Depth profiles of resistivity and spectral IP for active modern submarine hydrothermal deposits: a case study from the Iheya North Knoll and the Iheya Minor Ridge in Okinawa Trough, Japan
© The Author(s) 2017
Received: 31 May 2017
Accepted: 31 July 2017
Published: 16 August 2017
Exploration and exploitation of submarine hydrothermal deposits in Japan is becoming increasingly important for the steady supply of metal resources to Japanese industry. Metallic elements such as iron, copper, lead, and zinc are usually included in these deposits as sulfide minerals, such as pyrite, chalcopyrite, galena, and sphalerite, respectively. It is well known that most sulfide minerals, except for sphalerite, exhibit an anomalous signature of the induced polarization (IP) effect (e.g., Pelton et al. 1978). Revil et al. (2015a, b) investigated complex electrical properties of disseminated sulfides with a wide range of frequencies of alternating current (AC) and provided the most recent theory of the IP effect based on semiconduction mechanism (migration of hole and electron) inside the sulfide particles. The investigators showed that the IP effect makes the sulfide particles very conductive at high frequencies and perfectly insulating at low frequencies. In general, electromagnetic investigations are believed to be effective in finding submarine sulfide deposits and have been conducted in order to reveal conductive anomalies related to the presence of deposits (e.g., Kowalczyk 2008; Okada et al. 2017). However, the IP effect makes it possible for sulfide-bearing sediments to be both conductive and insulating, depending on the frequency of the current. Therefore, understanding of the IP effect of rock and sediment samples taken from submarine hydrothermal deposits is crucial for reducing uncertainties in interpretations of geophysical structure.
The present study measured the electrical properties of drillcore samples taken from Iheya North Knoll and the Iheya Minor Ridge, the active modern submarine hydrothermal systems in Okinawa Trough, Japan. The resistivity and IP signature obtained were compared to other physical, lithological, and material data, in order to examine their controlling factors.
Site C9017 at Iheya Minor Ridge is a flat mound area with some active hydrothermal vents. Unaltered fresh basalt composes the uppermost unit (0–10 mbsf). Vesicles in this basalt are often filled with sulfides. The lower units (10–105 mbsf) consist of hydrothermal sediments with a variety of grain sizes and clay mineral components. Fine-grained pyrite is disseminated throughout the sediment, contributing to its blackish color. Relatively coarse-grained pyrrhotite, with an average diameter of approximately 1 mm, is present only at a depth of ca.70 mbsf (Fig. 1b).
Site C9021 is located between the Natsu and Aki Sites of Iheya North Knoll. Its seabed surface is mainly composed of unaltered pumice with soft hemipelagic sediment. The upper unit (0–67 mbsf), composed of unaltered to weakly altered pumiceous gravel (Fig. 1c), overlies a lower unit (67–98 mbsf) of altered and silicified volcanic rock. Sulfide mineral is negligibly present in the whitish matrices that are found almost entirely throughout the deeper unit, corresponding to the relatively small semiquantitative sulfide contents; some of the altered volcanic rocks in the lower unit are partly silicified.
Site C9023 is located at the active hydrothermal mound of the Aki Site, Iheya North Knoll. At this site, the upper blackish hydrothermal sediments (0–79 mbsf) overlie greyish silicified volcanic rock (79–200 mbsf, Fig. 1d). Fine-grained sulfide mineral is disseminated, and pyrite, chalcopyrite, sphalerite, and galena are identified in the upper unit. In particular, massive sulfide rocks dominated by sulfide minerals are present only in the uppermost part of the upper unit (Fig. 1e). Based on the visual core description, the mode of occurrence of sulfide minerals seems to decrease with depth.
Promptly performed resistivity and IP properties measurements
IP parameter estimation
Resistivity and IP features
Chargeability m in IP parameters
Recent significant progress in IP studies has shown that the Cole–Cole exponent “c,” the time constant “τ,” and the chargeability “m” are essentially functions of the grain-size distribution, the mean radius of sulfide particles, and the volumetric content of sulfide minerals, respectively (e.g., Wong 1979; Dias 2000; Revil et al. 2015a). The present study focuses on the features of m, because understanding sulfide abundance and its mineralization is one of the major research objectives of the cruise on which the data were collected.
Figure 4 shows the results of IP parameter estimation, together with some fitting of the spectra to the Cole–Cole model (Fig. 4d). When focusing on chargeability m, the pyrrhotite-bearing sediment with high phases up to approximately 400 mrad (70 mbsf, site C9017) exhibits a high chargeability of approximately 0.8. In contrast, low chargeability is estimated for pumiceous sediments with low phases (i.e., less than a few tens of mrad from site C9021). Estimated chargeability and sulfide mineral content are clearly correlated with each other. For instance, the depth profile of chargeability in the core from site C9023, exhibiting a gradual decrease with depth, corresponds well to the transition from the upper sulfide mineral-bearing layer to the lower silicified rocks with lesser amounts of sulfide.
In general, resistivity is controlled by the combined factors of connectivity of fluids (a function of porosity), fluid properties (salinity, temperature), surface conductivity of clay minerals, and capacitive behavior due to the presence of clay minerals and sulfide particles (Wong 1979; Revil et al. 1998, 2015a; Zisser et al. 2010). In order to characterize the resistivity and IP properties of the study area, we examined the contribution of these factors to resistivity and IP properties, the factors causing low-resistivity anomalies, and the applicability of IP exploration techniques to submarine hydrothermal deposits.
Features of resistivity (1 Hz) and total chargeability
It is notable that the apparent weak correlation between total chargeability and clay content (Fig. 5d) is considered to be due to the amount of sulfide minerals contained in clay-rich sediments. It is also worth noting that the estimated total chargeabilities are already high values even with small sulfide contents of several wt%. An inhomogeneous distribution of sulfide particles near electrodes might result in an unexpectedly high IP anomaly because of the high sensitivity to data near the electrodes.
Factors controlling resistivity
In the present study, chemical analysis shows that the pore fluid is near-neutral pH and has high salinity similar to typical seawater (Kumagai et al. in prep.); this could cause enhanced electrical conductivity. Further, surface conductivity of clay minerals should also contribute to decreasing bulk resistivity even under high-salinity conditions, because of their high cation exchange capacity (CEC), except for kaolinite (Revil et al. 1998). Hydrothermal clay is predominantly composed of illite, chlorite, and kaolinite in the present study, whereas smectite is found in some samples obtained from depths of 20–50 mbsf at site C9017, and from depths of 60–67 mbsf at site C9021. Nevertheless, as shown in Fig. 5a, c, resistivity is more sensitive to porosity change than to clay content. This suggests that connectivity of pore fluids primarily controls bulk resistivity in the study area and that clay mineral is a secondary factor affecting resistivity change.
As mentioned previously, resistivity does not correlate with sulfide concentration in terms of the correlation coefficient (Fig. 5c). When focusing on data with sulfide content greater than 5 wt%, resistivity seems to increase with an increase in sulfide concentration. This might correspond to an increase in insulating sulfide at a low frequency such as 1 Hz; this is because fine-grained sulfide particles have high critical frequency for the IP effect (Revil et al. 2015a, b). In contrast, the massive sulfide rock exhibits relatively low resistivity despite its low porosity and high sulfide concentration (Fig. 5b, c), which could be explained by large sulfide particles with the rather low critical frequencies (Fig. 4d). These facts suggest that both the presence of fluids and massive sulfide bodies play an important role in decreasing resistivity when conducting ordinary DC resistivity surveys (i.e., those using a duty cycle with one second current on and off).
High temperature: a possible additional factor in decreased resistivity
In a deep-sea submarine hydrothermal system such as that found at the study site, high temperatures reaching approximately 350 °C can easily decrease the resistivity of saline fluids up to approximately two orders of magnitude compared to room temperature (e.g., Ussher et al. 2000). Therefore, in active modern submarine hydrothermal deposits, EM and DC resistivity surveys could detect not only massive sulfide bodies, rich in metallic and sulfidic materials, but also high-temperature fluid reservoirs, as low-resistivity anomalies. Notably, the maximum temperature of hydrothermal fluid recorded by ROV observations at Iheya North Knoll is 311 °C (Kawagucci et al. 2013). In particular, at sites C9017 and C9023, relatively small resistivities were obtained by logging while drilling (LWD); these resistivities are up to one order of magnitude smaller than those measured from the drillcore at room temperature (Kumagai et al. in prep.). At both sites, the drill hole discharged high-temperature hydrothermal fluids after drilling, suggesting that in situ high-temperature fluids could be related to the low resistivity values obtained by LWD. Consequently, special care should be taken when interpreting a resistivity structure for the purposes of exploration and exploitation of submarine hydrothermal deposits.
Features of phase peaks
It is well known that the frequency of phase peak depends on the size of sulfide particles and that larger particles cause a phase shift to lower frequencies (Dias 2000; Revil et al. 2015a, b). In the present study, as shown in Fig. 3 and described in the results, the spectral IP property includes two conflicting features: The majority of the samples have phase peaks at high frequencies, although low frequencies were measured in a few samples. For instance, the former and the latter correspond to the samples bearing fine-grained pyrite and those containing relatively coarse-grained pyrrhotite with a radius of approximately 1 mm (found at 70 mbsf at site C9017), respectively. Therefore, the different features of the phase peak appear to be qualitatively explained by the size of the sulfide particles. Likewise, the phase peak with low frequency below 0.1 Hz for the massive sulfide sample (Fig. 4d) could be explained by the presence of the very coarse sulfide mineral.
In addition, the hydrothermally altered clay layer from 25 to 35 mbsf at site C9017 shows two-phase peaks with low and high frequencies, suggesting a bimodal size distribution of sulfide particles, as shown in Fig. 3a. However, this clay layer contains fine-grained pyrite, and no large sulfide particles were observed in visual core description. Instead, this layer includes patchy mottled textures rich in fine-grained sulfide minerals (Fig. 1f); these textures might produce the same spectral IP signature as large sulfide grains.
Applicability of the IP techniques to submarine hydrothermal systems
In the present study, the IP property measurements show that total chargeability is quite sensitive to sulfide mineral content (Fig. 5e), suggesting that IP surveys in submarine hydrothermal systems could detect sulfide-rich bodies as effectively as land-based IP surveys can. In particular, the distinctive IP property of the massive sulfide rock in this study (Fig. 3c) is similar to that observed in terrestrial massive kuroko ore samples of volcanogenic massive sulfide deposits by Yoshikawa and Yoshikawa (1978), who performed time-domain IP measurements and found that the interfaces behave as a kind of Warburg impedance. According to this IP property, they proposed that a long-term injection of current could be effective in identifying massive sulfide bodies on land. Therefore, this methodology would also be effective in seabed surveys.
Notably, high temperatures could also modify spectral IP properties (Zisser et al. 2010), so that IP parameters m, c, and τ depend on temperature. Further investigations in terms of IP dependence on temperature may be needed to examine the robustness of using IP parameters for detection of sulfides in high-temperature hydrothermal systems.
Conclusions and future prospects
The present study successfully determined the depth profiles of resistivity and spectral IP properties of strata composing modern active submarine hydrothermal deposits and identified their primary controlling factors. These outcomes are a valuable guide for a precise interpretation of resistivity structures from marine electromagnetic surveys and provide a constraint on ore-formation models by combining geochemical and mineralogical evidence. Further investigations using drillcore samples will examine the contributions to spectral IP signatures of the species and particle size distribution of sulfide minerals, and the effects of high-temperature environments. It is expected that the progressive accumulation of knowledge about submarine hydrothermal deposits will play an important role in decreasing the risk and cost of exploration and exploitation of these submarine deposits.
SK and YO performed complex resistivity measurements. YM, WT, JT, and MM measured other properties onboard such as porosity and chemical composition of drillcores. OT conducted XRD analysis and provided the constituent mineral data. LM, TN, JI, and HK organized the drilling project and observed the measurements. All authors discussed the results of the above measurements. All authors read and approved the final manuscript.
We are grateful to the Captain, OMI, OSI, and crewmembers of the CK16-01 Cruise. We would like to thank the laboratory technicians of Marine Works Japan, Ltd., for supporting our measurements on-board. We also thank S. Takakura (AIST) for providing non-polarizable electrodes and technical support, and Y. Mitsuhata, T. Yokota, K. Takahashi, H. Oda, M. Sato (AIST), and T. Goto (Kyoto Univ.) for valuable discussions. We would like to express our appreciation of A. Revil and an anonymous reviewer for critical review and constructive comments, and Y. Ogawa for editorial support. This work was supported by the Council for Science, Technology and Innovation (CSTI), the Cross-ministerial Strategic Innovation Promotion Program (SIP) “Next-generation technology for ocean resources exploration” (Lead agency: JAMSTEC).
The authors declare that they have no competing interests.
Availability of data and materials
The data supporting the above findings are presently not available due to a moratorium on the publication of data prescribed by JAMSTEC and will be made available to the public following the expiry of the moratorium.
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Consent for publication
This work was supported by the Council for Science, Technology and Innovation (CSTI), the Cross-ministerial Strategic Innovation Promotion Program (SIP) “Next-generation technology for ocean resources exploration” (Lead agency: JAMSTEC).
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