Relation of Offshore and Onshore Mineral Resources to Plate Tectonics

ABSTRACT The Pacific and Atlantic are natural laboratories to study relations between mineral resources and plate tectonics. The distribution of mineral deposits about convergent lithospheric plate boundaries may be best observed in the Pacific. The Pacific is surrounded by convergent plate boundaries where oceanic crust is consumed by subduction along Benioff zones. Areas of offshore petroleum potential are associated with the convergent plate boundaries which createoceariic trenches along the eastern Pacific and small ocean' basins along the western Pacific. Precious, base, iron and ferro-alloy metal deposits occur onshore from the convergent plate boundaries on continents in the eastern Pacific and on island arcs and continents in the western Pacific. The distribution of mineral deposits about divergent plate boundaries is less known than about convergent plate boundaries because the former are submerged oceanic ridges. The Atlantic is bisected by the Mid-Atlantic Ridge where oceanic crust is created by sea floor spreading. Certain metals are being concentrated in oceanic crust at divergent plate boundaries by hydrothermal processes. The TAG Hydrothermal Field, an active hydrothermal area discovered by the NOAA Trans-Atlantic Geotraverse (TAG) on the MidAtlantic Ridge at 26°N, is providing information of how metals are concentrated in oceanic crust at divergent plate boundaries. Present Jrodels which treat the relation between mineral deposits and lithospheric plate boundaries are largely interpretive, explaining the observed distribution of deposits. The development of these models may lead to the discovery of new deposits both offshore and onshore. INTRODUCTION The Atlantic is an opening ocean basin that is inferred to have been growing wider at a rate of several centimeters per year for approximately the past 200,000,000 years. The Pacific is a closing ocean basin that is inferred to have been diminishing in size at a rate comparable to that of the opening of the Atlantic. The contrasting histories of the Atlantic and Pacific are explained according to the theory of plate tectonics in terms of motions of lithospheric plates (Fig. 1) about divergent and convergent plate boundaries (Fig. 2). The occurrence of mineral deposits including petroleum may be related to plate motions and, in particular, to geological processes associated with the boundaries between the plates. Differences in plate motions have resulted in different distributions of minerals in the Atlantic and Pacific regions that may be Characteristic of divergent and convergent ' plate boundaries. DIVERGENT PLATE BOUNDARIES Relations between the opening of an ocean basin about a divergent plate boundary and theoccurrence of mineral deposits may be inferred from the development of the Atlantic as follows:At an early stage of opening following continental rifting a sea is formed with its circulation restricted by tectonic conditions and by the positions of the surrounding continents 1 Conditions in this sea favor both the accumulation of organic matter that may become petroleum and of rock salt that may subsequently form domes to trap the petroleum2. Metalliferous sediments and possibly massive stratabound sulfide bodies are concentrated by hydrothermal processes at the divergent plate boundary. An example of this early stage of opening is the Red Seas3.