GEOLOGY 101 – Skarn deposits, Part 1

Skarn deposits are significant sources of tungsten, copper, iron, gold, molybdenum, lead, zinc and tin, as well as being minor sources of silver and bismuth. Industrial minerals, including wollastonite, mica, talc and graphite, are also produced from skarn deposits.

Based on the elements that are present, skarns can be divided into seven categories: iron, tungsten, copper, zinc-lead, molybdenum, gold and tin.

Mineral deposits containing skarn typically form at or near the contact between predominantly carbonate-rich rocks (limestone or dolomite) and an igneous intrusive body, or in carbonate veins along faults or fractures.

They form when hot magmatic fluids from the intrusion react with the host carbonate-rich rock, producing calcium, iron, manganese and magnesium silicates (also known as calc-silicates). This process is called metasomatism, meaning that new minerals grow in the host rocks when chemically active pore fluids are introduced into it from an external source. This new growth causes only minor textural or structural disturbances in the original rock.

The new minerals are typically coarse-grained crystals that grow over or replace the fine-grained or massive host rock. The calc-silicate minerals include garnet (calcium-rich grossularite and andradite to magnesium-rich pyrope), pyroxene (diopside to hedenbergite), epidote, olivine (forsterite to fayalite), wollastonite, amphibole (actinolite-tremolite to hornblende) and scapolite.

Garnet and pyroxene are the predominant minerals in most skarns, but not all other minerals develop in every skarn. The mineralogy of the skarn depends on factors including the composition of both the intrusive and carbonate rocks; the structural or relative permeable nature of the host rocks; and the level of intrusion.

In order for skarns to form, host rocks must be permeable so that metasomatic fluids can flow into and through them. If the host rock is impermeable to fluids, the build-up of heat from the cooling intrusion will cause thermal metamorphism, which bakes the rocks and leads to the formation of hornfels (fine-grained rock in which new minerals are created by thermal metamorphism of existing mineralogy).

Although hornfels is typically fine-grained, it can be overgrown by such coarse-grained minerals as andalusite or cordierite. Because of the differences in the permeability of the host rock, carbonates develop skarns, while impermeable calcareous shales develop hornfels.

Skarns are classified as either calcic, if they formed in a limestone, or magnesian, if they formed in a dolomitic host rock. Silicate skarns form when intrusives come into contact with calcium-rich silicate rocks, such as amphibolite. Endoskarn is skarn that develops in the intrusive, whereas exoskarn develops in the surrounding carbonate-rich rocks. Endoskarn is igneous rock-hosted; exoskarn, sedimentary rock-hosted.

Typically, skarns are zoned, their mineralogy changing with distance from the intrusion. Closer to the intrusion, garnet is more abundant than pyroxene. Farther from the intrusion, pyroxene becomes more abundant before grading into unaltered carbonate host rocks.

There are also subtle changes in the chemical compositions of the minerals, particularly in the iron-to-manganese ratio in pyroxene. Closer to the intrusive, pyroxene is iron-rich; farther away, it becomes manganese-rich. Garnets in copper and other skarns change in colour from dark-brown nearest the intrusive to yellow at greater distances.

Skarns form in three stages: First, country rock is heated by an intrusive magma, resulting in thermal metamorphism of the rock into hornfels. Dissolved metals are then deposited during a water saturation phase that follows crystallization in the magma. This process is similar to that of the boiling phases that form porphyry deposits. This vapour-fluid phase infiltrates permeable country rock, causing metasomatism, and leads to skarn formation. Metal deposition (typically as sulphides) takes place in the later, cooling stages of the metasomatic event. Finally, retrograde alteration occurs in the cooling of the system. This alteration develops through circulation of ground waters from country rock.

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