Resource estimation of the sulfide-rich deposits of the Yuhuang-1 hydrothermal field on the ultraslow-spreading Southwest Indian Ridge
Graphical abstract
Introduction
Since the first observation of a “black smoker” at the 21°N, East Pacific Rise in 1979 (Spiess et al., 1980), seafloor hydrothermal activity and its associated polymetallic sulfides, which are important potential mineral resources, have attracted extensive attention from the international community and scientists (e.g., Baker and German, 2004, Beaulieu et al., 2015, Hannington et al., 2010, Lusty and Murton, 2018, Petersen et al., 2016). Statistically, the total accumulation of seafloor sulfide deposits in submarine neovolcanic zones in the global oceans is up to ~6 × 108 tons; and the tonnage of sulfides on slow- and ultraslow-spreading (spreading rate <40 mm/yr) centers accounts for ~86% of the total mass of sulfide deposits at ridges (Hannington et al., 2011). For several hydrothermal fields located on slow-spreading ridges, studies of the distribution of sulfide deposits and estimates of their potential resource have already confirmed that the large-scale of sulfide reserves on this type of ridge system (e.g. Trans-Atlantic Geotraverse, TAG, hydrothermal field on the Mid-Atlantic Ridge, MAR; German et al., 2016, Hannington et al., 2011, Hannington et al., 1998, Murton et al., 2019).
Ultraslow-spreading ridges, with spreading rates of less than ~20 mm/yr, are characterized by stable tectonic environments, deep hydrothermal circulation, and large-scale water–rock reactions (Dick et al., 2003, German et al., 2016), which should lead to the formation of the largest SMS deposits. To date, >20 hydrothermal fields have been discovered and confirmed on ultraslow-spreading ridges (data from www.vents-data.interridge.org/). Among them, however, only for Mount Jourdanne deposit on the Southwest Indian Ridge (SWIR) size was reported, and estimated at <3000 tons using the area vs. tonnage relationship for the Solwara-1 deposit as a reference (Hannington et al., 2010, Hannington et al., 2011), which is much smaller than expected. Therefore, there is currently still a lack of studies characterizing the distribution and content of large sulfide resources on ultraslow-spreading ridges.
The Yuhuang-1 hydrothermal field (YHF) on the ultraslow-spreading SWIR was first reported in the DY21 cruise. Thence, the DY34, 39, 40, 43 and 58 cruises were conducted in this field to obtain estimates of the sulfide distribution and thickness using a deep-tow camera, TV-grab sampling, and self-potential surveys (Tao et al., 2014, Liao et al., 2018a; Zhu et al., 2020b). Photos and videos of the substrate were obtained, together with sulfide samples and self-potential anomalies, providing first-hand data to investigate the sulfide distribution and estimate the total resources of the YHF.
In this work, we calculated the sulfide resources for different areas of the YHF and studied the mineralization and content of selected samples from sulfide deposits, mineralized rocks, and hydrothermal sediments. Based on this integrative analysis, a preliminary estimate of the size and tonnage of the sulfide deposits and metal contents in the YHF is presented, providing a basis for sulfide exploration on the SWIR.
Section snippets
Geological background
The SWIR forms the boundary between the African Plate and the Antarctic Plate, running from the Rodrigues Triple Junction (RTJ) in the east, to the Bouvet Triple Junction (BTJ) in the west (Fig. 1A), with a total length of approximately 8000 km (Georgen et al., 2001, Tao et al., 2014). It is an ultraslow-spreading mid-ocean ridge, with a full spreading rate of 14–16 mm/yr (Dick et al., 2003). The axial rift valley of the SWIR is divided into several segments by a series of north–south transform
Geological survey
The sulfide distribution of the YHF was studied via an interpretation of photos and videos obtained from six comprehensive towed survey lines performed during the DY30 and DY34 cruises. The interval between the survey lines was approximately 300 m, and an ultra-short baseline (USBL) was used to control the positioning accuracy within ±5 m. During the survey, the sulfide distribution was also verified via TV-grab sampling and shallow drilling by a robotic lander-type seafloor drilling rig in
Sulfide-rich areas and host rocks of the YHF
Two main sulfide-rich areas, ~500 m apart, were identified in the YHF: (1) the southwest sulfide area (SWS) and (2) the northeast sulfide area (NES; Fig. 3A). The area of the SWS is ~48.5 × 104 m2, while the NES covering a relatively small area ~13.4 × 104 m2. No active vents or vent-endemic species were observed in either area.
Distribution of sulfide-rich deposits and host rocks
The substrate around the YHF is dominated by basalts and altered ultramafic rocks covered by a layer of brecciated rocks (ultramafic and basaltic), which may have been formed, based on their morphology, by tectonic activity at the seafloor (e.g., sample 40TVG16 and 21TVG20). This is similar to what has been described for other tectonic-hosted deposits (e.g., the Logatchev-1 and Rainbow hydrothermal fields, MAR, and the Longqi-1 hydrothermal field, SWIR) controlled by detachment faults (Fouquet
Conclusions
According to seafloor observations and sampling, the YHF is composed of two sulfide-rich areas, the SWS and NES, and three types of hydrothermal precipitates, i.e., sulfide-rich mounds, sulfide-rich breccia, and hydrothermal sediment. The absence of venting or vent-endemic species, the lack of active chimneys, and extensive oxidation and collapse, taken together, indicate that the mounds of YHF are likely extinct.
The total estimated resources of the YHF are ~10.6 × 106 tons including the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was financially supported by the National Key R&D Program of China (2018YFC0309900, 2017YFC0306603 and 2017YFC0306203), Natural Science Foundation of China (42006074, 42073010, and 41806076), Natural Science Foundation of Zhejiang Province (LQ19D060002), China Ocean Mineral Resources R & D Association Project (DY135-S1-1-02), and Macao Science and Technology Development Fund (FDCT-002/2018/A1). We thank the captains and crews of the Chinese Dayang cruises.
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