Physical Volcanology and Hydrothermal Alteration· of the Footwall Rocks to the Archean Sturgeon Lake Massive Sulfide Deposit, Northwest Ontario A Thesis Submitted to the Faculty of the Graduate School of the University of Minnesota by Peter K. Jongewaard In Partial Fulfillment of the Requirements for the Degree of Master of Science January 1989 Acknowledgements The author would like to acknowledge his advisor, Dr. Ronald L. Morton of the University of Minnesota-Duluth for his support and encouragement, as well as initiation of this thesis. Thanks are also extended to Ors. James A. Grant, John C. Green, and Paul Siders of the University of Minnesota-Duluth for serving on the advisory committee. Thanks are given to Mr. Frank Balint of Minnova, Inc., Thunder Bay, Ont., for access to the property, company records, and financial support. Also acknowledged are Dr. J.M. Franklin of the Geological Survey of Canada , Mr. George Hudak, Mr. J.S. Walker, and Mr. Chris Rog o f the University of Minnesota-Duluth for stimulating discussions in the field. Lastly, the author wishes to thank his family for constant support, encouragement, and patience. ABSTRACT The f ootwall rocks beneath the Archean Sturgeon Lake massive sulfide deposit consist of a 3 km thick sequence o f steep north-dipping subaqueous volcanic and volcaniclastic rocks intruded by mafic dikes and sills. Preserved primary textures, fragment composition, and regional stratigraphic correlation allow for the volcanic and volcaniclastic rocks to be subdivided into seven distinct units. Mafic volcanic rocks comprise the lowermost units, consisting of massive, pillowed, and amygdaloidal flows, with minor interlayered felsic flows and pyroclastic deposits. These mafic units are overlain in the west half of the field area by a coarse heterolithic volcanic breccia, which grades up into, and is intercalated with quartz-phyric felsic pyroclastic flow deposits. This felsic unit is locally overlain by bedded volcanic debris flows and associated epiclastic sediments. These are in turn overlain by a thick sequence of massive to bedded quartz-phyric felsic pyroclastic flow deposits, the Mattabi series, with minor intercalated volcanic debris flows. These rocks are overlain by the host rocks to the orebody which consist of bedded to massive quartz- & plagioclase-phyric pyroclastic flow deposits. The succession is capped by mafic to intermediate massive and amygdaloidal flows, and was later intruded by two large dioritic sill-like bodies which dilate the stratigraphy. The east half of the field area is underlain by the lowermost mafic units which are overlain by fragment-poor ash-rich felsic pyroclastic flow deposits. A NE trending structure, interpreted to be a synvolcanic normal fault, provides the break between east and west, and may be a major bounding fault to a large caldera system. Convective circulation of hydrothermal fluids within the succession resulted in the formation of distinct alteration assemblages, which include a) silicified; b ) Fe- chlorite; c) chloritoid; d) cordierite; e) aluminum silicate; f) sericite; and g) Mg-chlorite. Field and petrographic evidence suggest that two periods of hydrothermal activity affected the felsic volcanic rocks of the succession: 1) development of silicif ied rocks and Al-silicates from leaching of original constituents, and chloritoid/chlorite alteration by a process of Fe-enrichment, and 2) widespread sericitization from K enrichment, and cordierite alteration, by leaching of Fe and Mn, leaving the rocks enriched in Ca and Mg during the time of development of the Sturgeon Lake massive sulfide deposit. Subsequent regional deformation and metamorphism to upper greenschist facies produced a bedding-parallel foliation and development of metamorphic mineral phases, strongly dependent on pre-metamorphic altered compositions. i TABLE OF CONTENTS Abs tr act ................................................... i Table of Contents ................................. ....... ii List of Figures ........................................... iv Li st of Tables ........................... ............... vii List of Plates ........................................... vii I. Introduction ............................................. l Purpose of Study ........................................ 1 Location, Access, and Physiography ...................... J Methods of Study ........................................ 5 Regional Geology ........................................ 6 Previous Work ........................................... 9 II.Lithology and Stratigraphy ............................. 13 Introduction ........................................... 13 Description of Units ................................... 15 Precaldera Volcanic Rocks ............................ 15 Mafic Volcanic Rocks .............................. 15 Caldera-Related Units ................................ 22 Heterolithic Breccia .............................. 22 Quartz Pyroclastic Flow (QPYF) .... ..... 27 Debris Flow Deposits .............................. 31 Mattabi Pyroclastic Deposits .................. .. JS L-Series Pyroclastic Deposits ..................... 48 Post-Caldera Rocks ................................... 54 Mafic Volcanic Rocks ........................... ... 54 Mafic Intrusive Rocks ............................. 55 Volcanic Interpretation ......... ................. 59 Massive Sulfide Orebody ................................ 62 III.Hydrothermal Alteration ............................... 67 Introduction ........................................... 6 7 Alteration Assemblages ................................. 68 Silicified Assemblage ................................ 68 Iron-Chlorite Assemblage ............................. 68 Chloritoid Assemblage ................................ 72 Cordierite Assemblage ................................ 74 Aluminum Silicate Assemblage ......................... 77 Sericite Assemblage .................................. 78 Magnesium Chlorite Assemblage ........................ 81 Distribution of Alteration Assemblages ................. 82 ii IV.Effects of Metamorphism ................................ 85 V.Alteration Geochemistry ................................. 96 Silicified Assemblage ................................ 102 Fe-Chlorite Assemblage ............................... 104 Chlori toid Assemblage ................................ 107 Cordierite Assemblage ................................ 111 Aluminum Silicate Assemblage ......................... 114 Ser ici te Assemblage .................................. 118 Mg-Chlorite Assemblage ............................... 120 VI.Alteration Model ...................................... 123 VII.Summary and Conclusions .............................. 129 References Cited ......................................... 13 4 Appendix 1: Modal Mineralogy ............................. A-1 Appendix 2: Geochemical Analyses ......................... A-2 Appendix 3: Modal Mineralogy-Orebody ..................... A-3 iii LIST OF FIGURES 1. Location Map ............................................ 4 2. Location of Subprovinces ........................... ... 7 3. Regional Geology ........................................ 8 4. Areas of Study ......................................... 11 5. Outcrop of pillowed lower mafic unit ................... 1 7 6. Amygdaloidal sheet flow of lower mafic unit ............ 1 9 7. Photomicrograph of amygdaloidal mafic flow ............. 1 9 8. Pumice fragments in outcrop, slj-105 ................... 22 9. Blocks within breccia unit ............................. 2 4 10. Breccia within drill core ............................. 25 11. Fragments within breccia ............................ 25 12. Photomicrograph of QPYF matrix ........................ 3 0 13. Photomicrograph of amygdules within fragment in QPYF .. 30 14. Large pumice block within debris flow deposit ......... 33 15. Bombs within debris flow deposit ...................... 34 16. Bedded debris flow deposits ........................... 35 17. Thin laminated debris flow horizon .................... 36 18. Fragments in mafic debris flow deposit ................ 36 19. Drill core intersection of Mattabi pyroclastic rocks .. 4 0 20. Crystal- and ash-rich beds, Mattabi pyroclastic rocks .42 21. Outcrop of siliceous Mattabi pyroclastic rocks ........ 42 22. Mantled quartz phenocrysts, Mattabi pyroclastic unit .. 43 23. Photomicrograph of fragment and phenocrysts in Mattabi pyroclastic rock ...................... ....... 4 4 24. Photomicrograph of fine-grained Mattabi matrix ........ 44 iv 25. Photomicrograph of graded silt-sized quartz chips in Mattabi pyroclastic rock .............................. 45 26. Drill core intersection of coarse debris flow deposit within Mattabi pyroclastic deposit .................... 47 27. Outcrop of contact between L-series subunits .......... 50 28. Rusty mineralized horizon, L-series pyroclastic rocks.51 29. Photomicrograph of plagioclase phenocryst in L-series.51 30. Thinly laminated ash beds within L-series ............. 52 31. Photomicrograph of phenocrysts within L-series rock ... 53 32. Massive northernmost dioritic intrusion ............... 58 33. Thin dike cutting L-series exposure ................... 58 34. Diagrammatic section through orebody, DDH# SLM-156 .... 63 35. Graph of Sph-Cp zonation in orebody ................... 65 36. Graph of Py-Po zonation in orebody .................... 66 37. Outcrop of silicified L-series rock ................... 69 38. Drill core intersection of Fe-chlorite