NEW HORIZONS — Formation of ocean deposits

Although large, high-grade deposits recently drilled on the seafloor 200 miles west of Vancouver Island are unlikely to be developed in the near future, they nonetheless provide valuable insight into the formation of ancient seafloor deposits currently being mined on land.

The drilling was conducted last fall by the Ocean Drilling Program (ODP), which is funded by Canada and other countries. Representing Canada were myself and Jan Peter of the Geological Survey of Canada, which discovered the mineralization there in 1985.

Middle Valley is a hydrothermally active rift near the northern extremity of the Juan de Fuca Ridge, a spreading centre where new crust is generated at the boundary between the Juan de Fuca and Pacific plates. Because of its proximity to the continent, Middle Valley is covered by a thick sequence of turbiditic and hemipelagic sediment.

The deposits, named ODP (after the program) and Bent Hill, are zinc-copper massive sulphides and occur near Middle Valley in sediments overlying 300,000-year-old basaltic crust. They started forming more than 125,000 years ago from high-temperature, metal-rich hydrothermal fluids that were generated in the underlying basaltic crust. The north-south alignment of the deposits, as well as the occurrence of faults that truncate the western margin of an adjacent sediment hill, indicate that the sulphide deposits are controlled by extensional faults.

At the seafloor, the Bent Hill deposit is 100 metres long by 35 metres high, whereas the ODP deposit, 350 metres to the south, is 150 metres long by 12 metres high. Drilling shows that the former is up to 104 metres thick and 200 metres wide in an east-west direction, and extends 100 metres south of the mound’s summit. The ODP deposit consists of three stacked lenses over a stratigraphic interval of 170 metres.

Sulphide textures and mineralogy at each deposit vary as a result of sulphide infilling and replacement of massive sulphides in the zone of hydrothermal discharge. The combined tonnage is estimated at 15-20 million tonnes, with grades that exceed 20% zinc over 50 metres in the lowermost lens of the ODP deposit. This is significantly larger than the 200,000 tonnes typical of most modern seafloor deposits.

Massive sulphides at both deposits are underlain by well-developed sulphide feeder zones that consist of hydrothermally altered sediment containing pyrrhotite-isocubanite-chalcopyrite-pyrite veins and impregnations. The

sulphide content generally increases toward the massive sulphide zone. The sulphide veins are sub-vertical to bedding-parallel, anastomosing, and are of variable thickness. They display sharp contacts with narrow alteration selvages and represent hydraulic fracturing events. The juxtaposition of hydrothermal minerals that are not in equilibrium indicate a complex history of hydrothermal fluid discharge and seawater entrainment.

One of the highlights of the drilling program was the discovery, below the two deposits, of an area of 13-metre-thick, stratabound copper mineralization, named the Deep Copper zone. The structure is laterally extensive and contains up to 16% copper. The suphides of Bent Hill and ODP consist of an unidentified copper-iron sulphide mineral and pyrrhotite that infill and replace silicified and chloritized sandstone capped by hemipelagic mud. Unlike the sulphide feeder zone, suphide veins are rare. The unit is interpreted as a local hydrothermal reservoir which deposited copper-iron sulphides during cooling of high-temperature, metaliferrous fluids and which was episodically tapped by north-south extensional structures that supplied fluids to the deposits.

Other insights revealed during drilling include the following: n Sediments covering hydrothermally active rifts play an important role in reducing conductive and convective heat loss and focusing fluid discharge at long-lived vent sites. These conditions probably explain why the Bent Hill-OPD deposits are large in comparison with deposits formed at sediment-bare oceanic ridges and why ancient sediment-hosted deposits are, on average, larger than deposits hosted by volcanic rocks.

n The Deep Copper zone represents an important new exploration target characterized by laterally extensive, high-grade mineralization. This style of mineralization is not well-known in the ancient record but should be considered an exploration target in ancient rifts blanketed by interbedded turbidites and hemipelagic sediments with a high along-strata (but a low cross-strata) permeability.

— The author is a senior research scientist for the Geological Survey of Canada.