CHONOLITHS, STOCKS, PIPES, AND AGGREGATE PLUTONS: THE GEOMETRIC COMPLEXITY OF INTRUSIONS THAT HOST MINERAL DEPOSITS
Despite their economic significance, the mechanical and magmatic controls underlying the development of pipes, stocks and chonoliths remain unclear. For example, magmatic Ni-Cu sulfides of the Voisey’s Bay intrusion (VBI, Canada) are located within dikes and at the base of a geometrically complex magma chamber. The chamber could, a priori, be termed a chonolith; however, detailed 3D structural analysis of the chamber indicates that it consists of an amalgamation of tabular and sill-like magma bodies affected by intermittent local activity of faults. Dike-hosted mineralization is located within thickened portions of feeder dikes, which locally resemble mafic chonoliths in cross-section. The geometry of these dikes is strongly controlled by the interaction of pre-emplacement wall rock structure with intruding magmas, and dike widening by thermo-mechanical erosion of the surrounding wall rocks.
Both porphyry-Cu and magmatic Ni-Cu sulfide systems share other similarities, which indicate emplacement of their host intrusions by similar processes. In both cases, the host intrusions consist of relatively evolved or volatile rich magmas that are emplaced late compared to their parental magmas. Both porphyry-Cu and magmatic Ni-Cu sulfide systems develop in environments in which there is active deformation, typically in the brittle to brittle-ductile realm of the crust.