|
|
 |
|||||||||||||||||
|
|||||||||||||||||
 |
|||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Reviews |
"As we further integrate basin analysis with detailed geochemical, radiogenic and stable isotope studies and increase our understanding of the distribution of key elements within sediments, no doubt our understanding of metal sources and genetic processes responsible for sedimentary ore deposits will increase." With these words, volume editor David Lentz summarizes his rationale for the GeoText 4 volume of the Geological Association of Canada. Prior to any other comment, I must first admit that I only got through the full meaning of this statement after having penetrated deeply into the book. Albeit part of the reason for this may result from my lack of expertise in many of the issues covered therein, I believe that my skepticism was mostly due to underestimation of the fundamental contributions that the application of geochemical techniques to sedimentary rocks can provide to our understanding of ore-forming processes and settings. I feel I may not be alone among economic geologists concerning this particular pitfall. That is why I definitively give this book a strong recommendation.
The volume consists of a collection of 11 independent papers, all but one written by individual authors. All authors are acknowledged authorities in their particular fields of research, and thus each paper is a comprehensive review of the current state of knowledge of the various research topics. Collectively, the book reviews an impressive body of published literature. In addition, most papers have been complemented with case studies that often rely on the rich personal experience of their authors. The book is well written and well structured, the sequence of the papers corresponding to an appropriate order.
In an interesting introductory paper, Lentz places the field of geochemistry applied to sediments and sedimentary rocks in an historical context. He reviews the publications that have been benchmarks in this field over the last 40 years (following the major advances in analytical techniques), and takes the reader through the evolving concepts that have built our current understanding of modern and ancient sedimentary systems, and crustal evolutionary models. Finally, he stresses the geologic and genetic relationships between sedimentary sequences, sedimentary processes, and various types of mineral deposits, and anticipates the major lines for future research in this field of knowledge.
The second paper, by McLennam et al., reviews how major and trace element geochemistry and radiogenic isotopes may be used to approach the provenance and sedimentary history of terrigenous sedimentary rocks, and to constrain paleoclimatic, tectonic, and crustal models. The paper contains a wealth of information on sedimentary processes, and emphasizes the dating and tracing techniques based on the U-Pb and 40Ar/39Ar systematics of detrital minerals, and the application of the Sm-Nd, U-Th-Pb, Rb-Sr, and Lu-Hf isotope systems to sedimentary rocks and their components. Many case studies document both the investigation of particular phenomena (e.g., diagenetic control on the distribution of lanthanide and actinide elements), and of particular sedimentary sequences (e.g., orogenic controls on continental drainage in North America during the Phanerozoic, based on the Nd isotope evolution of sedimentary rocks). This large-scope review (more than 300 references cited) provides perspective for the more specific papers that follow.
In the third paper, Nesbitt addresses the effects of weathering, transportation, and diagenesis on mineralogical and bulk chemical compositions of siliciclastic sediments and sedimentary rocks, with emphasis on paleoclimatological interpretations. Special attention is paid to quantitative approaches to mineral proportions and bulk rock compositions. Some examples involving different types of weathering profiles have been presented and nicely illustrated. The next paper focuses on the isotopic evolution of seawater on geologic time scales. The author, Veizer, thoroughly reviews the state of the art concerning the isotopic seawater curves for strontium, carbon, oxygen, and sulfur, over millions to billions of years of geologic time. He shows that, because these chemical species have residence times in seawater in excess of the mixing rate of the oceans, which is approximately 1,000 years, their isotopic seawater variation curves may in fact be the best proxy reflecting the geologic evolution of Earth. Commonly, the isotopic composition of ancient seawater can only be indirectly estimated from seawater-derived sediments, carbonate and phosphatic shells, and biochemical minerals, such as carbonates or sulfates. The problems involved and the limitations of the various methods are discussed, with particular emphasis on the controversial interpretation of the oxygen isotope record of ancient marine carbonates. Finally, Veizer stresses the utility of seawater isotope trends as tracers of large-scale evolutionary features of Earth. Strontium, in particular, is believed to have the greatest potential as a tool to correlate Precambrian sequences, or Phanerozoic strata that have poor stratigraphic control (isotope stratigraphy).
Two relatively short papers review, respectively, the Rb-Sr, Sm-Nd, and Lu-Hf isotopic approaches to sedimentary provenance and crust-mantle evolution, and the use of the Re-Os system as geochronometer for organic-rich sediments. After a brief introduction on the Sm-Nd, Lu-Hf, and Rb-Sr systematics of sedimentary rocks, Patchett discusses the application of radiogenic isotopes, namely Sm-Nd and Lu-Hf, to studies devoted to the evolution of global sedimentary systems, and orogenic versus cratonic sedimentation. Creaser, in turn, introduces the Re-Os isotope system and, through various examples, highlights the potential of Re-Os data from ancient organic-rich sediments to yield both depositional ages and information of paleoceanographic significance.
The seventh paper, by Fralick, deals with the use of ratio techniques in evaluating and interpreting geochemical data from clastic sedimentary rocks. The paper contains three main sections, each one illustrated with relevant case studies. Firstly, the author examines the relative immobility of the various elements on the basis of ratio techniques, and gives insight into the application of ratios of immobile elements (whose primary mineral phases behave similarly in hydrodynamic systems) for defining sediment sources. Then, he demonstrates how ratio normalization techniques applied to clastic sedimentary sequences allow one to define and compare different geotectonic settings. The paper ends with a section that emphasizes the potential of ratio techniques in interpreting hydraulic sorting in paleoplacer deposits.
In the next paper, Cox reviews, discusses, and illustrates current imaging and geochronological techniques used for characterizing detrital zircon. By compiling information from a large number of sources, the author summarizes and tabulates the morphological and chemical criteria that can be used to distinguish zircon types in sedimentary rocks as a function of zircon source (different rock types and/or different geologic settings). Cox also addresses the major principles of U-Th-Pb systematics of zircon, and discusses the advantages and pitfalls of the various, single-crystal dating analytical techniques currently available, from ID-TIMS to fission-track, passing through ion-microprobe and LAM ICP-MS techniques.
The last group of three papers provides detailed reviews of three elucidative examples of the direct or indirect association among sedimentary systems and a number of ore-forming systems. Brown discusses the application of red-bed–sourced metals to current genetic models for sedimentary rock-associated copper, lead, and uranium-vanadium deposits. In this paper, he points out that all these types of deposits result from low-temperature aqueous solutions, commonly brines, which can account for significant transportation of metals under oxidizing and near-neutral pH conditions, such as those resulting from equilibration with red beds. Hence, red beds can constitute reservoirs for metalliferous solutions, which would precipitate their metals upon migration to nearby or distant redox boundaries. In the next paper, Conveney reviews the role metalliferous Paleozoic black shales play in the generation of several redox-controlled sedimentary ore deposits. The author uses examples resulting from his own research experience: the German and Polish Kupferschiefer, the U.S. Pennsylvanian shales, and the Chinese Cambrian shales. Last, but not least, Peter describes the geologic setting, mineralogy, geochemistry, and genetic aspects of iron-rich metalliferous sediments related to massive sulfide deposits (Algoma-type exhalites). Peter’s review relies on a very large number of relevant examples extracted from the literature, but particularly on his outstanding database from the Bathrust mining camp, Canada.
The book is carefully edited. Only minor editing mistakes have been noted. The overlap among the papers is never significant enough to be felt as an inconvenience. On the contrary, some minor redundancy looks intentionally admitted by the editor and contributes to increase the clarity of the text as a whole. The small size of the font used in printing the book makes some figures a bit hard to read. The book is supplemented by an index.
This volume represents a valuable contribution to readers with several different motivations. All papers provide concise introductions to the various aspects treated, which enlarge their intended audience. The book has the important virtue of putting together, in a comprehensive way, a wide range of geochemical techniques as applied to sedimentary rocks. By doing this, it provides an integrated view of a geologic discipline whose significance is frequently underestimated. I found, however, that the book might have benefited from the inclusion of two or three complementary papers dealing with other types of sediment-related ore-forming processes (e.g., base metal supply in sedex deposits), or present-day submarine hydrothermal activity (e.g., geochemical signatures of hydrothermal contributions to marine sediments close to active hydrothermal sites). The readership of this volume is primarily likely to include researchers specifically interested in geochemistry of sedimentary rocks, both academics and graduate students. But it also should include economic geologists intending to become more familiar with some quantitative tools and interpretative approaches that may lead to a renewed look over former interpretations of specific deposits, mineralization processes, or metallogenic provinces. This is certainly the way to enhance our current understanding of ore-forming models, and to succeed in tracing new vectors for mineral exploration.
Jorge M.R.S. Relvas
CREMINER/Departamento de Geologia
Faculdade de Ciências, Universidade de Lisboa
Edifício C6, Piso 4, Campo Grande
1749-016 Lisboa
Portugal
December 7, 2004
| ||||||||||||||||||||||||||||||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
