Journal of Petrology Volume 41 Number 12 Pages 1822-1823 2000
© Oxford University Press 2000
BOOK REVIEW |
Potassic Igneous Rocks and Associated Au–Cu Mineralization, 3rd edn, by D. Müller and D. I. Groves. Springer Verlag, Berlin, 2000. xiii + 252 pp. ISBN 3540663711. £44.50, US$79.95
Potassium-rich igneous rocks are one of the most intriguing study subjects in igneous petrology and geochemistry. They show a very wide range of composition, from mildly potassic shoshonitic suites associated with calcalkaline magmas in arc environments to ultrapotassic leucitites, kamafugites and lamproites. Major, trace element and isotopic compositions are very variable, although all show high to extreme enrichment in several trace elements, such as Th, U, Rb, Ba and LREE. Potassic and ultrapotassic rocks occur in various tectonic settings, from island arcs and convergent continental margins to cratonic areas. They also have an economic interest, as they may be associated with mineralization and may host precious minerals, such as the case of diamond-bearing lamproites. Because of all these factors, potassic rocks have attracted the attention of petrologists, geochemists and economic geologists. Yet, books on these rocks are virtually lacking.The book by Müller and Groves fills this gap by giving a review of classification, genesis, tectonic settings and economic aspects of potassic rocks. The book basically consists of two parts; in the first half (Chapters 1–4), the authors elucidate the aspects related to nomenclature, tectonic setting and petrogenesis of potassic rocks, and provide a short description of the main type localities from various tectonic settings. In the second part of the book (Chapters 5–19), the economic aspects are reviewed, placing emphasis on the association of potassic rocks with Au–Cu mineralizations.
Chapter 1 is an overview of the main compositional characteristics of potassic and ultrapotassic rocks, their nomenclature and mode of occurrence. Chapter 2 focuses on the tectonic setting of potassic rocks. Here, the authors show how some of the classical trace element discriminant diagrams do not work for potassic magmas and a new hierarchical scheme is erected to discriminate among potassic rocks from various tectonic environments, including continental arcs, postcollisional arcs, initial and late oceanic arcs and within-plate settings. Chapter 4 is a short description of selected type localities of potassic rocks from the five tectonic settings. Here, the Central Africa Rift Valley occurrence is used as an example of intraplate potassic magmatism, even though these rocks were not considered in Chapter 3, when the database was built up.
Chapters 5–7 face the controversy regarding the relative contribution of magmatic vs metamorphic and crustal vs mantle origin of fluids responsible for transport of precious metals associated with potassic magmatism. Relevant case histories of direct and indirect association between potassic rocks and Au–Cu deposits are described. The considered occurrences cover a wide range of ages and tectonic settings, from the Archaean mesothermal gold mineralizations of the Superior province (Canada) and Western Australia, to the Eocene Bingham (Utah) porphyry copper deposit, Miocene epithermal gold mineralizations of El Indio (Chile) and Pliocene–Quaternary mineralizations of Papua New Guinea. For each occurrence, information on age and nature of mineralization, regional geology, and age, petrology and geochemistry of associated potassic rocks are given.
Chapter 8 contains a short but informative discussion on the behaviour of halogens (F, Cl) in magmatic systems during partial melting and fractional crystallization, and on the role of halogen-rich fluids for the transport of metals in ore deposits related to potassic rocks. Particular attention is devoted to the role of redox conditions of magmas and mantle sources in determining the behaviour of these elements and their capability to transport metals.
In Chapter 9, petrological, geochemical and tectonic aspects discussed in the previous chapters are combined and reconsidered critically to explore the implications for strategies of mineral exploration.
Finally, Chapter 10 consists of various tables that report the main characteristics of various Au–Cu deposits, with details on age, type and exact location of mineralization, production and estimated reserves, texture, mineralogy and geochemistry of associated potassic rocks.
Academic geoscientists are mainly interested in the petrogenesis of potassic magmas and in the implications for composition and evolution of mantle sources. After reading this book, one looks at potassic rocks with very different eyes and better appreciates how many close relations exist between magmatology and economic geology.
In general the book is well written and clearly illustrated with well-drawn figures, and has several simple and informative tables. This makes the book easy to read.
There are also, however, a number of items that most people will probably consider to be questionable or unfounded. For instance, it is not clear why data on rocks from Central Africa were not considered in the database erected in Chapter 3. Yet Central Africa is a typical intraplate occurrence, as stated in Chapter 4, where the Virunga volcanic province is described as a typical example of intraplate potassic volcanism. An additional problem relates to the tectonic setting of various potassic occurrences, given the debate existing in the literature for many of them; for instance, is the Roman province an example of a continental arc or is it a post-collisional magmatic province as suggested by several researchers? The same problems apply to other occurrences, and raise doubts on the general validity of the discriminant diagrams presented by the authors. These, however, look interesting and appear as good tools to characterize several types of occurrences, independent of the tectonic significance they may have. I feel that these diagrams will be extensively used and will certainly stimulate further research on relations between geochemistry and tectonic setting of potassic magmas. Also, the statement that lamproites are typical intraplate rocks will not be shared by those scientists working on lamproites from the western Alps, northern Apennines, SE Spain and other orogenic areas.
Overall, the book is valuable, well written and clearly illustrated, and will be an important reference text to scholars and students interested in potassic magmatism, mineralization, igneous petrology and economic geology.
Angelo Peccerillo
Piazza Università, Perugia, Italy
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