What term describes a mineral deposit that occupies an existing fissure in rock?

The term "vein" is used to describe a mineral deposit that fills an existing fissure or fracture in rock. This geological feature is typically formed by the deposition of minerals from hydrothermal fluids, which travel through these cracks and solidify as they cool. The mineralization often occurs in a restricted area, following the pathway of the fissure, resulting in a concentrated deposit that can have significant economic value, especially in mining for precious and base metals.

While "mineral vein" might seem like a suitable option, it is essentially a more specific term that describes the same concept but is not as commonly used as just "vein" in the geological context. The other choices, such as "depository" and "crystalline cluster," refer to other geological or mineralogical concepts that do not precisely match the definition of a mineral deposit within a fissure. A depository, for example, suggests a broader accumulation of materials, and a crystalline cluster refers to a group of crystals that may not necessarily be related to the concept of fissures or mineral filling. Thus, "vein" is the most appropriate and commonly accepted term for this type of mineral deposit.