Metalliferous deposits are described from the eastern flank of the East
Pacific Rise (EPR) offshore Costa Rica, close to a basaltic seamount
called ``Dorado high''. Based on heat-flow data and porewater
profiles, the site is an area of active low-temperature hydrothermal
discharge. We focus on the mineralogical and chemical analysis from a
124 cm long gravity core (GC50), located on the northwestern slope of
the 100 in high Dorado. In this core, the sediments consist of detrital
clay minerals as well as authigenic minerals such as zeolites, apatites,
and Fe/Mn-rich oxyhydroxides. In contrast, the reference sediments from
adjacent areas without hydrothermal activity are olive gay hemipelagic
muds composed of volcanic glass particles, clay minerals, siliceous
microfossils, and some detrital quartz and feldspar.
Bulk sediment chemistry and chemical enrichment factors calculated with
respect to the reference sediment indicate that the most important
chemical changes occurred at the base of the core from 100 to 124 cm
bsf, with strong enrichments in MnO, CaO, P2O5, and Fe2O3. These
enrichments are correlated with the occurrence of authigenic
Fe-oxyhydroxide (goethite) and Mn oxide (todorokite and vernadite, at
100 cm bsf), and hydrothermal apatite (110-124 cm bsf). In the upper
section of the core from 12 to 70 cm, the sediment is composed of
abundant smectite and authigenic phillipsite, and only minor chemical
changes can be observed with respect to the reference sediments.
The ubiquitous presence of phillipsite suggests that the entire
sedimentary column of core GC50 was first affected by diagenesis.
However, below 70 cm bsf, these phillipsites are partially dissolved and
Fe oxides occur from 110 to 124 cm, followed upward by Mn oxides at 100
cm. This transition from Fe to Mn-rich sediments can be interpreted in
terms of an upward increasing redox potential. PAAS-normalized REY
patterns of GC50 sediments present clearly negative Ce and positive Y
anomalies inherited from seawater at the base of core GC50. These
anomalies decrease upward, which we interpret together with the
transition from Fe to Mn-rich sediments by an upward migrating
low-temperature hydrothermal fluid. Thus, after a first stage of
diagenesis, the discharge of a low-temperature hydrothermal fluid
occurred through the sedimentary column, leading to the precipitation of
hydrothermal compounds that are lacking towards the surface. (c) 2008
Elsevier B.V. All rights reserved.