Nickel Sulfide Ores and Impact Melts: Origin of the Sudbury Igneous Complex

By Peter C. Lightfoot

Nickel Sulfide Ores and Impact Melts: Origin of the Sudbury Igneous Complex presents a current state of understanding on the geology and ore deposits of the Sudbury Igneous Complex in Ontario, Canada. As the first complete reference on the subject, this book explores the linkage between the processes of meteorite impact, melt sheet formation, differentiation, sulfide immiscibility and metal collection, and the localization of ores by magmatic and post-magmatic processes.

The discovery of new ore deposits requires industry and government scientists and academic scholars to have access to the latest understanding of ore formation process models that link to the mineralization of their host rocks. The ore deposits at Sudbury are one of the world’s largest ore systems, representing a classic case study that brings together very diverse datasets and ways of thinking.

This book is designed to emphasize concepts that can be applied across a broad range of ore deposit types beyond Sudbury and nickel deposit geology. It is an essential resource for exploration geologists, university researchers, and government scientists, and can be used in rock and mineral analysis, remote sensing, and geophysical applications.

• Provides the only reference book to focus entirely on the Sudbury Igneous Complex
• Brings together an understanding of ore deposit and impact melts as a basis for future exploration
• Authored by a leading expert on the geology of the Sudbury Igneous Complex with 35 years of experience working on nickel sulfide ore deposits

• Language: English
• Publisher: Elsevier Science
• Release date: Aug 23, 2016
• ISBN: 9780128041055

Peter C. Lightfoot

Dr. Peter Lightfoot is a Chief Geologist with the Brownfield Exploration group at Vale Base Metals. He is responsible for technical aspects of the exploration programs at Sudbury, Thompson, and Voisey’s Bay and tracks the developments in understanding of the geology and global supply of metals from nickel sulfide and laterite deposits. He received his B.A. from Oxford University in 1980, his M.Sc. degree from the University of Toronto in 1982, and his Ph.D. from the Open University in 1985. Following post-doctoral studies at the University of Toronto, he joined the Ontario Geological Survey in 1987, and worked extensively on Sudbury and undertook a joint program of research on the Noril’sk Deposits. In 1996, Peter joined Inco as Senior Geologist and worked on Inco’s Voisey’s Bay Project and international project generation. With Vale’s acquisition of Inco in 2006, his responsibility continued to focus on global nickel project generation. In 2010, Peter returned to provide technical support in exploration within Vale’s mining camps in Canada. Peter is an Adjunct Professor at Laurentian University and Associate Editor of Elsevier’s Ore Geology Reviews. For 25 years, Peter has been based in Sudbury, and through his linkages to industry, government and academia has been positioned to develop and assemble the ideas presented in this book.

Book preview

Nickel Sulfide Ores and Impact Melts

Origin of the Sudbury Igneous Complex

Peter C. Lightfoot

Vale Base Metals Ontario, Canada

Table of Contents

Cover

Title page

Copyright

Dedication

Preface

Acknowledgments

Chapter 1: Sudbury – an introduction to the ore deposits and the impact structure

Introduction to Sudbury

History of discovery and production at Sudbury

The geology of Sudbury in a nutshell

Terminology used to describe the geology of the Sudbury structure

Sudbury geology at the center of important Earth Science debates

Magmatic nickel sulfides – an introduction and overview

Process controls in the formation of magmatic sulfide ore deposits

Evidence for impact origin of Sudbury

Synthesis of the approach used in this book

Appendix 1.1 Synthesis of Sudbury bibliography

Chapter 2: A synthesis of the geology of the Sudbury Structure

Introduction

An overview of the Regional Geological Setting of the Sudbury Structure

Geology of the Sudbury Structure

Chapter 3: Petrology and geochemistry of the Sudbury igneous complex

Introduction and objectives

The Main Mass

The Offset Dykes

The sublayer

The Sudbury Breccia

Metamorphic and partial melt textures in the footwall of the SIC

Melt segregations, dykes, and vitric fragments in the Onaping Formation

Parent magma composition, differentiation, and sulfide saturation history of the Sudbury melt sheet

Chapter 4: The mineral system characteristics of the Sudbury Ni-Cu-Co-PGE sulfide ore deposits

Introduction

Petrology and mineralogy of the sulfide mineralization at Sudbury

Geochemistry of the Sudbury ores

The offset and breccia belt mineral systems

Contact and footwall mineral systems

Process of formation of the Sudbury mineral systems

Chapter 5: The relationship between the impact melt sheet and the Ni-Cu-PGE sulfide mineral systems at Sudbury

Introduction and objectives

The impact record of the solar system

Regional controls on ore deposit endowment at Sudbury

The differentiation of sulfide melt

A guide to future applied research on Sudbury and impact craters

Chapter 6: Sudbury nickel in a global context

Introduction

The role of exploration

The decision process and the technology used to explore

Sudbury in the context of global nickel sulfide deposits

Case studies of other major magmatic sulfide mineral systems

The future of nickel production

References and Sudbury Bibliography

Index

Copyright

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ISBN: 978-0-12-804050-8

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Dedication

This book is dedicated to my wife, Nancy, who patiently tolerated my work on Sudbury.

Nancy provided the foundation of happiness that made this book possible.

Preface

Unusually large ore deposits are created by events that focus magma, fluids, energy, and metals into crustal containers and/or structures. These broadly magmatic–hydrothermal deposit types include porphyry copper, iron oxide copper gold, kimberlite-hosted diamond, and magmatic sulfide deposits. All of these deposits are marked by the generation of magmas and/or fluids in the mantle in areas of anomalous heat related to plume events and/or rifting or subduction of oceanic crust beneath continental margins. The energy driving the process comes from the mantle along with much of the mineral wealth in the ores. The world class orthomagmatic Ni-Cu-Co-PGE-Au deposits associated with the Sudbury Igneous Complex (SIC) are quite different in the sense that the energy source and genesis was triggered by a large meteorite impact event, and although there has been extensive work to try to identify mantle contributions, there is no compelling evidence that magma, fluid, energy, or metals come from deep inside the Earth.

This book explores the linkages between sulfide and silicate magmas generated by the 1.85 Ga Sudbury impact event which produced one of the largest Ni-Cu-PGE sulfide-ore-deposit camps which has now been mined for over 100 years. I examine the relationship between crustal melts on the one hand and magmatic sulfide ore deposits on the other. Normally magmatic sulfide ore deposits rich in Ni, Cu, and PGE require a mafic or ultramafic contribution of magma; at first sight the relationships at Sudbury constitute an oxymoron. Melts of average upper crust would be felsic in composition and bereft of metals normally found in ultramafic and mafic magmas that are normally required to form magmatic sulfide ore deposits. This book provides geological and geochemical evidence that the rocks of the SIC were produced by crustal melting, differentiation, and crystallization. The rocks record evidence for the formation of immiscible magmatic sulfide from the melt sheet, concentration of base and precious metals by equilibration of the sulfides with the silicate magma of the melt sheet and the gravitational concentration of these sulfides toward the base of the complex where they form the main ore bodies. The timing of processes can be established from the geological relationships between the rocks, and these observations inform models of ore genesis, and help explain the diversity in composition and scale of mineral zones which range from several million to just a few thousand tons of contained metal.

The impact process provided sufficient energy to melt the upper crust as well as produce the pseudotachylite breccias, syn-impact radial and concentric dykes, and an uneven crater floor covered by an impact melt that crystallized to form the Main Mass of the SIC. The scale on which ore deposits were formed is a direct function of the thickness of the primary melt sheet and the topography of the crater floor. Dense immiscible magmatic sulfides were localized in physical depressions and cracks in the crater floor where it was possible for the sulfides to concentrate. These relationships are examined in ore deposits which formed at the lower contact, in the immediate footwall of contact deposits, and in radial and concentric dykes generated in response to the migration of impact melt into the fractured crust.

The ore deposits of the SIC provide a wealth of variation in geometry, mineralogy, and chemistry that record evidence of the scale of primary segregation and concentration, and they record a protracted history of crystallization that sometimes permitted the segregation and localization of differentiated sulfide melts enriched in Cu, Ni, and PGE (the Footwall Deposits), leaving behind sulfides that are typically rich in Ni but have lesser Cu and PGE contents (the Contact Deposits). Geological relationships in ore bodies developed in radial and concentric structures show evidence for the emplacement of multiple different sulfide melts formed at different stages from the melt sheet; these deposits are often hosted in dykes (the Offset Deposits) and zones of extensive partial melting and brecciation associated with the embryonic development of dykes (in the South Range Breccia Belt). The details of these relationships offer a superb example of magmatic sulfide ore formation which provides new insight into the controls on the genesis and localization of magmatic sulfide ore deposits that can be applied elsewhere.

The preservation of the SIC is very good, and there is an excellent three-dimensional understanding of the complex above a depth approaching 3 km due to mining and exploration activity over 100+ years that provides a basis for the insights in this book. At least three major orogenic events have modified the rocks through deformation and metamorphism. These effects create an additional complexity in understanding the ore bodies. Tectonic displacement and solid-state migration of soft sulfides (kinesis) into spaces has created a superimposed range of relationships in the ores. The effects of these processes are often difficult to distinguish from syn-magmatic emplacement of sulfide melts into structures that were active during crater re-adjustment.

This book aims to unite an understanding of the process of melt sheet evolution with the formation of the magmatic sulfide mineralization. A sequence of events in the evolution of the SIC is constrained by geological and geochemical evidence, and the mechanisms of ore formation are placed into this context. Sudbury provides a wealth of lessons which helps inform studies of other magmatic sulfide deposits, and provides models which will continue to support future discovery of ore deposits at Sudbury. The elegance and simplicity of the basic scientific relationships at Sudbury provides an opportunity to present students with a unified theory of ore genesis in a crustal melt sheet. The theory remains part of focused research activity of Sudbury specialists, but it also provides a more general framework in which geologists can understand entire mineral systems where ore deposits are not treated in isolation from their host rocks.

In Chapter 1 the themes of the geology, impact origin, and the ore deposits of the Sudbury Structure are woven together in this book in a traditional approach, beginning with an account of the importance of Sudbury mines for nickel, copper, and precious metal production in a global context. The history of the Sudbury Mining Camp is discussed in terms of both the discovery of the ore deposits and the vigorous debate surrounding the origin of the host rocks. There has been a major shift in understanding from endogenic models requiring explosive magmatic events to the more recently accepted models of impact tectonics, melt generation, and ore formation. The theories of formation of immiscible magmatic sulfide ore deposits are introduced, with the supporting evidence that shows Sudbury to be part of a group of ore deposits normally associated with mafic and ultramafic rocks.

In Chapter 2 the geology of the Sudbury Structure and the associated igneous rocks are presented in a traditional way, and the information is designed to provide a foundation on which the ore deposits can be understood and their formation placed in the context of a sequence of events following the impact process.

Chapter 3 presents a detailed review of the petrology and geochemistry of the magmatic rocks that comprise the SIC that permits the origin and evolution of the silicate magmas to be more clearly understood in the context of crustal melting and differentiation. Selected topics introduced in this section include the source of the metals, the timing of emplacement of the Offset Dykes, and the differentiation of the rocks comprising the main body (Main Mass) of the SIC.

The ore deposits at Sudbury are described in detail in Chapter 4 as the products of gravitational settling and differentiation of the sulfide liquid. The details of ore body morphology and relationship to the magmatic rocks, breccias, and the footwall of the SIC are provided. Case studies of the discovery, geology, mineralization, and geochemistry are presented for important examples of the main styles of mineralization.

Chapter 5 provides a unified hypothesis for the formation of the SIC and the associated ores; it returns to look at the sequence of events and provides process models that examine the differentiation and evolution of the Sudbury melt, it’s sulfide saturation history, and the source of the metals.

Chapter 6 shows how the information is used to support exploration. Sudbury’s position amongst large, magmatic sulfide systems such as Noril’sk, Thompson, Jinchuan, and Voisey’s Bay is examined, and the similarities and differences in the process of formation of the ore deposits is highlighted. This chapter highlights the potential for future discovery of ore bodies at Sudbury, and it shows where Sudbury is placed in a global context of nickel sulfide and laterite deposits.

The aim of this text is to produce an overall understanding of the geology of the Sudbury impact structure and its ore bodies without writing an encyclopedic text that is inaccessible to students of geology. The elegance and simplicity of the basic ideas in impact geology and ore formation at Sudbury provides students with a grand theory of ore deposit formation with a minimal amount of the jargon and terminology that has evolved during 100 years of investigation.

This book is designed for an audience that includes not just those dedicated to finding the next generation of magmatic sulfide ore deposits, and understanding them, but also students of Earth Sciences who wish to gain a basic understanding of one of the most interesting and enigmatic ore-deposit systems on Earth—Sudbury.

Peter C. Lightfoot

Vale Base Metals and Laurentian University

Acknowledgments

Many individuals contributed in ways that brought this book to publication, and I name them because they deserve credit for influencing the development of the ideas presented in this book or ensuring that I completed a careful navigation through the approval process.

Vale is thanked for their permission to write the book. Scott Mooney provided the essential support required in Vale to bring this book to completion; Scott reviewed the book proposal, endorsed the concept to Vale, and then reviewed each chapter. I thank Scott for his support and his continued interest in understanding the rocks that comprise the Sudbury enigma. Scott Mooney and Kevin Graham are thanked for working with the author and Elsevier to secure the book contract as well as developing a path to publication. Vale endorsed and encouraged this contribution, and I thank Jennifer Maki, Conor Spollen, and Cory McPhee for their support and endurance through the technical details. I thank Heather Brown for helping to guide me through the development of the arcane three-way publishing contract required to realize this process and publication of the book.

The book proposal was reviewed by Tony Green, Reid Keays, Chris Hawkesworth, Franco Pirajno, Ed Ripley, and Scott Mooney; their input and advice helped me to focus on the applied value of Sudbury geology, and it ensured that I tried harder to extract the most important salient points from the wealth of data and historic work on the impact melts and associated ore deposits.

The readers who have helped me to improve the text through careful reviews comprise scientists and technical experts from academia, government, and industry. My thanks are extended to Sam Davies, Ian Fieldhouse, Anatoliy Franchuk, Lisa Gibson, Sandy Gibson, Reid Keays, Jason Letto, Glenn McDowell, Enrick Tremblay, Ben Vandenburg, and Xu-Ming Yang.

The graphic design, layout, and drafting of all images and diagrams was undertaken by Alex Gagnon on the basis of draft diagrams provided by the author; Alex’s skills in graphic design and conveying the geological concepts at the heart of this book make the final product so much better for all of his hard work and attention to detail.

Ben Vandenburg carefully selected and photographed all of the polished samples to illustrate the wealth of detail that supports an understanding of paragenesis; Ben also prepared the photomicrographs of the thin and polished sections for Chapters 3 and 4, and provided expert preparation so that rock samples could be photographed.

Lisa Gibson provided excellent 2D renditions of geological features evident in the 3D renditions of geological models that are difficult to convey without careful thought.

I will forever appreciate the mentoring and encouragement from Terry Morgan of Bishop Gore School. During my undergraduate work at Oxford, I was inspired to work on the geology of ore deposits by Professor Dick Stanton. Professor Tony Naldrett was my MSc supervisor and later a coinvestigator of the Siberian Trap and the associated Noril’sk mineral system. Professor Chris Hawkesworth was my PhD supervisor, and brought flood basalt geology and geochemistry into focus in the Deccan Trap. Both influenced my approach to Sudbury geology and they are thanked for sharing ideas, opinions, and data on Sudbury. I am deeply indebted to Professor Reid Keays who has been a colleague for over two decades and continues to be a source of ideas and inspiration that have contributed to my understanding of Sudbury geology.

The ideas expressed in this book have been developed over a period approaching 25 years of work on the Sudbury Igneous Complex and the associated ore deposits. I have benefited enormously from joint research with Tony Naldrett, Chris Hawkesworth, Reid Keays, Ed Ripley, and Will Doherty. Scientific interactions with many individuals have helped me to formulate these ideas; I am especially grateful to my colleague and friend, Professor Igor Zotov of the Russian Academy of Sciences, Moscow, who I miss greatly. I also extend my thanks to Drs. Doreen Ames, Sarah-Jane Barnes, Steve Barnes, Anthony Cohen, Fernando Corfu, Sarah Dare, Burkhard Dressler, Nick Gorbachev, Mike Lesher, Chusi Li, Des Moser, Gordon Osinski, Ulrich Riller, and Ron Sage.

I have been fortunate to have jointly supervised a number of graduate students and worked with postdoctoral fellows who have contributed to the understanding of magmatic sulfide ore deposits. My thanks goes to Emilie Boutroy, Mark Cooper, Lisa Cupelli, James Darling, Keith Farrell, Anatoliy Franchuk, Jian-Feng Gao, Kathy Hattie, Glen House, Grant Mourre, Mars Napoli, Jon O’Callaghan, Kostas Papapavlou, Steve Prevec, Aaron Venables, Yu-Jian Wang, Sheng-Hong Yang, and Mei-Fu Zhou.

I am privileged to have worked over the past 20 years with many world-class exploration geoscientists in the minerals industry whose dedication and enthusiasm for Sudbury geology has been an outstanding help in bringing applied focus to this book. These experts neither occupy the hallowed halls of academia nor are they experts from the government sector; they are applied exploration geoscientists. In particular I want to thank the following who have given freely of their ideas and helped me to gather or access the best samples and better understand Sudbury and its place in the spectrum of global magmatic sulfide ore deposits: Dick Alcock, Gordon Bailey, Mike Baril, Andy Bite, Cam Bowie, Charlene Brisbois, Sean Dickie, Sherri Digout, Dawn Evans-Lamswood, Catharine Farrow, Vivian Feng, Kevin Fenlon, Ian Fieldhouse, Carrie Forget, Brain Gauvreau, Lisa Gibson, Sandy Gibson, Paul Golightly, Graeme Gribbin, Roger Jackson, Alan King, Sasa Krstic, Andy Lee, Jason Letto, Herb Mackowiak, Hadi Mahoney, Michael McBurnie, Glenn McDowell, Bert McNabb, Chris Meandro, Rogerio Monteiro, Gord Morrison, Kyle Napoli, Mars Napoli, Krystal Oneill, Rob Palkovits, Ryan Paquette, Ed Pattison, Rob Pelkey, Clarence Pickett, Ben Polzer, Ashok Rao, Noelle Shriver, James Siddorn, Gary Sorensen, Peter Stewart, Rob Stewart, Matthew Stewart, Enrick Tremblay, Tony Vanwiechen, Shawna Waberi, Robert Wheeler, Christina Wood, Manqiu Xu, and Xue-Ming Yang.

From Elsevier, I wish to thank Tim Horscroft, Amy Shapiro, Tasha Frank, Marisa LaFleur, and Paul Prasad Chandramohan for all of their advice, input, and hard work on this book.

I thank my parents Irene and Bryan, my wife Nancy, my daughter Megan, and my family Betty, Fred, Isobel, Heather, Rob, Andrew, and Josh. They have all been a constant source of encouragement and understanding.

The author wishes to thank the following organizations and publishers for permission to use materials as follows:

Fig. 3.32a–d are from Ripley et al. (2015) is used with the permission of The Society of Economic Geologists.

Fig. 4.31b is used with the permission of the Archives of Ontario.

Figs. 4.77e and 5.1f are modified from Kenkmann and Dalwigk (2000), and is used with the permission of John Wiley and Sons.

Figs. 5.11a–e, 5.12a,b, 5.13a–e, and 6.22b,c are modified from Lightfoot and Evans-Lamswood (2015), and are used with permission of Elsevier.

Figs. 4.23a–c, 4.49a,b, 4.50a–c, 4.51a–e, 4.52a–c, 4.58a–d, 4.59, and 5.3a–c are modified from publications of the Ontario Geological Survey, and used under license to Elsevier.

Chapter 1

Sudbury – an introduction to the ore deposits and the impact structure

Introduction to Sudbury

Three facts about the Sudbury Structure place it among the crown jewels of planetary geoscience: (1) It is one of the largest, oldest, and best-preserved impact structures on planet Earth; (2) it was and remains the birthplace of important geoscience controversies in igneous petrogenesis, ore deposit geology, and impact cratering; and (3) it is home to Canada’s largest mining camp with more than 100 years of mining activity on deposits of high-grade nickel sulfide ores. Quite how these three themes and the many strands of science and discovery that underpin them have evolved into a holistic understanding of the geology of Sudbury make it a classic study in Earth Sciences.

Wagner’s views on continental drift, and the theory of plate tectonics were at the root of a revolution in Earth Sciences in the 20th century (Wegener, 1929), but the sudden and catastrophic events that change the planet in seconds to years rather than millions of years are at the crux of a shift in geoscience emphasis away from progressive (albeit rapid) change to sudden and profound shifts in the configuration of planet Earth. The Sudbury Structure is a case study in rapid change. Its formation was triggered by an impact event that lasted a fraction of a second followed by a crustal readjustment period likely lasting much less than 250,000 years. Exploration and mining activities over a period of more than 100 years in the Anthropocene underpin our current three-dimensional understanding of the geology of the shallow part of the Sudbury Structure above ∼3 km depth. The path to our current understanding has triggered many epiphanies and quite a few global revolutions in the Earth Sciences. Living in the footprint of an astrobleme, one has a privileged opportunity to understand the complex geology as an outcome of a catastrophic event that happened on a short time scale relative to the slow motion of plate tectonics and mantle plumes. This book is written to explain the importance of geoscience, exploration, and discovery as it relates to not just ore deposits, but the further understanding of the Sudbury impact structure.

The City of Sudbury, located in Northeastern Ontario, Canada, is one of the world’s principal sites of global nickel production; it is a city with a mineral industry that has evolved through almost 130 years. Since the discovery of the ore deposits, over 11.1 million metric tons of nickel and 10.8 million metric tons of copper together with by-products of cobalt, silver, gold, and platinum group elements (PGE) have been mined from the ore deposits (modified after Mudd, 2010). This wealth has been and continues to be generated from seven major mine complexes and 21 smaller ore deposits around the outer margin of the SIC which comprises part of the Sudbury Structure; the principal mines are owned and operated by international mining companies (Vale, Glencore, KGHM) and smaller mining companies (eg, Wallbridge). The high-grade ore deposits of the SIC are among the largest known historically mined and future resources of Ni–Cu–PGE sulfides, and comprise the foundation of the economic wealth of one of the largest mining camps in the world. The economic wealth generated at Sudbury in terms of just nickel and copper value at current metal prices (± March 2015 nickel price) is close to 215 billion US dollars. The Sudbury Nickel Camp has underpinned the growth of the economy of Canada and Ontario, inspired contributions to the science of magmatic ore deposit geology (Naldrett, 2004), triggered the development of exploration technologies such as airborne geophysics and down-hole electromagnetic geophysical tools (Polzer, 2000; King, 2007), and provided a foundation for the development of mining technologies to handle the challenges of extraction of mineral from deep mines, the process technology for sulfide ores, and the foundation for the future growth of a global service center in the City of Sudbury in Northeastern Ontario.

The Sudbury Nickel Camp is second in the world in terms of contained Ni in sulfide deposit (contained metal in historic production and unmined reserves and resources), behind that of the Noril’sk Camp, located in the low Arctic of Siberia. The latter deposits are controlled by the Russian mining company, Noril’sk Nickel. Fig. 1.1 shows the contained metal in the principal global nickel sulfide deposits and the position of the two main companies operating in the Sudbury Mining Camp. The figure shows only deposits with more than 500 kt contained nickel from past production plus unmined ores; there are remarkably few high-grade nickel sulfide ore deposits, but there are quite a large number of lower-grade deposits (<1% Ni) that are largely undeveloped as mines. Fig. 1.1 shows the current amount of contained nickel metal in reserves and resources plus the historic production of nickel for Sudbury (Vale and Glencore data are broken out and shown separately, but collectively they account for 99% of the production from the Sudbury Deposits). Total production plus unmined reserves and resources for the Noril’sk low-grade nickel deposits (Noril’sk Nickel), the Noril’sk high-grade nickel deposits, and other large developed and undeveloped nickel deposits are also shown. Production and untapped high-grade nickel resources from Sudbury (typically with nickel grades between 1.5 and 2.5%) exceed those of the Noril’sk Deposits (which typically have nickel grades of 1–3%), but in terms of overall contained metal in high-grade and low-grade deposits (ie, ≤1% Ni), Noril’sk exceeds Sudbury in contained nickel. Both Sudbury and Noril’sk have more than twice the contained nickel of the next largest deposits which include Pechenga (Kola Peninsula, Russia), Jinchuan (Gansu Province, China), Mt Keith (Western Australia), and Thompson (Manitoba, Canada) (Fig. 1.1; Naldrett, 2004).

Figure 1.1   Global Nickel Production (nickel contained in ore) and Reserves Plus Resources of Major Nickel Sulfide Deposits. Based on a compilation of global nickel reserves and resources in 2013 and production to the end of 2014.

The discovery of significant nickel sulfide deposits is quite a rare event as shown in a plot of discovery year versus total metal produced together with reserves and resources (Fig. 1.2); Fig. 1.2 illustrates the fact that large deposits such as Noril’sk and Sudbury are quite unique, whereas more recent discoveries tend to be mid-size or smaller. Mining companies continue to try to expand the reserve and resource base of all of these deposits, but the probability that recent discoveries will grow to the size of Sudbury or Noril’sk is very low, and the global trend has been toward the discovery of smaller deposits through time.

Figure 1.2   Past Production (nickel in ore), Unmined Reserves, and Resources of Deposits Plotted Against Initial Year of Discovery. The deposits are broken out into high grade (>1% Ni) and low grade (≤1% Ni). Based on a compilation of global nickel reserves and resources in 2013 and production to the end of 2014.

The Sudbury Mining Camp has had an enormous economic and social impact on not only the Sudbury Region but also on the economic development of Canada. The City of Sudbury grew up around the natural resources and mining industries, and is now a center for industry, government, academia, and health. The present-day skyline of Sudbury is still dominated by the Superstack which is 380 m tall (Conroy and Kramer, 1995), and was the tallest freestanding structure at the time. It was commissioned in 1972 to control the release of sulfur dioxide gas that contributed to the original discovery of acidification of lake waters in Kilarney Provincial Park (Beamish and Harvey, 1972; Wren, 2012). The multi-mine head-frames attest to the new depths to which ore is mined and extracted as well as the infrastructure of mills, smelters, and refineries which take raw ore and convert it to metal that is sold to market. Sudbury has inspired some landmark contributions to music like Sudbury Saturday Night (Connors, 1967). The growth of the community has largely been based on mining but it has now grown into a new regional center. The Sudbury Neutrino Observatory (SNO) is a recent facility designed to detect solar fission reactions; it utilizes the underground facilities in Vale’s Creighton Mine to detect and understand the flux of solar neutrinos at a depth of 2100 m (http://www.sno.phy.queensu.ca). The Nobel Prize in Physics (2015) was awarded to Dr. Takaaki Kajita (Japan) and Dr. Arthur B. McDonald (Canada), for their work at the SNO Laboratory on neutrinos.

In addition to its economic importance, Sudbury offers a unique opportunity to study the unique geological features and events that gave rise to the best-known examples of magmatic sulfide ore deposits. The metal endowment has a direct spatial relationship to a geological structure that was produced 1850 million years ago by an asteroid impact. This impact produced the conditions necessary for the ore deposits to form, and created an assemblage of rocks in the Sudbury Region that have puzzled geologists for over 100 years. The effects of the Sudbury impact event included a tsunami event and fall out of debris recorded in rocks of similar age over 1200 km away from Sudbury (Addison et al., 2010). This book examines the sequence of events that gave rise to the Sudbury Structure and its associated ore deposits. It is not designed to be a textbook in the conventional descriptive sense, but aims to tell the story of how detailed geology, petrology, and geochemistry of the Sudbury Structure help establish a sequence of events in the catastrophic impact event and the processes which resulted in the formation of the Sudbury ore deposits. The book is written from the perspective of a geologist who works with three-dimensional datasets in the exploration of the Sudbury Structure, and who works on the footprint of the Sudbury Mines.

History of discovery and production at Sudbury

The recognition of the geophysical manifestation of mineralization at Sudbury was first made by A.P. Salter in 1857 (Giblin, 1984a,b). Salter observed a deflection in his compass readings in an area close to the current Creighton Mine. The actual discovery of nickel in Sudbury occurred after construction of the Canadian Pacific Railway began near to the future location of Murray Mine (Fig. 1.3). In 1883, Thomas Flanagan noticed copper (nickel) sulfides in the area, which resulted in a detailed geological survey in an area which eventually became Murray Mine.

Figure 1.3   Distribution of Mines and Mine Complexes of the Sudbury Region (Historic Producing Mines, Present mines, and Undeveloped Deposits Scaled to Contained Nickel)

Early prospecting activities resulted in a number of discoveries (see Table 1.1 for a summary of the main events in the evolution of the Sudbury Mining Camp). The early development of these deposits tended to be ad hoc, with small open pits, drifts, inclined shafts, and surface block caving (Whiteway, 1990; Boldt, 1967). Early in the life of the camp, these ores were roasted and then shipped to foreign destinations for processing in well-established plants in locations such as Swansea in South Wales (Francis, 1881). These shipments underpinned the later parts of the industrial revolution in the United Kingdom and the position of Britain at the center of a global empire; it also triggered enormous environmental damage in industrial areas such as the Lower Swansea Valley. Only later on in the development of the camp were technologies developed to concentrate, smelt, and refine the metal in Sudbury from large open pit mine operations such as Frood-Stobie and Clarabelle and underground mine complexes serviced by shafts (Boldt, 1967; Table 1.1).

Table 1.1

Summary of Some of the Important Historic Events in the Discovery, Exploration, and Development of the Sudbury Ore Deposits

Table 1.2

The Sudbury Ore Deposits by Location, Size, and Geological Environment

a Reported reserves and resources come from the last public domain complete reports for 43-101 compliant reserves and resources. Companies do not always report resources. No correction has been applied for mined material.

b Production from the Levack Mine Complex prior to acquisition by FNX; smaller producers including KGHM (previously FNX and Quadra--FNX), and First Nickel produced 95 kt Ni from all Sudbury Basin properties to 2010 (Mudd, 2010).

c Production from Glencore Mines is not broken out by mine; total of 1824 kt Ni in ore produced is from Mudd (2010); the majority of the production came from the Falconbridge Deposit.

The first mine to enter production was Copper Cliff in 1886, with the first local smelting in 1888 (Table 1.1). Geological studies undertaken in 1886 led to the start of production in 1901 at Creighton mine which was owned and operated by the Canadian Copper Company. The discovery of nickel in the Sudbury ores spurred the creation of the International Nickel Company (Thompson and Beasley, 1960). These two divisions became known under the trade name Inco in 1919 (Table 1.1).

Early methods of processing of massive sulfide ore involved the use of open roasting yards where the mined ore was piled on wood and set on fire to burn off the sulfur in the ore and produce a very primitive product that could be shipped and refined. The roast yards were the cause of environmental destruction from the perspective of both the deforestation activity required to support the process, and the sulfur dioxide which killed local vegetation (Gunn, 2011). The early locally roasted product was shipped to the United States for copper refining and at the Orford refinery in New Jersey. The refined copper from the Orford refinery contained a contribution of nickel that had long been recognized as a contaminant in copper production. Nickel-bearing copper has the historic name Kupernickel, where the word Nickel had the meaning of a demon metal as it interfered with the production of a quality copper product.

The recognition of the importance of nickel in steel alloys resulted in a steady growth in the market for nickel, and by 1920 Sudbury produced 80% of the global nickel supply. A steady stream of new discoveries and new mines entered production over the 20th century, and these include the world-class ore deposits at Frood Stobie which began production in the 1920s under the ownership of Inco (Frood Mine) and Mond Nickel (Stobie Mine), eventually to be merged under Inco into the Frood-Stobie Mine Complex. During the 1930s, the copper cliff smelter was completed and this eliminated the need for heap roasting.

Falconbridge was incorporated in 1928 by Thayer Lindsley. Falconbridge acquired mining claims in the town of Falconbridge, and by 1930 the Falconbridge Mine began operation (Falconbridge, 1959). The Falconbridge Mine closed in 1984 once its ore reserves were exhausted.

In the 1940s, nickel production was increased by Inco as nickel was needed in the production of materials to support the war effort. In the 1950s, the construction of the world’s tallest smelter chimney was completed, measuring 195 m tall. This would later be overshadowed by the currently used superstack which was built in the 1970s and is 380 m tall (shown in the photograph at the front of the chapter). In the 1960s, Inco completed the sinking of what was then the world’s deepest shaft, Shaft No. 9 at Creighton Mine which reached a depth of 2175 m.

The Clarabelle Mill was constructed in 1971 and was capable of handling 35,000 tons of ore per day. This allowed multiple mines in Sudbury to produce ore and truck it to one central location. The year 1989 was recorded as a high-profit year for Inco, bringing in over $750 million.

In the last half century, the majority of production in Sudbury has been controlled by two principal companies; although the ownership has changed from Inco to Vale and Falconbridge to Glencore, the production of metal continues, and new discoveries are made as a result of exploration. The mines at Sudbury now exploit deep ore bodies having the deepest operation in North America at over 2400 m in Creighton Mine. As the shallower ore deposits were located and mined, the emphasis increasingly moved to the discovery of new ore bodies at greater depths.

The total historic production of nickel from Sudbury is shown in Fig. 1.5, where the production output of Sudbury (in kt nickel in ore) is shown together with the average yearly nickel and copper grade. The plot also shows the evolution of ownership of the principal deposits as the companies changed names and ownership (Tables 1.1 and 1.2). The principal historic events that had a major impact on the amount of nickel production coming out of Sudbury are also shown, and it is easy to see the major impacts of labor unrest, economic recessions, both the first and second world wars, and the cold war on nickel production. The effects of local and global events on the production profile explain the sudden departures in production from the steady increase through the 20th century that reflected the global increase in demand for nickel in the production of stainless steel, alloys, electroplating, and other industrial uses for nickel metal. The discovery of the Voisey’s Bay Deposit in Labrador in 1994 (Naldrett et al. 1996a,b; McNish, 1999) and the subsequent development of a mine at the deposit in 2005 led to the production of nickel metal in Sudbury from concentrates shipped from Voisey’s Bay to Sudbury for processing. The nickel in these shipped concentrates are not included with the Sudbury total, so the actual production of metal from Sudbury ore has dropped in the 21st century in response to the changing economics and availability of alternative high-grade sulfide concentrate feeds.

Figure 1.4   Synthesis of Sudbury Nickel Production, Nickel Grade, and Copper Grade From Discovery to the End of 2014

The data are sourced from Mudd (2010) with extrapolation to the end of 2014. The principal mining companies and major historical events controlling nickel supply and price are shown.

The historic production of nickel expressed as a ratio for Vale’s seven principal mine complexes and other smaller mines is shown in Fig. 1.5A on the basis of the average production of ore over 5-year intervals. The deposits at Copper Cliff, Frood-Stobie, Levack-Coleman, Garson, Murray, Creighton, and Crean Hill have contributed the lions share to Vale’s historic nickel production with more than 90% of the metal coming from these seven major deposits. Of these deposits, the Frood-Stobie Deposit has produced the largest contribution, followed by Creighton Mine Complex. Over the last 30 years, an increasing contribution of nickel has come from deposits discovered in the north part of the Sudbury Basin at Levack and Coleman. Fig. 1.5B shows the production on the basis of the principal style of mineralization which relates to the position of the ore bodies relative to the base of the SIC (i.e. ore bodies hosted at the base of the SIC are contact type, those in the footwall beneath the contact ore bodies are footwall type, those hosted in the Offset Dykes are offset type, and the remainder are a variant of the offset type referred to as the breccia belt type).

Figure 1.5   (A) Normalized historic production by mine complex based on data from Vale. (B) Normalized historic production from the four principal deposit types described in the text.

The discovery of new ore bodies at Sudbury has resulted from two approaches. One group of discoveries has been made as a result of following the known surface ore deposits to depth by carefully drilling at the edges of the ore bodies and along the trends of known mineralization; in the cases of the very large deposits such as Creighton, Frood-Stobie, Murray, Garson, Falconbridge, and Levack, this approach has been successful and in many cases has maintained a supply of new ores that has pushed the closure of the mine complex well into the future. Another group of deposits have been discovered as a result of exploration work where geologists have identified the possibility of mineralization that is not obviously attached to known ore bodies, and does not extend to surface; these discoveries are often challenging and harder to make, but the use of a combination of geological information, geophysical methods, and geochemistry ensures that the company can minimize both expenditure (to pay for expensive drilling) and risk (failure to find an economic ore body). Exploration success of this type yields new discoveries which, under the right economic conditions, can be mined. This exploration success is important because it shows the value of better datasets, new ideas and approaches, and careful exploration. Examples of this type of new discovery include the Kelly Lake, Onaping Depth, McCreedy East 153, Podolsky, Capre, Victor, Nickel Rim South, and Victoria Deposits. These discoveries together with the un-mined deposits and as yet unfound deposits are the future of the Sudbury Camp.

Position of the Sudbury Camp as a future producer of nickel

Increasing depth of mine operations coupled with the associated cost of mine development mean that the next generation of ore deposit discoveries at Sudbury need to be high-grade; the size and grade of the deposit will need to support the capital and operating costs of the mines and processing facilities. Clever approaches to finding more ore underpin this success, and the exploration geologist is a key player in growing the future development of Sudbury through the discovery of new ore deposits as well as incremental, yet strategic growth in the size of the mined deposits.

Continued pressure on sulfide nickel production comes from global developments. One of these has been the increased global production of nickel from near-surface laterite nickel deposits (Dalvi et al., 2004; Mudd, 2009) using well-tested technologies such as pyrometallurgy and Caron furnaces as well as new technologies such as high-pressure acid leaching of ores. The migration from sulfide sourced nickel to laterite nickel was initially driven by the large laterite resources which could be developed using inexpensive hydroelectric energy to power ferronickel smelters (PT Vale Indonesia operates the Soroako laterite mines and ferronickel plant in Sulawesi, Indonesia; Dalvi et al., 2004). More recently, there has been a transition to low-pressure and high-pressure leaching using hydrometallurgy plants that have been very expensive to develop and slow to ramp-up to full production (Mudd, 2009). Over the past 10 years, there has been a transition toward processing of nickel laterite ores shipped directly from Indonesia and the Philippines, principally to blast furnaces and electric arc furnaces in China. This shift in mining and processing activity was driven mainly by the enormous growth of the Chinese economy in the last 15 years, and their demand for nickel to produce low quality stainless steel (Lennon, 2007). Although many of the electric-arc furnace plants in China remain in operation, the Indonesian Government has now banned direct ore shipments, as it wishes to encourage companies to process ores within Indonesia and develop its industry and economy. This has led to an increase in the amount of directly shipped laterite production from the Philippines.

The position of sulfide nickel as a major component of future production is secured by the fact that laterites contain very little value-added metal (other than cobalt), whereas sulfide deposits continue to generate value from copper and by-product metals such as platinum, palladium, gold, and silver.

The geology of Sudbury in a nutshell

The history of the developments in science that have been triggered or supported by the geology of the Sudbury Structure is an important context to understanding the origin of the ore deposits. In order to explain these ideas, it is first necessary to briefly introduce the geology of the Sudbury Structure. In a nutshell, the Sudbury Structure comprises three principal rock associations which are shown in broad stratigraphic context in Figs. 1.6 and 1.7 together with the presently accepted interpretation of these rocks (based on Grieve, 1994), namely:

1. The 1.85 Ma SIC is a differentiated magmatic body comprising noritic, gabbroic, and granophyric rocks of the Main Mass (Fig. 1.7). The base of the SIC is irregular and the depressions are typically occupied by inclusion-rich mineralized norites and SLGRBX which have a quartz-feldspar matrix which comprise the Sublayer. The Main Mass is surrounded by a number of radial and concentric dykes composed of QD which often contain magmatic breccias in association with mineralization; these are termed the Offset Dykes; they tend to be discontinuous because of late faulting or due to the primary emplacement process which generates physical discontinuities, breaks, and jogs in the dyke; this is the origin of the term Offset Dyke at Sudbury (Figs. 1.6 and 1.7).

2. The surrounding and underlying country rocks are Archean and Proterozoic in age, and are heavily brecciated and disrupted by the Sudbury impact event. These rocks contain pseudotachylite veins, termed Sudbury Breccia (SUBX), that appear to be derived by shock-induced comminution and partial melting of the host.

3. A sequence of breccias which cap the SIC comprise the Onaping Formation, and these rocks grade into progressively upward into deep-water sedimentary rocks of the Onwatin and Chelmsford Formation (Fig. 1.7).

The Sudbury ore deposits broadly group into four types which are discussed in detail in Chapter 4. They include the following:

1. Contact deposits located at the lower contact of the Main Mass in association with physical traps and structures.

2. Footwall deposits which often occur in the country rocks below the contact deposits or the eroded remnants of contact deposits.

3. Offset deposits occurring in the radial Offset Dykes.

4. Deposits associated with wide domains of SUBX and partially melted to recrystallized country rocks.

Figure 1.6   Geological Map of the Sudbury Structure Highlighting Location Information, Terminology, and Nomenclature

Figure 1.7   Simplified Geological Stratigraphy of the SIC and Associated Country Rocks Modified after Grieve (1994) and Lightfoot et al. (1997b).

Terminology used to describe the geology of the Sudbury structure

The literature on the SIC and the associated ore deposits extends back to the early 20th century with classic papers describing the elliptical shape of the complex, and breaking out the rock units of the igneous sequence. It is necessary to introduce this terminology at an early stage so that it is familiar to the reader. The nomenclature at Sudbury was established long before the classification scheme of Streckeisen (1967), and so caution is required in interpreting specific rock names. The names of the rock types (QD norite, etc.) do not match the modern definitions, but it would be inappropriate to change the nomenclature of Sudbury geology after over 100 years of common usage.

The term Sudbury Structure refers to all of the rock formations created by the 1.85 Ga Sudbury impact event (Table 1.3). The SIC is an elliptical-shaped, differentiated igneous body comprising noritic, gabbroic, and granophyric rocks which comprise the Main Mass of the SIC (Figs. 1.7 and 1.8). The SIC is described in two sectors which are broadly the South Range, and the North and East Range (Fig. 1.6) where range refers to the elevated topographic expression within the SIC.

Table 1.3

Summary of Key Thematic Development in Debates on Sudbury Geology