicular to the RRF offshore Vietnam depends on the magnitude of left‐lateral displacement along the RRF. In general, the larger the left‐lateral displacement along the fault, the smaller the amount of E‐W extension. All purely strike‐slip models of the opening of the South China Sea that assume large displacements (&gt;250 km) along the RRF encounter major problems because they imply little extension, or even considerable shortening, offshore east Vietnam. This is inconsistent with the presence of large elongated basins offshore Vietnam. Using a plate tectonic model, we compare continental extension values implied by different magnitudes of displacement along the RRF with crustal stretching estimates derived from 2‐D profiles modeled from gravity data. We utilize 2‐D gravity forward models to restore the extended continental margin crust to its original position prior to extension. We find that substantial amounts of extension for offshore Vietnam can only be modeled assuming moderate displacements along the RRF compatible with the presence of a southward subducting proto–South China Sea. The total amount of ENE‐WSW extension offshore northern Vietnam constrained by our 2‐D gravity profiles and gravity inversion increases southward from 36 to 89 km along the Yinggehai Basin. These values of ENE‐WSW extension are consistent with 250 km of left‐lateral displacement along the RRF.

Development of a Synthetic Earth Gravity Model by 3D mass optimisation based on forward modelling

Several previous Synthetic Earth Gravity Model (SEGM) simulations are based on existing information about the Earth’s internal mass distribution. However, currently available information is insufficient to model the Earth’s anomalous gravity field on a global scale. The low-frequency information is missing when modelling only topography, bathymetry and crust (including the Mohorovicic discontinuity), but the inclusion of information on the mantle and core does not seem to significantly improve this situation. This paper presents a method to determine a more realistic SEGM by considering simulated 3D mass distributions within the upper mantle as a proxy for all unmodelled masses within the Earth. The aim is to improve an initial SEGM based on forward gravity modelling of the topography, bathymetry and crust such that the missing low-frequency information is now included. The simulated 3D mass distribution has been derived through an interactive and iterative mass model optimisation algorithm, which minimises geoid height differences with respect to a degree-360 spherical harmonic expansion of the EGM2008 global external gravity field model. We present the developed optimisation algorithm by applying it to the development of a global SEGM that gives a reasonably close fit to EGM2008, and certainly closer than a SEGM based only on the topography, bathymetry and crust.

Gravity evidence for a mafic intrusion beneath a mineralized zone in the Bondy gneiss complex, Grenville Province, Quebec — Exploration implications

A ground gravity survey over the Bondy gneiss complex and its mineralized iron oxide- and copper-rich hydrothermal system(s) in the Grenville Province of SW Quebec was undertaken to aid mineral exploration in mapping subsurface intrusions. Several kilometric-scale positive Bouguer anomalies were identified that coincide with outcropping mafic and intermediate intrusive rocks of the post peak-metamorphic, 1.17–1.16 Ga mafic to intermediate Chevreuil suite intrusions and a 1.09–1.07 Ga Rolleau ultramafic stock. An additional 4 × 3 kilometre positive gravity anomaly indicates a mafic body underlies part of the metamorphosed hydrothermal system in the area of magnetite , pyrite , pyrrhotite , and chalcopyrite mineralization . Advanced argilic alteration associated with sulphide enrichment here is however indicative of an epithermal system with a felsic intrusion fluid source. As a felsic intrusion cannot explain the positive Bouguer gravity anomaly both felsic and mafic bodies must be present beneath the mineralized zone. Our preferred interpretation based on integrating gravity data and 2D forward gravity modelling with the results of field and geochemical studies is that this anomaly corresponds to a ca. 500 m deep mafic 1.17–1.16 Ga Chevreuil suite pluton that may have provided the source for hydrothermal fluids associated with late ductile shear- and fault-related mineralization or remobilization of early mineralization associated with a felsic pluton into late structures. This interpretation is compatible with gabbro xenoliths in the 1.07 Ga Rivard lamprophyre dyke on the NW margin of the gravity anomaly that bear significant similarities with those of the Chevreuil intrusive suite. The presence of both early felsic and late mafic intrusions beneath a group of three mineral occurrences in the Bondy gneiss complex strengthens their prospectivity in comparison to other mineral occurrences in the area. That early, pre-metamorphic mineralization was upgraded late in the tectonothermal evolution during a subsequent igneous and deformation event agrees with interpretations of other IOCG-style deposits in the Lac Manitou area of the eastern Grenville Province. ► A ground gravity survey provides new data that aids mineral exploration. ► Most gravity anomalies are associated with mafic to intermediate intrusive rocks. ► A Gravity anomaly over mineralized outcrops may be generated by a deep gabbro. ► Mafic intrusion may be the driving source for mineralization or remobilisation.

Mineral potential targeting and resource assessment based on 3D geological modeling in Luanchuan region, China

In this paper, we used 3D modeling and nonlinear methods (fractal, multifractal, and probabilistic neural networks (PNN)) for regional mineral potential mapping and quantitative assessment for porphyry and skarn-type Mo deposits and hydrothermal vein-type Pb–Zn–Ag deposits in the Luanchuan region, China. A 3D geological model was constructed from various geological maps, cross sections, boreholes, and gravity and magnetic data. Geological features associated with mineralization were extracted using the 3D geological model and metallogenic models of porphyry and skarn-type Mo and Pb–Zn–Ag deposits. The multifractal method, principal component analysis, and power spectrum–area method were used to separate regional variability from local variability in the geochemical data. A 2.5D forward modeling of gravity and magnetic data was carried out to define the geometry, depth, and physical properties of geological bodies at depth. 3D visualization of the results assisted in understanding the spatial relations between the deposits and the other geological bodies (e.g., igneous intrusions). The PNN method was applied to represent and integrate multiple anomalies for mineral potential modeling. The concentration–area fractal method was used to classify the PNN mineral potential model. Three levels (ground surface and two subsurface horizontal planes) of mineral potential models were evaluated for undiscovered Mo and Pb–Zn–Ag deposits. Validation of the results shows that 3D modeling was useful for not only accurately extracting geological features but also for predicting potential mineral targets and evaluating mineral resources. The mineral potential targets identified consist of eight Mo potential targets and 15 Pb–Zn–Ag potential targets. Based on grade–tonnage data from the known Mo and Pb–Zn–Ag deposits and the results of 3D modeling, estimated potential resources of each of these types of deposits are 10.8 and 153.1 Mt (Pb+Zn is 152.9 Mt and Ag is 0.92 Mt), respectively.