Forming a porphyry copper-gold deposit

Porphyry copper-gold deposits and their broader mineral systems are a fascinating expression of the tectonic processes that shape the face of our planet. The interaction between water and the mantle, driven by the inexorable dynamic forces of global tectonics, leads to the development of the complex geological formations and incredibly rich ore deposits that we see at or near the surface today.

Porphyry copper deposits account for something like 70% of the world copper supply. These deposits are big, but generally low grade formed from vast disseminations of copper sulphide minerals that have precipitated from complex hydrothermal fluids in the subsurface. Understanding these deposits is the first step to understanding how to explore to find more of them.

I have just returned from a field trip across the Macquarie Arc, New South Wales. This region is a dissected belt of Ordovician calc-alkaline volcanic and related intrusive rocks that are interpreted to have formed in a magmatic arc setting related to subduction of the Pacific plate beneath Australia. The region hosts the Cadia and North Parkes copper-gold porphyry deposits and many other related deposits. It was a fantastic trip that went across the stratigraphy of the arc rocks and looked at the porphyry-style alteration and mineralisation as well. On the back of this trip, I wanted to share a few of thoughts on the general processes that form porphyry copper-gold deposits.

Subduction zones and metal sources

Subduction zones are like conveyor belts that transport water into the mantle. The olivine-pyroxene-plagioclase rich rocks in the upper mantle are hot and exist under tremendous pressures. Injection of fluids, in particular water, together with a host of other elements such as carbon, chlorine, fluorine, sulphur and oxygen has a dramatic effect on the stability of mantle minerals and causes all types of chemical changes. By processes such as dissolution, reprecipitation and partial melting, economically important elements like copper and gold can be liberated from their primordial mantle home, the atomic scale dislocations within the crystal lattice of the mantle’s silicate minerals.

Extracting copper and gold from silicate minerals is just the beginning of their journey however. High temperature fluids that form in the mantle above subduction zones percolate upwards by means of nano-pores and microfractures. If there is sufficient water in the mantle, these fluids and the metallic goodies that they carry can coalesce into larger melt networks and eventually be entrained into magmas that rise through bouyancy towards the crust.

With the right tectonic forces these magmas will break through the hot, dry deep crustal rocks and may pond and interact with the crustal material, possibly further enriching the magmas in economic elements or elements that help to transport them. The magmas may also undergo fractional crystallisation, which further modifies their composition leaving the more mafic minerals in deeper crustal reservoirs and producing more silica- and water-rich magmas in the process.

Oblique view of the eastern Pacific trench adjacent South America, formed by the subduction of Pacific oceanic crust beneath the South American continent. This process is the reason so many incredible porphyry deposits have formed in along the western margin of the continent since the Mesozoic. Google Earth image.

Extraction and upwards journey

Tectonic forces that affect the crust can cause these deep crustal magmas to rise towards the surface. These tectonic forces are largely a response to variable coupling between the upper and lower plates across subduction systems and the motion vectors of each of the tectonic plates. Where oblique convergence occurs, transpression and transtension occurs across and along typically steep structures that can transect the crustal column. Steep structures are important in facilitating the migration of magma upwards. If those structures are sufficiently large and throughgoing they may form conduits that enable tapping of deep crustal magma systems. Magmas may then in effect be evacuated from their deeper roots towards the brittle-ductile transition zone where they typically pond.

Silica-rich alteration overprinting Ordovician volcanic rock in the Macquarie Arc, NSW. This is part of an epithermal alteration system and may be the upper portion of a porphyry system

Fluid-rock interactions and sulphide crystallisation

Once again tectonic forces acting on these thermally perturbed regions with relatively high magma/rock ratios are primed for a final ejection from their mid-crustal settings and these magmas are able to escape upwards. In the process of this upward transport, magmas undergo spectacular changes in physical properties and chemical composition that can lead to initiation and stimulation of hydrothermal cells in surrounding rock packages, expulsion of volatile elements from the crystallising magmas and physical breaking of country rock through combinations of thermal shock caused by emplacement of hot, viscous magmas and superimposed tectonic forcing.

These magmas typically undergo rapid crystallisation. As a consequence, they often have a particular mineral texture which consists of some larger crystals entrapped within a finer-grained groundmass. Such a texture is called porphyritic from where we get the rock type qualifier ‘porphyry’. Many different types of magmas may crystallise porphyritic igneous rocks, and not every ‘porphyry’ will be associated with the formation of metallic ore deposits.

Hornblende-bearing monzonite porphyry from the Macquarie Arc, New South Wales. Photo by A. Reid.

There are several key chemical and mineralogical characteristics that indicate a magma is more ‘fertile’ for producing the fluids that form deposits. Typically porphyry copper-gold deposits form in relation to intermediate silica, high strontium, and water-rich magmas. Mineralogically these rocks are often endowed with iron and magnesium-rich (mafic) minerals, especially hornblende and biotite, along with more silica-rich minerals such as potassium feldspar and quartz.

Metallic elements are typically incompatible with silicate minerals. They are the wrong atomic size for the silicate mineral lattice and they are more chemically attracted to other bonding agents such as sulphur. In turn, sulphur likewise prefers not to be within silicate minerals and as a consequence metals and sulphur will seek each other out in a crystallising magma chamber. As the silicate minerals precipitate, the remaining magmatic fluid becomes more and more enriched in these incompatible elements.

Finally, these enriched fluids can be expelled into the surrounding geological environment as the porphyry intrusions move upwards, punching into the surrounding rock and into structural sites of low pressure at depths of just a few kilometres below the Earth’s surface.

The dynamic and violent introduction of magma into cold, low pressure rocks causes rapid, complex, highly non-linear chemical processes from which sulfur-rich fluids can quickly precipitate copper-rich sulphide minerals as they interact with oxidised or sulfur-bearing host rocks.

In situations where multiple phases of magmatic intrusion occur, these fluid-rock interactions are repeated and amplify the stark physical and chemical differences between the host rock and the magmatic fluids which can result in the formation of very rich copper and gold accumulations.

Exploration and geology

These and related processes are what form the orebodies that exploration geologists seek. These processes are what form the metal-rich mineral accumulations that we use to the metallic infrastructure that underpins modern society.

Geology is the science that seeks to understand these systems and deliver on the world’s ever-growing need for metallic mineral deposits. While geology is not only the science of mineral deposits, the science of mineral deposits is all geology. And geology it is an exciting interface between chemistry, physics and technology that applies these disciplines to real-world problems.

What a fantastic discipline to work in!