Polymetallic nodules are enriched in critical metals and possess high economic values. The authors observed, for the first time, high abundance of spherical polymetallic nodules on the abyssal basin around the Caiwei Guyot in the northwestern Pacific Ocean. In order to reveal their metal resource potential and long-term metallogenic process, the authors carried out high-resolution scanning electron microscopy (SEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray diffraction (XRD), transmission electron microscopy (TEM) of laminas, and marine chemical analysis. The polymetallic nodules are composed of columnar or lamina growth patterns with low reflectance, which can be subdivided into four types of laminas based on microtexture. The Mn/Fe ratios of the laminas increased in the order type 1.2 < type 1.3 < type 2 < type 1.1. The nodule laminas yielded low Mn/Fe ratios (<4.01), Ni (<12000 ppm) and MgO (<3.74%) contents but high contents of Co (average 8516 ppm), Pt (average 416 ppb), and rare earth elements and Y (REY, average 2887 ppm), relative to other polymetallic nodules in the global ocean. XRD and TEM analyses indicated that the polymetallic nodules are composed of Fe-vernadite. Evidences from micro-texture, geochemistry and mineralogy imply that the polymetallic nodules around the Caiwei Guyot have been forming from oxic bottom water since initial growth, due to oxygen-rich bottom water flowing-through and low surface productivity. The dissolved Mn and Fe elements in the bottom water are continuously precipitated into ferromanganese oxides under oxic conditions. The ferromanganese oxides absorb high contents of multivalent metals such as Co, Pt, and Ce, owing to surface oxidation of the manganate octahedral layer during precipitation. The northwest Pacific Ocean shows deep depth, slow sedimentary rates, moderate-to-high contents of dissolved oxygen, and low summer surface productivity. Based on this, the authors propose that the northwest Pacific Ocean is an ideal metallogenic belt for polymetallic nodules with abundant Co, Pt, and REY resources.
High apatite Cl and S contents have previously been proposed to be effective in discriminating fertile porphyry Cu systems from infertile ones. However, apatite Cl contents could be very low in syn-mineralization intrusions of some giant porphyry Cu deposits (e.g., <0.1 wt% at Yerington, USA and Yulong, China). The reason for this abnormal phenomenon remains enigmatic, hindering further application of apatite as potential indicator mineral in porphyry Cu exploration. To address this outstanding issue, we conducted a comparative study between mineralized and coeval barren magmatic suites in the Yulong porphyry Cu belt in eastern Tibet, with emphasis laid on their contrasting apatite volatile contents. Apatites from pre- and syn-mineralization intrusions at the mineralized Yulong suite have similar SO3 contents (0.35 ± 0.26 wt% and 0.34 ± 0.23 wt%, respectively) with but lower Cl contents (0.41 ± 0.37 wt% and 0.08 ± 0.02 wt%, respectively) than the coeval barren suites (SO3: 0.20 ± 0.12 wt%–0.95 ± 0.26 wt%; Cl: 0.49 ± 0.08–1.17 ± 0.06 wt%). The Yulong suite displays a sudden and coupled drop of apatite XOH and XCl values and surge of XF/XCl ratios from pre- to syn-mineralization intrusions, likely indicating extensive fluid exsolution. However, this trend is not observed in the barren suites. Zircon grains from pre- and syn-mineralization intrusions at the mineralized Yulong suite have higher 10,000*(EuN/EuN*)/Y ratios and lower calculated Ti-in-zircon temperatures (691° ± 49 °C and 703° ± 48 °C, respectively) than the barren suites (770° ± 68 °C–790° ± 76 °C), suggesting higher magmatic water contents for Yulong. This is consistent with higher abundances of hydrous minerals such as biotite and amphibole in the Yulong suite. Amphibole phenocrysts from pre- and syn-mineralization intrusions at Yulong have significantly lower crystallizing pressures (2.1 ± 0.6 kbar and 1.9 ± 0.5 kbar, respectively) than the barren suites (4.6 ± 0.4 kbar to 4.7 ± 0.5 kbar). A similar trend is also shown by crystallizing pressures estimated from biotite phenocrysts from these barren and mineralized suites. We propose that the barren suites were more Cl-rich because their source magma chambers have greater depth but lower water contents, leading to limited fluid exsolution and Cl loss. In contrast, the shallowly-emplaced, more hydrous Yulong suite was originally Cl-rich but underwent early fluid exsolution and Cl loss before/during apatite crystallization. As such, apatite Cl contents may not always be good fertility indictor for porphyry Cu systems. We demonstrate that high apatite SO3 contents are less affected by fluid exsolution due to buffering of saturated anhydrite in oxidized and S-rich magmas, and may be used to aid exploration.
The geochemical cycle of mercury in Earth's surface environment (atmosphere, hydrosphere, biosphere) has been extensively studied; however, the deep geological cycling of this element is less well known. Here we document distinct mass-independent mercury isotope fractionation (expressed as Δ199Hg) in island arc basalts and mid-ocean ridge basalts. Both rock groups show positive Δ199Hg values up to 0.34‰ and 0.22‰, respectively, which deviate from recent estimates of the primitive mantle (Δ199Hg: 0.00 ± 0.10‰, 2 SD)1. The positive Δ199Hg values indicate recycling of marine Hg into the asthenospheric mantle. Such a crustal Hg isotope signature was not observed in our samples of ocean island basalts and continental flood basalts, but has recently been identified in canonical end-member samples of the deep mantle1, therefore demonstrating that recycling of mercury can affect both the upper and lower mantle. Our study reveals large-scale translithospheric Hg recycling via plate tectonics.
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