EDITORIAL & OPINION — GEOLOGY 101 — Porphyry deposits, Pt. 1

Porphyry deposits are the quintessential low-grade, large-tonnage mineral deposit. These formations are called porphyries because they are commonly, but not solely, associated with intrusive igneous rocks with large, well-formed mineral crystals (typically feldspars) set in a groundmass of finer-grained crystals.

The intrusive rocks that usually host the deposits are generally felsic to intermediate, ranging from granite to quartz monzonite to granodiorite. The deposits are rare in mafic intrusions, such as gabbros.

The porphyritic texture indicates that the magmas intruded and crystallized near the surface. Because of their near-surface nature, these intrusions are termed epizonal, but can also be moderately coarse-grained with uniform-sized crystals or mesozonal.

Porphyry deposits can be subdivided into different types based on their metal content. These types include copper, copper-gold, copper-molybdenum and molybdenum. In general, copper- and gold-rich porphyries are associated with intrusions derived from mafic magmas in settings such as island arcs. Molybdenum-rich deposits are associated with felsic intrusions derived from magmas with a substantial component of remelted continental crust.

Porphyry deposits are related both genetically and spatially to igneous intrusions. There are usually several bodies of intrusive rock, emplaced in multiple events, and porphyry copper deposits are often associated with dyke swarms and breccias. The country rock intruded by the porphyry can be of any lithological type.

Both the intrusion and the country rock typically exhibit strong and pervasive fracturing. The only geological requirement for porphyry mineralization is that the host rock be rigid or brittle.

Mineralization and alteration can develop in both the intrusive and country rock. The core of the mineralizing system demonstrates the most intense alteration — called potassic alteration because potassium is added to the affected rocks. In the potassic zone the minerals biotite, potassium feldspar and quartz develop. The potassic zone grades outwards into the phyllic zone, which contains quartz and muscovite, usually in its fine-grained variety, called sericite. The phyllic zone then passes into the argillic zone, where quartz and clay minerals develop. The propylitic zone, containing chlorite, epidote and carbonate, develops next, grading outwards into unaltered country rock. These zones do not all show up in every deposit: any one can be missing. The argillic zone, typically the smallest, is often entirely absent.

Usually, mineralization has a low-grade core containing disseminated pyrite that grades out into the ore zone. In the ore zone, pyrite with lesser chalcopyrite (copper ore) and molybdenite (molybdenum ore) are present in veins and disseminations. Sometimes an outermost zone containing only pyrite develops, and then passes into unmineralized country rock.

Formation of these deposits seems to involve two processes.

One, the orthomagmatic process, involves a mechanism called “second boiling,” whereby water saturates the magma as a result of crystallization. With progressive crystallization of the magma, the volume of water dissolved in it increases at a relative rate since water will not seep into silicates. For example, suppose a magma contains 2% dissolved water: once 50% of the magma has crystallized into silicate minerals, the remaining magma would contain a dissolved water content of 4%.

Because water boils at 100C and the magma has temperatures exceeding 600-700C, excess water will essentially boil off (hence the term second boiling) if released near the earth’s surface. When this happens, sulphur, copper, molybdenum and gold can be concentrated in solution in this water. When the aqueous part of the magma boils off, the pressure can cause the intrusive and country rocks to brecciate and fracture, providing pathways for the solution to travel through the rock and be deposited. This type of brecciation and fracturing is sometimes called “ground preparation.”

The second means of formation, known as the “convective process,” starts when continued cooling of the intrusive magma causes groundwaters to circulate through the surrounding country rocks, much as water convects to seafloor volcanic vents and forms volcanogenic massive sulphide deposits. These late-circulating hydrothermal fluids can add more metals to the ore-forming system, or redistribute metals that had been previously deposited in the orthomagmatic stage so as to upgrade the concentration of the sulphides.

Porphyry deposits occur in a similar geological setting to epithermal-style gold deposits, and share many of the same characteristics and processes of formation. Some epithermal deposits are part of a larger porphyry-deposit system.

— The author is a professor of geology at Memorial University in St. John’s, Nfld.