Volcanic facies and hydrothermal alteration at the Norra volcanigenic massive sulfide deposit

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Volcanic facies and hydrothermal alteration at the Norra volcanigenic massive sulfide

deposit

Storuman area, Sweden

Meseret Gebreyesus

Geosciences, master's level (120 credits) 2020

Luleå University of Technology

Department of Civil, Environmental and Natural Resources Engineering

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Abstract

The Skellefte District, with a long and rich history of base metal extraction that commenced as early as 1924, is rated amongst the leading mining provinces in Sweden and Europe. The district represents a Paleoproterozoic volcanic arc hosting more than 85 known (gold-bearing) VMS deposits and several economic grade orogenic gold occurrences.

The Barsele property lies at the intersection of the Skellefte District and the Lycksele-Storuman (Gold Line) metallogenic trends. The Project includes the Central-Avan-Skiråsen intrusion- hosted gold deposits and the Norra VMS deposit.

The mineralizations at Barsele area were discovered by Terra mining in 1980, and since then the area has been the focus of various exploration companies including MinMet, Northland Resources, Orex Minerals and Agnico Eagle. Despite these historic interests, only little work has been done to combine volcano-sedimentary facies studies with lithogeochemistry to better understand the setting and genesis of the deposits.

This study intends to resolve the stratigraphic architecture of the Norra volcano-sedimentary succession and establish a better understanding of the alterations caused by fluid system which formed VMS mineralization. This was performed by integrating the results obtained from volcanic facies logging of two profiles and immobile-element lithogeochemical investigations which were supplemented by petrographic analysis.

On the basis of volcanic facies analysis it was revealed that, the massive to semi-massive sulfide mineralization at Norra is hosted within a sub-vertically dipping volcano-sedimentary succession. The footwall is comprised of sheared mafic and felsic porphyritic intrusions and lavas, foliated sulfidic and graphitic black shales and mudstones. The hanging wall is generally dominated by feldspar-porphyritic basaltic-andesite and dacitic intrusions.

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Lithogeochemical studies indicate that the coherent volcanic rocks belong to three distinct magma series. Feldspar+quartz-porphyritic dacite and basaltic-andesites are comagmatic, with transitional to calc-alkaline affinity, whereas feldspar-porphyritic basalt, which encloses the mineralized facies have a tholeiitic affinity. Post mineralization doleritic dykes belong to a third separate magmatic suite of transitional affinity. The mineralization occurs close to the lithological contact between the dacitic intrusion at the stratigraphic footwall and the feldspar-porphyritic basalt. The contact also marks a chemical transition from a footwall of transitional to calc-alkaline magmatic affinity to tholeiitic hanging-wall. Structural interpretation using chemostratigraphic correlation indicate that the ore lens sits on the north-eastern limb of an upright syncline.

Clast-rich massive to semi- massive sulfide ore textures, and the associated style and intensity of alteration suggest that subseafloor replacement was the dominant type of VMS mineralization.

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Kathol and Weihed, 2005). It is generally regarded as low to medium metamorphosed felsic magmatic region bordering a continental landmass to the North (Arvidsjaur Volcanics) and a deep-marine sedimentary basin to the South (Bothnian basin)(Allen et al., 1996). Bauer and Imaña, (2017) suggested that the metavolcanic rocks at Norra may possibly constitute a western continuation of the Skellefte Group metavolcanic rocks, which hosts or directly underlie most VMS deposits of the Skellefte District. However, little is known of the stratigraphic setting of the deposit and the structural geology of the area is complex (Giroux et al., 2012). Moreover a radiometric dating constrained from a metadacite from Barsele area, which was mainly noted to form part of the Bothnian Supergroup, gave age of 1959±14 Ma (Eliasson et al., 2001). This shows that the metavolcanic rocks at Barsele are considerably older than their counterparts at the Skellefte District.

The Barsele property sits at the intersection of the Skellefte District and the Lycksele-Storuman (Gold Line) metallogenic trends in Northern Sweden (Fig.2). The Lycksele-Storuman ore district is located southwest of the Skellefte District. It has been termed the ‘Gold Line’ because of a NW-trending linear Au anomaly on till geochemistry maps (Bark, 2008). It is situated in the northern margin of the Bothnian Basin which is a large structural depression extending from northern Sweden to southern Finland. The Bothnian Basin is believed to have formed in the early Paleoproterozoic when intra-continental rifting of the Archaean plate created a sediment trap on top of Archaean formations (Kumpulainen, 2009).

Several companies have carried out exploration campaigns in the Barsele area in attempts to unlock its economic potential. The deposit was first identified by Terra mining in 1980 through reconnaissance geochemical till sampling program. Additional fieldwork and drilling of till anomalies were carried between the years 1989-1998 which led to the discovery of the Barsele Central, Avan and Skiråsen zones and the Norra VMS occurrence.

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3 Fig. 2 Barsele Location Map in relation to the Skellefte District and the Lycksele-Storuman (Gold Line) metallogenic trends (Map Credit Giroux et al., 2012).

In 2003, MinMet took over the project after Terra mining abandoned it due to unfavorable estimation of the potential economics of mining low-grade gold resources. In the following years, Northland Resources became the main actor on the project by acquiring 60% interest in the property in 2004 and 100% in the year 2006. The property was 100% owned by Orex Minerals in the year 2010 and between the years 2010 to 2015 the company performed different exploration programs which include geophysical surveys, trenching, drilling, and calculated a NI 43-101 mineral resource estimate (Giroux et al., 2015). In 2015, Agnico Eagle acquired 55% of the Barsele project and have since then been solely responsible in managing an exploration program that has a near-term objective to discover and define a 2.0 to 2.5 million ounces gold deposit (Pelletier and Richard, 2018)

Despite the area being the target of historic exploration interests, there has been limited in-depth studies aimed at integrating petrographic analysis, volcanic facies logging and lithogeochemistry.

Furthermore, most work to date has focused on the intrusion-hosted gold deposit rather than VMS style mineralization such as Norra. Thus, the objectives of this study are:

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• To characterize the volcanic and sedimentary units of the host succession by using volcanic facies logging and immobile element lithogeochemistry.

• To investigate the mineralogical and geochemical changes resulting from hydrothermal alteration associated with sulfide mineralization.

• To define the geochemical vectors to the mineralization that may aid continued exploration.

• To test if the metavolcanic rocks at Norra are chemically similar to those of the Skellefte Group.

• To test and refine a conceptual model for a major fold structure in the study area based on modelling of chemostratigraphic groups by Agnico consultant Marcello Imaña.

• To assemble a ”reference sample collection” including the main lithological units of the local area for future reference.

2. Geology

2.1 Regional Geology

The Skellefte District an important ore-producing region located in northern Västerbotten and southern Norrbotten counties, northern Sweden. It consists of ~1.9–1.87 Ga supracrustal and associated intrusive rocks of the Fennoscandian Shield (Fig.3), which were deformed and metamorphosed during the Svekokarelian orogeny at 1.9-1.8 Ga (Lundström et al., 1997, 1999;

Kathol and Weihed, 2005).

The supracrustal rocks of the Skellefte District can be subdivided into the Skellefte and Arvidsjaur Groups which are mainly dominated by metavolcanic rocks and the predominantly metasedimentary Vargfors Group (Skyttä et al., 2012). Allen et al. (1996) suggested that the Skellefte District is made up of a simple stratigraphic succession consisting of the Skellefte Group metavolcanic rocks - being the lowest unit and the host of the majority of the VMS deposits of

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5 the district – and the overlying Vargfors Group metasedimentary rocks (Fig.4). It was further proposed that this stratigraphic succession was later exposed to folding, uplifting and intrusion of Jörn- and Revsund-type granitoids. The Skellete Group, a submarine metavolcanic rocks of felsic to intermediate composition, has 3km maximum measured stratigraphic thickness but its base is not exposed (Allen et al., 1996), even though some workers suggest that the Bothnian Supergroup could constitute the basement for the Group (Skyttä et al., 2012). The Bothnian Supergroup is further described below.

Vargfors Group mainly consists of metasedimentary rocks of turbiditic and coarse clastic nature which are believed to be the marine equivalent of the Arvidsjaur Group (Kathol and Weihed, 2005). The original grainsizes of the metasedimentary units varies from fine sandstone to coarse conglomerate and volcanic breccia-conglomerate units that have conformable contact relationships (Allen et al., 1996).

The Arvidsjaur Group metavolcanic rocks are of felsic to intermediate composition, have subaerial and continental depositional environments and occur north of the Skellefte Group (Kathol and Weihed, 2005). Various radiometric dating of parts of Arvidjaur Group and the Vargfors Group suggest that they have similar ages (Allen et al., 1996).

To the east and south, the Skellefte District borders the Bothnian Supergroup which consists of metasedimentary units, mainly consisting of volcanogenic turbiditic greywackes and argillites intercalated with metavolcanic rocks. This succession is interpreted to underlie the Skellefte Group. However, the nature of the transition between the Skellefte and Vargfors Groups in the north and the Bothnian Supergroup in the south is unclear and appears gradational (Kathol and Weihed, 2005).

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northwestern part of the lithotectonic unit. Well-preserved, metamorphosed siliciclastic sedimentary rocks of uncertain age, including arkose (Naggen group), occur in the southern- most part of the unit, whereas conglomerate, arkose and rhyolite (Dobblon group) are spatially and temporally associated with the 1.8 Ga plutonic suite in the NW (Figs 4.3&4.4).

The typically high-grade metamorphic rocks in the Both- nian Supergroup display broad, mildly negative to mildly positive magnetic anomalies over much of the lithotectonic unit (Fig. 4.5a). For this reason, the structural patterning expressed in the magnetic data is generally not very distinct. By contrast,

in the coastal areas in the northeastern parts of the unit, between Umeå and Skellefteå (Fig. 4.3), the metasedimentary rocks are characterized by an intense magnetic banding deﬁn- ing an overall north–south to NNE–SSW structural grain related to intercalations of graphite- and pyrrhotite-rich argil- litic horizons (Fig. 4.5a). Deviations in the prevailing trends are caused by deformation zones with an ENE–WSW strike, including, for example, the Burträsk shear zone, and by open large-scale folding close to Umeå (Fig. 4.5a). The metasedimentary rocks are also spatially associated with positive gravity anomalies (Fig. 4.5b).

500 600 700 800

6900700071007200730074007500

0 10 20 30 40 50 km

Luleå Norrvijaur

Bräcke

Naggen Revsund

Östersund Sollefteå

Skellefteå Burträsk Fäboliden

Älvsbyn

Jörn Moskosel

Norsjö Storuman

Vilhelmina

Åsele

Strömsund

Härnösand

Boden

Piteå

Dorotea Dobblon

Sorsele

Boliden

Sundsvall Arjeplog

Arvidsjaur

Malå

Lycksele Knaften Kristineberg

Robertsfors

Umeå

Örnsköldsvik

Timrå

Nordmaling

Bothnian Bay

Gulf of Bothnia BSZ KADZ

DN SZ

JGC VR

SS

VRSS

VR SS

ISZ HSZ

HSZ

HSZproto-HSZ Transition

Belt

Bothnian basin

Skellefte district Skellefte–Arvidsjaur magmatic province

BSZ KADZ

DN SZ

JGC VR

SS

VRSS

VR SS

ISZ HSZ

HSZ

HSZproto-HSZ Transition

Belt

Bothnian basin

Skellefte district Skellefte–Arvidsjaur magmatic province

Deformation zone, type and sense of movement not specified

Volcanic and subordinate siliciclastic sedimentary rocks (1.90–1.86 Ga) Plutonic rocks (1.90–1.85 Ga) Plutonic and subordinate supracrustal rocks younger than the Skellefte–

Arvidsjaur magmatic province and Bothnian basin (1.81–1.76 Ga)

Plutonic rocks (1.95–1.93 Ga) Siliciclastic sedimentary and subordinate volcanic rocks (1.96 or older to 1.86 Ga) Impact site

BSZ = Burträsk shear zone DNSZ = Deppis–Näsliden shear zone HSZ = Hassela Shear Zone ISZ = Ismunden shear zone JGC = Jörn granitoid complex KADZ = Karesuando–Arjeplog

deformation zone proto-HSZ = proto-Hassela Shear Zone VRSS = Vidsel–Röjnoret shear system

and its southern continuation to Nordmaling

NORWAY

F I N L A N D NORWAY

F I N L A N D

Fig. 4.2. Overview of the spatial distribution of the Bothnian basin, the Skellefteå–Arvidsjaur magmatic province and the metallogenic Skellefte district. The rocks are affected to variable degrees by metamorphism under low-pressure conditions.

BOTHNIA–SKELLEFTEÅ LITHOTECTONIC UNIT 85

by guest on January 13, 2020 http://mem.lyellcollection.org/

Downloaded from

Fig. 3 Regional geology and spatial relatioships of the Bothnian basin, the Skellefte District and Skellefteå-Arvidsjaur magmatic provice. The map also shows the geological relationship between Storuman area and the Skellefte distric. (Skyttä et al., 2020)

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DESCRIPTION OF REGIONAL GEOLOGICAL AND GEOPHYSICAL MAPS OF THE SKELLEFTE DISTRICT ... 27

1890 1880 1870 1820 1790

depositsAu

Vargfors Group

Sedimentary formation Upper volcanic formation

Lower volcanic formation

Skellefte Group Ma

VMS Porphyry

deposits depositsNi Mo, W,

Sn, Li

? Basement ?

Massive sulphide

Porphyritic volcanics

Porphyritic intrusive

GII-GIII

GI metamorphism

Conglomerate Lime cemented

conglomerate Greywacke

Reworked volcanics

Felsic intrusions and lava

Mafic volcanics

Felsic volcaniclastics Skellefte

granite Revsund granite

Phyllite Jörngranitoid

Gallejaur intrusive

Weihed et al. 1992

+ +

+ + + +

+ +

+ + + +

+ +

+

+ +

Bothnian Group

Vargfors Group

Elvaberg Formation

Revsund SE

Dödmanberg Conglomerate

Abborrtjärn Conglomerate Mensträsk Conglomerate

Skellefte Group Gallejaur

Volcanics Arvidsjaur

Group

Early Jörn LateJörn –

Gallejaur NW

1873 Ma

1882 Ma

1890 Ma

? ? ?

?

Disconformity Massive sulphide ore

Allen et al. 1996 Granitoid rocks

Jörn–Arvidsjaur granitoids Revsund granitoids Sorsele granitoids

Polymict conglomerate and sandstone Basaltic volcanic rocks

Skellefte Group

Basement

Arvidsjaur Group Vargfors Group Dobblon Group

Ultrabasic volcanic rocks Greywackes and pelitic sediments Rhyolitic–andesitic volcanic rocks Legend

Terrestrial environment Marine

environment

Lundberg 1980

E F

G

(PMS)

(Jörn GI suite)

(Bothnian Super- group)

Angular unconformity Stockwork vein ore

Fig. 8. Continued.

commonly turbiditic sedimentary rocks are interpreted to interfinger with subaqueous volcanic rocks and sedimentary rocks of the Skellefte and Vargfors Groups.

Upwards and laterally to the north, the Skellefte Group rocks pass into mainly subaerial volcanic sequences of the Arvidsjaur Group. The marine equivalent of the Arvidsjaur Group is the Vargfors Group, which consists mainly of coarse clastic and turbiditic sedimentary rocks and mafic volcanic rocks, deposited on the rocks of the Skellefte and Bothnian Groups or the lower parts of the Arvidsjaur Group.

Differences between the Skellefte, Vargfors, and Bothnian Groups are indicated by different geochemical affinities of basic volcanic rocks. However, as these rocks occur only sparsely, especially within the Bothnian Supergroup, the boundary between the Skellefte and Vargfors Groups on the one hand and

the Bothnian Supergroup on the other is drawn some- what arbitrarily on the map. It runs from lake Dob- blon (/), south of river Vindelälven and south of Kristineberg to Norsjö and Bastuträsk, from where it turns north. This artificial line is generally interpreted as a lateral transition from one group to the other. Rocks of the Skellefte and Vargfors Groups have their coeval counterparts within the Bothnian Supergroup. In places, the Skellefte/Bothnian Groups or Vargfors/Bothnian Groups contain laterally equivalent rock associations. In terms of sequence stratigraphy these associations should be considered as one unit, which probably consists of several, individual sequences.

At different stages and at different levels throughout the sedimentary and volcanic evolution within the map area, the supracrustal rocks have been intruded The geological setting of Skellefte District is a topic of a continuous debate. Most researchers, based on geochemical signatures regarded the Skellefete District as an oceanic island-arc (Allen et al., 1996). According to Vivallo and Claesson (1987) it was interpreted as an intra-arc rift within a continental margin. Allen et al. (1996), taking into consideration the overall high abundance of felsic composition of the Skellefte group, have argued that the Skellefte volcanic rocks are formed due to extension and extensive magmatism in a marine arc that emerged on continental or mature arc crust.

Fig. 4 A NW-SE cross-section of Skellefte District showing regional time-stratigraphy in relation with lithostratigraphy and location of VMS deposits (Allen et al., 1996)

2.2 Geology of Storuman Area

The Barsele Property is located within an area of Paleoproterozoic supracrustal rocks and associated intrusive rocks. Typical greywackes which are formed in a turbidite system dominate the area (Kumpulainen, 2009). The succession of metavolcanic and metasedimentary rocks at Barsele area are generally regarded to form part of the Härnö Formation (Pelletier and Richard, 2018) of the Bothnian Supergroup - supracrustal rocks which are dominantly greywackes and

7

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8 mudstones associated with subordinate coherent volcanic rocks of mafic to felsic composition including pillow lavas and volcanoclastics (Wasström, 2019). The Knaften area and the area between Lake Storjuktan and Gunnarn are localities where most of the mafic and metavolcanic rocks of the Bothnian Supergroup were documented in succession dominated by metasedimentary rocks (Kathol and Weihed, 2005). The volcanic rocks at Knaften area are commonly basaltic to andesitic in composition and have MORB- to island arc-type magamatic affinity (Wasström, 1990).

The area was intruded by three distinct granitoid units which have been described with respect to the Svecokarelian orogeny as early, late, and post (Claesson and Lundqvist, 1995). A calc- alkaline suite dominated by tonalite and granodiorite belonging to the early orogenic granitoids are considered most important in terms of their association with gold mineralization (Pelletier and Richard, 2018).

The Norra gold rich volcanic-hosted, semi-massive to massive sulfide is quite different from the intrusion-hosted gold mineralizations in the Barsele area in terms of mineralization style and host lithology. Quartz-phyric volcaniclastic unit, which occurs within successions metavolcanic rocks and as interbeds in pelitic metasedimentary rocks of the Härnö Formation, mainly host the mineralization (Pelletier and Richard, 2018). A detailed geological map of Barsele area is shown in figure 5.

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9 Fig. 5 Geological map of the Barsele area showing the main lithological units (modified after Krispinsson, 2018)

2.3 Volcanogenic Massive Sulfide (VMS) deposits

Volcanogenic massive sulfide (VMS) deposits also described as volcanic-hosted massive sulfide (VHMS) deposits, are stratiform or stratabound bodies of base metal sulfide ore and other accessory minerals, which are formed on or near the seafloor by precipitation from hydrothermal fluids in volcanically active marine environments through quenched mixing with cool seawater.

They are important sources of copper, zinc, lead, gold, and silver (Hannington, 2014)

The volcano-sedimentary rocks which are associated with VMS deposits are believed to have been leached by the percolating hydrothermal fluid to become the primary sources of metals, this has been confirmed through mass balance studies and through the consistency of metal assemblages in various VMS deposits with the expected metal contents and ratios in the associated

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10 primary volcanic rocks (Robb, 2005). But recent sulfur stable isotope and fluid inclusion studies of some VMS deposits imply that magmatic fluid component had also made important metal contributions (Robb, 2005).

VMS ores consists of >60 percent sulfides, predominantly pyrite and/or pyrrhotite with different amounts of Cu, Zn, and Pb sulfides (Hannington, 2014); gangue minerals commonly consists of quartz, barite, anhydrite, iron oxides, chlorite, sericite, and talc (Shanks and Koski, 2012).

Tornos et al. (2015) depending on previous numerous studies noted that VMS deposits show a limited number of mineralization style which include sulfide mounds and associated chimneys, stratiform exhalative bodies and replacive mineralization (Fig.6). They further suggested that these styles of mineralization are generally determined by the physical and chemical property of the host rock, the temperature and composition of the hydrothermal fluid, and the chemistry of the depositional environment.

Hydrothermal fluids which are responsible for the VMS ore formation also produce alteration zones that envelop the ore three-dimensionally (Robb, 2005). These zones are mainly large volumes of volcano-sedimentary rocks which are hydrothermally altered to variable intensity (Allen, 2002). Alteration zones related to VMS mineralization usually show characteristic zonation (Fig.7), which are represented by specific alteration mineralogy (Shanks and Thurston, 2012). The alteration zones in most cases show a uniform patterns of sericite-rich and sericite ± chlorite ± carbonate halos enclosing a relatively smaller zone, adjacent/close to the ore which is rich chlorite, pyrite, and quartz assemblages (Large et al., 2001).

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11 Fig. 6 Schematic diagram showing the different styles of VMS mineralization. (a) Mounds and black smoker chimneys in oxic environments; (b) Brine pools; (c) Mound and stratiform sulfides in regional anoxic environments;

(d) Subseafloor replacement (Tornos et al., 2015) .

VMS deposits are associated with a variety of tectonic settings predominantly in zones of extension and active volcanism along mid-ocean ridges, but also common at island arcs, back- arc basins and fore-arc troughs (Robb, 2005). Barrett and MacLean (1999) suggested that based on geochemical signatures VMS deposits formed within Phanerozoic terranes are associated with at least three general tectonic setting, (1) Continental crust rifting in marine basins where volcanism was affected more by the upwelling asthenosphere and associated continental crust contamination, than by subduction-related magmatism, (2) Rifted mature arc or early formed back arc basin where subduction-related influences on volcanism are high but continental crust contamination is limited and (3) Subduction-related island arc which is formed entirely within the oceanic crust.

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12 Fig. 7 Characteristic hydrothermal alteration zoning in a VMS district after Goodfellow (2007)

VMS deposits in the Skellefte District, despite their wide range mode of occurrence, are believed to have similar type of alteration and ore types which were deposited under similar geological setting. They are generally made up of sulfide-rich ore lenses which are associated with pyritic dissemination, with or without pyrite vein network mineralization (Allen et al., 1996).

Due to their compact nature VMS deposits are relatively small exploration targets (Gibson et al., 2007), combined with their multi-metal character and high metal prices in global markets make them economically viable exploration targets in present time and for some more years to come (Schlatter, 2007).

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13 3. Methodology

The results and interpretations in this thesis are generally based on detailed volcanic facies logging, petrographic and geochemical analysis of representative samples collected from drill cores during logging of two chosen boreholes NOR17007 and NOR18002.

3.1 Volcanic Facies Logging

The main goals of the volcanic facies logging at Norra are to (1) recognize and define the principal facies groups of the volcano-sedimentary succession and the relationships between them, with special emphasis placed on detailed documentation of contact relationships between the different units and facies (2) obtain a detailed textural composition analysis of the different facies groups, which can have significant implications for the understanding of the genetic processes and mode of emplacement and (3) identify the facies group which is proximal to the VMS mineralization at Norra and the facies group favorable for hosting of VMS.

The Norra volcano-sedimentary succession has been examined by detailed geological drill core logging from two bore holes (NOR17007 and NOR18002).The drill cores were logged by graphic logging techniques (Appendix 3). Special emphasis was placed on characterizing different alteration styles, identification of primary rock textures thereby distinguishing their facies characteristics using the guidelines described in McPhie et al. (1993).All rocks in the Barsele area are metamorphosed with the exception of minor late doleritic dykes. However, to emphasize the primary features of the rocks the meta-prefix was dropped. Hence rocks are described using their pre-metamorphic, lithological and lithofacies descriptive sedimentary and volcanic names.

Moreover, terminologies such as 'sandstone' and 'siltstone' are commonly used in describing rocks of sedimentary origin, nevertheless in the context of McPhie et al. (1993) only refer to grain-size classifications regardless their genetic implications.

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14 3.2 Petrographic Analysis

Rock textures, alteration and mineralogy have been analyzed from 39 petrographic thin sections from Norra. Lithological controlled sampling was carried out during core logging and the samples were made to represent all the lithological units identified. The petrographic samples were prepared at AGNICO Eagle’s core shack in Stensele and sent to Vancouver Petrographics, Canada, for production of thin sections with a preferred thickness of 30μm. The petrographic investigations was performed at Luleå University of Technology (LTU) using a NIKON Light Optical Microscope with a digital camera attached. The thin sections were examined through transmitted and polarized light and appropriate photos were taken to emphasize characteristic textures, alteration types and mineralogy.

3.3 Lithogeochemistry

The main purposes of the lithogeochemical study of the volcanic rocks at Norra is to (i) distiguish the chemostratigraphy of the rocks, (ii) analyze if there is any correlation between chemostratigraphic and volcanicfacies units,and (iii) understand, characterize, quantify, and interpret the style and intensity of alteration, and thereby to identify the relative location of the stratigraphic horizons which might host VMS-mineralizations (ore horizon).

A total number 40 drill core samples were taken which were believed to represent all the major rock types and alteration facies that were identified earlier through drill core logging. All samples were polished using sand paper to remove any contaminant from the drilling process. The samples were then sent to ALS Minerals, Piteå (Sweden), where further sample preparation was performed prior to chemical analysis at ACME commercial laboratory in Canada. For analytical quality control purposes certified reference materials were included with each sample batch.

Most of the major and trace elements were determined with inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis method after a lithium borate fusion and nitric acid digestion. Lithium borate fusion is generally used for samples which are difficult to dissolve in

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15 acid and is most appropriate for digestion of samples which have high content of aluminosilicates, carbonate rocks, some rare-earth phosphates, rare-earth fluorides and sulfides. Inductively coupled plasma mass spectrometry (ICP-MS) analysis method, which enables determinations of heavier trace element at extreme detection limits, was used, after aqua regia digestion, to determine REE and refractory trace elements. Total carbon and sulfur were determined by the LECO combustion analysis method.

Boron was analyzed at ALS laboratory in Vancouver, Canada using ICP-AES analysis after samples were digested using sodium hydroxide fusion technique.

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Fsp-porphyritic basalt

Fsp-porphyritic basaltic-andesite

Qz-Fsp-porphyritic dacite

Sulfide ore

Silicate facies iron formation

Peperite

Hyaloclatitic breccia

Doleritic dyke NOR17007

NOR18002

7216700N 7216800N

100Z

200Z

300Z

400Z

Sample points Boreholes

Crystal-rich pumiceous sandstone breccia SW NE

Fig. 8 Simplified geological cross-section of Norra volcano- sedimentary succession showing the massive sulfide lens and the associated volcanic and sedimentary units.

Scale: Each grid square is 100x100m NOR17007: N 7216853 E 617060 NOR18002: N 7216670 E 616878

Projection: SWEREF99 TM

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NOR17007 NOR18002

Fsp-porphyritic basalt

Qz-Fsp-porphyritic dacite

Fsp-porphyritic basaltic-andesite Banded mudstones

Banded graphytic shales

Sulfide ore Peperite

In situ hyaloclastitic breccia Silicate facies iron formation

Crystal-rich pumiceous sandstone breccis Lithic-rich graphytic shale

100m

200m

300m

400m

500m 400m 300m 200m 100m

Fig. 9 Digitized drill core logs of the profiles studied (NOR17007 and NOR18002) showing the different lithologies mapped at Norra. Position of the drill holes is given in Fig.8.

i i

a

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18 4. Results

4.1 Volcanic facies logging

The most abundant units in the Norra volcano-sedimentary succession, listed in descending order of their abundance, are (1) coherent porphyritic intrusions, (2) in situ hyaloclastite breccia and peperite, which occurs at the margins of many intrusions (3) black shales and sulfidic mudstones (4) normal-graded pumiceous breccia and crystal-rich breccia-sandstone (5) post mineralization doleritic dykes (6) massive to semi-massive sulfide ore and minor occurrences of silicate facies iron formations.

The Norra volcano-sedimentary succession is generally dominated by compositionally and texturally diverse porphyritic intrusions or lavas which is associated with hyaloclastite breccias and peperites. Non-volcanic sedimentary facies rocks such as black shales and mudstones occur usually intercalated with the volcanic rocks.

Volcaniclastic mass flow facies, which is the most common and most favorable VMS ore host facies group in many VMS occurrences in the Skellefte District (eg.,Allen et al., 1996), is a minor component at Norra.

Depending on their compositional and textural similarities the Norra volcano-sedimentary succession could be grouped into three main facies associations i.e. (i) Facies #1 (F1) the primary volcanic facies which include coherent porphyritic dacite, basalt, basaltic-andesite intrusions and associated breccia facies (Peperites and in situ hyaloclastites) and (ii) Facies #2 (F2) the volcanigenic and non-volcanic sedimentary facies association, which include black shales and sulfidic mudstones and (iii) Facies #3 (F3) resedimented volcaniclastic facies association which mainly represented by a pumiceous and crystal-rich breccia-sandstone .

The graphitic shale clast-supported sulfide breccia mineralization is enclosed in the feldspar- porphyritic basalt unit. The mineralization, which is intercepted by only one of the two boreholes

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19 studied, occur near the contact between the chlorite+carbonate-altered, feldspar-porphyritic, sheared basalt host and quartz-feldspar-porphyritic dacite-I unit.

The normal-graded bedding observed in a breccia-sandstone unit (NOR18002 @379m) is the only younging-direction indicator noted at Norra. Based on this information it is proposed that the feldspar-porphyritic basaltic-andesite and feldspar + quartz -porphyritic dacite-II units occur in the hanging wall of the VMS system and for that matter the feldspar + quartz -porphyritic dacite-I, which is associated with crystal-rich sandstone and pumiceous breccia-sandstone, occupies the stratigraphic footwall.

4.1.1 Qz-Fsp porphyritic dacite-I (F1)

This unit is the lowest observed unit in the footwall, 100m thick and is overlain by lithic-rich graphitic black shale, normal-graded crystal-rich and pumiceous sandstone sedimentary facies association. Chemically it is mainly of dacitic composition and it is generally dark grey, moderately foliated, strongly and evenly feldspar+quartz-porphyritic (Fig.10a) and contains abundant sulfidic mudstone intercalations with pyrrhotite laminations (~3vol%). Quartz phenocrysts with sizes 3 to 5mm comprise 5 to 15vol% of the rock whereas feldspar phenocrysts have size range of 1 to 4mm comprise 10 to 15vol% of the rock. Generally sericite+chlorite+calcite+quartz±epidote±zosite is the common alteration mineral assemblage.

calcite+epidote+chlorite fracture infill associated with pyrrhotite and chalcopyrite mineralization is also common (Fig.11i). Petrographic analysis of some thin sections from this unit show numerous primary volcanic microtextures such as embayed quartz phenocrysts which are occasionally broken (Fig.11a). It shows a sharp contact with the adjacent units.

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20 4.1.2 Normal-graded crystal-rich and pumiceous breccia-sandstone (F3)

The basal part of this unit contains a thin interval of lithic-rich graphitic black shale whereas the rest part of the succession, which makes up most of this facies association (~30m) (Fig.10s), is dominated by normal-graded crystal-rich sandstone alternating with pumiceous breccia- sandstone. Each unit show a sharp contact with the adjacent units.

The black shale is around 4m thick, strongly bedded and rich in clasts which are quartz+feldspar- porphyritic and of dacitic composition.

The pumiceous breccia-sandstone is light grey, massive with more or less angular pumiceous clasts and crystal-enriched in the matrix. Moderate, patchy to pervasive sericitization and fracture-controlled, moderate chloritization are the most common alteration types observed, however locally it is rich in lath-shaped amphiboles and biotite wisps replacing pumiceous clasts.

The sandstone unit is crystal-rich, massive to normal graded, and well sorted. This facies is overlain by the feldspar-porphyritic basalt.

4.1.3 Feldspar-porphyritic basalt (F1)

This unit is basically encloses the mineralized interval at Norra, and separates the two compositionally and texturally similar, porphyritic dacite units. Texturally it is strongly porphyritic, mostly has a fine grained dark-grey groundmass with visible texture of subhedral lath-shaped feldspar phenocrysts (3 to 5mm) ranging from 5 to 15vol% (Fig.10g & 10h).Three different contact relationships can be recognized, and these include (i) basaltic peperite contact towards mudstone, with gradual transition to the main coherent rock, (ii) sharp towards adjacent sedimentary units, typically graphitic black shales/mudstone and (iii) less commonly hyaloclastitic (Fig.10e). It is mainly fine grained beige to grey in color, strongly sheared and highly carbonate- , chlorite- and talc-altered at the contact towards mineralization. Common alteration mineral associations include chlorite+calacite+talc+epidote±zosite (Fig.11m). The rest of the unit is

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21 weakly foliated with moderately patchy to pervasive chlorite alteration. Quartz veins are commonly associated with calcite+chlorite veinlets. Blobs of arsenopyrite occasionally occur in association with sphalerite veins which have pyrrhotite rims and minor chalcopyrite disseminations.

4.1.4 Basaltic peperite (F1)

Basaltic peperite represent a clast-to matrix –supported unit containing monomict, blocky and sharply angular lava clasts which commonly exhibit jigsaw-fit texture. It consists of abundant concave-fractured clasts, which are clast shapes typically developed due to quench fragmentation (McPhie et al. 1993) (Fig. 10j), and they are generally of basaltic composition and occasionally show glassy chilled margins (Fig.10i). The sedimentary component of the peperite is generally massive without any visible bedding or laminations. Thickness of this unit ranges between 5 to 40m and mostly occurs in close association with the feldspar-porphyritic coherent basalt. It commonly has a sharp contact with the adjacent sedimentary unit but mostly a gradational contact to the coherent basaltic unit. Feldspar phenocrysts has epidote core. Fracture controlled to pervasive, moderate chloritization is the most common alteration.

4.1.5 In situ basaltic hyaloclastite breccia (F1)

The thickness of this unit range from 2 to 45m. It is generally massive to weakly foliated, greenish polyhedral clast-supported, monomict with jigsaw-fit texture and occurs typically at the margins of the feldspar-porphyritic basalt unit. Clasts are of basaltic composition with no visible phenocrysts (Fig.10e). Moderate to strong pervasive chlorite is the most common alteration with pyrrhotite veinlets which can constitute up to 15vol%. This unit alternates with 3 to 5m thick, sulfide mud matrix supported basaltic peperite. It has sharp contacts with the adjacent units; a graphitic black shale and pumiceous breccia sandstone. It is commonly restricted to the margins of basaltic intrusion and associated with the basaltic peperite.

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(a) (b) (c)

(d) (e)

(f)

NOR17007@019m NOR17007@142m NOR17007@270m

NOR17007@262m NOR18002@052m NOR18002@362m

Qz Cal

Qz Qz

Cal

Qz

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(g)

(h) (i)

(j)

(k) ))

(l)

NOR17007@440m NOR17007@440m NOR17007@066m

NOR17007@071m NOR17007@213m NOR17007@162m

Pl

Concave quench clast

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(m)

(n)

(o)

(p)

(q)

(r)

NOR17007@176m NOR17007@247m NOR17007@130m

NOR17007@381m NOR17007@077m NOR18002@069m

Cal

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(s) (t)

NOR18002@395m NOR18002@191m

Fig. 10 Photographs from drill core slabs of volcanic and sedimentary rocks from Norra. The white scale bars are 2cm long. (a) NOR17007@019m. Feldspar+quartz-porphyritic dacite-I (F1). Strong and evenly quartz-porphyritic with sericite-altered, fine-grained and homogeneous groundmass. Quartz phenocrysts are dark grey, rounded and broken. (b) NOR17007@142m. Feldspar+quartz-porphyritic dacite-II (F1). Strongly quartz-pophyritic with sericite-altered, fine-grained, homogenous groundmass. Quartz phenocrysts are dark grey, rounded and broken and have calcite in the rims and fractures. (c) NOR17007@277m. Dacitic hyaloclastite breccia (F1). The blocks are weakly porphyritic.

Quartz phenocrysts are dark grey, rounded occasionally with calcite rims. Abundant calcite veining. (d) NOR18002@052m. Feldspar+quartz-porphyritic dacite-II (F1). Strongly foliated and strongly porphyritic. Quartz phenocrysts are dark grey, rounded and occasionally with calcite rim.

Homogeneous groundmass with weak pervasive chlorite and fabric controlled sericite alterations. (e) NOR18002@362m. Feldspar-porphyritic basalt hyaloclastite (F1). Pale greenish, angular monomict and pervasively chlorite-altered clasts with glassy matrix. Pervasive jig-saw fit texture. (f) NOR17007@262m. Feldspar-porphyritic basaltic-andesite hyaloclastite (F1). Light grey, angular monomict and pervasively Sericite altered clasts with glassy matrix. Pervasive jig-saw fit texture.

Abundant calcite veinlets.

Qz

(u)

Apy

NOR17007@259m

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(g) and (h) NOR17007@440m. Feldspar-porphyritic basalt (F1). Strongly porphyritic. Feldspar phenocrysts are subhedral and aligned in a patchy chlorite-sericite-altered groundmass. (i) NOR17007@066m. Feldspar-porphyritic basalt peperite (F1). Pale greenish, monomict and pervasively chlorite-altered clasts with sediment matrix and chilled margins. Pervasive jig-saw fit texture. (j) NOR17007@071m. Basaltic peperite unit (F1). Glass matrix supported with concave quench clasts. (k) NOR17007@213m. Feldspar-porphyritic basaltic-andesite (F1). Strongly porphyritic. Feldspar phenocrysts are subhedral and randomly oriented in a patchy chlorite- epidote-altered, homogeneous and fine-grained groundmass. (l) NOR17007@162m. Lithological contact showing fine-grained basaltic dyke intruding feldspar+quartz-porphyritic dacite-II. The dacite is strongly foliated with abundant fabric controlled calcite veining. (m) NOR17007@176m.

Massive, lithic-rich graphitic black shale (F2) with porphyritic dacite clasts as large as 4cm. (n) NOR17007@247m. Strongly laminated graphitic shale (F2). Abundant and concordant pyrrhotite and sphalerite laminations. (o) NOR17007@130m. Strongly laminated sulfide rich mudstone (F2) with some faulted and boudinaged layers. Abundant concordant pyrrhotite and sphalerite laminations. (p) NOR18002@069m. Massive, medium grained and unaltered doleritic dyke. (q) NOR17007@381m. Silicate facies Iron formation. Garnet-amphibole rich discontinuous bands alternating with sediments rich bands. Abundant quartz+calcite veins. (r) NOR17007@077m. Sulfide breccia with monomict clasts of fine-grained, graphitic black shale and matrix of semi-massive sulfide mainly pyrrhotite, sphalerite and chalcopyrite. (s) NOR18002@395m. Crystal-rich pumiceous breccia sandstone (F3). Light grey sericite-altered and stretched pumice clasts with crystal rich matrix. (t) NOR18002@191m. Lithological contact of sulfidic shale and hyaloclastite breccia with distinct chilled margins. (u) NOR17007@259m.laminated graphitic shale (F2) with a large blob of arsenopyrite.

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27 4.1.6 Massive to semi-massive sulfide ore

The 12m thick zone of sulfide mineralization occurs as sulfidic breccia within the feldspar- porphyritic basalt unit that is associated with a peperite. It is composed of mainly monomict, massive and graphitic black shale clasts, scattered in a massive to semi-massive sulfide matrix dominantly composed of pyrrhotite (10-30vol%) and sphalerite (~30vol%) but also including chalcopyrite (~5vol%), with minor arsenopyrite (~3vol%) (Fig.10r). The ore typically have sharp contacts with the enclosing units and is tightly foliated at the contact with the underlying peperite unit. Extensive, pervasive to patchy chlorite+carbonate alteration was documented extending 20m in to the hanging-wall, but it was traced as deep as 100m in to the footwall. More intensive dolomite-calcite-chlorite-talc alteration was observed on the highly sheared coherent feldspar- porphyritic basalt unit which occur directly stratigraphically above the mineralization (hanging- wall).

4.1.7 Qz-Fsp porphyritic dacite-II (F1)

This unit is texturally and compositionally similar to the porphyritic dacite-I unit described above. The main difference between the two units appear to be in the percentage of phenocryst population. This unit is generally less porphyritic and it is spatially associated with in situ dacitic hyaloclastite breccias, dacitic peperites at its margins and iron formation intercalations.

Geochemical analysis also proved the two dacitic units to be comagmatic. Thickness of this unit ranges from 30m to maximum 130m. It is dark grey in color, massive to weakly foliated, evenly porphyritic, at the margins frequently peperitic and less commonly hyaloclastitic and occasionally sheared (Fig.10b & 10d). Quartz phenocrysts, which constitute 5 to 10vol% of the rock, are generally <5mm in size, rounded with distinct resorption embayment features and occasionally shattered occurring in a fine-grained and commonly weak pervasive sericite+quartz+chlorite- altered groundmass⁽Fig.11p). Feldspar phenocrysts are generally <2mm and constitute 2 to

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28 5vol% of the rock. In intensively altered rocks, feldspar phenocrysts are commonly zoned, corroded, stretched and partially or completely replaced by sericite. Sericite+chlorite +carbonate+quartz±epidote±zosite association is the common alteration which intensifies stratigraphically downwards towards mineralization. Disseminations of pyrrhotite which associated with minor chalcopyrite are also common.

4.1.8 In situ dacitic hyaloclastite breccia (F1)

The in situ dacitic hyaloclastite breccia unit is about 30m thick and occurs as massive, polyhedral clast-supported, monomict with pervasive jigsaw-fit texture. It consists of pale grey to green angular blocks that are quartz-phyric dacitic in composition with dark grey silicified groundmass (Fig.10c). Gradual transition to the massive feldspar+quartz-porphyritic dacite from one side and a sharp contact with graphitic shale matrix-supported peperitic dacite on the other side. Fracture- controlled weak chlorite alteration is common with minor pyrrhotite+chalcopyrite veinlets (0.5vol %).

4.1.9 Silicate facies Iron formation (F2)

These units occur as a narrow layers with thickness ranging from 6 to 8m, hosted within the feldspar+quartz-porphyritic dacite-II unit. They occur stratigraphically above the mineralized zone in the studied cross section, typically close to the contact with the mineralized facies, however no base metal sulfide mineralization was documented (Fig.10q). It is generally massive, dark grey to greenish with alternating microscopic-scale bedding of garnet-rich and quartzose sedimentary layers (Fig.11e). The garnets are round anhedral and less commonly subhedral grains with a dark core in a groundmass of dominantly amphibole (Fig.11f). This facies was not recognized prior to the current study, and not readily recognized during initial logging. Reasons

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29 why it had previously been missed may be the subtle banding and dark grey color, whereby it is readily mistaken for deformed basalt or shale. Geochemically, the facies is characterized by enrichment in Fe, Mn and Ca but depleted in alkali metals. The iron content (expressed as Fe2O3- total) is ~35%, MnO abundance ranges between 2-6%. Semi-quantitative chemical compositions of the garnets in the iron formation were obtained using scanning electron microprobe (SEM) analyses. The results suggest that the garnets contain primarily the spessartine (Mn3Al2 (SiO4)3) component.

4.1.10 Coherent basaltic-andesite (F1)

This unit occurs at the uppermost part of the Norra succession. The chemical composition ranges from basaltic-andesite to basaltic. Texturally it is strongly porphyritic with lath-shaped feldspar phenocrysts (1 to 5mm), which are occasionally aligned, constituting 5 to 25vol% of the rock (Fig.10k). Whereas the groundmass is mainly massive fine-grained, dark-grey with fracture- controlled chlorite+calcite alterations⁽Fig.11n). Quartz veins with calcite rims, which are occasionally associated with pyrrhotite+arsenopyrite+chlorite veinlets are common. Contacts with the adjacent graphitic black shales is typically sharp with chilled margins but it also shows occasional gradational peperitic contact.

4.1.11 Black shales (F2)

Black shales are commonly present as intercalated units between the volcanic rocks in multiple stratigraphic positions throughout the Norra volcano-sedimentary succession constituting distinct primary depositional units or repeated units due to folding. They usually occur as carbonaceous (graphitic) and/or sulfidic shale units (Fig.10m, 10n & 10o). Unlike the black shale clasts incorporated in the ore lens, most of the shale units are banded with concordant

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30 sphalerite+pyrrhotite laminae (Fig.), mostly weakly foliated and occasionally containing elevated base metal (Cu and Zn) concentrations relative to the adjacent rocks. Moreover black shales at Norra occur as the main sediment component in some peperite matrix. Typically show sharp contact with adjacent volcanic rock units.

4.1.12 Post mineralization doleritic dykes (F1)

These units are commonly dark grey, medium-grained, massive and unaltered group of dykes which have been observed frequently intruding many units at Norra volcano-sedimentary succession. They vary from about 0.5m to a maximum of 10 m in width. At the contacts with the adjacent units they exhibit fine grained to aphanitic chilled margins (Fig.10p).

Petrographically, they show plagioclase+pyroxene-dominated ophitic texture with anhedral to subhedral pyroxene crystals observed partially surrounding lath-shaped euhedral crystals of plagioclase⁽Fig.11o). Furthermore, fractured and pale green minor olivine crystals and disseminated opaque minerals were also documented.

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31 Fig. 11 Photomicrographs of thin sections from Norra (a) MG001 (NOR17007 @ 2.4m). Primary volcanic textures of sericite-altered, strongly deformed, feldspar+quartz porphyritic coherent dacite showing embayed and resorbed quartz phenocrysts and strongly deformed and elongated relict plagioclase phenocrysts in a strong pervasively sericite-altered fine groundmass. Microphotograph taken under cross-polarized light. (b) MG001 (NOR17007 @ 2.4m). Large, broken and rotated quartz

(a) (b)

(c) (d)

(e) (f)

Fsp

Qz Sericite+Cal cite+Qz

Sericite+Cal

cite+Qz Qz

Qz

Qz Sericite+Cal cite+Qz Sericite+Cal cite+Qz

Qz

Grt+Amp

Grt

Amp

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32 phenocrysts in a strongly deformed sericitic groundmass. Microphotograph taken under cross-polarized light (c) MG004 (NOR17007@019m). Sericite+calcite-altered, feldspar+quartz-porphyritic coherent dacite (Qz, 10-15%, <5mm; Fsp, 5%, <3mm). Abundant relict plagioclase phenocrysts which has been altered to sericite and large quartz phenocrysts in a sericite groundmass which contain fine grained quartz and feldspar. Microphotograph taken under cross-polarized light. (d) MG004 (NOR17007@019m).

Embayed large quartz phenocrysts in fine grained quartz+calcite-rich sericite groundmass.

Microphotograph taken under cross-polarized light. (e) MG017 (NOR17007@382m). Silicate facies iron formation showing dark grey band of garnet+amphibole aggregates alternating with quartz-rich bands which also contain minor garnets. Microphotograph taken under cross-polarized light. (f) MG017 (NOR17007@382m). Silicate facies iron formation showing a lens which is mainly composed of medium-grained garnets (commonly spessartine) and amphiboles as main components.Garnets exhibit

(h) (g)

(i) (j)

Qz

Po

Cal cite

Sericite+

Qz Chl orit e Fsp

Po Ccp

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33 a strong zoning, with light grey rims and an opaque core. Amphibole occur as lath-shaped prismatic crystals radiating from a common center. Microphotograph taken under cross-polarized light. (g) MG005 (NOR17007@041m) Sericite+calcite+chlorite altered, feldspar+quartz-porphyritic coherent

(m) (n)

(o) (p)

(k) (l)

Chl orit Cal e

cite

Cal cite

Qz

Cpx Pl

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34 dacite (Qz, 10-15%, <5mm; Fsp, 15%, <2mm). Embayed and resorbed large quartz phenocrysts.

Abundant relict plagioclase phenocrysts which have been replace by sericite, occasionally has Epidote/zoisite core with calcite rims. Microphotograph taken under cross-polarized light. (h) MG005 (NOR17007@041m). Large relict plagioclase phenocrysts which was replaced by sericite in a fine grained quartz+calcite+zosite+epidote groundmass. Microphotograph taken under cross-polarized light.

(i) MG005 (NOR17007@041m). Calcite+epidote+chlorite fracture infill associated with pyrrhotite and chalcopyrite mineralization. Microphotograph taken under cross-polarized light. (j) MG005 (NOR17007@041m). Pyrrhotite and chalcopyrite mineralization associated with calcite+epidote+chlorite fracture infill. Microphotograph taken under reflected light. (k) and (l) MG006 (NOR17007@118m). Chlorite+sericite-altered, strongly sheared basalt. Fine grained strongly deformed, pervasive and fracture controlled sericite+chlorite alteration. Microphotographs taken under

(q) (r)

(s) (t)

Cal cite Cal cite Fsp

Ep

Po

Qz

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35 cross-polarized light. (m) MG019 (NOR17007@441m). Feldspar-porphyritic basalt. Lath-shaped plagioclase phenocrysts (commonly labradorite) with first order grey color, commonly show twining and aligned in a preferred orientation. Abundant clinoamphiboles (commonly tremolite) which occur as intergranular grains between plagioclase laths, lines and microstructures cutting the plagioclases. Chlorite replaces amphiboles and plagioclase, fills fractures and also occur in alteration patches associated with other undifferentiated minerals. Sericite fills the interstitial spaces in the fine grained groundmass^. Microphotograph taken under cross-polarized light. (n) MG010 (NOR17007@214m) Feldspar- porphyritic basaltic-andesite. Plagioclase phenocrysts with first order grey color and minor altered pyroxene crystals with high interference colors embedded within a fine-grained and chlorite+sericite- altered groundmass. Lath-shaped plagioclase crystals, which are anhedral to subhedral, seem to be aligned in a preferred orientation and show simple twining^.Microphotograph taken under cross-polarized light.

(o) MG026 (NOR18002@069m). Doleritic dyke. Grey to whitish lath-shaped, subhedral, and polysynthetically twinned plagioclase with interstitial clinopyroxene are the main components. It also contains minor chlorite, calcite and biotite with disseminated opaque minerals. Generally medium grained and exhibit sub-ophitic to ophitic textures^.Microphotograph taken under cross-polarized light.

(P) MG0027 (NOR18002@074m). Feldpar+quartz-porphyritic dacite. Shattered, milky and dark grey colored, quartz phenocryst. Showing closely spaced fractures and fragments with jig-saw fit texture.

Fractures are commonly filled with calcite and occasionally chlorite. Microphotograph taken under cross-polarized light.⁽q) MG002 (NOR17007 @ 4.8m). Chlorite+sericite+epidote altered, feldspar- pophyritic basalt. Relict of plagioclase phenocrysts ~5%, <3mm which has been completely altered to sericite/epidote in a strong pervasively chlorite+sericite+epidote-altered fine-grained groundmass, Microphotograph taken under cross-polarized light. (r) MG002 (NOR17007 @ 4.8m). Abundant epidote grains at the core of a relict plagioclase phenocryst showing bright interference colors.

Microphotograph taken under cross-polarized light. (s)MG002 (NOR17007 @ 4.8m). A calcite vein cross cutting the basaltic unit. Microphotograph taken under cross-polarized light. (t) MG002 (NOR17007 @ 4.8m). Relict of plagioclase phenocryst which has been replaced by quartz, pyrrhotite,

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36 and chalcopyrite at the core sericite and epidote at the rims. The quartz sub-grains could indicated low grade deformation. Microphotograph taken under cross-polarized light.

Volcanic facies and hydrothermal alteration at the Norra volcanigenic massive sulfide deposit