CHONOLITHS, STOCKS, PIPES, AND AGGREGATE PLUTONS: THE GEOMETRIC COMPLEXITY OF INTRUSIONS THAT HOST MINERAL DEPOSITS

Several types of intrusion-hosted ore-deposits are formed within geometrically complex bodies. Felsic to intermediate porphyry-Cu systems, which typically form in the upper 4 km of the crust, and are often hosted within vertical pipes or stocks that are circular to elongate in plan view. Ni-Cu magmatic sulfide deposits hosted within mafic intrusions generally form in the top 10 km of crust. Many of such deposits are hosted within structurally complex feeder systems, some of which have been informally described as chonoliths (i.e., irregularly shaped intrusive bodies which do not share the geometrical characteristics of dikes, sills or laccoliths).

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.