Alteration and mineralization of an oceanic forearc and the ophiolite-ocean crust analogy

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doi: 10.1029/98JB00598
Author(s): Alt, Jeffrey C.; Teagle, D. A. H.; Brewer, Tim S.; Shanks, Wayne C., III; Halliday, Alex N.
Author Affiliation(s): Primary:
University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States
Other:
University of Leicester, United Kingdom
U. S. Geological Survey, United States
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 103(B6), p.12,365-12,380. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 79 refs.; illus., incl. 3 tables, sketch map
Summary: Mineralogical, chemical, and isotopic (O, C, S, and Sr) analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid-ocean ridges (MOR) and ophiolites. Ocean Drilling Program Hole 786B in the Izu-Bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows, breccias, and basal dikes. The rocks comprise boninites and andesites to rhyolites. Most of the section was affected by low-temperature (<100°C) seawater alteration, with temperatures increasing downward. The rocks are partly (5-25%) altered to smectite, Fe-oxyhydroxide, calcite, and phillipsite, and exhibit gains of K, Rb, and P, loss of Ca, variable changes in Si, Na, Mg, Fe, Sr, and Y, and elevated δ18O and 87Sr/86Sr. Higher temperatures (∼150°C) in the basal dikes below 750 m led to more intense alteration and formation of chlorite-smectite, corrensite, albite, K-feldspar, and quartz (±chlorite). A 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to Mg-chlorite, albite, and pyrite, and gained Mg and S and lost Si and Ca. Focused flow of hydrothermal fluids produced serialization halos (Na-K sericite, quartz, pyrophyllite, K-feldspar, and pyrite) along quartz veins at temperatures of 200°-250°C. High 87Sr/86Sr ratios of chloritized (∼0.7055) and sericitized (∼0.7065) rocks indicate involvement of seawater via mixing with hydrothermal fluids. Low δ34S of sulfide (-2 to -5.5 per mil) and sulfate (12.5 per mil) are consistent with input of magmatic SO2 into hydrothermal fluids and disproportionation to sulfide and sulfate. Alteration processes were generally similar to those at MORs, but the arc section is more intensively altered, in part because of the presence of abundant glassy rocks and mafic phases. The increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in MOR and the Troodos ophiolite. Copyright 1998 by the American Geophysical Union.
Year of Publication: 1998
Research Program: ODP Ocean Drilling Program
Key Words: 05 Petrology, Igneous and Metamorphic; 27 Economic Geology, Metals; Alteration; Andesites; Basins; Boninite; C-13/C-12; Carbon; Chemical composition; Crust; Dikes; Enrichment; Fore-arc basins; Hydrothermal alteration; Hydrothermal conditions; Igneous rocks; Intrusions; Isotope ratios; Isotopes; Izu-Bonin Arc; Leg 125; Magnesium ores; Metal ores; Metasomatism; Mid-ocean ridges; Mineral composition; Mineral deposits, genesis; North Pacific; Northwest Pacific; O-18/O-16; ODP Site 786; Ocean Drilling Program; Ocean floors; Oceanic crust; Ophiolite; Ore-forming fluids; Oxygen; Pacific Ocean; Petrography; Pyritization; Quartz veins; Rhyolites; S-34/S-32; Sericitization; Stable isotopes; Sulfides; Sulfur; Troodos Ophiolite; Veins; Volcanic rocks; Volcanism; West Pacific
Coordinates: N315227 N315229 E1411336 E1411334
Record ID: 1998042233
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.