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The author has granted a non- L'auteur a accordé une licence non exclusive Licence allowing the exclusive permettant la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or seii reproduire, prêter, distribuer ou copies of this thesis in microfom, vendre des copies de cette thèse sous paper or dectronic formats. la forme de microfiche/film, de reproduction sur papier ou sur format électronique.
The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts from it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. ABSTRACT
The Hess Offset is a steeply dipping dyke located 12-15 km north of the 1.85 Ga
Suc?huryigneous complex (SIC) within the 200-250 km diameter Sudbury impact structure.
It is up to 60 m wide and strikes subconcentricaily to the SIC for at least 23 km. The Hess
Offset consists of more than one phase. The main phase of the dyke is granodioritic, but it conforms with what is locdly referred to as quartz diorite: a term used for al1 the Offset dykes of the Sudbury impact structure. Rare earth element data shows that the Hess Offset is genetically related to the SIC and that it is most cloaely affiliated with an evolved felsic norite component the Main Mass and not bulk impact melt. This indicates that Hess was emplaced during fiactionation of the impact melt sheet, rather than immediately following impact. The main Quartz Diorite phase of the dyke comprises plagioclase + homblende + biotite * pyroxene, with minor quartz and granophync intergrowths of quariz and alkali feldspar. Pyroxenes are rarely preserved intact; most occur as uralitized relicts. Unaltered clino- and orthopyroxenes do survive in both Hess and especially the Foy adjacent Offset, but are rare and only occur in small isolated patches within the Offsets. The extensive alteration of the pyroxene implies metasomatic alteration. Metasomatism may have ken caused by the ingress of hot fluids fiom the wall rocks during their dehydration due to frictional heating and pseudotachylyte formation. Alteration was not pervasive, as discrete areas of more pristine dyke have been pteserved. Critically, the Hess Offset occupies a fault system that marks the northern limit of a pseudotachylyte-nch, shatter cone-bearing annulus about the SIC. This fault system was active during the modification stage of the impact process. ACKNOWLEDGMENTS
Funding for this work was provided through NSERC Research/Operating and
INCO/NSERC Collaborative Research and Development grants to my supervisor John
Spray. Logisticai support was provided by the Ontario Geological Survey (OGS) and inco
Exploration. 1am particularly grateful to the OGS for the use of one of their vehicles and for the helicopter time. Help and advice from Inco staff (and ex-staff), especidly Andy Bite,
Hadyn Butler, Bob Martindaie, Everett Makela, Gord Morrison, Mars Napoli, Ed Pattison,
Walter Peredery, Rick Lacroix, Peter Lightfoot and Dick Alcock. Many aspects of this project would not have been possible without the many years of experience and knowledge from the Inco geologists. Inco technical staff are also thanked for their cosperation and advice.
Clive Karnichaitis and Neil Pettigrew acted as very able field assistants for the 1996 and 1997 field seasons, respectively. John O'Connor, Wes Marsaw and Ron Nerpin are thanked for helping us to recover our truck from the Onaping River.
The Barringer Crater Company and the Geological Society of Arnerica, are thanked for travel awards that facilitated participation in conferences at Denver, Sudbury and
Houston. Discussion with many geologist at these conferences proved invaluable.
Constructive reviews by Alex Deutsch, Peter Lightfoot and Bruce Marsh helped to improve an earlier version of the Meteroitics manwript, helped shap some of the idea presented in this thesis.
This project would not have been possible without the support of my supervisor John
III Spray and the many discussions on impact geoiogy we have had (yes 1 did spell cheque this).
1 also wish to thank Richard Grieve for taking the time out of his busy schedule to be on my supervisory cornmittee and to both Ted Bremner and Amie McAllister for doing such a thomugh job on my examining cornmittee. Al1 the graduate students at WB are thanked for advice, fiiendship and entertainment value, discussions on al1 aspect of rocks and the meaning of life (42). In particular 1 wish to thank Jürgen Kraus (for his 'J' waves), Yvette
Kuiper, Heather Gibson, Lucy Thompson, James Whitehead, hdyStumpf, Bill Gray, Ali
Ahrnadi, Geoff Allaby, Lindsey Dunn, Ron Scott and John O'Comer etc etc ... Dave
Huntley's thought-provoking letters were appreciated and inspirational.
The technicai staff at UNB are thank for their help with many aspects of this project.
In particular 1 wish to thank, Calvin Nash and Anse1 Murphy for the excellent thin sections,
Jack of al1 trades Pirie for a bit of everything, Angel Gomez for Ming of figure Cor the
Meteoritics paper and Bob McCulloch for the plethora of photos. The help of the staffof the
Electron Microscopy Unit was invaiuable, in particular Douglas Hall. The office staff,
Merrill Beatty and Christine Lodge are thanked for keeping the department ming. Many members of the faculty are thanked for their advice, enthusiasm. Nina Chrzanowski is thanked for her inspiration and fiiendship.
1 wish to thank al1 my fnends spread around the four corners of the globe for their support and encouragement fiom afar, in particular Geoff, for encouraging me to start this
Master degree in the fmt place and having the faith in me to complete it. ktbut definitely not least, my family, for their support understanding and encouragement can not be thanked enough, for without you none of this would have been possible. LIST OF CONTENTS
AJ3STMCT...... II ACKNOWLEDGEMENTS......
LIST OF CONTENTS...... VI
LIST OF TABLES ...... X
LIST OF FIGURES ...... XI
cn~rfER ONE .LNTRODUCTION...... 1
INTRODUCTION...... ,,., ...... 1
GENERAL GEOLOGY OF THE SUDBURY BASEMENT...... **..**.*..*...*... 1
Supenor Province ...... I
Early Proterozoic Southern Province ...... 3
THE SUDBURY STRUCTURE...... S
The Sudbury Igneous Cornplex...... 5
The Main Mass of the SIC ...... 6
The Sublayer...... 8
Associated ores ...... 9
Footwall breccia ...... 9 Sudbury breccialPseudotachylyte...... 10
Whitewater Group...... ~*...... *...... 10
POST- SUDBURY EVENT DEFORMATION ...... 1 1 HISTORY...... 17
SUDBURY OFFSET DYKES ...... 18
CHAPTER TWO .GEOLOGICAL SETTING OF THE
HESS OFFSET DYKE ...... 22
ACCESS ...... 22
TERRAIN...... 24
WALL ROCKS ...... 24
HESS OFFSET...... 30
FOY OFFSET...... 42
FOYMESS PELATIONSHIP...... +...... 43
FAULTS ...... 45
CROSS CUTTDiG PHASES...... 48
CHAPTER THREE .HESS OFFSET CONTACT RELATIONS/STRUCTURE ...... 52
ASSOCJATIONWITH PSEUDOTACKYLYTE...... 52
PSLUDOTACHYLYTE VEINLETS ...... 56
BROADER PSEUDOTACHYLYTE ZONES ...... 60
CATACLASTIC ZONES ...... ,...... 65 CONTACT QUARTZ DIORITE ...... 68
HOST ROCKS AT THE CONTACT ...... 70
CHAPTER FOUR .CHEMISTRY ...... 71
SAMPLE LOCATIONS...... 71
RELATiONSHIP OF OFFSET DYKES TO THE SIC ...... 74
HESS OFFSET M RELATION TO THE NORTH RANGE MAIN MASS ...... 82
VARIATiONS WITHIN THE HESS OFFSET...... 85
QUARTZ DiABASE DYKES ...... 91
PARALLEL FOY ...... 93
INCLUSIONS M THE HESS OFFSET...... 94
Footwall-like breccio inclusions...... 94 . . Gneissic inclusion...... 95
Riven Option Basic hclusion...... -96
CHAPTER FlVE .MRWRALOGY OF THE HESS AND RELATED
OFFSET DWS...... 105
PLAGIOCLASE...... 106
PYROXENES AND THEIR ALTERAnON PRODUCTS ...... 115
PRIMARY AMPHIBOLES AND BIOTiTE...... 124 CONTACTPHASES...... 128
MTERNAL CONTACTS...... ,128
ALTERATION OF THE HESS QUARTZ DIORITE...... 130
lNCLUSIONS ...... 134 MINERALOGY OF THE PARALLEL FOY OFFSET...... 140 QUARTZ DIABASE...... 146
CHAPTER SIX .CONSTRAINTS ON THE ORIGIN AND
EMPLACEMENT OF THE HESS OFFSET ...... 150
DYKE EMPLACEMENT...... 150
TIMING OF OFFSET EMPLACEMENT...... 153
MINERALOGY OF THE HESS OFFSET ...... ,,...... 156
METAMORPHISM OF THE SUDBURY STRUCTüRE...... 157
PROPOSED CRYSTALLISATION SEQUENCE FOR
HESS QUARTZ DIORITE...... 159
ARGUMENT FOR HYDROTHERMAL OR DEUTERIC ALTERATION ...... 161
VARIATIONS R\J CHEMISTRY ...... 1 62
MODELS FOR EMPLACEMENT...... *...... **...... *...... 163
SUGGESTED FUTURE WORIC ...... 166
SUMMARY...... ,*...... ***...... 169 REFERENCES...... 172
APPENDIX 1 X-ray fluorescence preparation and analytical techniques...... 182
APPENDlX 2 Electmn microprobe anaiytical techniques and data ...... 188
APPENDIX 3 Additional pyroxene analyses...... 190
APPENDIX 4 Sample names and locations ...... 196 LIST OF TABLES
Table 4.1 a Major element data and CIPW normative mineraiogy for . . Hess quartz dionte...... 97
Table 4.1 b Trace and REE data for Hess quartz diorite...... 98
Table 4.2a Major element data and CIPW normative mineralogy for . Hess quartz dionte...... 99
Table 4.2b Trace and REE data for Hess quartz diorite...... 100
Table 4.3a Major element data and CIPW normative mineralogy for
Hess related lithologies...... 101
Table 4.3b Trace and REE data for Hess related lithologies...... 102
Table 4.4a Major element data for host rocks to the Hess Offset. and
average values for the North Range felsic norite and the
North Range Offsets...... 1 03
Table 4.4b Trace and REE data for host rocks to the Hess Offset. and
average values for the North Range felsic norite and the
North Range Offsets ...... 1 04
Table 5.1 SEM analyses of plagioclase nom the Hess Offset...... 1 11
Table 5.2 SEM analyses of inclusions in plagioclase hmthe Hess Offset ...... 114
Table 5.3 SEM andyses of orthopyroxene fiom the Hess and Foy Offxts...... 116
Table 5.4 SEM analyses of clinopyroxene hmthe Hess and Foy Offset...... 117
Table 5.5 SEM analyses of the alteration products of pyroxene...... 122 Table 5.6 SEM analyses of primary homblende and biotite...... *..*.*126 XI Table 5.7 SEM analyses of reaction products on the rims of pyroxenes
and amphiboles...... 127
0 Table 5.8 SEM analyses of plagioclase fiom inclusions in the Hess Offset,
footwall-like breccia inclusions, Parallel Foy and quartz
diabase dykes ...... 14 1
Table 5.9 SEM analyses of amphibole and biotite fiom the Parallel Foy
Offset and quartz diabase...... 145 LIST OF FIGURES
Figure 1.1 General geology of the Sudbury Structure...... 2
Figure 1.2 Petrographic variations in the Main Mass of the SIC...... 7
Figure 1.3 Cross section through the Sudbury Structure ...... 16
Figure 2.1 Geological setting of the Hess. Foy and Parallel Foy Offsets ...... 23
Figure 2.2 Photo demonstratingthe dangers of the Onaping River ...... 25
Figure 2.3 Photo showing the typical terrain in the Hess area ...... 25
Figure 2.4 Photo ofthe contact ofthe Hess Offset with Nipissing diabase...... 27
Figure 2.5 Photo of a granitoid inclusion in the margin of the Hess Offset...... 27
Figure 2.6 Photo of epidote veining in the Hess Offset ...... 31
Figure 2.7 Photo of an intemal contact within the Hess Offset ...... , ...... 31
Figure 2.8 Detailed map of the proximal Foy intersection ...... 32
Figure 2.9 Detailed map of the distal Foy intersection ...... 33
Figure 2.1 O Photo ofclast-packed footwall-like breccia inclusion ...... 35
Figure 2.1 1 Photo ofclast-packed fmtwall-like breccia inclusion...... 35
Figure 2.1 2 Detailed map of the Rivers Option...... 37
Figure 2.1 3 Rubble pile around the excavated pits at the Rivers Option...... 38
Figure 2.14 Quartz veining in the Host rocks of the Rivee Option ...... 38
Figure 2. 15 Large basic inclusion in Hess at the Rivers Option...... 39
Figure 2.1 6 Fine grained (Humnian?) inclusion in Hess at the Rivers Option...... 39
Figure 2.1 7 Detailed map of the Dehydration Ridge area...... 41
Figure 2.1 8 Footwall-like breccia inclusion. Maki Showing. Foy Ofkt...... 44 Figure 2.19 Quartz diabase dyke. crosscutting through Hess quartz diorite...... 44
Figure 2.20 Detailed map of the faulting at the Harty Option ...... 49
Figure 3.1 Photo of broad pseudotachylyte veins ninning subparailel to Hess ...... 54
Figure 3.2 Photo of anastomoshg pseudotachylyte veinlets ...... 54
Figure 3.3 Simplified bloc k diagram of general field relationships of Hess ...... 55
Figure 34 Photomicrograph of strained granite...... 57
Figure 3.5 Photomicrograph of quartz recrystailisation along the margin
of pseudotachylyte veinlets...... 57
Figure 3.6 Displacement ofquartz vein by micro-pseudotachylyte veins ...... 58
Figure 3.7 Photomicrograph showing assimilation of a felspar clast and complete
recrystallisation of quartz along the margin of a
micro-pseudoiachyl yte vein ...... 58
Figure 3.8 Displacement of feldspar twins by micro-pseudotachyl yte veins ...... 59
Figure 3.9 Photomicrograph of granite macroscopically pewasively
cut by micro-pseudotachylytevek ...... 59
Figure 3.10 Broad. sharped margined vein of pseudotachylyte...... 61
Figure 3.1 1 Phctomicrrrgraph of matrix and inclusions in a broad, shar~cd
margined vein of pseudotachyl yte ...... 6 1
Figure 3.12 Photomicrogmph of a defonned granite clast in a broad
zone of pseudotachylyte ...... 63 Figure 3.1 3 Photomicrograph of flow foliated pseudotachylyte...... ***63 Figure 3.14 Inegular zone ofpseudotachylyte with sharp to diffise margins ...... 64 Figure 3.15 Photomicrograph of the contact between highly altered granite and
diffusemargined pseudotachylyte...... -64
Figure 3.16 Photomicrograph of cataclasiticaily defomed granite ...... 66
Figure 3.17 Photomicrograph of cataclasitically deformed granite with
a pseudotachylyte Iike maûix...... A6
Figure 3.18 Photomicrograph of sphemlitic textured quartz diorite in
contact with granite ...... 69
Figure 3.19 Photomicrograph of sphemlitic texture in granite in contact . . with quartz dionte...... 69
Figure 4.1 Geological setting of the Hess, Foy and Parallei Foy Offsets,
with location of sarnples analysed ...... 72
Figure 4.2 AFM diagram for the Main Mas, the Hess Offset and related rocks...... 75
Figure 4.3 Al,O,/K,O venus SiO,/K,O diagram for the Main Mass, the Hess
Offset and related rocks ...... -76
Figure 4.4 MgOK,O versus FeO*/K,O diagram for the Main Mass, the Hess
Offset and related rocks ...... 77
Figure 4.5 Samarium versus Lanthanurn plot for the Main Mass, the Hess
Offset and related rocks ...... 79
Figure 4.6 REE concentrations for selected Offsets, normalised to North
Range felsic nocite ...... 8 1
Figure 4.7 REE concentrations for the quartz diorite of the Hess Offset,
normalised to North hgefelsic norite ...... 84 Figure 4.8 QAP diagram of the Hess Offset and related lithologies ...... 86
Figure 4.9 REE concentrations for average values of quartz diode.
footwdl-like breccia inclusions. and host rocks. normalised
to North Range felsic norite ...... 88
Figure 4.10 REE concentration of quartz diabase dykes comparai to the Hess
Offset quartz diode. nodisedto North Range felsic norite ...... 90
Figure 5.1 Subophitic to intersertial texture of the quartz diorite...... 107
Figure 5.2 inverted pigeonite hmthe Hess Offset ...... 107
Figure 5.3 Orthopyroxene partially replaced by actinolite and
nmrned by homblende ...... 108
Figure 5.4 Ferro-magnesio- homblende on the rim of actinolite...... 1 08
Figure 5.5 Ferro-actinoliteoccurring on the rim of ferro-magnesio hombiende...... 109
Figure 5.6 Interstitial granophyre eroding plagioclase...... 109
Figure 5.7 Cryptocrystalline inclusion in the rirn of plagioclase.
plane polarised light ...... 1 1O
Figure 5.8 Cryptocrystallineinclusion in the rim of plagioclase. crossed polars ...... 110
Figure 5.9 SEM photo of needle-shaped alignment of inclusions in plagioclase...... 113
Figure 5.10 SEM photo ofbleb like inclusions in plagioclase ...... 13
Figure 5.1 1 Pyroxene quadrilateral for pyroxene thennometry...... 1 19
Figure 5.12 Otthopyroxene partially replaced by calcite...... 123
Figure 5.13 Chlonte and biotite replacing actinolite...... 123
Figure 5.14 Contact quartz diorite with actinolite...... 129 Figure 5.15 Contact quartz diorite with acicular amphibole inclusion
altered to biotite ...... 129
Figure 5.16 Granophyric quartz diorite fiom the central portions of the dykes ...... 131
Figure 5.1 7 Altered quartz diode fiom the centre of the dyke ...... 131
Figure 5.1 8 Epidote. calcite veins cross cutting the dyke...... 133
Figure 5.19 Apparent igneous texture of the matrix of the
fwtwall-like breccia inclusions in Hess ...... 1 33
Figure 5.20 Hetroli thic nature of the footwall-like breccia inclusions in Hess, . . plane polarsied light ...... 136
Figure 5.2 1 Hetrolithic nature of the footwall-like breccia
inclusions in Hess, crossed polars ...... 136
Figure 5.22 Felsic clast in the 'gnessic breccia' of the Maki showing...... 138
Figure 5.23 Matrix ofthe 'gnessic breccia' of the Maki showing...... 138
Figure 5.24 Matrix of the non foliated 'gnessic' inclusion fiom the
distal Foy intersection...... 139
Figure 5.25 Ophitic basic inclusion fiom the Rivers Option...... 139
Figure 5 .26 Small inclusion of feldspar crystals, eastern end of the Hess Offset ...... 142
Figure 5.27 Decussate texture of small inclusion of feldspar crystals...... 142
Figrue 5.28 Relatively unalted Parallel Foy OfEset ...... 143
Figure 5.29 Altered Parallel Foy Offiet......
Figure 5.30 Two phases of the quartz diabase dyke, chilled against . . Hess quartz dionte...... 147 Figure 5.3 1 Plagioclase porphyroblast in quartz diabase ...... 147
Figure 5.32 Manix ofthe quartz diabase ...... 148
Figure 6.1 Ideaiised block diagrams representing possible modes of
emplacement of the Hess Offset dyke ...... 164 Chapter One
INTRODUCTION
The Sudbury structure is located in the southem part of northem Ontario, Canada. It comprises the Sudbury Igneous Complex (SIC), the Sudbury basin (the area interior to the
SIC), and variously brecciated basement rocks beneath the SIC (Fig. 1.1). The structure is approximately 200-250 km in diameter (Grieve, 199 1; St6mer and Deutsch et al., 1994;
Thompson et al., in press). To the north of the SIC, in the North Range, the basement predominantly consists of a Superior Province Archean granite-greenstone terrain. In the
South Range it comprises Southem Province Huronian Supergroup metavolcanics and metasedimentary rocks. Since the discovery of Coppet in the area in 1856 (Giblin, 1984).
Sudbury has created a great deal of interest, not only because of the large arnounts of ore deposits associated with the structure, but because its origin has been an enigma for so long.
Theories for the ongin of the Sudbury structure are varied and controversial.
CENERAL GEOLOGY OF THE SUDBURY BASEMENT
Superior Province
The Superior Province consists of middle Archean supracrusta1 and plutonic rocks of the Levack gneiss complex (Langford, 1960). Archean metasedimentary rocks and Offsets dvkeg Grenville Province granitoids (mainly Archean) 1 @ Hess 0 Foy (proximal) Whitewater Group greenstones Foy (distal) @ ParaIlel Foy Sudbury igneous Levack gneiss complex (SIC) @ Whistle-Parkin @ Macbnnan Huronian Supergroup / sublayer (mstrssdlmenbry rocka) @ Manchester 4 Offset dykes: Fiood-Stobie-Victoria Huronian Supergroup @ Copper Cliff (mstrvolanic rocks) / farilt @ Womiington 4 thrust @ Ministlc
Figure 1.1 - General geology of the Sudbury structure. The black rectangle marks the appmximate location of the large fold out map of the Hess Offset, in the back pocket. metavolcanics of the "Greenstone Belts" and the granitic Cartier Batholith, which is part of a group of composite felsic piutoiis col!ectively referred to as the Algoma granites (Card and inries, 198 1). nie greenstone belts are best exposed in an arcuate zone in the North Range of the Sudbuy structure (Fig. 1.1). The Benny Greenstone belt occurs in the NW (Card and
Imes, 1981) and the Parkin and Hutton Greenstone belts in the NE (Meyn, 1970). The
Levack gneiss complex occurs in an arcuate form immediately adjacent to the northem nrn of the SIC (Fig. 1.1). Krogh et ai. (1984) obtained a U-Pb zircon age of 271 1k7 Ma for a typical tonalitic unit within the Levack gneiss complex, whereas a leucosorne layer gives an age of 2647*2 Ma (Percival and Krogh, 1983), which is considered to be the age of regional high-grade metarnorphism and accompanying anatexis (Krogh et al., 1984; Card, 1994).
Geochemical modelling suggests that the bulk of the Cartier Batholith could have been the result of partially melting Levack mafic tonalite gneiss (Meldrum et al., 1997). Tectonism by the Kenoran Orogeny was most intense from about 2720 to 2660 Ma, with the end of the
Kenoran Orogeny marking the termination of the formation of the Superior Province craton
(Thwston, 199 1).
Early Proterozoic Southem Pnwince
The Huronian Supergroup makes up the eastem part of the Southem Province, which presently occurs as a wedge which thickens to the south. It is an Early Proterozoic assemblage of mainly clastic metasedimentary rocks with a local basal volcanic accumulation. The Copper Cliff Formation (2450I25 (Ma Krogh et al., 1984)), with correlative Thessalon Formation and layered intrusions, is interpreted to represent a related passive-rnargin, rift-associated igneous suite. The Matachawan dykes have been dated at
24522 Ma (Heaman, 1988), which is comparable in age to that of the Copper Cliff felsic volcanic rocks (Krogh et al., 1984). These dyke swarms have been interpreted as a failed-arm environment of an opening ocean prior to the development of the Huronian Supergroup
(Fahng, 1987).
The Huronian Supergroup is divided into four s~ratigraphicgroups (stratigraphy formalised by Robertson et al. (1 969). with more recent work by Debicki (1 991). The lowermost member is the Elliot Lake Group. It consists of volcanic rocks and clastic metasedimentary rocks. The remainder of the Huronian Supergroup is characterised by cyclic repetitions of conglomerate, mudstone, wacke and quartz feldspar arenites (Roscoe,
1969). Based on this repetition the sedirnentary sequences are divided into three groups:
Hough Lake Group, Quirke Lake Group and the Cobalt Group. Erosional remnants of the
Huronian Supergroup occur north of the SIC and have ken referred to as outlien (Dressler,
1984a). These outliers fonn an arcuate zone subconcentric to the SIC, which coincides with the location of the Archean greenstone belts indicating that a major stnictural basinal feature exists at this radial distance fiom the SIC (Fig. 1.1).
Deposition in the Southem Province ended with renewed tectonic defornation prior to the intrusion of gabbroic bodies in the early Proterozoic. These gabbroic rocks are collectively tenned the Nipissing diabase and are the focus of the 2.2 Ga Preissac dyke swarm. Buchan et al. (1989) have demonstrated, from paleomagnetic work, that the
Nipissing diabase sheets represent three distinct intrusive events, which occurred between
2.25 and 2.15 Ga. Magmatic and hydrothermal Cu-Ni-sulfides and platinum group elements (PGEs) have been recognised within and associated with the Nipissing intrusions (Lightfoot and Naldrett, 1996).
THE SUDBURY STRUCTURE
The approximately 200-250 km diameter Sudbury structure comprises the Sudbury
Igneous Complex (SIC), the Sudbury basin and the variously brecciated basement rocks around the SIC. The SIC is a layered elliptical body of norite/quartz-gabbro/granophp.This encloses the Sudbury basin. within which the heterolithic breccias, mudstones, siltstones and wackes of the Whitewater Group were deposited. Two main types of breccia are related to the formation of the Sudbury structure (Lakomy i 990), the footwall breccia that is around the lower contact of the SIC, and the Sudbury breccia (pseudotachylyte) which has ken found up to 80 km from the SIC (Peredery and Morrison, 1984; Thompson and Spray, 1994,
1996). Formation of shatter cones (Guy-Bray, 1966), shock metamorphic features in certain minerals (Dressler, 1984a),a ring fracture system encircling the SIC (Butler, 1994; Spray and
Thompson, 1995). brecciation of the basement (MIiller-Mohr, 1992) and the development of a contact metamorphic aureole around the SIC (Dressler 1984b) are some of the effects of the Sudbury event on the basement rocks.
The Sudbury Igneous Complex
The SIC is composed of the sublayer and the Main Mass. The Main Mass is a 2.5 km thick differentiated sequence of various noritic rocks grading into quartz gabbro and granophyre. The sublayer consists of noritic and gabbroic rocks occurring as discontinuous lenses beneath the Main Mass (Fig. 1 .l) (Naldrett et al., 1970). The Main Mass of the SIC
The Main Mass is divided into a lower, middle, and upper zone, each zone displaying distinct petrographic variations between the North and South Ranges. These petrographic variations are represented schematica~lyin Figure 1.2. This figure shows the modal mineralogy, variations in the Mg/(Mg+Fe) ratio (Mg#)of the pyroxenes and the anorthite
(An) content of plagioclase on sections through the North and South Ranges. The lower zone consists of the mafk and felsic norites in the North Range. The South Range has a marginal zone of quartz-rich nonte that grades into the South Range norite.
The mafic norite is characterised by 40 to 60% orthopyroxene. Much of this unit is an orthopyroxene curnulate in which orthopyroxene grains are enclosed poikilitically by plagioclase. The unit grades both upward and downward into a hypidiornorphic-textured mafic norite containing tabular grains of both plagioclase and orthopyroxene. The mafic norite grades upward into felsic norite with a progressive increase in the proportion of plagioclase through a transition zone of 7 to 13 m.
nie felsic notite is characterised by 20 % orthopyroxene and is a coarse grained, hypidiomorphic granular-textured rock that consists of cumulus plagioclase, cumulus orthopyroxene, and largely intercumulus clinopyroxene, and biotite, quartz and quartz- feldspar micrographic intergrowths. The quartz plus micrographic intergrowths average more than 25 modal %. Through the upper one third of the felsic norite, orthopymxene is absent and clinopyroxene has a tabular fom indicating that it is a cumulus phase.
The South Range norite has a medium- to couse-grained hypidiomorphic granular texture. The main primary minerals of this rock are cumulus plagioclase and orthopyroxene, No data
4215 m
!iWnophyre
3 015 m
- quartz gabbro quartz gabbro felsic nonte -
O m mafic norite 1 810 m
South Range norite Legend 0 Quartz + Micrographie intergrowth 0 Plagioclase //? I 0Hypersthene / HO.3 1 A027 Augitr P 61 1 0 Biotite +Prirnary Amphibole quartz rich norite / 1 Opaque Oxides + Apatite + Titanite I HOA 1 A0.35 Om / P 60 H 0.3 1 Fe/(Mg+Fe) atomic ratio in hypenthene South Range Section A 0.27 Fe/(Mg+Fe) atomic ratio in augite (horizontal distances) P 6 mole % Anorthite in Plagioclase
Figure 1.2 - Schematic diagram showing the variation in modal minenilogy, anorthite (An) content of plagioclase, and the MgOlMgO + Fe0 (Mg#)for pyroxenes, of the North and South Ranges of the Sudbury Igneous Cornplex, (after Naidrett et al., 1984). South Range section is measuring horizontal the distance from the contact of the sublayer, not the true thickness. and intercumulus clinopyroxene, quartz, titaniferous magnetite, and ilmenite. The transition fiom the South Range norite to the quartz-rich norite is marked by a progressive increase in modal quartz fiom 8 % in the South Range norite up to 20 % in the outer margin of the complex. Roughly, the upper 150 m of the South Range norite is equivalent to the felsic norite in the North Range.
The middle zone is a continuous unit of quartz gabbro. The base of the unit is marked by the appearance of cumulus Fe-Ti oxides apatite and titanite.
The upper zone is a single continuous granophyre unit. The transition from the middle to the upper zone is gradational and marked by an increase in the proportion of granophyric intergrowth of quartz-plagioclase and potassic feldspar.
The Sublayer
The sublayer is a fine- to a medium- grained norite and gabbro unit, which contains inclusions of adjacent country rock, mafic to ultramatic inclusions of unknown origin and sulfides. The sublayer occurs as discontinuous lenses at the base of the Main Mass. essentially parallel to the lower contact of the SIC and as protnisions and embayrnents into the footwall rocks at the base of the complex (Fig. 1.1). There is a sharp contact with overlying nontes and a sharp to gradational contact with the footwall breccia that locally underlies the sublayer. Sublayer distribution is apparently controlled by the morphology of the basal contact of the SIC rather than the base of the Main Mass. It is not known if the quartz diode of the Offset dykes has transitionai or sharp contacts with the sublayer
(Lightfoot et al., 1997b).
Sublayer rocks are distinguished fiom those of the Main Mass by their lower quartz content and higher pyroxene content (Naldrett and Kullenid., 1967; Naldrett et al., 1970;
Pattison, 1979). The geochemical distinction between the sublayer and the Main Mass norites cannot be explained by closed system fractional crystallisation or partial melting (Lightfoot et al., 1997a). The composition of the sublayer displays marked variations between different embayments, although each individual embayment is relatively homogenous. It is likely that these differences reflect interaction of sublayer magma with country rocks on a local scale
(Lightfoot et al., 1997b).
Associated Ores
Pyrrhotite, chalcopyrite and pentlandite, pyrite, and titanium-poor magnetite account for the bulk of Sudbury sulfide orebodies (Naldrett, 1984). They occur along the lower contact of the SIC, in the sublayer, the footwall breccia and/or the footwall rocks themselves, in the Sudbury breccia (pseudotachylyte) and in the Offset dykes. Marked compositional differences occur between the various Sudbury ore bodies and many of the deposits exhibit extensive compositional zoning.
Footwall Breccia
The footwall breccia is most common in the East and North Ranges. It occurs as a coherent clast-rich breccia layer in discontinuous lenses and sheets along the lower contact of the SIC. Small dykes or offshoots of footwall breccia occur in the brecciated crystalline basement rocks up to 250 m away fiom the SIC (Morny, 1990). The footwall breccia contains rock fragments that are mainly derived from the local basement rocks, but may include clasts of Sudbury breccia The lower contact between the breccia and the underlying footwall rocks is gradational for distances of up to 150 m. This gradational zone has been termed, "megabreccia" by Pattison (1 979). Where the footwall breccia is directly overlain by mafic norite, the contact is generally sharp, whereas there is a gradational contact between the footwall breccia and the sublayer. The composition of the matrix in the contact zone of the sublayer or Main Mass is diontic with intersertal textures, whereas beneath this zone the matrix is characterised by poikilitic to granular textures and a tonalitc to granitic composition. Thermal meaiing and partial melting of fine-grained clastic material by the overlying hot SIC resulted in a metarnorphic- to igneous-textured matrix, which crystallised below 1,040 OC(Lakomy, 1990).
Sudbury Breccid Pseudotachylyte
The Sudbury breccias occur in the footwall country rocks of the Superior and
Southem Provinces and are generally considered to be pseudotachylytes (Dressler, 1984a;
Thompson and Spray. 1994). Sudbury breccias form irregular or crudely tabular bodies ranging fiorn a few mm to 1 km thickness. The breccia dykes generally dip steeply to vertically. Fragments are generally of the sarne rock type as the local host rocks. Fragment size within the breccias depends on the size of the breccia body, srnaller breccia bodies and dykes containing proportionally fewer visible Fragments. The breccia is concentnited in four zones that define a radial to the SIC: at 0- 13 km, 25-35 km, 42-48 km, and 78-80 km
(Thompson and Spray, 1996). Breccia zones in the distal parts of the Sudbury structure tend to be less continuous, less abundant, thimer and more likely to be of a cataclastic origin
(Thompson and Spray, 1 994).
Whitewater Croup
This group is confined to the central part of the Sudbury basin. The group is made up of three formations, from the base to the top: the Onaping, Ontwatin, and Chelmsford
Formations (Fig.1.1).
The Onaping Formation consists of complex stratified breccia units and igneous- textured bodies. The basal contact with the SIC is gradational with common granophyre or sharp with the plagioclase-rich granophyre (Peredery, 1972; Muir and Peredery, 1984).
The Vermilion Member locaily drfines the base of the Onwatin Formation, a massive siliceous, carbonaceous argiilite. The remainder of the Onwatin Formation consists of argillite. silty argillite, fine to coarse grained siltstone, and graywacke. The unit grades upward into coarser sedimentary rocks of the Chelmsford Formation, which consists largely of wacke and siltstone (Rousell, 1984).
POST- SUDBURY EVENT DEFORMATION
The Sudbury Event has been dated at 185W1 Ma by the U-Pb method using zircon and baddeleyite from the SIC (Krogh et al., 1984). This event occurred during the Penokean
Orogeny, which was produced by the collision of an allochtonous terrain with the passive margin of the Superior Archean Cmton (Bennet et al., 1 99 1). These orgenic events lead to deformation and regional metamorphism of the Sudbury structure. Deformation is predomiriant in the South Range of the SIC defined as the area south of the South Range shear Zone. Recent detailed structural anaiysis of the South Range of the Sudbruy structure by Riller and Schwerdtner (1 997) demonstrates that the Penokean Orogen cmbe subdivided into two major tectonic pulses. the Blezardian and Penokean. The pre-impact Blezardian tectonic pulse, 2.4 - 2.2 Ga was responsible for the deformation and metamorphism of Huronian rocks and emplacement of the Murray and Creighton granitoid plutons. The
Huronian outlien to the NW, and the Vernon syncline to the West of the Sudbury basin, are thought to delineate a rim syncline to an elliptical dome structure cored by Archean basement. This dome is thought to have formed due to non-cylindncal folding during the
Blezardian orogeny (Riller and Schwerdtner 1997). Blezardian deformation of the Huronian rocks in the South Range was accompanied by amphibolite facies metamorphism. In contrast,
Penokean structures formed at 1.9-1.8 Ga such as the South Range shear zone, are characterised by shape fabrics formed at low to middle greenschist facies metamorphic grade
(Riller and Schwerdtner 1997). This is show by the breakdown of homblende and plagioclase porphyroclasts into the greenschist facies metamorphic assemblage of quartz + chlorite + epidote * zoisite I actinolite biotite defining S-C surfaces, and the lack of plastic deformation of feldspar (Tullis, 1983). There is thus evidence for a greenschist facies metarnorphic overprint in the Huronian Supergroup rocks in the South Range of the Sudbury structure (Rillcr and Schwerdtner 1997). The metamorphic overprint on the SIC decreases progressively northward (Cd1978). This regional metamorphic overprint is attributed to
Penokean thrusting (Riller and Schwerdtner 1997). Thomson et al. (1985) and Fleet et al.
(1987) describe local variations in metamorphic grade of the SIC rocks with metamorphic grade decreasing to the north to low- middle- greenschist facies in the North Range. The extent to which the SIC rocks have been altered varies locally. The patchy distribution of metamorphism is attributed to the low permeability of rnafic rocks prohibiting extensive ingress of fluids, thus limiting metarnorphic reactions (Fleet et al., 1987).
Shanks and Schwerdtner (1 991) interpreted the South Range shear zone as a splay of a large listric thmt that underlies the Southem Province immediately south of the Sudbury structure. An alternative explanation could be that deformation is a consequence of asymmetric folding of the Sudbury structure similar to folds produced in laboratory buckle fold experiments with soft matenals (Cowan and Schwerdtner, 1994, and references therein)). One limb of the fold progressively deforms by solid-body rotation and distortion
(South Range), while the other limb records little strain but tilts slightly (North Range).
Eventually a shear zone develops between the two lirnbs, probebly because their strains are incompatible.
Northwest- directed thnisting andor folding is thought to have displaced the South
Range of the Sudbury structure northwestwards by a minimum of 8 km (Shanks and
Schwerdtner, 1991) and possibly up to 20 km (Milkereit et al., 1992). Paleomagnetic data inciicate that erosion removed a cover of - 10 km in the Sudbury area over 1.85 Ga (Schwarz and Buchen, 1982). Normal faults were reactivated and became the loci of north-verging listric reverse faults during Penokean compression. Based on pressure-temperature data and regional structure, Zolnai et al. (1984) conclude that Huronian rocks were buried to mid- cnistal levels (- 15 km depth) south of the Murray Fault, but to no more than 5 km north of the Murrary Fault. There is scant evidence for a major regional metamorphic event above sub-greenschist facies in the basement rocks of the North Range, which is consistent with this relatively shallow burial.
Isotopic dating of North Range rocks (40~r/39Ar,Thompmn et al., in press; RbISr,
Deutsch, 1994) yield a broad range of apparent ages that do not coincide with any established tectonic or thermal events in the North Range. Thompson et al. (in press) have proposed a mode1 that best fits both their age data and the regional geology. They propose that low-grade
regional metamorphism is due to Penokean thin-skimed overthrusting occurring in the North
Range immediately following impact. This thnisting resulted in tectonic (geologically
instantaneous) burial to 5-6 km at 1850 Ma, yielding an appmximate footwall temperature
of 160" - 180°, assuming a geotherrnal gradient of 30°C km-'and an original surface
temperature of - 1 O". Heating (burial) lasted until - 1000 Ma, when there was exhumation
of the impact structure. The lack of ages younger than Grenvillian indicates that the impact
structure was elevated and probably exhumed at - 1O00 Ma, such that temperatures dropped
below - 150 OC.
Sub-greenschist metamorphism of the North Range cannot explain some of the
mineral assemblages observed in SIC-related rocks. These variations in mineralogy could be due to metasomatic events other than the Penokean, as oxygen isotope data are consistent
with either a local hydrothemal metamorphism ador a regional metamorphic overprint
(Thomson et al., 1985). Other than the Penokean Orogeny, there are several other possible events that may have extended into the North Range. These include at least two phases of anorgenic granitic magrnatism concentrated to the south of the Sudbury structure at 1750-
1700 Ma and 1500- 1450 Ma (Card, 1992; Davidson et ai., 1992). Albitisation of rocks east of the SIC has ken linked to this plutonism (Schandl et al., 1994). The Onaping formation
has undergone basin-wide hydrothermal alteration (Arnes et al., 1997). This hydrothemal
system has been confirmed by U-Pb dating of hydrothemal titanite within these rocks
indicates that this hydrothermal system is impact induced (Arnes et al., 1998).
The interpretation of high-resolution seismic reflection data dong the Lithoprobe Transect and gravity data across the Sudbury structure has provided new geomeûical comtraints on the shape of the Sudbury structure at depth (Mikereit et al., 1994; McGrath and Broome., 1994). Magnetic rock property measurements (Hearst et al., 1994) indicate the presence of a strongly magnetic phase of the Levack gneiss close to the contact with the
Sudbury structure, and that the Levack gneiss may consist of as many as four magnetically distinct units. Figure 1.3 shows a schematic diagram of the geometry of the Sudbury structure combining the results of these studies.
Deformation of the Sudbury structure may have been enhanced during the Grenville orogeny. The Grenville Province orogenic belt of - 1 LOO to 1070 Ma tnincates the Southern
Province to the south of the SIC (Easton, 199 1 ). The N W boundary of the Grenville Province is a 30 km wide zone known as the Grenville Front Tectonic Zone (GFTZ).
Olivine diabase dykes of the Sudbury Swarm are found throughout the Sudbury area, extending northward fiom the southwestern part of the Grenville Front Tectonic Zone. They trend W to NW, parallel to the regional faults of the Timiskaming System located to the east of the Sudbury structure. It is common for dykes to occupy these fault zones. The Sudbury dyke swmhas ken dated at 123814 Ma (Krogh et al., 1987). The Sudbury dykes transect lithotectonic trends in Archean and early Paleoproterozoic rocks, but are displaced by faults of the Grenville Front Tectonic Zone (GFTZ).The SIC and Archean rocks are displaced northwards to the east of both the Fecunis Lake and Sandcheny Creek faults of the NNW- trending Onaping fault System (Fig. 1. 1 and 2.1 b).
WISTORY
The origin of the Sudbury structure remains a subject of some controversy. The four main rnodels that have been proposed are: (1) volcanogenic, (2) impact, (3) impact-induced volcanism, and (4) hybrid (impact-generated plutonic activity). Early theories proposed the structure to be purely volcanogenic in ongin. This theory is still maintained by some geoiogias, such as Muir (1984). Ln such models the Sudbury breccias and other shock metarnorphic features are due to explosive eruption. Slumping of caldera wnlls formed the lower sections of the Onaping Formation, with the rest of the Formation king deposited by further eruptions, after the SIC was intruded into the footwall breccia.
Dietz (1 964), following recognition of shatter cones in the Sudbury area, suggested that the structure was the site of a major impact event, which caused the extensive brecciation of the country rocks. This was the initiation of many new theories for the formation of the Sudbury structure. He proposed impact and then generation of magma deep in the crust that was able to ascend to the Crater and differentisite to form the SIC. The
Onaping Formation was considered to have been formed by explosive degassing of the cooling magma.
Many Mer models of impact-generated igneous activity have been presented, variously attributing the Onaping Fom~tiûnto impact-ifiducedvolcanisrn (Stevenbon, 1W2), to the accumulation of a fatlback material (e.g., Peredery, 1972). Such models require a high thermal gradient at the time of impiict (French, 1WO), and extreme cnistal contamination to account for the unusual composition of the SIC (French, 1970; Peredery and Momson,
1984; and Naldrett and Hewins, 1984). The SIC has a REE pattern very sirnilar to the upper continental crust at the time of impact (Kuo and Crockett, 1979; Grieve et al., 1991). This indicates that the SIC is composed purely of impact melt and requires no made component. Nd mode1 ages for
Sudbury structure rocks are 2.5 to 2.9 Ga which corresponds with U-Pb zircon ages for the local basement rocks (Faggart et al., 1985; Deutsch et al.. 1989). In pure impact models, the
Main Mass of the SIC is the ciast free impact melt, the Basal member of the Onaping
Formation a clast rich melt breccia, the Gray member a suevitic breccia, the Green member melt-rich fallback layer and the Black member redeposited suevitic breccia.
Lightfoot et al. (1 997b)have proposed that although. there is no direct evidence for a mantle contribution, there could be a minor mantle component (<20 %) to the SIC. A primitive component to the SIC could possibly explain some of the more cryptic features of the SIC, such as the mafic-ultramafic fragments of olivine melanorite, websterite, wehrlite and pyroxenite that occur as "xenoliths" in the sublayer and the high S, Ni, Cu, Co and Pt group element (PGE)content.
SUDBURY OFFSET DYKES
Emanating from the SIC are the so-called Offset dykes (Coleman, 1905) which are disposed radially and concentrically about the SIC (Fig. 1.1). Ten such bodies are now known, many of which are directly associated with world-class Ni-Cu-POE deposits.
Coleman (1903)coined the term 'Offset' because the dykes, particularly the radial ones, can teminate dong strike, but reappear displaced by as much as a few km parallel to strike.
Some of the segmentation of the dykes has been caused by pst-emplacement faulting. Where no such faulting is evident the dyke segments have smooth lobate ends (Cochrane,
1984). Traditionally, the igneous material within the Offset dykes has been described as a variant of the sublayer. However Lightfoot et al. (1997b) suggested that the trace and rare! earth element chemistry of the Offsets show a greater afhity to the lower units of the Main
Mass of the SIC.
Al1 Offset dykes are steeply dipping to vertical and have sharp contacts with the host rocks. They consist predominantly of so-called quartz diorite, but may include significant proportions of footwall breccia. The Offsets are hosts to many of the Ni-Cu sulfide deposits in the Sudbury mining camp. Several of the dykes were injected into pre-existing dyke- like bodies of Sudbury breccia.
Most of the radial Offset dykes can be traced in the field back to the contact sublayer.
Typically, the radials connect to the Main Mass by trough shaped, sublayer-filled, footwall ernbayments up to several hundred metres wide. Within a few km distance f'rom the Main
Mas, the radial Offset dykes narrow in width. Generally only small arnounts of massive
Sudbury breccia are adjacent to these Offsets. Signifiant amounts of footwall breccia are included within some Offsets, such as the Foy Offset which in places consists entirely of footwall breccia (Pattison, 1979).
The parallel or concentric Offset dykes have no apparent connection with the Main
Mas. They occur as continuous to semi-continuousdykes of quartz diorite that strike parallel
to the lower contact of the Main Mass and are hosted by zones of massive Sudbury breccia.
They consist of discontinuous dyke segments, or elli psoidal pods of amphibole-biot ite quartz
dionte, within zones of massive, variably recrystallised Sudbury breccia. Discontinuouspods of quartz diorite not associated with Offsets also occur generally within embayment structures that are mostly filled with sublayer rocks.
Traditionally the term quartz diorite has been used to describe the igneous material filling the Offset dykes (Grant and Bite, 1984). in fact, most compositions lie in the quartz momdiorite field, of the QAP plot of their average anhydrous mineralogy as defined by the
IUGS (1974). North Range Offsets are transitional in the granodiorite field (Grant and Bite,
1984), even so the terni quartz diorite is iikely to remain in common usage.
There are several different varieties of the quartz diorite material that are associated with the Offsets, which are narned afier the dominant mafic minerals present (Grant and Bite,
1984). These are onhopyroxene-, clinopyroxene- and amphibole- biotite quartz diorite. The majority of quartz diorite varieties differ mainly in pyroxene content and degree of pyroxene alteration. The contact of the Offsets is typically a fine to very fine grained biotite- quartz diorite, which has been attributed to localised assimilation of siliceous country rock material
(Grant and Bite, 1984). Sphenilitic-textured quartz diorite also occurs at the contacts of some
Offsets. This texture is interpreted to be indicative of a quenched mck, which cooled rapidly against the country rock. It is particularly well developed at the margins of quartz diorite pods in the parailel Offset dykes.
Assimilation of country rock into the dykes can produce modification to the quartz diorite compositions. Assimilation of major elements is moa pmunced near country rock contacts, and can be directly related to the composition of the adjacent country rocks (Gnuit and Bite, 1984). Ostennann et al. (1 995) demonstrated that marginal facies of the Foy Offset had oxygen isotope values close to that of the local country rock, whereas sarnples From the centre of the Offset had values similar to those in the North Range norites (Ding and
Schwarz, 1984).
Oxygen isotope data indicate that the marginal facies of the Foy Offset contain an enhanced but variable contribution from local country rocks: at the centre of the Foy Offset,
6"O is 6.8%, values are similar to the North Range norites , whereas the marginal facies have 6"O of 7.9%. close to values for Levack material (Ostermann et al., 1995). Lighübot et al. (1 997b) believe that the minor differences in trace and REE between South and North
Range Offsets could be accounted for by the assimilation of the distinctly different country rocks of the North and South Ranges during emplacement.
Parts of the Hess Offset had been known to prospectors as a sulfide- bearing dyke for several decades, but it was only equated with the SIC as recently as the 1970s (Peredery,
1974), while its currently-established iength was realised in the 1990s (this work). In most previous work the Hess Offset has been described as part of the Foy Offset rather than as a distinct parallel Offset (Grant and Bite, 1984; Morris and Pay, 1981 ;Lightfoot et al., 1997a).
Like the other concentric Offsets, the Hess does ofien have good sphemlitic textures at the contact with the country rocks. Although the Hess Offset is associated with a zone of pseudotachylyte it does not consist of pods of quartz diorite which are contained in massive zones of pseudotachylyte like some of the South Range concentric bodies (e.g.Frood Stobie,
Manchester, McConnell, Kirkwood and Vermilion). Chapter 2
GEOLOGICAL SETTING OF THE HESS OFFSET DYKE
The Hess Offset occurs 12-15 hi north of the SIC in the North Range within the
Benny (Card and Innes, 198 1 ), Hess, Harty, Leinster and Tyrone Townships (Card and Meyn,
1969). It strikes for a length of at least 23 km as a essentially continuous 10-60 m wide dyke, that is oriented subconcentrically to the SIC (Figs. 1.1 and 2.1). The distal Foy extends at least 30 km beyond the northem margin of the Main Mass of the SIC. The Hess Offset intersects the Foy Offset, with the Foy being displaced to the east by 2 km. This naturally divides the Foy into southem (proximal) and northem (distal) portions (Fig. 1.1 and 2.1 b).
The Foy Offset in the area where it intersects the Hess Offset was also examined in this study, as was a small section of a radial Offset dyke located to the west of the Foy Offset. In this work this latter dyke is called the Parallel Foy.
ACCESS
Access to the Hess and Foy Offsets was by Highway 144 that links Sudbury and
Timmins, Ontario, and by grave1 roads extending east and West from Highway 144, by came through Clearwater Lake dom the Onaping River, and via Sandcherry Lake through
Schkowona Lake. Some dl-terrain vehicle trails are marked on the large fold out map (in the back cover). Many of these trails are thoroughly overgrown and can be dificult to fuid.
However, they can be useful for access by foot. The water level in the Onaping river is Figure 2.1 - Gmlogical setting of the Hess, Foy and Parallel Foy Offsets. Note; eastem end of (a) adjoins western end of (b). extremely variable and can be independent of local rainfall due to the hydro-dam located up- river on the Onaping Lakes. Great care should be taken when attempting to ford this river
(Fig. 2.2). TERRAIN
Hess is poorly exposed in relatively remote. inaccessible and forested terrain.
Topography is strongly controlled by the bedrock lithology, structure and the most recent
Pleistocene glaciation (Boissonneau, 1968). The temin altemates between rocky hills and depressions filled with glacial deposits and swamp (Fig. 2.3). The path of the Hess Offset can typically be traced as a modest topographic low, as it has been eroded more than the surrounding granitoids, which constitute the predominant host rock. Major NNW- trending faults (of the Onaping System) control the overall drainage patterns, probably due to the fact that this trend is parallel to that of the ice movement during Pleistocene glaciation. Drainage is generally southward via the Vermilion River and its tributaries. Local variations in drainage direction attest to the immature nature of the drainage regime. Fluctuations in stream and pond distributions are dependant on annual strengths of the beaver population.
WALL ROCKS
The Cartier granite constinites the predominant rock type in the area. These felsic plutonic rocks are pink to light grey, medium- to coarse-grained, and cornmnnly porphyritic.
Fine to medium grained 0.1 -0.5 mm equigranular granitic rocks, fine-grained aplite and coarse pegmatite also occur as dykes pervading the granite.
In outcrop, the Offset dykes have a bulbous blocky appearance due to weathering Figure 2.2 - Variable water levels in the Onaping River can make fording the river hazardous and caution should be used.
Figure 2.3 - The NE end ofDepot Lake, showing typical topography of the area. dong rectangular joint systems. Other basic to intemediate intrusions in the region include the pre-impact 2.45 Matachewan dykes (Heaman, 1 988), the 2.2 1 Ga Nipissing diabase suite
(Corfû and Andrews, 1986; Lightfoot et al.. 1993), as well as post-impact 1.24 Ga Sudbury and 1.14 Abitibi dykes (Xmgh et al., 1987). The Hess Offset can appear very similar to the other basic to intermediate intrusions in the area, but in particular to the Matachewan dykes and Nipissing diabase bodies. Great care has to be taken in distinguishing these intrusions fiom Hess Offset material, particularly when tnicing Hess through Nipissing diabase (e.g., as at Clear Lake, Fig. 2. la Fig. 2.4). In hand sarnpie the Offset material can be distinguished fiom Nipissing diabase and Matachewan dykes by the "salt and pepper" appearance, due to the subophitic to intergranular texture of the Offsets. Nipissing diabase and Matachewan dykes contain a higher proportion of pyroxenes andor amphiboles and have a more ophitic texture. The higher proportion of mafic minerals tends to give these rocks a greener hue and their crystal faces tend to be blockier. Offset dykes contain a greater proportion of biotite and typically have visible granophyric intergrowths of quartz and potassium feldspar, especiaily near dyke centres. Offset dykes can have a blue to pinkish hue, dependant of the amount of granophyre present. The Nipissing diabase can also contain significant proportions of granophyre, such as on the SW shore of Clear Lake (Card and Innes, 1981), but this tends to be coarser grained than that of the Offset dykes. The Hess Offset is also typified by lath- like to acicular pyroxene crystals (or amphiboles pseudomorphing pyroxene), especially Mar wall rock contacts. These acicular crystals can be up to 20 mm long and occur as radiating masses, or comprise fan-like arrangements of divergent, comrnody branching, fibres.
Humnian Supergroup rocks outcrop in a broad arcuate zone in the vicinity of the Hess Figure 2.4 - Hess quartz diorite (lefi) in contact with Nipissing diabase, south shore of Nipissing Island in Clear Lake. Both are cross cut by later epidote veins. Dashed line indicates the contact. N=Nipissing inclusion in the quartz diorite, G=Cartier granitoid inclusion in pseudotachylyte adjacent to the dyke, V=epidote veins. View to the north east, scale card length 8.5 cm.
Figure 2.5 - Contact of the Hess Offset quartz diorite (right) with Cartier granitoid (lefi), marked with dashed line. The quartz diorite contains a small inclusion of granite. North shore ofDrained Lake, west ofthe Rivers Option. View to the east, scale card 8.5 cm.
27 Offset. They have not been observed in direct contact with the Hess Offset, but they do occur predorninantly to the north of the Offset. The Huronian Supergroup consists of cyclical repetitions of three sequences of conglomerate, siltstone, wacke and arenite, with a basal group of felsic and mafic volcanics, wacke, arenite and siltstone (Debicki, 1991). The basal
Elliot Lake Group is completely absent from the area and formations in the overlying Hough
Lake Group, Quirke Lake Group and Cobalt Group are locally absent. The Huronian
Supergroup rocks in the area are generally termed outliers, and are remnants of a former, probably continuous Huronian sedimentary cover.
At the western end of the Hess Offset, hmClear Lake to the Fecunis Lake fault, there are large, partially fault-bounded blocks of Huronian Supergroup metasedimentary rocks (Card and Innes, 1981). This large arcuate zone of Huronian Supergroup metasedimentary rocks lies approximately parallel to the Hess Offset and between 1 and 4 km north of it. These blocks consists of al1 three formations of the Quirke Lake Group. and the lower two formations of the Cobalt Group (Card and Imes, 1981). To the north of these
Huronian Supergroup blocks, outcrops of metavolcanic rocks of the Benny greenstone belt and Levack gneiss occur (Card and Innes, 198 1). To the east of the Fecunis Lake fault there are no further large outcrops of Huronian Supergroup rocks, but the arcuate greenstone belt and Levack gneiss continue between 3 and 5 kilometre north of the Hess Offset (Card and
Meyn 1969).
Some smaller outcrops of Humnian Supergroup rocks are found between the large arcuate zone of larger Huronian Supergroup blocks and the Hess Offset. At the norihem end of north Depot Lake there is a large outcmp of Cobalt Group, Gowganda Formation, polymictic paraconglomerate and quartz-feldspar sandstone (Card and Innes, 1981). There are also smaller outcrops of siltstone and wacke, probably of the basal Gowganda formation of the Cobalt Group, just south of the quartz feldspar sandstone. These are only a few hundred metres north of the Hess Offset.
The Hess Offset appears to cut through a smail anticlinal enclave of the Serpent formation and underlying Espanola formation, both of the Quirke Lake Group, on the SW shore of Elation Creek (Fig 2.1). Metasedimentary rocks of were found to the south of where the Hess Offset is believed to strike, although no outcrop of Hess was found in the area.
Calcareous siltstone and wacke, probably of the Espanola formation. occur on the northern shore of Pterodactyl Lake. which is south of the Hess Offset (see fold out map back of thesis, UTM 464442, 5177952). However. there is a branch of Hess-iike materid surrounded by Sudbury breccia on the eastem side of the small strearn/swarnp entering
Pterodactyl Lake on the NE shore, north of these metasedimentary rock outcrops.
Pseudotachylyte (Sudbury breccia) is found cross cutting al1 Archean and early
Proteromic rocks within the area, but excluding the Offset dykes related to the Sudbury structure. Pseudotachylyte is more prevalent and obvious in the granites. but is commonly associated with pre-existing weaknesses in the host rocks, suc h as bedding and contacts between contrasting lithologies (Thompson, 1996). In the vicinity of the Hess Offset, pseudotachylyte occurs as ubiquitous anastomosing sub mm- to mm- scale veinlets and as larger sub- me= scale discrete veins. Broader zones of more cataclastic deformation also occur, some of which grade into zones of pseudotachylyte. HESS OFFSET
The main melt phase of the Hess Offset is a fine to medium grained quartz diorite typical of the Sudbury Offset dykes. Relatively sulfide-deficient and xenolith-poor marginal zones can enclose a core of more sulfide-rich and xenolith-rich quartz diorite. The central portions of the dyke typically contain more granophyre. Inclusions entrained in the central portions of the Offset are mainly granitic, but also include noritic, gneissic, anorthositic
rocks, and breccia. The contacts show a decrease in grain size and typically show extensive development of acicular pyroxenes. At the actual contact there are commonly inclusions of the host rock, most commonly Cartier granitoid (Fig. 2.5). The quartz diorite is commonly crosscut by epidote veins ranging frorn millimetre scale to several centimetres in width.
These veins are generally very distinctive due to their positive relief and pale green colour
(Fig. 2.6).
The western end of Hess is predominantly a relatively sulfide- and inclusion-poor
quartz diorite. Sulfides occur as very minor disseminated blebs up to 1 cm in diameter.
inclusions, where present, are generally felsic in nature and small (between 0.5-3 cm in
diameter). Other inclusions in the central portion of the dyke are rare. In the swamp to the
south of Elation Creek an apparent contact of quartz diorite against quartz diorite was found
(Fig. 2.7). Coarse grained, more granophyre- rich quartz diorite is in sharp contact with a
darker quartz-poor variety.
In the area of the Foy-Hess intersections (from West of Neil Lake to the distal Foy
intersection), Hess contains significantly more inclusions than does the western end of the
Offket. Detailed maps of the proximal and distal Foy intersection are show in Figs. 2.8 and Figure 2.6 - Epidote veins cross-cutting quartz diorite. Harty option, scale card 8.5 cm in length.
Figure 2.7 - Interna1 contact, within the Hess Offset, of lighter mon granophyrk quartz diorite (lower half of pichue) against normal quartz diorite, in the swamp south West of tlation Creek. Clinorneter 9 cm in length, view to the no&.
2.9, respectively. The majority of the inclusions in this area are heterolithic breccias with a dark apparent igneous-textured matrix. These inclusions are similar in appearance to the footwall breccia occumng as sheets and discontinuous bodies parallel to the lower contact of the SIC (Dressler et al., 1991), but also to the so-called footwall breccia infilling part of the proximal section of the Foy Offset (Pattison, 1979). There is a complete range in clast content within these footwall-like breccia inclusions from clast-rich (up to approximately
90% clasts) to clast-poor. Large clasts within these inclusions are randomly oriented. Al1 of the footwall-like breccia inclusions contain millimetre- to centimetre- scale felsic clasts that appear similar to those that can occur in the main phase of the Hess Offset. Clasts are generally subrounded to rounded and range in size from submiilimetre to approximately 0.3 metres (Figs. 2.10 and 2.1 1). The large clasts in the footwall-like breccia inclusions are mostly granitic or gneissic, although gabbroic. morthositic and other mafic inclusions do occur (Fig. 2.1 1). The clast-poor breccia can be mistaken for the quartz dionte, but the footwall-like breccia inclusions are distinguishable by the higher proportion of small felsic inclusions and the complete the lack of acicular or lath-like crystals. Similar breccia inclusions are also found in other Offset dykes, as in the Foy (Pattison 1979) and Whistle
Offsets (Dressler 1984b ).
Larger (0.3 m - 2 m) granite and gneissic inclusions also occur in the Hess-Foy intersection area. It is not always clear fiom field relationships as to whether these are large clasts within the breccia inclusions, or isolated inclusions in the quartz dionte. In most gneissic inclusions there is a distinct foliation defined by compositional banding.
Imrnediately West of the distal Foy-Hess intersection, approximately 8 metres in width, in the Figure 2.10 - Inclusion of clast-packed footwall-like breccia within the Hess Offset West of the proximal Foy intersection. Am>won scale card pointing north. Card length 8.5 cm.
Figure 2.11 - Inclusion of clast-packed footwall-like breccia, at the proximal Foy intersection (see Fig. 2.8, sample location 7- 193A). Compass 6 cm wide, pointing north. centre of the Hess Offset consists of an extremely fine grained dark zone, which other than a lack of gneissic banding has very similar mineralogy to the gneissic inclusions. No sharp contact was found with the quartz diorite. Similar outcrops were also found in the area of the proximal Foy intersection.
The Rivers Option is a small mineralised zone of the Hess Offset, on the 'peninsular' of a drained Iake in eastem Hess township (see detailed map and Fig 2.12, UTM 464952,
51783378). Four pits were excavated and the area was drilled as part of an exploration program undertaken by Inco in the 1950s and 1960s. Some drill collars can still be found in the area (Fig. 2.12). The main mineralised 'showing' on the 'peninsular' is surrounded by rubble due to blasting during excavation (Fig. 2.13). The host rocks to the north of the
'showing' are highly altered and extensively cut by quartz veins (Fig. 2.14). Imrnediately adjacent to the Offset and on the edge of the swamp to the east, the host rocks are essentially quartz with only minor feldspar, mica and chlorite. To the north they consist of quartz feldspar and minor arnounts of accessory minerals, but are still extensively cut by quartz veins. These brecciated rocks appear to have a gradational contact to the north and west with granite. Due to the pervasiveness of deformation, it is dificult to ascertain exactly whether the protolith for these rocks was granite or Huronian Supergroup metasedimentary rocks.
Between the two main water-filled pits there are several inclusions entraincd within the quartz diorite. These include two large, coanely- grained, slightly lighter-coloured inclusions, approximately 50 cm in diameter (Fig. 2.15) and one small, fine-grained, apparently sedimentacy, rectanplar inclusion approximately 4 cm by 8 cm (Fig. 2.16). The main mineralisation, apparent due to the amount of gossan present on the outcrop surface, Figure 2.12 Detailed map of the Rivers Option mineralised showing. Drill holes are fiom Inco Exploration fiom the 50's. Sample fiom the Rubble Pile not marked on this rnap are 7-108.6- . 122 and 7- 1 12. The large area through the centre of the map where there is no exposure is swamp. The 'showing' is the 'peninsular' in the centre. Figure 2.13 - Rubble pile around the pits, hmexploration by Inco in the early 1960s, at the Riven Option. View looking NW.Scale bar is 60 cm.
Figure 2.14 - Extensive quartz veining in the wall rocks, north ofthe pits at the Rivers Option. Scale (centre photo) card 8.5 cm in length, looking north east.
38 Photo 2.14 - Large coarse-grained lighter inclusions in the quartz diorite behueen pits at the Rivers Option (see Fig. 2.12), contacts indicated by dashed line. Scale card is 8.5 cm long, arrow pointing north. Dark hole in the upper left corner is a diarnond drill hole fiom Morris (1981).
Figure 2.16 - Fine grained (Huronian metasedimentary rock?) inclusion in quartz diorite between the two basic inclusions(above) at the Rivers Option (see Fig 2.12). Scale card is 8.5 cm long, mwpointhg north.
39 outcrops through the southem area of the pits.
Bifurcations in the Hess Offset can take the form of short splays, which generate claw-shaped apophyses (see detailed map of Dehydration Ridge, Fig 2.17). The small outcrop of what appears to be Hess-like material approximately 30 m south of other outcrop of Hess on the eastem side of the smdl strearn/swamp entering Pterodactyl Lake. This may also be a bifurcation in the Hess Offset. This is very similar in appeannce to Hess quartz diode and was identified in the field as such, though it is greener and more extensively cut by epidote veins. There is a broad area of pseudotachylyte adjacent to the outcrop. Pinches and swells and bifurcations of the Hess Offset may be more common, but are not obvious due to the relative paucity of outcrop. Apart from displacement of the Hess Offset due to regional faults
(Sandcherry Creek fault and the Fecunis Lake fault), and minor displacements either due to primary emplacement mechanisms or minor faults, the Hess Offset is essentially continuous, except for a 3.25 km section between Little Elbow creek and the oxbow on the Onaping
River where, despite good exposure, no outcrops of quartz diorite have ken found.
The most westerly outcrop of the Hess Offset so far known occurs on the western side of Nipissing Island in Clear Lake. Further west, in Clear Lake, there is a narrow arm of the lake that is on strike with the Hess Offset where boulders of quartz dionte have been found.
The southem shore of this arm is Nipissing diabase which, in places, is extremely granophyric. On the NW shore of this ann there are outcrops of the Quirke Lake Group,
Espanola Formation (Card and Innes, 198 1). The most easterly known exposure of Hess
Offset is in central Tyrone Township, approximately 400 m east of the intersection of the
Hess Offset with the distal portion of the Foy Offset. There is a large boulder-strewn area +++++++++++++*++++* +++++++++++++++++++ + ++++++++++++++++++- t~++++++++tt+++tttt + * ++++++++++++++++++ L++++t++ti+t+tt++ 8' ++++++++++++++++
+++++ +++++++ .+++++++ +++++++++++ ++++t+t+++
++++++++++ +++++++++ +++irt+ ++++++ ++++++++ t+ttt+t+ +++++++ +trr+++r t+-++++ +**+++r .+++r+++i 'Ti++++ .,++etc ++t+*+ tttttt .++++++*++ ++++++ +-++++
++++++++++Tt Y&+++*++* ,++++++++++++ +++++ +t+t+t+++++++ ttct,t *++++t+++++++ *++++ +*+++t+*+c++ Pt*** ++++++*+*+++ +tt++ +++++++++++ VTCCC t + * + + + + + + + + +-3 +++*+ extending about I km east of this outcrop, in which boulders of quartz diorite have been found. This indicates the Hess Offset may continue both east and West of those parts of the dyke mapped during this work. FOY OFFSET
There are no significant quantities of pseudotachylyte adjacent to the Foy Offset, but there is an abundance of footwall-like breccia occurring as inclusions in the Foy Offset
(Grant and Bite, 1984). The embayment at the mouth of the Foy Offset consists of a xenolithic Norite, the centre of which is quartz poor, mantled by progressively more quartz- rich varieties adjacent to the contacts with the host rocks. Approximately 3 km beyond the contact of the Main Mass with Noritic rocks of the Foy embayment, the Foy Offset consists of altemating sections of footwall breccia and quartz diorite. Pattison (1 979) suggested that this indicated sirnultaneous emplacement of leucocratic footwall breccia and quartz diorite, noting that the average chernical compositions of major elements in the footwall breccia and the quartz diorite are vinually identical. Sirnilar patterns of marginal quartz enrichment and altemation with footwall breccia have ken observed in the Whistle embayment (Pattison,
1979).
Ten kilometres from the contact with the Main Mass, the Foy Offset is up to 120 rn wide and consists of quartz diorite. Relatively sulfitie-deficient and xenolith-poor margins of quartz dionte enclose a core of sulfide-rich and xenolith-rich quartz diorite. These inclusions are pcimarily locally derived granitic, migrnatite, gabbroic and metavolcanic rocks
(Grant and Bite, 1984), but the suifide-rich cores also contain inclusions of metapyroxenite and melanorite (Farrell et al., 1995). At the Nickel Offset mine the quartz diorite is in places hosted in a zone of diorite breccia (Lightfoot et d.,1997a, Figure 178). o zone of quartz diorite breccia has been mapped adjacent to the Foy Onset at the Maki Showing immediately south of the proximal Foy intersection (Lightfoot et. al. 1997%sketch map bottom left corner of Map 2). The area of quartz diorite breccia is at least 40 by 20 metres and is well exposed.
In this study, outcrops of Foy quartz diorite were found on both sides of this zone of breccia, indicating that this is more likely to be an inclusion of footwall-like breccia material within the dyke. The inclusion comprises adark matrix surrounding 2-5 cm felsic inclusions, as well as occasional large inclusions, up to a metre in diameter, of granite, gneiss and more mafic inclusions (Fig. 2.18). The inclusions within the raftlblock are essentially the same as those seen in the other breccia inclusions in the Foy and Hess Offsets.
FOYMESS RELATIONSHIP
Where the contacts are able to be constrained the Hess Offset is generally between
10-30 m wide, but is occasionally up to approximately 60 m wide. l'he Hess Offset to the east of the distal Foy Offset rapidly pinches down to a width of approximately 10 metres
(Fig. 2.1). To the West of the proximal Foy intersection the Hess Offset is significantly wider than normal (approximately 60 rn), (Fig 2.Q and it gnidually mows to about 30 m on the
West side of Neil Lake. Hess nmws to between 30 and 40 m, immediately east of the proximal Foy intersection. Approximately 100 metres east of the proximal Foy intersection the Hess Offset pinches to a width of between 10 and 15 rnetres for a short distance then broaden out to 60 metres. The proximal section of the Foy Offset is also significantly wider in the immediate intersection area. Figure 2.18 - Footwall-like breccia inclusion at the Maki igure 2.19 - Vein of quartz diabase cross-cutting Hess quwtz Showing, with lerge granite, anorthosite, and mafic inclusions ,orionte at Dehydration Ridge. Dashed line indicates the contact, and small quartz blebs within the matrix. Compass is 6 cm ale card length 8.5 cm, looking north east. wide, The main melt phase of the Hess and Foy Offsets are indistinguishable in the field.
Although there is relatively good exposure at both the proximal and distal Foy intersections, no actual contact has been found between the Hess and Foy, if such a contact exists. There is a change in strike of both the Foy and Hess Offsets at the distal Foy-Hess intersection. The
Foy Offset changes from striking NNW (335 O) (general strike of the northern section of the proximal Foy Offset) to striking NNE (010") (general strike of the southem section of the distal Foy Offset.) The strike of the Hess Offset changes from 070" West of the distal Foy intersection to 110' east of the intersection. There is an apparent dextral displacement of 2 km of the Foy Offset by the Hess Offset. Though the sense is different, similar displacements occur between the Whistle and Parkin Offsets located to the NE of the SIC, concomitant with a comparable change in lithology from Cartier granitoids to Archean Parkin and Hutton
Greenstone belt and Huronian Supergroup metasedimentary rocks (Fig. 1A).
FAULTS
Most, if not all, faults in the area originated in the Early Precambrian. but were periodically reactivated during subsequent middle and Late Precambrian deformational events (Card and Imes, 198 1). The most conspicuous faults in the region are those of the
NNW- trending Onaping System. The Onaping fault system extends for at least 150 km across the southem Abitibi Subprovince (Buchan and Ernst, 1994). These include the Fecunis
Lake fault and the Sandcheny Creek fault in the Hess area, and the Upper Wanapitei River fault fbrther to the east. These structures are marked by prominent topographie lineaments and lake- strearn systems. The predominance of these feanires is probably attributable to the fact that these faults are approximately parallel to the direction of Pleistocene glacial ice movernent and were consequently the loci of Pleistocene erosion (Bamett, 1 99 1). The faults foming the Onaping System are relatively straight and apparently dip vertically. The Hess
Offset is displaced northwards to the east of both the Fecunis Lake and Sandcheny Creek faults of the NNW-trending Onaping System (Fig. 2.1). These two faults also displace the
SIC and the older rnetavolcanic and metasedimentary rocks of the Bemy Belt with the same sense (Fig. 1.1 )(Card and Meyn, 1969). Because the base of the SIC is known to dip to the south (Milkereit et al., 1994)- the relative displacement of its northem margin indicates that both the Fecunis Lake and Sandcherry Creek faults downthrow to the east.
NE-trending faults in the area are parallel to major regional structures such as the
Flack Lake fault and Montreal River fault. These trend between 055 O and 070°, and include the Geneva Lake and Clear Lake faults and Shingwak Creek faults (see fold out map in back pocket). These are essentially parallel to the Hess Offset. There is both sinistnil and dextral apparent horizontal movement on these faults as revealed by of Be~yBelt and Huronian rock displacement (Card and Innes, 198 1; Card and Meyn, 198 1).The significance of the NE- striking faults is uncertain; however tecionic events that have developed approximately NE- trending structures have resulted fiom, NW-directed thrusting of the Grenville Province (c. 1
Ga)(Easton, 199 1), re-activation of the Murray fault system of the Penokean folds and thnist belt and intracratonic thmsting in the Kapuskasing Structural Zone (Percival and Card,
1985). Two subpmllel ENE-trending lineaments have been described fiom the vicinity of
Geneva Lake (Kellet et al.. 1996). There is some evidence for a sinisual offset (4km) of the lineaments where they intersect the Onaping faults, implying movement on these faults pre-dates the most recent movement on the Onaping faults.
NW-striking faults, including the Gilbert Lake, Depot Creek, Banneman Lake,
Munster Lake, Michaud and Leinster Creek faults, are al1 parallel to the regional fault trend of the Timiskaming System centred to the east of the Bemy map area. Faults related to the
Timiskaming rifi are NNW-striking normal faults with an apparent sinistral component of displacement. These are part of acentral Superior Province-wide fault set (Jackson and Fyon,
1991). There is sinistral displacement of Benny Belt and of Huronim Supergroup rocks by some of these faults in the area. There is possible minor dextral displacement of the Hess
Offset by the Bannerman Lake fault through Elation Creek, and aiso through Depot Creek fault, but the lack of outcrop in this area makes this difficult to test. The late Precambrian olivine diabase dykes of the 1.24 Ga Sudbury swmare approximately parallel to these faults and the fault system is typically occupied by these dykes. The presence of an olivine diabase dykes through Little Elbow Creek and Big Elbow Pond may indicate the presence of Timiskaming suite faults in these areas as we!l (see fold out map in back pocket; Fig.2.1).
There are seveml localities where there is minor dispiacement of the Hess Offset. It not clear if this displacement is due to primary segmentation of the dyke due to emplacement mechanisms, or whether it is the result of later faulting. These include a 150 m sinistral displacement of dyke segments to the NE of Pterodactyl Lake. Here, there is a narrow swamp-filled valley sviking at approximately 140°, separating the two dyke segments. This valley may represent a fault, which is parallel to the Timiskaming system. Dehydration Ridge reveals approximately 30 m of sinistral displacement of two segments of Hess (Fig2.17). A
Matachewan dyke runs through this area, and this may have influenced the primary emplacement path of Hess. There is no evidence for faulting at this location. The Harty option is one such example of where there is sinistral displacement of the two dyke segments of approximately 190 metres (see detailed map Fig 2.20). The separation of the two segments occws along a long swamp-filled valley striking at approximately 150". There is extensive development of pseudotachylyte along the granite walls of this valley between the two dyke segments. The srnall section of Hess that outcrops to the NW of the Parallel Foy intersection with Hess appears to be displaced to the north of the rest of the Hess Offset. On the Inco exploration map this is marked as a branch of the main Hess Offset running parailel to it. No exposures of Hess Offset were found to the south of this section during this project, nor was it possible to trace out this northem section for any great distance due to low lying ground in the area. Like the Dehydration Ridge are& there is a Matachewan diabase dyke in the ma of displacement and no evidence of faulting.
Overall, such minor displacements across the Hess Offset could be more common than they appear due to the paucity of outcrop. When there is spacing of a couple of hundred metres between outcrops it is tempting, when drawing a map, to just join the outcrops together as a continuous dyke, whereas it could well be that this is not the reality of the situation. ui some areas this would become apparent with more detailed rnapping, but in areas of poor exposure, detailed geophysical data would be the only means of ascertaining continuity.
CROSSCUTTING PHASES
The Hess Offset is crosscut by later olivine diabase dykes of the Sudbury dyke swm.These dykes are generally 20-40 m wide and dip vertically. They are approximately parallel to the NW-trending faults of the Timiskaming System and are typically emplaced along these structures. They are displaced by late movement on NE-trending faults. Due to preferential erosion of the olivine diabase, these dykes are typically expressed as long, linear narrow streams or swarnp-filled valleys. These dykes cut across the Hess Offset in the
Elation Creek are% and in the vicinity of Little Elbow Creek (see large fold out map in the back cover). There is also a Matachewan dyke in this area and it is to the east of these outcrops that the Hess Offset disappears. There are several Sudbury dykes cutting through the area between Little Elbow Creek and the oxbow in the Onaping River where no outcrops of the Hess Offset were found during this study.
The Parallel Foy Offset also appears to crosscut the Hess Offset though no actual contacts are exposed due to the presence of Neil Lake. On the W shore of Neil Lake there is an outcrop of Parallel Foy with a large amount of recrystallised breccia associated with it, and outcrops of Hess Offset are found 1O metres SW.
An approximately 25 cm wide dyke of quartz diabase occua within the central portions of the Hess Offset on Dehydration Ridge (UTM465482,s 178602; Fig 2.1 9). Quartz diabase is easily distinguished by its finer grain size and the distinctive phenocrysts of plagioclase. The vein dips steeply to the east and is slightly chilled dong is margins. Quartz diabase was also found in the rubble pile at the Rivets Option. This dyke phase shows a slight decrease in grain size where it is in contact with the quartz diorite and one sample was found with quartz diabase clearly chilled against quartz diorite. The quartz diabase phase of
Hess is considered related through slightly later then the main yartz diorite phase. Relatively, older quariz diabase contains significant amounts of sulfides, whereas the younger is barren. The quartz diabase is considered to be late stage intrusive related to the
SIC (see geochemistry Chapter 5). Chapter 3
HESS OFFSET CONTACT RELATIONS/STRUCTURE
Hess occurs within a structurai zone that coincides with the northern margin of a suggested pseudotachylyte-rich annulus developed up to 13 km fiom the SIC (Spray and
Thompson, 1995; Thornpson and Spray, 1996). This limit corresponds to a ring-fault system defined by field studies (Thompson and Spray, 1994), and by a comprehensive remote sensing investigation (Butler, 1994). Outliers of Pmterozoic Huronian Supergroup metasedimentary sequences outcrop predominantly to the north of this ring fault, whilst
Archean Cartier granitoids and Levack gneisses dominate to the south. A northside downthrow/southside upthrow is thus implied, with granitoids and gneisses possibly representing an annular horst between the SIC and Hess, with a ring-graben developed north of Hess. Shatter cones are also predominantly found within this imer zone (Guy-Bray, 1966;
Gibson and Spray. 1998).
ASSOCIATION WITH PSEUDOTACHYLYTE
That the Hess Offset occupies part of a fault system is substantiated by the dyke's close association with pseudotachylyte (locally referred to as Sudbury breccia), which is known to form during high-speed slip associated with faulting (Thompson and Spray, 1996).
At Sudbury, as in other large impact stnictures, the immense sire ofpseudotachylyte bodies (developed up to 1 km thick and tens of km long) indicates that very large, single-slip
displacements have taken place: a behaviour that has been referred to as superfaulting (Spray,
1997). Pseudotachylyte is commonly associated with nattiral weaknesses in the host rocks, as such bedding and contacts between contmting lithologies (such as at Pterodactyl Lake).
In the case of Hess. pseudotachylyte commonly occupies the wall rock to the dyke, otherwise catacIastic/commuiuted margins of Cartier Granitoid predorninate. Pseudotachylyte pervades
the wall rock to the Hess Offset as discrete veins (up to 0.5 m wide) and as millimetre thick
to rnicroscopic veinlets. Immediately adjacent to the dyke the larger veins of pseudotachylyte
are more typically developed subparallel to the Offset (Fig. 3. l), although veins do also occur
at a high angle to the strike of Hess. The broader veins of pseudotachylyte are typically
vertical to subvertical and are generally preferentially eroded compared with the surrounding
granitoids. The smaller veinlets of pseudotachylyte anastomose and are more randomly
oriented (Fig. 3.2). Pseudotachylyte is less abundant than it is in many of the other Offsets,
such as at Manchester and the massive South Range breccia belt. So, while Hess does occur
within a zone of marked deformation, it does not do so within a particularly broad
pseudotachylyte zone. Only minor amounts of pseudotachylyte are associated with the Foy
Off'set, other than the nwnerous inclusions of footwall-like breccia within the Offset itself
(Pattison, 1979). Exotic (breccia) inclusions occur in the Hess Offset near the Foy
intersection similar to those described in the Foy Offset by Pattison (1979). Inclusions of
wall rock material are comrnonly found within Hess, especially near its margins, whereas
exotic and fwtwall breccia inclusions are more oflen found in the central portions of the
Offset. Figure 3.3 is a simplified block diagram showing the general field relationships for Figure 3.1 - Granite cliff immediately north of the Hess Offset. ?gure 3.2 - Millimetre- to centimetre-sca Hess outcrops out of the field of view to the bottom left of the seudotachylyte veins anastomosing through Carti picture. The stepped nature of the cliffcoincides with broad veins ranite host rock. of pseudoîachylyte running essentislly parallel to the Hess offset. Scale bar is approximately I m. footwall-li ke breccia inclusion
~artiergranitoid inclusion
Figure 3.3 - Simplified block diagram showing the field relationships of the Hess Offset. Brod pseudotachylyte veins typically occur sub-parallel to Hess. Veins of pseudotachylyte Vary in thickness along their length, and may die out or bifurcate. Minor amounts of cataclastically-defomed host rocks are found adjacent to the dyke. Small inclusions of Cartier granitoid host rock are comrnon at the dyke margins. Larger inclusions of breccia and exotics tend to occur in the centre of the dyke. the Hess Offset.
PSEUDOTACHYLYTE VEINLETS
Al1 granite in the area shows evidence of strain, with undulose extinction in quartz, and sutured grain margins, and in the development of tapering deformation twins in plagioclase (Fig. 3.4). Fine mm- scale pseudotachylyte, which tends to Vary in thickness along its length, is ubiquitous throughout the area of the Hess Offset. In places its appears to be associated with ill-defined shatter cone surfaces, as similarly described by Martini
(1992) and Gibson and Spray (1 998). Along the margins of these fine pseudotachylyte veins, quartz shows extensive dynamic recrystallisation, with relicts of larger old quartz grains showing undulose extinction and elongate subgrains passing laterally into domains of small
(0.05 - 0.1 mm) dynamically recrystallised grains (Fig. 3.5). This deformation is not pervasive within the host rock, but is restricted to a nmow zone associated with the margin of the pseudotachylyte vein. It indicates that shearing has taken place along vein margins
(Thompson and Spray, 1996). Displacement along some of these veinlets has been observed to be as much as a metre, but it is more typically in the order of centimetres or millimetres
(Fig. 3.6). Rock and mineral clasts within the pseudotachylyte veins are typically streaked along the vein and quartz clam are completely recrystallised (Fig -3.7). Where feldspar clasts are present they are usuall y found partial1y assirnilated and injected by the pseudotacliy 1yte matrix. Minor displacement of twin planes in feldspar occurs (Fig. 3.8).
Whete these fine pseudotachylyte veins are pervasive the host granite typically appears darker, is flintier and the crystal faces are less distinct. In such areas it can be Figure 3.4 - Granite showing evidence of strain; deformation twins in feldspar and undulose extinction in quartz. Crossed polars, field of view 2.5 mm. Sample 6-62.
Figure 3.5 - Dynamic recrystallisation of quartz dong the margin of a micro- pseudotachylyte vein, indicating that shearing has occmed dong the margin. Crossed polars, field ofview 2.5 mm. Sarnple 6-66.
57 Figure 3.7 - Pseudotachylyte veinlet showing Figure 3.6 - Displacement of quartz vein in granite, complete recry stallisation of quartz, and partial dong micro-pseudotachylyteveins. assimilation and injection of the pseudotachyiyte matrix in feldspar clasts. Crossed polars, field of view 2.5mm. Sarnple 6-94. Figure 3.8 - Pseudotachylyte veinl~iscutting granite and displacing twin planes in feldspar. Quartz crystals are completely recrystallised. Crossed polars, field of view 2.5m.m. Sarnple 6-66.
Figure 3.9 - Granite where the pseudotachylyte veinlets are mamscopically pervasive, showing extensive displacement of twin planes in feldspar (upper centre), to a hornfelsed feldspar texture (lower Ieft) and completely recrystallised quartz (lower right). Crossed polars, field of view 4.75 mm. Sample 6-1 75. dificult to distinguish recrystallised pseudotachylyte matrix fiom finely recrystallised quartz and feldspar. In outcrop these rocks cm appear similar to areas of more cataclastic defornuition. Truly cataclastic deformation is not comrnon at Sudbury (Thompson and Spray,
1996). The displacement of twin planes in feldspar crystals dong micro- fractures is more common and plagioclase is generally found more saussuritised and in places has been recrystallised to give a hornfelsed texture. Quartz shows higher strain as increased undulose extinction, but more commonly it is completely recrystallised into finer equigranular polygonal crystals (Fig. 3.9).
BROADER PSEUDOTACHYLYTE ZONES
Broader zones of pseudotachylyte (centimetres to metres wide) typically occur in areas that have been intensely cut by srnaller pseudotachylyte veinlets. These occur as both distinct veins with shq margins and as more irregular zones of anastomosing veins possessing sharp to diffise contacts with highly altered wall rocks.
The broad, distinct veins Vary in thickness dong their length, and they may die out or bifùrcate along their length. niey can usually be traced for several metres and up to tens of metres (Fig. 3.10). The distinct broader veins of pseudotachylyte have a crypto- to micro- crystalline matrix. Most typically the matrix consists ofa pst- impact retrograde mineralogy, but original igneous textures also occur. Clasts are predominantly quartz and are generally well rounded and completely recrystallised to an equigranular polygonal texture (Fig. 3.1 1).
Composite clasts are typically defomed and contorted, and are commonly sirnilar in appearance to the granite that has been pervasively cut by micro- pseudotachylyte veins (Fig. Figure 3.10 - Broad vein of pseudotachylyte cuning through granite running subparallel to the Hess Offset. The vein bifuricates in the right hand side of the picture undemeath the granite boulder. Scale card (centre of picture) 8 cm long.
Figure 3.1 1 - Matrix and quartz clasts (white), within a broad, sharp-margined pseudotachylyte vein. Dark areas are biotite. Plane polarised iight, field of view 2.5 mm. Sample 6-64.
61 3.1 2). Feldspar cry stals are usually found highly corroded by the pseudotac hy 1yte. They consist of finely recrystallised material. A flow foliation is sometimes observed within these broader pseudotachylyte veins. The foliation is typically defined by the presence of layers of different composition and the clasts are often streaked panille1 to the compositional layering
(Fig. 3.13). Many similar features have been noted in other pseudotachylytes in the Sudbury structure (Thompson and Spray 1994; Thompson and Spray 1996)
Irregular zones of anastomosing veins of pseudotachylyte can be interfingered with highly altered wall rock with sharp to difise contacts (Fig. 3.14). The weathered surface of the entrained wall rock is a darker orange than the typical granite in the area and also has a baked appearance. The pseudotachylyte veins are essentially the same as those seen in the broad pseudotachylyte veins with sharp margins. It is the highly altered wall rocks to these zones that are distinctive. Within these areas there is extensive recrystallisation of feldspars to a honifelsed texture, with quartz occurring as completely recrystallised contorted ribbons.
Small biotite andor chlorite crystals are otlen aligned dong the path of the recrystallised quartz or feldspar crystals, which may define fold like features (Fig. 3.15). These folds bear a resemblance to the folds seen in flow-foliated pseudotachylyte. The apparent macroscopically diffuse contact with the pseudotachylyte veins is revealed in plane polarised light to be a sharp contact.
The different 'types' pseudotachylyte have gradational contacts between each other.
One such example of this can be seen in the approximately 30 me& wide zone of pseudotachylyte on the eastem side of the small stream/swamp on the NE shore of
Pterodactyl Lake. A branch of Hess-like material occurs immediately West of this breccia Figure 3.12 - A deformed granite clast within a broad zone of pseudotachylyte. Quartz is entirely recrystalised and feldspar is homfelsed. Crossed polars. field of view 6 mm. Sample 6- 177,
Figure 3.13 - Pseudotachylyte showing flow foliation defined by two different coloured matru matetials, both containing quartz ciasts. Crossed polars, field of view 22 mm.Sample 6-76. Figure 3.14 - Irregular zone of pseudotachylyte with sharp to difise margins, interfingered with highly altered waIl rock.
Figure 3.15 - Photomicrograph of apparently diffise-margined pseudotachylyte (centre) with highly altered wall rock (right). Dark bands withiri the wall rock are ribbons of small biotite and chlorite crystals aligned along the margins of recrystallised quartz and feldspar clasts. Light ana on left side of picture is late-stage alteration of the pseudotachylyte due to epidote veinirig. Plane polarised light, field ofview 19 mm.Sample 6-328. 64 outcrop. The matrix of the pseudotachylyte grades fkom textures typical of the broad sharp-
margined pseudotachylyte veins, to granite that has been extensively cut by micro-
pseudotachylyte veins. A highly brecciated contact occurs between the granitoid and calcareous Huronian Supergroup metasedimentary rocks which occur immediately to the
south of this large breccia outcrop. The contact between the granite and the Huronian
metasedimentary rocks, represents a pre-existing weakness in the rocks, along which
pseudotachylyte has preferentially formed.
CATACLASTIC ZONES
Two large zones ofapparently cataclastic deformation have been observed associated
with the Hess Offset. The larger area is approximately 300 by 100 metres with good
exposure, other than in an adjacent marsh, and is immediately adjacent to the Hess Offset at
the Riven Option, in eastem :ess township (Fig 2.12 and fold out map in the back pocket).
The smaller area is on the southern end of Portage Hill between Clear and Depot Lakes and
is to the south of where the Hess Offset would be expected to outcrop, in SW Hess
Township. There is about 100 by 50 metres of discontinuous outcrop, which shows variable
development of cataclastic deformation and pseudotachylyte. Severai small areas of
cataclasticdly deformed granites have also been observed along the length of the Offset.
The cataclastic zones consist of angular hgments of quartz and feldspar ranging in
size from sub-millirnetre to centimetre scale. The microcrystalline matrix smunding the
hgments consists of white micas, feldspar and minor quartz. Overgrowths of biotite, titaaite
and chlorite occur (Fig. 3.16). Feldspar crystals of?en have kinked or displaced twin planes Figure 3.1 6 - Cataclastically deformed granite, with a complete range in clasts size from sub- millimetre to approximately 1 mm, within a mica-rich mabut. Crossed polars, field of view 2.5 mm. Sample 6-67.
Figure 3.17 - Cataclastically deformed granite but with more tnie pseudotachylyte-like ma&. Plane polarised light, field of view 2.5 mm. Sample 6-68.
66 and both quartz and feldspar clasts cm be pervaded by the matrix. The complete range in size distribution and the more angular nature of the clasts distinguishes these fiom the pseudotachylytes. Quartz fragments exhibit some undulose extinction and are partially recrystallised, but recrystallisation of quartz is not as extensive as in areas that have ken extensively pervaded by micro-pseudotachylyte veins. In some areas the matrix has the appearance of a more typicai pseudotachylyte rather than appearing as a cataclasite (Fig.
3.17).
At the Rivers Option, the entire area is extensively cut by quartz veins, which are generally between 5 and 10 cm wide. Some of the quartz veins show minor displacement along micro-fractures. The pseudotachylyte matrix has also been cross-cut by quartz veins.
The host rocks at the northem contact at the Riven Option consists essentially of recrystallised seriate quartz with only minor amounts of feldspar and mafic minerais.
Approximately 10 metres north of the contact, the host rocks consist of angular hgmental quartz with more significant amounts of feldspar in a fine-grained matrix of white mica, and minor amounts of chlorite and biotite. Macroscopically, the quartz veins show little sign of deformation, but in thin section minor displacements of the quartz veins have been observed, with extensive recrystallisation of quartz occming along the matgins. Approximately 100 metres north of the Hess contact, feldspar is more common and the ratio of hgrnents to matrix is less. The matnx appears more pseudotachylyte-like and exhibits flow features.
Quartz veins are small (millimetre scale) and relatively minor, but are observed both cross cutting and king cross cut by the pseudotachylyte-type ma&. Al1 these appear to have gradational contacts through to relatively unaltered granite over several metres to tens of metres. CONTACT QUARTZ DIORITE
Where observed the margins of the Hess Offset are always found chilled against the country rock. At chilled margins, the overall finer grain size is commonly accompanied by the development of conspicuous actinolite needles (up to 20 mm long), which are probably pseudomorphic after pyroxene (Gmt and Bite, 1984) (Fig. 3.18). Where there are no distinctive large acicular crystals, the contact quartz diorite is typicdly a fine- to very fine- grained biotite quartz diorite. This can be in direct contact with granite or with pseudotachylyte. Contacts with the country rocks appear to either be sharp, or gradational over a few centimetres. The contact with pseudotachylyte appears gradational due to the fine- grained nature of both the pseudotachylyte and the chilled quartz diorite and their similar minerdogies. Spherulitic-textured quartz diorite is the most cornmon contact phase of the
Hess Offset. This consists of radiating clusters of lathy plagioclase and actinolite. Spherulitic quartz diorite has been observed in other Offsets and is particularly well developed at the margins of quartz diorite pods in the South Range concentric Offset dykes (Grant and Bite,
1984). This is not a true spherulitic texture, in that no discrete spherical bodies are identifiable. Variolitic texture would be a more appropriate name as it consists of fan-like arrangements of divergent, ofien branching, fibres of actinolite (pseudomorphingpyroxene), and feldspar (Fig. 3.1 8). As the terni spherulitic quartz diorite has been used in the literature
(Grant and Bite, l984), its usage will be retained in this work. This spherulitic quartz diorite appears to be a quenched rock, which was presumably cooled rapidly against the country rock. Figure 3.18 - Acicular homblende pseudomorphing pyroxene in the Hess Offset quartz diorite, in contact with altered granite (left). Plane polarised light. field of view 29 mm. Sample 7-225.
Figure 3.19 - Spheditic texture in granite in contact with Hess Offset. Crossed polars, field of view 2.5 mm. Sample 7-203.
69 HOST ROCKS AT THE CONTACT
The apparently gradational contacts of Hess in thin section is revealed to be sharp
However, alteration of the gmnite in the contact zone gives the granite a similar appearance to the quartz diorite in hand sample. Alteration includes partial recrystallisation of the feldspars in the granite. Recrystallisation in both spherulitic (Fig. 3.19) and homfels textures occur. niese plagioclase cqstzils are ~picallyextensively sriussuritised. The recrystallised feldspar crystals in the granite are a similar size to the plagioclase crystals in the quartz diorite. The contact can be distinguished by the smdler proportion of mafic minerals in the altered granite. Biotite crystals tend to fom reaction rims mund remaining unmelted quartz crystals in the granite and quartz clasts that occur in the quartz diorite. Quartz crystals in the granite are anhedral and show relatively minor undulose extinction. These crystals range in size from submillimetre to 6 mm, and are recrystallised with highly irregular grain boundaries, possibly indicating grain boundary migration crystallisation. Similar textures are developed in granite clasts in the margins of the Hess Offset. Cbapter four
CHEMISTRY
Major, trace and meearth element data obtained as part of this study show that the
Hess Offset is genetically related to the SIC and that it is most closely afiliated with an evolved felsic norite component of the Main Mass and not bulk impact melt. This indicates that Hess was emplaced during hctionation of the impact melt sheet, rather than immediately following impact. All the Offset dykes in the Sudbury structure posses similar chemistries, though with some variation, especially near the wall rock contacts due to assimilation of the host rock (Grant and Bite, 1984). The most signiîïcant differences in chemistry in the Offset dykes occur between dykes of the North and South Ranges (Lightfoot et al., 1997b). There is some variation in the chemistry of the Hess Offset along its length and there are minor differences if Hess is compared with other North Range Offset dykes.
SAMPLE LOCATIONS
The locations of the analysed samples are show in Fig. 4.1. More detailed maps of the proximal and distal Foy intersections, the Rivers Option and of Dehydration Ridge are show in Chapter 2. Major element analyses of seventeen samples of the main phase of the
Hess Offset are show in Tables 4. la and 4.2a. Twelve of these samples were taken at intervals along the known exposed length of Hess; additional samples were also taken hm the Riven Option (H-4-8) (a more detailed location map for Riven Option is given in Fig.
2.1 1), and the proximal Foy intersection (H-l 1 & 12) (a more detailed location proximal Foy
Intersection is given in Fig. 2.7). Trace and nue earth element data for the main phase of the
Hess (sample H-1 to H-1 4) are presented in Tables 4. t b and 4.2 b. Major, trace and rare earth element data are also presented for the quartz diabase dykes (QDB-1 and QDB-2)associated with the main phase of the Hess Offset, and the Parallel Foy Offset (PF), (Table 4.3a and
4.3b).All rocks are compared with analyses of the Main Mass lithologies (Lightfoot et al.,
1997a) and other Offset dykes (Lightfoot et al., 1997a and b), in particular the North Range felsic norite (NRFN) ari the Average North Rmgc Offset dyke composition (NRO) (Table
4.4a and 4.4b). Some inclusions entmined within the Hess Offset were also andysed. These include (1) a basic xenolith (iNRO) from the Rivers Option, (2) the igneous-textured matrix of several footwall-like breccia inclusions (FBX)adjacent to the Parallel Foy Offset, two from the proximal Foy intersection and one from the distal Foy intersection, (FBXI to 4, respectively), and (3) a gneissic inclusion fiom the distal Foy intersection (GN) (Table 4.3a and 4.3b). These inclusions were studied in order to determine if tliey were earlier crystallising phases of the SIC entniined within die Hess Offset, passively transported inclusions of surrocinding footwall rocks or, if they are xenoliths. Nipissing diabase (N- 1 and
-2) was also analysed for cornparison with the Hess Offset (Table 4.4a and 4.4b). Additional analyses of Nipissing diabase (Lightfoot et al., 1993) and Cartier granitoid (Meldrum et al.,
1997) were also used to access the effects of contamination on Hess dyke chemistry (Table
44a and 4.4b). RELATIONSHIP OF OFFSET DWSTO THE SIC
The Offset dykes of the Sudbury structure have broadly similar compositions, with relatively minor variations king show between Offsets and within each individual Off&.
The sirnilarities of the Offsets are especially noticeable in the concentrations of the trace elements and REE. The differences between the Offsets in the North Range compared with those in the South Range are the most obvious. The North Range Offset dykes include Foy,
Ministic, Whistle-Parkin and Hess. These are intermediate in composition between North
Range quartz gabbro and the felsic norite of the Main Mass (Figs. 4.2,4.3 and 4.4). South
Range Offset dykes include the Worthington, Copper Cliff, Manchester and the South Range
Breccia Belt. The South Range quartz diorites tend to be lower in SiO,, A1203,K,O and
N+O, and higher in Fe20,, MgO, Ca0 and TiOz and MnO. These are intermediate in composition between the quartz gabbro and South Range norite (Grant and Bite, 1984). The differences in the compositions of the Offset dykes between the North and South Ranges are broadly similar to the differences shown by the Main Mass quartz gabbros (Grant and Bite,
1984). niere is less variation in composition of the North Range Offset dykes than in the
South Range Offszts (Grant and Bite, 1984). Différences in compositions could be due to variations in grade of metamorphkm (and metasomatism) andor due to the more complex geology of the South Range, compared to that of the North Range. Apparent diflerences could also be due to the South Range Offsets having ken studied in more detail.
Incompatible element concentrations of the quartz diorite and the felsic norite or South
Range norite are very similarand there are no quartz diorites that have incompatible element concentrations as hi& as those seen in the quartz gabbro or granophyre of the Main Mass. granophyre quartz gabbro
0 felsic norite mafic norite
Calc-Alkaline
Na,O + K,O
O Parallel Foy Offset
2 8 quartz diabase m~~eii, \ Hess quartz diorite rn8 footwalt-like breccia inclusions A Rivers Option inclusion A Nipissing diabase
Figure 4.2 - AFM diagrams, plotting the amount of MgO, FeO* (as total iron) and N40 and &O (tholeiitic/calc-alkaline dividing line after lrvine and Baraga., 1979). (a) is for the Main Mass of the SIC. (b) is for the Hess Offset and related rocks. O A o Parallel Foy Offset
i quartz diabase Hess quartz diorite 1 0 footwall-like breccia inclusions I 1 A Rivers Option inclusion * O -- A Nipissing diabase - + gneissic inclusion u Oe: -
O+ -
9
l I 1 IlIII I
Figure 4.3 Molar ratio plot of Al2O4IC20verses SiO&O. (a) is for the Main Mass of the SIC. (b) is for the Hess Offset and related rocks. ParaIlel Foy OIlbet
quartz dlabme
Hosi quartz diortta
foohMill-like brsccfa indurions RLmn Option !ndusion
Niplssing diabatw gnebric includon
Figure 4.4 Molar ratio plot of FeOZ/KZ0verses MgO/K,O,where FeO* is total iron. (a) is for the Main Mass of the SIC. (b) is for the Hess Offset and related rocks. Plots for incompatible elements for various units of the SIC Main Mass and Offset dykes reveal a linear my,as exemplified by La/Sm (Fig. 4.5).This trend may be the result of fiactional crystallization of a liquid, whereby the ratios of incompatible trace elements in the residual liquid and crystals, plus trapped liquid, remains constant (Lightfoot et al., 1997).
In such plots. the Offset dykes partly overlap with both the felsic norite and, to a lesser extent, the mafic norite fields of the Main Mass (Fig. 4.5). Most North Range Offset dykes are slightly enriched in LREE and Sr, and depleted in TiO,, compared with South Range
Offsets (Lightfoot et al., 1997b). The Manchester Offset in the South Range is grnerally an exception to this. Manchester haa chemistry similar to the North Range Offsets. The differences in major elernents in the North Range Offset compared with the South Range
Offsets reflects the differences in the quartz gabbro of the North and South Ranges (Grant and Bite, 1984). There are distinct isotopic differences between the Main Mass rocks of the
North and South Ranges (Kuo and Crocket, 1979; Dickin et al., 1996), but these do not directly rpflect the variation in isotopes ratios observed in the Offset dykes of the North and
South Ranges. The enrichment in large ion lithophile elements and light REE in the North
Range Offsets reflects the enrichment of these elements in the Archean footwall lithologies that constitute the dominant target rocks for the North Range of the Sudbury structure.
The regional differences in quartz diorite chemistry in the Offsets in the North versus the South Range could dso be due to assimilation of different country rocks duruig emplacement of the dykes (Lightfoot et al., 1997), or to heterogeneities inherited from the main body of the melt sheet (i.e., it did not fonn a homogeneous melt pool on impact, but retained compositional variations that nflect original target rock distribution). Lightfoot et 0 Granophyre quartz diorite
quartz gabbro footwall-like breccia quartz diabase O offsets Parallel Foy felsic norite Nipissing diabase mafic norite Riven Option inclusion sublayer gneissic inclusion
Figure 4.5 - Samarium versus Lanthanurn plot for the Hess Offset and related rocks, for sample nurnbers and locations see Fig. 4.1. Sbaded areas are for the Main Mass of the SIC. other Offset data fiom Lighifoot et al. (1997a). Nurnbered symbols refer to the sample numbers show in Figure 4.1. al. (1997a) demonstrate that the ratios of many of the studied isotopes reflect the isotopic compositions of the dominant host rocks; Archean granitoids and gneisses in the north and
Early Proterozoic sedimentary rocks and mafic volcanics the South. Subtle compositional variations within individual Offset dykes have been ascribed to local wall rock, ador inclusion assimilation. This is particularly common in the distal teminations of Offset dykes
(Grant and Bite, 1984). In the Foy Offset dyke, the central portion of the dyke has alB0 values similar to those of North Range norite, whereas the dyke margins show dl" values close to those of Levack gneiss, which constitutes the wall rock (Ostemann et al., 1995).
This implies that localised, marginal-dyke contamination has occurred, rather than bulk dyke contamination.
Figure 4.6 shows a REE plot for various quartz diorites, including average Hess, two examples fiom the South Range Breccia Belt (McCo~elland Vermilion), and an embayment (Creighton). Average North Range felsic norite (NRFN, Table 4.4b) is used to normalise al1 REE data for al1 REE plots because the NRFN is more closely affiliated with
North Range Offset dykes (Osterann et al., 1996). The quartz diorites show relatively flat patterns, with the exception of Creighton embayment and the Parallel Foy, which show enrichment in dl REF. Copper Cliff, Worthington, McConnell and Vermilion also show some HREE enrichment (though it should be noted that the plot in Fig. 4.6 is not logarithmic). The HREE enrichment shown by the South Range quartz diorites may be due to differences in North and South Range SIC compositions, implying that the North Range felsic norite and the North Range Offsets are relatively entiched in LREE, which may indicate a pnmary melt sheet inhomogeneity. The Offset dykes tend to show minor Figure 4.6 - Average REE concentrations for selected Offsets and embayments of the Sudbury Shcture, nomalised to the average REE concentration of the North Range felsic norite (NRFN; see Table 4.4b). Values for al1 Offset other than Hess hmLightfwt et al, (1 997a). enrichment in REE relative to felsic write, as well as negative Eu anomalies. This irnplies that the Offset dykes were emplaced after some fractionation of plagioclase had occurred within the Main Mass.
The Hess quartz diorites possess similar REE profiles to the Foy, Manchester,
Ministic and Whistle-Parkin Offset dykes (Fig. 4.6: Fig. 4.1 for locations). niese similarities indicate that the Offsets are not displaced very far from the composition of a cornmon source melt. This source melt most closely resembles the felsic norite of the Main Mass.
HESS OFFSET IN RELATION TO THE NORTH RANCE MAIN MASS
The composition of the quartz diorite of the Hess Offset is intermediate between that of the North Range felsic norite and the quartz gabbro (Lightfoot et al., 1997a). Compared with NRFN, Hess is relatively eMched in Cao, TiO,, Fe,O,, Mg0 and depleted in A120,
(Tables 4. la, 4.2a and 4.4a). The intermediate position of the Hess Offset between the North
Range felsic norite and the quartz gabbro of the Main Mass of the SIC can be seen in Figures
4.2 - 4.4. Figw 4.2a shows the variations in F = FeO, M = Mg0 and A = Na20 + &O on an AFM plot for the units of the Main Mass in the North Range. Figure 4.2b shows the same plot for Hess quartz diorite samples and related rocks. The Main Mass rocks of the North
Range lie either side of the line dividing tholeiitic and calc-alkaline fields of the AFM diagram. The Main Mass norites show alkali enrichment, which is typical of calc-alkaline crystallisation trends, whereas the quartz diorite shows iron e~chmentrelative to the felsic norite: a pattern more typical of tholeiitic-suite crystallisation trends. Tholeiitic crystallisation trends prevail for the quartz gabbro and granophyre. Figures 4.3a and b show molar ratio plots of Al,O,/K,O venus Si0,/K20 for the
North Range Main Mass rocks and the Hess quartz diode and related rocks, respectively.
Figure 4.3a shows a strong linear trend for the Main Mass rocks, which is largely the result of plagioclase fractionation (Cochrane 1984); but is aiso influenced by pyroxene crystallisation (Grant and Bite, 1984). More differentiated units of the Main Mass occupy positions closer to the origin. Other than the mafic norite and the sublayer, which show a wide spread in values, the extensive overlap of fields probably results, in part, from al!eration. The Hess Offset quartz diorites plot in between the felsic nontes and the gabbro samples. The Hess quartz diorite sample that is displaced to the highest values (plotting in the upper right of Figure 4.3b) is from Pterodactyl Lake, and is highly altered.
Figure 4.4 a and b are rnolar ratio plots of MgO/K,O versus FeOt/K,O. Two trends are apparent: a lower trend representing quartz gabbro and granophyre, and an upper, steeper trend representing the felsic and mafic norites. The steeper trend of the norite may largely result fiom pyroxene crystallisation. The quartz diorite sarnples of the Hess Offset appear to follow the trend of the norites. The shallower, higher uon trend clearly reflects the presence of significant arnounts of iron oxides in the quartz gabbro (Grant and Bite, 1984). The North
Range quartz diorite forms a trend with a slope somewhat intermediate between the norite and gabbro.
The composition of the Hess Offset is intemediate between that of the North Range felsic norite and the average value of the other North Range Offsets (Ministic, Foy and
Whistle-Parkin; (compositions from Lightfoot et al., 1997% see NRO Table 4.4a). Typically
Hess quartz diorite is enriched in FqO,, Mg0 and Ca0 and depleted in K20compared *th Figure 4.7 - North Range Felsic Norite normalised REE patterns of the Main Quartz Diorite phase of the Hess Offset. Sample number refer to location map Fig. 4.1. NRO.
VARIATIONS WITHIN THE HESS OFFSET
Major element data indicate a relatively uniform composition for Hess. Some compositional variation seen withir. the Hess Offset is due to assimilation of host rocks. This is most obvious in dyke samples taken less than a metre fiom the contact with the host rocks
(H2 and H13). Minor differences occur between the eastem and western ends of the dyke and, in particular, the Rivers Option and from the pond NE of Pterodactyl Lake (H-4).
The SiO, contents of the Hess samples range fiom 54.4 to 61.3 wt%, so they are al1
Litennediate in composition. nie normative mineralogy as plotted on a quartz - alkali feldspar - plagioclase (QAP) diagram, shows that the Hess samples are mainly granodiontes, with some classimng as quartz monzodiontes and tonalites (Fig. 4.8). Gmtand Bite (1 984) show that most cf their analysed Offset dykes for Sudbury as a whole are, in fact, quartz monzonites, with North Range samples king transitional into the granodiorite field. Despite the proper nomenclature for these rocks, al1 Offsets dykes tend to be referred to as quartz dionte (also commoniy abbreviated to QD) in Sudbury literanire. This usage is retained in this work for the main dyke phase of Hess.
The Cartier granitoids constitute the predominant host to the Hess Onset. If the quartz diorite assimilated some of the granitoid then enrichment in Si4,Na20 and K,O, and particularly the LREE, would be expected (Table 4.4a ana b, Fig.4.9). The slight enrichment in LREE relative to the North Range felsic norite in al1 of the Hess quartz diorite samples could be due to minor Cartier assimilation (Fig 4.7). Two samples were taken very near the contact with Cartier granitoid, one from the westem and one fiom the eastem ends of Hess
(HZ and H13, respectively). Both these samples show minor K,O enrichment and Ca0 depletion, but the SiO, and Na0values are similar to those in other quartz diorites (Table
4.la and 4.2a). Hl3 is enriched in al1 REE, particularly in LEE, whereas H2 shows negligible enrichment in LREE compared with other quartz diorite samples fiom the westem end of Hess (Fig. 4.7).
In cornparison with the Hess quartz diorites, Nipissing diabase possesses lower SiO,,
Na,O and K,O and higher Ca0 and Mg0 (samples N-1 and N-M in Table 2), thus critically verifying the distinction between samples H-1 and N-1 made in the field, where the Hess
Offset dyke cuts through Nipissing diabase at Clear Lake (Fig. 4.1). These samples of
Nipissing diabase are monzonite (N- 1) and quartz monzonite (N-2), which is consistent with the regiond variations shown by Nipissing diabase (Lightfoot et al., 1993). Where Hess cuts though the Nipissing diabase in Clear Lake there is no obvious effect of local contamination other than possible minor depletion in Na20,as Nipissing diabase has significantly less Na20 than the Cartier granitoids.
There are distinct differences between the eastem and western ends of the Hess
Oflit. The eastem end of Hess is typicaily enriched in SiO, and K,O and depleted in FqO, and Mg0compared with the western end. The lowest values of Siozare found in the quartz diorite samples at the Rivers Option. This decrease in SiOz at the Rivea Option is also accompanied by a decrease in Ti4and an enrichment in Fe203and Mg0 relative to the rest of the Hess Offset. The Rivers Option is mineralised as reveded by its elevated Ni and Cu content (Table 4.1 b). Where there is increased sulfide content there is also, in general, a + East average -c West average + fbx average
8 RO average +- gneissic average -+ average
r Cartavenge
Figure 4.9 - North Range felsic nonte nonnalised REE pattern of the gneissic inclusion fiom the Distal Foy intersection, the average of the igneous textured matrix of the four footwall- like breccia inclusions from the proximal and distal Foy intersections (Fbx average): average of four quartz diorite samples fiom the western end of the Hess Offset (west average); average of four quartz diorite samples from the eastem end of the Hess Offset (east average): average of four quartz diorite samples from the Rivers Option (RO average); The average values for the composition of the Cartier granitoids fiom Meldrum et ai. (1997): the Average value for theNipissing diabase from LightFoot et al. (1993; and this work). decrease in Al,O, and a enrichment in Fe,O, and MgO. The difference in major elements fiom east to West is also reflected in the trace elements and REE. Figure 4.7 shows NRFN- nomalised REE plots of the Hess quartz diorite. The eastem end of the Hess Offset is relatively edched in REE, especially the LREE, compared with the western end and, in particular, compared to the Rivers Option.
Al1 samples analysed east of the oxbow in the Onaping River (H6-H14) are more
'typical' of the eastem samples and bear a closer resemblance to the composition of the average North Range Offset (NRO) (Fig 4.10). Al1 samples analysed West of Dehydration
Ridge, (H 1-H5 and H 15-H 17) are more 'typical' of the western samples and bear a closer resemblance to the average value of the North Range felsic norite (NWN) (Fig. 4.1 0, flat line through 1). It is not known if this change in chemistry is abrupt or gradational as no sarnple between Hl 7 and H6 were obtained because no exposure of Hess could be found in the intervening area.
The Pterodactyl Lake sample (H4), identified in the field as Hess-related, is anomalous in tems of major eiements. It is relatively enriched in Mg0 and N%O and depleted in K,O and Ca0 compared with other quartz diorites in the area (H3 and H 1 5-Hl 7).
The major element chemistry suggests that H4 is not Hess related, whereas the REE data indicates it is SIC-related and has a pattern typical of Hess quartz diorite, with only minor enrichment of REE, in particular LREE. This intermediate igneous rock is similar in appemce to the Hess Offset, though greener and more extensively cut by fine epidote veins. There is a broad area of pseudotachylyte adjacent to the outcrop. Major element chemistry and petrographic appraisal indicate (see chapter five) that this outcmp of quartz -+ east average +k RO average + west average
t NRO average
Figure 4.10 - North Range felsic norite nomlised REE patterns for the quartz diabase dykes (QDB-1and 2); Parallel Foy Offset (PF); average of four samples of quartz diorite fiom the eastem end of the Hess Offset (east average): Average samples of quartz diorite from the western end of the Hess Offset (west average): Average samples of quartz qiorite fiom the Rivers Option (RO average): Average of 63 samples fiom the Ministic, Foy and Whistle- Parkin Offsets fiom Lightfoot et al. (1997b). diorite has been extensively altered. As other exposures of quartz diorite in the area are not similarly altered, it is unlikely that regional metamorphic processes are responsible for this alteration. Other processes that could have altered the rock are localised deuteric or hydrothermal alteration. As the dyke cooled, water fiom within the crystallising magma or fiom the surrounding host rocks, could lead to deuteric and hydrothermal alteration. The pseudotachylyte could be a potential source for metamorphic fluids, as dehydmtion dong the margins of the fault system must have accompanied pseudotachylyte formation. As the temperature pulse (from fictional heating) migrated outward, dehydration probably also produced large thermal and "H,O gas stresses to fracture the local rock, thus allowing the wall rock to develop "stem" leaks. Since the magma was emplaced soon afterwards, and the dehydration may have been conduction controlled (Le., slower), gas pockets were probably trapped intermittently. Once the dyke melt cooled suficiently to form a network of fine contraction cracks due to the AV of solidification and simple cooling these gasses could pervasively invade and react with the dyke (personal communication B. Marsh, 1997). The epidote veins could be evidence of such a process.
QUARTZ DIABASE DYKES
The quartz diabase phase of the Hess dyke is enriched in TiO,, Cao, Na@, P,O, and depleted in K,O and MgO, relative to quartz diorite (Table 4.3a). On the QAP diagram the quartz diabase dykes plot within the field of South Range quartz dionte, but they have higher modal proportions of alkali feldspar than the quartz dioste (Fig .4.8). Quartz diabase is also
Cu- but not Ni-e~chedcompared to quartz diorite (Table 4.3b). Quariz diabase shows hn enrichment trends compared with the Hess Offset and other SIC rocks (Fig. 4.2 and 4.4). In
Figures 4.3b, quartz diabase plots on the trend of the Main Mass norite and quartz gabbro.
Figure 4.2a shows the relative iron enrichment of the quartz diabase such that it appears to be a continuation of the tholeiitic crystallisation trend seen in the main phase quartz diorite. in Figure 4.4 b the molar ratio plots of MgO/K20 venus FeOS/K,O show that the quartz diabase samples plot on the same trend as the quartz gabbros and granophyre rather than of the quartz diorite or norites. The quartz diabase phase of Hess exhibits an SICIOffset dyke- related REE profile, but with overall enrichment in incompatible elements (QDBI and
QDB2, Table 4.3 b; Fig.4.1 O). This would suggest that it is a more evolved and incompatible- element-emiched melt phase of the SIC.
The quartz diabase associated with the Hess Offset is significantly enriched in SiO?, and NaO,and depleted in Fe203,Mg0 and Cao, compared with the quartz diabase dykes in the Victoria area of the South Range (Grant and Bite, 1984). The quartz diabase in the
South Range is intermediate in composition between the South Range quartz diorite and the dionte in the distal section of the Copper Cliff Offset. The quartz diabase dykes in the South
Range are east-west striking, near vertical dykes. These may be related to the gabbroic sills that occur at the contact between the Onwatin and Chelmsford Formations (Cooke, 1946).
Hem, a metagabbro sill, as much as 30 m thick, is exposed at three localities over a length of 4 km. This si11 occurs at the contact between the Onwatin Formation and the Chelmsford
Formation in northem Balfour Township. The rocks are altered, but primary pyroxene is
locally preserved. There are extensive quartz-carbonate veins and lenses within the
metagabbro sills. These veins are commonly intimately mixed with the metagabbro and significant pyrite, arsenopyrite and chalcopyrite mineralisation is associated with them
(Rousell, 1984). The quartz diabase from the Riven Option area of the Hess Offset has similar values of Cu, Ni, Cr, Ba and Pb to the metagabbro sills at the contact between the
Onwatin and Chelmsford Formations (see Table 4.3b and Rousell, 1984, Table 9.4). The quartz-carbonate material that cuts these metagabbro sills generally has high Cu and Zn values, and a modest Pb content.
The intrusion of the quartz diabase dykes into the quartz diorite may have contributed to the alteration of the Hess Offset. This could occur through the migration of late stage fluids from the quartz diabase dykes metasomatically altering the quartz diorite.
PGRALLEL FOY
The Parallel Foy Offset dyke (Figs. 1.1,1.4b,4.1 b) has elevated SiO2,TiO, and P20, and is depleted in Mg0 and AlzO,, Ca0 compared to the main phase quartz diorite. On the
QAP diagram the Parallel Foy Offset plots as a granodiorite that is richer in quartz than the other Offset dykes of the Sudbury structure, as field observations also indicated (Chapter 2).
MgO/K,O versus FeO*/X,O plot on the same trend as the quit tz gabbro or granophyre (Fig.
4.4). The Parallel Foy Offset is relatively enriched in REE compared with the NRFN,and to the Hess Offset. The REE chemistry of the parallel Foy is very similar to that of the quartz diabase (i.e., is more evolved), though it is not porphyritic and more closely resembles regular quartz diorite in the field. The Parallel Foy Offset dyke shows elevated SiO?, TiO,,
K20and P,O, and is depleted in Fe,O,, MgO, Ca0 compared to quartz diabase dykes. INCLUSIONS IN THE HESS OFFSET
Footwali-like breccia inclusions
Numerous inclusions of footwall-like breccia occur both east and West of both the proximal and distal Foy intersections, these range in size fiom several centimetres possibly to tens of metres in diameter. The major element chemistry of the matrix of the footwall-like breccia inclusions (FBX)in the Hess Offset is very similar to that of the quartz diorite, with only minor enrichments in SiO-,, Na20, and depletion of Fe,O,, Mg0 and Ca0 compared with the quartz diodes (Table 4.2a and 4.3a). These inclusions plot in the tonalite field of the QAP diagram, within in the area of the South Range quartz diorite. This highlights their similarity to the quartz diorite of the Hess Offset. Figure 4.9 shows the NRFN-normalised patterns for inclusions within the Hess Offset. The igneous-textured matrix of the footwdl- like breccia inclusions have surprisingly consistent major and REE compositions. The trend of the NRFN-nomalised REE patterns indicates that they are SIC related. In fact, the hctionation patterns are very similar to those of the quartz diorite in the area. These inclusions are not only similar in appearance to the footwall breccia occuning as sheets and discontinuous bodies parallel to the lower contact of the SIC (Dressler et al., 1991), but also to the leucocratic breccia infillingsome of the proximal sections of the Foy OfEset (Pattison,
1979). There is an altemation of this footwdl-like breccia phase (and quartz diorite filling the Foy Offset), for about 2 km near the Foy em bayment (Fig . 1.1 ). Further north dong the
Offset ftom the embayment, the quartz diorite contain inclusions of this fmtwall-like breccia phase. Pattison (1979) believed the altemation of these two phases indicated simdtaneous emplacement. The entrainment of footwd-like breccia inclusions in both the Foy and Hess Offset implies emplacement of the footwall-like breccia phase prior to the emplacement of the quartz diorite. Furthemore, the concentration of the footwall-like breccia inclusions in the Hess Onset adjacent to the Foy intersections implies the Hess Offset was being fed by the Foy Offset.
The quartz diorite in the area of the Foy-Hess intersections, where there are a large number of these footwall-like breccia inclusions, is enriched in SiO,, and depleted in Ti02,
Cao, Na,O, Fe203and Mg0 and shows marginally greater enrichment of LREE over HREE compared with other quartz diorite samples fom the eastem end of the Hess Offset. These features could be explained by partial assimilation of the footwall-like breccia inclusions by the Hess Offset, thus explaining the general trend of these features in the eastern part of the
Hess Offset.
Gneissic Inclusion
Many centimetre to metre scale inclusions possessing gneissic foliations occur in
Hess near the Distal and proximal Foy intersections. A large (minimum width 8 m) fine- grained inclusion located immediately West of the Distal Foy intersection, is unfoliated, but its mineralogy is sirnilar to other gneissic inclusions. This inclusion has elevated AlzO,, P209
Na,O compared with most Hess quartz diorite samples. The REE pattern deviates significantly fiom the SIC-related and Nipissing rocks, which tend to indicate that these are not related to either of these. These gneissic inclusions may be blocks of Levack gneiss, which is exposed Mersouth and north and probably locally underlies the Cartier granitoids
(Miikereit, 1994). These gneissic inclusions were probably passively transported with the quartz diorite or within the footwall-like breccia inclusions either hmdepth or via the Foy Offset fiom the south.
Riven Option Basic Inclusion
A one metre diameter inclusion fiom the Riven Option is basic and geochemically closer in composition to the Nipissing diabase than quartz diorite. Compared with the quartz diorite it has elevated levels of MgO, Al?O,, Ca0 and is relatively depleted in Si4, TiO,,
N-O, &O. Its CIPW nom shows that it is orthopyroxene normative (Le., noritic) (Table
4.3a). The inclusion is relatively depleted in REE compared with the North Range felsic norite and particularly depleted in LEE. The flat REE pattern could indicate that this inclusion is an early-fomed hctionate ofthe SIC, though the hctionation pattern is similar to that of the average Nipissing diabase (Fig. 4.9). The Nipissing suite is inhomogeneous, and shows a broad range due to in situ differentiation of the Nipissing magma, coupled with assimilation of the Huronian roof sedimentary rocks (Lightfoot et al., 1993).There are no exposures of Nipissing diabase in the immediate vicinity of the River Option, though there are exposures less than a kilometre to the north. It is unclear as to whether these inclusions are Nipissing diabase from the host rocks, or if this was a phase of the Main Mass of the SIC that had started to solidi@prior to the injection of Hess, that was tninsported itself by the quartz diorite.
The anomaious chemistry of the Rivers Option quartz diorite, (depletion in Si4and
Ti4, elevated Fe203and MgO), and the depletion in REE relative to the NRFN may, in part, be due to partial assimilation of such basic inclusions. It seems possible that some of the variation in chemistry of quartz diorite observed dong the length of Hess is due to variation in the inclusion population and assimilation~contaminationof these inclusions. - - - wt% Si4 57.06 56.83 57.62 58.00 55.21 54.57 58.20 54.38 58.51 Ti4 0.86 0.78 0.82 0.80 0.70 0.74 0.76 0.69 0.78 A1203 15.77 16.24 15.74 15.53 14.05 13.80 15.76 14.94 lS.W Fq03* 9.55 10.04 9.11 9.20 12.34 13.91 8.63 12.94 8.29 Mn0 0.14 0.13 0.15 0.12 0.13 0.12 0.12 0.13 0.12 Mg0 5.61 5.31 4.91 5.98 6.79 6.27 4.99 5.79 4.34 Ca0 6.81 4.35 7.02 5.43 6.17 5.70 6.60 6.29 5.80 Na@ 2.02 2.45 2.96 4.47 2.61 2.73 2.95 2.87 2.84 K20 1.93 3.70 1.49 0.28 1.86 2.01 1.83 1.75 1.90 P201 0.27 0.17 0.19 0.17 0.14 0.15 0.16 0.22 0.20 LOI 1.74 4.04 1.30 2.54 2.09 2.86 1.20 2.13 1.38 Total 10 1.74 104.04 101.30 102.54 102.09 102.86 101 -20102.13 99.21
Norm Q 16.08 25.25 14.67 12.95 11.80 11.57 14.52 11.00 17.81 C 0.00 4.46 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Or 11.45 2.26 8.78 1.69 11.02 11.88 10.81 10.34 11.48 Ab 17-16 21.44 25.04 38.12 22.08 23.08 24.93 24.32 24.56 An 28.46 2 1.18 25.29 2 1.63 2 1.13 19.49 24.37 22.69 23.19 Ac 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ks 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Di 0.99 0.00 4.69 1.61 5.16 4.44 4.01 4.03 2.02 wo 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Hy 13.59 13.68 10.06 12.43 14.52 13.55 10.56 12.55 10.1 1 Mt 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Hm 9.61 10.39 9.11 9.27 12.34 13.91 8.63 12.94 8.47 Il 0.30 0.29 0.31 0.26 0.28 0.26 0.26 0.28 0.26 Tn 1.72 0.00 1.61 1.64 1.34 1.47 1.53 1.34 1.62 Ru 0.00 0.66 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ap 0.65 0.42 0.44 0.41 0.33 0.36 0.38 0.52 0.48
Table 4. la - Major element data for the main phase of the Hess Offset dyke. For sample locations see Fig. 4.1. Major element chemistry detemined by x-ray fluorescence spectrometry. Fe,O,* = total Fe in femc state; LOI = loss on ignition. Nom = CIPW normative minedogy. Abbreviations for mineral narnes; Q = quartz; C = corundum; Or = orthoclase; Ab = albite; An = anorthite; Ac = acmite; Ks = potasium metasilicate; Di = diopside; Wo = wollastonite; Hy = hypersthene; Mt = magnetite; Hm = hematite; Il = Ilmenite; Tn = titanite; Ru = rutile; Ap = apatite. For anaiytical procedute see appendix 1. Trace (pprn) S Cl Sc v Cr Ni Cu Zn Ga As Rb Sr Y Zr Nb Ba Hf Ta Pb Th
Table 4. l b - Trace and rare earth elernent data for the main quartz diorite phase of the Hess Offset dyke. Analysis made by inductively coupled plasma-mass specaometry. For analytical procedute see appendix 1. Detcction limits for the REE are -0.1 chondrite values (0.005-O.S ppm). ND = not detected. For sample location see Figure 4.1. wt% SiOl 59.04 61.34 59.9 60.55 61.25 56.81 56.82 57.06 TiOz 0.82 0.77 0.79 0.77 0.76 0.8 1 0.85 0.84 A203 15.42 15.32 15.3 15.18 14.96 15.67 15.71 15.63 Fqo3* 7.94 7-77 8.42 8.77 8.08 8.94 8.77 8.9 Mn0 0.12 0.1 t 0.12 0.12 0.13 0.12 0.12 0.13 Mg0 5.02 3.94 4.35 3.93 4.13 5.82 5.53 5.52 Ca0 6.04 5.54 6.1 4.79 5.57 6.92 6.53 6.97 Na20 3.29 3.01 2.83 2.71 2.87 3 3.12 3.05 K20 2.04 2.02 2.01 2.97 2.08 1.67 1.99 1.65 P205 0.26 0.18 0.18 0.21 0.18 0.26 0.26 0.25 LOI 1.37 1.23 1.12 2.95 2.21 1.57 0.94 1.82 Total 101.37 101.23 101.12 102.95 102.21 101.57 100.94 101.82
Nom Min Q C Or Ab An Ac Ks Di wo HY Mt Hm Il Tn Ru AD
Table 4.2a - Major element data for the main phase of the Hess Offset dyke. For sample locations see Fig. 4.1. Major element chemistry determined by x-ray fluorescence spectrometry. For details of abbreviations see Table 4. la. Table 4.2b - Trace and tare earth element data for the main quartz diorite phase of thc Hess Offset dyke. Analysis made by inductively couplcd plasma-mass spectromeny. For further details set table 4.1 b. QDBI QDBZ PFOY INRO FBXI FBX2 FBX3 FBX4 GN wt% Si02 57.97 58.69 61.92 49.39 61.26 63.78 62.17 62.41 56.47 Ti02 1.57 1.33 1.74 0.57 0.74 0.69 0.73 0.75 0.75 Al203 13.87 13.85 13.26 17.88 15.28 15.46 15.51 15.45 17.1 1 Fe203* 11.74 11.58 9.6 9.56 8-28 6.89 7.56 7.38 8.89 Mn0 0.17 0.16 0.14 0.16 0.11 0.07 0.11 0.1 0.11 Mg0 2.98 3.08 2.3 9.49 3.94 3.48 3.74 3.81 4.57 Ca0 7.1 6.46 4.21 10.05 5.05 3.86 4.59 4.48 5.77 Na20 3.22 3.49 3.38 1.87 3.09 3.1 1 3.29 3.25 3.26 K20 1 1.1 2.8 0.99 2.05 2.5 2.15 2.16 2.56 P205 0.37 0.25 0.66 0.04 0.19 0.16 0.15 0.19 0.5 LOI 0.84 0.94 0.58 0.75 1.16 1.22 0.34 1.26 1.24 Total 100.84 100.94 100.58 100.71 101.16 101.22 100.34 101,26 101.24
Nom Min Q C Or Ab
Table 4.3a - Major element data for quariz diabase and other lithologies related to the Hess Offset dyke. For sample locations see Fig. 4.1. QDB = quartz diabase. PFOY = Parallel Foy Offset. INRO = basic inclusion in Hess, fiom the Rivea Option. FBX = footwall like breccia. For fiuther details see table 4.1 a. QDBl QDB2 PFOY iNRû FBXI FBXS FBX3 FBX4 GN Trace @pm)
Table 43b - Trace and rare earîh eletnent data for quartz diabase and other Iithologies telateâ to the Hess Offset dyke. For sample locations sec Fig. 4.1. QDB = quartz diabase. PFOY = Parallel Foy Off!MRO = basic inclusion in Hess, fiom the RivaOption. FBX = footwall like breccia. For Merdetails see table 4, l b, NIPi NIP2 NRO NRFN CART wt% Si02 51.83 52.27 58.98 57.1 70.26 Ti02 0.38 0.83 0.73 0.6 0.37 A1203 13.5 15.53 14.79 16.5 13.96 Fe203* 9.06 1 1.26 7.90 7.4 2.39 Mn0 0.17 0.16 0.12 0.1 0.03 Mg0 11.77 6.79 3.82 4.5 0.83 Ca0 11.78 10.53 4.90 6.2 1.39 Na20 1.19 1.9 3.O2 3 3.66 K20 0.28 0.6 1 2.36 1.8 4.58 P205 0.03 0.12 0.18 0.2 O. 16 LOI 0.44 1.22 1.93 1,8 0.9 1 Total 100.44 101.22 98.74 99.2 98.53
Table 4.4a - Major element data for the Nipissinp diabase NIP 1 and 2, for sarnple locations see Fig. 4.1. NRO = average (63 samples) quartz diorite value of the North Range Offset dykes from Lightfoot et al. (1997b). NRFN = average (5 1 samples) North range felsic norite fiom Lightfoot er al. (1997a). CART = average (7 samples) Cartier granitoid from Meldrum et al. (1997). For further details see table 4.la. NIPI NIP2 NIPAVG NRO NRFN CART Trace (ppm)
Table 4.4b - Trace and rare earth elernent data for the Nipissing diabase NIP I and 2, for sarnple locations se Fig. 4.1. NRO = average (63 samples) quartz diorite value of the North Range Offset dykes hmLightfoot et al. (1997b). NRFN = average (5 1 samples) North range felsic norite hmLightfmt er al. (1997a). CART = average (7 samples) Cartier granitoid hmMeldmm et al. (1997). For tiirther detaib see table 4.1 b. Chapter 5
MINERALOGY OF THE HESS AND RELATED OFFSET DYKES
The Hess OfEset is composed of more than one rock phase: predominately quartz diorite with minor amounts of quartz diabase. The latter pst- dates the former, though they are considered CO-magmatic.More than one phese of quartz diorite may occur as an intemal contact of quartz diorite against quartz diorite has been observed (Fig. 2.7). The dominant mineralogy of Hess quartz diorite is plagioclase + amphibole + biotite A pyroxenes, with minor quartz and granophyric intergrowths of quartz and alkali feldspar. Accessory phases include ilmenite, apatite, zircon and baddeleyite, have been optically identi fied and the presence of baddeleyite has been confirmed on the electron microprobe. Late stage igneous and secondary (metamorphic) phases are actinolite, chlorite, biotite, epidote, titanite and piemontite. Disseminated sulfides occur in varying proportions throughout the dyke. The main mineralogy is a variety of quartz diorite typical of many of the Offsets of the Sudbury structure (Pattison, 1979; Grant and Bite, 1984). Where the Hess Offset intersects the Foy
Offset there is no discemable difierence in the mineralogies of the two dykes, whereas the
Parallel Foy Offset is distinctive, due mainly to the greater arnount of granophyte present.
Minor disseminated sulfides occur throughout the dyke, but are more concentrated in some areas (e.g., at the Rivers Option). The main phases include chalcopyrite, pyrrhotite, pentlandite and pyrite. Minor amounts of PdBiTe (michewrite), CoAsS (cobdtite), (FeNi),AgS, (argentiferous pent landite) and Ag,Te (hessite) were also detected by electmn microprobe; the latter two phases being considered rare amongst Sudbury ore minerais
(Naldrett, 1984).
Textural relationships indicate that plagioclase is the first minerai to crystallise.
Orthopyroxene (or calcic amphiboles pseudomorphing pyroxene) generally fonns euhedral lath- like crystals, but it can be subophitic. Clinopyroxene (or calcic amphiboles pseudomorphing pyroxene) crystals are more subhedral to anhedral and are usually found on the rim of orthopyroxene crystals cr interstitially between plagioclase crystals (Fig. 5.1 ). Rare inveried pigeonite also occurs (Fig. 5.2). Ferro-magnesio homblcnde, with euhedral 'primary' biotite, occurs as a discontinuous rim on pyroxene (Fig. 5.3), or on the rirn of actinolite pseudornorphing pyroxene (Fig. 5.4), and also within granophyric intergrowths (Fig. 5.5).
Granophyric intergrowths of quartz and orthoclase are late crystailising phases. This texture is due to rapid crystailisation in the presence of water (Smith and Brown, 1988). in the quartz diorite the granophyre occurs intersertally, filling in between the larger minerais in characteristic triangular or irregular shapes and typically corroding adjacent plagioclase crystals (Fig. 5.6).
PLAGIOCLASE
Plagioclase is distinct in commonly exhibiting broad labradorite cores of relatively uniform composition (An,,,) (analyses 1 and 2, Table 5.1 :and narrow oligoclase rims (An=.
,,) (analyses 3 and 4, Table S. 1) (Fig.s 5.7 and 5.8). Some crystais have a more morthitic rim
(analysis 5, Table 5.1) The granophyric intergrowths in Hess occur as radiating fnnges Figure 5.1 - Subophitic to intersertial texture of the quartz diorite. O=orthopyroxene, C=clinopyroxene, H=homblende, A=actinolite. Note the partial alteration of the orthopyroxene in the middle lefi to actinolite. Two large darker crystals with positive relief are orthopyroxene, rimmed by clinopyroxene where marked with the arrow. Plane polarised light, field of view 1.25 mm. Sample 7- 198.
Figure 5.2 - hverted pigeonite SEM. Light areas are orthopyroxene, with dark clinopyroxene larnella. Lower left dark area is granophyre; upper right is plagioclase. Sample 7- 198. Figure 5.3 - Othopyroxene (O) rirnmed by clinopyroxene (C), partially reacted to actinoli te (A), with rim of ferro-magnesio homblende (Hb),which is rimed by ferro-actionlite (FeA), and biotite (B). plane plarised light, field of view 0.625 mm. Sample7- 198.
Figure 5.4 - Ferro-magnesio-homblende (Hb) on the rim of actinolite which is pseudomorphing pyroxene. Primary biotite (B) occurs on the rim of the homblende, and secondary biotite (B2) and tschennakitic homblende (Tsc)occur on the rim of the actinolite. Plane polarised light, field of view 6.25m.m.Sample 6-107. Figure 5.5 - Primary ferro-magnesio-homblende (Hb),occurring within granophyre, with a rim of ferro- actinolite (FeA) and an outer rim of biotite (B). Plane polarised light, field of view 0.625 mm. Sample 7- 198.
Figure 5.6 - Interstitial granophyre eroding plagioclase. Crossed polars, field of view 0.625 mm.Sample 6- 107. Figure 5.7 - Plagioclase crystal surrowided by granophyre showing the typical (red-brown) cryptocrystalline inclusions at the compositional break near the rim of the crystal. The dark patch on the rniddle left hand side of the photo is a mixture of sericite and epidote fomed by secondas, alteration of the plagioclase. Plane polarised light, field of view 0.625 mm. Sample 6- 107.
Figure 5.8 - Same as Figure 5.9 with mssed polars. Note the compositional break near the rim of the crystal, and the acicular-like inclusions of piemontite and epidote in the core of the plagioclase crystal. Crossed polars, fieM of view 0.625 mm. Sample 6- 107.
110 Sample 1 2 3 4 5 6 7 8 Wt% SiO, 53.03 54.06 65.45 62.80 56.62 54.43 66.98 48.28 TiO, 0.05 0.02 0.04 0.01 0.07 nd nd nd Al,O, 29.34 27.71 20.71 22.69 27.10 27.53 20.29 34.1 1 Cr20, 0.03 0.07 nd nd 0.03 nd 0.03 nd FeO, 0.56 nd 0.07 nd 0.82 0.74 O. 11 1.61 Mn0 nd 0.85 0.32 0.1 1 nd nd nd 3.26 Mg0 0.12 0.10 nd nd 0.06 0.06 0.02 0.98 Ca0 12.41 10.78 4.28 3.76 9.37 10.04 1.05 0.19 Na20 4.64 5.55 8.26 9.92 6.42 5.76 11.50 0.39 K20 0.32 0.3 1 0.05 0.09 0.16 0.12 0.05 9.75 Total 100.50 99.45 99.18 99.39 100.65 98.67 100.04 99.57 Stoichiometry Si Ti Al Cr Fe Mn Mg Ca Na K
Table 5.1 - Microprobe analyses of plagioclase fkom the Hess Offset dyke. Stoichiometry on the basis of eight oxygen. FaT=total iron as FeO. nd = not detected. An, Ab and Or are weight percent anorthite, albite and orthoclase respectively. Numbers in parentheses are the sample numbers (see fold out map in back pocket for location of samples). 1 = core (6-34); 2 = core (1 07-8); 3 = rim (6- 1 07); 4 = rim (6- 107); 5 = rim (6-34); 6 = core, contact quartz diorite (6-43); 7 = rim, contact quartz diorite (643); 8 = antiperthite lamellae in core (6- 107); 9 = feldspar inclusion (6- 1 07). around plagioclase, contiguous with their oligoclase rims. Alteration of plagioclase is relatively minor compared with that of the pyroxenes and amphiboles but, in places, the plagioclase laths are replaced by a fine-grained mixture of epidote and sericite. Near wall rock contacts and in the more granophyre rich centrai portion of the dyke, this alteration tends to be more pervasive and plagioclase tends to be more albitic (analyses 6 and 7, Table
5.1). Micro-antiperthite exsolution lamellae of potassium feldspar (e.g., Or,) (analysis 8,
Table 5.1) are cornmon within plagioclase. Fine (1 -5 pm), needle-like inclusions occur in the cores of some plagioclase crystds (Fig. 5.9). Some needles form a adpatterns within the host feldspar. SEM images of these inclusions reveal them to be individual crystals that are aligned to appear as needles. These inclusions range frorn wonesite mica, epidote, phlogopite and piemontite (analysis 1 - 4 respectively, Table 5.2). Larger inclusions in more altered plagioclase crystals include blebs of actinolite (analysis 5, Table 5.2), piemontite, epidote
(analysis 6, Table 5.2), biotitic mica (analysis 7, Table 5.2) and biotite (analysis 8, Table 53,
(Fig. 5.1 O).
A reddish-brown clouding coincides with a compositional break near the rim of plagioclase crystals, which is best seen in plane-polarised light (Fig 5.7). This clouding is distinct fiom the typical alteration seen in plagioclase crystals, which forms epidote and sericite. The clouding of plagioclase crystals is attributed to the presence to numerous cryptocrystalline inclusions, which scatter light inegularly; the red colour is generally indicative of haematite (Smith and Brown, 1988). Similar clouding has dso been noted in plagioclase crystals in the Main Mass of the SIC, particularly within the granophyre (Naldmtt et al., 1970). This change is associated with the appeanuice of larger epidote inclusions in Figure 5.9 - SEM photo of needle- shaped alignment of piemontite inclusions (diagonal lines) in plagioclase. Upper leA hand corner is granophyre. Sample 6- 107.
Figure 5.1O - SEM photo of bleb-like inclusions of piemontite and epidote (light grains), near the compositional break of the plagioclase crystal. Labradorite core is the lighter area to the lefk of the photo, oligoclase rim in centre in contact with granophye to the right. Sample 6-107. 113 ------Sample 1 2 3 4 5 6 7 8 Wt% SiOl 35.97 41.48 35.73 38.28 50.34 36.06 44.61 37.60 Ti4 0.73 nd 0.02 0.02 0.10 0.08 0.86 1.32 A1203 19.13 29.67 17.86 25.96 1.99 24.01 20.45 18.01 Cr203 0.02 nd 0.12 0.13 0.05 0.02 0.01 0.04 FeOT 24.83 6.00 O. 12 0.02 14.35 1 1.O7 12.97 19.85 Mn0 0.28 0.23 24.33 10.05 0.3 1 0.14 0.09 0.18 Mg0 5.10 0.05 10.61 0.02 14.46 0.00 7.31 10.09 Ca0 0.38 21.67 0.60 23.61 12.36 23.36 1.97 0.57 Na,O 0.54 0.78 0.95 0.13 0.13 0.05 3.58 0.35 K,O 5.34 0.05 4.69 nd 0.18 0.02 5.50 8.88 Total 92.30 99.94 95.03 98.22 94.27 94.80 97.35 96.89 Stoichiornetry (22) (13) (22) (13) (23) (13) (13) (22) Si 5.66 3.15 5.45 3.06 7.63 7.63 6.23 5.61 Ti 0.09 nd 0.00 0.00 0.01 0.01 0.09 0.15 Al 3.55 2.65 3.21 2.44 0.36 0.36 3.36 3.17 Cr 0.00 nd 0.01 0.01 0.01 0.01 0.00 0.01 Fe 3.27 0.38 0.02 0.00 1.82 1.82 1.51 2.48 Mn 0.04 0.02 3.14 0.68 0.04 0.04 0.01 0.02 Mg 1.20 0.01 2.41 0.00 3.27 3.27 1.52 2.24 Ca 0.06 1.76 0.10 2.02 2.01 2.01 0.29 0.09 Na 0.16 0.1 1 0.28 0.02 0.04 0.04 0.97 0.10 K 1.07 0.01 0.91 nd 0.03 0.03 0.98 1.69
Table 5.2 - Microprobe analyses of inclusions in plagioclase crystals fiom the Hess Onset dyke. Number in parentheses indicates number of oxygens uscd to calculate stoichiometry. Fe=total iron as FeO. nd = not detected. Numbets in parentheses are the sample nurnbers. 1 = wonesite mica (6428); 2 = epidote (6-34); 3 = phlogopite (6-107); 4 = piemontite (6- 107); 5 = actinoiite (6435); 6 = epidote (6-435); 7 = mica ??(6- 122); 8 = biotite (6- 122). the rim zone of the plagioclase crystal. It has been suggested that H,O reacts with the calcic plagioclase to produce albite, an epidote-group phase, quartz and Al,O, (Naldrett et al.,
1970). The breakdown products are favoured by increasing water pressure and decreasing temperature. A similar deuteric process appears to have affected the plagioclase of the Hess and Foy Offsets. Near the contacts with the wall rocks the reddish-brown clouding is typically more extensive, occurring throughout most of the crystal.
The plagioclase in the Main Mass of the SIC shows a general decrease in anorthite content with increased stratigraphie height. The anorthite component of plagioclase in the
North Range of the SIC decreases from 65 at the top of the mafic norite, to 58 in the upper third of the felsic norite to 53 at the top of the quartz gabbro (See Fig 1.2). An abrupt change in plagioclase composition from An, to An, occurs in the upper 100 rn of the quartz gabbro.
The plagioclase core compositions in both the Foy and Hess Offsets are comparable with the upper third of the felsic norite of the Main Mass.
PYROXENES AND THEIR ALTERATION PRODUCTS
Pyroxenes are nuely preserved intact; most occurring as uralitised relicts, although unaltered pyroxenes have been found in parts of both the Hess and Foy Offsets. Relatively unaltered pyroxenes were found in the Hess Offset West of the proximal Foy intersection, and
West of the Parallel Foy intersection (samples courtesy of Inco exploration) and in the Foy
Offset the Maki Showing and south of the proximal Foy intersection (sample number 7-1 98).
Table 5.3 and 5.4 shows the microprobe analyses for orthopymxenes pyroxenes and clinopyroxenes respectively. The stoichiometry was calculated on the basis of six oxygens. Total 99.37 99.31 100.05 100.17 101.90 Stoichiometry Si 1.96 1.93 1.97 1.97 1.97 Ti 0.01 0.00 0.01 0.01 0.01 Al 0.08 O 0.05 0.06 0.03 Cr 0.01 0.02 0.00 0.01 0.00 Fe3' 0.00 0.03 0.00 0.00 0.94 Fe2+ 0.47 0.36 0.70 0.79 0.01 Mn 0.01 0.01 0.01 0.01 0.02 Mg 1.41 1.50 1.19 1.07 0.95 Ca 0.06 0.06 0.07 0.09 0.07 Na 0.00 0.00 nd 0.00 0.00
Wo 3.40 3.20 3.70 4.50 3.90 En 72.70 77.90 60.50 54.70 48.40 Fs 24.00 18.90 35.80 40.80 47.70 others 5.08 7.70 3.62 4.3 1 4.25 Temperature (OC) 1080 1095 1000 1130 850
Table 5.3 - Microprobe analyses of orthopyroxene fiom the Hess and Foy Offsets. Stoichiometry on the basis of 6 oxygen. FeO, = total iron as FeO. nd = not detected. Wo, En and Fs are the percentage of wollastonite, enstatite and femsilite components, respectively. "Others" are the percentage of non-pyroxene quedrilaterial components, calculated fiom the projection scheme of Lindsey and Anderson ( 1 983). Crystalistion tempemes estimateci nom Lindsey's graphical thennometer (see figure 5.1). Numbers in parenthesis are the sample numbers: 1 = core (7-1 95); 2 = core (Inco, 70); 3 = rim (Inco 69); 4 = dm @CO,69); 5 = rim (7-1 98). Sample 1 2 3 4 5 6 7 Wt% Si4 51.80 51.55 51.41 53.12 51.37 50.02 51.80 TiOl 0.46 0.38 0.50 0.15 0.55 0.63 0.57 A1203 1.90 1.86 1.94 1.68 1 .O5 1 .71 1.75
- -- %al 100.12 100.54 99.84 97.08 101.96 99.76 10148 Stoichiometry Si 1.94 1.92 1.94 2.03 1.96 1.93 1.97 Ti 0.01 0.0 1 0.0 1 0.00 0.02 0.02 0.02 Al 0.08 0.08 0.09 0.08 0.05 0.08 0.08 Cr 0.00 0.00 0.00 0.00 nd 0.00 0.00 Fe 3+ 0.03 0.07 0.02 0.00 0.68 0.03 0.00 Fez' 0.37 0.38 0.42 0.44 0.0 1 0.60 OS9 Mn 0.01 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1
others 7.52 9.69 7.72 5.17 6.09 7.89 6.36
Table 5.4 - Microprobe analysis of clinopyroxene hmthe Hess and Foy Offsets dykes. Stoichiometry on the basis of 6 oxygens. Fe= total iron as FeO. nd = not detected. For Merdetails see Table 5.3. Numbers in parenthesis are the sample nunbers. 1 = core (Inco 69); 2 = core with amphibole lamellae (7- 198); 3 = core (7- 195); 4 = core with amphibole lamellae (Inco, 70); 5 = rim (7-198); 6 = rim (Inco, 69); 7 = rim (Inco, 70). Results are plotted on the pyroxene quadrilateral (Fig. 5.1 1) using the projection scheme of
Lindsley and Andersen (1983). Orthopyroxene crystal are zoned. Core compositions are relatively consistent, but the rirn compositions show more variation. The cores of orthopyroxene crystals possess a significantly higher Mg# than do coexisting clinopyroxenes.
Using Lindsley's ( 1983) graphical pyroxene thermometer, the tie- line positions indicate that the rims of the orthopyroxene were in equilibriurn with the cores of the coexisting clinopyroxene (Fig 5.1 1). This implies that crystallisation of the orthopyroxene rims with the clinopyroxene occurred simultaneously, which is in agreement with textural observations
(Fig. 5.1 1).
The cores of the orthopyroxenepossess reasonably consistent compositions with Mg#
0.75-0.80, (analyses 1 and 2, Table 5.3) (for fiirther pyroxene analyses see appendix 3). The rims are typically narrow (less than 0.2 mm) and have a wider compositional range, typically
Mg# 0.75-0.58, (analyses 3 and 4, Table 5.3), with some rims showing extreme zoning
(analysis 5. Table 5.3). Clinopyroxenes have a more variable core composition, especially in the arnount ofwollastonite (Wo) component. The Mg# ofclinopyroxene cores ranges fiom
0.71-0.64 (analyses 1,2,3 and 4, Table 5.4). Rim compositions show even greater variation in Mg#, but less variation in the Wo component (analyses 5,6 and 7, Table 5.4)
The Mg# of clinopyroxene cores in the Hess Offset are very similar to the augite in the felsic norite of the Main Mass (Naldrett et al.. 1970). In the North Range, the augite has an Mg# of 0.70 at the base of the felsic nonte, and 0.69 towards the top of the felsic Norte.
In con-, orthopyroxene cores in the Hess and Foy Offsets have considerably higher Mg# than any of the orthopyroxenes in the Main Mass. In the North Range of the Main Mass, the
Mg# of orthopyroxene decreases hm0.72 at the top of the rnafic norite, through 0.67 at the base of the felsic norite, to 0.62 towards the top of the felsic Norte. This trend of decreasing
Mg# continues through the quartz gabbro and the granophyre (Naidrett et al., 1970). The downward decrease in Mg# in orthopyroxene in the mafic norite, is accompanied by an increase in zoning. This is similar to the zoning observed in the orthopyroxenes in the Hess and Foy OfXsets, with relative Fe- e~chmentin the rims.
Using Lindsley's (1983) thennometer at one atmosphere, the cores of the orthopyroxene plot at fairly consistent temperatures, ranging fiom 1000°C to 1 100°C
(average 1070°C, from 19 analyses Table 5.3 and appendix). The rim temperatures are variable, ranging from 850°C to 1 175 OC. Clinopyroxene core temperatures range from
1,220°C to 1,O 10 OC. Clinopyroxene rim compositions show a similar range in temperame from 980°C to 1175°C. Tie line orientations, and textural relationships indicate that the orthopyroxene rims are in equilibrium with the cores of the clinopyroxenes but the clinopyroxenes indicate a higher crystallisation temperature. The differences and the range in temperatures seen in the clinopyroxenes are essentially controlled by the arnount of Wo component. Variation in Wo content in clinopyroxenes may be due to the presence of amphibole lamellae in the pyroxenes, which may well not be visible with an optical microscope (Veblin and Buseck 1981). Partial alteration of the pyroxene to actinolite is comrnon. It is particularly prevalent nea.crystal margins so typically effects clinopyroxene more than orthopyroxene. Due to the similarity of the chernical composition of the pxenes and actinolite, intergrowth of the two minerds would be difficult to detect on the SEM. The distinctive TEM diffraction patterns of the two minerals could be used to resolve the intergrowth.
From Figure 5.1 1 it can be seen that the clinopyroxene cores containing the higher
Wo component yield temperatures of 1,00O0C-1 100°C (cf. analyses 1 and 3, with andyses
2 and 4, Table 5.4). These are most likely to represent relatively pure cores without amphibole lamellae. Such pyroxenes have - 35 % Wo component, compared with the pyroxenes plotting at much higher temperatures + 100-200 OC which have - 26-34 % Wo component. Veblin and Buseck (1 98 1) calculate that a mixture of 90% pyroxene and 10% optically unresolvable amphibole would lower the temperature by about 120°C, compared with that of a pure pyroxene, be would still yield reasonable stoichiometry. No such phenomenon is observed in the orthopyroxene, as the conversion of orthopyroxene to cwnmingtonite would not significantly alter the Ca0 composition, so therefore this does not significantly affect the orthopyroxene temperature calculation.
Even in areas of the Offset that contain relatively unaltered pyroxenes, there is a patchy distribution of pyroxenes that show varying degrees of alteration to actinolite (Figs.
5.1 and 5.3). Only minor amounts of cummingtonite were found intergrown with actinolite
(analysis 1, Table 5.5). Textural relationships tend to indicate that actinolite after clinopyroxene (analyses 3 and 4, Table 5.5) has a higher calcium content and a lower Mg# than actinolite resulting fiom the alteration of orthopyroxene (analysis 2, Table 5.5). The
Mfls of the actinolite are similar to those of the respective precursor pyroxenes.
Quartz dioritejust West of the proximal Foy intersection contains remnants of altered pyroxenes smunded by calcite (Fig. 5.12). These samples are relatively quartz rich and the plagioclase crystals are extensively saussuritised, giving hem a khaki colour. Interstitial Total 101.5 1 100.89 100.39 100.54 98.47 97.29 95.32 94.76 Stoichiometry Si Ti Al Cr F&+ F$+ Mn Mg Ca Na K
Table 5J - Microprobe analyses of alteration products of pyroxene fiom the Hess and Foy Offsets. Number in parentheses indicates nurnber ofoxygms used to calculate stoichiometry. Fe& = total iron as FeO. nd = not detected. Nurnbers in parentheses (below) are the sample numbers. 1 = calcian-cummingtonite (hum, 70); 2 = actinolite, ahothopyroxene (Inco, 70); 3 = actinolite, aAer clinopyroxene (Inco, 70); 4 = actinolite after clinopyroxene (7-1 98); 5 = actinolite from Quartz Diorite containing no pyroxene (6- 107); 6 = actinolite hmQuartz Diorite containing no pyroxene (6- 122); 7 = secondary biotite on actinolite (7- 198); 8 = seçondary biotite on actinolite (6- 122). Figure 5.12 - Orthopyroxene (in the centre), partially replaced by calcite (C), from the Hess Offset. Plane polarised light, field of view 1.25 mm. Sample Hess III.
Figure 5.13 - Chlorite (CM)and secondary biotite (B2) replacing actholite in the Hess Offset, rimmed by primary hornblende (Hb) and biotite (B) with minor tscherrnakitic homblende (Tsc) occurring on the rim of the prhary homblende. Plane polarised light, field of view 1.25 mm. Sample 6-34. 123 biotite remains and minor amounts of chlorite are associated with the pyroxene and calcite, but no amphibole is present. This possibly suggests that there was a calcium-rich fluid migrating through the rock. This metasomatism could either be the result of deuteric or hyârothermal alteration.
Actinolite in more extensively altered quartz diorite (where no pyroxenes are found) has a more intermediate composition between that of the two actinolites from the different pyroxenes in the less altered sarnples (analyses 5 and 6, Table 5.5) (Figs. 5.4 and 5.12). This implies interchange of calcium, magnesiurn and iron between the pyroxenes during uralitisation. This might be expected if there is a flow of a calcite-rich fluid through the system. Actinolite is cornmonly found partly replaced by chlotite (analysis 7, Table 5.5) and secondary biotite (analyses 8 and 9, Table 5.5) (Fig. 5.13).
PRIMARY AMPHIBOLES AND BIOTITE
Ferro-magnesio homblende, with euhedral pnmary biotite, occurs on the rirns of ortho- and clino-pyroxenes (Figs. 5.1 and 5.4) and on actinolite pseudomorphing pyroxene
(Figs. 5.4 and 5.13), and also within the granophyric intergrowths (Fig. 5.5). These varieties of homblende and biotite are considered to be late magmatic (deuteric) in origin. Primary homblende is easily distinguished fiom actinolite by its dark brown-green pleochroism and its euhedral non-fibrous form. Primary, dark greenbrown, euhedd biotite is easily distinguished fiom secondary biotite: the latter is idioblastic to xenoblastic and is found in association with actinolite and chloritised amphiboles (Fig. 5.13). Secondary biotite is depleted in TiO, and there is minor calcium e~chmentcompared to primary biotite (analyses 1,2, and 3, Table 5.6, secondary biotite analyses 8 and 9, Table 5.5).
The composition of ferro-magnesio homblende that is developed on the rims of
pyroxenes (analysis 4 and 5, Table 5.6) is essentidly the same as those occurring on the rims
of actinolite (afier pyroxene) (analysis 6,7 and 8, Table 5.6). The wide range in composition
implies that reactions between the minerals did not go to completion and equilibrium was
not achieved.
Blue-green amphibole occurs as optically continuous rims on the ferro-rnagnesio
hornblende and on pyroxene (andor actinolite &er pyroxene). In areas where the pyroxenes are relatively unaltered, these rims tend to be ferro-actinolite (analyses 1,2 and 3, Table 5.7),
(Fig. 5.3). Where no pyroxenes remain the hornblende tends to be more tschermakitic
(andysis 4,5,6, and 7, Table 5.7), (Fig. 5.4). In the centre of the Offset dyke, where more granophyre is present, the deveiopment of blue-green amphibole is more pervasive. Similar
features were reported by Fleet et al. (1 987) in the Foy Offset near the Main Mass, and in the
Ministic Offset, where incomplete replacement of magmatic brown edenite by optically continuous blue-green actinolite was documented. Fleet et al. (1 987) demonstrated that the colour of optically continuous cdcic amphibole changes progressively with replacement
from brown to green-brown to green to pale blue green. Ti and Fe contents appear to be
important factors in colour development, which suggest that Fe2+to Ti " charge transfer
might be more important than ~e*'to Fe '' charge transfer. They interpret this alteration of
the primary hornblende to be due to regional prograde metamorphim. . -- Sample 1 2 3 4 5 6 7 8 Wt% SiOz 38.17 33.78 37.67 44.01 43.62 47.1 48.38 44.16 Ti4 2.91 2.39 1.69 1.53 1.59 O 0.4 1.78 A120, 13.86 15.16 15.61 7.52 7.59 5.77 5.56 6.99 Cr203 0.07 0.01 0.06 0.03 0.00 0.02 0.03 0.02 Fe4 22.45 27.5 20.49 25.60 28.58 20.74 18.12 23.84 Mn0 O 0.15 0.23 0.18 0.48 0.24 0.44 0.44 Mg0 10.39 6.98 11.3 7.13 5.56 9.46 12.11 7.24 Ca0 0.18 0.01 0.2 10.54 10.38 12.18 11.1 1 10.53 Na20 0.05 nd 0.04 1.94 1.45 0.6 0.83 1.14 K20 8.96 9.29 9.35 1.11 0.96 0.21 0.45 0.81 Total 97.05 95.26 96.64 99.59 100.21 96.32 97.43 96.95
Table 5.6 - Microprobe analyses of primary homblende and biotite 6om the Foy and Hess Offsets. Numbers in parentheses indicate number of oxygen used to calculate stoichiometry. FeO, = total iron as FeO. nd = not detected. nc = not calculated. Numbers in parentheses (below) are the sample nunbers. 1 & 2 = biotite, fiom quartz diorite containing pyroxenes (Inco, 69); 3 = biotite, From altered Quartz Diorite, containing no pyroxenes (6-122). 4 & 5 = ferro-homblende, hmquartz diorite containing pyroxenes (7- 198); 6 = faro-homblende, hmaltered quartz dionte, containing no pyroxenes (6-34). 7 = magnesio-homblende, hm altered quartz diorite, containing no pyroxenes (6-1 22); 7 = fm-homblende, hmaltereâ quartz diorite, containing no pyroxenes (6-34). -- Total 96.s99.63- 9872 97.78 98.01 97.7 1 96.68 Stoichiomeüy Si 7.67 7.56 7.44 7.55 6.41 6.25 6.21 Ti 0.02 0.00 0.05 0.02 0.00 0.02 0.05 Al 0.36 0.42 0.61 0.56 4.82 2.35 2.34 Cr 0.00 nd 0.01 nd 0.01 nd 0.01 Fe3' 0.00 0.58 0.00 0.25 0.00 0.79 0.98 F$+ 2.60 2.40 2.83 1.59 1.73 1.97 1.68 Mn 0.06 0.04 0.04 0.03 0.02 0.04 0.05 Mg 2.29 1.99 2.04 3.00 nd 1.58 1.69 Ca 2.23 1.89 2.48 1.96 4.24 1.89 1.79 Na 0.04 0.07 0.14 0.12 0.07 0.39 0.40 K 0.04 0.02 0.07 0.01 nd 0.15 0.19
Table 5.7 - Microprobe analyses of alteration products developed on the rims of primary homblendes and on pyroxenes or actinolite pseudomorphing pyroxene. Stoichiometry dculated on the basis of 23 oxygens. FeO, = total iron as FeO. nd = not detected. Numbers in parentheses are sarnple nurnbers. 1 = ferro-actinolite (Inco, 69); 2 = ferro-actinolite (7- 198); 3 = fem>-actinolitic homblende (7-198); 4 = actinolite (6-34); 5 = alumino-fm- tschemakitic homblende (6-34); 6 = fmo-tshemakitic homblende (6- 107); 7 tschermakite (6-34). CONTACT PHASES
The margins of the Hess Offset are always found chilled against the country rock, defined by a marked decrease in grain size or the development of a spheditic texture of elongate plagioclase and pyroxene (or actinolite pseudomorphing pyroxene) crystais. The development of these chilled margins is usuaily accompanied by subtle changes in rnineraiogy. In the absence of large acicular pyroxenes or their pseudomorphs, the contact quartz diorite is typically a fine- to very fine- grained biotite quartz diorite. It consists of subhedral plagioclase, patchy biotite. significant amounts of epidote, titanite and interstitial quartz, with or without granophyre and minor amphibole and chlorite. Actinolite in the sphenilitic-textwed contacts rarely remains unaltered (Fig. 5.14). being more typically ni;:aced by a patchy aggiugate of biotite crystals, but the rough outline of the origiaally acicular crystals cm still be discemed (Fig. 5.1 5). The mineralogy of this contact phase is essentially the same as that observed in the non-spherulitic chilled margins. Plagioclase tends to show more extensive development of the red-brown crypto-crystalline inclusions, which is typically accompanied by more extensive saussuritisation. The prevalence of biotite in the contact quartz dionte may imply the assimilation of K rich wall rocks.
INTERNAL CONTACTS
A macroscopically sharp contact of quartz-rich quartz diorite against quartz- poor quartz diorite was observed in Elation Creek (Fig. 2.6). Microscopically, the contact between granophyre-rich and nomal quartz diorite is diffise. In the granophyre- rich quartz diorite there is extensive development of blue-green aluminium-rich amphibole and chlorite (Fig. Figure 5.14 - Contact quartz diorite of the Hess Offset, with acicular plagioclase, showing minor developement of the red-brown crypt~~rystdlineinclusions near the crystal margin. Actinolite pseudomorphing pyroxene, rimmed by ferro-magnesio hornblende and biotite. Plane polarised light, field of view 1.25 mm. Sample 7-203.
Figure 5.15 - Contact quartz diorite of the Hess Offset. With acicular plagioclase there is extensive development of the red-brom cryptocrystalline inclusions throughout the plagioclase crystal. The acicular pyroxene crystals have ken pseudomorphed by a patcby biotite aggregate. Plan polarised light, field of view 1-25 mm. Sample 7-1 14. 5.16). Plagioclase typically shows more extensivedevelopment ofthe red-brown colouration and is more saussuitised and quartz is more predominant. Similar features have been observed in other areas where the central portion of the Offset is exposed (e.g. Bear Print
Creek, UTM 463482,5 177463). Slower cooling might concentrate the volatiles (including
H,O) in the central portions of the Offset. The H20 could then extensively alter the early crystallising stages such as plagioclase, pyroxene and amphibole. The reactions ofthese early mineral phases with the volatiles will be more extensive than in more marginal phases of the quartz diorite, as they have more tirne to react and equilibrate. The late crystallising phases, such as the quartz and orthoclase would also be concentrated in the central portions of the
Offset. In the final stages of crystallisation these would crystallise rapidly in the presence of
H,O to form the granophyre.
ALTERATION DISTRIBUTION HESS QUARTZ DIONTE
Different areas of the Hess Offset show local variations in pyroxene venus amphibole development, and in the pervasiveness of homblende-actinolite alteration. Plagioclase is generally less altered than the coexisting mafic phases. Relatively unaltered pyroxenes in
Hess have only been found near the proximal Foy intersection. Even in this area the distribution of unaltered pyroxenes is patchy. It is not clear from field relationships whether the pyroxene-bearing quartz diorite of the Hess Offset is a separate phase of the Hess Offset, or if it is just a less altered phase of the actinolite-karing quartz diorite. Even where pyroxenes are present there are varying degrees of alteration to actinolite, both intercrystaily and intraerystally, and fiom different areas of the same outcrop, tending to indicate patchy
130 Photo 5.16 - Extensively altered quartz diorite. From the central portion of the Hess Offset dyke. Blue-green amphibole predomhtes over actinolite, and plagioclase is extensively saussuritised. Plane polarised light, field of view 1.25 mm. Sample ELC.
Photo 5.17 - Highly aitered quartz diorite cross eut by epidote vein (dark band in upper part of the photo), fiom the Pterobctyl Lake am. There is extensive alteration of both plagioclase and amphiboles. Plane polarised light, field of view 1.25 nim. Sample 7- 1 15. alteration rather than distinct intrusive phases.
The quartz diorite fiom the Pterodactyl Lake area differs fiom other Hess quartz diorite in that it shows extensive plagioclase as well and amphibole alteration. The amphibole is sirnilar in appearance to the amphibole in the granophyric centrai portions of
Hess. The REE element plot for this Pterodactyl Lake sample (Fig. 4. 7), shows that it is
Hess-related, but that it has probabiy been metasomatised. The opticaliy determined anorthite content of the plagioclase (An,.,,), which is consistent with Na-metasomatism. In outcrop this rock diffea ficm the typical Hess quartz diorite in that it is greener and more extensively cut by fuie (mm scale) epidote veins. ln thin section it is texturally similar to typical Hess quartz diorite (Fig. 5.17). Plagioclase crystals are blocky and the granophyric intergrowths occur as radiating fnnges contiguous with their rims. The amphiboles are blue-green and are similar to those in the granophyric central portions of Hess and are associated with extensive chlorite. Epidote is common not only within the veins but throughout the rock, and is typically associated with the amphibole (Fig. 5.17).
Epidote veining similar to that seen in the Pterodactyl Lake outcrop is similar to that seen throughout the Hess Offset and in inclusions entrained within Hess (Fig. 2.5). Calcite is often also associated with epidote veins, which range in scale fiom submillimetre to several centimetres in thickness (Fig. 5.18). Where epidote and calcite veining is present more of these minerals also occur throughout the sample and especially in association with mafïc minerals (Fig. 5.12 and 5.17).
Green-blue homblende occurs on the rims of primary ferro-magnesio homblende and occasionally on the rims of pyroxene or actinolite. The development of this pale green-blue Photo 5.18 - Late stage quartdcalcite veining crosscutting altered quartz diorite, in the Pterodactyl Lake area. Crossed polars, field of view 0.625 mm. Sarnple 6- 1 1 5.
Photo 5.19 - Apparent igneous texture of the matrix of the breccia inclusions in the Hess-Foy intersection area. There is a bimodal distribution of plagioclase crystals which form a decussate texture. Plan polariseci light, field of view 1.25 mm. Sample 6-4 10. honiblende is more prevalent in the central granophyric portions of Hess. Fleet et al. (1987) attribute the development of the green-blue homblende to regional metarnorphism. The fact that it is more extensively developed in the central portions of the dyke where slower cooling andor more water is present tends to indicate that this is deuteric auto-metamorphism occurring within the dyke.
The biotite-rich marginal phases of the Hess Offset are compositiondly similx to the biotite quariz diorite described by Grant and Bite (1984), the development of which was attributed to alteration or assimilation of siliceous country rock material. Grant and Bite
(1984) note, however, that there is no textural evidence to suggest this mineralogy is due to alteration. In the Hess Offset the patchy appearance of the biotite in the contact phase tends to indicate that both alteration and assimilation have occurred (see geochemistry Chapter 4).
INCLUSIONS
Numerous footwall-like breccia inclusions are entrained within both Foy and Hess in the area of both the proximal and distal Foy intersections. The breccia inclusions are so narned due to the presence of clasts, although the matrix has an apparent igneous texture.
However, because the rnatrix has also undergone static recrystallisation, metarnorphic ierminology is used to describe these annealed rocks. The inclusions are sirnilar in appearance to the fmtwall breccia, occurring around the base of the Main Mass (personal communication Andy Bite and Mars Napolli Inco Exploration, 1997). This breccia forms discontinuous lenses and sheets dong the footwall in the North and East Ranges of the SIC.
Because the Hess-Foy inclusions are similar in appearance to the footwall breccia they are referred to here as footwall-like breccia. These footwall-like breccia inclusions range in size from several centimetres to tens of metres. These are heterolithic and are characterised by a bimodal plagioclase distribution, which occur as small stubby laths (Fig. 5.20). These laths fondecussate textures, or occur as small clustea or as single crystals in poikilobastic host minerals. Granoblastic textures of polygonal plagioclase and quartz, and a granophyric intergrowth of quartz and feldspar, also occur in the breccia rnatrix. Several minerals occur as porphyroblasts in the matrix. Quartz and plagioclase are the most common porphyroblasts, but biotite, homblende, orthopyroxene,clinopyroxene, magnetite, apatite and orthoclase also occur. Pyroxenes and amphiboles typically occur as poikioblasts; the ophitic texture with small plagioclase inclusions gives them a blotchy appearance (Fig. 5.21 ). There are alteration features in the breccia that are similar to those seen in the Hess Offset these include: (1) alteration of pyroxene to actinolite, (2) ferro-magnesio homblende on the rim of pyroxene crystals, and (3) cryptocrystalline inclusions in plagioclase crystals giving them a distinctive reddish-brown colouration. Microscopically, the matrices of clast-packed and clast-poor breccia are the sarne. The clasts do not show any pronounced alignrnent; this observation, coupled with their local derivation, suggests in situ brecciation without flow. The fact that clasts of non-local derivation (gneiss, anorthosite, and mafic clasts) are found in the breccia implies transport of the inclusions. It is not clear if the transport medium for the breccia inclusions was the quartz diorite, or if the breccia was emplaced as a dyke prior to the emplacement of the quartz diorite.
Lightfoot et al. (1997a) map so-called "quartz diorite breccia", occturing adjacent to the Foy Offset at the Maki showing in the proximal section of the Foy Offset. This is actually Figure 5.20 - Heterolithic inclusion of footwall-like breccia fiom the central portion of the Hess Offset. Left-hand side is a large clast of consisting of an aggregate of biotite and amphibole crystals. Dark minerals in centre are biotite. Dark blebby crystal on the right-hand side is pyroxene. Plagioclase crystals in matrix show extensive development of cryptocrystalline red-brown inclusions sirnilar to those in the quartz diode. Plane polarised light, field of view 1.25 mm. Sample 7- 194A
Figure 5.2 1 - Same as Figure 5.17 with crossed polars.
136 sunounded by tme quartz diorite so is an inclusion. This is similar in appearance to the footwall-like breccia occming in the Hess Offset. The majority of the clasts in this inclusion appear to be from the Levack Gneiss complex and are similar in appearance tu gneissic inclusions seen in the Hess Offset, and "gneissic breccia" might be a more appropriate name.
A diffise margined clast from the footwall-like breccia in the Maki showing contains felsic inclusions of completely recrystailised quartz clasts and recrystallised feidspars (Fig. 5.22).
The quartz occurs as highly xenoblastic, contorted aggregates of small, polygonal equigranular crystals and the feldspars crystals are hornfelsed. The texture of the feldspar crystals is sirnilar to those associated with granite which has been extensively pervaded by pseudotachylyte veinlets. The matrix mund these felsic clasts consists of blebby feldspars and actinolite with only minor quartz (Fig. 5.23). The actinolite is partially replaced by chlorite and biotite.
The mineralogy of the 8 metre wide, fine grained dark unfoliated inclusion in the
Hess Offset immediately west of the distal Foy intersection (Fig. 2.8), is similar to the foliated gneissic inclusions in the Hess Offset. In thin section the inclusion appears very similar to the gneissic breccia of the Maki Showing (Fig. 5.24, cf. Fig. 5.23). The matrix consists of annealed feldspar and secondary biotite and chlorite on actinolite, with only minor amounts of quartz. The actinolite shows much more extensive alteration to chlorite and biotite than those at the Maki Showing.
nie two well-rounded 50 cm in diameter inclusions hmthe central portion of the
Rivers Option (Fig. 2.13) are ophitic to sub-ophitic. They essentially comprise plagioclase and uralitised pymxene (Fig. 5.25). Occasional relicts of pyroxene are found within the cores Figure 5.22 - Felsic clast within the gneissic breccia inclusion. Quartz is completely recrystallised and the feldspar has a homfelsed texture. Crossed polars, field of view 2.5 mm. Sample 7- 1 74
Figure 5.23 - Ma& of gneissic breccia inclusion. Amphibole partialiy replaceci by biotite and homfelsed feldspar. Plane polarised light, field of view 2.5 mm. Sample 7- 174.
138 Figure 5.24 - Matrix of a large inclusion West of the distal Foy intersection with similar textures to the quartz diorite breccia inclusions at the Maki Showing. Upper left: hornfelsed feldspar, darker areas: secondary biotite and chlorite pseudomorphing actinolite. Plane polarised light, field of view 1-25 mm. Sample 7-2 1 8.
Figure 5.25 - Ophitic inclusion from the central portion of the Hess Offset at the Rivers Option. The pale area in the centre of the photo is where the pyroxene has ken completely altered to actinolite as seen by the cleavage. Darker areas to the left and Iower right are relict pyroxene which is oniy pareially reacted to actinolite. Plane polarised light, field of view 1.25 mm .Sample 7-93. 139 of actinolite crystals. Actinolite is partially replaced by chlonte. Their normative mineralogy shows that orthopyroxene predominates over clinopyroxene (Table 4.3a), so this was probably onginally a noritic, rather than a gabbroic, inclusion. Rocks with similar textures to the breccia inclusions at the FoyRIess intersection areas have ken found within the mbble pile at the Rivers Option. The small rectangular inclusion at the Rivers Option (Fig. 2.14) consists essentiaily of actinoiite and biotite, with minor amounts of titanite and epidote.
Small blebs which are essentially feldspar crystais (analysis 1, Table 5.8) with minor amounts of other accessory minerals (analysis 2, Table 5.8) occur with in the Hess Offset
(Fig.s 5.26 and 5.27). These range fkom a few millimetres to 2 cm in diameter (Fig. 5.26).
The high anorthite content of these plagioclase crystals implies that they were an early crystallizing phase of the Hess Offset, or are inclusions transported fiom the Main Mass of the SIC . In the field these can appear very similar to other blebs of feldspars within the granophyre, which are obviously late stage crystallisation products.
MINERALOGY OF THE fARALLEL FOY OFFSET
The cross cutting relationship of the Parallel Foy Offset to the Hess Offset and the more differentiated chemistry indicate that this dyke postdates the Hess Offset. The Paralle1
Foy Offset dyke is more equigranular and the proportions of granophyre significantly pater than in the Hess and Foy Offsets. Plagioclase tends to be more saussurhiseci, and sirnilar in appearance to plagioclase in the contact zone and central portions of Hess, though the degree of alteration is extremely variable even within one sample (Figs. 5.28 and 5.29). The cons
(analysis 3, Table 5.8) are only slightly more sodic than those in the Hess and Foy Offsets, Sample 1 2 3 4 5 6 7 8 wt % Si4 51.50 37.66 57.71 62.91 65.19 53.23 56.37 59.77 Ti& 0.02 0.01 0.07 0.03 0.04 0.05 0.04 0.08 A&0330.23 25.34 26.27 23.96 18.72 28.23 25.89 24.32 Cr,O, nd 0.15 0.02 0.01 0.01 0.02 nd 0.01 Fe4 nd 0.02 0.59 0.17 0.05 0.51 0.7 0.61 Mn0 0.53 9.95 nd nd nd nd0.06 nd Mg0 0.03 0.05 0.08 0.01 0.01 0.1 1 0.06 0.05 Ca0 13.1 1 23.74 8.68 4.75 0.08 12.09 8.53 6.48 Na20 4.43 nd 6.27 8.95 2.56 4.94 6.67 7.97 K,O 0.04 nd 0.51 0.07 12.74 0.1 0.13 0.16 Total 99.89 96.90 100.19 100.85 99.4 99.28 98.44 99.46 Stoichiometry Si 2.36 3.05 2.59 2.76 2.99 2.44 2.58 2.69 Ti 0.00 0.00 0.002 0.001 0,001 0.00 0.00 0.00 AI 1-63 2.42 1.39 1.24 1.01 1.52 1.40 12.88 Cr nd 0.01 0.0007 0.0003 0.0004 0.00 nd 0.00 Fe nd 0.00 0.02 0.006 0.002 0.02 0.03 0.02 Mn 0.02 0.68 nd nd nd nd 0.00 nd Mg 0.00 0.01 0.005 0.001 0.0007 0.01 0.00 0.00 Ca 0.64 2.06 0.42 0.22 0.004 0.59 0.42 0.31 Na 0.39 nd 0.55 0.76 0.23 0.44 0.59 0.69 K 0.00 nd 0.03 0.004 0.75 0.01 0.01 0.01 An 61.90 nc 42 22.5 0.4 57.1 41 30.7 Ab 37.80 nc 54.9 77 23.3 42.2 58.1 68.3 Or 0.20 nc 2.9 0.3 76.2 0.05 0.7 0.09
Table 5.8 - Microprobe analyses of plagioclase inclusions in Hess, feldspars hmthe Parallel Foy, phenocrysts and maüix in quartz diabase. Stoichiometry of plagioclase on the bais of eight oxygen, of piemontite on the basis of 13 oxygens. F4= total ùon as FeO. nd = not detectcd. nc = not calculated. Number in parentheses are the sample numbers. 1 = plagioclase in plagioclase rich inclusion, Hess Offset (6- 107); 2 = piemontite, main accessory phase in plagioclase rich inclusions (6-107); 3 = core plagioclase, Parallel Foy Offset (6- 428); 4 = rim plagioclase, Parallel Foy Offset (6-428);5 = orthoclase in granophyre, Parallel Foy Offset (6428); 6 =plagioclase phenocryst core, quartz diabase (643 1 b); 7 = plagioclase phenocryst rim, quartz diabase (6-43 1 b); 8 = plagioclase in matrix, quartz diabase (6- 122). Figure 5.26 - Small inclusion of feldspar crystals and rninor accessory minerals within the quartz diorite of the Hess Offset. Partially crossed polars, field of view 3.5 mm. Sample 6- 107.
Figure 5.27 - Decussate texture of the feldspar crystals within the inclusion, within the Hess quartz diorite. Crossed polars, field of view 0.3 125 mm. Sarnple 6-107. Figure 5.28 - Relatively unaltered Parallel Foy with saussuritised plagioclase and extensive granophyre development. Plane polarised light, field ofview 0.625 mm. Sample 6408.
Figure 5 .î9 - Altered Parallel Foy with patchy ferro-rnagnesio homblende anâ biotite. Plane polarised light, field of view 1 Zmm. Sample 6-428. whereas the rirns (analysis 4, Table 5.8) going into the gnuiophyre (analysis 5, Table 5.8) are highly sodic. In more extensively altered areas of the Parallel Foy Offset, plagioclase breaks dom completely to a mixture of epidote, albite and amphibole, as hau also been observed in highly altered samples of felsic norite (Oliver, 195 1; Fleet et al., 1987).
Amphiboles in the Parallel Foy Offset have aconsiderably lower Mg# compared with amphiboles in the Hess Offset. Other than this iron enrichment, apparently primary ferro- magnesio homblendes (analyses 1 and 2, Table 5.9) have similar compositions to those in the Hess Offset. They are typically found in association with 'primary' biotite, which also shows iron e~chmentcompared with the primary biotite of the Hess Offset (analyses 3 and
4, Table 5.9) (Fig. 5.28). The rims of the pnmary ferro-magnesio homblende show aluminium e~chment(analysis 2, Table 5.9) compared with the cores (analysis 1, Table
5.9). The Parallel Foy is pyroxene normative, but any pyroxene onginaily present has been completely altered to a patchwork of blue-green ferro-actinolitic homblende (analysis 5,
Table 5.9) and blue-green ferro-pargasite (analysis 6, Table 5.9) and biotite (Fig. 5.28). In thin section the appeatance of the Parallel Foy Offset is very similar to that of the more distal section of the Copper Cliff Offset in the South Range (Andy Bite personal communication,
Inco Exploration Ltd., 1997).
The Parallel Foy Offset, like the Hess Offset, shows varying degrees of alteration.
The more highly altered sampies of the Parallel Foy Offset contain no homblende, only patchy aggregates of biotite. The plagioclase is more extensively altered to a mixture of clear albite, epidote and hornblende. (Fig. 5.29). -- - Sample 1 2 3 4 5 6 7 8 9 Wt% Si4 42.8 42.39 35.34 37.22 47.9 40.07 53.23 44.05 44.35 Ti02 1.1 0.2 1.69 1.49 0.34 0.1 1 0.03 0.19 0.61 A1203 7.29 10.13 15.22 14.91 5.2 13.18 2.07 11.07 8.43 Cr203 0.07 0.04 0.01 0.01 0.04 0.02 0.16 0.3 0.03 FeO, 29.69 25.88 27.52 26.76 22.54 25.82 14.43 21.66 19.91 Mn0 0.55 0.47 0.27 0.32 0.42 0.41 0.29 0.35 0.22 Mg0 3.74 4.94 6.39 6.48 7.42 3.88 14.89 8.03 9.96 Ca0 10.06 1 1.55 nd 1.77 11.6 11.49 12.37 11.57 9.83 Na20 1.22 0.99 nd 0.09 0.54 1.31 0.26 1.18 0.68
Total 97.59 97.74 95.72 95.8 96.49 97.65 97.86 98.92 95.54
Table 5.9 -Microprobe analyses of amphibole and biotite fkom the Parallel Foy Offset and quartz diabase. Number in parentheses indicates number of oxygen used to calculate stoichiometry. FeO, = total iron as FeO. nd = not detected. nc = not caicuiated. Numba in parentheses (below) are the sample numbers. 1 & 2 = fm-homblende, Parallel Foy Ofkt (6-428); 3 & 4 = biotite, Parallel Foy Onset (6-428); 5 = altered homblende, fm-actinolitic homblende, Parallel Foy Offset (6-428); 6 = altered homblende, ferro-pargasitic actinolite, Paraiiei Foy Offset (6-428); 7 = actinolite pheoncryst, quartz diabase (6-122); 8 = ferro- homblende, quartz diabase matrix (6- 122); magnesio-homblende, quartz diabasc matrix (6- 122). QUARTZ DIABASE
In conmt to the quartz diorite, the quartz diabase phase of Hess is plagioclase porphyritic and relatively finer grained. Its normative mineralogy indicates that clinopyroxene dominates over orthopyroxene. It is typically extensively altered. The plagioclase phenocrysts are a charactenstic feature of quartz diabase (Andy Bite, personal communication Inco Exploration Ltd.) (Fig. 5.30). 'Ihese phenocrysts are of the same composition as, and show similar zoning to, the plagioclase crystal in the quartz diorite
(analyses 6 core, and 7 rim, Table 5.8) (Fig. 5.3 1). Crypto-crystalline inclusions occur at the compositional break. Rare phenocrysts of actinolite crystals rimmed by ferro-magnesio homblende also occur (malysis 7, Table S.9), otheMlise actinolite is absent from the quartz diabase. The groundmass consists of ferro-magnesio homblende (analyses 8 and 9, Table
S.9), plagioclase, biotite, quartz and oxides (Fig. 5.32). Plagioclase crystals in the groundmass are ofien extensively saussuritised compared with the plagioclase phenocrysts
(analysis 8, Table 5.8). The oxide content is much higher than that of the quartz diorite.
The mineralogy of the Hess Offset reveals that it is a variety of amphibole-biotite quartz diorite typical of many of the Offsets of the Sudbury structure (Pattison, 1979; Grant and Bite, 1984). This variant probably originated as a two-pyroxene-bearing magma that underwent deuteric aiteration during the later stages of its crystallization. The above indicates that the Hess dyke (and probably the Foy, Parailel Foy and quartz diabase dykes) originated as primary igneous assemblages comprising quartz + plagioclase + homblende + biotite + clinopyroxene + orthopyroxene. The later stages of crystallization and solid-state cooling Figure 5.30 - Two phases of quartz diabase chilled against the Hess Offset quartz diorite (nght hand side). The more coarsely crystallised quartz diabase in the centre of the photo contains disseminated sulfides, the more finely crystallised quartz diabase to the lefi is chilled against the coarse quartz diabase and is essentiaily sulfide Free. Plane polarised light, field of view 30 mm. Sample 7- 108.
Figure5.3 1 - Quartz diabase with porphyroblast ofplagioclase, which appears similar to the plagioclase crystals found in the quartz qiorite of the Hess Onset. The compositional break near the rim of the plagioclase crystal coiocides with the red-brown cryptocrystalline inclusions and the apparentiy acicular inclusions of epidote and piemontite in the centre of the plagioclasecrysta1. Plane polarised light, field of view 1.25 mm. Sample 6- 122. Figure 5.32 - Matrix of quartz diabase. Plagioclase crystals are extensively saussuritised and consist of patchy aggregates of homblende and biotite. Plane polarised light, field of view 1.25 mm. Sample 6- 122. involved the reaction of melt andor wall rock waters with the primas, assemblage to mate deuteric alteration products: oligoclase + epidote reaction rims on more calcic plagioclase, uralitization of clino- and orthopyroxene, and granophyric quartz-alkali feldspar intergrowths. The additional formation of actinolite, chlorite, secondary biotite and possibly titanite is probably due to a distinct post-impact, low-grade thermal event (or events), as yet unidentified in the North Range (though see Thornposn et ai., in press for further discussion). Chapter 6
CONSTRAINTS ON THE ORIGIN AND EMPLACEMENT OF THE
HESS OFFSET
The chemistry and mineralogy of the Hess confirrn that it is an Offset dyke relatcd to the SIC. Field relations show that it was emplaced into a major zone of deformation (a pseudotachylyte-bearing fault systern) that strikes subconcentrically with the northern marpin the SIC. There is more than one melt phase associated with Hess. The main phase filling the
Hess Offset is classified as quartz diorite. This is similar to the quartz diorite in other Offset dykes of the Sudbury structure. Variation in the alteration of mafic minerals is the main difference in mineralogy observed within the Hess quartz diorite. Relatively unaltered pyroxenes are only found at the eastem end of the Hess Offset. Granophyric quartz diorite is associated with the central portions of the Hess Offset and has been found in sharp contact with nonnal quartz diorite. Quartz diabase has been found as concordant dykes within the
Hess Offset; it thus appears to pst-date the main dyke phase. The Foy - Hess Offset dykes are indistinguishable in the areas where the two dykes intersect.
DYKE EMPLACEMENT
The Offset dykes of the Sudbury structure are so named for the en echelon pattern observed in some of these dykes. Some of the segmentation of the dykes is caused by pst- emplacement faulting. Where no such faulting is evident the dyke segments have smooth lobate ends (Cochrane, 1984) and segmentation is due to primary emplacement. En echelon patterns of dyke segments are thought to be near-surface manifestations of a continuous dyke at depth. At shallow depths changes in the stress field or the host rock rheology, cm cause the rotation of the dyke segments (Pollard, 1987).
Dyke geometry is related to the orientation of the local and regional stress state of the rock into which the dyke is being intruded (Emst et a1.,1995). A large overpressurised reservoir of magma will produce a localised stress state around itself which decreases with increased distance fiom the reservoir. Close to the reservoir, where the stress caused by the pressure in the magma charnber will be the strongest, dyke emplacement will be controlled by this local stress field and will typically produce a characteristic radial pattern (Emst et al.,
1995). At the dacethe local stress field can differ from the stress field at depth. The dyke may rotate to align itself with this local stress field, which causes dyke segmentation and an en echelon surface pattern (Pollard, 1987). As the dykes propagate farther outwards away fiom the chamber the magnitude of the local stress perturbation due to the pressurisecl chamber decreases and, therefore, the relative importance of any regional differential stress field increases. If the difierential stresses are minimal, dykes may propagate in any direction, perhaps determineci by the characteristics of the wall rocks (Delaney, 1986). However, if the regional stress field is rnuch greater and differs fiom the stress field caused by the pressurised magma chamber, the süike of the dyke will change to realign itself with the regional stress field (Ernst et al., 1995).
The change in strike of the very distal part of the Foy Offset may be due to
151 realignrnent of the dyke with the regional stress field as at this distance fiom the SIC the local stresses caused by the SIC will be minimal. The sinistd displacement of the Hess
Offset by the Sandcherry Creek and the Fecunis Lake faults of the Onaping system are clearly due to post-emplacement movernent on these faults, as they displace the SIC with same sense. In other areas where the Hess Offset is displaced it is ambiguous as to whether the displacements are due to pst-emplacement faulting or to primary emplacement mechanisms. The sinistral displacement of Hess, at the Harty showing (Fig. 2.19) and to the
NE of Pterodactyl Lake (Fig. 1.4), both occur along narrow swarnp filled valleys. The orientations of the valleys are similar to the regional fault trend of the Timiskarning System, which normal faults with an apparent sinistral component of displacement. However, at the
Harty Showing there is extensive development of pseudotachylyte along the walls of the valley that separates the two sections of the fault, which rnay indicate this displacement is related to primary emplacement mechanisms. Changes in rheology have clearly affecteci
Hess in some areas. These include Dehydration Ridge and the eastem end of the in the Harty showing, where the dyke is displaced or narrows where it cuis across an earlier Matachewan dyke. On the eastem end of the South western section at the Harty showing there is a sirnilar jog or segmentation of the Hess Offset. Aithough there are no earlier dykes in the area, this could also be due to local perturbations in the stress field. The strike of the distal and proximal sections of the Foy Offset are distinctly different, which rnay indicate that then is a change in the local stress field where these intersect Hess.
Systematic bifurcation (that is, preferential splitting of dykes in one direction) can be used as a flow direction indicator (Rickwood, 1990). Such bifitrcatiom do exist in the
lS2 Hess Offset, these include the NW side of Pterodactyl Lake (Fig 2. l), Dehydration Ridge
(Fig. 2.16) and possibly NW of Padlel Foy intersection (though lack of outcrop here, make it unclear if this is a bifùrcation) (Fig. 1.4). Unfortunately the lack of consistent bifurcations in Hess make any inference on flow direction for the Hess Offset invalid.
TIMING OF OFFSET EMPLACEMENT
If the emplacement of the Offset dykes was vimially instantaneous following impact, as has been suggested by physical models (Melosh, 1989), then the Offset dyke melts would be expected to be representative of the original bulk impact melt composition. However, instantaneous collapse does not explain how many of the Offset dykes are mineraliseci, nor does it account for their hctionated REE patterns with negative Eu-anomalies, when nonnaliseci to the North Range felsic norite (Fig. 4.6 and 4.7). Time would be required to crystallise and settle out Ni-Cu sulfides fiom the melt sheet in order for thern to accumulate and then be transporteci with the quartz diorite melt into footwall fault systerns. Nor does instantaneous collapse explain how norite blocks have been incorporated into certain Offset dykes (e.g., at Whistle Offset (Lightfoot et al., 1997a); and possibly in the Hess Offset at the
Rivers Option): Again, time would be required to crystallise out components of the SIC Main
Mass prior to Offset dyke emplacement. These obsexvations indicate that transient cavity collapse and gravitational modification of the mter may have taken place over a more protracted wodmaps hundreds of thousands of yem) and not immediately followlng impact or that dyke emplacement is due to ooier processes such as tectonic stress or isostatic rebound. In this situation, rather than being repmentative of the parental impact melt, the
153 Offset dykes ernbody more evolved melt compositions derived afier the hctionation of the
Main Mass. In their themal modelling of the cooling history of the SIC, Ivanov and Deutsch
(1 997) calculate that the melt sheet would have reached its liquidus - 100 ka after impact, and its solidus -300 ka after impact. This suggests that the Offset dykes were ernplaced 100-300 ka after impact.
Most of the Offset dykes are very similar in composition, particularly in reference to the trace elements and REE (Fig 4.6). The only Offsets that differ significantly are the
Parallel Foy, the distai Copper Cliff and the Creighton. The Creighton embayment is not a hue Offset dyke, though it does contain quartz diorite, and is strongly mineralised and bears a greater resemblance to the mineralised sublayer than the Main Mass melt (Lightfoot et al.,
1997). Thus, similar chernistries and mineralogies of the Offset dykes imply that the must have been emplacement more or less simultaneously.
The granophyric central portion of the Hess Offset may represent remobilisation of the late stage crystallisation products, or further injection of more magma, due to readjustrnent of the mater or dyke. Lack of a chilled margin implies that normal quartz diorite had not completely cooled before the granophyric quartz diorite was intnided. The similarity of the granophyric phase in sharp contact with normal quartz diode, with the central granophyre-rich portion of the Offset, which has gradational contacts with normal quartz diorite, and the lack of any chilled margin, would tend to indicate remobilisation of late melt phase, which is high in water and SiO, content. Further later stage readjustments may have given rise to the intrusion of the Parallel Foy Onset and the quartz diabase dykes, which are both related to the Main Mass of the SIC, but are more hctionated that the quartz
154 diorite.
The quartz diabase appears to represent a separate later SIC-related intrusive phase, as its rnargins are chilled against the quartz diorite of the Hess Offset. The quartz diabase dyke at Dehydraton Ridge appears to be a concordant to the main quartz diorite phase of the
Hess Offset. The quartz diabase dykes in the South Range cut across the quartz diorite of the
Offsets and have relatively consistent easterly trends (Grant and Bite, 1984). Due to the lack of exposure of quartz diabase dykes in the Hess Offset area, it is unclear whether the quartz diabase dykes are all intruded concordantly into the Hess Onset, or if some cut across it. The quartz diabase dykes may be related to the gabbroic sills that occur between the contact of the Onwatin and Chelsford Formations of the Whitewater group, but due to an almost complete lack of published information on these gabbro bodies, it was not possible to investigate this connection any further, but it is indeed an intriguing possibility. 1s it coincidental that the quartz diabase is seen in the area of greatest mineralisation? Intrusion of the quartz diabase implies readjustments of the Crater after the emplacement of the main phase of the Offset dykes, thus, allowing for further melt injection into the region of the Hess
Offset. The Parallel Foy Offset also represents a later intrusion of melt that was emplaced afier the main phase of Hess, as it is more hctionated than and crosscuts the Hess Offset.
There are distinct di fference in the major element chemistry between the Parallel Foy and the quartz diabase dykes, implying that these have separate sources, although their timing could be coeval.
Initiation of dykes is not well understood, but a melt pressure higher than the lest compressive stress is needed for dyke intrusion (Stevenson, 1989; Sleep, 1985). Fracturing
155 and faulting effects will initially propagate outward fiom the impact site (Melosh, 1989).
There must be huge pressure gradients present to facilitate intrusion of melt into these fiachues. The radial pattern of the radial dykes adjacent to the SIC implies the least compressive stress is also radial to the SIC, or the melt pressure at the source (the SIC) is greater than the regional stresses. If the Offset dykes are not formeci in the initial collapse of the transient cavity an initiation mechanism for dyke emplacement is requited. This could be due to Mermodification of the Crater due to isostatic rebound (Wichman and Schultz.
1993), and/or tectonic loading by Penokean thnist sheets (Deutsch et al., 1995; Thompson et al., in press). It has been suggest that there could be a small component of a mantle- derived pircrite magma involved in the melt generation within the Sudbury structure
(Lightfoot et al., 199%). Upwelling of mantle-derived pricnte magmas could give rise to doming and extension of the cmst, thus, increasing the pressure in the melt chamber and giving rise to a mechanism for dyke emplacement.
MINEUOGY OF THE HESS OFFSET
The Hess Offset can be classified as a pyroxene or amphibole-biotite quartz diorite typical of the Sudbury structure. The Offset dykes typicall y consist of plagioclase, granophyric intergrowths ofquartz and orihoclase, biotite, and either amphibole or pymxene as the main mafic phase. Only rare examples of relatively unaltered pyroxenes are found in the Hess Offset, these are found in the vicinity of the proximal Foy intersection and West of the Parallel Foy Offset. Typically, the main mafic phase in Hess quartz diorite is actinolite.
Actinolite itself is generally partially to completely teplaccd by chlorite and biotite. Primary
156 hornblende occurs on the rims of pyroxenes or its pseudomorph. Minor amounts of blue- green homblende typically also occur on the rims ofpyroxene or its pseudomorphs, or on the rim of the brown-green homblende.
The variation in mineralogy within the Hess Offset could be due to metasomatic alteration or regional metamorphism. The variation in amphibole chemistry seen with in the
Offset dykes has generally been attributed to regional metamorphism (Fleet et al., 1985;
Thomson et al., 1984). However, this work reveais a patchy and highly variable alteration state along the length of Hess. This would support deuteric alteration rather than regional metamorphism as the source of mineralogical variation. The prevalence of biotite in quartz dionte in contact with the host rocks is attributed to assimilation of the host rock (Grant and
Bite, 1984).
METAMORPHISM OF THE SUDBURY STRUCTURE
The mineral assemblages seen in the Offset dykes are not uniquely characteristic of metamorphism but may also be formed by deuteric or hydrothermal processes. Oxygen- isotope data are consistent with either a local hydrothermal metamorphism or a regional metamorphic overprint (Thomson et al., 1985). The composition of the calcic amphiboles in the SIC is considered to be an important indicator of metamorphic grade (Thomson et al.,
1985; Fleet et al., 1987). The presence of alminous (tschmakitic) blue-green calcic amphibole in the North Range Offset dykes has been considered indicative of epidote amphibolite facies or medium-grade conditions (Fleet et al., 1987). Previous workers have assumed that the mineral assemblages observed in the Offset dykes were indicative of
157 regional metamorphism, due to the apparent prograde nature of the metamorphism
(alminous blue-green amphibole after actinolite) and the similarity of compositionally equivaient mineral assembiages in the adjacent host rocks of the Huronain Supergroup
(Thomson et al., 1985). Local variations in mineralogy were ascribeà to the low permeability of the mafic rocks, such that metamorphism was not pervasive and so gave rise to patchy alteration of the rocks (Fleet et al., 1987).
Metamorphism of the Sudbury region has generally been attributed a single metamorphic event: the Penokean Omgeny, which is considered responsible for metamorphosing the South Range to low amphibolite facies, with metamorphism fading to the North (Fleet et al., 1987; Dressler l984a; Card, 1992). Recent work has demonstrated that there were at least two separate metamorphic events: the pre-impact Blezardian tectonic pulse and the pst-impact Penokean orogeny (Riller and Schwerdtner, 1997). In the North
Range there is little direct evidence for a major pst-impact metamorphic event, it appears to have only been affected by low grade, sub-greenschist facies metamorphism (Deutsch.
1994; Thompson et al., in press). In light of this it seerns probable that the mineral assemblages observeci in the Hess Offset are the product of deuteric or hydrothermal alteration related to the progressive cooling of the melt.
0ththan the Penokean Orogeny, there are several possible mechanisms for very low grade regional metamorphism. These include at least two phases of anorgenic @tic magmatism concentratai to the south of the Sudbury structure at 1750-1 700 Ma and 1500-
1450 Ma (Card, 1992; Davidson et al., 1992). Albitisation of rocks east of the SIC has been linked to this pluto~mi(Schandl et al., 1994). The Onaping Formation has undergone basin-
158 wide hydrothemial alteration as seen in vertically stacked, serniconformable alteration zones in which the rocks have undergone, hmbase to top, silicification, albitization, chloritzation, carbonatization and, in the uppermost zone, complex feldspathization (Ames et al., 1997).
U-Pb dating of hydrothennal titanite within these rocks constrains the timing of the hydrothermal system responsible for the alteration of the Onaping rocks to 1848+3.8/-1.8 Ma
(Ames et al ., 1998).
The significant dehydration dong the matgins of the fault systems that accompanies pseudotachylyte formation is another potential source of fluids involved in the metasomatism of Hess. The later intrusion of the quartz diabase into the Hess Offset may also have conûibuted to the aiteration of Hess.
PROPOSED CRYSTALLISATlON SEQUENCE FOR HESS QUARTZ DIORITE
Textuml relationships show that plagioclase wes the first minerai to crystallise.
ûrthopyroxene generally forms euhedral lath-like mystals but can be subophitic around plagioclase. Chopyroxene crystallises later than the orthopyroxene, as can be seen by the texturai relationships (Photo 5.1) and the difference in Mg# between ortho- and clino- pyroxene (Fig. 5.1 1). Ferro-magnesio homblende and euhedral biotite occur on the rims of the pyroxene crystals and textural relationships indicate that these are primary. The last main phases to crystallise are granophyric intergrowths of quartz and feldspar. There is a sharp compositional break, near the rim of plagioclase crystals, with the rims being more sodic than the cores. A reddish bmcolouring of the plagioclase coincides with the oligoclase- andesine compositional break, the colouring has bem attributed to the presence of numemus
159 cryptonystalline inclusions within the nystal (Smith and Brown, 1988). It has been suggested that H20reacts with the calcic plagioclase to produce albite, an epidote-group phase, quartz and A1,Q (Naldrett et al., 1970). The breakdown products are favoured by increasing water pressure and decreasing temperature. During late stage cooling, when the water content is high, the plagioclase begins to break down to more sodic plagioclase and in doing so releases Ca0 and Al,03. Excess Al fiom the breakdown of plagioclase may be utilised in the production of more aluminous homblendes as see on the rims of the ferro- magnesio homblendes. The excess Ca fiom the breakdown of plagioclase crystals was probably used in the production of epidote-gmup minerais. Some Ca may have also been used in the hydration of orthopyroxene to actinolite. Where insufficient water was present for the cornplete hydration of pyroxene to actinolite, the excess Ca crystallised as intersertial calcite, as observed West of the proximal Foy intersection (Photo 5.12). Breakdown of actinolite to chlorite and biotite would also require addition of water Al and K, and would produce Ca. Excess Ca may have been consumed in the crystallisation of titanite andlor late stage homblende. Some elernents consumed in these late stage alteration possesses could have been introduced with migrating waters invading the dyke fiom the wail rock.
In the central, mon granophyric portions of the Hess Offset, the development of blue- green aluminium rich amphibole is more extensive and plagioclase crystals typically show more extensive development of the red-bmwn colouration as well as being more saussuritised. The slower cooling would concentrate the volatiles, including &O, in the central portions of the Offset. The H,O could extensively alter the early crystallising stages
such as plagioclase, pyroxene and amphibole. The reactions of these early mind phases
160 with the volatiles will be more extensive than in more marginal phases of the quartz dionte, as they have more time equilibrate. The late crystallising phases, such as the quartz and orthoclase would also be concentrated in the central portions of the Offset and, in the final stages of crystallisation, these would crystallise rapidly in the presence of H,O to form the granophyre.
ARGUMENT FOR HYDROTHERMAL OR DEUTERIC ALTERATION
Near the contacts with the wall rocks the reddish-brown clouding in plagioclase crystals and the breakdown of amphibole to biotite and chlorite are typically found to be more extensive. This may indicate greater accessibility to fluids fiom the host rock, as well as evidence for K-metasomatism. This implies that water fkom the host rocks did enter the dyke, at least in the contact region, and that this water introduced ceain eiements (e.g. K).
The most extensively aitered sample of quartz diorite fiom the Hess Offset was found sunoundeci by pseudotachylyte on the Northem Shore of Pterodactyl Lake. Field relationships and the REE chemistry indicate that this is quartz dionte, even hou& the major element chernistry and its mineralogy are quite distinct nom other samples of quartz diorite. The fact that this is surrounded by significantly more pseudotachylyte than other samples of the Hess Offset may indicate that the pseudotachylyte maybe the source of the fluids responsible for its extensive alteration.
The Parallel Foy and the quartz diabase dykes both show extensive breakdown of the mafic phases essentiaily to homblende and biotite, although both are pyroxene normative
(set Table 4.4a). The clouding and alteration of plagioclase crystals is also more extensive
161 in both the quartz diabase dykes the Parallel Foy Offset. As these are later in fkactionates of the SIC the melt may initially have contained more water for deuteric alteration. The Parallel
Foy has normative amite (NaFeSi,O,). Alteration of this could give paragasite, showing the
Na-rich nature of the Paralle1 Foy, aithough it is unclear if the high sodium content is a primary feature of the Parallel Foy Offset or it is due to Na-metasomatisrn similar to that seen in the Hess quartz dionte in the Pterodactyl Lake area. The rims of fmo-magnesio homblende in the Parallel Foy Offset show enrichment in Al, similar to that seen in the main phase of the Hess Offset, so it is probable that the Parallel Foy (and quartz diabase dykes) underwent similar deuteric or hydrothermal alteration as did the Hess Offset.
VARIATIONS IN CHEMISTRY
The geochemistry of the eastem and western ends of the Hess Offset are distinct.
Some of the variation in geochemistry along the length of the dyke maybe a feature of partial assimilation of inclusions entrained within the quartz dionte. At both the Rivers Option and the Foy intersections the differences in both major and trace elements of the quartz diorite partially reflect the chemistry of the inclusions seen in these areas (Table 4. la and b, and 4.2a and b). Partial assimilation of the host rocks in the contact regions has occdas well and the ingress of volitiles has altered the composition of the quartz diorite near the contacts.
Some variations in the geochemistry of the quartz diorite are probably dependant on the degree of metasomatic alteration. MODELS FOR EMPLACEMENT
Figure 6.1 shows a number of models constructeci to explain the existence of Hess.
Figure 6.1 does not allude to the timing of Offset dyke emplacement relative to impact, melt sheet formation and crystallization. Figure 6.1 a illustrates the simplest scenario: hctures in the footwall beneath the SIC are merely filled gravitationally by melt. This seems unlikely as a gninodioritic melt is less dense than the host rock; dyke emplacement was more probably laterai or vertical (upward) (Rubin, 1995). Hess could have also benfed passively via the Foy Offset (Fig. 6.lb). Since the SIC is assumeci to essentially be an unbuffered magma chamber, (i.e. the pressure in the chamber will decrease during dyke emplacement
as the melt is lost to the dykes and is not being replaced) (Ernst et al, 1995), sufficient
pressure to form the Offsets by 'passive' emplacement seerns unlikely. Further wuntering these fint two models is the presence of pseudotachylyte along the margins of the Hess and
Foy Offsets (and most of the Sudbury Offsets). Hess thus occurs along a major zone of disruption and deformation coincident with what can be marked changes in lithology either
side of this zone. The pressure in the melt charnber (the SIC) would have to be significantly
higher than the regional stress for such long radial and concentric dykes to be formed
simultaneously.
Figure 6. lc shows the Hess Offset lying in the plane of a superfault that facilitated
transient cavity coilapse and tenace fornation. This mode1 accounts for the association of
Offset dykes with pseudotachylyte, whereby a large, single-slip displacement (possibly
reaching kms) produces copious arnounts of fnction melt dong the fault walls (Spray and
Thompson, 1995; Spray, 1997). The collapsing wedge plunges into the SIC melt sheet,
163 - --- 1 SIC
Figure 6.1 - Idealised block diagnuns representing possible modes of emplacement for the Hess Offset dyke: (a) downward injection of SIC melt into footwall Eractures (note present erosion level); (b) lateral injection of SM: melt into concentric footwail fracture via the radial Foy Offset fracture; (c) collapse of transient cavity wall into SIC Melt Sheet via superfaulting to form slumped tenace. SIC melt back-injected up superfault zone; (d) variant of (c) with graben generated by antithetic faulting; (e) formation of concentric horst (? Peak ring) between SIC and Hess, with Hess Occupying horst's northem fault system and king fed by the proximal Foy via breached horst. Approximate width of horst is IS km. Al1 blocks in figure are of comparable size. raising the pressure of the SIC, resulting in the injection of melt back up the relatively weaidpermeable fault zone. Figure 6. Id is a variant of 6. lc, showing additional antithetic faulting which results in a graben structure developing between the superfault scarp and main ternice. This would explain the prevalence of Proterozoic Huronian Supergroup rocks to the north of Hess (in the graben), with predominantly Archean granitoids and gneisses being exposed to the south (relative horst). Neither Figure 6.1 c nor 6. ld requires the feeding of
Hess via the Foy Offset. Instead, Hess would have been fed from beneath via a listric supdadt that intersects the basal regions of the SIC melt body. Simultaneous emplacement of the radial Offset is also compatible with tliis model, as both the radial and concentric dykes are the product of radially outward flow from the SIC. Figure 6. le creates the same lithological distribution, but via the genenition of an annular horst between the SIC Main
Mass and Hess. Such a horst could be the expression of a peak ring structure, a fature comrnon in larger impact basins (e.g., Melosh, 1989). In this model the proximal Foy penetrates the horst via a fault/hcture breach and feeds the Hess Offset through the horst's northem P~uItsystem. Large, rapid displacements would be required on the horst to generate pseudotachylyte, but this would be compatible with rebound and various isostatic adjustments following hypervelocity impact (e.g ., Wichman and Schultz, 1993). This would readily explain the concentration of inclusions in the Hess Offset, similar to those in the Foy
Offset, around the Hess-Foy intersections.
Of the five models presented, Figures 6. ld and e are preferred as these best fit the geological setting of Hess. The above are simple models and they could acntally be a combination of two or more of these. One such scenario could be the initial injection of the
165 radial Offsets (e.g. the Foy) leading to a decrease in volume of the melt chamber, such that inward collapse could occur, similar to the collapse of a caldera andor to models 6.1 c and d. Further pseudotachylyte could be produced on the concentric hcture fomed by the collapse (Spray, 1997). The inward collapse of the crater walls would increase the pressure of the SIC causing futther expulsion of magmas dong the fractures produced in the inward collapse of the crater (such as 6. ld in the west and 6. le in the east which might also account for the geochemical differences between the east and West ends of Hess).
SUGGESTED FUTURE WORK
Boulders of Hess quartz diorite have been found both to the east and west of the known extent of the Hess OfEset dyke suggesting that the dyke may continue both east and west of those parts of the dyke mapped during this work. To the west, the Hess Offset probably continues through the nmow arm at the southwestern end of clear Lake.
Unfortunately, the shore of the lake is boulder strewn and there are numerous faults in the area. At the most easterly known exposure of Hess there is a large boulder-strewn area extending about 1 km east of this outcrop. Further field work would be difficult and might be futile, but geophysical methods might prove useful. On satellite images of the North
Range it is not dificult to imagine that the Hess Offset does continue to the West and east, possibly joining up with the Ministic adorWhistle-Parkin Offsets, respectively.
The difference in major and trace element chemistry between the eastern and westem ends of the Hess Offset is intriguing. These could be primary features or could be due to the assimilation of inclusions within the dyke, to late-stage deuteric, hydrothermal or metamorphic alteration or a combination of the above. The differences in the geochemistry partially reflect the geochemistry of the inclusions entrained within the Offset, although it is not clear if this is the main controlling factor. More analyses spaced along the length of the Offset rnay help to determine whether or not the change in chemistry fiom east to west is gradational or sharp. Subsequent intrusion of the quartz diabase dykes rnay also have affected the chemistry of the Hess Offset. A detailed study across the width of the Hess
Offset in the Dehydration Ridge area, where a quartz diabase dyke cuts through the quartz diorite may help to determine the degree of alteration due to the quartz diabase intrusion.
More detailed study of the footwall-like breccia inclusions in the Foy-Hess
Intersection area would be beneficial to establishing whether these are linked to the leucocratic footwall-like breccia filling the Foy Offset Mersouth, and to the main footwall breccia at the base of the Main Mass. The origin of the footwall-like breccia entrained in the
Hess Offset is unclear. These inclusions may have been created in situ (as has been suggested for the footwall breccia occurring around the base of the SIC, Deutsch et al., 1989), or they may have originated in the mater and were then transported out to their cwrent location,
either as a dyke in the Foy Offset prior to the emplacement of the quartz diorite or
transported with the quartz diorite. More detailed studies of both the matrix and the clasts
entrained in these footwall-like breccia inciusions could resolve this.
The gneissic inclusion seen in the Hess Offset and in the footwall-like breccia are
similar in appearance to large non-foliated inclusions in Hess and the gneissic breccia (quartz
diorite breccia, Lightfmt et al., 1997a) of the Maki showing in the Foy Offset. A cornparison
167 of these inclusions would establish whether they were related and may help to detennine their ongin and also the ongin of the footwall-like breccia inclusions in which they are often entrained. There are no known outcrop of gneiss in the irnmediate vicinity. Possible sources for these inclusions are the Levack gneisses, which may also occur at depth below the granites (Milkereit et al., 1994), and +ich actcr to the smth around t!x SIC.
Ai-otropy ofmagnetic susceptibility (AMS) has been successfhlly used to determine the flow directions of dykes (e.g., Ernst and Baragar, 1992). Similar work could elucidate the flow direction of the quartz diorite of the Hess Offset and to test that it is consistent dong the length of the dyke. The flow direction(s) in the area of the Hess-Foy intersections would be of particular interest, as this should indicate whether or not the Hess was fed by the Foy
Offset. AMS of the footwall-breccia like inclusions in the Hess and Foy Offset and the footwail breccia filling the Foy Offset firther to the south could ascertain whether these were emplaced as a dyke pnor to the emplacement of the quartz dionte, or if they were carried into their present position by the quartz diorite.
Whole rock and mineral separate isotope data may help to detmine the metarnorphic history of the Hess Offset, as would more detailed work on the regional metamorphism of the North Range of the Sudbury structure.
Very little has been published on the quartz diabase dykes related to the Sudbury structure. More detailed mineralogy and geochemistry of the quartz diabase dykes in both the North and South Ranges and how these compare to each other would be helpfùl. Also, the metagabbro in the Whitewater Gmup needs to be studied to detennine whether or not there is any relationship between the metagabbros and the quartz diabase dykes. ûther than
168 in-house Inco Exploration reports, nothing hm previously been published on the Paralle! Foy
Offset. In this work this dyke was only examineci in the area irnmediately adjacent to the
Hess Offset. No detailed work was done on this dyke to try to determine the full extent of the body, or whether or not it joins the Main Mass of the SIC to the south and whether or not it contains any other phases or inclusions.
SUMMARY
(1) The Hess Offset dyke is at least 23 km in length and up to 60 m wide. It is the most distal concenhic Offset dyke so far known. It occurs 12- 15 km north of the SIC and süikes subconcenüically with the SIC'S norihm margin. It intersects the Foy Offset and continues east of Foy for at least 0.5 km.
(2) nie Hess Offset dyke was ernplaced into a fractured, cornminuteci and pseudotachylyte-bearing zone of defomation that marks the northem edge of a fault system that strikes subconcentrically to the SIC.
(3) Major, trace and REE data indicate that Hess is genetically related to the SIC.
Senru stricto, Hess is a quartz monzodiorite to granodiorite, but the bulk of Hess can be classified as quartz diorite typical of the Offset dykes as defineû by geologists working in the Sudbury basin. Hess shows strong similarities with the felsic norite of the Main Mass.
This composition lies between that of the norite and granophyre of the Main Mass, indicating that Hess (and al1 Offsets) is representative of a more evolved (plagioclase-hctionated) derivative of the bulk melt sheet. Later phases of Hess include quartz diabase.
(4) The Hess quartz diorite in the vicinity of the Foy Offset dyke intersection is
169 indistinguishable fiom Foy quartz diorite. The proximal Foy probably fed Hess, and Hess, in tum, probably fed the distal Foy. Hess widens in the vicinity of Foy and is more inclusion- nch in the intersection area. The narrowing of Hess beyond the Foy may have created a
'bottleneck', thus trapping larger (> 10 m) inclusions in the intersection area.
(5) The presence of large footwall-like breccia inclusions with igneous-textwed matrices and gnessic inclusions in the Foy-Hess intersection area (as well as to both the west and east ofboth intersections) and the similarity of these inclusions to inclusions seen in the
Hess Offset implies at least some transport fiom the Foy Offset.
(6) The Hess quartz diorite comprises a quartz + plagioclase + homblende + biotite
-+ clinopyroxene + orthopyroxene assemblage that chssifies it as either pyroxene or amphibole-biotite quartz dionte variant. The original assemUage may well have comprised significant amounts of clinopyroxene and orthopyroxene, but ksephases were usurped by
homblende and biotite during late (deuteric) stages of crystallization. Hot rock - fluid
interaction is also considered responsible for the development of granophyric intergrowths
of quartz and alkali feldspar, and for the clouding of plagioclase rims (with epidote and more
albitic plagioclase replacing labradorite). Subsequent low gracle metamorphism due to the
Penokean Orogeny, and/or hydrothennal alteration due to the ingress of fluids from the host
rocks, produced secondary biotite, actinolite, epidote, titanite and chlorite.
(7) The Hess Offset dyke originated during the modification stage of impact basin
formation, due to either (a) transient cavity collapse via listric superfaulting, with resultant
back-injection of SIC melt up the fault zone (Fig. 6. ld); or (b) annular horst @eak ring)
formation during rebound, with the Hess Offset dyke injecting the horst's northem fault
170 system via the Foy Offset (Fig. 6. le).
(8) The evolved nature of the source melt for Hess indicates that its injection did not occur irnmediately ahimpact, but up to 300 ka later.
(9) As work on Hess has dernonstnited, there rernains considerable potential for extending known Offset dykes and even discovering new ones within the Sudbury impact structure. Detailed field work rernains the key to realizing this potential. REFERENCES -Ames D. E. and Gibson H. L. (1997)Crater-fil1 sequence, Onaping Formation: Petrographic and Geochemical Signature of regional Hydrothennal alteration. in Conference on Large Meteorite Impacts and Piunetory Evolution (Sudbury 1997), pp. 1 -2. LPI contribution No. 922, Lunar and Planetary Institute, Houston.
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WHOLE ROCK GEOCHEMICAL ANALYSIS
Major element analysis was made by X-ray fluorescence at both UNB and at
NSERC's Regional Analytical Facility at Memorial University, Newfoundland. Al1 trace elements analysis was obtained by XRF at Memorial University, and rare earth element analysis was made by ICP-MSat Memorial University.
Sample Preparation
The samples were first cut into small fragments, whose weathered rims were removed, and a steel jaw cnisher was used to produce smaller fragments of each sample. The finest fraction was discarded before powdering to eliminate any iron and cross-sample contamination fkom the jaws. This process was repeated until the pieces were small enough to fit into the tungsten carbide shatter box, in which they were ground for a period of approximately two minutes to produce a fine powder. The shatterbox was cleaned with silica sand and was thorough washed between each grinding.
X-RAY FLUORESCENCE (XRF) PREPARATION AND ANALYTICAL TECHNIQUES
Quartz diorite samples Hl, H2, H8, Hg, H 10, Hl 3, Hl 6 ami Hl7, the two Nipissing diabase sarnples (NIPI and NiPZ), and one of the fmtwall-like breccia sarnples (FBX4), were analysed by X-ray flurescence (XRF) spectrometry, using a modified Phillips PW 1410 spectrometer at UNB. Major element analysis was performed on fused borate glass disks prepared according to a recipe revised after Noms and Hutton (1969). The disks were analysed at 10 mA and 40 kV br major elements. The raw data was reduced using linear regression calibration lines prepared using 20 international standards (Abbey 1983). Losses on ignition were also determined at UNB.
Accuracy and Procision
Sample were analysed in one run in the spring of 1997. The accuracy was determined fiom the deviation of the oxide concentrations measured in the standards fiom the accepted values of Govindaraju (1 994). These are presented in Table 1A. 1 bclow:
Standard gBS-688 BR BCR-1 BHVO Prec Acc Prec Acc Prec Prec SiO2 O. 13 2.13 0.09 6.90 0.39 A120, 0.15 0.86 0.00 0.93 1.31 TiO, 0.00 2.50 0.00 3.85 3.54 Fe203 0.07 1.43 0.00 0.70 2.90 Mg0 0.2 1 7.69 0.08 0.83 5 A6 Mn0 0.00 4.38 0.00 10.00 5 S6 Ca0 0.00 2.29 0.0 1 0.22 O .43 Ni0 0.23 6.32 0.04 4.75 1.52 K20 0.00 3.89 0.0 1 3.21 0.00 P,O, 0.01 10.67 0.01 15.87 13.89
Table lA.l Prescision and accuracy of XRF analysis of known standards at the
University ofNew Brunswick for four different standards. Accuracy is the percentage
deviation of the average analyses of the standards fiom the composition as reported
by Govinjaru (1984). The precision of the analyses was determine by triplicate analysis of standards (20). A standard was also analysed with each run of samples to determine accuracy of that run.
XRF Major Element Disk Preparation
Following the crushing, the samples were dried for at least two hours at 1 10°C. Sample weight losses were not calculated during this heating (to yield H2OV),as H,O content reveals more about the ambient humidity than anything inherent to the sample.
Disks were prepared using the following recipe:
7.000 * 0.005 g of Li#,O,
0.500 * 0.002 g of La,O,
0.500 a 0.005 g of ammonium nitrate mH4N03)
1.000 * 0.002 g of sample
Pnor to preparation of the fused disks, lithium tetraborate (Li,B,O,) and lanthanurn oxide
(&O,) were preheated for at least six hours at 500°C and 850°C, respectively. Each sarnple was homogenized thoroughly and glas disks fired in Pt-Au ailoy crucibles at - 1O00 OC in a Claisse fluxer. During heating the Claisse fluxer was, 10 minutes stationary; 5 minutes gyrating 10 minutes stationary. The dioks were then poured and lefi for a further 3 minutes stationary to encourage annealing. Glass disks were stored in a desiccator.
Loss on Ignition
Loss on ignitions were determined by calculating the difference in weight of approximately 0.5 g of sarnple (acnial weight was determineci accurately) after heating in a mufne hacefor one and a half hours at 1100°C, to drive off al1 structuraily bond water-
Sample crucibles were fmt heated at 1100°C in a rnufflefwnace until their weights remaineci constant. Loss on ignition was calculated for each sample as a percent of the difference in weight of the total weight of the sample. Al1 remaining samples quartz dionte samples
(H3, H4, HS, H6, H7, H 1 1, H 1 2, H 14), the iwo quartz diabase samples (QDB1 and QDB2), the remaining footwall-like breccia samples (FBX2, FBX3 and FBX4), and the gneissic inclusion (GN) analyses for major elements were made by XRF at Memorial University.
Major element analysis was performed of fused borate glass disks, using 1.5000g of sample with 7.52000 of flux which included 0.02g of LiBr.
Accuracy and Precision
A pure quartz reagent blank and four certified geological reference standards (SY-2-
G, SY-3-G) were prepared and analysed with the samples. The accuracy was detemined fiom the deviation of the oxide concentrations measured in the standards fiom the accepted values of Govindaraju (1 994). The precision of the analyses was detemined by quadruple anaiysis of standards (20). These are presented in Table 1A.2 below:
Table 1A.2 - Precision and accuracy for XRF data hmMemoial University hm
the standards SY2G and SY3G. The upper row of numbers for each standard is the
precession and lower rows the accuracy. Accuracy is the percentage deviation of the
average analyses of SY2G and SY3G respectively fiom the composition as reporieâ ICP-MSREE DATA
Al1 ICP-MSanaiysis were preformed at Mernorial University, Newfoundland.
Anaiytical procedure: ICP-MS:
ICP-MSsample preparation involved: (1) sintemng of a 0.2 g sarnple aliquot with sodium peroxide, (2) dissolution of the sinter cake, separation and dissolution of REE hydroxide- bearing precipitate, (3) analysis by ICP-MSusing the method of internai standardisation to correct for matrix and drift effects. The advantage of the sintering technique is that it practically ensures complete digestion of resistant REE-bearing accessory phases (e.g., zircon, fluorite), which may not dissolve during an acid digestion. Full details of the procedure are given in Longerich et al., (1 990).
Cornparison of ICP data with XRF data for certain key elements was made as this can be indicative of analytical problems. The latter technique, king a solid sample method, is not prone to the dissolution and solution stability problems that can plague ICP-MS.The agreement between XRF and ICP-MS methods for almost al1 sarnples above the XRF's detection lirnit is excellent. Most notably is the good agreement for Y, good Y agreement virtually guarantees that the REE data are also good @ersonal communication Simon
Jackson, 1997).
Accuracy and Precision
A pure quartz reagent blank (BLANK) and the certified geological reference standards (gabbro MRG-1 and besalt BR-688) were prepared and anaiysed with the samples.
Reagent blank concentntians are generally insignificant and have not been subtracted from sarnple concentrations. Table 1 A.3 show the accuracy and precision of duplicate analyses of these standards.
Table 1A.3 - Precision and accuracy for ICP-MSdata fiom Mernorial University
From the standards MRGl and BR688. The upper row of numbers for each standard
is the precession and lower rows the accuracy. nc = not calculated. Accuracy is the
percentage deviation of the average analyses of the standards fkom the composition APPENDIX 2
JEOL 733 ELECTRON MICROPROBE
Microprobe analysis was performed at UNB using a JEOL 733 Microprobe. The
JEOL 733 microprobe was operated at an accelerating potential of 15 kV and a sample current of 1On.A. X-rays were selectively analysed in a gas proportional counter following selective diffraction by a four-crystal wavelength dispenive spectrometer (WDS). intensity ratios were processed on-line by the program CITZAF form California Institute of
Technology, Division of Earth and Planetary Sciences.
Background counts were always collected, except when anaiysing several minerals that were compositionally simiiar, or when analysing seveml spots of a single cornpositionally unzoned mineral. An analysis count time of between 30 and 40 seconds was used. Each time the microprobe was used the microprobe was calibrated by analysing several standards. Analyses of standards were also interspersed among unknown sample analyses.
The accuracy ofthe analyses is based on the composition of these standards. A representative sample of the analyses for several standards taken from different sets of microprobe data were used to calculate the genenil accuracy of analyses fiom the microprobe. These are repnsented in Table A2 for the various different standards used. The precision of analyses is calculated from repeat analyses of the same standard these are also presented in Table M. Precision Accuracy Precision Accuracy Range 0.40 0.32 0.50 0.97 1.62 0.19 1.20 0.67 1-23 2.07 0.10 0.26 0.02 107.14 0.05 0.17 1.O8 0.04 0.00 0.12 0.14 1.63 0.0 1 7.30 0.05 0.05 8.06 0.02 125.00 0.08 0.02 100.00 0.0 1 142.86 0.02 0.12 2.8 1 0.1 1 0.06 0.34 0.09 3.80 0.06 0.74 0.18 0.04 0.93 0.0 1 0.00 0.04 9
Precision Accuracy Range Precision Accuracy SiO2 0.70 0.39 2.5 1 0.0 1 1.21 ~1~0, O, 19 0.65 0.50 0.33 29.4 1 TiO, 0.0 1 0.00 0.0 1 0.03 141.18 Fe0 0.34 0.00 0.80 0.85 15.58 Mg0 0.0 1 0.00 0.00 0.00 5.16 Mn0 0.0 1 0.00 0.03 0.25 19.35 C-0, 0.04 0.00 0.1 1 0.00 79.4 1 Ca0 0.0 1 0.00 0.02 0.0 1 0.73 Na20 0.03 4.35 0.05 0.02 3.95 - K,O 0.16 0.40 0.52 0.06 150 n 7
Table A2 - Accuracy, precision and range data for analyses preformed on the JEOL 733 microprobe. Kkhbd = homblende standard; plagbyt = plagioclase bytownite standard; OR1
= orthclase standard; CPXS 1 1 = clinopyroxene standard. n =the number of analyses used for these calculations. Accuracy is the percentage deviation of the average analyses of the standards fkom the published compositions of the standards. ADDITIONAL PYROXENE ANALYSES
Pyroxene analyses used in Figure 5.1 1. 1 2 3 4 5 6 7 8 9
Total 100.86 99.18 99.62 99.23 99.21 99.76 99.18 99.14 100.41 Stoichiometry Si 1.94 1.94 1.92 1.96 1.96 1.95 1.94 1.93 1.98 Ti 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.00 Al 0.09 0.10 0.11 0.08 0.07 0.09 0.10 0.09 0.05 Cr 0.01 0.01 0.02 0.01 0.01 0.01 0.02 0.01 0.01 Fe2+ 0.42 0.40 0.38 0.42 0.47 0.40 0.42 0.38 0.44 ~ej' 0.01 0.00 0.02 0.00 0.00 0.00 0.00 0.03 0.00 Mn 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Mg 1.45 1.46 1.47 1.45 1.41 1.46 1.46 1.49 1.45 Ca 0.06 0.06 0.06 0.06 0.07 0.06 0.06 0.06 0.07
others 6.21 6.33 8.24 5.18 4.49 6. 6.48 7.47 3.35 Temp (Co) 1080 1090 1100 1030 1100 1030 1000 1060 1060
Table A3.1 Microprobe analyses of orthopyroxene cores fiom the Hess and Foy Offsets used in Figure 5.1 1. Stoichiometry on the basis of 6 oxygens. FeO, = total iron as FeO. nd = not detected. Wo, En and Fs are the percentage of wollastonite, enstatite and ferrosilite components, respectively. "Others" are the percentage of non-pyroxene quardrilaterial componmts calcdated hmthe projection scheme of Lindsley and Anderson (1983). Temperatures estimitted hmLindsley's graphical thennometer (see Figure 5.1 1). Total 99.58 99.37 100.22 99.30 100.30 99.94 99.67 100.19 Stoichiometxy Si 1.97 1.94 1.96 1.94 1.95 1.95 1.96 1.95 Ti 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.00 Al 0.06 0.09 0.07 0.10 0.08 0.09 0.07 0.08 Cr 0.01 0.01 0.01 0.02 0.01 0.01 0.00 0.01 Fe2+ 0.45 0.43 0.45 0.40 0.45 0.42 0.45 0.45 Fe3+ 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 Mn 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Mg 1.43 1.44 1.41 1.45 1.42 1.45 1.43 1.43 Ca 0.07 0.07 0.07 0.07 0.07 0.06 0.07 0.06 Na 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 Fe/(Fe+Mg) 0.24 0.23 0.24 0.22 0.24 0.23 0.24 0.24
Table A3.2 Microprobe analyses of orthopyroxene cores fiom the Hess and Foy Offsets used in Figure 5. 1 1. Stoichiometry on the basis of 6 oxygens. See table A3.1 for details. 1 2 3 4 5 6 7 8 9 10 wt % SiO2 54.23 53.96 52.18 53.48 54.3 1 52.97 52.09 52.14 52.66 52.65 TiO, 0.11 0.17 0.17 0.17 0.19 0.14 0.19 0.26 0.12 0.18 A40, 0.92 1.45 1-14 1.60 1.60 1.53 1.32 0.99 1.40 1.19 Cr,03 0.07 0.20 0.07 0.18 0.10 0.26 0.27 0.12 0.23 0.12 Fe0 17.23 16.33 25.32 15.81 16.79 19.22 23.13 22.70 21.09 22.45 Mn0 0.39 029 0.44 0.33 0.29 024 0.43 0.46 C.29 0.36 Mg0 24.79 25.16 192% 25.58 25.85 23.70 20.65 20.66 22.98 21.32 Ca0 1.98 1.78 1.78 1.85 1.85 1.55 1.87 2.15 1.54 1.78 N@ nd 0.03 0.04 0.03 nd nd nd nd nd nd K,O 0.02 nd nd nd nd 0.01 nd nd nd nd Total 99.74 99.38 100.42 99.04 100.99 99.63 99.96 99.49 100.32 100.05 Stoichiometry Si 1.98 1.97 1.97 1.96 1.95 1.96 1.96 1.97 1.94 1.97 Ti 0.00 0.01 0.01 0.01 0.01 0.00 0.01 0.01 0.00 0.01 Al 0.04 0.06 0.05 0.07 0.07 0.07 0.06 0.04 0.06 0.05 Cr 0.00 0.01 0.00 0.01 0.00 0.01 0.01 0.00 0.01 0.00 Fe2' 0.53 0.50 0.80 0.48 0.49 0.59 0.72 0.71 0.61 0.70 Fe3+ 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.04 0.00 Mn 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 Mg 1.35 1.37 1.09 1.40 1.39 1.30 1.16 1-16 1.26 1.19 Ca 0.08 0.07 0.07 0.07 0.07 0.06 0.08 0.09 0.06 0.07 Na nd 0.00 0.00 0.00 nd nd nd nd nd nd Fe/(Fe+Mg) 0.28 0.27 0.42 0.26 0.26 0.31 0.38 0.38 0.33 0.37 Mg/(FeeMg) 0.72 0.73 0.58 0.74 0.74 0.69 0.62 0.62 0.67 0.63 wo 4 4.5 3.7 3.8 3.7 3.2 3.9 4.5 3.2 3.7 en 69.1 59.2 55.5 71.6 71 66.8 59.3 59.2 65.2 60.5 fs 26.9 36.3 40.8 24.6 25.2 30 36.8 36.3 3 1.6 35.8 others .d4 3.74 3.81 4.78 4.86 4.63 4.76 3.74 5.91 3.62 Temp(CO) 1105 1040 1000 1100 1080 1000 1150 1010 1000 1000
Table A3.3 Microprobe analyses of orthopyroxene rims from the Hess and Foy Offsets used in Figure 5.1 1. Stoichiometry on the basis of 6 oxygens. See table A3.1 for details. SiO? 52.57 51.67 51.18 51.95 51.16 51.74 51.66 51.01 51.78 TiO, 0.49 0.53 0.47 0.37 0.49 0.39 0.51 0.49 0.46 A1,0, 2.13 1-91 1.99 1.60 1.99 1.68 1.88 1.94 1.78 Cr,O, 0.08 0.02 O. 13 0.10 0.01 nd 0.05 0.06 0.06 Fe0 12.27 13.69 12.57 14.21 1325 13.16 12.75 14.12 13.50 Mn0 0.30 0.33 0.26 0.36 0.22 0.34 0.31 0.32 027 Mg0 14.77 13.83 14.66 14-49 13.58 14.75 14.51 13.74 14.69 Ca0 18.09 18.19 17.91 16.94 18.65 17.17 17.88 18.01 17.71 N%O 0.29 0.25 0.26 0.26 0.26 0.27 0.23 0.27 0.30 K,O 0.01 nd nd nd nd nd 0.01 nd nd Total 100.99 100.43 99.43 100.28 99.61 99.50 99.78 99.96 100.55 Stoichiometry Si 1.95 1.94 1.93 1.95 1.93 1.94 1.94 1.92 1.93 Ti 0.01 0.02 0.01 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 Al 0.09 0.08 0.09 0.07 0.09 0.08 0.08 0.09 0.08 Cr 0.00 0.00 0.00 0.00 0.00 ;id 0.00 0.00 0.00 Fe2+ 0.38 0.40 0.35 0.42 0.38 0.38 0.38 0.39 0.37 Fe3+ 0.00 0.03 0.05 0.03 0.04 0.04 0.02 0.06 0.05 Mn 0.01 0.01 0.01 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 Mg 0.82 0.77 0.82 0.81 0.77 0.83 0.81 0.77 0.82 Ca 0.72 0.73 0.72 0.68 0.76 0.70 0.72 0.73 0.71 Na 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 Fe/(Fe+Mg) 0.32 0.34 0.30 0.34 0.33 0.3 1 0.32 0.33 0.3 1 M g/(Fe+M g) 0.68 0.66 0.70 0.66 0.67 0.69 0.68 0.67 0.69 wo 34.8 35.8 34.7 33.5 36.7 34 35.3 34.9 34.3 en 44.6 42.2 45.8 43.9 42.2 45.3 44.1 43.3 45.3 fs 20.6 21.9 19.4 22.7 21.1 20.7 20.6 21.8 20.4 others 7.14 7.81 8.99 7.07 8.28 7.63 7.34 9.47 8.82 Temp(CO) 1095 1045 1100 1\15 1015 I IO5 1075 1080 1110
Table A3.4 Microprobe analyses of clinopyroxene cores fiom the Hess and Foy Offsets used in Figure 5.1 1 . Stoichiomee on the basis of 6 oxygens. See table A3.1 for details. nd nd 0.01 nd nd nd nd 0.08 Total 99.03 100.08 99.86 99.74 99.59 99.64 99.59 100.05 Stoich iometry Si 1.98 1.94 1.93 1.94 1.92 1.96 1.94 1.95 Ti 0.02 0.0 1 0.02 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 Al 0.00 0.08 0.09 0.09 0.08 0.08 0.08 0.08 Cr O .O0 0.00 O .O0 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.4 1 0.4 1 0.37 0.40 0.4 1 0.42 0.4 1 0.43 FeJ+ 0.00 0.03 0.04 0.02 0.06 0.00 0.03 0.0 1 Mn 0.01 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 Mg 0.83 0.87 0.84 0.89 0.87 0.86 0.83 0.85 Ca 0.75 0.63 0.68 0.62 0.62 0.63 0.68 0.65
en 41.6 47.6 46.4 48.1 48.4 46.9 44.9 45.5 fs 20.9 22.2 20.5 21.8 22.6 22.7 22.1 22.8 others 2.28 7.97 9.18 7.14 9.19 7.75 7.47 6 Ternp (Co) 1000 1175 1130 1170 1 200 1170 1 120 1140
Table A3.4 Microprobe analyses of cfinopyroxene cores fiom the Hess and Foy Offsets used in Figure 5.1 1. Stoichiometry on the basis of 6 oxygens. See table A3.1 for details. Stoichiometry Si 1.93 1.94 1.94 1.92 1.94 Ti 0.02 0.02 0.02 0.02 0.0 1 Al 0.09 0.09 0.08 0.08 0.09 Cr 0.00 0.00 0.00 O .O0 0.00 Fe2' 0.52 0.44 0.5 1 0.52 0.45 Fe3+ 0.37 0.03 0.03 0.07 0.03 Mn 0.0 1 0.0 1 0.0 1 0.0 1 0.0 I Mg 0.67 0.73 0.8 1 0.75 0.72 Ca 0.70 0.74 0.59 0.63 0.73 Na 0.02 0.02 0.02 0.02 0.02 Fe/( Fe+ Mg) 0.44 0.38 0.38 0.4 1 0.38
others 9.12 7.9 7.9 9.6 7.75 Temp (Co) 1055 1010 1175 Il50 1050
Table A3.5 Microprobe analyses of clinopyroxene rims from the Hess and Foy Offsets used in Figure 5.1 1. Stoichiometry on the basis of 6 oxygens. See table A3.1 for details. APPENDIX 4
UTM COORDINATES OF SAMPLES AND PHOTO LOCATIONS
Chapter Two Figure # Description Coordinates Fig. 2.1 Ford in Onaping River 467342 5 182692 Fig. 2.3 North eastern Depot Lake 460929 5 176160 Fig. 2.4 Quartz diorite cross cutting Nipissing Island, Clear Lake 459341 5 174748 Fig. 2.5 Granite inclusion in quartz diorite near the contact 464918 5178412 Fig. 2.6 Epidote veins in quartz diorite Harty Option 472939 5 182478 Fig. 2.7 Lntemal contact in quartz diorite Elation Creek swamp 462987 5 177237 Fig. 2.10 Clast packed footwall-like breccia inclusion 476685 5183866 Proximal Foy intersection Fig. 2.1 1 Clast packed footwall-like breccia inclusion east of Neil 475881 5183679 Lake Fig. 2.13 Rubble pile from pit excavation 464952 5 178378 Fig. 2.14 Quartz veins in host rocks Rivers Option 464976 5 17843 1 Fig. 2.15 Basic inclusion in quartz diorite at the Rivers Option 464952 5 178378 Fig. 2.16 Huronian inclusion in quartz diorite at the Riven Option 464952 5 178378 Fig. 2.18 Footwall-like breccia inclusion fiom the Maki showing 476464 5 183 172 Fig. 2.19 Quartz diabase dyke crosscutting quartz diorite
Chapter Three
Figure # Description Coordinates Sample # Fig. 3.1 Pseudotachylyte veins running subparallel to 463955 5 17778 1 Hess Fig. 3.2 Anaston.sing pseudotachylyte veinlets in granite Fig. 3.4 Stiained granite Fig. 3.5 Recrystallisation of quartz dong pseudotachy 1yte veinlets Fig. 3.6 Displacement of quartz veins dong pseudotachylyte veinlets Fig. 3.7 Assimilation of feldspar crydsby pseudotachylyte matrix Fig. 3.8 Displacement of feldspar twins by micro- pseudotachylyte veinlets Fig. 3.9 Grantie pervasively cut by micro pseudotachylyte 460 193 veinlets Fig. 3.10 Brod s harp margined pseudotachy 1yte vein 4652 11 Fig. 3.1 1 Matrix and inclusions in broad sharp margined 465446 pseudotachylyte veins Fig. 3.12 De fomed granite clast in pseudotachy1 yte 46 1093 Fig. 3.13 Flow foliated pseudotachylyte 464665 Fig. 3.14 Irregular zone of pseudotachylyte with sharp to 470296 diffuse margins Fig. 3.15 Contact of highly altered granite with apparently 470299 di fhemargined pseudotachyl yte Fig. 3.16 Cataclastically deformed granite 464976 Fig. 3.17 Cataclastically de fomied granite with partial 464879 pseudotachylyte like matrix Fig. 3.1 8 Chilled margin in quartz diorite 475454 Fig. 3.19 Sphenilitic texture in granite in contact with 47668 1 Hess quartz diorite
Chapter Four
Chapter sample # Coordinates H-1 45934 1 5 1 74748 H-2 461010 5174748 H-3 463622 5 177574 H-4 464339 5 177971 H-5 464952 5 178378 H-6 464952 5 178378 H-7 464952 5 178378 H-8 464952 5 178378 H-9 469324 5 189797 H-10 476703 5 183847 H-1 1 476718 5183893 H-12 477088 5 183999 H-13 478085 5 186042 H-14 478276 5 186042 H-15 464577 5 178249 H-16 464802 5 1783 18 H-17 464442 5177952 Wl 459313 5174851 ROINCL 464952 5 178378 FBX- 1 475829 5 183639 FBX-2 476685 5 183866 FBX-3 FBX-4 PFOY QDB-1 QDB-2
Chapter Five
Figure # Description Coordinates Sample # Fig. 5.1 Subophitic to intersertial texture 476662 5 183798 C7- 198 Fig. 5.2 inverted Pigeoni te 476662 5 183 798 C7- 198 Fig. 5.3 Orhtopyroxene partiail y re placed by 476662 5 183798 C7-198 actinolite Fig. 5.4 Ferro-magnesio-homblende ,77952 CQ-107 Fig. 5.5 Ferro-actinolite ,83798 C7-198 Fig. 5.6 Interstitial granophyre ,77952 Cd- 1O7
Fig. 5.7 Cryptocrystailine inclusions in plagioclase t 77952 C6- 1O7 Fig. 5.8 Cryptocrystalline inclusions in plagioclase ,77952 C6- 1O7 Fig. 5.9 Aciculnr crystals in plagioclase ,77952 C6- 1O7 Fig. 5.10 Bleb like inclusions in plagioclase ,77952 C6- 1O7 Fig. 5.12 Orthopyroxene partiaily replaced by calcite ,83836 HESS iII Fig. 5.13 Chlorite and biotitc replacing actinolite ,78249 C6-34 Fig. 5.14 Contact quartz diorite 183890 C7-203 Fig. 5.15 Contact quartz diorite ,77971 C7-114 Fig. 5.16 Central granophyric quartz diorite 177237 ELC Fig. 5.17 Central altered quartz diorite ,77574 C6-250 Fig. 5.18 Epidote/caicite veins cross cutting :77971 C6-115 quartz diorite Fig. 5.19 Apparent igneous texture of footwall-like breccia Fig. 5.20 Heterolithic nature of footwdl-like breccia Fig. 5.2 1 Weterolithic nature of footwall-like breccia Fig. 5.22 Felsic clast in Maki gneissic breccia Fig. 5.23 Matrix of gneissic breccia at Maki showing Fig. 5.24 Matrix of gneissic breccia at the distal Foy intersection Fig. 5.25 Ophitic basic inclusion in Hess Fig. 5.26 Feldspar inclusion in Hess Fig. 5.27 Decussate texture of small inclusion of feldspar Fig. 5.28 Relatively unaltered Parallel Foy Offset Fig. 5.29 Altered Parallel Foy Offset Fig. 5.30 Quartz diabase chilled against quartz dionte 464952 5 178378 C7-108 Fig. 5.3 1 Plagioclase phenocryst in quartz diabase 464952 5 178378 C6-122 Fig. 5.32 Matrix of qua.diabase 464952 5 178378 C6-122 NOTE TO USERS
Oversize maps and charts are microfilmed in sections in the following manner: LEFT TO RIGHT, TOP TO BOTTOM, WlTH SMALL OVERLAPS
This reproduction is the best copy available.
UMI
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Dl Legend Lote Recambrian Oli
Middle Recambrian Assumed Outcrop I Pa OR
Nil
Cobilt Oroup Lo
Gc Quiilre Lake Oroup Se
m Es Br Early Recambriun Mc
Cs
Lc
LI Detailed map of the Hess Offset and surrounding area Legend
Olivine diabase
Ofket dyke quartz diorite . . true north *<- .. ;A$$ ;A$$ &'Y%< Nipissing diabase Nortti 1 Sroup American ûatum 927 Universal Transverse Mercator (67projection Lorrain Formation Zone 1 7. Cenfol ~kidian8 1 O W. Grid interval ,i00(h Contour interval 1Om 3s Gowganda Formation mte Oroup O 500 Serpent Formation METRES
Espanola Formation
Water '
Swamp 1 Matchawan gabbro Fault . Cartier granite Road
) Levack gneiss ATV trait ~railloverarcknroad
uiivine criaDase
Parallel Foy Offest
Offset dyke quartz diorite true north Nipissing diabase North American Datum 1927 Universal Transverse Mercator (6")projection Lorrain Formation Zone 1 7. Central Meridian 8 1" W. Gtid interval 1 Oûûm Contour interval 10m Gowganda Formation ra Lake Oiwp Serpent Formation
Espanola Formation
+ .,. . -:y ,# . . ,.%<.. . .,a Bruce Fomation . Water cambrian Swamp a Matchawan gabbro - Fault Cartier granite Road
Levack gneiss ATV trail Traillovergrown road
tnt Rail .