OCEANIC CORE COMPLEXES AS SITES OF SEAFLOOR HYDROTHERMAL DISCHARGE: 
APPLICATIONS TO THE GEOLOGICAL RECORD
The recognition that oceanic core complexes (OCCs) occur along slow spreading ridges is a recent contribution from the DSDP. When spreading rates are slow, seawater ingress of normal faults penetrates to the mantle causing serpentinization. Buoyant rise of oceanic crust over serpentinized mantle is accommodated by crustal scale detachment faults. OCCs lack the sheeted dykes and pillow basalt members typical of ophiolite sequences, exposing the basal serpeninite-gabbro complexes at surface. Detachments comprise a brittle/ductile interface that is a ready conduit for hydrothermal discharge and account for some of the largest seafloor hydrothermal fields discovered to date. Detailed studies of such fields demonstrate these differ significantly from typical black smoker settings, lacking well defined discharge pipes, forming carbonate/sulphate mounds as well as sulphide deposits, and have associated hydrothermally altered footwall serpentinite and gabbro.
	Seafloor detachment-related mineralization is likely to prove more broadly represented in the geological record than is currently realized. The Greens Creek Deposit on north Admiralty Island is a prime Upper Traissic example. The high grade polymetallic sulphide ore occurs at the base of Carnian/Norian age pyritic-graphitic argillite overlying serpentinite-gabbro plumbed Devono-Mississippian greenstone. The footwall assemblage is extensively altered to green barian mariposite and manganiferous carbonate. High-Ag barite replacement ores form footwall to laminated Pb-Zn-Ag exhalite and overlying bedded pyrite. Detachment control of hydrothermal discharge well accounts for a fundamental structural break between the host argillite and underlying transpositional hydrothermal phyllonite.  Hydrothermal breccia-conglomerate, sourced in footwall phyllonite, marks the ore horizon away from the site of mineralization and is contemporaneous with a major regional unconformity in stratigraphy. This Greens Creek example supplies several elements likely to be common to detachment controlled hydrothermal sites on ancient seafloors -in particular, bedded exhalative ores overlying ductiley deformed stratiform footwall hydrothermal alteration. Perhaps the extensive footwall chloritites giving rise to alumino-silicate rich porphyroblastic schist units at such Archean VMS camps as Manitouwadge and Snow Lake relate to hydrothermally altered detachments. The occurrence of footwall talc schist rather than a copper-rich chlorite pipe zone at Matagami Lake also suggests detachment-fault control. Similar stratiform footwall alteration schists occur at Neoproterozoic-Paleozoic sulphide camps in the southern Appalachians, the Mineral and Andersonville camps within the Central Virginian Gold-Pyrite Belt are prime examples.
	The recognition of oceanic core complexes not only adds new complexity to our understanding of the make-up of oceanic crust but also supplies a new seafloor setting for focused hydrothermal discharge. Although identified on slow spreading ridge segments, such complexes are also known to develop during initial continental break-up, accounting for OCCs in the continental margin rifts formed during opening of the North Atlantic. One might also consider extensional arc settings, such as at Greens Creek, prime for OCC development. Detachment-fault controlled hydrothermal discharge may therefore accompany a variety of ancient seafloor settings and may have been operative within some SEDEX as well as VMS environments of sulphide deposition.