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ABSTRACT: Supra-Subduction Zone Ophiolites: Economic Mineral Affinities

Graciano P. Yumul, Jr., Guillermo R. Balce, Raymundo S. Punongbayan

Supra-subduction zone (SSZ) ophiolites are defined as ophiolites formed directly above subducted oceanic lithosphere. They have transitional island arc-MORB characteristics. Studies on SSZ and MORB ophiolites in the Tethyan, Caledonian, and West Philippine basin regions show that the SSZ ophiolites have more chromite and massive sulfide deposits than MORB ophiolites. PGEs also occur in higher concentrations in SSZ ophiolites than in MORB ophiolites. The Zambales Ophiolite Complex in Luzon Island, Philippines, an example of SSZ ophiolite, contains these types of mineral deposits.

Several mechanisms are responsible for the formation and preservation of such mineral deposits. High degree of partial melting, expansion of the olivine-spinel phase volumes caused by the presence of water, incongruent melting of clinopyroxene to spinel plus liquid, changes in f02, temperature, pressure, and magma mixing can explain the chromite deposits in SSZ ophiolites. The presence of sediments and volcanics overlying massive sulfide deposits and the tectonics of emplacement are believed to be critical in the preservation of SSZ ophiolite-hosted massive sulfides since marginal basins (= SSZ ophiolites) are easier to emplace compared to the large, open sea oceanic basins that are usually subducted. Lastly, PGEs are believed to be derived from multistage meltin events that basically characterize SSZ ophiolites.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990