Abstract
This is a great book. In the Preface, the author writes:
This book is designed for readers with an interest in gold, science, the mineral industry, and natural curiosity. It is based on the author's career researching and teaching about the formation of gold deposits; and is written particularly for geoscience graduates, postgraduates, and professional geologists from the mineral industry. The book will be relevant to company leaders, stockbrokers, curious prospectors, and retired scientists.
The book is well written, comprehensible, and very readable. Step for step patient explications of geoscience should be welcome by the second group of above suggested readers. At the same time, it is highly original in its scientific contents. The author does not hesitate to contradict or correct majority opinions, to the point of at least once employing the ‘dirty’ word falsification. So even if you are not one of the author's selected group of readers, you should enjoy the book.
The author's skill in mingling short tales with science starts right in the Introduction (Section 1) with an underground visit to a small gold mine (Water Tank Hill mine in Western Australia). There, gold occurs at the contact of quartz veins crosscutting banded iron formation, revealing that gold was younger than the iron beds – a moment of illumination. Furthermore, we learn some basics about gold, the purpose of the book in satisfying pure curiosity, but ranging to the serious matter of assisting the annual $100 billion gold industry. Gold ore and deposits are sketched. Also, the binary gold-only and gold-plus classes of gold are first introduced, which the author calls an unreasonably effective idea (i.e. providing unexpected solutions). The two are marked by different oxidation states (Au1+ and Au3+) and association with economic basemetals in the second case. Figure 1.1 shows an artistic painting that illustrates effects of weathering on the distribution of near-surface gold.
Setting the scene, Sections (2) and (3) approach essentials about mining, its techniques and economics, and the nature of gold ore. On page 10, the author introduces a standard gold mine with a resource of 3 Moz (3 million Troy ounzes = 100 tonnes) of gold, that he comes back to several times throughout his book. Using typical figures, he demonstrates calculating costs and profits, and possible risks of investing in a gold mine. Tables 1 and 2 contain information on minerals and terms that are required for further reading. Figure 3.1 illustrates that most of all-time global gold production came from the gold-only class of deposits. The sequence of major components of the book in the following main part is painted in colourful Figure 3.3 (p. 19). In order to explain the ‘Formation of Gold Deposits’, sections span a great arc from (4) Provinciality of Goldfields that includes detailed global historic and present production data to final Section 21, Examples of Gold Deposits including discovery histories.
Allow me to list a personal selection of some highlights of the book:
The book is rich in information on many globally important gold provinces, fields (Table 4.3), and deposits, presented in the text and often supported by data in tables and imaged in figures. Laying the base for the source – transport – trap gold deposit formation model, the author cites gold contents in average continental crust with about 0.002 ppm (g/t) Au (=2 ppb), occurring as an extremely diluted trace element (Section 5); background Au in different crustal rocks (Table 5.2) varies between 0.5 and 10 ppb; its nanoscale dispersal prevents economic recovery although 1 km3 of average rock holds approximately 6 tonnes of gold. Therefore, the scale of a gold deposit formation system must be measured in kilometres (more than 16 km3 of rock must be leached to produce the above-mentioned standard deposit/mine). Economic gold ore, typically at 5–10 ppm, requires an up to 10, 000 times enrichment. For gold, partitioning between rocks, and hot fluids or melts (p. 55) is the most likely process. Two ore fluid types occur in gold deposits: Low salinity CO2 + H2S + H2O fluid inclusions are found in gold-only deposits, whereas gold-plus deposits are characterised by saline fluids and brines. Considering that gold-only deposits are predominant and that base metals such as Cu-Pb-Zn in crustal rocks are much more common than gold, it is clear that there must be processes that separate (‘segregate’) Au from the base metals. Likely agents are selective dissolution or selective precipitation. In Section 6, the author sketches the effects of segregation, i.e. the different enrichment of base metals in gold-only and gold-plus deposits. Geological observations that allow the determination of relative timing between gold mineralisation, alteration of host rock, metamorphism, deformation and structures (such as the BIF banding and replacement of magnetite by pyrrhotite in Water Tank Hill mine; Figure 7.3) are important controls in genetic studies of the formation of gold deposits. Radiometric age dating may not always provide clear answers (cf. Carlin p. 212). Commonality and Diversity are important features for understanding and classifying gold deposits. The first is expressed by alike deposits forming provinces, and similar enrichment, segregation, timing and ore fluids, mainly controlled by conditions and processes in the source. Diversity arises from proximal influences near the deposit such as host rock and structures. In Section 10, the author investigates the hypothesis that gold deposits can originate from crustal magma; reporting studies from various gold provinces (Abitibi, Zimbabwe, Barberton, Charters Towers, and Victoria), he concludes that true granitic origin is not feasible. Most former assignations were based on spatial co-occurrence of granites and gold deposits, which is no proof that melts were directly involved in gold concentration. Apart from a short note on page 160, the author disregards mantle melts, however, that do form gold ore, e.g. in gold-only prophyries. Many explications in this book are supported by chemical information. In Section (10) Fluids in the Earth's Crust, the polarity of water molecules, and its dielectric constant are cited, which facilitate dissolution of metals, essential in metallogenesis. The author's treatment focuses on metamorphic water because this is a major agent in the formation of gold deposits (Section 15). Devolatilisation of giant rock volumes by increasing metamorphic temperature (T) and pressure (P), and simultaneous dissolution of nanometric gold is the source process system of gold-only deposit formation. Figures 11.11 and 11.12 provide thermodynamic plots of the critical transition of metabasalt from greenschist to amphibolite facies, and the mole % of aqueous fluid and CO2 released. The author emphasises that these findings explain three main characteristics of gold-only deposits: the scale of provinces, timing, and the fluid composition. Section 14, the Hydrothermal Transport of Gold, holds rich data and information that support understanding gold deposit formation from the metal's solubility to its migration and deposition. Devolatilisation requires heat because the reactions are endothermic. Upflowing fluids form metamorphic gold-only deposits at about 350 ± 50oC and at 0.5–5 km depth (see also page 1.18 ff.). Heat pulses originate mainly in accretionary and supra subdduction settings (p. 160). Hydrothermal alteration of host rocks by ore fluids is a useful guide and vector to ore. A case study of the giant goldfield Kalgoorlie (Section 13) is highly instructive. We learn of the geological setting, rocks and structures, chemical and mineralogical alteration, and of its zoning, and the causes of the giant Au accumulation. The mode of Au deposition in the Golden Mile is paradigmatic (precipitation by reaction of fluids with Fe2+ in dolerite). Remarkably, giant Carlin and Witwatersrand are also described and presented as gold-only deposits. The treatment of features and the genesis of gold-plus, copper-gold deposits (Section 19; IOCG, porphyries and volcanic-epithermal) concentrates on crustal processes; subduction or other mantle-related processes are briefly mentioned. Since 1980, the Archean Yilgarn Craton in Australia is one of the World's leading gold producers, due to a steady stream of discoveries. Exploration success was mainly supported by regolith science and gold geoscience. In Section (20) the author reports on the intellectual input, thought processes, and nuanced decisions that triggered and upheld the performance. Section 21 tells in much details the discovery of Fosterville Deeps deposit in the Victorian Gold Province. The site was one of the thousands of minor gold deposits discovered after the rushes in 1851. The shallow free-milling oxidised heap-leaching Au ore at Fosterville had earlier been wrongly interpreted as of hydrothermal-epizonal origin, alike to most Carlin ore. In the Carlin gold province, the presence of primary sulphide ore beneath a thick regolith of friable clays and iron oxides was first recognised in the Screamer zone beneath the Goldstrike deposit, marked by auriferous quartz veins, calcite, pyrite, and arsenopyrite (p. 202). Similarly at Fosterville, deep gold ore was predicted, based on 30 years of published research, including the author's. Deep drilling from 2015 to 2020 yielded multiple core intersections of primary sulphide ore grading over 1000 g/t Au. Today, a low-cost high-grade mine is established. In 2020, the operation produced a record 640, 467 ounces of gold at an average grade of 33.9 g/t Au and average recoveries of 98.9%.
Phillips is an experienced and independent thinker whose reasoning, opinions and judgement concerning gold deposit formation must be seriously considered. In most cases, his deviations from majority views are based on detailed arguments that can be examined and weighed. And, he has seen and studied the geology of many mines that he describes. I recommend The Formation of Gold Deposits by Neil Phillips without reservation.