The Noosa Quartz Diorite pluton, emplaced in Mesozoic sandstone, is composed of plagioclase, hornblende, quartz, and other minor minerals. It contains at least three types of quartz-feldspar veins:
1. Narrow quartz-plagioclase veins, less than ½ cm thick, containing slightly more quartz and less biotite and chlorite than the enclosing rock.
2. Aplite veins, a few centimetres thick, composed of potassic feldspar, sodic feldspar, and quartz.
3. Composite veins, a few centimetres or tens of centimetres thick, normally composed of a central granitic vein rock which contains more quartz than the enclosing quartz diorite, and a marginal vein rock composed mainly of potassic feldspar and quartz. A transitional vein rock, whose composition is intermediate to that of the granitic and marginal vein rocks may also be present. Some of the composite veins, when traced along strike, grade into the enclosing quartz diorite.
Mechanisms of vein emplacement are considered and discussed. It is suggested that some vein rocks have crystallized from a silicate melt which moved to the site of vein formation by fluid flow, while other vein rocks have crystallized from a dispersed phase which moved to the site of vein formation by diffusion through solid quartz diorite.