The Mauken gold project, Northern Norway

Transcription

1 The Mauken gold project, Northern Norway Newly discovered gold deposit, situated in a Paleoproterozoic greenstone belt By Mikkel Vognsen Scandinavian Highlands

2 Introduction The Mauken greenstone belt project is situated in an unexplored and untested terrain in Northern Norway. The investigated area is a Paleoproterozoic basement window exposed in the allochtonous Caledonides. The main economic mineral potential in the Mauken greenstone belt is associated with orogenic and epithermal gold mineralisation. The deposit consists of three types of auriferous mineralisations: I) silicification of mafic tuffites with gold-bearing arsenopyrite-pyrite mineralisation. II) gold-bearing arsenopyrite-pyrite en echelon quartz vein and quartz-stockwork mineralised system. III) shear-hosted gold-bearing chalcopyrite-pyrite mineralisation in a quartz-ankerite alteration system. The geology and metallogeny of the greenstone belt is similar to other gold deposits and parallels can be drawn to deposits in Proterozoic rocks in the Svecofennian Province and in the Canadian Shield. The gold mineralisations in the Mauken greenstone belt was discovered late autumn 2008 and was further investigated during the 2009 field season. The target is currently only sampled on surface. There is no previously systematic geochemical sampling or drilling in the licence area. Exploration drilling is expected to take place during

3 Table of contents Introduction...1 Tenure...3 Physiography and climate...3 Regional geological setting...3 Geology...6 Mineralisations...7 Alteration...8 Grades...10 Geophysical surveys...11 Airborne TEM / MAG survey...11 Ground-based MAG survey...13 Potential based on the geophysical data...15 Geological parallels...17 Economic potential...17 References

4 Tenure Northern Highlands ApS, a wholly owned subsidiary of Scandinavian Highlands Holding A/S, presently holds a 57.3 km 2 exploration licence area, covering the majority of the Mauken greenstone belt. Physiography and climate The basement window is defined as an elongated 5 x 40 km NW SE orientated fell ridge. The majority of the greenstone belt is situated above the timber line (approximately 500 masl). The morphology of the belt is dominated by steeply dipping rocks forming a plane eroded mountain ridge, flanked on either side by rivers. The licence area is easy accessible using Euro-route E6, national road 854, and/or 87 which run through and along side the area. The nearest international airport is located in Bardufos (Bardufos National Airport) just five kilometres south of the licence area. Above the timber line the exposure is close to 100 %. Bedrock is only in few places covered by 1 2 m overburden. Exposure is generally poor below timber line due to vegetation. Located 280 km north of the artic circle the licence area has midnight sun from mid May to mid July. The sun is below the horizon from late November to mid January. Field season duration is from May to October. Regional geological setting The Mauken greenstone belt is part of the West Troms Basement Complex (WTBC), which is composed of Neoarchaean to Palaeoproterozoic tonalitic gneisses, igneous and metasupracrustal rocks. Structurally the greenstone belt is associated with two crustal scale deformation zones (Fig. 1): I) the NW SE-trending Bothnian-Senja fault zone (BS), which again is part of the larger NNW SSE Mesozoic De Geer Zone, a reactivated off-shore mega-shear system. The south-eastern extension of the BS fault zone is connected to the Archeaen-Proterozoic terrain boundary termed the Raahe-Lado zone. II) the NNW SSE-trending Malangen-Onege lineament, which can be followed from Northern Norway to the Onega Lake in western Russia. A trans-scandinavian fracture zone running in virtually a straight line, having a gravity anomaly associated with it. The Mauken Greenstone belt is situated between two major fault structures (Fig. 1). The strike of the belt is parallel with the overall NW-SE orientation of the Precambrian terranes in northern Scandinavia. The belt lies in the western extension of gold-bearing shear zones in central Lapland, Finland, which host several large gold deposits, e.g. along the Sirkka 3

5 deformation zone. The significance of structures as factors controlling the occurrences of gold is well demonstrated in the Canadian Shield and Central Lapland, where practically all gold deposits are connected with deformation zones and their conjugate shear and fault zones. The bedrock situated between these prominent structures and crustal boundaries is dominated by reworked Archeaen rocks. From the ore prospecting point of view the association with terrain boundaries, continental and regional deformation, fault and shear zones conforms well to the above description and places the Mauken greenstone belt in a very favourable position in terms of gold mineralisations. 4

6 Figure 1. Simplified geological map of the Fennoscandian shield with continetal and regional structures. Selected major Scandianvian ore deposits are shown. Inset map: Map of northen Norway with location of the De Geer and Senja fault zones. KNDZ Kiruna-Naimakka deformation zone, KISZ Kiistala Shear Zone, NDZ Nautanen deformation zone. The map is modified from Koistinen et al. (2001), with structural data from Touminen et al. (1973); Edfelt, (2007). 5

7 Geology The greenstone belt is dominated by basalts, amphibolites, chlorite- and sericite schists with lesser carbonate lenses and metaarkosic sandstone. Silicification (i.e. addition of silica), carbonatisation and quartz veining are the dominating alteration types. The greenstone belt is intruded by Paleoproterozoic Ga granodiorites (Fig. 3). Metamorphic grades from greenschist facies to lower amphibolite facies. The internal geometry of the Mauken greenstone belt is characterised by two main rock units: an underlying mafic basalt (pillowed and massive lava) unit and a mafic to intermediate banded and laminated amphibolite unit (hanging wall). The stratigraphic contact between the two units, is well imaged by ground magnetic geophysical data, and has been traced along a strike of over 6 km. The contact zone is highly altered by carbonate- quartz veining and extensive carbonate brecciation. The stratigraphic contact has acted as a permeable zone, allowing migration of mineralised hydrothermal fluids. In addition, a series of conjugate shear and fault zones are passing through the stratigraphic contact at a low angle. The contact zone represents the principal metallogenic target on the Mauken property. Figure 2. Simplified structural map of the Mauken greenstone belt district showing the main geological structures and lithological units. 6

8 Figure 3. Simplified geological map the Mauken greenstone belt showing the main geological features and the location of the gold mineralised stratigraphic contact. Mineralisations The gold mineralisation in the Mauken greenstone belt is hosted by mafic metavolcanic rocks affected by extensive alteration (Fig. 3). The alteration assemblage has unique characteristics, controlled by the host rocks and is associated with silicification, Fecarbonatisation and the introduction of finely disseminated arsenopyrite, pyrite and chalcopyrite. The most promising gold discovery is an Au-As mineralised silicic horizon, referred to as Main Zone. Main Zone mineralisation is located along the stratigraphical contact between the underlying basalt unit and the hanging wall amphibolite unit. It can be followed for more than 1750 m along a gently warping strike and varies in thickness from a few centimeters up to 5 m. On some localities the Au-As mineralisation splays out in several sub meter wide parallel mineralised horizons. Gold is refractory, primarily within arsenopyrite and lesser 7

9 pyrite, which has a disseminated appearance (Fig. 4). Main Zone mineralisation is syn- to late deformational. A second gold mineralisation type is located within the hanging wall amphibolite unit, stratigraphic up from Main Zone. It is a slightly discordant en echelon quartz vein system (dilatant) with auriferous arsenopyrite-pyrite mineralisation. The quartz veins measure up to 10 m, mostly less than 1 m wide. A second system of Au-mineralised stockwork quartz vein is associated with the dilational system. The dilational zones are all located within competent silicified amphibolite sequences. The third gold mineralisation type is a shear-hosted quartz-ankerite vein system. The host rock is a flat laying sericite schist with quartz-ankerite vein and veinlets, both folded and discordant. The remobalised low grade Au mineralisation is presumably syn-genetic with the Caledonian shearing of the chlorite-sericite schist. Chalcopyrite is the most common sulphide, with minor pyrrhotite and pyrite. Near the margin of the intrusive granodiorite, another significant mineralisation type is located. It is a monolithic clast-supported sulphide-oxide breccia. The breccia matrix is dominated (> 95%) magnetite with lesser amounts of pyrite and pyrrhotite. The breccia width ranges between 1 to 2 m were it is most well developed. In the adjacent granodiorite rock, only weak brecciation has taken place, but can be followed up to 30 m away. Flouspatt-rich felsic pegmatites are associated with the breccia zone. The sulphide-oxide breccia is a late hydrothermal event and emphasizes the intrusive origin of the granodiorites. The breccia is hosted entirely within the granodiorite. Alteration The proximal alteration halo enveloping the Main Zone mineralisation is characterised by chloritisation, carbonatisation and lesser seritisation. Channel samples across the Main zone mineralisation show enrichments in CO 2, K 2 O and S, and leaching of Na 2 O in the silicified host amphibolite rock. Distal alteration is characterised by chloritisation and notably a sulphidisation of smaller (tens of meters) sequences of the adjacent carbonate brecciated pillow basalt. The carbonate alteration occurring along the stratigraphic contact between the basalt and amphibolite unit is traceable for several kilometres along strike and can also be followed into both rock units up to hundreds of metres. 8

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11 i Figure 4. a) Gold mineralised arsenopyrite-rich silicified rock with secondary quartz veining. b) Close-up of gold mineralised arsenopyrite (silvery needles) rich silicified rock. c) Ankerite-calcite vein with late quartz fracture fillings in sericite schist. d) Ankerite-calcite veined sericite schist with low grade auriferous chalcopyrite mineralisation. e) Flat lying chlorite-sericite schist, shear zone rock f) Fecarbonate and magnetite alteration in breccia form in pillow basalt unit g) Granodiorite hosted magnetite breccia. h) Flourite-bearing feldspar-quartz pegmatite intruding the magnetite filled breccia zone. i) Profile photo of Fe-carbonate-magnetite alteration breccia zone in pillow basalt. Grades Mineralised surface rock samples have been collected along a 1750 m almost straight line. The typical Au content of the samples, range between 1 4 ppm. Presently the maximum Au grade is 5.79 ppm in a rock grab from an arsenopyrite rich silicified amphibolite. Gold is refractory, mainly occurring as invisible gold in arsenopyrite and pyrite (Fig. 5). The dilatant zone with the en echelon quartz vein and quartz stockwork systems show a remobilised arsenopyrite and a lower gold grade compared to Main Zone, typical Au contant range between ppm. The shear zone hosted chlorite-sericite schist with the ankerite-calcite alteration system show a low-grade Au mineralisation, but may prove to be significant in size. a b Figure 5. a) Back scattered electron image of euhedral poikiloblastic arsenopyrite nuclei, rimmed by Aubarren Ti-Ru-bearing arsenopyrite. Inclusions: native gold, pyrite, titanite with rutile inclusion and quartz. b) Euhedral poikiloblastic pyrite with native Au inclusion. 10

12 Geophysical surveys Airborne TEM / MAG survey A total of 75.5 line km was flown with a spacing of 750 m covering ca 47 km2. The survey was designed so 12 NNE SSW trending lines were tied together by two NNW SSE tie lines. The survey was conducted in the early days of our exploration activities and the 750 m spacing was knowingly far apart, but the survey was only indented to be a rough screening of the prospect area. Two areas with anomalous TEM highs were revealed in the survey (Fig. 6) and the surface areas were subsequently investigated. The northern elongated NW SE trending anomaly (Fig. 6, Anomaly 1) is attributed to the Caledonian thrust zone. Graphite schists are known to outcrop in that area and currently no economic potential is believed to be associated with this anomaly or structure. The second anomaly (Fig. 6, Anomaly 2) has been focus for much attention. The area is dominated by low grade Proterozoic greenschists, metabasalts and lesser carbonates. The conducting material giving the high TEM anomaly is yet unexplained, but within the anomaly area several quartz vein systems with Fe-sulphides has been located. These are not the course for the EM anomaly, but indicative of sulphide mineralisation. Stream sediment samples in the area also indicate an increased copper content. Potential for a VMS type mineralisation is present. 11

13 Figure 6. SkyTEM survey resitivity chart ( m). Coordinate system UTM (WGS84) Zone 34N. Two pronounced high anomaly areas are revealed. The collected magnetic data from the airborne survey show a prominent high magnetic anomaly area (Fig. 7). The magnetic data can to some extent be correlated with the EM data (i.e. Anomaly 1) from the SkyTem survey. The TEM / MAG anomaly is attributed to the Caledonian thrust zone. Another interesting anomaly zone is where the MAG and SkyTEM signals coinciding in the south eastern corner of the survey area. This anomaly indicates highly conductive mafic basement rocks. The MAG anomaly ( nt) is not as clearly delineated and seems to extend beyond the survey area. 12

14 Figure 7. MAG data measured during SkyTEM survey. Total magnetic field [nt]. Red/pink shows highest magnetic respons. Coordinate system UTM(WGS84) Zone 34N. Ground-based MAG survey Using ground based magnetic equipment (Geometrics G-858 Cesium Vapor and G-856 Proton magnetometer), a 4 km 2 large area has been covered by foot giving excellent resolution (Fig. 8). The survey was designed with a 50 m line spacing with fill-in 25 m perpendicular to the strike of the known Au mineralisation. The differences in the magnetic characteristics of the greenstone belt rocks are ideal for mapping. The survey revealed that the gold mineralisation at the Mauken property is structurally controlled and associated with extensive alteration in the contact zone between the two major lithological units i.e. the basalt and the amphibolite unit. Regional geomagnetic field removal was based on the International Geomagnetic Reference Field (IGRF). 13

15 The magnetic survey has amplitudes ranging between hundreds of nanotesla (nt). In general, high magnetic values correspond to topographic elevated areas and the low magnetic values correspond to topographic depressions. The low magnetic zone ( nt) appears to be coincident with the basalt-amphibolite contact. The intermediate to low magnetic area is constituted by altered rocks situated along the contact zone. The topographic elevated area on the south western side of the low magnetic area is constituted of homogeneous and pillowed basalts ( nt), and the elevated area on the north eastern side is constituted chiefly of banded and laminated amphibolites ( nt) (Fig. 8). Figure 8. Ground magnetic survey. North up. The different lithologies are without difficulty distinguished. Red/dark brown shows highest magnetic response indicating mafic volcanics. The gold mineralisation is situated within this low magnetic zone along an almost 1750 m linear strike. In the south eastern part of the MAG survey the mineralisation is associated with smaller isolated intermediate magnetic areas ( ) (Fig. 9). The principal target zone, oriented NNW SSE, extends for at least 5 km across the property and is open in both ends. 14

16 Figure 9. Topographic map with the ground based MAG survey. There is a clear coherence between topographic troughs and low magnetic characteristics. Graded in-situ rock samples show the long strike of the gold mineralisation. Potential based on the geophysical data The detailed ground based magnetic survey can be linked with the lesser detailed aeromagnetic survey data. And the EM anomalies revealed by the SkyTEM survey can to some extent also be explained. Interpretations based on geophysical data must naturally be followed up with field work investigations. A comparison of the magnetic data sets suggests a continuation of the low-magnetic alteration zone. In north western direction the low-magnetic contact zone believed to continue for at least 5 km. The south eastern direction the zone is disappearing underneath the shallow dipping Caldonian cover, reappearing in the south eastern apex of the belt (Fig. 10). 15

17 Figure 10. Topographic map with airborne magnetic data in the southeastern part of the area and ground based magnetic survey data covering a smaller area to the north east. Target zones for exploration are indicated with red circles. 16

18 Geological parallels Parallels can be drawn from the newly discovery of the gold deposit in Mauken greenstone belt to large and well investigated gold deposits in Scandinavia and Canada. The potential for an even larger mineralisation system than what is already found is very much present in the Mauken greenstone belt. Mauken greenstone belts structural association with the crustal and regional scale deformation, fault- and shear zones are significant and essential. Several other gold deposits are distributed along the trans-scandinavian structures. Furthermore is the presence of other deposit types in the Mauken district also a favorable factor. Geological analogous can be drawn to Finland where the Central Lapland Greenstone Belt is hosting the Kittilä greenstone belt with its many gold mineralisations. The prominent Suurikuusikko gold deposit (Agnico-Eagle Mines Limited, 2009) is in many ways comparable. The deposit is characterised by a conjunctive shear zone cross cutting a lithological contact and with an alteration assemblages similar to Mauken greenstone belt. The Suurikkusikko ore zone is also situated between a stratigraphic contact and the facies gradient are similar. The potential of Mauken greenstone belt is further enhanced when compared to Canadian gold deposits. The geology and alteration types allows for a comparison with several gold deposits e.g., Madsen Mine and Campbell Mine in the Red Lake Mining District, Canada (Zhang et al. 1997; Dubé et al. 2000). Economic potential The gold mineralisation in Mauken greenstone belt is closely associated with the stratigraphic contact zone. The results of stratigraphic-structural mapping further show that the contact zone is spatially associated with silicification, carbonatisation and quartz veining. The stratigraphic contact has been tracked for > 6 km, and is open in both ends, extending the prospective for further Au mineralisation. The economic potential of the newly delineated shear zone is highlighted by the identification of several auriferous chalcopyrite mineralised ankerite-quartz alteration zones with intensities and assemblages similar to greenstone belt hosted quartz-carbonate vein deposits in Canada (Dubé et al. 2007). The main economic mineral potential in the Mauken greenstone belt is for orogenic gold and epithermal gold mineralisations. As described above, the association with large scale structures and Mauken greenstone belts location and alignment with these is very favourable. The geology i.e. rock types, alteration types, metamorphic facies and mineralisation is directly comparable to well known gold deposits in greenstone terrains world wide and is emphasizing the significance of the Mauken gold project. 17

19 References Edfelt, Å. 2007: The Tjårrojåkka Apatite-Iron and Cu (-Au) Deposits, Northern Sweden, Products of One Ore Forming Event. Doctoral Thesis. Luleå University of Technology, Luleå, Sweden. Dubé, B., Balmer, W., Sanborn-Barrie, M., Skulski, T., Parker, J. 2000: A preliminary report on amphibolite-facies, disseminated-replacement-style mineralization at the Madsen gold mine, Red Lake, Ontario, Geological Survey of Canada. Current Research C17. Dubé, B., Gosselin, P. 2007: Greenstone-hosted quartz-carbonate vein deposits, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit- Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods. Geological Association of Canada, Mineral Deposits Division, Special Publication. No 5, Tuominen, H. V., Aarnisalo, J., Söderholm, B. 1973: Tectonic patterns in the central Baltic Shield. Bulletin of the Geological Society of Finland. 45, Weihed, P., Arndt, N., Billström, K., Duchesne, J-C., Wilu, P., Martinsson, O., Papunen, H., Lahtinen, R. 2005: Precambrian geodynamics and ore formation: The Fennoscandian Shield. Ore Geology Reviews, 27, Zhang, G., Hattori, K., Cruden, A.R. 1997: Structural evolution of auriferous deformation zones at the Campbell mine, Red Lake greenstone belt, Superior Province of Canada; Precambrian Research. 84,