Question

Suppose that you are an exploration geologist hired to interpret the metamorphic rocks of two islands to determine the prospect for mining metals. You are looking for hydrothermal deposits, a common source of metal ores. Island 1 shows a circular ring of disturbed rock with coesite (a dense form of quartz), tiny diamonds, and glass, surrounded by ejected material. Island 2 is dominated by parallel bands of phyllite and garnet-mica schist. Are the rocks on either island typical of hydrothermal metamorphism? Explain.

Short answer

Neither island shows typical characteristics of hydrothermal metamorphism; Island 1 suggests impact metamorphism, and Island 2 indicates regional metamorphism.

Step-by-step solution

Identify typical indicators of hydrothermal metamorphism

Hydrothermal metamorphism typically results in the presence of minerals like quartz, as well as ore-forming minerals such as sulfides (e.g., pyrite, chalcopyrite) due to the action of hot, mineral-rich water. Hydrothermal systems can generate specific textures and alteration patterns in rocks that geologists recognize.

Analyze Island 1's characteristics

Island 1 features a circular ring of disturbed rock with coesite, tiny diamonds, and glass, surrounded by ejected material. These characteristics are indicative of high-pressure, high-temperature environments usually associated with impact events like meteorite impacts, not hydrothermal systems.

Analyze Island 2's characteristics

Island 2 is dominated by phyllite and garnet-mica schist, which are metamorphic rocks formed under medium-grade regional metamorphic conditions. These rocks suggest a history of intense pressure and temperature but do not specifically indicate the involvement of hydrothermal fluids.

Conclusion based on analysis

Neither Island 1 nor Island 2 displays the typical characteristics of hydrothermal metamorphism. Island 1 suggests impact metamorphism due to the presence of high-pressure minerals, whereas Island 2 features regional metamorphic rock bands, which are more indicative of tectonic or burial metamorphism.

Key concepts

Hydrothermal Metamorphism

Hydrothermal metamorphism is a fascinating geological process where rocks are altered by hot, mineral-rich waters. These waters circulate through cracks and seams in the rock layers, often originating from magma or the Earth's deep thermal zones. This interaction can change the chemical composition and mineral structure of the rocks. The hallmark of hydrothermal metamorphism is the formation of new minerals like quartz, and sulfide ores including pyrite and chalcopyrite. These minerals are indicators of the metal-rich environments that are sought after in mining. The rock textures produced often show evidence of mineral alteration and replacement.
This type of metamorphism is critical in forming ore deposits, making it important for miners and geologists seeking precious metals. Hydrothermal systems can also form along mid-ocean ridges and volcanic arcs, where seawater interacts with hot rocks.

Impact Metamorphism

Impact metamorphism occurs when a sudden, high-energy event, such as a meteorite impact, alters the Earth's surface. This can create unique conditions of extreme pressure and heat, resulting in distinctive minerals and rock structures. Coesite, a dense form of quartz, and tiny diamonds are typical minerals formed under such conditions. These are found in Island 1 and serve as clear indicators of impact metamorphism.
The process forms shocked rocks and is often surrounded by brecciated, or fragmented, rock material. Glass can also be an indicator, as the high temperatures from the impact can melt rock that later solidifies quickly. Except for mining rare impact-related minerals like diamonds, impact metamorphism is typically not associated with significant ore deposits.

Regional Metamorphism

Regional metamorphism happens over large areas and is usually related to tectonic processes like mountain building or the collision of tectonic plates. This type is characterized by high pressures and temperatures over extensive regions, often deep within the Earth's crust. Island 2, with its prominent bands of phyllite and garnet-mica schist, is a good example of this process.
These metamorphic rocks are typically formed under moderate to high-grade conditions. Phyllite is notable for its silky sheen and layered appearance, while garnet-mica schist, known for its schistosity, contains garnet crystals and shiny mica flakes.

• Phyllite and garnet-mica schist are formed under increased pressure and temperature, suggesting an environment governed by regional forces rather than fluid activity.
• These rocks offer insights into the thermal history of a region, but are not primary indicators of hydrothermal metal deposits.

Phyllite

Phyllite is a type of metamorphic rock that forms under conditions of medium-grade metamorphism. It sits between slate and schist in terms of metamorphic intensity. Characteristically, phyllite has a foliated, or sheet-like, appearance with a shiny surface due to the abundance of fine-grained mica.
This rock type originates from the transformation of clay-rich rocks and indicates a rise in pressure and temperature beyond the conditions that create slate. Its presence often suggests a history involving tectonic pressure, making it a typical product of regional metamorphism. Phyllite’s foliation, or layering, is typically less pronounced than that of schist, but its silky sheen is highly distinctive. Geologists often study phyllite to better understand the conditions and processes at play during the metamorphosis of rock materials.

Garnet-Mica Schist

Garnet-mica schist is a metamorphic rock that forms under medium to high-grade metamorphic conditions. This rock is rich in garnet crystals and mica minerals, which reflect high degrees of metamorphism with intense pressure and heat. The presence of garnet, a well-known metamorphic mineral, adds an extra mineralogical detail that can indicate specific temperature and pressure conditions during its formation.
In garnet-mica schist, the abundance of mica gives it a foliose texture, allowing the rock to easily break along the mica layers. The schist often sparkles due to its mica content, while the garnet adds a distinctive reddish hue. This type of rock provides insights into the intense geological environments it has endured, typically associated with mountain-building events. Therefore, while garnet-mica schist suggests processes of regional metamorphism, it does not necessarily indicate the hydrothermal environments rich in ore minerals that geologists might prospect.