Iron-Copper-Zinc Isotopic Compositions of Andesites from the Kueishantao Hydrothermal Field off Northeastern Taiwan

The studies of iron (Fe), copper (Cu), and zinc (Zn) isotopic compositions in seafloor andesites are helpful in understanding the metal stable isotope fractionation during magma evolution. Here, the Fe, Cu, and Zn isotopic compositions of andesites from the Kueishantao hydrothermal field (KHF) off northeastern Taiwan, west Pacific, have been studied. The majority of δ56Fe values (+0.02‰ to +0.11‰) in the KHF andesites are consistent with those of MORBs (mid-ocean ridge basalts). This suggests that the Fe in the KHF andesites is mainly from a MORB-type mantle. The Fe-Cu-Zn isotopic compositions (δ56Fe +0.22‰, δ65Cu +0.16‰ to +0.64‰, and δ66Zn +0.29‰ to +0.71‰) of the KHF andesites, which are significantly different from those of the MORBs and the continental crust (CC), have a relatively wide range of Cu and Zn isotopic compositions. This is most likely to be a result of the entrainment of the sedimentary carbonate-derived components into an andesitic magma. The recycled altered rocks (higher δ56Fe, lower δ66Zn) could preferentially incorporate isotopically light Fe and heavy Zn into the magma, resulting in relative enrichment of the lighter Fe and heavier Zn isotopes in the andesites. The majority of the δ56Fe values in the KHF andesites are higher than those of the sediments and the local CC and lower than those of the subducted altered rocks, while the reverse is true for δ66Zn, suggesting that the subseafloor sediments and CC materials (lower δ56Fe, higher δ66Zn) contaminating the rising andesitic magma could preferentially incorporate isotopically heavy Fe and light Zn into the magma, resulting in relative enrichment of the heavier Fe and lighter Zn isotopes in the andesites. Thus, the characteristics of the Fe and Zn isotopes in back-arc and island-arc volcanic rocks may also be influenced by the CC and plate subduction components.

How carbonate dissolution facilitates sediment-hosted Zn-Pb mineralization

Most of the world’s Zn and Pb is extracted from sediment-hosted Zn-Pb deposits. The Zn-Pb deposits hosted in carbonate rocks are hypothesized to form by mixing of acidic metal-bearing brines with reduced sulfur-bearing fluids while dissolving sedimentary carbonate. To test the role of carbonate in this process, we conducted hydrothermal experiments simulating ore formation by reacting Zn ± Pb ± Ba–bearing brines with H2S and SO42– produced by native sulfur, with and without carbonate minerals (calcite or dolomite crystals), at 200 °C and water-saturated pressure. Sphalerite, galena, and barite (or anhydrite) crystals formed only when carbonate was present in the experiment, accompanied by carbonate dissolution. The textures of sphalerite clusters are similar to those observed in ancient and modern hydrothermal deposits. Thermodynamic modeling at 150 °C and 250 °C demonstrates that mixing of metal-rich brines and H2S causes most of the Zn in solution to precipitate as sphalerite only when carbonate dissolution occurs to buffer the pH, consistent with the experimental observations. The need for a pH buffer increases with increasing temperature, and different pH buffers may play a role for different deposit types. We propose that carbonate-buffered fluid mixing is a critical process for forming post-sedimentary Zn ± Pb ± Ba deposits in sedimentary carbonate rocks.

Recycling of Marine Carbonate Induced Calcium Isotope Heterogeneity of Arc Magmas in Subduction Zones

Calcium (Ca) is an essential element constituting sedimentary carbonate in subducting sediments. Ca isotopic characteristics of subduction-related rocks could provide insight into the behavior and budget of carbonate and carbon cycles in subduction zones, due to the distinctive δ44/40Ca ranges of sedimentary carbonate with respect to the mantle. Here, we studied the Ca isotopic compositions of arc magmas from the Northern Luzon arc (NLA), which are evolved from a depleted mantle metasomatized by slab-derived fluids and sediment melts. The δ44/40Ca values range from 0.76 ± 0.04‰ to 1.01 ± 0.03‰ and cover the typical ranges for bulk silica earth (BSE, ~ 0.94‰) and fresh mid-ocean ridge basalt (MORB, ~ 0.83‰). The Ca isotopes of NLA volcanics are not dominantly determined by the effects of mantle partial melting or fractional crystallization, nor significantly modified by secondary alteration. Instead, the δ44/40Ca values of NLA volcanics are controlled by the subduction-related metasomatism. The metasomatism by slab-derived fluids (mainly expelled from altered oceanic crust, AOC) dramatically elevated the contents of fluid-mobile elements (e.g., Ba and Pb) with respect to fluid-immobile elements (e.g., Ce). This process, however, rarely modified the Ca isotopes, possibly ascribed to the δ44/40Ca similarity between AOC and the depleted mantle. The δ44/40Ca values significantly correlated with subduction indicators (e.g., Sr-Nd isotopes, Ba/Nb, Ce/Pb, and Nb/La), demonstrating the Ca isotopes of NLA volcanics are mainly controlled by the metasomatism of sediment melts subducting from the South China Sea (SCS). Based on the thermal structures and chemical compositions of sediments subducting into global trenches, we propose that carbonate Ca isotopic signals can only be observed in the arcs with high sedimentary Ca fluxes and temperature-pressure conditions well beyond the solidus of H2O-saturated sediment melting, e.g., NLA, Nicaragua, Guatemala, Colombia, Peru, South Chile, North Vanuatu, New Zealand, and Kermadec. The absence of such signals in other arcs suggests either limited sedimentary fluxes or much of the subducting sedimentary carbonate has been survived during plate subduction to enter the deep mantle.

Massive carbon storage in convergent margins initiated by subduction of limestone

Remobilization of sedimentary carbonate in subduction zones modulates arc volcanism emissions and thus Earth’s climate over geological timescales. Although limestones (or chalk) are thought to be the major carbon reservoir subducted to subarc depths, their fate is still unclear. Here we present high-pressure reaction experiments between impure limestone (7.4 wt.% clay) and dunite at 1.3–2.7 GPa to constrain the melting behaviour of subducted natural limestone in contact with peridotite. The results show that although clay impurities significantly depress the solidus of limestone, melting will not occur whilst limestones are still part of the subducting slab. Buoyancy calculations suggest that most of these limestones would form solid-state diapirs intruding into the mantle wedge, resulting in limited carbon flux to the deep mantle (< ~10 Mt C y−1). Less than 20% melting within the mantle wedge indicates that most limestones remain stable and are stored in subarc lithosphere, resulting in massive carbon storage in convergent margins considering their high carbon flux (~21.4 Mt C y−1). Assimilation and outgassing of these carbonates during arc magma ascent may dominate the carbon flux in volcanic arcs.

Hydrotermal mineralization hosted by Cambrian sedimentary rocks asevidence of kimberlite-hosting structure, Syuldyukarskoye field, Yakutia

The paper describes diamondiferous kimberlite area within a new Yakutian Syuldyukarskoye fi ld and presents detailed mapping results of ore-hosting shear evidence, veinlet bleaching of redbeds, outcrops of metagrained pyrite, pyrite-calcite and calcite veinlets hosted by Cambrian terrigenous-carbonate rocks where kimberlites occur. Kimberlite localization is shown at fault junction as well as kimberlite long axis combination with west-northwest orehosting shear. These tectonic structures combine with veinlet bleaching halos, those of pyrite-calcite and calcite veinlets, and calcite druses characterized by red photoluminescence and phosphorescence. Red, blue and partially white photoluminescence is caused by manganese concentration in calcites (&gt; 0,1%). Hydrothermal calcite nature is supported by C and O isotope composition variations, which reflect the input of medium temperature formational and meteoric waters, carbon of sedimentary carbonate rocks and deep hydrocarbons. Anomalous Ba, Cr, Ni and La content is recognized in hydrothermal calcites from near-kimberlite environment. Kimberlite position in the southeastern part of endogenous mineralization halos and greater diamond potential of the western kimberlite body, which is larger compared to the eastern one, allow forecasting of new productive bodies.

Carbonate Replacement as the Principal Ore Formation Process in the Proterozoic McArthur River (HYC) Sediment-Hosted Zn-Pb Deposit, Australia

The McArthur River (HYC) Zn-Pb-Ag deposit in the Carpentaria Zn belt, northern Australia, is one of the world’s largest and most studied sediment-hosted base metal deposits, owing to its lack of deformation and preservation of sedimentary and ore textures. However, the ore formation process (syngenetic vs. epigenetic) is still a subject of controversy. In this paper we focus on key characteristics of the HYC deposit that remain unexplained: preservation of sedimentary carbonate (dolomite) and its association with Zn, and the role of thallium (Tl) and manganese (Mn) distribution in the orebody. Our findings demonstrate a sequence of events during ore formation: Tl is hosted almost exclusively within euhedral pyritic overgrowths around early diagenetic pyrite; sphalerite mineralization occurred after Tl-bearing pyrite overgrowths, in association with acid dissolution (replacement) of laminated and nodular dolomite across the subbasin; and outer rims are enriched in Mn on preserved dolomite at the dissolution reaction front in contact with sphalerite. New thermodynamic fluid chemistry modeling demonstrates the metal distribution and paragenesis can be explained by acidic, oxidized ore fluids entering the pyrite-dolomite host lithology, allowing reduction and pH buffering by acid carbonate dissolution, resulting in stepwise metal deposition in an evolving fluid. We argue this represents strong evidence for epigenetic ore formation at HYC. Furthermore, the primary control on ore deposition is not synsedimentary faulting in the subbasin; rather, the chemical potential of sedimentary carbonate within reduced, sulfidic lithologies appears to be of critical importance to precipitation of sphalerite.