Rapid crack-seal growth of Faden quartz

Faden quartz is characterized by a central thread-like fluid-inclusion-rich zone surrounded by a wide, clear, faceted rim. Typically found in fractures (veins) within low-temperature metamorphic rocks, the origins of Faden quartz remain contentious. We use scanning electron microscope-based cathodoluminescence and charge contrast imaging microscopy to reveal that Faden quartz threads consist of closely spaced, narrow microfractures (gap deposits) filled with quartz, which traps fluid inclusions and is surrounded by postkinematic lateral quartz deposits. Gap deposits form by the crack-seal mechanism of sequential breakage, fracture opening, and quartz precipitation. Faden quartz forms by the same mechanism as crack-seal quartz bridges found in some fractures formed under diagenetic conditions. Most fluid inclusions trapped within crack-seal gap deposits of Faden crystals from the Zhob region, Pakistan show a narrow range of homogenization temperatures between 140 °C and 147 °C and salinities of 3.5–5.0 wt% NaCl equivalents. Spanning quartz results in fracture-normal lengths of several tens of centimeters composed of narrow (5–10 μm) crack-seal deposits. Rapid widening relative to quartz accumulation and apertures enabled these fractures to remain open and function as fluid conduits in the subsurface for millions of years.