Metal Deposits in Relation to Plate Tectonics

475 Pages·1990·17.091 MB·English

by  Dr. Frederick J. Sawkins (auth.)

Minerals, Rocks and Mountains 17
Frederick J. Sawkins
Metal Deposits in
Relation to Plate
Tectonics
Second Edition
Minerals and Rocks 17
Editor in Chief
P. J. Wyllie, Pasadena, CA
Editors
A. EI Goresy, Heidelberg
W. von Engelhardt, Tiibingen . T. Hahn, Aachen
F. J. Sawkins
Metal Deposits
in Relation to
Plate Tectonics
Second Revised and Enlarged Edition
With 246 Figures
Springer-Verlag Berlin Heidelberg GmbH
Dr. FREDERICK J. SAWKINS
Department of Geology and Geophysics
University of Minnesota
108 Pillsbury Hall
310 Pillsbury Drive S. E.
Minneapolis, MN 55455, USA
Volumes 1 to 9 in this series appeared under the title
Minerals. Rocks and Inorganic Materials
ISBN 978-3-662-08683-4 ISBN 978-3-662-08681-0 (eBook)
DOI 10.1007/978-3-662-08681-0
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Preface to the Second Edition
Almost all the reviews of the first edition of Metal Deposits
in Relation to Plate Tectonics have been positive, some en
thusiastically so. The dissenters either represent the dimin
ishing ranks of those still dubious about the far-reaching im
plications of plate tectonic theory, or they seem to be tied to
rather special interests. Whatever the level of acceptance of
the first edition, I am keenly aware of certain serious short
comings, some related to omissions, others to matters of
balance in emphasis.
This edition attempts to rectify these faults and to incor
porate at least some of the significant advances in the avail
able data and conceptual thinking that the last five years
have brought to the science of economic geology. In addi
tion, in recognition of current and, almost certain, future
emphasis on gold within the exploration community, I have
paid particular attention to the geological settings and
generative models of gold deposits of all kinds.
There doubtless will be much scepticism in certain
quarters regarding the emphasis that has been placed on the
importance of magmatic-hydrothermal processes for the
vast majority of the metal deposits that are formed in arc
systems. My position on this matter has been long standing,
influenced by due consideration for the geology of arc
related metal deposits and for the specifics of their distribu
tion in time and space, as well as by the constraints of
geochemistry. Geochemical modelling certainly has its place
in the spectrum of ore deposits research, but its use without
tight geologic constraints and concern for physical processes
has led some investigators up some very blind alleys.
The basic format of the first edition has been retained,
including the suggestions for exploration. I fully appreciate
that parts of these latter segments will seem naive to ad
vanced practitioners of the exploration business, but, as ex
plained in the original preface, my intent is only to help the
student or more general reader to appreciate the fundamen
tal connection between genetic concepts and their utilization
in exploration strategy. Few books dealing with ore deposits
offer this additional information.
VI Preface to the Second Edition
As before, I have attempted to assemble a bibliography
that is as current as possible and that is not replete with en
tries that have limited availability. One is always conscious,
however, that all around the world a great deal of excellent
work is either in publication or in progress and thus lies un
discovered by the writer. R. H. Sillitoe again kindly agreed to
edit and review my initial, somewhat raw, typescript, and in
the process readily shared his enormous experience and ex
pertise regarding the occurrence and formation of hydro
thermal metal deposits.
Finally, my thanks to those who, either through pub
lished reviews or personal contact, have voiced their support
for my efforts. It is in large measure because of them that
I find encouragement to attempt a second edition relatively
soon after the appearance of the original.
Frederick J. Sawkins
Preface to the First Edition
I attempt this volume with no small degree of trepidation,
for despite the near universal acceptance of plate tectonic
theory, and the undeniable sweep and power of the concepts
involved, points of controversy and uncertainty still abound.
This is especially true with regard to the limited extent to
which many ancient geologic terranes, however well studied,
lend themselves to plate tectonic interpretation. There is also
considerable controversy regarding the genesis of many
metal deposits, and this impacts upon the degree to which
such deposits can be meaningfully related to the tectonic set
tings in which they occur.
It is relatively simple matter to point out that certain
types of ore deposits exhibit impressive time-space associa
tions with certain kinds of more recent plate boundary en
vironments. It is also all too easy to posit plate tectonic-ore
deposit relationships in ancient geologic terranes. What is
more challenging is the recognition of the boundaries
beyond which speculation is idle, but within which useful
new insights regarding the relationship of certain ores to
their lithologic and tectonic environments may emerge.
My interest in plate tectonic-metal deposit relationships
was initially motivated by a desire to find a suitable
framework within which students could be introduced to the
wide variety of distinctive metal deposit types. Somewhat
later it was fostered by the desire to investigate the degree to
which such concepts could be used in creative planning of
exploration programs. However, I have encountered a certain
degree of confusion about metal deposits and their divergent
types amongst students, exploration geologists, and geo
chemists interested in ore genesis problems. We all have
much to learn regarding these matters, but an attempt at
synthesis seems warranted at this time, especially in view of
our increasing dependence on earth resources. I m~ke no
claim that application of plate tectonic concepts can pin
point new metal deposits. The main tool available to the ex
ploration geologist is that of analogy with respect to other
deposits and to their lithologic settings, i.e., certain types of
VIII Preface to the First Edition
metal deposits occur in association with certain types of
rocks. The importance of plate tectonics is simply that plate
interactions spawn various types of lithologic assemblages
and, thus, such concepts can considerably sharpen our
perceptions and interpretations of geologic terranes. As
such, they can aid the exploration geologist in his (her) eval
uation of various lithologic sequences and the types of metal
deposits that might have been generated within them.
The study of metal deposits has accelerated a great deal
in the last two decades, in particular through geochemical
research. The application of fluid inclusion, stable isotope,
and various experimental techniques to the study of metal
deposits has broadened our insights into the chemical and
hydrodynamic aspects of ore formation. These advances
have, in turn, allowed the formation of more realistic con
ceptual models for various types of ore generating systems.
Such models are of considerable aid to the exploration and
mining geologist in areas of known mineralization, but are
of more limited utility in terms of the search for new mineral
districts.
The problem of the geographic distribution of metal
deposits must be addressed initially in terms of geologic and
tectonic environments rather than the nuances of ore solu
tion geochemistry. Endeavors in this field, it seems, have not
kept pace with those of a more geochemical nature (e.g.
Barnes 1979). In part, this is because geochemical research
tends to be more 'tidy' and amenable to institutional fund
ing and publication than the rather less rigorous and more
empirical aspects of regional synthesis and compilation. The
latter have to be based on some combination of personal ex
perience and literature research. An important point in this
regard is that metal deposits and the systems that generate
them need to be viewed not as geochemical accidents, but
rather as fortunate culminations of normal geologic and
geochemical processes.
No one has made this point with more eloquence than
Wyllie (1981), who states that metal deposits are not "the il
legitimate offspring from random couplings of rocks and
fluids from indeterminate sources:' but "have respec
table . .. family lineages, with ancestors deep within the
continental crust, or below ie'
The extent to which ore generating systems can proceed
to fruition and where they develop depends on a host of
local factors, but the broad environmental controls in each
case will be tectonic. It follows that if these tectonic controls
result in the main from plate interactions, and if plate tec-
Preface to the First Edition IX
tonics have operated throughout much of earth history, then
a variety of environments favorable for ore generation must
have repeated themselves through geologic time. A conse
quence of these observations, if correct, is that major ir
regularities in the time distribution of various metal deposit
types require explanation.
Ore deposits can be broadly divided into those generated
by endogenetic processes and those generated by exogenic
processes. The former are invariably associated with thermal
processes and, in general, can be related more readily to
magmatic and tectonic events instigated by plate activity.
Deposits formed by surficial processes such as weathering or
shallow marine sedimentation will have relationships to their
tectonic environment that are more tenuous.
This volume represents an attempt to provide a rational
basis for the observed time-space distribution of metal
deposits, at least those of endogenous type. Such deposits
can form by a wide variety of mechanisms in a highly diverse
spectrum of geologic environments, and it follows that their
relationships to plate tectonics will vary from significant in
some instances to more tenuous in others. In fact, it is im
portant to realize that even in cases where an impressive as
sociation between plate tectonics and certain ore types is
manifest, plate interactions merely provide the master con
trol for the particular geologic environments within which
such deposits tend to form.
What I propose to do in the chapters that follow is deal
with specific plate tectonic environments and cover the ore
deposits that can be associated with each. Descriptions of
individual type examples for which abundant data exist are
provided to add substance to the volume. Additional ex
amples are catalogued, where feasible, but any exhaustive
compilation of world metal deposits is clearly beyond the
scope of this book. Certain of my interpretations of distant
ore deposits garnered from the literature will inevitably be in
error, and possibly enrage some of those with first-hand
local knowledge. To such people I can only offer an advance
apology and an exhortation to publish and set the record
straight. The line between creative synthesis and idle
speculation is an extremely fuzzy one. I have included,
wherever feasible, some thoughts on how the relationship be
tween tectonics, geologic terranes, and metal deposits can be
used in the context of exploration planning. Inevitably, such
thoughts will be of a speculative nature, but the literature on
ore deposits is notably lacking in attempts to do this. A con
certed attempt has been made to include up to date ref-
x Preface to the First Edition
erences and to avoid references pertaining to obscure sources
or unpublished material, so that the interested reader with
access to a good geological library can backtrack on any
particular subject.
It is hoped that this book will prove useful to students
at the advanced undergraduate and graduate levels, and to
mining and exploration geologists. lowe no small debt of
gratitude to various company geologists around the world
for their willingness to share freely of their time and local
expertise with an itinerant academic. Without their enor
mous, and sometimes unheralded, contributions, the science
of economic geology would be sad shape indeed, and in
recognition of this, I dedicate this volume to them. I am also
indebted to the front line troops such as Richard Sillitoe,
who, not only have covered vast amounts of territory, but
have written lucidly and imaginatively about their observa
tions.
Finally, it is hoped that we will be able eventually, to use
"fundamental ore deposit geology to elucidate plate tec
tonics, not (only) the reverse" (Guilbert 1981).
Acknowledgments. The initial typescript of this book was produced by
Kathy Ohler, for whose competent, patient, and always cheerful help I am
most grateful. The figures were drafted by Jim Kiehne, and I thank him for
his timely aid.
lowe a special debt of gratitude to Richard Sillitoe who reviewed nearly
all stages of the manuscript and corrected a number of my misconceptions.
Discussions with my colleagues at the University of Minnesota were also
most helpful. Finally, my thanks to my wife Patricia Davis for many sound
editorial suggestions, and to Peter Wyllie for initially suggesting the project
and for encouragement during its evolution.
Frederick J. Sawkins