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A huge amount of legacy drilling data is available in geological survey but cannot be used directly as they are compiled and recorded in an unstructured textual form and using different formats depending on the database structure, company, logging geologist, investigation method, investigated materials and/or drilling campaign. They are subjective and plagued by uncertainty as they are likely to have been conducted by tens to hundreds of geologists, all of whom would have their own personal biases. dh2loop (https://github.com/Loop3D/dh2loop , last access: 30 September 2021​​​​​​​) is an open-source Python library for extracting and standardizing geologic drill hole data and exporting them into readily importable interval tables (collar, survey, lithology). In this contribution, we extract, process and classify lithological logs from the Geological Survey of Western Australia (GSWA) Mineral Exploration Reports (WAMEX) database in the Yalgoo–Singleton greenstone belt (YSGB) region. The contribution also addresses the subjective nature and variability of the nomenclature of lithological descriptions within and across different drilling campaigns by using thesauri and fuzzy string matching. For this study case, 86 % of the extracted lithology data is successfully matched to lithologies in the thesauri. Since this process can be tedious, we attempted to test the string matching with the comments, which resulted in a matching rate of 16 % (7870 successfully matched records out of 47 823 records). The standardized lithological data are then classified into multi-level groupings that can be used to systematically upscale and downscale drill hole data inputs for multiscale 3D geological modelling. dh2loop formats legacy data bridging the gap between utilization and maximization of legacy drill hole data and drill hole analysis functionalities available in existing Python libraries (lasio , welly , striplog). [ABSTRACT FROM AUTHOR]

Two high-resolution seismic reflection profiles acquired north and south of Chibougamau, located in the northeast of the Abitibi subprovince of Canada, help understand historic volcanically hosted massive sulfide (VMS) deposits and hydrothermal Cu–Au mineralization found there. Major faults crossed by the profiles include the Barlow fault in the north and the Doda fault and the Guercheville fault in the south, all targets of this study that seeks to determine spatial relationships with a known metal endowment in the area. Common-offset DMO corrections and common-offset pre-stack time migrations (PSTMs) were considered. Irregularities of the trace midpoint distribution resulting from the crooked geometry of both profiles and their relative contribution to the DMO and PSTM methods and seismic illumination were assessed in the context of the complex subsurface architecture of the area. To scrutinize this contribution, seismic images were generated for offset ranges of 0–9 km using increments of 3 km. Migration of out-of-plane reflections used cross-dip element analysis to accurately estimate the fault dip. The seismic imaging shows the thickening of the upper-crustal rocks near the fault zones along both profiles. In the northern seismic reflection section, the key geological structures identified include the Barlow fault and two diffraction sets imaged within the fault zone that represent potential targets for future exploration. The south seismic reflection section shows rather a complicated geometry of two fault systems. The Guercheville fault observed as a subhorizontal reflector connects to a steeply dipping reflector. The Doda fault dips subvertical in the shallow crust but as a steeply dipping reflection set at depth. Nearby gold showings suggest that these faults may help channel and concentrate mineralizing fluids. [ABSTRACT FROM AUTHOR]

Two high-resolution seismic reflection profiles acquired north and south of Chibougamau, located in the northeast of the Abitibi subprovince of Canada help understand historic volcanic-hosted massive sulfide (VMS) deposits and hydrothermal Cu-Au mineralization found there. Major faults crossed by the profiles include the Barlow fault in the north and the Doda fault and the Guercheville fault in the south, all targets of this study that seeks to determine spatial relationships with known metal endowment in the area. Common-offset DMO corrections and common-offset pre-stack time migrations (PSTM) were considered. Irregularities of the trace midpoint distribution resulting from the crooked geometry of both profiles and their relative contribution to DMO and PSTM method and seismic illumination were assessed in the context of the complex subsurface architecture of the area. To scrutinize this contribution, seismic images were generated for offset ranges of 0 -9 km using increments of 3 km. Migration of out-of-plane reflections used cross-dip element analysis to accurately estimate the fault dip. The seismic imaging shows the thickening of the upper crustal rocks near the fault zones along both profiles. In the north seismic reflection section, key geological structure identified include the Barlow fault and two diffraction sets imaged within the fault zone that represent potential targets for future exploration. The south seismic reflection section shows rather a complicated geometry of two fault systems. The Guercheville fault observed as a subhorizontal reflector connects to a steeply dipping reflector. The Doda fault dips subvertical in the shallow crust but as a steeply dipping reflection set at depth. Nearby gold showings suggest that these faults may help channel and concentrate mineralizing fluids. [ABSTRACT FROM AUTHOR]

Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyzed a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold-bearing massive metabasalts (supracrustals) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), Western Dharwar Craton, southern India. The study involves the following steps: (1) analyzing the internal magnetic fabric, using anisotropy of magnetic susceptibility (AMS) studies, and determining strength of the host metabasalts, (2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R′), stress ratio (ϕ), and susceptibility to fracturing, and (3) deciphering the paleostress condition using fault-slip analysis. We interpret the NNW–SSE to NW–SE (mean 337/69 ∘ NE) oriented magnetic fabric in the rocks of the region as having developed during regional D1/D2 deformation on account of NE–SW shortening. However, D3 deformation manifested by NW–SE to E–W shortening led to the sinistral movement along CSZ. As a consequence of this sinistral shearing, fractures with prominent orientations formed riedel shear components, with CSZ as the shear boundary. Subsequently, all the pre-existing fabrics along with the riedel shear components were reactivated and vein emplacement took place through episodic fluid pressure fluctuation from high to low Pf at shallow depth (∼ 2.4 km). However, NNW–SSE orientations were prone to reactivate under both high- and low- Pf conditions, thereby attaining maximum vein thickness along these orientations. The deduced paleostress from fault-slip analysis along with the kinematics of the fractures and veins are in good agreement with previously estimated regional tectonics. Thus, integrating multiple domains of studies helps in the logical interpretation of fluid flow conditions and vein emplacement mechanisms in the study area that has not been ventured before. [ABSTRACT FROM AUTHOR]

We propose a methodology for the recovery of lithologies from geological and geophysical modelling results and apply it to field data. Our technique relies on classification using self-organizing maps (SOMs) paired with geoscientific consistency checks and uncertainty analysis. In the procedure we develop, the SOM is trained using prior geological information in the form of geological uncertainty, the expected spatial distribution of petrophysical properties and constrained geophysical inversion results. We ensure local geological plausibility in the lithological model recovered from classification by enforcing basic topological rules through a process called "post-regularization". This prevents the three-dimensional recovered lithological model from violating elementary geological principles while maintaining geophysical consistency. Interpretation of the resulting lithologies is complemented by the estimation of the uncertainty associated with the different nodes of the trained SOM. The application case we investigate uses data and models from the Yerrida Basin (Western Australia). Our results generally corroborate previous models of the region but they also suggest that the structural setting in some areas needs to be updated. In particular, our results suggest the thinning of one of the greenstone belts in the area may be related to a deep structure not sampled by surface geological measurements and which was absent in previous geological models. [ABSTRACT FROM AUTHOR]

Studying the uppermost structure of the subsurface is a necessary part of solving many practical problems (exploration of minerals, groundwater studies, geoengineering, etc.). The practical application of active seismic methods for these purposes is not always possible for different reasons, such as logistical difficulties, high cost of work, and a high level of seismic and acoustic noise. That is why developing and improving passive seismic methods is one of the important problems in applied geophysics. In our study, we describe a way of improving the quality of empirical Green's functions (EGFs), evaluated from high-frequency ambient seismic noise, by using the advanced technique of cross-correlation function stacking in the time domain (in this paper we use term "high-frequency" for frequencies higher than 1 Hz). The technique is based on the global optimization algorithm, in which the optimized objective function is a signal-to-noise ratio of an EGF, retrieved at each iteration. In comparison to existing techniques, based, for example, on weight stacking of cross-correlation functions, our technique makes it possible to significantly increase the signal-to-noise ratio and, therefore, the quality of the EGFs. The technique has been tested with the field data acquired in an area with a high level of industrial noise (Pyhäsalmi Mine, Finland) and in an area with a low level of anthropogenic noise (Kuusamo Greenstone Belt, Finland). The results show that the proposed technique can be used for the extraction of EGFs from high-frequency seismic noise in practical problems of mapping of the shallow subsurface, both in areas with high and low levels of high-frequency seismic noise. [ABSTRACT FROM AUTHOR]

We propose a methodology for the recovery of lithologies from geological and geophysical modelling results and apply it field data. Our technique relies on classification using self-organizing maps (SOM) paired with geoscientific consistency checks and uncertainty analysis. In the procedure we develop, the SOM is trained using prior geological information in the form of geological uncertainty, the expected spatial distribution of petrophysical properties, and constrained geophysical inversion results. We ensure local geological plausibility in the lithological model recovered from classification by enforcing basic topological rules through a process called "post-regularisation". This prevents the three-dimensional lithological model from violating elementary geological principles while maintaining geophysical consistency. Interpretation of the resulting lithologies is complemented by the estimation of the uncertainty associated to the different nodes of the trained SOM. The application case we investigate uses data and models from the Yerrida Basin (Western Australia). Our results generally corroborate previous models of the region but they also suggest that the structural setting in some areas need to be updated. In particular, our results suggest the thinning of one of the greenstone belts in the area may be related to a deep structure not sampled by surface geological measurements and which was absent in previous geological models. [ABSTRACT FROM AUTHOR]

The Olondo greenstone belt locates in the western part of the Aldan Shield, the largestbasement of the Siberian craton. This Archean greenstone belt is unique by its abundantmafic-ultramafic rocks (>30%) comparing to other greenstone belts worldwide, and one ofthe best preserved areas that provide a window into an ancient geological past. In this study,we focus on mafic rocks that underwent greenschist to amphibolite facies metamorphism,composed of hornblende-tremolite-actinolite-chlorite-epidote mineral assemblage. The maficrocks comprise komatiitic and high-Fe tholeiitic basalts, characterized by variable Mg#(0.44-0.74) representative of both differentiated and near-primary magma. On the basis ofchondrite-normalized REE pattern, these basalts are subdivided into two groups. Group I ischaracterized by flat to slightly enriched LREE (La/SmN=1.4 – 2.5; La/YbN=1.3 - 3.2) andLILE, as well as depleted in HFSEs (Nb, Ta, Ti), suggesting either their mantlesource might be influenced by subduction-related processes or result of crustalcontamination. Group II is characterized by depleted LREE pattern (La/SmN=1.0-1.3;La/YbN=0.7-1.8) with no Nb-Ta depletion, similar to the REE pattern of mid-ocean ridgebasalts (MORB). Trace-element modeling suggests the rocks were likely derived by10-30% partial melting of a depleted mantle or primitive mantle-like source with themelting temperature around 1420-1540oC. The initial 87Sr/86Sr ratio is between0.702045 and 0.705961 together with positive initial ɛNdi (+0.09 to +3.91), areindicative of heterogeneous depleted mantle with no significant crustal contamination.The new geochemical data above suggest typical modern tectonic processes suchas sea-floor spreading and subduction might be in operation in the Archean time. [ABSTRACT FROM AUTHOR]

One of the major consequences of photosynthesis is locally increased carbonate alkalinity, which stimulates the precipitation of carbonate minerals in aqueous environments. On the modern Earth, reef–type ecosystems occupy the most productive end of the oxygen– and carbonate–producing spectrum. However, prior to about 700 million years ago, photosynthetic bacteria fulfilled this role, building reef–like microbialite (e.g., stromatolite) structures, and eventually whole carbonate platforms, out of the carbonate minerals they help precipitate as a consequence of their alkalinizing effect. The most ancient examples of these systems, dating back to ca. 3.0 Ga, remain little explored. The EARTHBLOOM project examines in detail the stratigraphy, major element and trace element geochemistry of the 2,940 +/- 2 Ma to 2,925+/- 3 Ma Ball assemblage, Red Lake Greenstone belt, N. Ontario, Canada (dates from Corfu and Wallace, 1986, Can. J. Earth. Sci. 23, 27–42). This deposit contains over 400m of metasedimentary rocks that include over 200m of stromatolitic calcite and dolomite, making it the earliest known significant accumulation of carbonate on the planet. Field studies and examination of multiple industry drill cores reveal that in the east, deep-water facies (BIF, chert) are overlain by carbonates with pervasive microbial fabrics, while in the west, carbonates showing few microbial textures were deposited on and interbedded with abundant siliciclastics. Trace element data indicate general anoxia during precipitation of carbonate lithofacies. Relatively abundant pyrite and potential pseudomorphs after gypsum suggest evaporative concentration of sulfate, and sulfur isotope studies are currently underway. Carbonate C and O isotope compositions are consistent with precipitation from Archean seawater. Carbonate C isotopes, however, do show some variation between facies, with lighter values in deeper water facies. Organic carbon isotopes show a significant range, with values spanning from -30 to as heavy as -10 permil. The statistical distribution of organic carbon isotope data appears to suggest different organic carbon sources in near-shore carbonate-dominated environments versus off-shore cherty or shaley environments. A diagenetic origin of this signal appears unlikely based on C- and O- isotope systematics. The ensemble of data available to date indicates that the Red Lake carbonate platform represents an evaporitic setting, perhaps with locally concentrated sulfate, and remarkably, with little evidence for the presence of cyanobacteria during carbonate precipitation. [ABSTRACT FROM AUTHOR]

Rock complexes of Precambrian to early Mid Palaeozoic age dominate the bedrock geology of Finnmark, northern Norway. These can be readily divided into (1) a series of nappes and thrust-sheets that constitute the Caledonide orogen, and (2) a mid-crustal lithospheric basement made up of both granitoid and greenstone complexes that range in age from Neoarchaean to late Palaeoproterozoic. This basement extends into northern Finland and northwestern Russia, forming the northern margin of the Fennoscandian Shield. In several places in the Caledonides the older Precambrian rocks are also found to be incorporated as thrust slices in the Caledonian nappes. Some of the older rock complexes, notably the Palaeoproterozoic Kautokeino Greenstone Belt, also appear in antiformal tectonic windows within the Caledonides. During the period 2011-2015, new high-resolution aeromagnetic data from the Caledonides and the Archaean- Palaeoproterozoic crystalline basement of Finnmark and North Troms have been acquired from fixed-wing and helicopter-borne surveys conducted as part of NGU's MINN (Mineral Resources in North Norway) programme. The new data have provided spectacular and confirmatory evidence for the continuation of the diverse Precambrian greenstone belts and granulite terranes beneath the magnetically transparent Caledonian nappes.The main objective of the current study has been to estimate the depths to the main magnetic sources beneath the nappes using 3D Euler and 2D Werner deconvolution methods. The study area comprises both deep and shallow structures. Shallow structures in the region are mainly due to prominent and extensive, ultramafic-mafic complexes, each with associated sets of mafic dykes. Elsewhere, in several thrust sheets, there are significant numbers of mafic dykes with diverse trends and different ages, and locally these reach up to swarm proportions.In the current study we have specifically targeted the depth-to-basement beneath the nappes and purposely excluded the anomalies deriving from shallow structures such as dykes and faults. The results from both methods (i.e., 2D and 3D) are similar and show an appreciable increase of depth to the Precambrian basement rocks from south to north. In the south, basement rocks are either exposed on the surface southeast of the Caledonian front or reappear in tectonic windows. On the Porsanger Peninsula a prominent magnetic anomaly is inferred to represent an extension of the Repparfjord window. The depth-to-basement beneath nappes close to the Caledonian front is estimated to 500-2000 m whereas in the northern part of the study area the depths reach 3000-4000 m. In some cases, depths are estimated to between 5000 and 6000 m. However, these latter interpretations should be considered with extreme caution as there are not enough clustering solutions for those particular depth estimates. Nevertheless, by employing the 3D Euler and 2D Werner deconvolution methods we are thus able to estimate the approximate thicknesses of the nappes in this northernmost part of the Norwegian Caledonides. [ABSTRACT FROM AUTHOR]

In this study, we developed a novel three-dimensional (3D) crustal model for the Kautokeino Greenstone Belt (KKGB) in North Norway. This approach is based on the 3D density modelling and aeromagnetic data, integrated with qualitative interpretation of geophysical and geological data. Incorporation of the high-resolution geophysical data with surface observation allows for improving the existing geological maps by identifying several new structures. Moreover, integrating modern and accurate geophysical analysis techniques with the existing petrophysical databases enables to pertain the shallow and deeper structures throughout the area. To have a better control over the regional setting of the KKGB, we expanded the study area, including the northern part of Finland and Sweden. Integrated Gravity and magnetic measurements enabled us to follow the KKGB underneath the covering sedimentary rocks of Caledonian nappes and link them with their outcrop in the tectonic windows in the northern part of the belt. The proposed 3D crustal model shows that the Kautokeino Greenstone Belt is longer (and possibly broader) than what has already been suggested. In this respect, this approach suggests that the belt reaches depths of approximately 5-6 km and is located as a varying dipping structure between the Jergul Gneiss Complex (JGC) in the east and Ráiseatnu Gneiss Complex (RGC) in the west. The Raiseatnu Complex (RGC) to the west of the KKGB is characterised by magnetic anomalies trending NNW–SSE directions. This is different from the JGC and the RGC is more similar to the KKGB in terms of the greater number of anomalies and their shapes. This gneiss complex shows short-wavelength and circular anomalies compared to the more subdued and smoother anomaly pattern of the JGC. In the new 3D model, a large portion of the greenstone belt assumed to be part of the RGC complex either migmatized portion of KKGB or the latest phase of it. A set of high- and low-density structures assumed for modelling of the measured gravity anomalies of the region. This leads to find out most of the high gravity anomalies that are due to dense rocks of amphibolite-facies volcano-sedimentary rocks. While the low gravity anomalies are originated by a combination of quartzite and granitoids. In a large scale, this approach shows a similarity of the Proterozoic greenstone belts to their older Archean counterparts such that most of the deformation is caused by gravitational tectonics. This study shows that integration of geological and geophysical data can considerably improve the 3D understanding of complex, poorly exposed terranes of the Finnmarksvidda region in Norway. Integrated 3D density modelling using gravity data constrained by surface petrophysical samples assists to refine and establish realistic 3D subsurface models. [ABSTRACT FROM AUTHOR]

Four decades have now passed since Schidlowski et al. [1] proposed that a light carbon isotope signature in metasedimentary rocks, as found in the 3.7 Ga Isua Greenstone Belt (IGB) of southwest Greenland, might be used to indicate the presence of life in some of the earliest preserved geology. Isotopic evidence was relied upon because the host rocks have been metamorphosed and deformed to a degree that precludes preservation of unambiguous biomorphic features, such as those seen in the ca. 3.4 Ga algal mats at Strelley Pool, Western Australia [2]. Since Schidlowski et al.'s seminal work, a number of claims have been made for low δ13C indicating life in the Eoarchean, including those from Isua [3] and Akilia [4] in west Greenland, and the Nuvvuagittuq Greenstone Belt [5] and Saglek region, Labrador [6] in northern Canada. Such cases, often hailed at the time of publication as evidence for 'the Earth's oldest life', attract great publicity and debate, and are remarkable not least because of the overlap with a time when the rate of meteorite bombardment was considerably greater [7], with a concomitantly increased "impact frustration" to the establishment of terrestrial life [8].A number of key criteria, required for the credibility of claims for early life, were proposed in a review paper by Whitehouse and Fedo (2007) [9] and have recently been reiterated by Whitehouse et al. [10]. Along with the veracity of the carbon isotopic evidence itself, upon which most Eoarchean life claims rely, these criteria include (1) the suitability of host rocks as possible environments in which life might have developed and (2) temporal constraints provided either directly by the isotopic dating of the host rock or through field relationships with well-dated lithologies. Using the case studies cited above, this presentation will stress the important of both unambiguous geological relationships and geochronology in the quest for finding evidence of Earth's earliest biogenicity, and will review the status of such claims. It is noteworthy that the preservation of fossil evidence in all claims prior to the Paleoarchean is severely limited by the effects of tectonism and metamorphism, and sole reliance on chemical or isotopic evidence cannot provide an unambiguous basis for biogenicity.References: [1] Schidlowski et al. (1979) Geochim. Cosmochim. Acta 43, 189. [2] Allwood et al. (2007) Precam. Res. 158, 198. [3] Rosing (1999) Science 283, 674. [4] Mojzsis et al. (1996) Nature 384, 55. [5] Dodd et al. (2017) Nature 543, 60. [6] Tashiro et al. (2017) Nature 549, 516. [7] Bottke and Norman (2017) Annu Rev Earth Pl Sci 45, 619. [8] Maher & Stevenson, (1988) Nature 331, 612. [9] Whitehouse & Fedo (2007) Developments in Precambrian Geology (Elsevier) 15, 841. [10] Whitehouse et al. (2019). Precam. Res, doi.org/10.1016/j.precamres.2019.01.001. [ABSTRACT FROM AUTHOR]

Orogenic gold occurrences within the Central Lapland greenstone belt (CLGB), are located in shear zones within Paleoproterozoic mafic-ultramafic metavolcanic rocks and metasedimentary successions. In the mineralized zones, monazite and xenotime of hydrothermal origin are typical accessories in alteration zones and ore veins, and thus allow direct dating of the mineralization. We present U-Pb data from three different gold deposits within the CLGB: Iso-Kuotko, Levijärvi, and Saattopora. All three record several stages of hydrothermal activities along faults which were re-activated several times during the 1.92-1.8 Ga Svecofennian orogeny. The oldest, 1.91-1.90 Ga monazite occur at Levijärvi and all locations have grouping of ages at 1.87-1.85 Ga. At Saattopora, there is also a prominent monazite population with a 1.82 Ga age. However, both xenotime and monazite from the main gold ore stage are characterized by crystallization ages of 1.78-1.77 Ga. Thus the timing of gold mineralization in these deposits is connected to the late to post-orogenic stages of the Svecofennian tectonic evolution.Electron microprobe analyses of the phosphates reveal that especially monazite and to a lesser extent also xenotime have compositional variations according to age. This suggests that the hydrothermal fluid sources varied over the ~140 Ma time span of repeated hydrothermal activities, in accordance with the metamorphic-tectonic evolution of the region. [ABSTRACT FROM AUTHOR]