I took this photo on my vacation in Finland showing white stripes in the rock going different ways. How are these patterns born?
One can only speculate based upon a photograph - however they look very much like mineralized fractures. At some time in the past this rock mass may have fractured in response to thermal or tectonic stresses. Fluid may have then penetrated relatively long distances along the fractures into the area and infiltrated shorter distances into the wall rock along small scale porosity such as cracks and grain boundaries (and also possibly diffusions.) The fluid and rock were most likely not in chemical equilibrium when they came into contact and there were chemical reactions that produced additional minerals along the trace of the original fracture. This is probably what you see - the result of a process of fracture, mass transport, and fluid-rock chemical reaction. But a geologist would need much more information about this system (than available from one photograph) to describe the details.
Seeing something like this in the field is always very exciting because it raises so many questions. Where did the fluid come from? What was the temperature? When did the rock fracture?