The Ackley City batholith of southeast Newfoundland is an oval-shaped granitoid of approximately 5400 km2 which intruded Ordovician and Precambrian rocks of the Gander and Avalon tectonic zones, respectively, about 345 Ma ago. It is a composite body, consisting mainly of K-feldspar megacrystic granite and alaskite. Spatially related to the southeast contact of the alaskite are younger aplites and pegmatites within which are six separate molybdenite showings. The showings vary in their type of host rocks and in features of mineralization and alteration, but can all be classed as being of aplite–pegmatite type.This notably felsic (SiO2 > 71.8%) pluton has alkaline affinities and features indicative of derivation from an igneous source (I-type). Some major and trace elements exhibit considerable variations. These variations and other geological features indicate that aplites and pegmatites formed by in situ fractional crystallization of the alaskite at shallow depths (1.8 to 3.7 km) to produce a roof-zone complex. Mo is localized in these rocks, which formed from the final residual melt and coexisting vapour phase, and is considered to be closely genetically related to them.