Applied Earth Science: Transactions of the Institutions of Mining and Metallurgy

Discovery of a goldfield is a rare and difficult event, which contrasts with the normal outcome of exploration that is non-discovery. As there are many gold provinces globally and numerous discoveries annually, the recent and historical records of success are a great source of ideas to learn from and to enhance the rate of discovery.

Over the last two decades, important gold discoveries have been made in well-established gold provinces such as Australia, Canada and USA, regions with a long history of production like Ghana, and some further countries such as China, Mali, Tanzania and Peru that received lesser attention during the twentieth century. In contrast to the dominance of gold-only deposits throughout the last millennium of gold production, a significant proportion of recent discoveries are gold-plus, especially Cu–Au.

There have been some golden periods of exploration success in various parts of the world and as well as providing inspiration for exploration, they are sources of learning. Examples include the discoveries in Victoria (1850s), in the Witwatersrand (1886) including Carletonville (1930s) and Welkom (1940s), in the Carlin gold province of Nevada (1961 and 1980s), in the Yilgarn Craton (1980–1990s), and SW Pacific (1980s). Any area that is the focus of one of these golden periods attracts increased exploration activity and funding at the expense of less productive areas.

There is additional knowledge to glean from non-success. Despite being the world's major source of gold for the twentieth century, South Africa stands out for its lack of recent exploration success with no new goldfields discovered since Evander in 1951. This lack of exploration success has led to an 80% fall in gold production since the 1970 peak, and one significant consequence is a major decrease in revenue, and hence employment, in that industry.

Important ingredients in exploration success are area selection, appropriate exploration technologies and models, and skilled and motivated people. Some commentators have added luck to this list though this may be mis-guided. Instead, the thesis presented here is that doing exploration is like doing science; mineral discovery – like scientific breakthrough – is a rare event, and the way teams think and interact is a very important determinant of success in both mineral exploration and in science. If this thesis is valid, then teaching explorers to think and creating environments in which they can do so should favour discovery. It cannot be taken for granted that systematic thinking processes will be either taught or learned at all institutions, but rather there are places and people who enhance the development of thinking skills.

Exploration teams need to safe-guard against groupthink by the involvement of self- and external critical evaluation. Unorthodoxy will always have an important place in exploration while discovery remains a rare event well-removed from the norm; this is parallel to scientific revolutions which often owe their origin to unorthodox thinking and attention to minor anomalies. Introducing simple practices such as a fostering of a single-minded focus on discovery, maintaining a line-of sight from activities back to one's aims, encouraging unorthodoxy and avoiding the pernicious mentality of groupthink can be adopted at little cost.

For applied geologists working in the minerals industry the tasks of problem formulation, observation and data collection, interpretation and modelling invoke various philosophical considerations whether the practitioner is aware of them or not. A primary goal of applied geologists is to build models that accurately predict reality to an acceptable degree. In this paper, we describe the key philosophical frameworks proposed for conducting scientific investigations and relate them to the field of applied geology. We consider the very important differences in the types of problem confronted in experimental sciences (such as physics and chemistry) compared to the historical sciences, such as geology, where the processes studied are unique and only evidential traces of past events are available. The prediction quality of models is likely to be materially improved if the geologist is firmly and consciously practiced in the scientific method. In addition, if the predictions are framed and presented in terms of the underlying science, the quality of decisions made based on those predictions will likewise be improved. The implications for creating additional value to a project or operation can be very significant when geological models are constructed and used by a practitioner with an understanding of the philosophical basis of the activities constituting a scientific investigation. The method of multiple working hypotheses is particularly important when working in historical sciences. We argue that working within the framework of multiple working hypotheses can provide a valuable insurance against the adoption of, or persistence with, flawed models.

The abundance of gold in crustal rocks is an important constraint on the formation of gold deposits. Gold concentrations in unmineralised igneous, sedimentary and metamorphic rocks range from 0·05 to 20 ppb with average concentrations commonly between 0·5 and 5 ppb. Analytical methods with ultra-low detection limits are required to observe the full range of concentrations. Gold concentrations in igneous rocks are strongly controlled by the behaviour of sulphur. Higher gold concentrations occur in sulphur-undersaturated rocks compared to sulphur saturated igneous rocks. Mid ocean ridge basalt has lower gold concentration than ocean-island and volcanic-arc basalt, due mainly to lower oxygen fugacity at MOR settings that causes sulphur saturation. Gold concentrations in sedimentary rocks increase with increasing abundance of diagenetic sulphide minerals and organic matter. Gold concentrations in metamorphic rocks decrease systematically with increasing metamorphic grade. Amphibolite facies rocks commonly contain between 50 and 80% less gold than their unmetamorphosed protolith rocks.

The analysis of copper production data from the Iberian Pyrite Belt allows the identification of different phases of copper mining in the last 70 years, corresponding to technological and economic changes. The statistical data indicate that the belt has passed its copper production peak. This is the result of the depletion of existing reserves, with mining of ores with progressively lower grades. These reserves are only being replaced at a slow rate, because exploration success has been decreasing. This is despite the large exploration effort, as evidenced by the length of recent exploration holes. This probably implies that the belt will enter into a new cycle of its long mining history, while the world as a whole will eventually reach its copper peak.

The opening of the Yandal gold province during the 1990s provides a detailed case history of repeated exploration successes. First highlighted in 1992 with the discovery of the Bronzewing gold deposit, the resources of the Yandal gold province grew rapidly through the 1990s with a series of discoveries. By 2000, the gold endowment of the province was 470 tonnes (t), and it was producing 25 t Au p.a. from the Jundee, Bronzewing and Darlot mines. The exploration activities in the deeply weathered and mostly covered Archaean Yandal greenstone belt during this period set the standard for Australian gold exploration in deeply weathered terrains through a twofold approach that involved drilling 100 000 holes, and at the same time making a strong commitment to ore deposit research. Confidence came from an understanding of Archaean gold deposits and the regolith, including the effects of gold dispersion in the weathered zone and the necessity to properly penetrate the Cainozoic cover for effective drill testing. Special emphasis was placed on gold distribution, alteration, lithologies and structural geology, and these soon became the focus of both data collection and drill targeting during the exploration process. Aeromagnetic data provided the platform for interpolation between geologically constrained areas in this province of less than 10% outcrop. Rapid and effective communications and data handling were critical issues for managing large exploration programs involving many geologists in remote areas.