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Mineralogy and Chemistry State of Oregon Department of Geology and Mi neraf Industries 910 State Office Building 1400 SW Fifth Avenue Port1 and, Oregon 97201 OPEN-FILE REPORT 0-89-09 MINERAL DEPOSITS RECOVERED FROM NORTHERN GORDA RIDGE: MINERALOGY AND CHEMISTRY Martin R. Fi sk and Katherine J. Howard CoI1 ege of Oceanography Oregon State Uni versi ty, Corval 1 i s , Oregon 97331 Final Report for Contract No. 63-630-8803 Submi tted to: Oregon Department of Geology and Mineral Industries and the Gorda Ridge Technical Task Force Released June 1989 This report is based on results of a research progrm directed by the joint federal-state Gorda Ridge Technical Task Force, managed by the Oregon Department of Geology and Hineral Industries, and funded by the Minerals Pbnagement Service, U.S. Deparbmt of the Interior, thtough Cooperative Agremmt. Opinions expressed are those of the authors and do not constitute endarsement by the qwtsoring agencies or the Task Force. Ple Oregon Departmmt of Geology and Mineral Industries is publishing this paper because the subject matter is consistent with the mission of the -t. To facilitate thly distribution of information, ame era-ready cqy submitted by the authors has not h edited by the staff of the -on Wpmtwnt of Geology and nineral Industries. TABLE OF CONTENTS List of Fig- ........................................................................................... 3 ListofTables ............................................................................................. 3 Absbact. ..................................................................................................... 4 Induction........................ .. ................................................................. 4 Gorda Ridge.............................................................................. 4 Dive Imations............ *... ............................................................... 7 Samples....................................................................................... 9 Methods ..................................................................................................... 1 3 X-ray diffraction........................................................................ 1 3 Atomic absorption..................................................................... 1 3 Results........................................................................................................ 1 3 Discussion ................................................................................................ 1 7 SummaIy ................................................................................................. 2 2 Acknow1edgement.s.............................................................................. 2 3 References................................................................................................ 2 4 ABSTRACT The Gorda Ridge is the only submarine volcanic ridge within the newly proclaimed Exclusive Economic Zone of the United States. In 1988 a team of divers discovered hot springs on the northern segment of Gorda Ridge and recovered rock and mineral samples from this hydrothemat field. Here we report the mineral and chemical composition of these samples. INTRODUCTION Gorda Ridge Gorda Ridge is a reIatively short (300 km) segment of volcanic, ocean ridge that extends from the Mendocino Fracture Zone (40' 20' N) to the Blanco Fracture Zone (43' 00' N) off the coast of Oregon and California (Figure 1). The whole length of the ridge is within 200 nautical miles of the United States. The shape and average depth of the ridge is similar to slowly spreading ridges in other oceans, such as the Mid-Atlantic Ridge. The Gorda Ridge axial valley is more than 3,000 m deep and same depressions are as deep as 3,800 rn (Clague and Wolrnes, 1987). As seen in Figure 1, the Gorda Ridge is divided into three segments by two short offsets, and the northern segment extends from 42' 26' to 43' 00' N. This northern segment of Gorda Ridge has been the center of extensive study for the past nine years. Cruises there in 1980, 1981, 1983, 1985, and 1986 mapped the ocean floor with SEABEAM and SeaMARC II sidescan sonar, dredged basalts from the axial valley of Gorda Ridge and from the Blanco Fracture Zone, collected sediment cores and water samples, and measured watet temperature. One dive with the submersible ALVIN was made to the sea-floor (Malahoff, 1985). These studies Ied to five dives with the submersible SEA CLIFF in 1988, and the discovery of a new hydro- thermal vent field (Rona et al., 1988). The northern Gorda Ridge appears to 'be a single rift volcano that has its highest elevation (3,000 rn water depth) at 42' 47' N (Figure 2). At this location the rift valley is narrowest (about 7 km) and axial valley walls are only about 500 rn high (CIague and Holmes, 1987). Both to the north and south, from 42' 47' N, the va1Iey gets wider and deeper and the walls of the axial valley are higher. At the intersection of the ridge and the Blanco Fracture Zone the axial valley is more than 15 km wide and has a depth of 3,500 rn. South of the saddle the axial valley descends to its greatest depth of 3,800 rn (Clague and Holmes, 1987) which suggests that the magma supply is Iow in this area. WASHINGTON FUCA PLATE .-.-.-.-.-.-I-.--- CALIFORNIA Figure 1 Features of the northeastern Pacific Ocean basin near Gorda Ridge. The rates of motion in mmlyr are given for the Pacific Plate and the Juan de Fuca Plate south of the BZanco Fracture Zone. The northern section of the Gorda Ridge (labeled N) is shown in detail in Figure 2. The approximate location of the Cascadia subduction zone is indicated by the solid line with triangles. Cascade volcanos are indicate by isolated triangles. Figure from Fisk and Howard (1989). Figure 2 Topographic features of the northern segment of Gorda Ridge. Sites that were suspected to have hydrothema1 activity based on studies in 1985 are indicated by squares and labeled GR-14 and GR-15 after Collier et a!. (1986). Locations of dredged rocks are indicated by circles. The locations of two dives by MaIahoff (1985) are indicated by triangles and labeled A1406 and A1407. GR-14 was the site of SEA CLIFF dives 767 to 771 that coIIected the samples discussed in this report. Contour interval is in meters. Figure from Fisk and Howard (1989). Basalts were initially collected from this segment of the ridge in 1968 and 1976 by Oregon State University and their chemistry and mineralogy were reported by Kay et al. (1970) and Wakeham (1978). The first indication of hydro-thermal activity on the northern Gorda was the presence of slightly elevated temperatures, high manganese concentrations, and increased particle concentrations in the water within 400 m of the sea-floor at 42" 44.7' N, 126" 43.7" and ad 42O 57.1' N, 126" 35.0' W (Collier et al., 1986, Baker et al., 1987). These were designated Site GR-14 and Site GR-15, respectively, (Figure 2) and both sites continued to exhibit these anomalous signals later the same year (Collier and Baker, 1989), Another indication of hydrothermal activity on the northern Gorda was the presence of barite and pyrite on basalts dredged from 5,775 m at the south end of this segment (Clague et al, 1984). Both of these minerals ate associated with hydrothermal activity on other oceanic ridges, In 1985 photographs of the axial valley near hydrothermal site GR-14 showed that some rocks appeared to be covered with bacterial mats that often grow near areas of hydrothermal activity (Clague and Rona, 1989). Changes in sediment color and texture were also observed in the photographs which suggested the presence of hydrothermal precipitates. One sediment core from the area also contained high copper content, another indication of hydrothermal action (Nelson 1987, personal communication). Thin, black to brightly colored Iayers of minerals were removed from rocks colIected from the axial valley near Site GR-14 in 1985 (Clague and Holmes, 1987) and from rocks that had previously been collected from this area (Howard, 1989). These Iayers proved to contain minerals and chemical indicators that 'suggested the presence of hydrothermal activity in two areas neat GR-14. This wen was again dredged in 1986 and basalts were recovered with a 1 to 3 mm-thick coating of boehmite. Boehmite had previously been reported on the Middle Gorda (Clague et al., 1984; Clague and Holmes, 1987) but it was not known whether it was a hydrothermal precipitate or a weathering product of the basalt. Close examination of the sample indicated that the boehmite was a product of weathering of the basalt by reaction with silica-poor water that was 200' to 300°C (Howard and Fisk, 1988). Dive locations Five dives were made in the vicinity of Site GR-14 (Figure 2). Three of these (767, 768, and 769) were on a long hill with its axis parallel to the ridge and two dives were in the deepest part of the valley near Alvin dive 1407. The tracks of dives 767 and 768 are shown in Figure 3 and dive Figure 3 Schematic diagram of the eastern side of the axial vaIley at GR-14. The long hill is indicated with the light stipple. The approximate path and end points (x) of SEA CLIFF dives 767 and 768 are shown. The hydrothermal field from which samples from dives 767, 768, and 769 were colIected is shown with the dense stipple. Contour interval Is in meters. 769 traversed the southern end of the hydrothermal field. The first dive traversed eastward from east side of the axial valley until reaching the long hill and then turned south and maintained a depth of about 2750 rn.
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