Skarns
Introduction
Skarn is a type of metamorphic rock that forms under high temperature and pressure conditions when silicate minerals react with surrounding carbonate rocks. Skarns are typically rich in calcium, iron, and magnesium, and often contain valuable ore minerals such as copper, tungsten, and gold.
Formation
Skarn formation occurs in the contact zone between igneous and sedimentary rocks, particularly where limestone or dolomite beds are intruded by granitic magma. The heat and fluids from the magma cause the surrounding carbonate rocks to undergo metamorphism, resulting in the formation of skarn. This process, known as contact metamorphism, is characterized by high temperatures and pressures, but relatively low shear stress.
The formation of skarn involves two main stages: prograde and retrograde. During the prograde stage, the carbonate rocks are heated and begin to recrystallize, forming new minerals such as garnet and pyroxene. The retrograde stage occurs as the system cools and the newly formed minerals react with the cooling fluids to form other minerals such as amphibole, epidote, and quartz.
Mineralogy
Skarns are complex and varied in their mineralogy, reflecting the diverse conditions under which they form. The primary minerals in skarn are calc-silicate minerals, including garnet and pyroxene. Other common minerals include feldspar, mica, and quartz. Skarns can also contain ore minerals such as chalcopyrite, sphalerite, and magnetite, making them economically important for mining.
Classification
Skarns are classified based on their mineralogy and the type of surrounding rocks. There are two main types of skarn: exoskarn and endoskarn. Exoskarn forms in the country rock that surrounds an igneous intrusion, while endoskarn forms within the intrusion itself.
Exoskarn is typically composed of garnet and pyroxene, with lesser amounts of other minerals such as feldspar, mica, and quartz. Endoskarn, on the other hand, is more varied in its mineralogy and can contain a wide range of minerals, including scapolite, vesuvianite, and wollastonite.
Economic Importance
Skarns are economically important as they often contain valuable ore minerals. The high temperatures and pressures associated with skarn formation can lead to the concentration of metals such as copper, tungsten, and gold. As a result, many skarns are mined for their metal content.
In addition to their economic value, skarns are also of interest to geologists as they provide valuable insights into the processes of metamorphism and ore formation.