Which hydrothermal mineral is indicative of formation at great depth and at high temperature?

• A. Garnet
• B. Olivine
• C. Quartz
• D. Pyrite

Answer: (A)

Formation at great depth with high temperature leaves a distinctive mineral signature, and garnet is a classic indicator of those conditions. Garnet is stable in high-temperature, high-pressure metamorphic rocks (high-grade facies) and often crystallizes from hydrothermal fluids in deep, hot settings. This makes it a telltale sign of intense thermal energy and depth in a hydrothermal system. In contrast, olivine is more characteristic of mantle-based or primary igneous rocks and doesn’t uniquely point to deep, high-temperature hydrothermal formation. Quartz is extremely stable across a wide range of depths and temperatures, so it isn’t diagnostic for high-T, deep formation. Pyrite forms over a broad spectrum of temperatures and settings, including lower-temperature hydrothermal veins, so it doesn’t specifically indicate great depth and high temperature.