Mineralogical and Geochemical
characterization of the Fe-Cu-Occurrence and associated Hanging wall and Footwall
Alteration halo of the Viscaria D-Zone, Kiruna District, Northern Sweden
Madelen Estholm
Geosciences, master's level (120 credits) 2019
Luleå University of Technology
Department of Civil, Environmental and Natural Resources Engineering
Mineralogical and Geochemical characterization of the Fe-Cu- Occurrence and associated Hanging wall and Footwall Alteration halo of the Viscaria D-Zone, Kiruna District, Northern Sweden
Madelen Estholm Luleå University of Technology
Master Thesis in Ore Geology and Environmental Geochemistry
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Abstract
Northern Norrbotten County is one of the three major ore producing districts in Sweden. Based on the predominance of epigenetic Cu-Au and Fe-oxide mineralization this region is regarded as a typical IOCG province. The massive to layered Viscaria Cu-deposit is proposed to be a VMS deposit of Besshi-type and is unique in deposit type of the region. The volcaniclastic rocks of the Viscaria Formation hosting the Viscaria deposit belong to the rift related Kiruna Greenstone Group. The Viscaria deposit consists of three stratiform-stratabound mineralized zones: A-, B- and D-zone.
Sulphide mineralization of the D-zone differs in structural features, host rock, mineralization style and Fe-oxide dominance over Cu-sulphides compared to the main Cu-ore in the A-zone. These differences between A- and D-zone mineralization styles raise the subject that the D-zone could be of a different origin.
The Kiruna area is mainly covered by glacial-till, which contributes to limited bedrock exposure. This becomes a challenge when exploring for new deposits and highlights the importance of good geological knowledge obtained from existing deposits to carry through successful exploration programs. The objective of this study is to characterize the Fe-Cu-occurrence and the associated alteration halo of the Viscaria D-zone. Alteration halos can reach several kilometres away from the main ore zone and provides mineralogical and chemical signatures that extend the target area significantly in exploration for new deposits.
Detailed core logging, optical microscopic studies, lithogeochemistry and electron microprobe analyses was utilized to contribute to a better genetic understanding of the D-zone and the associated hanging wall and footwall alteration features. The study shows that the main ore minerals of the D- zone consist of magnetite and chalcopyrite, and minor pyrite and hematite. The major alteration minerals associated with mineralization are calcite, actinolite/tremolite, epidote, chlorite and also biotite and scapolite in the hanging wall. The most prominent potential ore vector is calcite veins and biotite, as the abundance increase towards the D-zone. D-zone are further characterised by low REE concentrations, similarly to the calcic-dolomite host.
The study also shows that the alteration halo of the D-zone is different in the hanging wall compared to the footwall, which is confirmed by the lithogeochemistry and mineral chemistry. The footwall is characterized by spilitization and chlorite alteration. The lithogeochemistry of the D-zone and the hanging wall reveals element mobility of Mg, Ba, Fe, Na, K, Cu and Zn. Mineral chemistry of epidote, amphibole and chlorite shows chemical changes in Fe/Al- and Mg/Fe-ratios. According to the result of this thesis and earlier studies, the D-zone mineralization is suggested to be part of the VMS system forming the stratigraphically above laying A-zone of the Viscaria Cu-deposit.
Contents
1.! Introduction! 4!
1.1.#Objectives# 5!
1.2#Background# 5!
1.3#Hydrothermal#alteration#systems# 5!
1.3.1!VMS)Systems! 6!
1.3.2!IOCG)Systems! 7!
1.4#Regional#Geology# 8!
1.4.1!Major!regional!events!of!metallogenetic!importance! 8!
1.5#Local#Geology# 10!
1.6#Viscaria#Formation# 11!
1.6.1!Viscaria!ore!deposit!and!Alteration!Features! 12!
2. Methodology! 14!
2.1#Core#Logging# 14!
2.2#Sampling# 14!
2.3#Petrography# 15!
2.4#Lithogeochemistry# 15!
2.5#Mineral#Chemistry# 16!
3. Results! 17!
3.1#Macroscopic#observations# 17!
3.1.1!The!D)zone! 17!
3.1.2!The!Footwall! 18!
3.1.3!The!Hanging!wall! 20!
3.1.4!Structural!and!Alteration!Features! 22!
3.2#Microscopic#Observations# 23!
3.2.1!The!D)zone! 23!
3.2.2!The!Footwall! 24!
3.2.3!The!Hanging!wall! 25!
3.3#Lithogeochemistry# 28!
3.3.1!Lithogeochemical!data! 28!
3.3.2!Lithogeochemical!element!trends! 30!
3.3.3!Box!plot! 32!
3.3.4!Rare!Earth!Elements! 33!
3.4#Mineral#chemistry# 34!
3.4.1!Nomenclature!of!minerals! 36!
3.4.2!Chemical!variation!of!minerals!across!the!D)zone! 38!
4. Discussion! 39!
4.1#Genetic#aspects#of#the#DOzone#Mineralization# 39!
4.1.1!Geochemical!Characterisation!of!the!D)zone! 39!
4.2#Alteration#features#related#to#the#DOzone# 40!
4.2.1!Alteration!minerals!and!mineral!paragenesis! 40!
4.3#Summary#Alteration#Features#of#DOzone#Hanging#wall#and#Footwall# 43!
4.4#Formation#of#the#DOzone# 44!
4.4#Suggested#further#studies# 46!
5. Conclusion! 47!
6. Acknowledgements! 47!
7. References! 48!
8. Appendix! 51!
8.1#Core#log# 52!
8.1.1!Alteration!Features! 54!
8.2#Core#Sampling# 55!
8.3#Microscopy# 57!
8.3#Lihogeochemistry#data# 58!
8.3.1!Lithogeochemistry!reference!data! 65!
8.3.2!Lihogeochemical!trends! 66!
8.4#Mineral#Chemistry# 70!
8.4.1!Microphotographs! 73!
8.5#Classification#diagram# 74!
1. Introduction
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The Northern Fennoscandian Shield is an economically important metallogenic province in Europe. Northern Norrbotten County, in northernmost Sweden is one of three major ore producing provinces in Sweden and is dominated by epigenetic deposits of Cu-Au and Fe- oxides ores (Bergman et al., 2001; Martinsson et al., 2016). Based on the deposit style together with the regional extensive scapolite and albite alteration the region has been regarded as a typical IOCG (Iron oxide copper gold) province (Billström et al., 2010). The majority of the ore deposits were formed during Palaeoproterozoic time and many of them have been classified as end-members of IOCG-type deposits such as the Fe-oxide dominated Kiruna iron-apatite deposit (1.98 Ga) and the Fe-oxide poor Aitik Cu-Au (1.9 Ga) deposit (Hitzman et al., 1992; Weihed et al., 2005; Wanhainen et al., 2003). Although some of the deposits in the area may just shear a few characteristics of an IOCG system (Billström et al., 2010; Martinsson et al., 2016).
The massive layered Viscaria Cu-deposit is located four kilometres west of the Kiruna town, and is unique in its place and mineralization-style. The deposit is hosted by the Viscaria Formation, which is part of the rift related Kiruna Greenstone group of Palaeoprotozoic age.
Viscaria Formation includes three mineralized lenses of stratiform-stratabound style named A-zone, B-zone and D-zone from the stratigraphically uppermost to the lowermost. These units have later been tectonically tilted in the current sub-vertical position. The deposit has been classified as a syngenetic exhalative VMS-deposit (volcanogenic massive sulphide) of Besshi-type (Martinsson, 1997).
At Viscaria an alteration zone characterized by plagioclase destruction and phyllosilicate formation occurs in the footwall to the Viscaria A-zone, which also encloses the stratigraphically lower B-zone (Martinsson, 1997). The D-zone in the stratigraphically lowest part of the Viscaria Formation is different in mineralization style compared to the above laying Viscaria A- and B-zone horizons. The D-zone has lower Cu-grade and is dominated by Fe-oxides and occurs in a deformed and sheared dolomite rock sequence. Instead of being a part of the Viscaria ore system, the D-zone could possibly be a sheared related epigenetic IOCG-type mineralization.
The character of the D-zone ore and the alteration features of the host rocks are crucial in the genetic understanding of the deposit and would also contribute knowledge for further exploration in the Kiruna area. Kiruna area are largely covered by glacial till, which provides limited bedrock information, which makes it important to get maximum of information from each drilled core in exploration drilling programs.
1.1. Objectives
This thesis is a following up study to Martinsson (1997) and Masurel (2011) in the process towards an improved understanding of the ore forming processes that have occurred within the Viscaria Formation. This thesis has two objectives (1) to identify mineralogical and geochemical signatures of the Viscaria D-zone ore lens and (2) to establish the mineralogical and geochemistry signatures of the ore related alteration halo within the hanging wall and footwall to the D-zone. The aim is to generate results, which will contribute to a better understanding of the Viscaria deposit and also to be useful in exploration of new ores/mineralized zones in the Kiruna district.
1.2 Background
The Viscaria copper-deposit was discovered in 1972 using geobotanical methods, in particular the presence of the flower Viscaria Alpina in the area (Martinsson, 1997). The flower Viscaria Alpina (Lychnis alpina) is tolerant to high metal content and used as an indicator of anomalous copper rich ground (Nordal et al., 1999). The first hole was drilled in 1973 intersecting the Viscaria A-zone (Martinsson, 1991). Mining started 1983, and 12.54 Mt of ore with a Cu grade of 2.3%, were processed from the A-zone the B-zone by the mining companies LKAB (1983-1985), and later by Outokumpu OY (1985-1997). The D-zone has not been mined and is estimated to contain 0.74Mt at 1.5% Cu grade, with cut off at 1%
(Martinsson, 1997).
1.3 Hydrothermal alteration systems
A subsurface heat source (intrusion) could generate a hydrothermal system including a recharge zone with progressive heating of fluids during vertical and lateral transport through the rock (Fig. 1C) (Galley et al., 2007). The hydrothermal fluid provides gain and losses of elements, including metals and sulphur, as a result of interaction between hydrothermal fluids and rocks (Fig. 1B) (Piercey, 2009). Alteration refers to chemical and mineralogical changes of the rocks and could give essential contributions to understand ore genesis. Metasomatic (hydrothermal alteration) systems could be complex as the systems are commonly considered as chemically open systems.
Alteration character, extent and zoning in wall rocks depend on several parameters such as wall rock structures, fractures, porosity and permeability, rock composition (chemistry), system conditions (pressure and temperature), fluid composition and the fluid /rock ratio (Large et al, 2001). The most intense and pervasive alteration features are commonly found proximal to the ore zone, and fades away in distal parts caused by temperature drop and lower fluid/ rock ratio. The size and morphology of the alteration zone is depending on the character and size of the hydrothermal system and on the wall rock character. Regional alteration zones may extend several kilometres in scale and have a patchy character and are mainly caused by local unfocused hydrothermal fluids (Gemmell and Fulton, 2001; Galley et al., 2007; Piercey, 2009). The alteration halos also provide mineralogical and chemical signatures that extend the target area significantly in exploration for new deposits.
1.3.1 VMS-Systems
VMS-deposits constitute commonly small, limited targets of generally 5-50Mt of ore. They are a commonly occurring as stratiform-startabound, semi-masive to massive, sulphide lenses.
The deposits are important sources of Cu and Zn with varied contents of Pb, Ag, Au, Cd and other metals (Galley et al., 2007) and commonly have a well-developed metal zoning of Fe- Cu, Cu-Pb-Zn and Pb-Zn-Ba. VMS-systems are subdivided based on their host rocks, which reflecting the depositional environment, such as; volcanic-associated massive-sulphide (Cyprus-type in mafic rocks, such as opholite), volcanic hosted (Kuroko-type in bimodal- sequences, with felsic>mafic rocks), or volcano sedimentary hosted–deposits (Besshi-type mafic and siliciclastic rocks). Tectonic settings are divergent and are related to mid oceanic ridges, back arc settings or intracontinental rifts (Galley et al., 2007).
Active tectonic rifting contributes to crustal thinning and mantle depressurization, which generates basaltic melts. The subsurface heat source (intrusions) generates hydrothermal convection cells with seawater transformed into ore fluid by leaching metals from the sub-seafloor rocks. The fluids migrate to the sea- bottom along fracture zones in the footwall rock. The precipitation of metals occurs when metal-rich fluids rapidly cool as they reach the sea-bottom and generates lenses of massive sulphide-ores. The alteration zones are most prominent in the footwall of the deposit, due to post-mineralization formation of the hanging wall.
Proximal alteration is characterized by high water/rock ratio and high temperature (400 °C) generating
pervasive/intense alteration (Gemmell and Fulton, 2001; Piercey, 2009). The proximal alteration zones are characterized by sulphide-silicate vein stockwork, greenschist-grade minerals, element enrichments of Fe, Mg, Si, K, S and base metals (Gemmell and Fulton, 2001; Galley et al, 2007; Piercey, 2009). High temperature zones are addressed by feldspar destruction and the formation of phyllosilicates resulting in Na and Ca depletion (Galley et al., 2007).
A regional alteration zone is commonly expressed by zeolite, greenschist and amphibolite - metamorphose mineral assemblages (Fig. 1C) (Galley et al., 2007). Typically these more regional alteration zones show epidotization, silicification, spilitization and, actinolite-
Figure 1. From Galley et al. (2007) illustrating the ore genetic model formation of VMS-deposits and associated alteration zonation of mineral assemblages.
clinozoisite-magnetite minerals, indicated by enrichments of Mg, Na, Ca, Fe and Si and distinct base metal depletion (Galley et al, 2007; Piercey, 2009). Associated chlorite is generally Mg-rich, carbonate mineral occurs to be Fe-Mn-Mg poor, epidote has a high Fe/Al ratio and mica is commonly Na-rich (Piercey, 2009).
1.3.2 IOCG-Systems
The IOCG deposit type was recognized by Hitzman et al. (1992) and includes a range of sub- types. The metal association includes commonly iron oxides (magnetite and/or hematite) of low titanium content (generally 0.5wt%), Cu-sulphides, Au, U and REE (in apatite or as specific REE-minerals). The IOCG deposits are formed from hydrothermal convective systems with fluids of magmatic- (alkaline or oxidized K-rich magmas) or non-magmatic (metamorphic- or surface/basin derived) origin, which are oxidized and highly saline (Barton and Johnson 2000). The tectonic setting is diverse and generally associated to extensional, compressional or arc environments. Characteristic is also regional sodic-calcic alteration and Cl-enrichments suggested to be products of mobilized evaporate sequences (Barton and Johnson 2000).
Ore formations occur in shallow crust (4-6 km depth) and are related to deep- seated, volitaile rich hydrothermal systems (Hitzman et al., 1992). The IOCG-deposits occur on several continents and varies generally in size from 100 Mt to 1000 Mt (e.g. Ernest Henry 167 Mt, in Australia, Candelaria 384 Mt, in Chile and Salobo 994 Mt, in Brazil). An exception is the giant Olympic Dam deposit in South Australia containing
>2000 Mt (Groves et al., 2010).
The mineralization style is varied and ore minerals may occur as veins, breccia infill, stockworks, skarn hosted or ironstone hosted (Pollard, 2000). The morphology of the mineralization and the alteration is varied, and largely controlled by high permeability zones such as faults, shear zones or intrusive contacts. Alteration features depend mainly on the host rock, although generally sodic alteration occurs at deep-seated levels and potassic alteration at intermediate to shallow levels. Silicification and sericitization occurs at the most shallow levels (Fig. 2). Alteration minerals commonly include K-feldspar, biotite, clinopyroxene,
Figure 2. Figure illustrating ore genetic model of IOCG-deposits and its associated alteration zones by Hitzman et al., (1992).
scapolite, albite, amphibole and titanite (Pollard, 2000). The metal association is diverse, but is commonly characterized by enrichments of; P, Ba, F, Co, Ni, As, Mo, Ag, LREE and U (Zn and Pb may be enriched or depleted compared to the host rock) (Williams et al., 2005;
Pollard, 2000; Hitzman et al., 1992).
1.4 Regional Geology
The Northern part of the Fennoscandian Shield has a complex geological history of rifting, subduction, microcontinent accretion and subsequent continent – continent collision tectonics.
In Northern Norrbotten the crust consists dominantly of Precambrian crystalline rocks (2.8- 1.8 Ga) (Bergman et al., 2001). The oldest preserved rock is the Archaean basement dominated by metagranitoids (2.8 Ga). The Archaean basement is unconformally overlain by the Karelian rock succession (2.5-1.96 Ga). The Karelian is subdivided into the Kovo Group (2.5-2.3 Ga) and the Kiruna Greenstone Group (2.2-2.0 Ga) (Bergman et al., 2001). The Kovo Group consists of clastic metasediments and metavolcanic rocks of intermediate-mafic composition (Martinsson, 1999; Bergman et al., 2001). The Kiruna Greenstone Group is related to the Jatulina rifting event at 2.2-2.0 Ga, and comprises mafic volcanic- and volcanosedimentary sequences. These rocks consists more specific of tholeiitic metabasalts and metavolcaniclastic rocks, metakomatiites and minor metasedimentary intercalation, which includes carbonate rocks, iron formations and graphite schists (Martinsson., 1997;
Martinsson, 2004). The supracrustal Svecofennian rocks (1.9 Ga) are deposited on top of the Karelian units. They are subdivided into the Porphyrite Group comprising calc-alkaline and metaandesitic rocks followed by the overlaying Kiirunavaara Group consisting of bimodal metavolcanic rock of mafic to felsic composition. Further, metasedimentary rocks overlie these rocks and are dominated by quartzites (Bergman et al., 2001; Martinsson, 2004). The region has six different suites of magmatic intrusive rocks, including the 1.9 Ga Haparanda and Perthite Monzonite Suites and the 1.8 Ga Lina Suite (Bergman et al., 2001). The Vendian and Cambrian clastic sedimentary rocks occurs in the western part of Norrbotten, which unconformally cover the Palaeoproterozoic rocks. These rocks were affected by thrusting during the Caledonian orogeny at 0.5-0.4 Ga (Fig. 3) (Bergman et al., 2001).
1.4.1 Major regional events of metallogenetic importance
The Northern part of the Fennoscandian Shield experienced two major rock-forming stages in Palaeoprotozoic time, which are of major metallogenetic importance in the region. These are the Kalerian development (2.5-2.0 Ga) and the Svecofennian event (2.0-1.75 Ga) (Gaál, 1990). The Kalerian development represents rifting of the Archaean crust, which began around 2.5-2.44 Ga and includes the formation of the 2.1 Ga Kiruna Greenstone Group. The Kiruna Greenstone Group was formed within an aulacogen related to a triple junction created by a mantle plume (Martinsson, 1997). This is supported by the occurrences of epicontinental clastic sediments, evaporites, and mafic dyke swarms (Martinsson, 1997). Repeated intraplate rifting resulted in tectonic subsidence, which ended with submarine eruptions of MORB-type pillow basalts (Martinsson, 1997).
Svecofennian subduction occurred around 1.9 Ga, which generated calc-alkaline volcanic rocks and volcaniclastic sediments of mainly andesitic composition forming the Porphyrite Group. The overlaying Kiirunavaara Group is bimodal in character and consists of mafic to felsic volcanic rocks and reflects magmatism possibly related to a mantle plume (Perdahl, 1995; Martinsson, 2004). The volcano sedimentary sequences were deformed during the Svecokarelian orogeny around 1.9-1.8 Ga (Bergman et al., 2001; Weihed et al., 2005). The deformation event contributed to deep-seated shear zones in the area, for instance, the KNDZ:
Kiruna-Naimakka Deformation Zone, and PSZ: Pajala Shear Zone. Such deep-seated shear zones may be favourable settings for the formations of IOCG-deposits (Hitzman et al., 1992) and the evaporite sequences in the Kiruna Greenstone Group could act as an important source of Cl- contributing to high solubility of metals in hydrothermal systems (Barton and Johnson 2000; Robb, 2005).
The regional metamorphism is of low-pressure type and varies generally from upper greenschist facies to upper amphibolite facies. However, the age and grade of metamorphism in the region is not completely understood but seem to be related to the main magmatic events at 1.88 and 1.80 Ga (Bergman et al., 2001).
Figure 3. Simplified geological map of the Northern Norrbotten, in Northernmost Sweden. From Bergman et al.
(2001).
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1.5 Local Geology
The Kiruna area is covered largely by glacial till. The underlying bedrock includes a well- preserved sequence of the Karelian greenstone succession, the Svecofennian porphyries and the younger clastic sediments. The Kiruna Greenstone Group (Fig. 4) is subdivided into six formations, in order from the base; Såkevaratjah Formation, Ändnamvare Formation, Pikse Formation, Viscaria Formation, Peuravaara Formation and Linkaluoppal Formation (lithological description in Table 1). These formations are described in more detailed by Martinsson (1997). The Greenstones are overlaid by the Kurravaara Conglomerate and the Kiirunavaara Group (1.89-1.88 Ga). Intrusion of grantioids and gabbro are dated to 1.89-1.87 Ga and subordinate syenites at 1.79 Ga (Romer et al., 1992; Bergman et al., 2001; Westhues et al., 2016). The rock sequence is dipping steeply towards the southeast.
Table 1 Characteristic lithology features of the six main Formations of the Kiruna Greenstone Group (Martinsson, 1997).
Figure 4. Geological map of the bedrock distribution of the Kiruna area, including Kiruna Greenstone Group from Martinsson (1997). Black square indicates the study area of the Viscaria, west of the Kiruna town, in Sweden.
Formation (Greenstone Group) Lithology (Characteristic) Upper Linkaluoppal Volcaniclastic basaltic composition
Peuravaara Basaltic pillow lavas
Viscaria Submarine basaltic pyroclastic flows
Pikse Subareal basalt flows
Ändnamvare Komatiites and basaltic komatiites
Lower Såkevaratjah Amygdaloidal basalt (scapolite altered) Viscaria'Formation Unit Lithology'(Characteristic)
Upper A Felsic'ash'tuff'of'dacitic'composition'including'black'schist'and'mafic'volcanclastics B4 Subaquesous'pyoclastic'flow'(basal'and'coarse'grained)
B3 Several'intercalations'of'black'schists'(5%'graphitic'content) B2 Volcaniclastics'with'graded'bedding'(cm'to'dm'scale)' B1 Volcaniclastic'rocks'('fine'grained'rarely'layered'tuffite)
C Black'schist'('10%'graphite'content)'similar'composition'as'the'tuffite D2 CalcDalkaline'andesitic'tuffite'with'thin'carbonates'intercalations Lower D1 Dolomite'unit
The rock succession is metamorphosed in the upper greenschist to lower amphibolite facies and main minerals in the metabasalts are albite, actinolite and chlorite (Martinsson, 1997). In general the rocks are almost undeformed and primary structures are mostly well preserved with pillows and amygdules occurring in the basaltic lava flows. Strongly deformed rocks are related to local shear zones in the area (Kiruna-Naimakka Shear Zone), but more ductile rocks such as komattites and volcaniclastic rocks may exhibit penetrative foliation (Martinsson 1997).
1.6 Viscaria Formation
The Viscaria Cu-deposit is hosted by the Viscaria Formation and is situated between the overlying Peuravaara Formation and the underlying Pikse Formation (Martinsson, 1997).
Peurvaara Formation is characterised by basaltic pillow lava of MORB-composition with minor intercalations of pyroclastic material and iron-rich chemical sediments (Martinsson, 1997). The Pikse Formation comprises several massive tholeiitic basalt flows with MORB to LKT chemical character with minor intercalations of chemical sediments (Martinsson, 1997).
The basaltic flows are of two types with low TiO2 (<1 wt%) and high TiO2 (>1 wt%) contents (Martinsson, 1997). The Viscaria Formation consists of approximate 400 m thick sequence of volcaniclastic tuff of mainly basaltic composition, which was deposited in a subaqueous environment (Martinsson, 1997; Masurel, 2011). Intercalations of carbonates and graphitic schist occur in the volcaniclastic tuff sequence.
The subdivision of the Viscaria Formation into lithologically different members is described in detailed by Martinsson (1997). Here a general illustration is presented in Figure 5 and description in Table 2. The Viscaria Formation contains several mafic dyke intrusions. These are suggested to be subvolcanic intrusions related to the overlaying pillow basalts of the Peuravaara Formation (Martinsson 1997).
Figure 5. The A-zone occurs at top of the volcaniclastic Viscaria Formation while the D-zone occurs at the base. The footwall of the D-zone is the Pikse Formation basalt. Illustration from Martinsson (1997).
Table 2 General description of the characteristic lithology and features of the eight members within the Viscaria Formation (described in more detail by Martinsson (1997)). The position of the different units is illustrated in Figure 5.
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1.6.1 Viscaria ore deposit and Alteration Features
The Viscaria A-zone is the economically most important ore horizon that has been suggested to be a Cu-dominated stratiform VMS-deposit of Besshi-type (Martinsson, 1991, 1997). The main ore minerals are pyrrhotite, chalcopyrite, magnetite and minor amounts of sphalerite.
The B-zone contains several thin and mainly low grade mineralized horizons that only have been mined in small scale. The C-zone consists of a black schist horizon with uneconomic sulphide contents. The D-zone is stratabound in character and hosted by a deformed dolomite,
Viscaria Formation
Unit Lithology (characteristics)
A Felsic ash tuffite of dacitic composition including black schists and mafic volcaniclastics B4 Basaltic subaqueous pyroclastic flows
B3 Several intercalations of black schist (5% graphite content) B2 Basaltic tuff, layered with graded bedding
B1 Basaltic tuff (lacking layering) C Black schist with 5-15% graphite
D2 Andesitic tuffite of calcalkaline composition and containing carbonate intercalations D1 Dolomite
thus different to the A- and B-zones. It is dominated by magnetite occurring semi-massive banded or more massive ironstone with irregular disseminated pyrite and chalcopyrite.
The A zone has an extensive alteration zone in its footwall, which has been investigated by Martinsson (1997). This alteration zone extends through the B- and D-zone and ends in the upper part of the Pikse Formation (Martinsson 1997). The volcaniclastic tuff unit is affected by plagioclase destruction and biotite formation with more local occurrence of scapolitization, albitization and potassic alteration. In the A zone the potassic alteration (high K/Na ratio) is most obvious close to the Cu-rich part and extends more than 150 meter below the ore zone.
The A zone footwall is geochemically characterized by enrichment of K, Ba, Rb, Cs and depletion of Na and Ca (destruction of feldspar and creation of biotite). Metals enriched are Zn and minor Fe, Mn, Cu, Sb, As, Au, U and LREE. The most immobile elements are Ti, Al, Th, Zr, HREE, Y, Sc and V (Martinsson 1997).
2. Methodology
The objective of the thesis is to characterize the mineralogy and geochemistry of the D-zone and the associated alteration features of the alteration halo within the hanging wall and footwall. The study was performed on four representative diamond drill cores with a total length of 1883 meters (Table 3). The methods used in the study are visual core logging, petrography using optical microscopy, lithogeochemistry and mineral chemistry by electron microprobe analysis.
Figure 6 Field map of the study area illustrating drill core positions (blue dots) in relation to the D-zone.
Table 3 Drill core information of depth, geographic location (coordinate system in RT90) and elevation. The cores representing the footwall stratigraphy are VDD0165 and VDD0147. The cores representing the hanging wall startigraphy are VDD0186 and VDD0189 (see illustrated stratigraphy in Fig. 7). The cores were obtained from Avalon Minerals Ltd drilling program that was preformed in 2013 and 2015.
2.1 Core Logging
Detailed geological information about the footwall and hanging wall rocks were obtained from visual core logging at Avalon Minerals LTD, in Kiruna. The logging information included primary lithology, alteration minerals, alteration intensity (weak, moderate, strong/intense), alteration style and deformation intensity. The alteration features were classified as pervasive, patchy (selective), disseminated, vein or veinlet.
2.2 Sampling
Samples for mineralogical and geochemical studies were obtained from the four cores during the core logging. The samples were selected to represent different alteration features (see Table 9 and 10 in Appendix) of unweathered, homogenous lithology sections at various distance to the D-zone, also including the D-zone itself. The sampling intervals were varied from 60 meters in distal parts to shorter intervals proximal to the ore zone (from ten to one meter intervals). The sample length varied from 1 to 0.4 m of half or quarter core (approximate 6-4 cm core diameter). The samples were prepared and cut by a diamond saw at Avalon Minerals, in Kiruna.
Hole ID Core length (m) East North Elevation
VDD0147 360 1680824 7537525 510.9
VDD0165 402.28 1680594 7537395 508.9
VDD0186 563.2 1680988 7537061 535
VDD0189 557.7 1681157 7537209 532.5
The D-zone strike northeast - southwest (Fig. 6). Two cores drilled from east to west represent the hanging wall stratigraphy. Two other cores drilled from west to east represent the footwall stratigraphy. These four cores represent two cross-sections occurring approximate 250 meters apart along the D-zone mineralization.
Figure 7. A general cross section of the footwall and the hanging wall stratigraphy of the D-zone. The lithostratigraphy here is simplified and described in more detail by Masurel (2011).
2.3 Petrography
A petrographic study using optical microscopy was done to document mineral contents, mineral textures, minerals relationship, and mineral paragenesis. Petrographic thin sections were mainly selected from samples used for geochemical analysis. The rock samples were prepared at Luleå University of Technology. Thin sections were made by Vancouver Petrographics Ltd, in Vancouver, Canada including 31 polished and 13 covered thin sections (Table 10 in Appendix). A Lecia petrographic microscope was utilized to examine thin sections under transmitted and reflected light at Luleå University of Technology.
2.4 Lithogeochemistry
Lithogeochemistry was used to obtain the chemical composition, geochemical characteristics and examine element mobility patterns in the wall rocks associated to the D-zone. A total of 46 samples were analysed from the hanging wall and 30 samples from the footwall (Table 9 in Appendix). These samples were prepared at Avalon Minerals LTD, in Kiruna, and sent for analysis to ALS laboratory in Piteå, Sweden. The analytical procedure included; Total carbon, Base metals by 4-acid ICP-AES, Lithium Borate fusion ICP-AES and ICP-MS, Loss of ignition and total sulphur by Leco. The accuracy is estimated to be high with systematic quality controls including standard samples through the analysis.
2.5 Mineral Chemistry
Quantitative chemical analyses of minerals were performed utilizing an electron microprobe analyser (EMPA) Cameca SX 100, at geological research-centre (GTK) in Espoo, Finland.
The EMPA is an analytical tool, utilized for spot measurements of mineral chemistry in polished thin section and was used to determine the chemistry of minerals within the D-zone alteration halo.
The analyser contains a focused electron beam bombarding the sample to produce X-rays. A gas flow proportional counter measures the intensity of emitted X-rays at the wavelength characteristic to the elements being analysed and using standards for calibration. The equipment is able to make analysis of five elements at the same time.
Analysis of major elements, mainly of silicate minerals were performed on 13 polished thin sections from the hanging wall and the footwall (VDD0186 and VDD0165). All samples were coated with a thin graphite layer using a vacuum gauge. The graphite layer acts as a conductive material during analysis. The micro probe beam current was 20 nA with a diameter of 5 µm and had an accelerating voltage of 15 kV. The electron microprobe was calibrated with natural standards. The precision was obtained by the counting statistics within the sample and the analytical session, which showed high correlating results. The accuracy of the measurements was difficult to measure and is directly defined by the standards that were used (provided by GTK).
3. Results
The results presented here including the core log (macroscopic observations), microscopic observations, lithogeochemistry and mineral chemistry.
3.1 Macroscopic observations
!
This section describing the results obtained during the core log. A general description of the lithology has been divided into D-zone, footwall and hanging wall. A more detailed description of the wall rock lithology is made by Masurel (2011). See summarized core log in Figure 14, and for complete core logs see appendix Figures 29 and 30.
During collecting of the geological data further subdivisions were applied to the wall rocks with “proximal D-zone” and “distal D-zone”. The Proximal zone was defined by increased presence of ore-minerals and pervasive alteration features, which was observed during core logging. The proximal zone has a gradual boundary and begins in the core approximated 100 m from the D-zone in both the hanging wall (fig. 14, 270-300 m) and the footwall (fig. 14, 150-170 m). The distal zones were based on patchy alteration patterns, and extending from the proximal zone and away from the D-zone.
3.1.1 The D-zone
The D-zone is tabular in shape and occurs parallel to the stratigraphy, however it is irregular in thickness (obtained information from several cross sections of the D-zone). It occurs within the stratigraphically lower part of the calcitic-dolomite unit (D1), which is affected by shearing and ductile deformation. The D-zone comprises of oxides and sulphides, which occurs in order of abundances; magnetite, chalcopyrite, pyrite and minor hematite. The mineral of economic interest is chalcopyrite and magnetite. Magnetite occurs as massive to semi massive bands within the carbonate unit (Fig. 9). Chalcopyrite occurs mainly aggregated in carbonate veins, fine-grained disseminated, as infill replacement and commonly within semi massive magnetite (Fig. 8). Chalcopyrite and pyrite are associated with magnetite occurrence, and the sulphides occasionally cross cut the magnetite banding (Fig. 8 D).
Common alteration features within the D-zone are; carbonate, quartz, chlorite, talc and amphibole-actinolite. Quartz and actinolite are associated with magnetite. Carbonate is associated with the sulphides.
Figure 8 Chalcopyrite and pyrite occurs in varied styles, commonly with calcite veins and as fine-grained disseminated in sheared structures. A) Breccia: Sulphides as replacement texture (infill/replacement of dissolved calcite). B) Tuff: Sheared fine-grained (disseminated) sulphides. C) Tuff: Disseminated fine-grained sulphides in sheared structure. D) D-zone: Cross cut sulphide bearing calcite veins in a massive sheared magnetite assemblage. E) D-zone: Disseminated coarse-grained sulphides within massive magnetite. F) D-zone: Ductile deformation of massive magnetite ore with disseminated chalcopyrite and sheared calcite veins.
Figure 9. D-zone: A) Magnetite occuring as veins togeteher with sulphides in the tuff unit. B) Banded semi-massive magnetite (dark grey) with quartz (left side dusty white) and dolomite (white coloured to the right). C and D) Thin sections with irregular bands of magnetite (dark grey) in dolomite (white).
3.1.2 The Footwall
The footwall consists of fine-grained metabasalt of greenschist-facies belonging to the Pikes Formation. The sequence consists of several massive basaltic flows with locally amygdaloidal textures with the former vesicles commonly filled with quartz, calcite, epidote or chlorite. The sequence is intruded by a basaltic dike (generally 10-20 meters wide). A breccia (10-15 meters thick sequence) occurs at the upper contact and consists of medium to strongly weathered polymict clasts (>50%) supported by a carbonate or fine-grained sedimentary matrix (Fig. 11). The lower lithological contact of the breccia is gradual from massive basalt to angular clasts sometimes with a jigsaw texture, and changes to more round and oval clasts in a preferred alignment towards the upper contact.
3.1.2.1 The Footwall Alteration Features Footwall distal to the D-zone
Basaltic rocks in the footwall to the D-zone are generally affected by chlorite-epidote-albite- carbonate alteration. Selective and partly pervasive epidote and chlorite alterations are the most prominent features of the basaltic unit. Epidote occurs in short intervals as weak to moderate patchy character but partly also as veinlets. Chlorite occurs as weak to moderate alteration with patchy to pervasive character and is commonly associated with local shearing.
Albite alteration is pervasive in intervals, commonly over several meters of examined drill cores. Veins in the footwall are dominantly of calcite, K-feldspar, epidote and quartz. Calcite veins occur as minor features, with increasing intensity towards the proximal zone and the D- zone. Chalcopyrite and pyrite are sometimes minor constituents in veins but occur mostly disseminated associated with local shear zones. Minor magnetite occurs disseminated and commonly associated with sulphides.
Footwall proximal to the D-zone
The distal zone graduates to the proximal zone, approximate 100 m from the D-zone. The proximal zone is defined by increase concentrations of ore minerals, and affected by intense/pervasive alteration including epidote, carbonate, albite, chlorite and rarely biotite.
Epidote alteration is partly of pervasive character occurring as a halo around calcite veins.
Chlorite occurs commonly as patchy and pervasive alteration within sheared structures. Albite alteration is varyingly developed in the examined cores and occurs pervasive, or is absent.
The intensity of calcite veins increase toward the main mineralization. Hematite staining occurs locally in fractures.
The fine-grained mafic dyke is mainly affected by patchy epidote alteration. Euhedral porphyroblasts of carbonate are associated with chlorite alteration (Fig. 10). This feature is mainly observed in the dike and more rarely in the main basalt lithology. No major shear structures or mineralization have affected the mafic dike.
Figure 10 A) Calcite porphyroblasts associated with chlorite B) Microphotograph of euhedral calcite porphyroblast.
The breccia contains native copper and hematite staining, a result from weathering and oxidation processes (Fig. 11B). The breccia sequence contains locally crosscutting calcite- and magnetite-veins. Occasionally sulphides and oxides occur as infill/replacement minerals or as individual fragments in the carbonate matrix (Fig. 11A).
Figure 11. Hand specimens of breccia from VDD0165 A) Breccia with chalcopyrite as infill mineral or replacement of carbonates. B) Angular polymict clasts and hematite staining.
3.1.3 The Hanging wall
The hanging wall sequence consists of laminated volcaniclastic tuffaceous siltstone (tuff) of andesitic composition (Masurel, 2011). The sequence includes an intercalation of graphitic schist (C-zone) in its upper part and the contacts are gradual from grey shades to black colour reflecting increasing graphite contents. The hanging wall is intruded by mafic sills (I and II), which are of dolorite character. The contacts between the mafic sills and the tuff are diffuse and are defined by approximate one meter short interval of fine-grained rock. Locally larger carbonate veins of approximate one meter thickness occur in the hanging wall.
3.1.3.1 The Hanging wall Alteration Features Hanging wall distal to the D-zone
The hanging wall is affected by weak epidote-carbonate-chlorite-albite alteration. Biotite and scapolite alteration seems to be associated and occurs pervasive in contact of the mafic sill and the tuff. Chalcopyrite, pyrite and less common pyrrhotite occur mainly in calcite veins.
Disseminated, fine-grained sulphides are also present in local shear zones, as well as magnetite. Pyrite extends further up in the hanging wall than chalcopyrite, and pyrrhotite is mainly observed in the distal hanging wall (Fig. 13F).
In general epidote alteration is partly pervasive and commonly forms alteration halos enclosing sulphide bearing calcite veins. The amount of calcite veins increase towards the proximal zone. Albite alteration is locally patchy or pervasive in several meters long intervals.
Chlorite is strongly associated to mafic lithologies, and occurs pervasive with locally increased intensity associated with foliated and sheared structures. Moderate to weak biotite alteration is a major feature of the tuff unit and occurs with semi-pervasive to patchy character (Fig. 12). Increase in alteration intensity occurs in contact zones between mafic intrusions and tuff, and also towards the D-zone. Biotite occurs locally in bands associated with chlorite, and is often successively replaced by chlorite towards local shear structures.
Biotite alteration is also associated to scapolite porphyroblasts in contact zones between the tuff and the mafic intrusions. The mafic sills are affected by pervasive scapolite-biotite alteration. The graphitic schist has thin stripes of calcite and chlorite and is locally affected by pervasive albite alteration. Pyrite is occasionally present as clusters (30-70 mm) within the foliation of the rock.
Hanging wall proximal to the D-zone
The proximal hanging wall is defined by a gradual increase of pervasive biotite alteration and carbonate veining, that occurs along the core approximate 100 m from the D-zone. Biotite is gradually and locally replaced by chlorite, occasionally grading into chlorite schist in local sheared structures (Fig. 12). The chlorite schist could change into patches of talc, especially at the border of the dolomite unit and in the D-zone. Epidote occurs as minor patchy/pervasive alteration, as veinlets, and commonly as a halo to calcite veins. Red-feldspar veins (Fig. 13, B, E and G) with hematite staining are less important, but a distinct increase of vein intensity in the tuff unit is observed towards the D-zone. Quartz clusters are locally present in the dolomite unit and in the D-zone, and are commonly associated with coarse-grained magnetite.
Magnetite occurs also together with disseminated chalcopyrite and pyrite, and as veins.
Figure 12. Hanging wall tuff lithology with ‘alteration including biotite, scapolite, chlorite and epidote (VDD0186) The cores are approximate 4,6 cm wide. A) Dominantly biotite alteration with minor scapolite and chlorite. B) Chlorite with patchy biotite and calcite. C) Intense chlorite alteration with calcite veins. D) Sheared and chlorite altered tuff. E: Calcite vein with patchy epidote. F: Pervasive biotite and scapolite alteration. G: Pervasive chlorite alteration and sheared calcite.
Figure 13. Alteration features of the hanging wall (HW) and footwall (FW) of the D-zone A) Tuff with relict lamination B) Red- feldspar veins in the HW mafic sill C) Amygdales in the FW basalt D) Pervasive scapolite alteration in the mafic sill (HW) E) Red- feldspar veins adjacent to calcite vein (HW) F) Pyrrhotite in calcite vein (HW) G) Red feldspar as a rim on calcite vein with sulphides (HW). H) Epidote halo at calcite vein (HW).!
3.1.4 Structural and Alteration Features
The D-zone seems to occur within the centre of a shear zone, which gradually changes into foliation (moderate strain) in the wall rocks and reaches partly into the mafic sill (II) in the hanging wall. The breccia sequence within the footwall is affected by deformation expressed by weak flattening and a commonly preferred alignment of fragments.
Weathering processes in the wall rock were mostly observed close to the present surface, at lithological contacts, fractures and permeable zones in the breccia unit and locally in the dolomite unit. Staining of goethite and hematite, and clay coatings are associated to fractures.
The footwall and the hanging wall have different alteration features. The most common alteration minerals in the examined cores are; calcite, actinolite, epidote, albite, chlorite, biotite and scapolite (fig.14). Minor alteration features are K-feldspar, quartz and talc. Red- feldspars veins, hematite stained calcite veins and quartz veins are of minor importance and occurs through the whole examined stratigraphy.
Figure 14. A summarized and simplified lithological cross section showing the intensity and distribution of alteration minerals related to the D-zone mineralization. Solid line represents pervasive/intense alteration character and dashed line represents patchy/less intense alteration. Calcite porphyroblasts are marked as * (Fig. 10). Abbreviations and core log in Appendix.
3.2 Microscopic Observations
List of thin section samples and a summary of the microscopic results are given in appendix, Table 10 and 11, respectively.
3.2.1 The D-zone
Oxides: Magnetite grains commonly appear as subrounded and subhedral grains in the D-zone and occurs both as fine and coarse-grained (Fig. 15). Magnetite is commonly associated with calcite, chalcopyrite, K-feldspar or quartz. Hematite occurs locally as alteration rims on magnetite grains or as staining in feldspar veins. Magnetite occurring in the wall rocks commonly has a subhedral crystal shape.
Figure 15 Microphotograph of thin sections (TS) of the D-zone in reflected light A) TS165-348: Aggregates of subhedral magnetite. Anhedral chalcopyrite and pyrite interstitial to magnetite grains. B) TS165-353: Subhedral fine- grained magnetite with coarse-grained chalcopyrite replacing the carbonate gangue. C) TS165-339: Footwall breccia with clast containing disseminated magnetite and fine-grained chalcopyrite. Chalcopyrite and subhedral magnetite occurs in the breccia matrix. D) TS165-348: Chalcopyrite and pyrite occurs interstitial to coarse-grained magnetite.
Sulphides: The chalcopyrite grains are subhedral to anhedral and commonly coarse grained (Fig. 15). They occur interstitial to magnetite grains and commonly has inclusions of subrounded magnetite grains or. Pyrite is commonly juxtapose or adjacent to the chalcopyrite grains. The pyrite crystals are commonly subhedral although locally with an euhedral crystal
shape, commonly in local deformation zones and mostly without any major inclusions.
Associated to the sulphides are calcite, epidote, actinolite and chlorite.
3.2.2 The Footwall
Basalt
The least altered basalt consists dominantly of plagioclase and hornblende. The alteration minerals consist mainly of calcite, K-feldspars, epidote, actinolite, sericite, chlorite, with some associated magnetite, hematite, chalcopyrite and pyrite. Accessory minerals are titanite and apatite.
The plagioclase crystals occur in a relict intergranular texture together with amphibole.
Euhedral plagioclase laths commonly have inclusions of small epidote crystals. The amphiboles vary in composition from hornblende to actinolite and may be replaced by chlorite and epidote in varying extent, with later formed epidote at crystal rims (Fig. 17c).
Actinolite occurs mainly as an alteration surrounding calcite veins together with epidote, chlorite and K-feldspar (Fig. 17).
Calcite occurs disseminated, as amygdaloidal infill, porphyroblasts and as veins that commonly contains sulphides. Calcite is commonly in contact with quartz and feldspar, and these minerals are suggested to be paragenetically late. Chlorite occurs in most of the samples, but in varied modal content. The mineral appears often as brown to green flaky crystals (Fig. 16). Chlorite is a late stage, possibly retrograde, mineral overprinting amphiboles, biotite and other Fe-rich minerals. Epidote is an alteration mineral together with calcite, actinolite, chlorite and sericite (Fig. 16 and 17). Epidote occurs commonly in aggregates, disseminated in matrix, as porphyroblasts, and as reaction rims to veins with calcite, chlorite and feldspars (Fig. 17).
K-feldspar occurs as a late mineral, commonly disseminated and in veins together with sericite, carbonate and epidote. Some feldspar veins have locally a reddish tint, suggested to be hematite staining. Sericitization of feldspars (occurs generally more intense in the distal zones) is commonly pervasive in the basalt unit and within K-feldspar veins.
Fine-grained sulphides and oxides occur commonly disseminated in the basalt unit and coarse-grained in calcite or feldspar veins. Magnetite occurs commonly as fine-grained dissemination of subhedral to anhedral grains and is often rimmed by hematite, especially in contact with quartz. Pyrite and chalcopyrite occur mainly disseminated or aggregated commonly with anhedral to subhedral shape.
Breccia
Primary minerals are mostly absent in the breccia due to alteration and weathering. Most commonly observed minerals are coarse-grained feldspar, quartz, calcite and amphibole in a fine-grained calcite matrix. Relict clasts often have an internal cryptocrystalline texture.
Hematite staining and goethite are products of oxidation. Coarse grained quartz and sulphides are commonly fractured. Cubanite is observed as inclusions in chalcopyrite grains.
Figure 16. Microphotographs of hydrothermally altered footwall basalt. A) TS165-174 Epidote and chlorite alteration. B) TS165-008 partly developed epidote and chlorite alteration.
3.2.3 The Hanging wall
Dolomite
The dolomite is generally fine grained and intersected by calcite veins. Coarse crystals of actinolite/tremolite occur disseminated in a weak aligned crystal direction and associated with sulphides and magnetite (Fig. 17 F). The amphibole crystals have weak to none pleochroism suggesting them to be near tremolite in composition. Other alteration minerals are quartz, feldspar, biotite and chlorite. The quartz has a recrystallised granoblastic texture, sometimes with straight grain boundaries but also bulging recrystallized texture (limited grain recrystallization).
Tuff unit
Alteration minerals occurring of the tuff are epidote, chlorite, K-feldspar, sericite, calcite, quartz, oxides and sulphides. Feldspar, epidote and chlorite commonly occur with the sulphides. The matrix is fine-grained and consists of K-feldspars, quartz, epidote and biotite.
Biotite occurs commonly in a preferred alignment, outlining the foliation of the rock. Chlorite is commonly replacing biotite. The sulphides have locally inclusions of epidote (Fig. 15 A).
Graphitic schist
The graphitic schist is very fine-grained and observed minerals are graphite, biotite (Fe-rich) chlorite (Mg-rich), quartz and K-feldspar. The sulphides appear aggregated and disseminated in a preferred alignment parallel with the fabric of the rock.
Figure 17. Microphotography in transmitted light of the most common alteration minerals in the wall rocks. A) Tuff TS186-473: Gangue minerals of chlorite, epidote and K-feldspar surrounding chalcopyrite and pyrite. B) Tuff TS186- 473: Coarse calcite grains surrounded by epidote in a fine grained matrix of biotite, feldspars and quartz. C) Basalt TS165-008: Amphiboles coexisting with epidote and calcite grains. D) Basalt TS165-050: Calcite vein with actinolite, chlorite and feldspars crystals in the rim. E) Basalt TS165-050: Sulphide bearing feldspar vein with red staining.
Chlorite and epidote grains occur at the rim of the vein. F) Calcic-dolomite TS186 -507: Coarse actinolite crystals surrounded by magnetite, chalcopyrite and pyrite.
Mafic Sill, I, and II
The mafic intrusions have an intergranular texture with preserved tabular plagioclase crystals and relict pyroxene crystals. Alteration minerals are epidote, amphiboles, chlorite and biotite.
Biotite with a red-brownish colour is common and partly replaced by chlorite. Epidote and chlorite occur commonly at the edges of plagioclase and amphibole. Ilmenite occurs as an accessory mineral commonly with titanite coronas (see Fig. 38 in appendix).
