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Genesis of an Archaean Quartz-Feldspar Porphyry

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Genesis of an Archaean Quartz-Feldspar Porphyry GENESIS OF AN ARCHAEAN QUARTZ-FELDSPAR PORPHYRY A thesis submitted to the Department of Geology in partial fulfillment of the requirements for the degree Bachelor of Sciences (ii) Bachelor of Sciences (1985) McMaster University (Geology) Hamilton, Ontario TITLE~ GENESIS OF AN ARCHAEAN QUARTZ-FELDSPAR PORPHYRY AUTHOR~ Ian S. Cooper SUPERVISOR: Dr. R. H. McNutt NUMBER OF PAGES: 4J, v SCOPE OF CONTENTS: Three conformable units of fine-grained quartz­ feldspar porphyry were mapped in the Berry River For­ mation, Warclub Group, Northwestern Ontario. The largest unit (Unit 1) is compared geochemically and petrographically to quartz-feldspar porphyry intru­ sions and tuffs in the area with the aim of deter­ mining the method of emplacement of the porphyry unit (Unit 1), and consequently the other two units. (iii) ACKNOWLEDGEMENTS I would like to thank the Ontario Geological Survey for allowing time for data collection. I would especially like to thank Mr. G. Johns of the OGS for his aid in the production of this thesis, and for his interesting correspondences (ONGYS). Some of the mapping was done by Mr. G. Johns and Mr. J. Davison. I would like to express my gratitude to Dr. R. H. McNutt for his instruction and supervision of this thesis. 0. Mudrock aided in the preparation and analysis of samples by XRF. L. Zwicker aided in the prepara­ tion of thin sections. J. Whorwood prepared the photo­ raphic plates. Finally, special thanks to Miss w. Mertick, who believed in me, and also to the geology students at Mac, especially the graduating class of 1985, for making the past four years bearable. "Obscurum per obscurius" ••• ( iv) TABLE OF CONTENTS CHAPTER ONE: INTRODUCTION AND GENERAL GEOLOGY 1.1 Location and Access 1 1.2 Method and Purpose of Study 1 1.J Regional Geology 6 1.4 Geology o:f the Warclub Group 8 1·5 Structural Geology 11 CHAPTER TWO: GEOLOGY OF THE BERRY RIVER FORMATION 2.1 Porphyries 12 2.2 Pyroclastics 1.3 CHAPTER THREE: GEOCHEMISTRY .3·1 Analytical Procedures 18 J.2 Major Oxide Characteristics and Classification 22 .3·.3 Trace Element Characteristics and Classifica­ tion 24 CHAPTER FOUR: PETROGRAPHY 4.1 Porphyries 29 4.2 Pyroclastics .32 4.J Porphyry Intrusions .3.3 CHAPTER FIVE: DISCUSSION AND CONCLUSIONS 35 REFERENCES CITED 41 ( v) LIST OF FIGURES 1.1 Thesis area location map 2 1.2.1 Thesis area outcrop data 4 1.2.2 Interpretation of Figure 1.2.1 5 1.3 Savant Lake-Crow Lake Belt 7 1.4 Area stratigraphic map 10 2.2.1 Pyroclastic terminology 14 2.2.2{a) Measured section A34 16 2.2.2(b) Measured section A22 17 3.1.1 XRF analysis results 19 Calculation of Fe o , and FeO 20 2 3 3.1.2 Analytical error calculation 21 3.2 Jensen cation plot 23 3.2(table) Barth norms 25 3.3.1 Zr/Tio2 vs Si02 27 3.3.2 Nb/Y vs Zr/Ti02 28 4.1.1(table) Point count data )0 4.1.2(table) Matrix mode approximations 31 5.1(table) Field criteria of the greenschist facies 37 CHAPTER ONE: INTRODUCTION AND GENERAL GEOLOGY 1.1 Location and Access The map area for this study is approximately 9 km 2, bounded roughly by Latitudes 49°26'00" Nand 49°27'00" Nand by Longitudes 93°58'30" Wand 94° 01'00" w. The village of Sioux Narrows is 10 km to the south on Highway 71 (see location map, Fig. 1.1); the Town of Kenora lies 50 km to the northwest. Access to the area east of Highway 71 is via the Maybrun Road, an Ontario Ministry of Natural Resources Forest Access Road, from Highway 71 itself, as well as by canoe from Long Bay, Lake of the Woods. The thesis area is contained by the Maybrun Road and Berry Lake to the north; by the Berry River to the east; by Robert's Road to the south and by Long Bay to the west. Outcrop exposure is roughly 10%. 1.2 Method and Purpose of Study The field area was mapped over a period of seven days during July"and August, 1984. Preliminary trav­ erses were completed before detailed mapping and sam­ pling were undertaken at selected points. Three stratigraphically sequential units of fine- grained, felsic, porphyritic rock were the main subject of interest. They all conform to regional strike but show no clear contact relationships with the surround­ -2­ N E ..:.:: z ~ a u l.ll 0 w ~ <{ ...J >­ 0:: 0:: w co >­ co4: ~ </) ~ </) a u Q) Q) ..c '­ i= f-4: col.ll 0 ...J I I '' • I ' .{-4­ I 'YP.,'A~,O '­ ~ ' 1 7y '----­ / I I Figure 1.1 Thesis area (shaded) location map. -}­ ing pyroclastic sequences. They have similarities with both the crystal tuffs and intrusive porphyries found in the region. These three porphyritic units have been interpret­ ed as ash-flow tuffs in a facies model proposed for the region (Johns, in prep.). However, no detailed work has been done to establish whether or not these porphyritic units are ash flow de.posits. The purpose of this paper is to establish the genesis of these porphyritic units. Three possible methods of emplacement are considered: (i) ash flow (ii) intrusion (iii) lava flow. To achieve this, a.map of the field area was produced (see Figure 1.2.1) followed by laboratory studies in geochemistry and petrography. It was necessary to establish the field relation­ ships between the porphyries and the neighbouring pyroclastics (it should be noted here that the term "porphyry" is used thoughout this paper to indicate a texture alone--there is nQ genetic connotation in the use of this term). Therefore, field studies were concentrated at the contacts to determine their nature --conformable; intrusive (baked or chilled margins); gradational with pyroclastic sequences; erosive, etc. Representative hand samples were collected from -4­ ~ ~ j LI I .j :? .. I < ~ :r..., . Q \ :'--:,: •.. ~ "''; "" =l v C> :· ~:1. ~ .. /I I -:. .; l ' ..C) ~I f ti ~-- ,~ / / -~:;:;=r- ~ ;/ JII II II ?r ¥'/ a:: It ,~ ~ w \\v" CD \\ \1 < II ~ 1/ !!< >­ <! CD 0 i= <J) co 0 ...J ~ co ~z -.......----~z 0 ...J C> Figure 1.2.1 Thesis area outcrop data. 1=mafic meta- • volcanics, 2=intermediate to felsic metavolcanics, 6=clastic metasedimentsf 8=mafic intrusive 9= meta­ morphosed porphyry intrusionP Some data frqm Johns (198Ja,b) -5­ I. '"' I .. .r,.t I -:------z + + + + • • I 1· · [ I . "' I · I 1 + + + l.. "J"'t • .,., ""0 0 C!J • QJ ...2:!)q ""0(/)<Vc -.... a.o '-o a. E:.;; ::::1.­ .... 0 Cl Ill+­ 2 1/)g ou c cr>­ QJQJ cr fVl w co ., • ...t . I . ('") < • ., I I I E ~ .......N - .!!! 8 ~ V1 co <!) z 0 __.J 0 Figure 1.2.2 Interpretation of Figure 1.2.1, showing geochemistry sample locations and measured section locations. a=interrnediate to felsic metavolcanics; b=clastic metasediments; c=metamorphosed porphyry intrusion; d=mafic metavolcanics; e=metamorphosed mafic intrusion (gabbro). -6­ the metavolcanics and porphyries for thin section. As well, samples for geochemistry were collected at regular intervals (every 65 m ) in a traverse perpen­ dicular to strike across the largest porphyritic unit (Unit 1 in Figure 1.2.2). This procedure was followed to determine whether there are any systematic varia­ tions across strike in the units not visible on out­ crop scale. Unit 1 was chosen as being the most likely to show variation because of its size. Due to the sim­ ilarity between the three porphyry units, it was as­ sumed that conclusions drawn for Unit 1 would be equally applicable to the other two units. Samples also were collected from two intrusive quartz-feldspar porphyries in and around the map area for petrographic and geochemical comparison with the porphyries under study. Hand samples from all porphyries are very dense. ~ Regional Geology The geology of the region is summarised from Stockwell (1970), and Trowell, et al. (1980). The map area is situated in the northwest portion of the Savant Lake--Crow Lake Belt in the Wabigoon Subprovince of the Archaean Superior Province (see Figure 1.J). The Wabigoon Belt is characterised by arcuate supracrustal volcano-sedimentary sequences deformed by younger batholithic intrusions. These volcanics and sediments were metamorphosed, faulted, folded and in­ -7­ + + E ~· lU :t + 0 t­ ~ 'Z-1wo ~ .j. Cs::r 1­- ~ "'(' .. .,. ~~ .. ... + >-X a:t­ a::­ 1IJ .J rDO >-"J: + «~ Oco v I + I I + Figure l.J A portion of the Savant Lake-Crow Lake Belt, NW Ontario. Figure simplified from Trowell, et al. (1980). Thesis area shaded. -8­ truded by granitic rocks during the Kenoran Orogeny. Major shear zones, such as the Pipestone-Cameron Fault (see Figure 1.J) may be late-stage effects of this orogeny. In most areas, the volcanics are older and are overlain by the sediments, but in some areas (such as this map area) the sediments occur at the base of the exposed sequence (implying more than one volcano-sedimentary cycle). Volcanics often grade along strike into sediments. Maximum thicknesses in the Wabigoon Subprovince vary from belt to belt, and within individual belts, ranging up to 15 000 m. The thickness of the volcano­ sedimentary sequence near the map area has been est­ imated at 8200 m by Moorhouse (1965)· Volcanic rocks comprise the majority of the thicker succes­ sions, dominated by basalts, with lesser amounts of intermediate to felsic lavas, tuffs and breccias. The intermediate to felsic portions tend to occur locally as thick sequences. Felsic, porphyritic, fine-grained hypabyssals commonly occur in or near the volcanics, and may be contemporaneous with volcanic activity. 1.4 Geology of the Warclub Group The area was previously mapped by Burwash (19)4) as part of a survey covering a larger area.
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