Using Magnetic and Electromagnetic Data Processing to Map

Saskatchewan Geological Survey 1 Summary of Investigations 2017, Volume 2

Using Magnetic and Electromagnetic Data Processing to Map Sub-Phanerozoic Basement Features in the Flin Flon Area

Omid Mahmoodi 1 and Ryan Morelli 1 Information from this publication may be used if credit is given. It is recommended that reference to this publication be made in the following form:

Mahmoodi, O. and Morelli, R. (2017): Using magnetic and electromagnetic data processing to map sub-Phanerozoic basement features in the Flin Flon area; in Summary of Investigations 2017, Volume 2, Saskatchewan Geological Survey, Saskatchewan Ministry of the Economy, Miscellaneous Report 2017-4.2, Paper A-7, 15p.

Abstract The Flin Flon area of Saskatchewan and Manitoba hosts a variety of Paleoproterozoic rocks, some of which host world-class volcanogenic massive sulfide (VMS) deposits. Publicly available airborne magnetic and electromagnetic (EM) data were processed to characterize the distribution of physical units (rocks with similar petrophysical properties) and structures in the rocks extending southwest of Flin Flon under Phanerozoic cover, which are potential hosts of VMS deposits. A compilation of processed magnetic data maps, including a total magnetic intensity (TMI) map, a residual magnetic map, a high-pass filtered magnetic map and a tilt derivative map, was used to delineate lineaments that were categorized as faults and other deformational features. A tilt derivative map was used to locate magnetic domain boundaries. These domains were further characterized based on their lineament fabric and magnetic intensity. Deformed linear features formed the dominant internal fabric of some of the magnetic domains. To estimate the depth, dip and dip direction of some of these magnetic sources, two sections along the TMI map were used for forward modelling. The magnetic domain boundaries, together with major lineaments, a depth to magnetic source map, two modelled profiles, and limited drillhole information, were used to create an initial geological model to be used as a starting model for three-dimensional magnetic data inversion. The inversion was undertaken in several steps to modify the magnetic susceptibility of the model. The resultant distribution of magnetic susceptibility gave a more detailed image of the geometry of magnetic sources. Versatile time domain electromagnetic (VTEM) data from the same publicly available survey were also inverted to create a conductivity model. Maps derived from this processing provided supplementary information about deformational features. Using conductivity-depth sections and EM profiles, several distinct conductive targets were selected. The information obtained from magnetic data processing can also be used to evaluate the conductive targets based on their proximity to the magnetic features.

Keywords: sub-Phanerozoic, Flin Flon, volcanogenic massive sulfide, magnetic, VTEM electromagnetic, inversion

1. Introduction The Flin Flon belt is known to be the most productive Paleoproterozoic volcanogenic massive sulfide (VMS) district in the world (Syme and Bailes, 1993). The belt extends westward from Manitoba into Saskatchewan and is covered by Phanerozoic rocks in the south, which limits knowledge of the distribution of basement rocks that host VMS deposits. Previous sub-Phanerozoic geological mapping projects undertaken in the area, were mainly based on aeromagnetic data, low-resolution gravity data, and limited drillhole information (Leclair et al., 1997; Morelli, 2010). The limited number and uneven distribution of drillholes in the map area significantly limits the ability to produce a detailed two-dimensional (2-D) bedrock map or build a three-dimensional (3-D) geological model. However, the available geological information (observed or interpreted), although limited, is sufficient for generation of an initial geological model to be used for geophysical inversion. This initial model can then be modified through inversion to produce a more holistic model that is consistent with the observed geophysical data. Although the resultant model is not a definitive image of the subsurface geology (due to non-uniqueness of the inversion process), it can be modified or updated as additional geological information becomes available (e.g., Williams, 2008; McLean et al., 2009; Spicer et al., 2011).

1 Saskatchewan Ministry of the Economy, Saskatchewan Geological Survey, 1000-2103 11th Avenue, Regina, SK S4P 3Z8 Although the Saskatchewan Ministry of the Economy has exercised all reasonable care in the compilation, interpretation and production of this product, it is not possible to ensure total accuracy, and all persons who rely on the information contained herein do so at their own risk. The Saskatchewan Ministry of the Economy and the Government of Saskatchewan do not accept liability for any errors, omissions or inaccuracies that may be included in, or derived from, this product.


Remote bedrock mapping methods are mainly based on interpreting geophysical data, particularly potential-field data. Interpreted patterns and signal attributes of geophysical anomalies can provide information to characterize physical units (parts of the model with distinct and characteristic petrophysical properties), which can then be translated into geological information such as rock units at depth, structural features, or key exploration parameters for economic mineralization (McLean et al., 2009; Tschirhart et al., 2017). For the latter, which mainly includes parameters for base metal deposits in the Flin Flon area, electromagnetic (EM) data represent important supplementary information, as the anomalous EM responses can be associated with alteration zones, sulfide mineralization, graphitic shear zones, and fault zones. Information obtained from magnetic data interpretation can help to interpret EM data and/or screen targets and prioritize positive targets (Thomas et al., 2000; Ford et al., 2007).

In this work, airborne magnetic data are interpreted to better constrain the distribution of sub-Phanerozoic rocks associated with VMS deposits in the Flin Flon area of Saskatchewan. A detailed 2-D airborne magnetic data interpretation is performed in order to identify magnetic domains based on magnetic intensity and magnetic fabric, and then this information is used to build an initial geological model for magnetic data inversion. Each geological unit is initially assumed to be homogeneous and have a specific magnetic susceptibility value. The inversion is performed to update the magnetic susceptibility of the model so that it is consistent with the observed geophysical data. The versatile time domain electromagnetic (VTEM) data are also processed to locate conductive targets. Finally, there is a discussion of how information obtained by interpreting the magnetic data can be used to screen several selected conductive targets based on EM data inversion.

2. Geological Setting The study area is in the eastern part of Saskatchewan, on the western side of the Paleoproterozoic Flin Flon greenstone belt. The southern part of this belt is covered by gently south-dipping Lower Paleozoic carbonate and clastic rocks of the Western Canada Sedimentary Basin (WCSB) and subordinate overlying recent glacial drift. The cover thickens southward and varies from 10 m to 100 m at the southern boundary of the study area. The Flin Flon belt is within the Trans-Hudson Orogen and consists of tectonostratigraphic assemblages of arc and ocean-floor affinity stitched by calc-alkaline plutons emplaced during successor arc magmatism (Syme et al., 1999). The juvenile arc assemblages contain mafic to felsic volcanic rocks that are the main hosts of the VMS deposits discovered to date (Syme and Bailes, 1993). Juvenile arc assemblages are separated by major faults or intervening ocean-floor rocks (MORB-like basalts), turbidites, and plutons. Figure 1 shows a regional total magnetic intensity (TMI) map and a geological map of the part of the Flin Flon belt that includes the geophysical survey from which the data for this study were taken.

3. Dataset Geotech Ltd. was contracted by Exploration Syndicate, Inc. to conduct an airborne VTEM survey over the McKenzie Lake area in east-central Saskatchewan in 2006 as part of their search for VMS deposits in the sub-Phanerozoic portion of the western Flin Flon belt. Magnetic and electromagnetic (EM) data were collected at an along-line spacing of approximately 2 m, along east-west traverse lines that were separated by 200 m. The nominal terrain clearance was 50 m and 70 m for the EM and magnetic sensors, respectively. Using a 30 Hz repetition rate, secondary EM field measurements were recorded in the transmitter off-time at 26 time windows centred at times ranging from 130 μs to 7450 μs after transmitter current turn-off (Geotech Ltd., 2009). The magnetic and EM data are publicly available through Government of Saskatchewan mineral assessment file 63L02-0030.

Levelling of the magnetic data using tie lines with 4000 m nominal spacing was performed by the ‘Geophysics Levelling’ extension of the Oasis montaj software developed by Geosoft Inc. The levelled data were gridded with a 50 m cell size using the minimum curvature algorithm. A smaller rectangular area (Figure 1B – dashed line) was selected and cropped from the original airborne survey to facilitate a more detailed interpretation. This smaller study area is host to known VMS mineralization, including the Archibald Lake deposit (Saskatchewan Mineral Deposits Index #2741; Morelli and Prokopiuk, 2014). The resulting total magnetic intensity grid of this detailed study area is shown in Figure 2.


Figure 1 – A) Regional total magnetic intensity map. Yellows and reds indicate areas of higher magnetic intensity; blues and greens are areas of lower magnetic intensity. B) Location of the airborne versatile time domain electromagnetic (VTEM) survey (red outline) overlain on the regional geological map of the Flin Flon belt (modified from NATMAP Shield Margin Project Working Group, 1998); southeastern part of surveyed area (dashed rectangle) was used for more detailed study. The black solid line demarcates the northern edge of the Western Canada Sedimentary Basin. A detailed legend for the geological map is described in NATMAP Shield Margin Project Working Group (1998).


Figure 2 – Total magnetic intensity map of southeastern part of VTEM survey area used in detailed study. The two solid black lines indicate profiles Aa and Bb used for forward modelling.


4. Data Processing and Results a) Two-dimensional (2-D) Modelling Lineaments Linear, continuous features observed on a magnetic map are referred to as lineaments, and may represent deformation zones, boundaries between units of contrasting physical properties, or physical property heterogeneities within a unit. If the primary magnetic mineral in a rock is magnetite, deformation zones can be observed on magnetic maps as a) low magnetic susceptibility in situations where reducing fluid (metamorphic or hydrothermal) intrudes into the rock and destroys magnetite; b) high magnetic susceptibility where oxidizing metamorphic fluid intrudes into the rock and results in the formation of magnetite; or c) low magnetic susceptibility as a result of weathering and magnetite destruction in fracture zones (Paananen, 2013).

A series of processing techniques are used to enhance subtle signatures related to local gradients in potential-field images for locating ridges, edges or textural contrasts. Features detected along edges or ridges delineate physical property contrasts that may correspond to a geological contact, or linear structures. Source edge detection (SED) methods provide images that can be used together to map the contacts and texture of magnetic images (Pilkington and Keating, 2009). The Tilt angle (Miller and Singh, 1994), the Theta derivative (Wijins et al., 2005), and the horizontal gradient (TDX) (Cooper and Cowan, 2006) are different normalized magnetic derivatives that are widely used in structural and contact mapping. Their advantage over a potential-field image is that weak, small-amplitude anomalies can be amplified relative to stronger ones. The contact of a magnetized body corresponds to zero values on a Tilt map, and maximum values (equal to 1) on a Theta derivative map. On the other hand, a Tilt map shows positive values over strongly magnetized zones and negative values over weakly magnetized zones. Subtle features within highly magnetic units are strongly enhanced on a Theta map. The TDX changes rapidly over a contact where the zero values are bracketed by positive and negative peaks (Fairhead and Williams, 2006; Fairhead et al., 2007; Paananen, 2013).

To apply SED algorithms, the data were reduced to the pole (RTP) to ensure the anomaly is centred over the causative body (Baranov and Naudy, 1964). To illustrate how SED algorithms enhance linear features, the transparent TDX grid, in colour, has been draped over a greyscale Theta grid (Figure 3). This map and similar maps were used to compile a remote predictive geological map.

Linear features on these SED maps were digitized and are shown in Figure 4. Based on the shape, texture, structural style and continuity of anomalies (Holden et al., 2008; Pilkington and Keating, 2009), they were categorized into deformational features, including minor and major faults (minor faults were those linear features that disappeared in an upward-continued magnetic map, which illustrated they are related to relatively shallow structures). To detect magnetic domains and remove the high frequency internal variations within units, the RTP map was upward-continued to 1000 m (Gunn, 1975). Then, the tilt derivative map of upward-continued data was produced and the tilt-zero contours were plotted (with a minor modification) to delineate the magnetic domains.

Collectively, the domains were characterized based on their fabric and magnetic intensity. Identified magnetic domains are labeled on Figure 4, and their geological and geophysical characteristics are described below. A northeast-trending fault with considerable strike length is labeled as NEF on Figure 4. It is represented by a thick red line to distinguish it from other, minor faults with relatively shorter strike length and limited depth extent. The domain names and geological information in the discussions that follow are mainly from the project previously undertaken in the area by Morelli (2010), though the boundaries of magnetic domains proposed here differ slightly from that study.


Figure 3 – Map of detailed study area showing transparent coloured horizontal gradient (TDX) grid placed over greyscale Theta grid.

Figure 4 – Interpreted magnetic domains, faults and deformational features illustrated on the reduced to the pole (RTP) magnetic map. Distinct magnetic domains and the major faults are labeled by letter codes; these features are described in detail in the text.


Magnetic Domains Magnetic domain MDh corresponds to the relatively magnetic, northwestern portion of Morelli’s (2010) McKenzie geophysical domain and shows a strong internal north-northeast–trending linear magnetic fabric. Minor faults are observed as magnetic lows intercepting linear magnetic highs. From geological mapping (Morelli, 2010), this domain is believed to be dominated by clastic sedimentary rocks that are intensely folded and locally sheared. No drillcore was available at the time of Morelli’s (2010) study, so the linear features with high magnetic susceptibility have an unknown source (Morelli, 2010). Magnetic domain MDe is located immediately northeast of domain MDh and has a similar magnetic intensity and lithological composition; however, the trend of the linear magnetic features differentiates these two units. On the boundary between the two domains, there are minor faults, which might explain this change in the trend of magnetic features. The trend of curvilinear positive anomalies for MDe varies from northwest on the west side to northeast on the east side. As with the MDh domain, the source of the linear features with high magnetic susceptibility in domain MDe is unknown. Magnetic domain WD represents the eastern extent of Morelli’s (2010) low magnetic susceptibility and low-gravity Windy Lake Domain, which was interpreted as comprising felsic intrusive rocks that were possibly derived from highly metamorphosed sedimentary rocks, based solely on its geophysical character (Morelli, 2010). The linear features in domain WD are similarly oriented to those observed in domain MDh but have generally lower magnetic susceptibility.

Magnetic domain MBl corresponds to the eastern portion of Morelli’s (2010) McKenzie Domain (trending from the lower west to the upper centre of Figure 4), and the contiguous southwestern portion of his Burrison Lake Domain (at the upper east). It is bounded to the west by magnetic domain MDh and to the east by a major northeast-trending fault. The McKenzie Domain portion has a low magnetic susceptibility and, based on drillcore, is dominated by upper amphibolite–facies metamorphosed clastic sedimentary rocks (Morelli, 2010). The portion comprised of the Burrison Lake Domain has low to medium magnetic susceptibility and contains upper greenschist to amphibolite facies mafic to intermediate volcanic and plutonic rocks with minor argillite (Morelli, 2010). Interpreted narrow linear magnetic features were originally in a north to north-northwest orientation and then rotated into a northeast trend along the later shear zone that defines the domain’s southeastern margin (Figure 4). In the low magnetic zones, the magnetic features are weak and attenuated, but are still observable, particularly on the SED images (Figure 3). There are two zones with anomalously high magnetic susceptibility within domain MBI (labelled INT on Figure 4). The sources of these anomalies are not clear yet, although they have magnetic signatures similar to intrusive bodies (a high magnetic intensity core surrounded by a low magnetic intensity halo). The trend of linear magnetic features slightly changes within the INT domains, which implies a change in the physical characteristic of this unit compared to surrounding rocks.

Domain CP is characterized by medium to low magnetic intensity, and is interpreted to mostly represent the Cumberland Pluton Domain (Morelli, 2010). Linear magnetic features are frequently disrupted by the presence of interpreted minor faults (Figure 4). Magnetic domain ADm broadly corresponds to Morelli’s (2010) Archibald Domain and is characterized by medium to high magnetic intensity. It consists of abundant greenschist facies mafic to intermediate volcanic and tuffaceous rocks and gabbro. Linear magnetic features in this domain show a dominant northwesterly trend, though they rotate into a northeasterly orientation along the west side of domain ADm to become parallel to the major northeast-trending fault to the north (Figure 4). A distinct, high magnetic susceptibility zone within the ADm domain (labelled as HM) has similarly oriented linear features. Although the source of this higher magnetic susceptibility is yet to be explained, one possibility could perhaps be its larger proportion of mafic rocks compared to MDe, MDh and MBI. In terms of lithology, domains ADm and ADb are classified into one lithological unit by Morelli (2010). The folded nature of the magnetic linear features in ADb appears to extend into ADm, especially on the eastern and northern sides of the fold; however, the higher magnetic intensity of domain ADb differentiates it from domain ADm.

Forward Magnetic Data Modelling Forward modelling results in a major oversimplification of the subsurface geology, but provides fundamental information on the regional dip and dip direction of magnetic sources that form the internal features of magnetic domains. The ‘GM-SYS Solutions’ package was used for 2.5-dimensional forward modelling of the magnetic data. The initial set of magnetic sources was individually located below the anomaly borders detected by tilt-zero contours.


The approximate depth of the magnetic sources was estimated based on a map created by the ‘Source Parameter Imaging’ (SPI) tool in Oasis montaj (Thurston and Smith, 1997). The magnetic susceptibility, shape, and depth of magnetic sources were iteratively modified to obtain the best fit between the observed and computed magnetic data. Remanence information wasn’t available, and, if present, could impact the modelled dip. The locations of two profiles are shown in Figure 2. Forward modelling along profile Aa (Figure 5A) implies that almost all of the magnetic features in the central and west part of the area are steeply inclined to the west. The dip direction of magnetic features within the western limb of domain ADb in profile Aa is different from the steep easterly dip on the eastern limb, where the domain is intersected by profile Bb (Figure 5B). It seems that features within domain ADb could be impacted by a regional fold structure, as their dip direction varies with a rotation in their trend. The major northeast-trending fault (NEF) is situated at the boundary between magnetic domains ADb and MBI (Figure 4), and the forward modelling of profile Aa suggests that it is steeply northwest dipping. The geometry, depth, and magnetic susceptibility of magnetic sources derived from 2-D forward modelling was used to build a starting model for inversion.

Figure 5 – Forward models of magnetic data along profiles Aa (A) and Bb (B) that are shown in Figure 2. On profiles: observed data are displayed by black dots; calculated data are shown by solid black curves; and red lines represent data mis-fit between observed and calculated data. Interpretation of the lithology and structures underlying the area covered by these profiles is given in the text. Abbreviations for magnetic domains, shown below the profiles, are the same as in the text.

b) Three-dimensional (3-D) Inversion of Magnetic Data The ‘VPmg’ package developed by Fullagar Geophysics Pty Ltd and licensed by Mira Geoscience was used for 3-D magnetic data inversion. This software can be run multiple ways, depending on the purpose of the study (e.g., Fullagar et al., 2008). For this study, an initial geological model was built using the interpreted magnetic domain borders (extracted from the tilt-derivative of the upward-continued magnetic data), the geometry of magnetic sources (as indicated in the 2-D forward-model profiles and extended based on zero-contour of the tilt), major interpreted faults, and limited information on the cover thickness (from drillholes). The model, encompassing a volume of 28 km by 24 km by 2 km, was discretized into 100 m by 100 m cells with 50 m height, with each successively deeper cell having a height increased by a factor of 1.02. A 3-D view of the initial model is shown in Figure 6.


Figure 6 – A 3-D view of the initial geological model based on the magnetic data interpretation.

The magnetic data were upward-continued 100 m to remove high-frequency anomalies related to surficial sources. Inversion of the magnetic data was performed in several steps, and the resultant model of each step was used as the reference model for the subsequent step. A series of images shown in Figure 7 illustrates the successive steps of inversion undertaken to fit the data, which are described below.

A) The regional inversion was conducted to remove the regional magnetic field generated by sources beyond the volume of the 3-D model. A regional model extending 44 by 34 by 5 km and surrounding the local model was used for inverting regional magnetic data. The model was discretized into cells with 500 by 500 by 200 m sizes. Once this regional model was determined (Figure 7A), the susceptibility values were fixed. For the following inversion steps, the local model was inserted into the resultant regional model, and became the active model for inversion. A similar procedure is described in more detail by Mahmoodi et al. (2017).

B) Any remaining long-wavelength magnetic field that could not be fit into the model was treated by running an inversion for a heterogeneous unit below the model (called VP basement). The calculated data of the VP basement is shown in Figure 7B.

C) Other than the Phanerozoic and Quaternary cover, all units were subjected to inversion to obtain an estimate of the average magnetic susceptibility of each unit. Each was considered a homogeneous unit at this stage. Figure 7C shows the calculated data of the inversion.

D) All homogeneous units from the last step were converted to heterogeneous units to allow magnetic susceptibility variation vertically and horizontally within each unit. Inversion was performed for deformational features, HM zone, and the two units that were interpreted to be intrusive bodies (INT) for two iterations. These units were selected because they were relatively small and considered to be the main sources of magnetic anomalies. The calculated data is shown in Figure 7D. After two iterations, the other units (other than the cover) were incorporated into the inversion process (Figure 7E).

E) Finally, high-frequency information (previously removed by upward continuation) was added back to the observed data and the inversion was run allowing heterogeneous changes only within the cover in order to fit the data. Figure 7F represents the calculated data of the inversion, which contain magnetic features similar to the observed data (Figure 7A).


Figure 7 – Steps in regional inversion of magnetic data for study area: A) observed magnetic data; B) calculated data from VP basement unit inversion; C) calculated data from homogeneous unit inversion; D) calculated data from heterogeneous deformational features and intrusive unit inversion; E) calculated data for heterogeneous unit inversion other than the cover; and F) calculated data from inversion of all heterogeneous units.


Calculated data from the VP basement inversion (Figure 7B) reveals the existence of deep features such as the major fault represented by the low magnetic corridor running from the bottom left to top center, and several highly magnetic zones, which were interpreted as intrusive bodies.

Figure 8A shows a 3-D view of the inversion model. Because a geological model was used to constrain the inversion, detailed structures and geological features such as linear magnetic sources are preserved in the magnetic susceptibility model; otherwise, the result would be a smooth, structureless model. The linear magnetic anomalies are modelled as features extending to depth, where appropriate. The tops of linear magnetic sources were initially positioned at a constant depth below the surface in the starting model; a side view from the centre of the model (Figure 8B) shows how the depth to top of these magnetic features was modified to match the observed data. Figure 8C represents a 3-D view of the iso-surface encompassing magnetic susceptibly values greater than 0.016 SI, which shows how the simple geometry of magnetic sources has been modified through inversion. The surfaces are coloured based on the elevation for visualizing the differences in the tops of the magnetic sources. Shallow sources are red at the top; deeper sources are in shades of blue. At the centre of the model (indicated by a dashed-line rectangle), which corresponds to the low magnetic zone that contains subtle linear features on the magnetic map, the magnetic sources (indicated by the iso-surface) are modelled at relatively greater depth.

Figure 8 – A) A 3-D view of magnetic susceptibility distribution resulting from inversion, with two sections and one horizontal slice shown for the same area as Figure 6. B) A side view from the magnetic susceptibility model. C) A 3-D view of the iso-surface encompassing magnetic susceptibly values greater than 0.016 SI.


c) Inversion of Electromagnetic (EM) Data Electromagnetic data can be a valuable tool to supplement the magnetic data for lineament detection. The location of the major fault apparent in the magnetic data (Figure 4) is also clear on the map of the 17th VTEM time window (the dark red line on Figure 9), where there is an abrupt truncation in many features in the measured secondary EM field. The minor faults extracted from the magnetic data are indicated by thin black lines on Figure 9. There is little evidence for some of these features on the map of the 17th VTEM time window. Several deformational features with varying strike length are detected by the VTEM system and are shown in Figure 9. This clearly shows that the VTEM data can be used to confirm the location of lineaments extracted from the magnetic data.

Figure 9 – Map of the secondary field (dB/dt) recorded by the VTEM system in the 17th time window.

In addition to lineament interpretation, it is widely known that EM data are useful for the direct detection of conductive VMS mineralization (Ford et al., 2007). The VTEM data from this study area were inverted using the ‘VPem1D’ package to locate discrete conductive targets, which may be typical of VMS targets in the area. Unlike the deformational features with considerable strike lengths, discrete conductors are those anomalies that have limited size and strike lengths (i.e., anomalies are observed on a limited number of survey lines) consistent with the sizes of VMS targets. An initial model of 1 millisiemens per metre (mS/m) was used for an unconstrained inversion. The model was discretized into vertical prisms of 40 m by 200 m dimensions, extending to 1300 m depth. The prisms were vertically subdivided into 10 m cells. The first four time windows of VTEM data were excluded from the inversion to put more emphasis on conductive zones at depth. Several conductivity sections resulting from the inversion are shown in Figure 10. EM profiles and conductivity sections were used to detect several discrete targets (Figure 11).


Figure 10 – A 3-D view of the conductivity-depth sections derived from inversion of VTEM data for the study area.

Figure 11 – The locations of identified conductive targets, possibly reflecting VMS-style mineralization, based on VTEM data overlain on the RTP magnetic map and interpreted linear features from the magnetic data.


Identified conductive targets are plotted on the 2-D magnetic map with the interpreted linear features (Figure 11). On this image, the conductive targets can be screened to some degree, based on their magnetic intensity and spatial proximity to the linear magnetic features. VMS deposits, if not altered or weathered, typically have high magnetic susceptibility, which can produce a positive magnetic anomaly mainly due to the common presence of pyrrhotite (3200 x 10-3 SI) and pyrite (5 x 10-3 SI). However, where they are located close to faults and conduits for hydrothermal fluids, they can be altered, which consequently weakens the magnetic anomaly. Linear features (minor and major faults) are likely related to structures that might be suitable conduits for fluids. However, the proximity of anomalies to deformational features is not always considered a positive factor for VMS targeting, as the conductive zone could be related to a graphitic shear zone rather than a distinct sulfide mineralization zone.

5. Conclusions The data used in this work were public geoscience data (i.e., from assessment files) provided by the Saskatchewan Ministry of the Economy. The processing of such data is a valuable resource for aiding mineral exploration. The processing of airborne magnetic data has resulted in a detailed interpretation of lineaments and structures hidden in the Precambrian rocks of the Flin Flon belt under the cover of flat-lying Phanerozoic rocks. Magnetic data along two profiles were forwardly modelled to estimate the geometry of the deformation-related linear magnetic features. Information obtained from source edge detection methods and 2-D forward modelling was used to build an initial geological model. Inversion of the magnetic data, which was constrained by this geological model, resulted in a 3-D magnetic susceptibility model that provides a better understanding of the distribution of magnetic sources at depth. It was shown that the major northeast-southwest fault in the area is a deep-seated structure. The geometry and magnetic susceptibility of the deformational linear magnetic features were modified through inversion. The model can be updated as more information becomes available, and eventually be classified to provide a predictive 3-D geological model. Electromagnetic data complements the magnetic data in helping to detect lineaments and structures. The EM data can also be used for direct detection of possible VMS deposits, and the information extracted from the magnetic data is complementary to the EM data, providing additional information to evaluate potential exploration targets.

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http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/45/087/45087752.pdf?r=1

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