Major rock types of the Irish Creek district are gneisses and schists, intruded by granodiorite. All these rocks are believed to be Precambrian. The ore deposits are fissure veins consisting largely of quartz veins bordered by greisen, and enriched by recurrent deposition. From field and microscopic evidence six stages of mineral formation have been deduced:
1. Crystallization of the granodiorite with the formation of hypersthene, augite, hornblende, andesine, orthoclase, microcline, and quartz; and accessory apatite, rutile, ilmenite, titanomagnetite, and zircon.
2. Metamorphism of the granodiorite and gneiss with formation of uralitic actinolite, epidote, and sphene.
3. A period of fracturing and formation of quartz veins.
4. Greisenization of the host rock producing first: coarse muscovite, brown biotite, cassiterite, beryl, wolframite, and quartz; then producing a second group of minerals-fluorite, green biotite, chlorite, phenakite, siderite, ankerite, parisite, sphalerite, sulfides, including aikinite and galeno-bismutite, leucoxene, fine muscovite, and quartz.
5. Formation of late vein minerals, clinozoisite, calcite, nontronite, and montmorillonite.
6. Formation of surficial alteration products, kaolinite, vermiculite, hematite, limonite, and scorodite.
The italicized mineral names indicate the minerals heretofore unreported from Irish Creek; parisite, aikinite, and galenobismutite are new minerals for Virginia. Included in the detailed descriptions of all the minerals is a chemical analysis of beryl.
