The quartz vein incrementally filled a tiny fault surface, which was simultaneously slipping and opening up. The crystal fibers of quartz record the combination of the two movements, in the form of shear and dilation. The lines are mineral lineation, the penetrative presence of which documents that the vein was continuously deforming while it was being precipitated along a fault surface. The black with tinges of red is hematite and manganese, the products of precipitation of silica, iron, and manganese from hydrothermal fluids containing silica and iron. This all could have happened when the Tucson Mountain caldera was being formed.
I collected this while mapping the geology of Brown Mountain with Evans B. Mayo in 1971. There are dozens and dozens of these veins, which stand out sharply as milky lines in contrast to the red host rock. I never identified any fault offsets along these veins, based on looking for offset marker beds. Now, looking back, I think it might be possible to figure out slip direction by analyzing the angles between quartz fibers and the fault surfaces. I like how tightly ‘welded’ this vein is attached to the host rock, and am attracted to the white/black streaking, which accentuates the crystal-fiber growth direction. This rock also reminds me of some unfinished business I have yet to do on Brown Mountain. It is easy to ignore a single isolated exposure of a streaky quartz vein, but not a hillside with hundred and hundreds.