Greenstone is not a specific mineral but rather a descriptive term applied to metamorphic rocks exhibiting a greenish hue. This coloration typically arises from the presence of minerals such as chlorite, epidote, and actinolite, which are common in rocks that have undergone low to moderate grade metamorphism. The term is widely used in geology, particularly in field settings, to quickly identify a rock that might host valuable minerals or provide clues to past tectonic events.
Mineralogical Composition and Origin
The fundamental nature of greenstone lies in its mineralogical makeup. While the green color is the defining visual characteristic, the specific minerals can vary significantly. Chlorite, a phyllosilicate, is frequently responsible for the green coloration in many instances. Epidote, another calcium-aluminum-iron silicate, also contributes to green shades, often in more intermediate to high-grade metamorphic settings. Actinolite, an amphibole mineral, is commonly associated with green volcanic rocks and greenschist facies metamorphism.
The Link to Metamorphism
Greenstone is almost exclusively a metamorphic rock, meaning it originated as another rock type—such as basalt, andesite, or sedimentary shale—that was altered by heat and pressure beneath the Earth's surface. This process, known as metamorphism, recrystallizes the minerals without melting the rock entirely. The specific category of metamorphism, often greenschist or amphibolite facies, dictates the mineral assemblage and ultimately the precise classification of the greenstone.
Metamorphic Facies and Classification
Geologists classify greenstone based on the metamorphic facies it represents. A rock rich in chlorite and actinolite belongs to the greenschist facies, indicating moderate temperatures and pressures. If the mineralogy shifts to include grunerite or more calcium-rich plagioclase, the rock may be classified as an amphibolite, representing higher-grade conditions. This classification is crucial for understanding the thermal history of a region.
Geological Significance and Occurrence
Greenstones are significant geological markers. They are frequently found in ancient mountain belts, known as orogens, where continental collisions have occurred. These rocks often form the core of these belts, providing evidence of deep burial and deformation. Notably, greenstone belts are common in some of the world's oldest continental crust, such as the Canadian Shield and the Pilbara Craton in Australia, where they sometimes host significant ore deposits.
Economic Importance and Associated Resources
Beyond their geological interest, greenstones are economically important. They are often associated with volcanogenic massive sulfide (VMS) deposits, which are sources of valuable metals like copper, zinc, lead, gold, and silver. The altered volcanic rocks within greenstone belts act as conduits for metal-rich fluids, concentrating these elements into mineable ore bodies. Consequently, the search for greenstone formations is a key exploration strategy for mining companies.
In essence, identifying what type of rock is greenstone requires looking beyond the color to its texture, mineral content, and geological context. It serves as a vital window into the dynamic thermal and tectonic history of the planet, linking surface observations to deep Earth processes.
