MASTER'S THESIS
Geochemical study and identification of a felsic porphyritic unit, Sjöliden, Gold Line, Northern Sweden
Vincent Combes
Master of Science (120 credits) Exploration and Environmental Geosciences
Luleå University of Technology Department of Civil, Environmental and Natural resources engineering
Abstract
The present work identifies and classifies a felsic meta igneous rock mapped at Sjöliden in Västerbotten, northern Sweden. The studied area is located five kilometers North East of the Svartliden gold mine. Situated within the Gold Line, the Sjöliden area is highly interesting from an economic point of view. This thesis has three main objectives, first to find out whether the porphyritic felsic unit has an intrusive or an extrusive origin, and secondly to give a geochemical classification to this rock. Also, a facies analysis of the metasedimentary lithologies is done. Geochemical data, thin sections, core logging and mapping were used for this work. The metasedimentary unit is a turbiditic sequence classified as continental island arc metagreywacke showing primary bedding with a great variation of graphite and sulphide content, grain size and color. It is a deep sea sediment. The felsic porphyritic unit is a meta- trachyte. It presents euhedral to subhedral sanidine phenocrysts, a fine grain groundmass with quartz veining, pyrrhotite, arsenopyrite and gold. Using immobile element and considering the fact that immobile elements show an alteration trend, the geochemical study classifies the felsic porphyritic unit as a trachyte. One of the main characteristic of the rock is its high value of sodium. This can be explained by a high concentration in the protolith, a sodic alteration (Na enrichment and K depletion) and by the deposition in deep ocean (Na rich environment). The most likely theory for the origin of this trachytic unit would be sedimentation of felsic volcanic ash on top of deep sea sediments. The contact is interpreted as a sedimentary cold depositional contact; hence the unaffected sediments. According to the geochemical study, the geodynamic formation of the metagreywacke is continental island arc whereas the trachyte is from a within plate setting. The meta-trachyte and the metagreywacke formed at the same time but the volcanic ash has not a local origin. The volcano was situated in a within plate environment, the ash transport can be up to 1000 km. This hypothesis would explain the fact that trachytic material is uncommon in the region, this trachytic unit having a remote origin. When compared with regional felsic units (intrusive rocks from the Gold Line and extrusive rocks from the Skellefte district), the Sjöliden trachyte presents a different geochemical signature. The comparison of immobile elements ratios highlights the singularity of the unit in the region.
Table of Contents
1. Introduction ...... 1
2. Geological settings ...... 2 2.1. Geology of the Fennoscandian Shield ...... 2 2.2. Regional geology of the Gold Line ...... 3 2.3. Regional metamorphism ...... 4
3. Generation of porphyritic rocks, a literature review ...... 5 3.1. Volcanic origin ...... 5 3.1.1. Felsic lava flow ...... 5 3.1.2. Subaerially eruption with subaqueous pyroclastic flows ...... 6 3.1.3. Submarine eruption of pyroclastic flow ...... 7 3.1.4. Ash layer deposition in the sea ...... 8 3.2. Intrusive hypothesis ...... 8 3.2.1. Sill or dyke ...... 8 3.2.2. Peperite: ...... 9 3.3. Determination of the type of feldspar for determination of the origin ...... 9 3.4. Phenocrysts ...... 10
4. Geology of Sjöliden ...... 10 4.1. Methodology ...... 10 4.2. Location of studied outcrops ...... 11 4.3. Geophysics ...... 13 4.4. Petrology ...... 13 4.4.1. Metasedimentary unit ...... 13 4.4.2. Felsic porphyritic unit ...... 15 4.4.3 Contact between the two units ...... 21 4.5. Geochemistry ...... 23 4.5.1. Metasedimentary unit ...... 23 4.5.2. Felsic porphyritic unit ...... 24 4.5.3. Alteration ...... 31
5. Discussion ...... 34 5.1. On the origin of the metasedimentary unit ...... 34 5.2. Geochemical classification of the felsic unit ...... 34 5.3. On the source of sodium and alteration ...... 35 5.4. On the origin of the felsic porphyritic unit ...... 35 5.5. On the peperite or mixing feature along contact ...... 37 5.6. On the comparison of Sjöliden porphyritic rock ...... 38
6. Conclusions ...... 41
7. Acknowledgements ...... 42
8. References ...... 43
9. Annexes ...... 47
1. Introduction
The aim of this thesis is to identify and classify a porphyritic felsic meta igneous rock occurring at Sjöliden in Västerbotten, northern Sweden. The studied area is located five kilometers North East of the Svartliden gold mine.
This area is situated within the Gold Line, which is a positive gold anomaly in glacial till striking NW-SE, where several gold deposits have been identified. Sjöliden is highly interesting from an economic point of view. The geological interpretation and understanding of the area obtained from the present thesis is therefore very important.
This thesis has two main objectives, first, to find out whether the porphyritic felsic unit has an intrusive or an extrusive origin, and to give a geochemical classification to this rock. Secondly, an attempt to construct a stratigraphical sequence from the drill core and a facies analysis of the metasedimentary lithologies is presented by combining volcanology and sedimentology. Meeting these aims will help greatly future exploration campaigns in the region. This paper will first present the geological settings: geology of the Fennoscandian Shield and geology of the Gold Line. A literature review with a volcanological approach is carried out supporting all possible origin hypotheses for the felsic unit. Here, the determination of the type of feldspar for determination of the origin is displayed. The geology of the Sjöliden area is studied using geophysical observations, field mapping, facies description, phenocrysts abundance logging, core logging and petrography studies. Using these data, the petrology of the metasedimentary units and the felsic porphyritic unit is analyzed and the contact between the two units is examined. A geochemical study of the metasedimentary unit and of the porphyritic unit is done together with an alteration analysis of the porphyritic unit. A comparison of the Sjöliden porphyritic rock with other regional similar units is presented. Finally the origin of formation and final hypotheses are discussed.
Figure 1 Location of Sjöliden, Northern Sweden, modified from Worldatlas.com (2012)
1 2. Geological settings
2.1. Geology of the Fennoscandian Shield
The Fennoscandian Shield represents the western part of the East European craton and is located from western Russia to Norway at the West. It is jointed to the West by the Caledonian orogenic belt. The Fennoscandian Shield comprises Archaean to Neoproterozoic units. There are three main crustal units, the Archaean, Svecofennian, and Southwest Scandinavian domains (Gaál and Gorbatschev, 1987). The orogeny that occurred between 1.9 and 1.8 Ga is called the Svecokarelian. The Svecofennian province is the supracrustal rocks that were set from 1.95 to 1.85 Ga (Geological Survey of Sweden, Wahlgren et al., 1996).
The Archean basement is created during the Saamian orogeny at 3.1-2.9 Ga and the Lopian orogeny. This province is made of gneisses, intrusions of granitoids and greenstones. The Svecokarelian orogeny gives formation of the present crust (Gaal and Gorbatschev, 1987). At 1.95 Ga, the Bothnian Basin starts to form where sedimentation occurs; this creates the metasediments of the Bothnian supergroup (Nironen, 1997). The Archean craton collides with the Arvidsjaur-Kiruna arc at ~1.875 Ga and the Skellefte arc during the Svecofennian orogeny (Juhlin et al., 2002). The Skellefte district is composed of a volcano-sedimentary basin bordered by the Bothnian province to the East and South, a continental domain to the North (Weihed et al., 1992) and related to subduction, South of the present day Skellefte district (Lahtinen et al., 2008; Lahtinen et al., 2005; Nironen, 1997; Weihed, et al., 1992).
Figure 2 Geological map of the Fennoscandian Shield modified after Rutland et al., (2001).
2 After a first collision, a subduction beneath the Skellefte arc took place (Juhlin et al., 2002). The closing time of the basin is dated at 1.87 Ga according to Nironen (1997). During the subduction phase, there are intrusions of granitoids from 1.89 to 1.85 Ga. The Skellefte district is formed at a convergent margin. It is characterized by a northward subduction under the Skellefte-arc at about 1.9 Ga (Weihed et al., 1992).
The gold deposits were formed during crustal shortening at 1.90-1.86 and 1.85- 1.79 Ga (Weihed et al., 2005). The host sequence has undergone metamorphism peaking at mid-amphibolite facies (Kathol and Weihed, 2005). An interpretation of the geology of this area can be seen in figure 3.
Figure 3 Geological Interpretation after Weihed et al. 1992 combined with magnetic airborne map from SGU
2.2. Regional geology of the Gold Line
The Swedish part of the Svecofennian province contains a few orogenic gold deposits. Several gold occurrences have been discovered and presently the Svartliden deposit is actively mined.
At a local scale, the area is located along a straight line with the gold mine Svartliden and Faboliden at the South, called the Gold Line. The Gold Line is a North West trending linear feature on till-geochemistry anomaly maps of Au and other normally Au- associated elements (Bark and Weihed, 2003). Three gold deposits (Sjöliden, Mevjankilen, Svartliden) within the studied area are represented in the following geological map modified and simplified after Bark and Weihed (2007), (figure 4 A).
3 A B
Figure 4 A Bedrock map of the Gold Line showing granitoids, Skellefte and Revsund type, 1.86-1.75 Ga, mafic metavolcanic rocks, 1.88-1.86 Ga, metavolcanic and metaigneous rocks, 1.91-1.88 Ga, and metagreywackes, 1.95-1.87 Ga. modified and simplified after Bark and Weihed (2007).
4 B Geological map at a smaller scale showing granitoids in pinks, metagreywacke in blue, felsic unit in yellow and mafic unit in green (coordinates in RT90) modified after Kero and Björk (2001)
2.3. Regional metamorphism
The Svecofennian province is characterized by a high temperatures and low pressures metamorphism (Gaál and Gorbatschev, 1987; Weihed et al., 1992; Weihed et al., 2002). P-T analyses confirm that northern Sweden is a low to intermediate pressure province, with pressures of 2–4 kbars (Bergman et al., 2001). All units in the Skellefte District are metamorphosed at greenschist to lower amphibolite facies conditions. There is an increase in metamorphic grade towards the Bothnian Basin (Weihed et al., 1992).The metasedimentary units in the Bothnian Basin have been metamorphosed into amphibolite facies (Allen et al., 1996) but also into granulite facies in some places (Hallberg, 1994; Lundström, 1998).
Amphibolite facies conditions were identified in the Storuman area, (Lundström, 1998) which is located to the North of the studied area. The metagreywackes in that area have been metamorphosed at 500–580°C (Lundström, 1998).
4 3. Generation of porphyritic rocks, a literature review
To better understand the formation of the felsic porphyritic rock, a review of possible hypotheses for the origin (table 1) is done. The two possible origins are intrusive or extrusive. But these terms have to be seen as end members; indeed a subvolcanic origin can also be possible. In theory intrusive igneous rocks generally present larger crystals than extrusive rocks because of the amount of time spent during mineral crystallization. Extrusive rocks are formed at the surface, and generally display smaller mineral crystals, or no crystals at all (obsidian), because of the rapid cooling due to the great variation of temperature. Chemically, an intrusive and an extrusive rock can be identical. It is therefore the size of crystal which can discriminate them (McPhie et al., 1993).
Table 1 List of hypothesis of event giving a fine grain groundmass and phenocryts according to literature
Origin Type Description Contact Phenocrysts Intrusion of felsic material Porphyritic if Subvolcanic intrusive within the sedimentary Chilled margin shallow depth sill or dyke host rock intrusion extrusive Lava Felsic lava flow Breccia on top Rare Subaerially eruption with Possible Peperite, extrusive Pyroclastic Subaqueous pyroclastic crystal rich breccia flows, turbidite layers Submarine eruption with Possible Peperite, extrusive Pyroclastic subaqueous pyroclastic crystal rich breccia flows, turbidite layers Sharp, Ash layers deposition on Volcano- sedimentary Basal part extrusive the sea floor, sedimentary contact, cold with crystals sedimentation material
3.1. Volcanic origin
3.1.1. Felsic lava flow
It is a very rare case where an eruption of a highly viscous material with high gas content gives a lava flow. This highly felsic material can produce obsidian or rhyolite, (Ayres and Peloquin, 2000).
Figure 5 Dome and thick rhyolite lava flow California near Mono Lake. (From http://earthds.info)
5 The lava of a felsic flow has low temperature, high gas content and high viscosity. According to Manley (1992), felsic units described as lava flows can be found in Southwestern Idaho (Bonnichsen, 1982), Yellowstone National Park (Christiansen and Hildreth, 1988), and Trans-Pecos Texas, USA (Henry et al., 1988), the Bushveld (Twist and French, 1983), southeastern Australia (Dadd, 1989). Yet, those conclusions have been highly debated. Characteristics of a felsic lava flow are the presence of crumble breccias at the basal part but also at the flow front. The youngest, most studied, rhyolite lava units worldwide are characterized by a small size (Walker, 1973). In North America, well-exposed flows are generally no larger than 1 km3 and usually less than 0.5 km3 (data from Heiken, 1978; Clough et al., 1982; Sieh and Bursik, 1986; Scott, 1987). The longest rhyolitic lava flow is located in Glass Mountain, at Medicine Lake Highland Volcano in northern California; the flow extends only 3 km downslope from its vent (Fink, 1980). There are other lava flows classified as felsic lava flows but they are less well-known, like the 6-km-long units of early rhyolite (Bailey et al., 1976) in Long Valley Caldera, eastern California, the 9-kmlong Banco Bonito obsidian flow (Self et al., 1988), Valles Caldera, New Mexico and the 13-km-long Rhyolite of Comb Peak (Christiansen and Lipman, 1966), Timber Mtn. Caldera, Nevada. Contact gives columnar jointed lava lobes and breccias.
3.1.2. Subaerially eruption with subaqueous pyroclastic flows
In this case, a subaerially eruption occurs. Then a pyroclastic flow is going into the ocean. It is a highly explosive eruption. A crystal rich layer at the base can be found according to Wright and Mutti, (1981). The following sketch (figure 6) from Wright and Mutti (1981) shows a model with a subaerially erupted pumice flow entering the sea.
Figure 6 Model of a pyroclastic flow entering the ocean (from Wright and Mutti, 1981)
Pyroclastic flows can occur in different ways, as seen in figure 7. It can be: a large ignimbrite flow (A), a small flow stopped rapidly by cooling from water at the sea floor (B) or a ash flow giving a turbidity current of ash going far from the vent (C).
6 It can also give an epiclastic process with slumping as the slope of the volcano is steep, with collapse of the flank of the volcano (D). In this case the deposition can be far from the volcano. In all cases, the style of deposit is violent, giving not a smooth contact with the seafloor and possible erosion of underlying sediments.
Figure 7 Model for the passage of pyroclastic flows into the sea, and the transportation of volcanoclastic material from the volcano to the ocean basin showing the welded ignimbrite, the pumice flow deposit, ash flow deposit and ash turbidite. (From Wright and Mutti, 1981)
According to McCoy and Cornell (1990), the most common volcaniclastic sediments are volcaniclastic turbidites. There are several origins, like a generation from pyroclastic gravity flows entering the sea, a reworking of tephra fall, and a reworking of pyroclastics during the repose period between volcanic events. The content of volcanic glass and presence (or lack) of sediment structure are the two characteristics to classify the sediments into four sediment-deposit types: tephra fall, volcaniclastic turbidite, debris flow or volcanic sand.
According to Stix (1991), subaqueous mass flow deposits often show a relative enrichment of crystals, especially at the bases, from flotation and suspension of the vitric material and ingestion of water at the head of the flow.
3.1.3. Submarine eruption of pyroclastic flow
In this case, unlike the previous hypothesis, the eruption is submarine. A crystal rich layer can also be found (Wright and Mutti, 1981). The felsic material is then colder when the deposition on the sea floor occurs.
7 3.1.4. Ash layer deposition in the sea
When an eruption occurs, a large amount of ash is produced. This felsic material can fall in the ocean and form a deposit on the sea floor. The following log (figure 8) from Wright and Mutti (1981), represent the Minoan ash layer core taken 170 km South East of Santorini, Greece. From base to top, they interpreted the section as plinian fall, fine ash fall; two repeated ash turbidites, laminated ash layers and on top pelagic mud mixed with ash.
Figure 8 Log modified from Wright and Mutti (1981) showing an increase of the crystals content at the basal part. 3.2. Intrusive hypothesis
3.2.1. Sill or dyke
The felsic unit could be an intrusive porphyritic unit like a sill or a dyke. The typical contact between an intrusion and its host rock is studied by Jerram and Petford (2010). The host rock is affected by the warmer intrusive rock; there is more effect on the host rock than on the intrusive rock. The scale of this baking can be centimeter to kilometer long. According to Skilling (2001), effects on the host rock when the magma intrudes are melting, cementation, dewatering, vesiculation, fracturing, compaction, fragmentation, fluidisation, and liquefaction. Also, the host rocks frequently show effect of thermal metamorphism of a few centimeters to few meters. The intrusion can have a subvolcanic origin when the intrusion occurs near the surface; the igneous material has then a lower temperature and a fast cooling. The contact of an intrusion is generally fine grained or glassy. The cooling of the intrusion can be determined by studying the grain size variation. A large variation in grain size means emplacement of hotter magma into cooler country rock, if no variation, there is a lower degree of variation.
8 3.2.2. Peperite:
According to Skilling (2001), the rock called peperite is an indicator for interaction of igneous material (intrusions, lavas and hot volcaniclastic deposits) in wet, unlithified rocks. A peperite is the mixture of igneous clasts within sediments. Clasts and host sediment are mingled and dispersed (figure 9). Its formation is due to fluidisation of host sediment, hydromagmatic explosions and forceful intrusion of magma. Peperite is common along contacts between sediment and intrusions, lavas and hot volcaniclastic deposits.
Figure 9 Model of a peperitic contact with fragmentation of felsic rock modified after, Goto and McPhie (1998). 3.3. Determination of the type of feldspar for determination of the origin (Steiger and Hart, 1967)
There are 4 principal varieties of K-feldspar: microcline KAlSi3O8 (low temperature), orthoclase KAlSi3O8 (medium temperature), sanidine (K,Na)AlSi3O8 (high temperature) and anorthoclase (Na, K)AlSi3O8.
The determination of the type of potassium feldspar of the phenocryts of a rock is very important to find out whether it has an intrusive or an extrusive origin. Indeed, microcline is present in felsic plutonic rocks but not in volcanic rocks, and in some metamorphic rocks. Orthoclase is common in many silicic intrusive rocks, but it is less common in volcanic units. It is also found in metamorphic rocks. Sanidine is the typical K feldspar in felsic extrusive rocks. Microcline is a potassium rich alkali feldspar. It contains some minor amounts of sodium. It is a common mineral in granite and pegmatites which forms during slow cooling of orthoclase. Microcline is more stable at lower temperatures than orthoclase. Microcline is characterized by cross-hatch twinning. This forms after the transformation of monoclinic orthoclase into triclinic microcline. Orthoclase is often present in granites and other felsic igneous rocks. Sodium-rich albite lamellae are formed by exsolution when the temperature decreases, enriching the remaining orthoclase with potassium. The intergrowth of the two feldspars gives a pertitic texture. Sanidine is the high temperature form of potassium feldspar. Sanidine is typical of rocks like obsidian, rhyolite and trachyte. The crystal system of sanidine is the monoclinic system. Orthoclase is the polymorph stable at lower temperatures. Sanidine can contain more sodium in its structure than microcline and orthoclase. Sanidine and high albite constitute a solid solution serie. The
9 intermediate composition is the anorthoclase. Anorthoclase is stable at temperatures of 600 °C and above. Anorthoclase has a triclinic symmetry, whereas sanidine and orthoclase have monoclinic symmetry. Microcline is often characterized by a cross-hatched twinning with twin domains that pinch and swell. Orthoclase and sanidine can have simple twins; grains are divided into two domains. When untwinned, distinguishing the different K-feldspar varieties may be difficult.
3.4. Phenocrysts
If a unit has an extrusive origin, why are there phenocrysts? According to Wright and Mutti (1981) the formation of crystal-rich pyroclastic and epiclastic volcanoclastic deposits can occur in the airfall ash, in the lower part of the eruption column, or at the basal part of ignimbrites.
If a unit has an extrusive origin, the phenocrysts size should follow a sorted distribution due to deposition of the pyroclastic material, if there is a random distribution or fewer crystals near the contact, it would suit more an intrusive origin.
4. Geology of Sjöliden
4.1. Methodology
Field mapping:
The field mapping of the area has been done during the summer 2011, from June to August while working for Dragon Mining Sweden (DMS). Based on airmag and existing SGU bedrock maps, traverses were planned crossing distinct geomagnetic signatures. Rock types were determined from outcrops studies (grain size, magnetic susceptibility, colors, mineralogy, and weathering). Eighteen different lithologies were defined based on observations done on 217 outcrops. The outcrop sizes vary widely, ranging from 1 m2 up to 100 m2. All the results of these three months of mapping were combined in a written report and a MapInfo workspace.
Sampling:
Approximately fifty samples were collected during the summer 2011. Metasediments and felsic porphyritic rocks hand samples were collected during the field mapping (See chapter 4.2. Location of studied outcrops) for geochemical analysis and thin sections studies.
Drill Core:
Drillcores from Dragon Mining Sweden were used for various purposes during this work as seen in table 2. These cores come from two areas, Sjöliden 1 (2011) and Sjöliden 2 (2012) as seen in figure 11.
10 Table 2 Purpose of study for each drillcore
Study purpose Drillcore Thin section contact SV 11484 3 meters geochemical assay SV 11476 8 meters felsic unit/ 4 meters sediment SJ 11609 Thin sections felsic unit SV 11476 / SV 11483 Thin section metasediment SV 11484 Phenocrysts abundance study SV 11474 Phenocrysts abundance study SJ 11614 Density study SJ 11614 Photo metasediment /felsic unit SV 11484 / SV 11474
Thin section:
14 polished thin sections were manufactured by Minoprep Sweden during the winter 2012. 4 thin sections from different types of metasediment and 10 thin sections of the felsic unit were made; with one showing the contact with metasediment (core SV 11484). Also, ten existing thin sections from DMS were used. These sections, of felsic material were cut from the drill core SV11476 in 2011.
Geochemical analyses:
4 meters of metasediment from the core SJ 11609, 8 meters of the felsic unit from the core SJ 11609 and 41 meters of the felsic unit from the core SV 11476 were sent for geochemical analysis at ALS Chemex Minerals Piteå. From the resulting data, main and trace elements were plotted in discrimination diagrams using the plotting software Petrograph and Excel. The complete characterization package (CCP-PKG01) from ALS was used. This package uses a whole rock analysis by combustion furnace to quantify the major elements in the sample. Trace elements including rare earth element suites are reported from three digestions: a lithium borate fusion for the resistive elements, a four acid digestion for the base metals and an aqua regia digestion for gold and other economic trace elements (ALS laboratory documentation, 2012). Two type of analysis were done, MEMS41 and MEMS81. 41 meters of the felsic unit (core SV 11476) analyzed by MEMS41 were used to study the thorium and cobalt content. MEMS41 is less reliable for more resistive materials (like zircon) as they are not completely dissolved; data are reported from an aqua regia leaching process. The MEMS81 is more reliable and was used for characterizing the rock (15 samples). The process is a lithium borate fusion of the sample prior to an acid dissolution and an ICP-MS analysis (ALS laboratory documentation, 2012).
4.2. Location of studied outcrops
Like most of the North of Sweden, the area is relatively poor in outcrops, but more than 200 outcrops were found. In this flat area, outcrops are relatively small and cover by vegetation. Reference outcrops are visible in figure 10 for the metasedimentary unit and in figure 11 for the felsic unit. The two drilling sites Sjöliden 1 and Sjöliden 2 are also marked.
11
Figure 10 Reference outcrops of metasediments with geological map modified from Kero and Björk (2001)
Figure 11 Satellite image showing in red dots the reference outcrops of the felsic unit
(including outcrop R08, see petrography chapter) and the 2 drilling sites:
Sjöliden 1 and 2, image modified from Google Earth.
12 Outcrops of the felsic unit (visible in previous image) were only found on two main topographic levels as visible in the topographic map below.
Figure 12 Topographic map with in red dots outcrops of the felsic unit modified from GSD Blue Map National Land Survey of Sweden.
4.3. Geophysics
A density study of the felsic unit has been done on eight meters of the core SJ 11614. An airborne map from Dragon Mining Sweden was used for the field mapping in order to localize structures (faults, folds) and geological domains by interpreting magnetic structures in the area.
4.4. Petrology
4.4.1. Metasedimentary unit
In the studied area, metasediments outcrops (figure 16) are relatively common. According to field observations, all metasediments found are parts of a turbiditic sequence of deep sea sediments. It gives high variation of grain size, structure, color and composition. The main features can be described as following with photos from thin sections.
Meta arenite is present as small beds in the Sjöliden area within the meta argillite. With a homogeneous matrix and some white bands, this unit is dark or light grey. No mineralization is present and outcrops appear medium to fine grained and massive. In places, outcrops can show schistosity.
13 Graphite rich metasediment is a minor unit occurring as lenses within the meta argillite. It shows no sharp geomagnetic signature so the extent of this unit is poorly defined. It is pyrrhotite, pyrite and graphite bearing as seen in the thin section photo at right. It can be brecciated. Some outcrops showed quartz veining. It is dark in color and fine grain. The susceptibility ranges from low to medium (~0.5). The schistosity of the rock is generally high but outcrops with lower schistosity are also present.
Figure 13 Graphitic sulphidic metasediment
Meta argillite/mudstone is a massive metasedimentary unit dominating the central part of the area. It is dark grey in color when fresh and some areas show clasts which seem to be of quartz. It is rich in pyrrhotite resulting in a high susceptibility (>1). This unit is biotite rich and can contain some graphite. It shows primary bedding and variation in composition of beds, with a groundmass of quartz, biotite, plagioclase and graphite in the photo at right from core SV 11484.
Figure 14 Meta argillite showing primary bedding and variation of grain size
Meta argillite with bedding is present mainly in the South East part of Sjöliden. It consists of alternating dark and white bands of argillite sediment visible in the thin section photo at right. In certain areas the white bands are dominant. The magnetic signature is low to medium. It is very fine grain. The bands are folded. Certain outcrops show quartz veining. The unit has no contact to the felsic unit and shows primary bedding together with some white pumiceous clasts.
Figure 15 Meta argillite showing strong schistosity
14 The following sedimentary facies are interpreted as parts of the Bouma sequence (see discussion chapter): very fine grain homogeneous silt, graphite and pyrite rich layer, folded and banded, sandy layer, graded bedding (see figure 14), clasts and light grey massive homogeneous arenitic layer.
Figure 16 Example of metasedimentary outcrop. Photo from DMS 2011.
Figure 17 Primary bedding and variation of grain size on the core SV11484, photo from DMS 2011.
4.4.2. Felsic porphyritic unit
This unit is significant for its potential gold mineralization. Outcrops of this unit have been the main target during the field mapping. It is a porphyritic igneous rock with quartz veins. Here, this rock will be studied for both hypotheses concerning its origin (intrusive and extrusive). Outcrops are found along the same topographic trend. In two outcrops the contact to the metasediments is exposed. The color is light grey to white depending on alteration intensity. In the core, the felsic unit can be found as lenses, as alternating sequences within metasediment or as a single horizon. This felsic unit is relatively small, up to 10-15 meters thick.
15 The felsic unit has a relatively high content of phenocrysts. The rock is rich in arsenopyrite found in two forms, disseminated (forming black spot in the groundmass) and filling fractures with pyrrhotite. The unit is also rich in quartz veins. Fractures are often oriented perpendicular to the contact with the metasediment. Locally the unit presents a breccia appearance with a high content of chlorite. Characteristics of this unit are presented in the core SV11474 photos in figure 18 and 19.
Figure 18 Photo of the felsic unit from core Sjöliden 1, showing from left to right: Pyrrhotite and arsenopyrite disseminated and filling fractures; Milky quartz vein; 4mm large K feldspar phenocryst with rim (see petrography chapter).
Figure 19 Variations of phenocryst content within the core SV11474.
16 The density of the felsic porphyritic rock in the core SJ 11614 has been measured on eight meters. The data are presented in table 3.
Table 3 Density measurements for eight meters of core of the felsic unit.
Meters Density from g/cm3 3-4 2,72 9-10 2,79 16-17 2,68 20-21 2,78 29-30 2,63 31-32 2,67 41-42 2,65 49-50 2,64
The mean value is 2.69 g/cm3, with a variation within the two lenses. The first lens (from meter 3 to 21) has a mean density of 2.74 g/cm3 whereas the second lens has a value of 2.65 g/cm3. According to Daly et al. (1966) and Johnson and Olhoeft (1984), average densities for rhyolite is 2.51, for granite 2.66, for trachyte 2.57 and for syenite 2.75. Those data are for unmetamorphosed rocks.
Quartz veining
The porphyritic unit is characterized by a large amount of quartz veining. These veins seem to have two main orientations (Dip direction/dip: 81/58 and 110/50) and can be up to 50 cm wide. Some of them can be milky.
Phenocrysts abundance study:
A study of the variation of abundance, size and shape of the phenocrysts was done to see any correlation between distance from metasediment contact and abundance of phenocryts. If there are less phenocrysts near the contact it would indicate more of an intrusive origin, if more phenocrysts occur at the basal part near the contact, it would indicate a pyroclastic origin. Cores SV11474 and SJ11614 were used as there is an alternation of the two units and several contacts within the core are visible. Following a rate of 25 cm distance, the number of visible phenocrysts on the surface of the core was counted. The results are presented in the following figures. Some quartz vein rich parts of the unit were avoid to not give a false result while counting the content of phenocryst.
17
Figure 20 Phenocrysts abundance on core surface of felsic unit with respect to core depth.
18
Figure 21 Phenocrysts abundance on core surface of felsic unit with respect to core depth. (Vertical scale not respected for better visual)
Petrography
The petrography of the felsic porphyritic unit was made using thin sections from core SV11476, core SV 11483 (at 21.9m) and the hand sample from outcrop 043 of Sjoliden 2. These thin sections were studied with cross-polarized light microscope at the University of Lulea. The mineralogy, size-shape of the phenocrysts, and texture of groundmass were analyzed to find indication of the origin of the unit.
This unit has a porphyritic texture comprising euhedral to subhedral alkali feldspar phenocrysts. This K-feldspar phenocryst is identified as sanidine but this statement is discussed in the discussion chapter. The amount of those phenocrysts is around 5-10%; their dimension is about 4 mm max in size and they may have a rim of albite due to exsolution of sodium (see figure 22 B and discussion chapter). The groundmass contains plagioclase (albite), biotite and some quartz. Less common minerals present in the rock are sphalerite,
19 pyrrhotite and arsenopyrite. Some phenocrysts are present at the contact with the metasediments. Even if the unit is altered and metamorphosed, there is still a primary texture, as it is a relatively low metamorphism. Indeed, most of the phenocrysts are unaffected. Some bending has been identified but seems to be of tectonic origin. This porphyritic texture means two stages of cooling. A first stage of slow cooling, during which the large grains developed, is followed by a period of more rapid cooling, during which the smaller grains formed. The main petrographic characteristics of the felsic unit are presented in the following figure with photos A and B from core SV 11483, photos C and D from hand sample outcrop R08 and photos E and F from core SV11476.
Figure 22 A Albite rich groundmass at the left top with metasediment at right (see chapter contact) B Exsolution albite from sanidine K feldspar: sodium goes to form albite around the phenocryst giving rims, C, D Subheudral K feldspar phenocrysts, E and F Perthitic features
20 4.4.3 Contact between the two units
The contact between the metasediment and the felsic unit is extremely sharp and curved; both units seem to not be affected. The metasediment has the same appearance at the contact and far from it: no alteration or baked contact characteristics are visible. It is the same observation concerning the felsic unit. A perfectly exposed contact is visible in the core SV 11476 (see figure 23). When a succession of felsic material and metasediment is visible, in the core SV 11474, the same sharp contact is present on top and below the felsic lens. In the core SJ 11604 and 03, (figure 23) at 22.43 m and 33.55 m, a fluid passing along the contact due to the impermeability of the metasediment can be observed. In one observed case, in the core from Sjöliden 2 the contact could represent a peperitic or “mixing” feature. (See chapter discussion).
Figure 23 Variation in the type of contact from sharp to mixed with fluid circulation in cores SV 11476, SJ 11604 and SJ 11603.
21 Petrography
A thin section was made to study the contact in the core SV 11484 at 16.5. The groundmass of the two units looks similar in microscope view (figure 24). There are less phenocrysts near the contact but one phenocryst is set 1 mm from the contact. The graphite allows the discrimination between the two rocks, indeed there is a high content of graphite in the metasediment groundmass and no graphite in the felsic rock. The contact itself measures 100µm. It is made of graphite, biotite and quartz. The two groundmasses are extremely fine grain.
Figure 24 The metasediment at left and the felsic unit with whiter and bigger crystals at right, in black the graphite visible within the metasediment. Contact in core SV 11484 at 16.5
Figure 25 Microscopic view of the contact, showing dissolution features and phenocryst near contact. Contact in core SV 11484 at 16.5
22 4.5. Geochemistry
4.5.1. Metasedimentary unit
Even if the sediments present in this area have been affected by metamorphism, samples can be plotted in a sedimentary discriminative diagram for classification of terrigenous sandstones and shales, from Herron (1988), to get a general geochemical identification. The samples, taken in a drill core from Sjöliden 2, plot in the greywacke field. Therefore the unit can be called metagreywake.
Figure 26 Discriminative diagram: The classification of terrigenous sandstones and shales, from Herron (1988).
In the In the La–Th–Sc tectonic discriminative plot for greywackes (Crook 1986), all samples plot in the continental island-arc field (figure 27).
Figure 27 Samples plotting in the continental island arc in tectonic discrimination diagram from Crook, (1986). 23 Table 4 Main elements (in %) and trace elements (in ppm) of 4 samples of metasediments using MEMS81
Sample Sample SJ2 68 SJ2 75 SJ2 76 SJ2 77 SJ2 68 SJ2 75 SJ2 76 SJ2 77 No No
SiO2 (%) 64.10 62.60 66.60 59.20 Ni 62.00 83.00 81.00 97.00
Al2O3 15.25 14.50 14.30 15.50 Pb 18.00 28.00 20.00 17.00
Fe2O3 5.95 7.27 6.69 8.06 Pr 10.40 7.29 8.12 8.03 CaO 1.69 1.93 1.79 1.47 Rb 111.50 155.00 155.00 185.00 MgO 2.11 2.91 2.66 3.35 Sm 6.30 5.73 6.26 5.94
Na2O 5.06 4.21 3.68 2.90 Sn 1.00 3.00 1.00 2.00
K2O 2.08 2.82 3.04 4.07 Sr 272.00 216.00 253.00 209.00
Cr2O3 0.02 0.02 0.02 0.02 Ta 1.20 0.90 0.90 0.90
TiO2 0.57 0.69 0.68 0.71 Tb 0.71 0.72 0.77 0.77 MnO 0.06 0.09 0.05 0.05 Th 16.65 9.83 10.75 11.10
P2O5 0.11 0.14 0.14 0.09 Tl 0.70 1.00 0.90 0.90 SrO 0.03 0.02 0.03 0.02 Tm 0.37 0.39 0.41 0.42 BaO 0.04 0.05 0.05 0.06 U 7.13 3.66 3.70 4.31 Ag (ppm) <1 <1 <1 <1 V 105.00 141.00 129.00 166.00 Ba 405.00 478.00 493.00 588.00 W 35.00 2.00 1.00 2.00 Ce 94.00 58.10 65.60 63.80 Y 24.20 24.00 25.70 25.60 Co 12.60 16.60 17.00 20.60 Yb 2.38 2.52 2.61 2.66 Cr 130.00 170.00 170.00 160.00 Zn 112.00 151.00 145.00 170.00 Cs 6.11 7.74 9.62 11.75 Zr 260.00 160.00 180.00 140.00 Cu 43.00 59.00 53.00 95.00 Ag <0.5 <0.5 <0.5 <0.5 Dy 4.08 4.34 4.66 4.58 As 15.00 39.00 16.00 21.00 Er 2.46 2.53 2.78 2.71 Cd <0.5 <0.5 <0.5 0.80 Eu 1.27 1.17 1.30 1.24 Co 13.00 16.00 15.00 19.00 Ga 21.30 20.10 19.00 22.40 Cu 47.00 60.00 55.00 100.00 Gd 4.71 4.76 4.89 4.94 Mo 2.00 <1 1.00 4.00 Hf 6.50 4.40 5.00 3.90 Ni 55.00 70.00 69.00 84.00 Ho 0.84 0.87 0.95 0.93 Pb 20.00 28.00 22.00 19.00 La 50.50 28.30 32.50 32.00 Sc 12.00 16.00 14.00 18.00 Lu 0.39 0.38 0.40 0.42 Zn 109.00 149.00 139.00 171.00 Mo 3.00 <2 2.00 5.00 LOI 1.37 1.98 1.32 1.63 Nb 18.10 12.80 11.60 12.50 Total 98.44 99.23 101.05 97.13 Nd 35.00 26.90 29.90 29.60
4.5.2. Felsic porphyritic unit
The geochemical study of the felsic porphyritic unit was made using analyses from 8 meters of the drillcore SJ11609 from Sjöliden 2 and analyses from 3 meters of the drillcore SV11476 from Sjöliden 1. These chemical data can be seen in table 6. As seen in the petrographic study of this unit, the rock is altered. Samples are plotted (figure 28) in the K2O + Na2O vs K2O/ (K2O + Na2O) diagram of Hughes (1973) to discriminate the least altered samples. Samples plotting in the igneous spectrum are the least altered.
24 The 3 least altered samples are SJ2-71, SJ1-19 and SJ1-20. Their mean content of SiO2, Na2O and K2O are 67.13 %, 6.59 % and 3.69 % respectively.
14 Na-altered
12 SJ2-71 SJ1-20 SJ2-69 SJ2-73 10 SJ2-72 SJ1-19 SJ2-79 SJ2-70 SJ2-80 8 SJ1-15 O (wt%) 2 SJ2-81 Igneous 6
O +K spectrum 2 K-altered Na 4
2
0 0 10 20 30 40 50 60 70 80
K2O / (Na2O + K2O) x 100 (wt%)
Figure 28 Data plotted in the igneous spectrum (Hughes, 1973), showing the least altered samples among samples SJ2-69, SJ2-70, SJ2-71, SJ2-72, SJ2-73, SJ2-79, SJ2-80, SJ2-81, SJ1-15, SJ1-19 and SJ1-20.
The plotting of the samples in a TAS diagram can give some information. Even if the rock is altered it reflects a general trend for their protolith (see figure 29 and 30). It shows that the unit is intermediate to felsic: the 3 least altered samples plot in trachyte if extrusive rock and alkaline rich granite plotting at the border with syenite if the rock is intrusive. As the mean value of SiO2 is 67.13 % for the least altered samples and increase for the more altered samples (up to 77.70 % of SiO2), the protolith has therefore at least 67.13 % of SiO2. The protolith cannot be a rhyolite. The use of discriminative diagram using oxides is not very convenient here so immobile elements are preferred. According to Hastie, 2007, the K2O–SiO2 diagram, used to subdivide volcanic arc rocks into rock type (basalts, basaltic andesites, andesites, dacites and rhyolites) and volcanic series (tholeiitic, calc-alkaline, high- K calc-alkaline and shoshonitic), is particularly susceptible to the effects of alteration. By using Th as a proxy for K2O and Co as a proxy for SiO2 (Hastie, 2007), it is possible to construct similar diagram that gives the same discrimination. But it is more robust for weathered and metamorphosed rocks. Such diagram is very suitable here. Indeed the variation in the content of potassium due to alteration and the possible silicification of the rock would give a false characterization of the rock in a K2O versus SiO2 diagram.
25
Figure 29 Classification diagram for extrusive rocks, samples plot in the trachyte-rhyolite field after Le Maitre et al. (1989).
Figure 30 Classification diagram for intrusive rocks, samples plot in the granite field after Cox et al. (1979).
26 43 values from 43 meters of cores SV11476 and SJ11609 are plotted in figure 31. These values plot on the dacite- rhyolite-trachyte field if it is an extrusive rock or syenite- granite if it is an intrusive rock. According to this plotting, the rock belongs to the high-K calc-alkaline series or to the shoshonite series. This conclusion is fairly reliable as it uses very immobile elements and because forty three meters of a core are used to avoid local anomalies.
100
10
1 Th (ppm) Th 0,1
0,01 50 40 30 20 10 0 Co (ppm)
Figure 31 Th versus Co diagram, samples plotting in the felsic high-K calc-alkaline field (Hastie, 2007)
This unit presents an uncommon high value of thorium for the region, from 25 to 40 ppm as seen in the following table. No correlation is visible between the Th content and alteration.
Table 5 Thorium content in 11 meters of core Sjöliden
Sample SJ2- SJ2- SJ2- SJ2- SJ2- SJ2- SJ2- SJ2- SJ1- SJ1- SJ1- No 69 70 71 72 73 79 80 81 15 19 20 Th 40.0 35.8 33.4 36.4 35.1 36.0 29.9 25.3 26.0 34.2 32.4 (ppm)
Nb, Y, Zr and Ti remained chemically immobile during alteration, and metamorphism. Therefore these immobile elements allow the identification of the precursors of altered rocks. Diagrams SiO2 versus Nb/Y of Winchester and Floyd (1977) and Zr/TiO2 versus Nb/Y of Pearce (1996) are very suitable for this study as the unit is altered. These diagrams are widely used as immobile elements proxy for the TAS diagram. Using values from 11 meters of core (values in ppm), we obtain the diagrams in figure 32. These samples plot on the trachyte field if it is an extrusive rock or syenite/granite if it is an intrusive rock. In the SiO2 vs Nb/Y, the least altered samples plot in the trachyte field and an enrichment of SiO2 is visible.According to this discriminative plot, this unit is evolved and alkaline.
27 80
Rhyolite Comendite 75 Pantellerite
70 Least altered Rhyodacite sample Dacite 65
Trachyte %
60 2 Andesite Trachyandesite Phonolite SiO 55
50 Sub-alkaline basalt Alkali basalt
45 Basanite Nephelinite 40 0,01 0,1 1 10 Nb/Y (ppm)
Figure 32 Discriminative diagram SiO2 versus Nb/Y and Zr/TiO2 versus Nb/Y showing a trachyte syenite protolith from Winchesterand Floyd (1977) and Pearce (1996)
28 When plotted in the diagram Y vs Zr, the least altered sample are located in the middle of the alteration trend showing mass gain and mass loss for the more altered sample. The content of Zr is related to the content of silica as the more SiO2 rich samples are the more depleted in Zr. The least altered samples have a Zr content between 529 and 550 ppm.
50
45
40 Least altered samples 35 SiO enriched 30 2
samples
) 25 Y (ppm 20
15
10
5
0 0 100 200 300 400 500 600 700 800 Zr (ppm)
Figure 33 Y versus Zr diagram showing mass gain and mass loss for the altered samples. Geodynamic study
Samples are plotted in felsic tectonic discriminative diagrams in figure 34. These diagrams from Pearce et al., (1984) and Pearce (1996) are made for granite but their use for a trachytic rock seems reliable in the present case. Trace elements are used: Rb against Y+Nb, Nb against Y and Ta against Yb giving discrimination between syn-collisional, within plate, volcanic arc and ocean ridge setting. Using the same 11 samples as previously, the three diagrams gives a within plate tectonic origin. Indeed, in this type of plot the sample set in the middle of the alteration line is the least altered and therefore the more reliable. For Ta vs Yb, the three least altered sample plot in the border between within plate and volcanic arc fields, (see discussion chapter). According to Leat et al., (1986), the Nb content of the felsic rocks gives information about the tectonic setting, here the value is relatively high (about 30 ppm) meaning a rifted continental crust whereas low value of Nb points toward magma formation in a subduction-related setting.
The result from this plotting suit well the previous conclusion that the rock is of trachytic composition, indeed trachyte are typical of within plate tectonic settings like continental rifting.
29
1000 Syn- collisional Within- plate
100
Rb (ppm) Volcanic arc Ocean- ridge
10 1 10 100 1000 Y+Nb (ppm)
1000
100 Within-plate
Syn- Nb (ppm) collisional 10 Ocean- Volcanic arc ridge
1 1 10 100 1000 Y (ppm)
10 Within-plate
Syn- collisional
1
Ta (ppm) Ocean- Volcanic ridge arc
0,1 0,1 1 10 100
Yb (ppm)
Figure 34 Tectonic discrimination diagrams showing a within plate origin for the felsic unit when focusing on the least altered samples, from Pearce et al., (1984) and Pearce (1996) 30 4.5.3. Alteration
In the previous chapter, the protolith is identified as a trachyte or syenite. By plotting the protolith of the felsic rock in the following diagram, Na2O versus K2O, with the 11 altered samples, the alteration can be defined. It shows a Na enrichment and K depletion. Also the protolith had more potassium than the altered samples otherwise it would not have crystallized K feldspars. Data for the protolith field are from Le Maitre (1976).
10
9 8 Sodium enrichment 7
6
5
Na2O % 4 Potassium Trachytic protolith depletion 3 2 1
0
0 5 10 15 20
K2O %
Figure 35 Na and K alteration characterisation in the Na2O vs K2O diagram.
By plotting Al2O3 against TiO2 and TiO2 against Zr, an alteration trend outlined by these altered samples is visible (figure 36 and 37).
Besides an albitization, a late chlorite alteration of the felsic unit is visible in the core.
20 18 16 14
12 (%)
3 10
O 2 8 Al 6 4 2
0 0 0,5 1 1,5 2
TiO (%) 2
Figure 36 Alteration line in the Al2O3 vs TiO2 diagram
31 0,6
0,5
0,4
%
2 0,3
TiO 0,2
0,1
0 0 200 400 600 800 Zr (ppm)
Figure 37 Alteration line in the TiO2 vs Zr diagram
Table 6 Geochemical composition of 11 samples from core SJ11609 using MEMS81:
Sample No SJ2-69 SJ2-70 SJ2-71 SJ2-72 SJ2-73 SJ2-79 SJ2-80 SJ2-81 SJ1-15 SJ1-19 SJ1-20 SiO2 (%) 65,50 67.10 69.40 67.00 68.60 68.70 73.70 77.70 72.60 67.00 67.30
Al2O3 17.40 16.35 16.85 16.65 16.45 15.70 14.45 11.95 13.25 15.75 16.05
Fe2O3 4.50 2.72 2.62 2.47 2.60 3.27 2.12 2.27 2.14 2.42 2.36 CaO 1.06 0.80 0.75 0.70 0.82 0.84 0.58 0.45 0.64 0.92 0.84 MgO 0.20 0.17 0.14 0.09 0.14 0.10 0.11 0.07 0.08 0.12 0.11 Na2O 9.28 7.35 7.46 8.99 7.78 8.68 8.04 6.79 6.78 6.59 6.91 K2O 0.68 2.52 3.21 1.00 2.37 0.67 0.34 0.18 1.16 3.69 3.37
Cr2O3 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01
TiO2 0.23 0.21 0.21 0.21 0.20 0.20 0.18 0.14 0.16 0.19 0.20 MnO 0.06 0.08 0.07 0.04 0.07 0.02 0.02 0.02 0.04 0.06 0.06
P2O5 0.04 0.03 0.03 0.02 0.03 0.02 0.01 0.01 0.03 0.02 0.01 SrO 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 BaO 0.01 0.03 0.04 0.02 0.03 0.01 <0.01 <0.01 0.01 0.03 0.03 LOI 1.39 0.93 0.90 1.00 0.94 0.99 0.92 0.72 0.80 1.28 0.88 Total 100.37 98.31 101.70 98.21 100.05 99.21 100.48 100.31 97.70 98.10 98.10
32
Table 6 Geochemical composition of 11 samples from core SJ11609 using MEMS81: (continued)
Sample No SJ2-69 SJ2-70 SJ2-71 SJ2-72 SJ2-73 SJ2-79 SJ2-80 SJ2-81 SJ1-15 SJ1-19 SJ1-20 Ag (ppm) <1 <1 <1 <1 <1 1.00 <1 <1 <1 <1 <1 Ba 120.50 257.00 327.00 140.00 257.00 123.00 38.90 19.10 126.00 289.00 253.00 Ce 213.00 201.00 203.00 199.00 210.00 196.00 177.00 150.50 157.00 199.00 205.00 Co 2.00 1.10 0.90 1.10 0.90 4.50 1.30 0.90 0.90 0.80 1.00 Cr 10.00 10.00 10.00 10.00 10.00 10.00 10.00 20.00 10.00 10.00 10.00 Cs 0.33 1.07 0.93 0.27 0.89 0.23 0.21 0.13 0.31 0.81 0.70 Cu 19.00 <5 5.00 10.00 6.00 17.00 18.00 46.00 16.00 12.00 12.00 Dy 3.95 3.61 3.51 3.40 3.46 3.78 2.93 2.46 2.60 3.56 3.29 Er 2.35 2.17 2.10 2.15 2.09 2.23 1.88 1.40 1.66 2.23 2.10 Eu 1.51 1.23 1.23 1.30 1.29 1.37 1.00 0.97 0.95 1.27 1.22 Ga 31.30 29.20 28.90 29.80 29.20 28.60 26.70 22.50 23.60 29.00 29.10 Gd 5.16 4.62 4.71 4.63 4.69 4.78 3.92 3.39 3.26 4.39 4.07 Hf 14.40 12.40 11.10 12.90 12.10 12.70 10.80 8.70 9.10 11.70 11.30 Ho 0.76 0.70 0.70 0.68 0.68 0.75 0.60 0.47 0.52 0.70 0.67 La 120.50 111.50 111.00 111.00 116.00 110.50 100.00 85.90 93.10 117.00 121.50 Lu 0.43 0.40 0.37 0.38 0.38 0.39 0.32 0.25 0.29 0.38 0.36 Mo 2.00 2.00 3.00 2.00 <2 <2 <2 <2 <2 <2 <2 Nb 44.70 38.00 33.80 33.30 33.80 38.50 30.90 26.20 26.30 29.30 31.10 Nd 60.30 55.20 55.80 55.60 57.00 55.60 48.70 42.40 43.40 55.50 57.20 Ni <5 <5 <5 <5 <5 <5 <5 <5 <5 5.00 <5 Pb 21.00 38.00 38.00 16.00 30.00 19.00 10.00 8.00 13.00 20.00 18.00 Pr 20.80 19.20 19.40 19.20 19.80 18.90 16.75 14.80 14.95 18.90 19.35 Rb 19.00 68.00 70.20 22.80 55.90 15.20 10.40 5.80 24.90 82.10 82.10 Sm 8.36 7.53 7.56 7.62 7.81 7.71 6.57 5.79 5.60 7.39 7.72 Sn 2.00 1.00 1.00 <1 1.00 <1 <1 <1 1.00 2.00 2.00 Sr 224.00 177.00 202.00 174.50 196.00 128.50 127.00 92.80 107.50 112.50 110.00 Ta 2.10 1.80 1.70 1.80 1.70 1.90 1.50 1.30 1.40 1.30 1.50 Tb 0.74 0.65 0.66 0.65 0.66 0.70 0.54 0.46 0.48 0.64 0.62 Th 40.00 35.80 33.40 36.40 35.10 36.00 29.90 25.30 26.00 34.20 32.40 Tl <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 Tm 0.38 0.34 0.33 0.35 0.32 0.36 0.29 0.23 0.25 0.33 0.31 U 20.50 18.60 17.80 19.30 17.70 17.90 14.00 11.75 14.50 21.20 18.60 V 9.00 6.00 <5 <5 5.00 5.00 <5 <5 <5 <5 <5 W 5.00 5.00 1.00 20.00 11.00 15.00 12.00 23.00 12.00 6.00 6.00 Y 26.30 23.90 23.20 23.70 22.80 24.30 20.70 14.70 16.40 23.10 22.60 Yb 2.60 2.38 2.30 2.36 2.28 2.47 2.04 1.53 1.90 2.51 2.35 Zn 36.00 107.00 80.00 31.00 66.00 63.00 47.00 33.00 37.00 101.00 74.00 Zr 700.00 600.00 550.00 640.00 560.00 610.00 520.00 410.00 407.00 540.00 529.00
33 5. Discussion
5.1. On the origin of the metasedimentary unit
The metasedimentary unit is interpreted as parts of a turbidic sequence. The sedimentary facies observed in the field can be linked to the Bouma sequence as visible in the following figure.
Figure 38 Field observations at left matching the Bouma sequence at right, modified after Bouma (1962).
5.2. Geochemical classification of the felsic unit
The geochemical study, using immobile elements and considering the fact that immobile elements show an alteration trend, classifies the felsic porphyritic unit as a trachyte or syenite depending on the type of formation. According to Y versus Zr diagram, showing mass gain and mass loss for altered samples, a rhyolitic protolith is unlikely. Indeed, the more altered samples plot in the rhyolitic field, and the least altered in the intermediate trachytic field. The least altered samples have a mean value of SiO2 of 67.13 %. The silica content increases for the more altered samples. This demonstrates that the protolith cannot be highly felsic and therefore not rhyolitic.
34 5.3. On the source of sodium and alteration
One of the main characteristic of the rock is its high value of sodium. This can be explained by a high concentration in the protolith as the rock is relatively alkalin (trachytic) and some unaltered trachyte can present similar content of sodium. But in the present case, the high content of Na is also due to a sodic alteration (figure 35 chapter alteration). This alteration can be relatively late but an early enrichment in sodium can be explained by the intrusion or deposition in a deep ocean environment, rich in sodium. In a petrographic scale, Na is within sanidine phenocrysts (sanidine being the most Na rich K feldspar) and in albite (as rims around phenocrysts and in the groundmass).
5.4. On the origin of the felsic porphyritic unit
According to the literature review, five hypotheses can give a felsic fine grained rock with phenocrysts. It can be a shallow subvolcanic intrusive sill or dyke within a sedimentary host rock. Three possible hypotheses were identified for an extrusive origin: A felsic lava flow, a pyroclastic flow deposit from subaerial or submarine eruption giving turbidite and a sedimentation of volcanic material like a ash layer on the sea floor giving a sedimentary contact.
• Interpretations
Various characteristics are against an intrusive origin: the grain size of the groundmass is very fine and the host rock is not affected. A dyke would have affected the host rock: alteration or baking and the host rock frequently shows effect of thermal metamorphism (centimeter scale to few meters). But according to the thin section study, no contact metamorphism evidence and no contact metamorphic minerals have been found near contact.
Also, the phenocrysts are interpreted as sanidine. This K feldspar is not formed in intrusive rocks. Finally, phenocrysts can be found 1mm from the contact with the metagreywacke unit and no shear features (typical of magma intrusion) have been identified along this contact.
A possible hypothesis would be a subvolcanic origin where the intrusion occurs near the surface. The igneous material has then a lower temperature and a faster cooling giving a finer grain groundmass.
The hypothesis of a pyroclastic flow origin is unlikely. Indeed, when a succession of felsic material and metasediment is visible, the same sharp contact is present on top and below, a pyroclastic flow does not follow this characteristic. Again, a hot pyroclastic flow would have affected the sediments during deposition. Also, such a deposit is violent so it would not give a smooth contact.
The rare case of a felsic lava flow origin is also unlikely: the upper contact with the metasediments does not present breccias, and no columnar jointed features or lava lobes has been identified in the field. Also, some units are too thin to be part of a felsic lava flow sequence.
35 • Theory: Volcano-sedimentary origin
The most likely theory for the origin of this trachytic unit would be a sedimentation of felsic volcanic material on top of deep sea sediments. Many characteristics support this theory. The very fine grained groundmass (as seen in the petrography study chapter) can be explained by a volcanic ash. The phenocrysts interpreted as sanidine is formed in extrusive rocks (and is typical of trachybasalt, trachyandesite and trachyte). The contact is interpreted as a sedimentary cold depositional contact; hence the unaffected sediments. As seen in chapter 3, phenocrysts are common in volcanic ash layers, especially at the basal part. The studied case could represent the basal part of an ash layer interlayered in a deep sea turbidite sequence. The meta-trachyte is present only as relatively small lenses. This can be due to erosion of the ash layer before sedimentation on top of it. The volcanic ash can have filled local depressions in the pre-existing topography of the sea floor or submarine channels.
• Volcanic ash
When ash particles are formed from the vent of the volcano, they form at high velocity an eruptive column. This column will first rise and then start moving laterally. This lateral dispersion will depend on:
− Eruption column height − Particle size of the ash − Wind direction − Wind strength − Wind humidity
The ash could be deposited hundreds to thousands of kilometers from the volcano. In the atmosphere, fine ash can stay during several days and be dispersed later by high-altitude winds.
• Geodynamic and rarity of trachytic unit in the region
According to this geochemical study, the geodynamic formation of the metagreywacke is a continental island arc whereas the trachyte is from a within plate setting. For the trachytic rock, the geodynamic setting was studied using 3 diagrams from Pearce et al., (1984) and Pearce (1996). Results from this study are reliable as the least altered samples plot in the within plate field for two diagrams. For Ta vs Yb, the three least altered sample plot at the border between within plate and volcanic arc fields.
Two hypotheses can explain the fact that the metasedimentary unit and the felsic unit have different tectonics settings:
Post-sedimentation There are two different stages: first sedimentation in a continental island arc setting followed by a later intrusive event in a within plate environment. Yet, as mentioned earlier in this chapter, the intrusive hypothesis is not preferred. The two units (meta-trachyte and metagreywacke) are believed to be of the same age.
36 Also a peperite or a mixing feature has been identified in one contact. If there is a late intrusion in compact metasediments then a peperite would not be formed: peperites occur in wet unconsolidated host rock.
Syn-sedimentation The meta-trachyte and the metagreywacke formed at the same time but the volcanic ash has not a local origin. The volcano was situated in a within plate environment, the ash transport can be up to 1000 km. This hypothesis would explain the fact that trachytic material is uncommon in the region, this trachytic unit having a remote origin.
The syn-sedimentation is therefore the most likely theory as it means an extrusive origin for the trachyte and a same age for both rocks.
• Phenocrysts
The type of K feldspar is believed to be sanidine but its identification can be discussed. But the rock is a trachyte and the typical trachyte phenocryst is sanidine. Also sanadine is richer in sodium than the other K feldspars and can present a perthitic texture with exsolved albite: two characteristics found in the studied rock.
Conclusions from the phenocryst abundance study are not really clear. The first lens follows a pyroclastic flow distribution model with increase of the phenocryst content towards the basal part whereas the second lens follows more an intrusive model with less phenocryst near the contact. Also, some quartz vein rich parts of the core have not been studied to avoid false result (the phenocryst content being decreased by the presence of quartz veining). Any conclusive interpretation is therefore not possible from this phenocryst abundance study.
5.5. On the peperite or mixing feature along contact
These features are not against an extrusive origin hypothesis but shows that the contact was made at the same time as sedimentation (syn-sedimentation deposition) as peperite occurs in water saturated sediments, Skilling at al., (2002).
According to White et al., (2000), the term peperite can refer to mixtures generated by contacts of lavas and other hot volcaniclastic deposits with wet unlithified sediments. Also, according to Branney and Suthren (1988), processes like deposition of pyroclasts on top of sediments, resedimentation of volcaniclastic deposits, and infiltration of sediment into volcaniclastic deposits can all produce mixtures of igneous clasts and sediment that look like peperite. According to Westhuizen and Bruiyn (2000), in T’Kuip (Northern Cape Province, South Africa), the deposition of a high temperature ash-flow onto water saturated sediments gives similar features that are elsewhere described as peperites.
37 • Conclusive interpretation for the origin of the trachyte
The interpreted formation of the felsic porphyritic unit as an airborne volcanic ash deposition on the surface of the ocean can be seen in figure 39.
Figure 39 Sketch showing an airborne volcanic ash deposition on the surface of the ocean (modified from Duggen et al., 2009).
5.6. On the comparison of Sjöliden porphyritic rock
• With unmetamorphosed trachyte
Mean values of 11 meters of core of the meta-trachyte are compared with unmetamorphosed trachyte average composition from Le Maitre (1976). It shows that the Sjöliden trachyte is richer in SiO2 and Na2O but depleted in K2O and CaO. This is the product of the alteration of the rock.
Table 7 Sjöliden meta-trachyte (SJ) and unmetamorphosed trachyte average composition from Le Maitre (1976) (AC) SJ(%) AC (%) SiO2 69.50 62.61 Al2O3 15.50 17.26 Fe2O3 2.70 3.07 CaO 0.80 2.34 MgO 0.10 0.95
Na O 7.70 5.57 2 K2O 1.70 5.08
38 • With local granite
When compared with local granite, the trachyte is different: The Revsund granite (figure 40) has a volcanic arc setting whereas the Skellefte-Härnö granite is from syn- collisional setting (Andersson, 2012). The Svartliden granite plots more irregularly but none of them belongs clearly to the within plate field like the trachyte.
Figure 40 Granites localization around Svartliden, 5km South West of Sjöliden from Andersson (2012).
• With regional felsic units
Using the Y versus Zr diagram (figure 41), 11 samples from the Sjöliden trachyte (in blue) are compared with 2 felsic dykes samples from the Knaften area from Wasström (2005), (in purple) and 8 rhyolite samples from Maurliden, Skellefte District, from Montelius et al., (2005). Two alteration lines are displayed, one for the altered rhyolite samples and one for the altered trachyte samples. This shows different magmatic compositions, the Sjöliden unit does not plot like a Skellefte rhyolite.
When one immobile-element ratio is plotted against another (figure 42), the effects of alteration are removed. The different rock types plot as separate clusters. 4 immobile elements Zr, Yb, Nb and Y are used here. Data from rhyolites from the Skellefte district (Montelius et al., 2005), felsic dykes from Knaften (Wasström, 2005), Revsund, Skellefte-Härnö and Svartliden granites from the Svartliden area (Andersson, 2012) are compared with the least altered samples of the trachyte from Sjöliden 1. Results from this plotting show the geochemical rarity of the Sjöliden trachyte, and support the theory that this unit as a remote origin and is uncommon in the region.
39 60 Skellefte district rhyolite
50
40
30 Sjöliden trachyte Y (ppm)
20 Knaften felsic dykes
10
0 0 100 200 300 400 500 600 700 800 Zr (ppm)
Figure 41 Different magmatic composition for the rhyolites from the Skellefte district, the dykes from Knaften and the Sjöliden unit, more intermediate in composition.
300
250 Sjöliden trachyte
200
150 Zr/Yb Skellefte rhyolite 100 Svartliden, Revsund and Skellefte-Harno granite 50
Knaften felsic dyke
0 0 1 2 3 4 5 Nb/Y
Figure 42 Geochemical rarity of the Sjöliden trachyte in the plot Zr/Yb against Nb/Y
40 6. Conclusions
The main conclusions drawn from this paper are the following:
• The metasedimentary unit is a turbiditic sequence classified as continental island arc metagreywacke showing primary bedding with a great variation of graphite and sulphide content, grain size and color. It is a deep sea sediment.
• The felsic porphyritic unit is a meta-trachyte. It presents euhedral to subhedral sanidine phenocrysts, a fine grain groundmass with quartz veining, pyrrhotite, arsenopyrite and gold.
• One of the main characteristic of the rock is its high content of sodium. This can be explained by a high concentration in the protolith, a sodic alteration (Na enrichment and K depletion) and by the deposition in deep ocean (Na rich environment).
• The most likely theory for the origin of this trachytic unit would be a sedimentation of felsic volcanic ash on top of deep sea sediments. The contact is interpreted as a sedimentary cold depositional contact; hence the unaffected sediments.
• According to the geochemical study, the geodynamic formation of the metagreywacke is continental island arc whereas the trachyte is from within plate. The meta-trachyte and the metagreywacke formed at the same time but the volcanic ash has not a local origin. The volcano was situated in a within plate environment, the ash transport can be up to 1000km. This hypothesis would explain the fact that trachytic material is uncommon in the region, this trachytic unit having a remote origin.
• When compared with regional felsic units, the Sjöliden trachyte has a different geochemical signature (Immobile elements ratios).
For further work, the dating of both the metasediment near the contact and the meta-trachyte would give a better understanding of the origin together with deeper drilling.
41 7. Acknowledgements
I would like to thank Dragon Mining Sweden AB for funding this project. I would also like to thank geologist Henrik Ask, Kateřina Schlöglová and Roman Hanes; but also my supervisor Olof Martinsson for the help and support during the completion of this thesis. Finally, I thank Per Samskog for the summer mapping in Sjöliden.
42 8. References
Allen, R., L., Weihed, P., and Svenson, S.-Å., 1996. Setting of Zn-Cu-Au-Ag Massive Sulphide Deposits in the Evolution and Facies Architecture of a 1.9 Ga Marine Volcanic Arc, Skellefte District, Sweden. Economic Geology, 91, pp 1022-1053.
Andersson, J., 2012.The Svartliden granite- petrography, whole rock geochemistry and stable isotope composition. University of Lulea, Sweden. Master thesis.
Ayres, L. D., & Peloquin, A. S., 2000, Subaqueous, Paleoproterozoic, metarhyolite dome- flow-cone complex, Flin Flon greenstone belt, Manitoba, Canada. Precambrian Research 101 (2000), pp 211–235
Bark G., Weihed P., 2003, The new Lycksele-Storuman gold ore province, northern Sweden; with emphasis on the earlyProterozoic Fäboliden orogenic golddeposit In: Demetrios G. Eliopoulos et al.(ed.). Mineral Exploration and Sustainable Development. Millpress. Rotterdam, pp. 1061-1064.
Bark, G., and Weihed, P., 2007. Orogenic gold in the new Lycksele-Storuman ore province, northern Sweden; the Palaeoproterozoic Fäboliden deposit. Ore Geology Reviews, 32, pp 431-451.
Bark, G., 2008. On the origin of the Fäboliden orogenic gold deposit, northern Sweden (2008:72). Doctoral thesis, Luleå University of Technology, 142p. ISSN 1402-1544.
Bailey Roy. A. 1976. Volcanism, Structure, and Geochronology of Long Valley Caldera, Mono County, California U.S. Geological Survey, Reston, Virginia 22092
Bergman Weihed, J., 2001. Palaeoproterozoic deformation zones in the Skellefte and Arvidsjaur areas, northern Sweden. In: WEIHED, P., (Ed.), Economic Geology Research 1999-2000. Sveriges Geologiska undersökning, C833, pp 46-68.
Bhatia et al., 1986. Trace element characteristics of greywackes and tectonic setting discrimination of sedimentary basins. Contributions to Mineralogy and Petrology 92, pp 181– 193
Bjork, L., and Kero, L., 2001. Berggrundskartan 22H Järvsjö NO skala 1:50000. Sveriges Geologiska Undersökning, Ai nr.145.
Bouma, A.H., 1962. Sedimentology of Some Flysch Deposits. A Graphic Approach to Facies Interpretation. Elsevier, Amsterdam, 168pp.
Branney, M., Suthren, R., 1988. High-level peperiticsills in the English Lake District: distinction from block lavas, and implications for Borrowdale Volcanic Group stratigraphy. Geolological journal. J. 23, p. 171-187.
Cas, R.A.F., 1978. Silicic lavas in Paleozoic flyschlike deposits in New South Wales, Australia: behavior of deep sub- aqueous silicic flows. Geological Society of America Bulletin 89, pp 1708–1714.
43
Cas, R.A.F., Wright, J.V., 1988. Volcanic Successions: Modern and Ancient. Unwin Hyman, London, 526 p
Cox, K.G.; Bell, J.D. and Pankhurst, R.J., 1979, The interpretation of igneous rocks: George Allen and Unwin Ltd. London, 450 p
Daly, R. A., G. E. Menger, and S. P. Clark, Jr. 1966. Density of rocks. In: Handbook of Physical Constants (S. P.Clark, Jr., ed.). Geological Society of America. Memoir, 97: pp 19-26.
De Rosen-Spence, A.F., Provost, G., Dimroth, E., Gochnauer, E., and Owen, V., 1980, Archean subaqueous felsic flows, Rouyn-Noranda, Quebec, Canada, and their Quaternary equivalents: Precambrian Research. 12, pp. 43–77.
Duggen S., Hoernle K., Hauff F., Klügel A., Bouabdellah M., Thirlwall M.F. 2009, Flow of Canary mantle plume material through a subcontinental lithospheric corridor beneath Africa to the Mediterranean: Geology, 37, pp. 283–286
Duggen N. Olgun, P. Croot, L. Hoffmann, H. Dietze, P. Delmelle, and C. Teschner 2009, The role of airborne volcanic ash for the surface ocean biogeochemical iron-cycle: a review. Journal: Biogeosciences , vol. 7, no. 3, pp. 827-844
Fink, J.H., 1980, Gravity instability in the Holocene Big and Little Glass Mountain rhyolitic obsidian flows, northern California: Tectonophysics, v. 66, p. 147-166.
Gaal, G., and Gorbatschev, R., 1987. An outline of the Precambrian Evolution of the Baltic Shield. Precambrian Research, 35, pp 15-52.
Goto, Y., McPhie, J., 1998. Endogenous growth of a Miocene dacite cryptodome, Rebun Island, Hokkaido, Japan. Journal of Volcanological Geothermal Research. 84, pp 273-286.
GSD Blue Map National Land Survey of Sweden. 2011. Topographic map.
Hastie, A.H., Kerr, A.C., Pearce, J.A., Mitchell, S.F., 2007. Classification of altered volcanic island arc rocks using immobile trace elements: development of the Th–Co discrimination diagram. Journal of Petrology 48, pp 2341–2357.
Heiken G, 1978. Characteristics of tephra from Cinder Cone, Lassen Volcanic National Park, California. Bulletin Volcanology, 41: 119-130
Herron, M.M., 1988, Geochemical classification of terrigenous sands and shales from core or log data: Journal of Sedimentary Petrology, 58, pp. 820–829.
Hughes C.J., 1973, Spilites, keratophyres, and the igneous spectrum. Geological Magazine. 109, pp 513-527.
Jerram and Petford, 2011, Field Description of Igneous Rocks The Geological Field Guide Series 2 - Practical Approach Book. John Wiley & Sons.
44 Johnson, G., and Olhoeft, G., 1984. Density of rocks and minerals. In Carmichael, R. (Ed.), CRC Handbook of Physical Properties of Rocks (Vol. Ill): Boca Raton, FL (CRC Press).
Juhlin, C., S.-Å. Elming, C. Mellqvist, B. Öhlander, P.Weihed, and A.Wikström, 2002, Crustal reflectivity near the Archean-Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area: Geophysical Journal International, Volume 150, Issue 1, pp180–197.
Kathol, B., and Weihed, P., 2005. (Eds.) Description of regional geological and geophysical maps of the Skellefte District and surrounding areas. Sveriges Geologiska Undersökning, Uppsala. 197 p..
Lahtinen, R., Korja, A. and Nironen, M., 2005. Palaeoproterozoic tectonic evolution of the Fennoscandian shield, in Lehtinen, M., Nurmi, P., and Rämö, T., The Precambrian Bedrock of Finland – Key to the evolution of the Fennoscandian Shield. Elsevier Science B.V, pp 418- 532.
Lahtinen, R., Garde, A.A., and Melezhik, V.A., 2008. Paleoproterozoic evolution of Fennoscandia and Greenland. Episodes, 31, pp 20-28.
Leat PT, Jackson SE, Thorpe RS. and Stillman CJ. 1986. Geochemistry of bimodal basalt - subalkaline/ peralkaline rhyolite provinces within the Southern British Caledonides. J Geol Soc London 143: 259-273
Le Bas et al., 1986. A chemical classification of volcanic rocks based on the total alkali silica diagram. Journal of Petrology 27, pp 745–750
Le Maitre R W., 1976, Some problems of the projection of chemical data into mineralogical classifications; Contributions to Mineralogy and Petrology pp 181–189.
Le Maitre, R.W., 1989. A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of igneous rocks. Blackwell, Oxford, 193 p.
Lundström, H., 1998. Metasedimentary rocks in the district of Storuman, Västerbotten: Geochemistry, petrography and geothermobarometry (Publ. B163). Projektarbete, University of Gothenborg, ISSN 1400-3821, pp 44.
Maclean, W.H., and Barrett, T.J., 1993. Lithogeochemical techniques using immobile elements. Journal of Geochemical Exploration, 48, pp 109-133.
McCoy, Floyd W; Cornell, Winton, 1990: Volcaniclastic sediments in the Tyrrhenian Basin. In: Kastens, KA; Mascle, J; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 107, 291-305
Montelius C., 2005; The genetic relationship between rhyolitic volcanism and Zn-Cu-Au deposits in theMaurliden volcanic centre, Skellefteå district, Sweden: Volcanic facies, lithogeochemistry and geochronology. Ph.D. thesis, Luleå University of Technology, Sweden
45 Montelius, C., Allen. R., & Svenson, S-Å., 2004: Polymetallic massive and network sulfi de depsotis hosted by a crystal-rich rhyolite pumice deposits, Maurliden, Skellefte district, Sweden. In: Allen, R.L., Martinsson, O. & Weihed, P. (Eds) Volcanic associated Zn-Cu-Au- Ag deposits, Magnetite-Apatite deposits, Sediment-hosted Pb-Zn deposits, and Intrusion- associated Cu-Au deposits in northern Sweden. Society of Economic Geologists Guidebook Series, volume 33, pp. 95-109.
Nironen, M., 1997, The Svecofennian Orogen: a tectonic model (1997). Precambrian Research, 86, pp 21-44.
Pearce, J.A., 1996, Sources and settings of granitic rocks; Episodes, v.19, pp.. 120-125.
Pearce, J. A., Harris, N. B. W. & Tindle, A. G.,1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology 25, pp 956–983.
Rutland et al., 2001, Nature of a major tectonic discontinuity in the Svecofennian province of northern Sweden. Precambrian Research 112, pp 211–237.
Stix, J., 1991. Subaqueous, intermediate to silicic composition explosive volcanism: A review. Earth Science Reviews 31:pp 21–53
Skilling I.P.. White, J. McPhie, 2002, Peperite: a review of magma sediment mingling. Journal of Volcanology and Geothermal Research, 114, pp 1-17
Steiger and Hart, 1967, From the microcline orthoclase transition within a contact aureole. American Mineralogist. 52, 87-116.
Vernon R. H and Williams P. F. 1988, Distinction between intrusive and extrusive or sedimentary parentage of felsic gneisses: Examples from the Broken Hill Block, NSW. Australian Journal of Earth Sciences, 35, pp 379-388
Wahlgren C., Bergman S., Lundström I., Stephens M.,1996, Regional berggrundsgeologisk undersökning, sammanfattning av pågående undersökningar 1995. Sveriges Geologiska Undersökning. Rapporter och Meddelanden. 84. pp. 154.
Walker, G. P. L. 1973. Lengths of lava flows. Philosophical Transactions of the Royal Society, London, 274, pp 107–118.
Wasström A., 2005, Petrology of a 1.95 Ga granite-granodiorite-tonalite-trondhjemite complex and associated extrusive rocks in the Knaften area, northern Sweden, GFF, 127 , pp 67-82
Weihed, P., Bergman, J., and Bergström, U., 1992. Metallogeny and tectonic evolution of the Early Proterozoic Skellefte district, northern Sweden. Precambrian Research, 58, pp 143-167.
Weihed, P., Billström, K., Persson, P.O., and Bergman, J., 2002. Relationship between 1.90- 1.85 Ga accretionary processes and 1.82-1.82 Ga oblique subduction at the Karelian craton margin, Fennoscandian Shield. GFF 123. , s. 163-180. 18 s.
46 Weihed, P., Arndt, N., Billström, K., Duchesne, J-C., Eilu, P., Martinsson, O., Papunen, H., and Lahtinen, R., 2005. 8: Precambrian geodynamics and ore formation: The Fenonoscandian Shield. Ore Geology Reviews, 27, pp 273-322.
Westhuizen W.A., de Bruiyn H., 2000, High temperature ash flow–wet sediment interaction in the Makwassie Formation, Ventersdorp Supergroup, South Africa. Precambrian Research, 341, 351 p.
White, J.D.L., McPhie, J., Skilling, I.P., 2000. Peperite: a useful genetic term. Bulletin of Volcanology. 62, pp 65, 66.
Winchester, J,A, and Floyd, P,A., 1977, Geochemical discrimination of different magma series and their differentiation products using immobile elements: Chemical Geology, 20, pp, 325-343
Wright J. V. and Mutti, 1981, The Dali ash, islands of Rhodes, Greece: A problem in interpreting submarine volcanogenic sediments. Bulletin of Volcanology. 44 pp 153–167.
9. Annexes
Photo of hand samples of the felsic porphyritic unit taken during the field mapping in Sjöliden (2011).
47 Table 8 Value of Thorium and Cobalt for 40 meters of the core SV 11476 using ME-MS41 in ppm:
meter Co Th meter Co Th (from) (ppm) (ppm) (from) (ppm) (pmm) 0 0.80 36.00 21 0.80 34.30
1 0.90 35.50 22 0.80 36.20 2 0.90 36.70 23 0.90 35.50 3 0.80 34.60 24 0.90 32.90 4 0.70 38.50 25 0.70 21.80 5 0.70 30.40 26 0.90 35.60 6 0.80 33.50 27 1.10 35.70 7 0.70 33.40 28 1.00 35.80
8 0.90 36.30 29 0.90 33.10 9 0.80 33.50 30 1.50 25.10 10 1.00 32.60 31 1.30 25.30 11 0.80 33.70 32 0.70 23.60 12 0.80 35.70 33 1.00 26.00 13 0.60 21.10 34 0.90 28.70 14 1.20 34.30 35 0.80 30.30
15 0.90 28.60 36 0.80 29.40 16 0.70 32.20 37 0.80 28.00 17 0.80 34.50 38 1.10 31.00 18 0.80 36.60 39 1.20 32.00 19 0.70 36.90 40 8.90 21.90 20 1.00 35.70
48