The Vargbäcken orogenic gold deposit, Skellefte district, northern Sweden: mineralization style, alteration and setting of gold

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(1)2007:084 PB. MASTE R’S THESIS The Vargbäcken Orogenic Gold Deposit, Skellefte District, Northern Sweden: Mineralization Style, Alteration & Setting of Gold. Promise Nkwachukwu Alioha. M.Sc. in Exploration and Environmental Geosciences CONTINUATION COURSES Luleå University of Technology Department of Chemical Engineering and Geosciences Division of Ore Geology Universitetstryckeriet, Luleå. 2007:084 PB • ISSN: 1653 - 0187 • ISRN: LTU - PB - EX - - 07/084 - - SE.

(2) The Vargbäcken Orogenic Gold Deposit, Skellefte District, Northern Sweden: Mineralization Style, Alteration & Setting of Gold. Alioha, Promise N.

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(4) Abstract The Vargbäcken orogenic gold deposit is located in the Palaeoproterozoic part of the Fennoscandian Shield within the western part of the Skellefte mining district, northern Sweden. The Skellefte district is a product of a ca. 1.90 Ga Palaeoproterozoic volcanic arc with numerous massive sulphide deposits and orogenic gold deposits of which Vargbäcken is one with an indicated resource of ca. 1.2 Mt with 1.44 ppm Au. Mineralized veins exhibit several structural styles, ranging from breccias to vein stockworks predominantly within competent host rocks close to the metasediment-diorite contact, and laminated quartz veins in brittle-ductile shear zones. The quartz veins are of two distinctive generations, the milky and opaque fine to medium grained quartz veins with a high amount of visible, coarse grained gold and the smoky dark translucent coarse grained quartz veins with disseminated sulphides and low amount of visible gold. The ore zone is situated on the metagreywackes-diorite contact, and the silicified diorite close to the contact. The best mineralized areas include zones where changes in strike and dip of the contact occur, and structural deformation zones along the contact, or within the diorite. Only sphalerite and rarely pyrrhotite is visibly correlated with the gold which is found in small cracks or interstices at grain boundaries between quartz grains. Gold is also seen as free grains in the silicate matrix of the host diorite. The vein related alteration is considered to be spatially restricted and most quartz veinlets have thin or no alteration envelopes. Alteration mineral assemblage suggests a greenschist metamorphic facies environment, indicating mesozonal conditions during mineralization. The metamorphism was overprinted by the hydrothermal alteration in Vargbäcken, thus the possible age of at least the last stages of mineralization at Vargbäcken is suggested to be ca. 1.80 Ga..

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(6) Table of contents 1. Introduction .............................................................................................................................1 1.1 Brief introduction to orogenic gold deposits ....................................................................2 1.2 Methods.............................................................................................................................3 2. Regional geology ....................................................................................................................3 2.1 Overview of orogenic gold deposits in Skellefte district, the Vindel-Gransele area and the Gold line ............................................................................9 3. Geology of the Vargbäcken deposit ......................................................................................12 4. Mineralization style ..............................................................................................................12 4.1 Vein timing relationships ................................................................................................18 4.2 Structural interpretation from geophysical data ..............................................................18 5. Alteration...............................................................................................................................19 6. Setting of gold .......................................................................................................................20 6.1 Structural microanalysis..................................................................................................22 7. Summary and conclusions ....................................................................................................23 Acknowledgements ...................................................................................................................24 References .................................................................................................................................25 Appendix A................................................................................................................................31.

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(8) Panteleyev, 2004). Vargbäcken is a discovery made by North Atlantic Natural Resources (NAN) prospectors (now Lundin Mining) in September 1997 at a locality with no previous mining history. Visible gold was recognized in rusty rock fragments from bedrock exposed in a freshly excavated ditch alongside a logging road (Panteleyev, 2004). A total of 66 drill holes (31 DD and 35 RC) have been drilled as part of the exploration in Vargbäcken. The Vargbäcken area is characterized by spars outcrops, and thin till cover usually 3-5 m in thickness. Recent exploration has been limited to the outcrop areas, providing excellent potential for the discovery of near surface but presently blind deposits. The use of immobile element geochemistry has generated good results in areas of thin till cover. This recent technology is easy and economical, allowing exploration focus to be quickly gained when applied in combination with structural interpretations of magnetic data (Panteleyev, 2004). Sequel to the increase in the gold price prior to the 1980s, and a renewed interest in gold, there has been an unprecedented wave of new research on orogenic gold deposit also within the Fennoscandian shield (Bark and Weihed, 2007). The understanding of structures, as well as host-rock rheology, composition, and distribution, are clearly realized to be key factors for development of an exploration strategy within Precambrian and Phanerozoic terranes. Orogenic gold deposits are characterised by quartz-dominant vein systems occurring in orogenic belts formed during terrane collision and cratonization (Groves et al., 1998). The Vargbäcken gold deposit which occurs in a quartz-dominant vein system is considered to be one of many orogenic gold deposits in the Skellefte district.. 1. Introduction The raison d’être of this work is to present the geology, mineralization style, alteration mineral assemblages and the structural settings of the Vargbäcken orogenic gold deposit. The indicated resources stand at approximately 1.2 Mt with 1.44 ppm Au (Mawson Resources AB, technical report 2006). It is essential to highlight significant structural controls of the mineralization, define an appropriate gold mineralization model and provide some useful guidelines for local mineral exploration. This thesis work is based on assessment reports and unpublished exploration reports by Mawson Resources AB and Blomquist and Leijd (1999) coupled with field observations, drillcore logging, microscopy and SEM analysis. The mineralization style, alteration, and setting of gold are described through petrographic and SEM studies. Albeit future work may lead to the revision of some preliminary opinions expressed herein. Gold deposits have been known in the Skellefte district since the last century, but only from the past two decades has the Skellefte district become a significant lode gold producing region as well as volcanic massive sulphide mining district (Weihed and Mäki, 1997). Most of the previous publications were on arsenopyrite dominated quartz veins within metasedimentary rocks. Vargbäcken is one of a group of gold deposits discovered during the past 20 years in the Skellefte polymetallic massive sulphide district that are now recognized as orogenic gold deposits, an important style of gold mineralization in their own right (Panteleyev, 2004). Björkdal and Åkerberg were discovered in 1985 and 1988 and put into production in 1988 and 1989, respectively. Svartliden, a 1994 prospecting discovery about 50 kilometres to the southwest of Vargäcken, has commenced production in 2004 (Weihed and Mäki 1997; 1.

(9) 1.1 Brief introduction to orogenic less than 10 km thick during rapid burial in a convergent orogeny (Beaudoin and Therrien, gold deposits 2005). Burial of a pile of mixed volcanosedimentary rocks will generate a short-lived pulse of metamorphic fluid that moves upwards along zones of transient permeability due to rapid burial during plate convergence (Goldfarb et al., 1991; Miller et al., 1994). This produces ‘Connolly’s fluid wave’ (Connolly, 1997) that transports gold to the area of precipitation that subsequently forms the gold mineralization. Earthquakes along crustal scale shear zones cause dilation near jogs that draw fluids and deposits gold in an interconnected network of shear zones. Fluids are trapped by an impermeable barrier separating the hydrostatic and lithostatic fluid regimes, (Beaudoin and Therrien, 2006), (Figure 1.). Orogenic or mesothermal gold deposits are a unique class of mineral deposit, which are the source for much of the world gold production. Orogenic gold deposits are found in metamorphic belts throughout the world (Goldfarb et al., 2001; Pitcairn et al., 2006). They are typically located adjacent to crustal-scale shear zones which themselves are completely lacking significant mineralization. The crustal scale shear zones draws gold-bearing hydrothermal fluids which are then channelled to the network of smaller shear zones that host mineralization (Goldfarb et al., 2001; Groves, 1993; Groves et al., 2003). It is estimated that approximately 1000 km3 of hydrothermal fluid could be released from devolatilization of 1000 km2 rock column. Fig. 1. Idealized composite depositional model for Archean lode gold deposits (Colvine et al., 1988).. 2.

(10) Gold deposits are usually located in second- or third-order structures, creating an environment suitable for the precipitation of gold from hydrothermal fluids close to the crustal-scale shear zones, in brittle, brittle–ductile, and ductile deformational environments (Eilu and Groves, 2001). Host rocks are commonly regionally metamorphosed in greenschist through loweramphibolite facies. Essentially, the ores develop synkinematically, with at least one stage of penetrative deformation of the country rock. The deposits inevitably have a strong structural control involving faults, shear zones, folds and/ or zones of competency contrast. Consistent geological characteristics include deformed and variably metamorphosed host rocks; low sulphide volume; carbonate–sulfide”sericite”-chlorite alteration assemblages in greenschist-facies host rocks; low salinity, CO2-rich ore fluids with δ18O values of <10‰; and usually a spatial association with large-scale compressional to transpressional structures (e.g., Colvine et al., 1984; Hodgson, 1993; Robert, 1996). Orogenic gold deposits commonly consist of abundant quartz veins rich in carbonates and show evidence for fluid formation during supralithostatic pressures (Goldfarb et al., 2001). The mineralized lodes are developed over a uniquely broad range of upper to mid-crustal pressures and temperatures, between about 200–650ºC and 1–5 kbar (Groves, 1993; Goldfard et al., 2001). Despite this high variability, the alteration and geochemical features of these deposits are strongly consistent (Eilu and Groves, 2001) (Table. 1).. of thin sections for optical and SEM studies. The outcrop in the Vargbäcken study area was mapped. This essentially covers the exposed metasediment-diorite contact enhanced by trenching. A total of 4 drill cores (3 out of 4 drill cores in a profile) were logged. The drill cores have a diameter of 42 mm were mostly half the core had been assayed for gold. Samples ca. 5-10 cm long were collected for thin sections. All the sampling was not confined to goldbearing rocks, but included various areas of characteristic alteration and quartz veins. From the drill core samples, 29 polished thin sections and 4 ordinary thin sections were prepared by Vancouver Petrographics Ltd, Canada and all the polished thin sections (including the 4 ordinary thin sections) were studied optically. Scanning electron microscopy (SEM) was carried out at Luleå University of Technology on 4 polished thin sections. The field study visits in essence took a total of twentysix (26) days.. 2. Regional geology. The Vargbäcken gold project is located in the Palaeoproterozoic part of the Fennoscandian Shield within the western part of the Skellefte mining district, Northern Sweden (Fig. 2). The Fennoscandian Shield constitutes the western part of the East European craton and covers an area from western Russia to Norway. It is bordered to the west by the Caledonian orogenic belt. To the east and southeast the Precambrian rocks are covered by Phanerozoic sequences (Bark and Weihed, 2007). The Svecokarelian 1.9-1.65 Ga domain makes up the central part of the Fennoscandian Shield (Gee and Zeyen, 1996). 2.8 to 2.7 Ga orthogneisses of tonalitic to granodioritic composition dominate the 1.2 Methods Archaean rocks of the north-easternmost The methods used in this thesis work include Fennoscandian Shield (Öhlander et al., 1993), detailed outcrop mapping of the study area, generally not of the tonalite–trondhjemite– logging of the drill core from the diamond drill granodiorite (TTG) series, but may have been holes in a profile, using ArcGIS/MapInfo to view derived from TTG series rocks by partial melting the spatial distribution of structures, preparation 3.

(11) Table 1 Summary of Orogenic Gold deposits and its characteristics compiled from Ash and Alldrick (1996), Groves et al., (1998), McCuaig and Kerrich (1998). Main Charateristics. Orogenic gold deposits. Age of mineralization. Post-peak metamorphism (i.e. late syncollisional) with gold-quartz veins abundant in the Late Archean and Mesozoic.. Tectonic setting. Geological setting. Moderate to gently dipping fault/sutured zones related to continental margin collisional tectonism and transcrustal structural breaks within stable cratonic terranes. Veins form within fault and joint systems produced by regional compression or transpression, second and third-order splays, and within brittle-ductile transition zone.. Structural setting. Intense carbonate alteration along second order or later faults, late syncollisional, structurally controlled intermediate to felsic magmatism, and rheological units.Veins at a high angle to the main collisional fault zone.. Host rocks. Greenschist metamorphic grade eg. gabbro, diorite, graywacke, argillite etc. Metamorphic grade of host rocks. Greenschist facies, but subgreenschist to lower amphibolite facies. Mineralization style Morphology. Tabular fissure veins in more competent host lithologies, veinlets and stringers forming stockworks in less competent lithologies. Veins with sharp contacts with wallrocks, showing ribboned/banded, stockworks and massive texture.. Ore mineralogy. Au, chalcopyrite, pyrrhotite, pyrite, arsenopyrite, galena, sphalerite, tellurides, scheelite, bismuth, stibnite, molybdenite etc. Gangue mineralogy. Quartz, carbonates, albite, sericite, muscovite, chlorite, tourmaline, etc. Alteration mineralogy Metal zoning PT conditions. Ore fluids Weathering. Ore genetic model Geochemical signature. Silicification, pyritization and potassium metasomatism, carbonate alteration, sericite, tourmaline and scheelite are common where veins are associated with felsic to intermediate intrusions. Vertical extent of up to 2 km, and lack pronounced zoning. 0.5-5 kilobars and 150o to 700 oC at ca. 2-20 km CO2-H2O-rich (5-30 mol% CO2), low salinity (<3 wt% NaCl) with high Au, Ag, As, (±Sb, Te, W, Mo) and low Cu, Pb, Zn metal contents. Orange-brown limonite due to the oxidation of Fe-Mg carbonates cut by white veins and veinlets of quartz. Quartz in abundant float in overburden. Deep transcrustal fault zones that develop in response to terrane collision act as conduits for CO2- H2O -rich (5-30 mol% CO2), low salinity (<3 wt% NaCl) aqueous fluids driven by a cycle of lithostatic and hydrostatic pressure build-up. Gold is deposited within the brittle- ductile transition zone caused by sulphidation due to fluid-wallrock reactions. Elevated values of Au, Ag, As, Sb, K, Li, Bi, W, Te and B ± (Cd, Cu, Pb, Zn and Hg) in rock and soil, Au in stream sediments.. Geophysical signature. Faults indicated by linear magnetic anomalies. Areas of alteration indicated by negative magnetic anomalies due to destruction of magnetite as a result of carbonate alteration.. Typical grade. ca.30 000 kg with grades of 16 g/t Au and 2.5 g/t Ag.. Economic limitations. Veins are usually less than 2m wide.. 4.

(12) principle phases of compressional deformation with intervening extensional events that involve early thrusting followed by a reverse oblique slip movement at the regional scale. This history is reflected in the local structures in the Skellefte district, and is described in detail below. The stratigraphic sequence of the Skellefte district has been discussed by many previous workers (Lundberg, 1980; Richard, 1986; Weihed et al., 1992; Weihed and Mäki, 1997 and Allen et al., 1996) and is illustrated in Fig. 3). The area consists in essence of ca 1.89 to 1.88 Ga subaqueous volcanic rocks (Skellefte Group), overlain by 1.87 Ga mafic volcanic rocks intercalated with a younger sedimentary sequence (Vargfors Group) and subaerial volcanics (Arvidsjaur Group) similar in age to the Vargfors Group, emplaced laterally to the north and on top of the metavolcanic rocks of Skellefte Group (Weihed et al., 2003). The Arvidsjaur Group is interpreted as a lateral facies variety of the Vargfors Group (Allen et al., 1996). The Skellefte, Vargfors and Arvidsjaur Groups are considered as various tectonic facies generated within the same volcanic arc or back arc rift setting (Claesson, 1985; Bergström, 2001). The Skellefte Group is interpreted to have been deposited from ca 1.90 to 1.88 Ga, in an essentially marine extensional arc setting that was followed by a compressional arc event during which the 1.88 - 1.87 Ga subaerial Arvidsjaur Group volcanic rocks were emplaced (Bergström, 2001). The Skellefte Group could be interpreted as the lowest stratigraphic unit dominated by juvenile volcaniclastic rocks, porphyritic intrusions, and lavas. The intercalated sedimentary rocks within the Skellefte Group consists of grey to black mudstone, volcaniclastic siltstone, sandstone and breccia-conglomerate, plus rare volcaniclastic rocks with a lime matrix and limestone (Allen et al., 1996). Rhyolitic rocks are abundant in the Skellefte Group although. during orogenesis (Gaal, and Gorbatschev, 1987). The Palaeoproterozoic Svecokarelian orogeny generated the Svecofennian supracrustal sequence, i.e. early orogenic sedimentary and volcanic rocks (Kathol and Weihed, 2005), that were intruded by various kinds of calc-alkaline granitoids and some gabbros (Svecokarelian intrusive) at ca 1.9 to 1.8 Ga due to the orogenic processes (Bark and Weihed, 2007). The Skellefte district is an Early Proterozoic geological terrane approximately 150 X 50 km in size oriented in a NW-SE orientation. It is a product of a ca. 1.90 Ga Palaeoproterozoic volcanic arc that formed at the margin of an Archaean craton to the north and the Palaeoproterozoic Bothian basin (Bark and Weihed, 2007) see Fig. 2. This district consists dominantly of submarine, felsic meta-volcanic units, and metasedimentary rocks that were intruded by several generations of granitiods that span the age range from 1.90 to 1.80 Ga (Weihed et al., 1992; Sundblad et al., 1993). The lowermost supracrustal unit in the Skellefte District is the Skellefte Group with an extremely variable stratigraphy (Weihed et al., 2002). Overlying the Skellefte Group is the Vargfors Group, both volcanosedimentary sequences (Allen et al. 1996), without a major unconformity. The rocks in the Skellefte district have undergone regional metamorphism at greenschist to lower amphibolite facies, and the regional metamorphic grade increases toward the south and east (Weihed et al., 2003). The Skellefte district hosts a number of orogenic gold deposits discovered prior to the 1990s in various geological settings, including shear zones. The area also hosts minor porphyrytype, low-grade Cu-Au mineralization e.g. Tallberg, (Weihed et al., 1992). Massive sulphide deposits, essentially pyritic Zu-Cu-Pb deposits, which are often Au-rich, e.g. Boliden and Holmtjärn, (Allen et al., 1996) are the main ore type in the area. Structurally, Weihed et al. (1992) and Allen et al. (1996) have defined a history consisting of two 5.

(13) 4˚. 58˚. 12˚. 70˚. 20˚. I. Fig. 1. Conglomerates and sandstones, polymict (Vargfors and Ledfat Groups) c. 1.87–1.85 Ga. Major faults and shear zones. Synform with plunge. Antiform with plunge. Subaerial to shallow water mainly rhyolite and dacite minor basalt-andesite (Arvidsjaur Group),c. 1.88–1.87 Ga Mainly marine rhyolite, dacite, andesite, basalt (Skellefte Group), c. 1.89–1.87 Ga. Synvolcanic granitoids of I-type, c. 1.89–1.85 Ga. Major gold deposits. K. 10 km. Mudstone, black shale, sandstone turbidites, (Bothnian Group,Vargfors Group, Skellefte Group)c. >1.95–1.85 Ga. Boliden. Åkerberg. Gabbro, diorite, ultramafic intrusions. 1700000. 8. Björkdal. K. Major massive sulfide deposits. J. Norsjö. Jörn. 1700000. Basalt-andesite, minor dacite lavas and sills (Vargfors Group), c. 1.88–1.86 Ga. 1650000. Am Gree n ph ibo schi st lite. Glommerträsk. J. Greensc hist Amphibol ite. Post-volcanic granitoids, c. 1.82–1.78 Ga. I. 28˚. Malå. Kristineberg. Adak. 1650000. Fig. 2 Geological map of Skellefte district modified from Allen et al. (1996). Insert shows the Fennoscandian Shield. 7200000. 23. 7250000 7200000. 7250000. 23. Fig. 4. l. Ca. es. nid. ed o. Fe n. ian. nd. ca. no s. d. iel. Sh. 6.

(14) the proportions of rhyolite, dacite, andesite and basalt vary within various domains (Allen et al., 1996; Weihed et al., 2002). The total thickness of the Skellefte Group is more than 3 km in the Petikträsk domain (Allen et al., 1996).. The transgressive Vargfors Group is best exposed in the western and central part of the Skellefte district, where it consists essentially of fluviatile and near-shore coarse epiclastic rocks (some with pebbles of Jörn granitoid) and basalts-andesites (Weihed et al., 1992). ............ Fig. 3 Regional time-stratigraphic relationships of Skellefte district (Allen et al., 1996). 7.

(15) is present which is interpreted as subparallel to the bedding in the supracrustal rocks and may have formed during a D1 deformation (Allen et al., 1996). Folds related to this early foliation have been observed in polished thin sections from the sedimentary units west of Kristineberg (Bergman Weihed, 1997, 2001). The early foliation may have developed during an early recumbent phase of folding and this has been suggested for the Långdal area (Weihed and Mäki, 1997). The second major phase of deformation resulted in gentle to open folds (F3) with axial surfaces striking N-NE, and fold axes coaxial with the early folds, indicating E-W convergence (Bark and Weihed, 2007). A spaced axial planar S3 crenulation cleavage is developed locally. In the central part of the Skellefte district, the intensity of the third folding phase increases towards the south with larger amplitude folds and a more penetrative cleavage. A number of north-striking shear zones are present in the Skellefte district and surroundings areas. These have a dominating reverse dip-slip movement and they are interpreted to have formed during or after the second major (D3) phase of deformation (Weihed et al., 2002b). Shearing was important during both phases of deformation. Early shear zones are mostly parallel to the axial surfaces of the early (F2), upright folds that are present everywhere in the Skellefte district. In the central part of the district, the shear zones strike NW and have a reverse oblique slip movement usually with the south side up (Weihed et al., 2003). The gold-bearing structures probably formed during the later stages of the second major (D3) phase of deformation. These structures involve e.g. semi-brittle shear zones and en échelon quartz veins in Grundfors (Weihed et al., 2002b). The timing of the D3 events is estimated from field relationships, to predate - to syndate the Revsund granitoids, and to post date the ca. 1.88 Ga Sikträsk granitoids, while the. The volcanic units are related to the Gallejaur structure (Weihed et al., 1992), and consist of green, moderate to high Mg basalt lavas and intrusions and andesite, dacite and rhyolite with interbedded sedimentary rocks. The moderate to high-Mg basalts in the upper part of the Skellefte Group and the Elvaberg Formation are considered as feeder sills and dykes related to the Gallejaur volcanism (Allen et al., 1996). Subaerial volcanic rocks which comprise the Arvidsjaur Group to the north of the Skellefte district represent a continental environment coeval with the volcanic arc. To the south, the large area of metamorphosed greywackes may be interpreted as a fore-arc environment (Weihed et al., 1992). The Arvidsjaur and Vargfors Groups are interpreted as lateral equivalents and the lower part of the Arvidsjaur group interpreted by many previous workers as subaerial lateral equivalent of the Skellefte Group (Allen et al., 1996). A three-phase deformation scheme has been proposed for most of the area, when the main deformation is denoted D2 and D3 (Bark and Weihed, 2007). The second phase of deformation generated tight to isoclinal folds (F2) with E-NE-striking, upright, axial planes and variably plunging fold axes, on average 60oE in the eastern and western part of the Skellefte district and W-NW in the central part of the district (Weihed et al., 1992, 2003,). An axial planar foliation is developed as a penetrative grain shape cleavage in coarser volcanic rocks and as a crenulation cleavage in laminated tuffs and schists. Slip along the cleavage is common in this area and shear zones occur in the same general orientation as the axial surfaces. Most of these shear zones have a reverse oblique-slip movement where the southern side has moved upwards over the northern side (Bergman Weihed, 1997). The presence of a crenulation cleavage as an axial planar cleavage to the first major identified folds shows that an earlier foliation 8.

(16) second phase of deformation D3, is estimated at 1.81Ga. The second major phase of folding (F3) at least locally affects the Revsund granitoids and therefore interpreted to be syn- to postRevsund in age (Bark and Weihed, 2007). Tectonic settings for the Skellefte district is considered to involve exotic terrains, evoking a model of either single or multiple subduction zones as proposed by Heitanen (1975), Gaal (1990), Weihed et al., (1992), and Juhlin et al., (2002). Northward subduction has been proposed based on a magnetotelluric survey (Rasmussen et al., 1987) which identified a low-resistivity slab dipping north beneath the Skellefte district. Furthermore, a seismic reflection profile in the Bothnian Bay also identified a north-dipping reflector east of the Skellefte district (BABEL group 1990). The volcanic rocks in the Skellefte district (Skellefte Group) are thus, as discussed above, interpreted to represent some kind of volcanic arc (Weihed and Mäki, 1997), and the felsic volcanic rocks as products of various arc settings (Gaal, 1990). Minor Archaean contribution to the source of the magma is detected in εNd values and through detrital zircons in the greywackes which indicate that Archaean greenstone crust was present in an erosional position possible during the formation of the Bothian basin (Weihed and Mäki, 1997). On a regional scale, the Vindelgransele area (Fig. 4) is underlain by an anticlinorium of felsic metavolcanic rocks of rhyolite or rhyodacitic composition of the Skellefte Group overlain by metasedimentary rocks of the Vargfors Group (Öhlander and Markkula, 1994). The rocks in the western part of the Vindelgransele area are dominated by metagreywacke and few felsic volcanics rocks. The metavolcanic rocks are generally porphyritic with phenocrysts of microcline, plagioclase or quartz. The supracrustal rocks have been folded with fold axes dipping steeply to the west (Öhlander and Markkula, 1994). The Vindelgransele area, like. much of the western parts of the Skelefte district, was metamorphosed in greenschist facies, and the metavolcanic rocks in the area are sericitized and chloritized. Bergström and Sträng (1997) have proposed a turbiditic origin for the Vargfors Group due to its high volumes of greywacke. The Vargfors Group in the Vindelgransele area consists of a localized fine-grained metabasaltic komatiite unit, overlain by an extensive thick sequence of turbiditic greywackes termed the Malå Group by Bergström and Sträng (1997). 2.1 Overview of orogenic gold deposits in Skellefte district, the Vindel-Gransele area and the Gold line Orogenic gold deposits in the Fennoscandian Shield generally are structurally controlled. Most deposits where structural information is available are hosted in second- to lower-order shear or fault zones, at their intersections, or at the intersections between antiforms and crosscutting fault and shear zones, all suggesting a compressional to transpressional regime at the time of emplacement (Weihed et al., 2005). Some Fennoscandian orogenic gold mineralization have been interpreted as discrete quartz-vein hosted deposits often adjacent to intrusions with weakly developed wall-rock alteration, and enriched with As, Bi, Te and W (Nurmi et al., 1991). Gold mineralization in the Svecokarelian domain occur in rocks metamorphosed, altered, and mineralized under lower greenschist to upper amphibolites facies P-T conditions similar to that in Precambrian shields in Canada and Western Australia (Eilu et al., 2003). Svecokarelian orogenic gold mineralization is the result of deformation and metamorphism in conjuction with accretion of the Lopian continent (now delineated by the Raahe-Ladoga line) that generated metamorphic fluids, which are interpreted to be responsible for the formation of the orogenic gold deposits, mainly along second-or third order shear zones (Sundblad, 2003). 9.

(17) Fig. 4 Regional Geology map of Vindel-Gransele area (Modified data from Geological Survey Sweden). 7220. 4. 7225. 79. 82. 70. kl. 65. 80. Bjurbäcksliden 80. 80. 1610. 5. Ap. 70. 80. 85. 80. 80. M. Ap. c. c. 80. 80. 77. t. 80. 75. 80. 80. 80. 80. 75. 80. 80. 80. kl. 80. 521. 80. 85. k. 82. 70. 80. 73. 75. Au. 80. 70. Fäbodliden. 85. 70. 80. c. a. a. 80. 80. c. 82. 80. k. 85. M. 70 63. Au. 80. 85. Jägarliden. Au. 70. 1615. 7230. c 80. m. 70. 75. B. Klodden. 80. 67. Middagsberget. 80. 80. Näverliden 70. Brännholmen. 75. R. 80 80. 80. 80. 80. R. d. 60. k. M. 58. 58. 75. 82. 72. 80. 80. 80. 80. 30. 58. 48 50. 50. mt. k. 73. 60. 80. Grundforsen. 48. 80. 78. ak. Granselliden. Kroktjärnmyran. Vindelgransele. Au. R 80. Vargbäcken. 75. R. ak. 1620. 10. Au. 0. Faults. 1. 2 km. Metarhyolite–metadacite, plagioclase and quartz porphyritic, lava or subvolcanic intrusion, (Skellefte Group) ca 1.89–1.86 Ga. Metabasalt–metaandesite, lava or sill / dyke (Skellefte Group) ca 1.89–1.86 Ga. Metaargillite, locally graphite- and sulphide bearing left, quartz veined right (Malå Group, Mörkliden Formation) ca 1.88–1.86 Ga. Metagraywacke, sandstone–siltstone, turbidite with preserved graded bedding (Malå Group, Fäboliden Formation) ca 1.88–1.86 Ga. Metadioritoid. Metaultramafic rock (Malå Group) ca 1.88–1.86 Ga. Metabasaltic komatiite, lava left, sill right (Malå Group, Bjurås Formation) ca 1.88–1.86 Ga. Granite, even–unevengrained (Revsund–Adak Suite) ca 1.86–1.75 Ga. Granite, coarsely porphyritic (Revsund–Adak Suite) ca 1.86–1.75 Ga. Hydrothermally altered areas. Gold mineralization.

(18) gold mineralization associated with thin quartz veinlets at the contact of diorite and metagreywackes (Blomquist and Leijd, 1999). A new interesting ore province, the so called Gold Line is currently being explored in the northern parts of the Bothnian Basin between Lycksele and Storuman in metagreywackes of the Bothnian Supergroup. The Gold Line mineralizations which include Stortjärnhobben, Fäbodliden, Knaften, Svartliden, and Barsele (Bark and Weihed, 2007) are structurally controlled. Some of the Gold line deposits are associated with arsenopyrite-löllingite and hosted in quartz veins in greywacke-and amphibolitedominated units. Gold mineralization associated with quartz veins and fractures in metavolcanic and metasediments of the Bothnian Supergroup are recognised in the Grundfors Södra, Krångfors Västra and Östra deposits (Bergman, 1992). Most quartz veins, especially the larger ones are restricted to intercalated amphibolites, while the metasediments host only a minor part. Sulphide content is low and disseminated and gold grades are essentially low and erratic (Kathol and Weihed, 2005) .. In the Skellefte district, gold mineralization is mainly structurally controlled (Kathol and Weihed, 2005). The orogenic gold deposits in the Skellefte district (Table. 2) share many properties with similar deposits in Finland (Nurmi et al., 1991) i.e. Kangaskylä (Mäkelä et al., 1988; Sundblad et al., 1993), Laivakangas (Mäkelä and Sandberg, 1985), Pöhlöl (Mäkelä et al., 1988) and Haveri (Mäkelä, 1980; Karvinen, 1997). The Björdal and Åkerberg deposit are both situated in the eastern part of the Skellefte district and are hosted by intrusive rocks in N-S striking metagreywackes of the Bothian supergroup (Kathol and Weihed, 2005). Several areas in the Skellefte district have gold mineralization hosted by small intrusions. In the Vindelgransele area, there are a number of deposits (i.e. Middagsberget, Fäbodliden A-C, and Vargbäcken) with arsenopyrite dissemination, hosted by quartz monzodioriticsill-like bodies that have intruded sedimentary rocks of the Vargfors group (Bergström and Weihed, 1991; Sundblad et al., 1993; Öhlander and Markkula, 1994). The Vargbäcken deposit has been interpreted as low sulphide. Table 2 Grade and Tonnage of Selected Gold (-bearing) deposits in the Skellefte District No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30. Deposit name Åkerberg Ångesdal Barsele 1 Barsele 2 Björkdal Blaiken Brokojan Duobblonbäcken Eriksmyran Ersmarkberget Fäbodliden Fäboliden A Fäboliden B Fäboliden C Grundträsk Gubbträsk Högås Klippen Knaften Kyrkviken Långtjärn Middagsberget Myrträsk Sandviksträsk Smilaliden Stortjärnhobben Svartliden Tjålmträsk Vargbäcken Vinliden. Status Tonnage (Mt) closed mine >1 Prospect Mineralization 7.6 Prospect Active mine >20 Mine on hold 0.85 Prospect Prospect Prospect Mineralization 0.9 Active mine 54.2 Prospect <0.01 Prospect Prospect Prospect Mineralization 0.7 Prospect Prospect Prospect Prospect Prospect <0.01 Prospect <0.01 Prospect Prospect Prospect Mineralization Active mine 1.5 Prospect Mineralization 1.2 Prospect. Au (ppm) 3. Ag (ppm). Cu (%). Zn (%) Pb (%). 1.6 2.6 4.3. 3.6 1.23 3. 4.5. 2 1.6. 11. 1 3. 4.5 1.44. † Data obtained from company website and annual reports. 11. 1.2. 1.1. References Weihed and Mäki, (1997) Mawson Resources, (2006)† Northland Resources, (2007)† Kathol and Weihed, (2005) Weihed et al., (2005) ScanMining AB, (2007)† Mawson Resources, (2006) Kathol and Weihed, (2005) Mawson Resources, (2006) ScanMining AB, (2004)† Lappland Goldminers AB, (2007)† Kathol and Weihed, (2005) Kathol and Weihed, (2005) Kathol and Weihed, (2005) Beowulf Mining plc, (2006)† Lappland Goldminers AB, (2007) Lappland Goldminers AB, (2007) Ovoca Gold plc (2006) Lappland Goldminers AB, (2007) Mawson Resources, (2006) Kathol and Weihed, (2005) Kathol and Weihed, (2005) Mawson Resources, (2006) Lappland Goldminers AB, (2007) Mawson Resources, (2006) Lappland Goldminers AB, (2007) Dragon Mining, (2007)† Lappland Goldminers AB, (2007) Mawson Resources, (2006) Kathol and Weihed, (2005).

(19) 3. Geology of the Vargbäcken deposit The Vargbäcken gold deposit is approx. 400 x 40 m in size and poorly outcropping. A simplified geological map of Vargbäcken (also showing the locations of the drillholes discussed) is presented in Figure 5. The area is covered by an overburden of glacial till about 3 m thick, with a few outcrops occurring to the north and east on the north-westerly slopes of Granseliden Mountain. The felsic volcanic rocks are interpreted to be well sorted, medium to coarse grained and fragment bearing in style. They are essentially juxtaposed by faults with attenuated metagreywackes. This contact is mostly coincident with the Vargbäcken Western Fault Zone, where intense brittle deformation obscures any original correlation. The metagreywackes of the Bjurås Formation of the Vargfors Group comprises a sequence of laminated, grey-brown greywacke with few interbedded pelite, trace to minor sulphides, including pyrrhotite, and pyrite present especially in darker and more carbonaceous lithologies (Fig. 6). The metagreywackes of the Malå Group are intruded by magnesium-rich ultramafic to mafic (komatiitic) sills. A suite of diorite intrusives, which forms the main host to quartz-vein style gold mineralization, is a member of this group (Blomquist and Leijd, 1999). The Vargbäcken diorite is considered to form a bedding concordant, but discontinuous silllike body, 100 to 200 m wide and at least 1.5 km long cutting the folded supracrustal rocks in a NE-SW direction, parallel to the trend of the northern limb of an anticlinal axial trace (Panteleyev, 2004). Several smaller diorite sills and dykes are present within the hanging-wall lithology above the host diorite sill. These small sills and dykes are usually fine-medium grained and sometimes associated with brecciation. Few mafic intrusive rocks are present within. the hanging-wall of the host diorite. They are medium to coarse grained, massive equigranular and moderately to strongly foliated. The mafic intrusive in Vargbäcken show a characteristic quartz-carbonate and actinolite vein alteration style, minor disseminated chalcopyrite, and a retrograde metamorphic mineral assemblage, and could be referred to as mafic-biotite schist. Some appear to cross-cut primary bedding, while others are bedding concordant tabular sills identified as gabbro intrusives, located within the footwall sedimentary sequence to the host diorite. The Vargbäcken diorite is generally medium to coarse grained, massive equigranular to weakly foliated. Lateral and vertical compositional zonation is considered to be present in the host diorite with a more mafic composition at the base and extremes. The diorite-metagreywacke contact is undulating and steeply dipping, characterised by intense alteration, including silicification and quartz brecciation within the contact zone (Blomquist and Leijd, 1999). Due to the different rheologic behaviour, or competency contrast, during the deformation and shearing, the more brittle dioritic intrusions are fractured and are the main hosts for gold mineralization. Gold mineralization is closely related to sulphides, indicating a temporal relationship between quartz formation, gold mineralization, and intrusive activity (Billström et al., 1997).. 4. Mineralization style. 12. Structural mapping reveals that small-scale shearing and micro-faults are dominant in the area. Mineralized veins exhibit several structural styles of quartz vein, ranging from breccias to vein stockworks predominantly within competent host rocks close to the metasedimentdiorite contact, and laminated quartz in brittleductile shear zones (Fig. 7a-c). The veins lack an obvious causative intrusion while some of the larger veins have sheared margins but no.

(20) Fig. 5 Geological map of the Vargbäcken deposit (Data from Mawson Resources).. 13.

(21) sil. tst. on. s. ted. e. Fo ld. na s. e. on. t ilts. n. mi. La. mi. d ate. La. Pho to c. N. Photo b e. Shear zon. Quartz vein. one. Contact z. 0. 1. 2. 3. 4. 5m. Fig. 6 Sketch of the Outcrop Mapped (Metasediment). The figure shows the metasediment of the mapped outcrop grading from grayish to more mafic near the contact with the diorite. The folds are generated during shearing within the main high strain zone, indicating that the metasediment has flowed in a ductile manner near the contact and more brittle-ductile at the grayish part with intense deformation. 14.

(22) Fig. 7 Outcrop photos ( for location see Fig 6.) of vein cross cutting relationship with corresponding sketch tracing. (a) and (b) coarse deformed quartz vein, showing cross cutting relationship within the host diorite, V1 overprinted by V2 in both cases. (c)Laminated siltstone assigned to Bjurås Formation of the Vargfors Group within the metasediment of the outcrop mapped.. individual large, banded, sutured or layered shear and fault-fill veins that typically form in ductile zones have been noted in the drill core that were logged (Panteleyev, 2004). This implies a synchronous vein formation and gold deposition within a large area. High-angle shear zones that either splay from, or are cut by, major shear zones are an integral structural component of the Vargbäcken deposit and are common zones of mineralization. Veins of the Vargbäcken area have irregular strikes. and dips. It is possible to obtain the strike and dip of the vein from the oriented drill core of some of the drill cores. The quartz veins strike between 035ºand 065º; and dip 70º and 80º NW. The stereographic projection shows a movement vector with a plunge ca. 55º NE (Fig. 8a-b). Some mylonitic foliation of quartz is present in a few cases and represents pervasive recrystallization of veins in shear zones during overprinting deformation. Many quartz vein textures indicate extension,. 15.

(23) a. b. Fig. 8 Stereographic projection plots of quartz veins of an oriented drill core VBNDD06077 (vbn77); (a) Poles to vein axis (b) Contoured equal-area projections.. displaying quartz and carbonate fibres at high angle to the vein walls with multiple stages of mineral growth. Some of the veins are laminated, with rather massive fine grained quartz, up to a centimetre thick and fibres sub-parallel to the vein walls (Fig. 9a). In hand specimen, the laminations appear as black (tourmaline) planar zones separated by domains of pure white quartz. In the thinner veins, laminations are confined to margins of the vein. Elongated wall rocks fragments are in places included in the veins. Shallow-dipping extensional veins occur within shear zones, in which case they are relatively small and sigmoidal in shape, apparently difficult to determine from the Vargbäcken drill cores if they are planar and laterally extending outside the shear zone. There is a marked asymmetric to disharmonic, smallscale folding observed in the northwestern part of the outcrop mapped. These folds were generated during shearing within the main high strain zone, in which the metasediment has flowed in a ductile manner (Fig. 6). The abundance of thin extension veinlets in the gold mineralized zone shows the tectonic and deformational control on the structural development, as well as the. overall kinematics (Panteleyev, 2004). These veins sporadically contain gold mineralization and have extensive carbonate-alteration halos. The presence of dynamically recrystallized biotite and epidote is a possible indication that this process must have taken place under greenschist facies conditions. Surface mapping, mainly in part of the trench, suggests that a conjugate set of north-south and west-northwest/east-southeast veins are the dominant ones. These veins might contain the bulk of the gold within the broader silicified and hydrothermally altered northeast-trending zones at, or near, the intrusive contact (Panteleyev, 2004). Changes in quartz vein thickness across shear zone, formation of splays on one side of shear zone, difficulty in vein correlation across shear zones, and sudden change in direction of veins as they approach shear zones are striking evidence for a close relationship between vein formation and movement within brittle-ductile shear zones (Weihed et al., 2003). These features are attributed to local stress changes caused by shear zones being weaker than the surrounding country rock (Weihed et al., 2003). The quartz veins do not continue more than a few metres into the turbiditic greywacke sediments, now. 16.

(24) Fig. 9 Drill core photos showing veins, vein alteration, stockworks and vein mineral inclusion. (a) Laminated quartz vein cutting dark fine grained diorite, showing internal quartz laminae are separated by dark (tourmaline), chlorite and amphibole, Semi-continuous wall rock septa and slip surface. (b) Diorite showing radial porphyritic texture. (c) Stockwork of two subparallel sets of extensional veins. (d) Laminated massive quartz vein with multistages of mineral growth (epidote and chlorite) infilling at the centre of the vein and wall rock, note the breakup at the center of vein. (e) Stockwork composed of two subparallel sets of extensional vein grading into a breccia texture in the right part of the photograph. (f) Small extensional quartz vein (4 cm), showing regular planar walls selvages, as well as open-space filling internal textures. Thin albite alteration (less than 1 cm) with porphyritic texture enveloping the vein within the host diorite. (g) Fine grained quartz vein, showing patches of pyrrhotite and minor chalcopyrite. (h) Quartz vein dominated by sphalerite, gold (encircled) sporadically distributed at the grain boundaries.. 17.

(25) extensively recrystallized to fine grained, biotite- the veins were all formed during one progressive rich metagreywacke or phyllite (Panteleyev, deformational event. 2004). 4.2 Structural interpretation from 4.1 Vein timing relationships geophysical data The quartz veins are of two distinctive generations, the milky and opaque fine to medium grained quartz veins with a high amount of visible, coarse grained, gold and the smoky dark translucent coarse grained quartz veins considered to have disseminated sulphides and low amount of visible gold. Systematic crosscutting relationships are relatively common among different veins sets within the deposit (Fig. 9a-b). The smoky dark translucent coarse grained quartz veins exist as extensional veins cutting across a fault fill milky and opaque fine to medium grained quartz vein at a few location. These cross cutting relationships between the two veins sets indicate that the veins are not coeval and a systematic vein chronology can be established between them. The veins show different degrees of overprinting deformation along the major shear zone within the diorite, also indicating a multi-phase quartz paragenesis of Vargbäcken. It is however, still possible that. On the aeromagnetic and IP anomaly maps (Fig.10a-b) moderate-low magnetic linear features represent deformation zones where the original magnetite in the rock has been oxidized. These linear features trend north-east and could be used to delineate Vargbäcken host rock. In figure 10b, there is an apparent magnetic offset of the Vargbäcken host rock, interpreted as higher-order deformation zones. The magnetic offset is interpreted as a major deformation zone dividing the main north-east striking linear structures into areas H1 and H2 coinciding with a fairly strong to moderate IP chargeability anomaly.. a. (Fig.10). The aeromagnetic and IP anomaly maps helped to delineate the main exploration target, to define the structural setting, and to interpret the tectonic events in the area. The moderate to low magnetic area H2 north of the major deformation zone is gold mineralized in contrast to high magnetic areas east of the H2 area. b. 0. 1. 1 km. 2 km. Fig. 10(a) Aeromagnetic map (b) IP anomaly map zoomed in on the same structures dotted cross in areas Fig. 10a show Vargbäcken deposit Offset in the IP anomaly is interpreted as a major deformation zone separating H1 and H2 coinciding with fairly strong to moderate IP chargeability anomaly. Purple line is metasediment – diorite contact.. 18.

(26) 5. Alteration In the Skellefte district hydrothermal alteration related to many gold deposits is generally developed in narrow alteration halos rarely exceeding 30 centimetres in width around the quartz veins (Weihed and Mäki, 1997). The most widespread alteration occurs within and adjacent to areas of intense deformation or shear zones and is usually characterised by biotite, chlorite, carbonates, albite and sericite, along with sulphide minerals (Weihed and Mäki, 1997). These alteration mineral assemblages characteristic of greenschist metamorphism has. been reported from many gold deposits within the Skellefte district (i.e. Björkdal, Tallberg, Middagsberget and Fäbodliden) (Weihed, 1992; Öhlander and Markkula, 1994). Replacement of biotite by chlorite was recorded in most shear zones in Vargbäcken (Fig. 11). K-feldspar occurs frequently together with epidote as fissure filling. Most veins in Vargbäcken are narrow veins, or veinlets, 1 to 4 centimetres wide. They exhibit features of planar extensional structures, those in which dilational fractures are infilled with granular quartz and the veins margins can have. VBN97006-65.20 m. VBN98014-51.60 m. VBN98015-190.00m. VBN98016-50.4 m. Fig. 11 Transmitted light photomicrograph showing veins and alteration in polished thin sections. (a) Extensive carbonate-alteration halos with interstitial intergrowth of subgrained quartz. (b) Biotite altered to chlorite, note the residual brownish color of biotite at the upper left with pleochroic haloes being surrounded by pale green chlorite. (c) Elongated subgrain, undulose extinction of quartz with fine grained layer of mica at extreme right both dynamically recrystallised. (d) Quartz vein showing elongated seriated subgrain and undulose extinction.. 19.

(27) granular quartz in a few cases intergrown with vein related alteration minerals. The vein related alteration is considered to be restricted, 1-2 cm wide, carbonate-albite alteration envelopes around the veins (Fig. 9f). Replacements of Caplagioclase by biotite or albite were associated with thin veinlets. Several generations of fractures filled with variable quantities of quartz, carbonate, sericite, chlorite, albite and sulphides occur in the alteration zones enveloping the quartz veins. Cross-cutting relationships show that these fractures in most cases are clearly younger than the major quartz veins. In some quartz veins tourmaline is present. However, most quartz veinlets have thin or no alteration envelopes.. 6. Setting of gold The ore zone occurs at the metagreywackediorite contact, mainly within the silicified diorite intrusive. This contact acted as a permeable zone for the introduction of fluids from which precipitation of hydrothermal quartz and the gold occurred. The best mineralized areas include zones where changes in strike and dip of the contact occurs, and where other structural disturbances such as shearing, fracturing and some brecciation have taken place along the contact, or within the diorite (Panteleyev, 2004). The gold in the quartz veins at Vargbäcken is coarse grained in place and often visible to the naked eye. In contrast to other gold deposits in the Vindelgransele area, where gold is closely associated with arsenopyrite (Blomquist and Leijd, 1999), the arsenopyrite content at Vargbäcken is relatively low. Pyrrhotite is the dominating sulphide present in quartz veins but can be patchy and rarely exceeds 10 % by volume. Other sulphide minerals including chalcopyrite, arsenopyrite, pyrite, sphalerite, molybdenite and galena are generally below 1 % in total abundance; rare native copper has also been reported.. The Vargbäcken gold mineralization is hosted in fractured competent units that have acted as fluid conduits during or after deformation. A petrographic study of Vargbäcken indicates that gold occurs both as free milling and within the sulphides, especially pyrrhotite and sphalerite, and also show an unequal distribution within the system as a result of both major depositional controls and subsequent tectonic remobilization (cf. Dubé et al., 2006). The free gold is commonly found in massive, deformed, medium to fine grained (milky) quartz veins as coarse visible gold grain (up to 2 mm in size) without sulphides and in places form nuggets. Gold can be observed in quartz veins that lack sulphides or have only a small pyrrhotite content. Only sphalerite and rarely pyrrhotite is visibly correlated with the gold. The gold in Vargbäcken, based on scanning electron microscope (SEM) studies is associated with increased content of Au, Ag, K, S, Cd, Bi, Ti and Te, although, no regular zoning or distribution pattern of the elements was found. Some of the gold is associated with sulphides, located at grain boundaries, on micro fractures within grains, or forming fine grained inclusions. Rarely, coarse visible gold is found within the sulphide-quartz veins. Gold associated with sphalerite and pyrrhotite tends to be more disseminated, often fine grained (20-50μm in size) and gold grades are lower than in the quartz veins. Some veins are low grade or even barren. Importantly both coarse-grained visible gold and fine-grained gold is present within sulphides in microfractures. Infact, the coarse-grained nature of gold at Vargbäcken suggest that gold has indeed been mobilized. The highest density of gold bearing quartz veins is found within the diorite along the north-western diorite-metasediment contact where visible gold was found in drill holes at ca. 800 m strike length and the highest density of visible gold bearing structures is close to the contact and extends ca.40m into the diorite (Panteleyev, 2004). SEM analysis shows that. 20.

(28) the gold is always alloyed with silver (gold to and by SEM. SEM-results are attached as electrum in composition). Gold to Silver ratio appendix A and have been summarized in Table 4. ranges from 2.3-5.3, (n=11). Table 3 shows the summary of four polished thin sections that where analysized both optical. Table 3 Summary of polished thin sections analyzed. Optical and SEM studies where carried out on the selected four polished thin sections.. Sample. Silicate phases. Opaque phases. Comment. VBN98016-46.90. Qz, C hl, Dol. Po, Sph, Au. Ore zone. VBN98016-50.40. Qz, C hl, Mic. Po, BiT, A u. Ore zone. VBN98025-178.40. Qz, B io. Cp. Diorite (lower part). VBN98025-187.40. Qz, Plg , Dol. Po, Cp, (Sph) , Ga. Near contact. Silicate phase: Qz-quartz, chl-chlorite, Dol-Dolomite, Mic-mica, Bio-biotite, plg-plagioclase, Opaque phase: Au-gold, Po-pyrrhotite, Sph-sphalerite, Cp-chalcopyrite, Ga-galena, BiT- bismuth telluride. Table 4 Summary of elements identified in the SEM in weight %. SEM DATA VBN98016-46.90 Spectrum Elements 1' 2' 3' 4' 1 2 3 4 5 6 7 8 9 10 11 C (wt %) 11.90 10.41 19.60 14.82 7.22 7.51 6.92 6.73 5.56 6.61 6.28 4.95 6.65 7.92 O (wt %) 3.27 8.37 18.30 4.58 2.77 10.41 5.18 3.74 3.05 2.66 10.86 5.40 3.07 1.15 21.11 Na (wt %) Al (wt %) Si (wt %) 4.00 8.31 24.09 4.75 1.66 8.95 6.38 4.16 2.40 1.98 9.68 7.55 4.10 3.04 32.90 S (wt %) 15.18 3.85 11.45 8.77 9.62 10.74 10.57 7.97 7.95 21.39 8.77 K (wt %) Ca (wt %) 4.84 4.75 9.09 2.79 8.63 7.66 6.08 8.63 14.88 10.84 3.79 1.80 3.23 Ti (wt %) Fe (wt %) 21.19 6.01 12.24 7.25 15.95 10.70 17.86 16.01 23.04 18.36 8.04 9.96 10.71 32.59 18.33 Zn (wt %) 18.69 5.62 6.15 6.51 12.76 7.79 9.09 11.27 10.20 10.33 7.25 9.66 12.89 29.27 7.74 Pb (wt %) Cu (wt %) Tc (wt %) 10.91 17.16 Te (wt %) 11.02 42.10 35.38 Bi (wt %) Ag (wt %) 4.07 12.93 7.25 12.47 7.61 8.02 7.04 8.13 12.85 9.31 Cd (wt %) 2.74 4.10 Au (wt %) 18.96 43.52 38.16 33.54 38.19 32.80 31.90 32.71 30.16 39.31 Au/Ag (wt %) 4.65 3.35 5.26 2.69 5.61 4.09 4.53 4.02 2.34 4.22. Elements C (wt %) O (wt %) Na (wt %) Al (wt %) Si (wt %) S (wt %) K (wt %) Ca (wt %) Ti (wt %) Fe (wt %) Zn (wt %) Pb (wt %) Cu (wt %) Tc (wt %) Te (wt %) Bi (wt %) Ag (wt %) Cd (wt %) Au (wt %) Au/Ag (wt %). VBN98016-50.40 Spectrum 1' 2' 1 2.70 11.77 4.51 4.54 12.26 8.64. 2 3 4 1.62 28.69 1.47 12.84 16.90. 11.39 17.82 14.25 4.48 20.70 34.10 3.68 11.19 4.21 6.27 5.49 14.57 27.57. 5 1.01 3.86. VBN98025-178.40 VBN98025-187.40 Spectrum Spectrum 1' 1 2 1' 2' 1'' 3.92 54.81 6.59 5.72 7.37 6.50 21.36 34.43 6.56 36.56 5.38 0.42. 10.52 15.94 20.36 10.73 12.21. 54.94 16.09 12.69. 57.72. 9.25 9.96. 2.26 39.34 81.24 31.30 42.74. 73.89 51.59. 3.20. 9.84. 4.04 54.67. 9.99. 47.71 14.43 19.09 6.97 9.98 23.27 2.33. 21. 30.72 0.73 36.19. 12 13 14 8.24 29.15 44.17 23.23 8.28 4.42. 33.16 6.25 7.85 10.11 6.17 15.66. 15 3.69 1.69. 2.69 2.64 9.79 18.93 4.67. 3.49. 13.88 18.71 22.83 63.16 7.47 11.83 11.43 6.40. 1. 2. 3. 4. 5. 3.82. 2.51. 4.22 0.42 1.14 8.65 9.23 1.94 0.72 0.41 73.27. 24.74 0.46 1.53 41.04 3.43 3.77 1.97. 16.29 0.56 4.90 23.69 4.20 15.70. 23.06. 33.82. 0.80 0.63 7.96 5.75 12.66 11.02 1.75 1.44 0.53 0.36 63.36 78.30. 9.12. 0.84.

(29) 6.1 Structural microanalysis The microstructures have been studied with optical microscopy on polished thin sections. The quartz grain boundaries show undulose extinction. This reveals a jaggedness of the quartz grain boundaries which corresponds to the shape of new grains formed at the expense of the old grain, possibly through the process of progressive subgrain rotation (Fig. 11c-d). In some places, the grains are elongated, seriated and subgrained, which are relicts of the vein’s original coarse-grained microstructure that has been progressively stretched by deformation indicating that the original grains have been dynamically recrystallised.. Irregularly shaped grains of gold often occupy small cracks and fissures in the quartz or follow laminations or planes of shearing either in the quartz or along its contacts with the wall rock. Free-milling gold is rare in the case of sulphide associated ores. The gold associated with sulphide displays irregular shapes and occurs most commonly in, or in close proximity to pyrrhotite and sphalerite, or in small cracks or interstices at grain boundaries between quartz grains (Fig. 12a-d). The free-milling gold is often situated both within quartz vein and at the necks of the quartz- and sulphide veins (Fig. 12c), this could indicate that at least the late stage of the mineralization was syn- and latekinematic.. VBN98016-50.40 m. VBN98016-46.90 m. Fig 12 Reflected light photomicrograph showing Au in polished thin sections and SEM photos showing Au and associated minerals. (a) Photomicrograph of gold and pyrrhotite in a fracture. Gold is located at the tip of the pyrrhotite grain.(b) Gold situated within the grain boundaries of a sphalerite dominated quartz vein. Note that gold is erratically distributed since some of the gold occurs as inclusions. SEM photos (c) and (d) showing gold and associated minerals of the same polished thin sections.. 22.

(30) 7. Summary and conclusions The Vargbäcken gold deposit can be classified as an orogenic gold deposit. The majority of orogenic gold deposits worldwide have been deposited from low-salinity (<3 wt% NaCl) H2O-CO2±CH4 fluids at 200–650°C and <1–5 kbar, where gold was transported as a reduced sulphide complex (Groves, 1993, Groves et al., 1998). One of the major limitations in deformed and metamorphosed terrains such as Precambrian greenstone belts is that the primary characteristics may have been obscured to a large extent by overprinting deformation and metamorphism so that they are difficult to recognize (Dubé and Gosselin, 2006). Since the Vargbäcken gold deposit is syn- to late main phase of deformation, the primary features of the mineralization are in most cases relatively well preserved. The deep-seated, gold-transporting metamorphic fluid has been channelled to higher crustal levels through major crustal faults or deformation zones (Poulsen et al., 2000). Along its pathway, the fluid has been enriched by dissolving various components - notably gold - from the volcano-sedimentary system, including a potential gold-rich precursor, which then precipitate as vein material or wallrock replacement in second and third order structures at deep crustal levels through fluid-pressure cyclic processes and temperature, pH and other physico-chemical variations (Dubé and Gosselin, 2006). Carbonic hydrothermal fluids replace calcium, magnesium and iron silicates of the wall rocks with carbonate minerals, together with potassium-mica, quartz, and sulphides. The components essentially added are CO2, H2O, K, and S. Silica was redistributed or added from wall rocks into veins. It is suggested that the Vargbäcken gold deposit is related to metamorphic fluids from accretionary processes, generated by prograde metamorphism and thermal re-equilibration of subducted volcano-sedimentary terranes. Peak. metamorphism in the area occurred at 1.85-1.80 Ga (Weihed et al., 1992; Billström and Weihed, 1996; Weihed et al., 2002a), coeval with at least two major deformation events, at 1.85-1.84 and 1.80 Ga (Bergman Weihed, 2001). The metamorphism is interpreted to be overprinted by hydrothermal alteration in Vargbäcken. Thus, the possible age of at least the last stages of mineralization at Vargbäcken is suggested to be ca. 1.80 Ga. The presence of growth and dynamically recrystallized quartz, biotite partly chloritized, epidote and albitization at the walls of the small extensional quartz vein are evidence suggesting that the mineralization formed under greenschist facies condition, close to the metamorphic peak (Fig. 13). Relatively low-temperature ore and gangue minerals, such as bismuth, tellurides, Ag-rich sulphosalts, in the alteration assemblage are also indicative of mesozonal conditions; 6-12 km, 1.5-3 kbars and 300-475°C (GebreMariam et al., 1995). Probably, an estimated P-T condition for the Vargbäcken gold deposit could be within these ranges 1.5-4 kbar and 300-500°C based on the above speculation. However, a multistage model is proposed since the Vargbäcken gold bearing quartz veins were apparently formed episodically. The first stage of mineralization containing gold, chalcopyrite, bismuth, and tellurides and was followed by a second stage that involves redistribution of gold and associated minerals (pyrrhotite, sphalerite, and less galena) to higher structural levels during metamorphism and deformation episodes within the shear zone. The fineness of the Vargbäcken gold decreased in the second stage of the ore-forming processes and was apparently superimposed on the first stage of gold mineralization. It would be interesting to undertake further investigation on the whole rock geochemistry and fluid inclusions with an attempt to characterise the chemistry and fluid source of the oreforming fluids. Investigation on the quartz vein. 23.

(31) Fig. 13 Schematic section showing the crustal continuum, metamorphic grade of host rocks of orogenic gold deposits and proposed terminology to describe the various P-T components. Modified after GebreMariam et al., (1995).. paragenesis is required to determine the fluid characteristics and its sources. More trenches at Vargbäcken will help expose the structures for detailed studies of the deformation history and kinematic studies which will improve the understanding of the spatial relationship between structures, magmatism and ore.. Finally, I would like to take this opportunity to appreciate all those whose suggestions, love, support, and encouragements helped in the production of the thesis. My parents, family, and friends are fondly appreciated for their love and believing in me when I stayed away from home.. Acknowledgements I would like to thank Mawson Resources AB for financial support and granting access to Vargbäcken project. Division of Ore geology and applied geophysics, Luleå University of Technology, did also provide finances for the project. For the review of the manuscripts and making valuable suggestion I thank my Supervisor Pär Weihed (Professor). I am grateful to SGU (Sveriges Geologiska Undersökning) Malå for using their logging facilities. 24.

(32) Bergman Weihed, J., (1997): Regional deformation zones in the Skellefte and Arvidsjaur areas. Sveriges Geologiska Allen, R.L., Weihed, P. and Svenson, S-Å., Undersökning Final research report of (1996): Setting of Zn-Cu-Au-Ag massive SGU-Project, 03-862/93, 39 p. sulfide deposits in the evolution and facies architecture of a 1.9Ga marine volcanic Bergman Weihed, J., (2001): Palaeoproterozoic arc, Skellefte District, Sweden. Economic deformation zones in the Skellefte and Geology. 91: 1022-1053. Arvidsjaur areas, northern Sweden. In: Weihed P. (Ed) Economic geology Ash, C. and Alldrick, D. (1996): Au-quartz research 1999 2000. Sveriges Geologiska Veins, in Selected British Columbia Undersökning, C833, p. 46-68. Mineral Deposit Profiles, Volume 2 Metallic Deposits, Lefebure, D.V. and Bergström, U. and Weihed, P., (1991): Structural Hõy, T, (Eds), British Columbia Ministry aspects of some gold mineralizations in of Employment and Investment, Open the Skellefte District, northern Sweden. File 1996-13, p 53-56. Geol. Fören. Stockholm Förh. 113:42–44. References. BABEL Working Group, (1990): Evidence for Bergström, U. and Sträng, T., (1997): early Proterozoic plate tectonics from Karlbladen 231, Malå, In C. H., Wahlgren seismic reflection profiles in the Baltic (ed.): Regional berggrundsgeologisk shield. Nature, 348: 34-38. undersokning sammanfattning av pagaende undersokningar 1996. Sveriger Bark, G. and Weihed, P., (2007): Orogenic gold in Geologiska Undersokning, Rapporter oc the new Lycksele-Storuman ore province, meddelander, v. 89, p. 47-55. northern Sweden: the Palaeoproterozoic Fäboliden deposit, Ore Geology Reviews. Bergström, U., (2001): Geochemistry and v. 32, issue 1-2, p. 431-451. tectonic setting of volcanic units in the northern Västerbotten county, northern Beaudoin, G. and Therrien, R., (2005): Sweden. In: Weihed P. (Ed) Economic Formation of orogenic gold deposits by geology research 1999-2000. Sveriges short-lived pulses of metamorphic fluids Geologiska Undersökning, C833, p. drained by major crustal shear zones. GSA 69-92. Annual Meeting, Program with Abstracts #93991. Billström, K. and Weihed, P., (1996): Age and provenance of hostrocks and Beaudoin, G. and Therrien, R., (2006): 3D ores in the Paleoproterozoic Skellefte numerical modelling of fluid flow in the district, northern Sweden: ECONOMIC Val-d’or orogenic gold district: major GEOLOGY, v.91, p. 1054-1072 crustal shear zones drain fluids from overpressured vein fields. Mineralium Billström, K., Broman, C. and Jonsson, E., Deposita v. 41, p. 82-98 (1997): Evidence for a prolonged fluid history at the Björkdal Au deposit, northern. 25.

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(34) A global synthesis: Ore Geology Reviews, Pitcairn, I.K., Teagle, D.A.H., Craw, D., Olivo, v. 18, p. 1-75. G.R., Kerrich, R. and Brewer, T.S., (2006): Sources of metals and fluids in orogenic gold deposits: insights from the Otago and Groves, D. I., (1993): The crustal continuum model for the late-Archaean lode-gold Alpine Schists, New Zealand. Economic deposits of the Yilgarn Block, Western Geology 101: 1525-1546. Australia. Mineralium Deposita, v. 28, p.366–374 Öhlander, B., Skiöld, T., Elming, S-Å., Claesson, S., Nisca, D.H. and Babel Working Group., (1993): Delineation and Character of Groves, D. I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G. and Robert, F., the Archaean-Proterozoic boundary in (1998): Orogenic gold deposits: a proposed Northern Sweden. Precambrian Research. classification in the context of their crustal 64: p. 67-84. distribution and relationship to other gold deposit types. Ore Geology Reviews. 13: Öhlander, B. and Markkula, H., (1994): 7-27. Alteration associated with the goldbearing quartz veins at Middagsberget, northern Sweden Mineralium Deposita, v. Groves, D. I., Goldfarb, R. J, Robert, F. and Hart, C. J. R., (2003): Gold deposits in 29, p. 120-127 metamorphic belts: overview of current understanding, outstanding problems, Gaál, G. and Gorbatschev, R., (1987): An future research, and exploration Outline of the Precambrian Evolution of significance. Econ Geol 98:1–29 the Baltic Shield. Precambrian Research 35: 15-52. Karvinen, W.O., (1997): The Haveri copper-gold mine property, south finland: Geological Gaa´l, G., (1990): Tectonic styles of Early Survey of Finland Guide, v. 44, p. 20-22. Proterozoic ore deposition in the Fennoscandian Shield. Precambrian Kathol, B. and Weihed, P., (Eds.), (2005): Research 46, 83– 114. Description of regional geological and geophysical maps of the Skellefte Gebre-Mariam, M., Hagemann, S.G. and District and surrounding areas. Sveriges Groves, D.G., (1995): A Classification geologiska undersökning Ba 57. Scheme for Epigenetic Archean Lodegold Deposits; Mineralium Deposita, v, Lundberg, B., (1980): Aspects of the geology 30, p. 408- 410. of the Skellefte field, northern Sweden: Geologiska Föreningens I Stockholm Goldfarb, R. J, Snee, L. W, Miller, L. D, Förhandlingar, v. 102, p. 156-166. and Newberry, R. J., (1991): Rapid dewatering of the crust deduced from ages of mesothermal gold deposits. Nature Mäkelä, K., (1980): Geochemistry and origin of Haveri and Kiipu, Proterozoic strata354:296–298 bound volcanogenic gold-copper and zinc mineralizations from southwestern Goldfarb, R.J., Groves, D.I. and Gardoll, D., (2001): Orogenic gold and geologic time: 27.

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(38) Weight%. 11.90 3.27 4.00 15.18 21.19 18.69 4.07 2.74 18.96. 100.00. Element. CK OK Si K SK Fe K Zn L Ag L Cd L Au M. Totals. Spectrum processing : No peaks omitted. 37.60 7.76 5.40 17.97 14.40 10.85 1.43 0.93 3.65. Atomic%. Sample: Sample 4 Type: Default ID: VBN 98016_46_90. Project 1. 18/09/2007 13:08:56. Appendix A. 31.

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