Developing sustainable mineral exploration targeting tools: titanite as an exploration guide and probe of porphyry ore system fertility

Ore deposits are the source of metals that underpin modern society. In a world with a changing climate, they are critical for the technologies that reduce our carbon footprint. However, ore deposits are scarce and increasingly difficult to find, with the best quality (lowest impact) resources likely to be buried beneath up to a kilometre of barren rock. Only one in every thousand prospects that are discovered is eventually developed into a mine, and the identification of such prospects has already impacted the environment and consumed significant resources. Consequently, recognizing the potential endowment ('fertility') of a particular belt of rocks at an early stage of exploration is essential for reducing risk, costs, energy use and environmental impacts. The challenge of then locating deposits at depth represents the new frontier of sustainable mineral exploration.

Porphyry-type ore deposits source much of the copper, molybdenum and gold utilized by humankind, and are also important repositories of critical elements used in key environmental technologies. Their formation is related to magma generation in subduction zones, the place where most of Earth's continents are created. Magmas produced in these arcs are mostly erupted to form volcanoes or are emplaced at depth as barren plutons. Rarely, they are transformed in such a way that they become capable of generating vast volumes of highly metalliferous waters. However, our understanding of what triggers this change is limited.

Titanite is a common accessory mineral in igneous rocks and often occurs in porphyry systems. It is also found in the hydrothermal alteration zones that surround these deposits. It is of interest because it could record the trigger events that fertilise arc magmas, and it could also act as a remarkable probe of hydrothermal evolution, tracking the changes in properties of fluids as they migrated away from the ore deposit. In exploration, such gradients can be tracked back 'upstream' to the orebody itself. Because it is a fairly hard mineral, it survives weathering and can become concentrated in stream sediments, thus could be used in regional exploration screening.

The objectives of this project are to establish titanite chemistry and its crystallization history in igneous rocks from selected barren and well-endowed magmatic arcs and from across the alteration zone of a well-constrained mineralized system. We will also utilize a remarkable archive of titanite grains from more than 25 sites worldwide, provided by the CASE partner. Analysis of this unique sample suite will allow us to identify and understand differences in magmatic evolution that control arc fertility and to characterize the mineral chemistry gradients present around mineralized systems. Our primary scientific aim is to develop a process model that explains how typical arc magmas become transformed into ore-generating systems. In practical terms, the recognition of a fertility fingerprint and of mineral chemistry gradients would provide powerful tools for exploration that could impact significantly on risk and costs. These are of paramount importance for porphyry-Cu explorers which is why Rio Tinto are willing to commit cash and in-kind resources of approximately 76,000 pounds. Rio Tinto, a member of the International Council on Mining and Metals, committed to the United Nations Global Compact and its supporting principles, is a global, multi-commodity resource company that pioneers socially and environmentally responsible mining. Large porphyry Cu-(Au-Mo) systems are a primary focus on four continents. Thus, research into the genesis of large porphyry Cu systems is directly relevant to its activities.

Governments now recognise that society must respond urgently to the increasing pressures on natural resources. To meet this challenge, society must discover and develop resources in a sustainable way and our proposal aims to make a contribution to these important objectives.