Dating and Tracing the History of Ore Formation - Chapter 13

Geochemistry & Hydrochemistry

Stein generated Chapter 13.4.1:  A Holistic Approach to Ore Geology in 2014. He stated in the introduction that Economic geologists study ore deposits. Two pieces of information are critical for their study – the timing of ore deposition and source rock. The compositions of ore-forming fluids, whether measured from fluid inclusions or estimated from ore minerals, are used to back our way into the question of source rock. But in reality, these data are merely in situ end measurements based on long and arduous journeys of fluids in the crust.

Timing of ore deposition has been tackled through traditional U–Pb and ⁴⁰Ar–³⁹Ar dating of minerals associated with magmatic–hydrothermal processes, and more recently with Re–Os geochronology where the sulfides themselves are directly dated. In addition, Re–Os offers a tracer isotopic ratio, ¹⁸⁷Os/¹⁸⁸Os, that provides information on the ore fluid source and/or its history. Timing and source are the pieces that allow us to move from description to interpretation at a scale beyond deposit, district, and mineral belt, and allow us to decipher the processes that move and distribute material among Earth reservoirs (Figure 1).

Breakthroughs and new understanding of ore deposits have come largely from outside the traditional discipline of economic geology. Most traditional economic geology literature concludes simply that ore fluids are either mantle-derived or crustally derived based on tallies of isotopic or other geochemical data at the deposit scale. But the many processes that move metals from their true source through the crust to a welcoming locality for ore deposition are not easily accessed by our data assemblies. A better understanding of those processes will improve our identification of source rocks. In the next decade, skill sets that account holistically for downward metal transport from aerosols to photic zone to seabed, balanced by cycling of metals from crust to mantle and mantle to crust, will be in demand.

Dynamic plate reconstructions pinned to absolute time based on direct dating of sulfide are essential to understanding metallic resources. And conversely, reconstructions must be informed by the many well-organized reserves of descriptive information that exist for ore deposits. Rather than geographically outlining mineral belts, it is essential to place craton, subcraton, and craton–margin processes on reconstructed paleocontinents to interpret the metallogenic past. Many compilations of metallogenesis through time are based on amassed literature age data without informed understanding of quality, context, and technique employed. These compilations are often carried out by those without any expertise in geochronology.

This approach can lead us astray. Meaningful geochronology requires ages for samples placed in well-constrained spatial and paragenetic contexts. We now have the technology to build real time bins for metallogenic processes from hard isotopic data acquired directly from the ore minerals. To bring ore geology to a new level of enlightenment requires that economic geologists merge their expertise with workers from diverse fields, from mantle dynamics to isotope geochemistry.

To understand ore deposits, we need to understand metal mobility through time in a three-dimensional (3D) whole-Earth perspective. This book is specific to mineral resources. Therefore, the emerging field of Re–Os isotope geochemistry that enables direct dating of ore minerals is emphasized. The details and systematics for other traditional dating methods can be found in textbooks and in the literature. Re–Os dating of sulfides and oxides is used to illuminate the fundamentals of radioisotope geochronology. This chapter presents issues tackled to bring Re–Os dating into the field of economic geology and, in turn, shows how sulfide chronology has begun to contribute to disciplines external to economic geology. Real examples are used as illustrative material. These examples will assist the economic geologists without a background in geochronology to work more knowledgably and effectively with age information.

Finally, many of the articles cited in this chapter were selected, in part, because they fall outside the cycle of literature quoted in traditional economic geology journals. The aim is to draw attention to highly relevant literature in ancillary disciplines – literature that offers much toward understanding ore genesis. This provides a holistic foundation for understanding fluids in the crust and their journey toward ore formation.