Stockholm University Bachelor Thesis (15 hp) November 2012 Sulfide mineralogy in the Ballachulish contact metamorphic Aureole Ossian Åström View towards Beinn a’ Bheithir, Ballachulish Igneous complex, across Loch Leven. ©Bob Hamilton. Abstract 16 samples of increasing metamorphic grade from the Ballachulish Igneous Complex and Aureole, located in the west of Scotland, were studied in order to analyze the sulfide mineralogy and to what extent they were affected by contact metamorphism. The samples were collected from two lithologies, the Creran Succession and the Ballachulish Slate lithology, as well as from the igneous complex. The sulfides of main interest in the samples are pyrite and pyrrhotite. At the onset of contact metamorphism, pyrite disappears while pyrrhotite gets more abundant as metamorphic grade increases. Pyrrhotite also undergoes multiple changes such as 1) elongation and thinning of the grains, 2) development of 120° grain-boundaries, 3) development of pyrite-zones within the pyrrhotite and 4) the decomposition of pyrrhotite and alignment of pyrite along its grain-boundaries at high temperature. The elongation of the grains occurs in both the Creran Succession and the Ballachulish Slate. The rest of the textures, however, can only be found in the Creran Succession. The two lithologies differ by the high graphite content in the Ballachulish Slate. The elongated grains as well as the pyrite inclusions in the pyrrhotite both are strong evidence of recrystallization. The absence of pyrite in the Ballachulish Slate was most probably caused by the buffering properties of the graphite-rich fluid in these rocks, causing more reducing conditions. There is evidence against a heavy, pervasive fluid flow through the aureole. However, the inner contact zone seems to have been affected by a more pronounced fluid flow. This could have been caused by the metamorphic fluid working in conjunction with fluids released from the intrusion. Regarding the mobility of S in the aureole, no strong evidence could be found, other than the decomposition of pyrrhotite grain-boundaries in the high-grade metamorphic samples. 1 Contents Abstract .............................................................................................................. 1 Introduction ....................................................................................................... 3 Significance of project ..................................................................................... 3 Background literature review .......................................................................... 3 Aims of the project .......................................................................................... 5 Geological setting ............................................................................................... 5 The Dalradian metasedimentary belt .............................................................. 5 The Ballachulish Igneous Complex .................................................................. 7 Contact metamorphism of pelites ................................................................... 7 The role of fluids during contact metamorphism ............................................. 10 Evidence for fluid flow in the Ballachulish Igneous Complex and Aureole ..... 10 Evidence for fluid flow in other contact aureoles .......................................... 11 Ore deposits associated with contact metamorphism .................................. 11 Locations and sampling .................................................................................... 12 Methods ........................................................................................................... 16 The SEM ........................................................................................................ 16 Results and sample descriptions ...................................................................... 17 Discussion ........................................................................................................ 44 Sulfides mineralogy outside the contact aureole .......................................... 44 Textural changes of sulfides during contact metamorphism ......................... 46 Creran Succession compared to the Ballachulish Slate .................................. 47 Evidence for sulfur and metal mobility .......................................................... 49 Conclusion ........................................................................................................ 50 Acknowledgements .......................................................................................... 50 References ....................................................................................................... 51 2 Introduction Significance of project The Ballachulish igneous complex and contact metamorphic aureole is one of the world’s most well studied contact metamorphic systems (Pattison & Harte, 2001). The igneous complex intrudes host rocks of variable chemical composition, which has led to formation of a diverse mineralogy during contact metamorphism (Pattison & Harte, 1997). However, the sulfide minerals in the metamorphic aureole have not been previously investigated. The hydrothermal fluids produced during contact metamorphism may react with the host rock in such a way that sulfur and metals mobilize, which may play an important role in the formation of ore deposits (Wilkinson, 1991). This study will investigate the sulfide mineralogy in the contact metamorphic zones of the Ballachulish aureole. Transects within the compositionally different Creran and Ballachulish Slate successions may also show whether bulk rock composition has affected the sulfide paragenesis. Background literature review The Ballachulish Igneous Complex and Aureole are located in the west of Scotland, in Argyllshire, by the south-east junction of two lochs, Loch Linnhe and Leven close by the Great Glen Fault (Fig. 1). The igneous complex was formed at 412±28 Ma (Pattison & Harte 1997), during the main period of calc-alkaline magmatism in the Caledonian Orogeny. The Caledonian Orogeny was a sequence of tectonic events that occurred during the Silurian and Devonian period, which has shaped much of the British Isles into what is seen today. These events involved not just only terrane accretion but also metamorphism and igneous activity (Pattison & Harte, 2001). The two dominant series of metamorphic rocks, the Moinian and Dalradian Series, form the Scottish Highlands with the Moinian Series located north of the Great Glen Fault and Dalradian to the south of it. Igneous intrusions are frequent in both series, labeled as early or late granitoids depending on their location in relation to the amount of regional deformation (Pattison & Harte, 2001). The three main intrusive rock types of the Ballachulish Igneous Complex using the IUGS classification scheme are monzodiorites, quartz diorites and granites (Fig. 1) (Pattison & Harte, 2001). The monzodiorites and quartz diorites form together a zoned envelope with a core consisting of porpyhritic granite. The entire igneous complex covers an area of 7.5 X 4.5 km², with the granite core being exposed over an area of about 8 km² (Pattison & Harte 1997). The host rock comprises of the Leven Schist, Ballachulish Limestone, Appin Quartzite, Appin Limestone, Appin Phyllite, Cuil Bay Slate and the Creran successions of the Dalradian metasedimentary terrane (Pattison & Harte 1997). These metasedimentary units were regionally metamorphosed during the Caledonian Orogeny to biotite- garnet zone greenschist facies (Pattison & Harte, 2001). 3 4 Through the extensive contact metamorphism caused by the igneous complex and its intrusion, the regional mineral assemblages became overprinted with the new contact metamorphic ones. These minerals begin in the pelitic to semi-pelitic rocks with the development of cordierite. Cordierite forms from recrystallization of Al-Fe-Mg minerals under low pressure, high temperature conditions. Between the regional metamorphism and the intrusion of the Ballachulish Igneous Complex, considerable uplift took place and at the same time erosion of the overlying rocks (Pattison & Harte, 1997). Contact metamorphism can be detected in the field as “spots” of cordierite, often occurring in pelitic schists. The extent of contact metamorphism can be measured using the cordierite isograds. The Ballachuish aureole is 1700 m wide between its eastern and western flank, where the host rock is quartzite. The northern and southern flanks of the aureole marks the narrowest part at less than 400 m in width, and are hosted by pelites. Quartzite conducts heat more efficiently than pelites, which explains why the aureole show more width in the quartzite hosted areas. Furthermore, these pelites occur in contact with the most fractionated and also the lowest temperature part of the igneous complex (Pattsion & Harte, 1997). The mineral assemblages in the aureole tell of a varying extent of fluid flow between rock units. The fluid seems to have moved between rock units through cracks, faults and bedding planes. The inner contact zone seems to have hosted the most abundant fluid flow as the dehydration of the pelites took place. Isotopic evidence along with the variable retrograde alteration suggests that the fluid had a metamorphic origin instead of a magmatic or meteoric one (Pattison & Harte, 1997). Aims of the project The aim of this project is to investigate the sulfide paragenesis in the Ballachulish