Ruthenium in Komatiitic Chromite

The distinction between Ru in solid solution and Ru-bearing inclusions is essential for the predictive modeling of platinum-group element (PGE) geochemistry in applications such as the lithogeochemical exploration for magmatic sulfide deposits in komatiites. This study investigates the role of chromite in the fractionation of Ru in ultramafic melts by analyzing chromite grains from sulfide-undersaturated komatiites and a komatiitic basalt from the Yilgarn Craton in Western Australia. In situ analysis using laser ablation ICP-MS yields uniform Ru concentrations in chromites both within-grain and on a sample scale, with concentrations between 220 and 540. ppb. All other platinum-group elements are below the detection limit of the laser ablation ICP-MS analysis. Carius tube digestion isotope dilution ICP-MS analysis of chromite concentrates confirms the accuracy of the in-situ method. Time resolved laser ablation ICP-MS analyses have identified the presence of sub-micron Ir-bearing inclusions in some chromite grains from the komatiitic basalt. However, Ru-bearing inclusions have not been recognized in the analyzed chromites and this combined with the in situ data suggests that Ru exists in solid solution in the crystal lattice of chromite. These results show that chromite can control the fractionation and concentration of Ru in ultramafic systems

Ruthenium Variation in Chromite from Komatiites and Komatiitic Basalts -- A Potential Mineralogical Indicator for Nickel Sulfide Mineralization

More than 390 chromite grains from komatiites and komatiitic basalts from the Yilgarn craton of Western Australia and the Finnish part of the Fennoscandian Shield were analyzed using in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to identify ruthenium (Ru) signatures in chromite associated with nickel sulfide-bearing rocks. Results indicate a potential method to discriminate mineralized and barren komatiite and komatiitic basalt units based on Ru concentrations in chromite and indicate potential for chromite to be used as a resistate indicator mineral in exploration for komatiite-associated nickel sulfide deposits. Chromites from barren komatiites and komatiitic basalts display Ru concentrations mostly between ~150 and 600 ppb. Chromites from mineralized units have distinctly lower Ru contents (\u3c150 ppb). These results can be interpreted in terms of the much higher partition coefficient for Ru into sulfide liquid compared to that of Ru into chromite, resulting in much lower Ru concentrations in chromite where both chromite and sulfide liquid are present and competing for Ru. As a result, the Ru content of chromite can be used to determine if a komatiite melt equilibrated with a sulfide liquid during crystallization, and therefore, if a system and/or sequence is prospective for nickel sulfide mineralization. The strength of this method compared to previous whole-rock exploration techniques derives from combining (1) the geochemical properties of a chalcophile element that records an ore-forming process while being strongly immobile during postmagmatic processes, with (2) the in situ analysis of a mineral that is generally preserved even in highly altered and mildly weathered komatiites and that is a common constituent of detrital heavy mineral samples. Chromite Ru content has potential as a prospectivity indicator, applicable to a wide range of media including bedrock, laterites, and detrital resistates heavy mineral samples

Mass transfer of fluids and metals in the deep Earth

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Transition Metals in Komatiitic Olivine: Proxies for Mantle Composition, Redox Conditions, and Sulfide Mineralization Potential

We present the results of a comprehensive study on the concentrations of first-row transition elements (FRTE: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), as well as Ga and Ge, in liquidus olivine from 2.7-3.5 Ga old Al-undepleted and Al-depleted komatiites from the Kaapvaal and Zimbabwe Cratons in southern Africa, the Yilgarn Craton in Australia, and the Superior Craton in Canada. The sample set includes komatiites that remained sulfur-undersaturated upon emplacement, as well as komatiites that reached sulfide saturation owing to assimilation of crustal sulfur. All olivine grains display concentrations of Mn, Zn, Ge, Co, Fe, Mg, and Ni similar to the Bulk Silicate Earth (BSE) values, with significant negative anomalies in Sc, Ti, V, Cr, Ga, and Cu. Olivine from the studied Al-depleted komatiites displays on average higher 100 x Ga/Sc ratios ( \u3e 5) than olivine from Al-undepleted komatiites ( ≤ 5). Because garnet preferentially incorporates Sc over Ga, the data suggest that elevated Ga/Sc ratios in komatiitic olivine are indicative of garnet retention in the source region of komatiites, highlighting the potential of olivine trace element chemistry as a proxy for the depth of komatiite melting and separation of the magma from the melting residue. Copper concentrations in the studied olivine grains are controlled by sulfur saturation of the host komatiite during olivine crystallization. Olivine from sulfur-undersaturated komatiite systems displays Cu concentrations mostly between 1 and 10 ppm, whereas olivine from sulfide-bearing komatiites has Cu contents of \u3c 0.5 ppm. Because komatiites contain some of the world\u27s highest metal tenor magmatic Ni-Cu sulfide deposits, the Cu variability in olivine as a function of the sulfide-saturation state highlights a potential application of olivine chemistry in the exploration for sulfide ore deposits. Olivine from the Paleo-Archean (3.5-3.3 Ga) komatiites displays overall higher V/Sc ratios (V/Sc = 2.1 ± 0.96; 2 S.D.) than olivine from their Neo-Archean (2.7 Ga) counterparts (V/Sc = 1.0 ± 0.81, 2 S.D.). Vanadium and Sc behave similarly during partial melting of the mantle and are similarly compatible in majorite garnet. However, V is redox-sensitive and its compatibility in olivine increases as the system becomes less oxidized, whereas Sc is redox-insensitive. We argue that olivine from the studied Paleo-Archean komatiites crystallized from more reduced magmas than their Neo-Archean counterparts. Elevated Fe/Mn ratios in olivine from Paleo-Archean komatiites mimic the V/Sc signatures and are interpreted to reflect that Fe2+ is more compatible in olivine than Fe3+. These results imply that V/Sc and Fe/Mn in komatiitic olivine may potentially provide insight into the evolution of the oxidation state of the Archean mantle. Additional studies that integrate the chemistry of komatiitic olivine with those of relict interstitial glass and melt/fluid inclusions are encouraged to fully understand and quantify the potential of FRTE in olivine as a proxy for the oxidation state of the mantle sources of komatiite magmas

On the formation of magmatic sulfide systems in the lower crust by long‐lived mass transfer through the lithosphere: Insights from the Valmaggia pipe, Ivrea Verbano Zone, Italy

The lower crustal domain of the Ivrea\u2010Verbano Zone (NW Italy) hosts five ~300\u2010m\u2010wide pipe\u2010like ultramafic intrusions that are metasomatized and exhibit Ni\u2010Cu\u2010PGE sulfide mineralization. To better constrain the role of metasomatism in the ore genesis, we studied the best\u2010preserved pipe at Valmaggia which was emplaced 249 Myrs ago. Phlogopite 40Ar/39Ar analyses show that the pipe was infiltrated by metasomatic fluids derived from the subcontinental lithospheric mantle (SCLM) in two pulses at ~208 Ma and ~189 Ma which introduced sulfides into the pipe. Consequently, the pipe repeatedly acted as a preferred path for mass transfer from the SCLM into the lower crust over >60 Myrs (i.e., emplacement to second metasomatic pulse). Uplifted block margins, such as the Ivrea\u2010Verbano Zone, are potentially important exploration targets for magmatic sulfides. We argue that exploration strategies should focus on structures such as pipes that can focus metasomatic agents during ascent through the lithosphere

Early Global Mantle Chemical and Isotope Heterogeneity Revealed by the Komatiite-Basalt Record: The Western Australia Connection

Although the heterogeneous nature of the chemical composition of Earth\u27s mantle is now well established, the origin and longevity of the heterogeneities continue to be debated. In order to further study early-Earth heterogeneities, we present a set of Sm-Nd, Lu-Hf, Re-Os, and Hf-W isotope and lithophile and siderophile element abundance data for komatiites and basalts from the ∼3.53 Ga Coonterunah, ∼3.34 Ga Kelly, and ∼3.18 Ga Ruth Well and Regal systems of the Pilbara Craton in Western Australia. The Sm-Nd, Lu-Hf, and Re-Os isotope data yield isochrons consistent with the accepted emplacement ages of the respective komatiite-basalt lavas. The mantle sources evolved with long-term 147Sm/144Nd = 0.200 to 0.214 and 176Lu/177Hf = 0.0355 to 0.0395, spanning the entire range of the time-integrated Sm/Nd and Lu/Hf measured in the Archean and Proterozoic komatiite-basalt systems to-date. Unlike with the other early Archean komatiites and basalts, the coupled 143Nd-176Hf isotope systematics of the Pilbara lavas provide no evidence for the involvement of early magma ocean processes in the evolution of their mantle sources. Episodes of variable degrees of partial mantle melting and melt extraction can account for the observed large variations in the time-integrated Sm/Nd and Lu/Hf ratios in the early Archean mantle domains. In contrast to the highly variable Nd-Hf systematics, the initial γ187Os values vary within a narrow range from +0.9 to -0.4 indicating that the Pilbara mantle sources evolved with chondritic time-integrated Re/Os. The apparent discrepancy between the depletions in incompatible lithophile trace elements and near-chondritic Re/Os observed globally is reconciled via a model whereby early low-degree mantle melting events fractionated Sm from Nd and Lu from Hf, but had little effect on the Re/Os ratio. This in turn would imply early formation and long-term isolation of a basaltic crust highly enriched in incompatible lithophile trace elements. The calculated total HSE abundances in the komatiite mantle sources range from ∼30% in the Coonterunah to ∼70% in the Regal system, of those in the estimates for the modern BSE, indicative of a 2.4x increase in HSE abundances from 3.53 to 3.18 Ga. All four komatiite-basalt systems exhibit positive 182W anomalies ranging between +11.4 and +7.7 ppm. The 182W/184W compositions and calculated HSE abundances in the Pilbara komatiite-basalt sources are inversely correlated and are most consistent with grainy late accretion of large differentiated planetesimals. Regression of the combined 182W-HSE data for the komatiite systems allows an estimate of the W isotopic composition of the pre-late accretion BSE of +17 ± 7. This estimate is similar to that of the Moon of +25 ± 5 and lends further support to the notion regarding an initially common W isotopic composition in the Earth-Moon system. Regression of the available HSE abundance data for komatiite mantle sources worldwide provides an estimate for the time of complete homogenization of late accreted materials within the mantle by 2.5 ± 0.2 Ga. Calculations indicate an average survival time of late accreted planetesimals in the Earth\u27s mantle of 1.9 ± 0.2 Ga, which constrains the average mantle stirring rates for the HSE in the Hadean and Archean

Findings Report: Virtual Workshop on ‘Resilient Supply of Critical Minerals’

Executive SummaryOn August 2-3, 2021, the Thomas J. O’Keefe Institute for Sustainable Supply of Strategic Minerals at Missouri University of Science and Technology (Missouri S&T) hosted the NSF-funded virtual workshop ‘Resilient Supply of Critical Minerals’. The workshop was convened via Zoom and attracted 158 registrants, including 108 registrants from academia (61 students), 30 registrants from government agencies, and 20 registrants from the private sector. Four topical sessions were covered: A. Mineral Exploration and Source Diversification.B. Supply Chain and Policy Issues.C. Improving Mineral Recycling and Reprocessing Technologies.D. Technological Alternatives to Critical Minerals. Each topical session was composed of two keynote lectures and followed by a breakout session that was designed to identify promising pathways towards increasing critical supply chain resilience in the United States. During each breakout session, participants were asked to address five questions: Q1. What are the roadblocks that affect the resilient supply of critical minerals?Q2. What are the most pressing research needs?Q3. What opportunities can lead to the fastest and biggest impact?Q4. What skills training is required to meet future workforce demands?Q5. What other questions should be asked, but are commonly overlooked? Several issues that limit critical mineral supply chain resilience in the United States were identified and discussed in all breakout sessions, including: 1. Insufficient understanding of domestic critical minerals resources. To address this issue, workshop participants highlighted the need for (i) more geologic research to identify new and evaluate existing resources; and (ii) a qualitative and quantitative assessment of critical minerals that may be recovered as by/co-products from existing production streams.2. Technical limitations of current mineral processing and recycling technologies. To address this issue, workshop participants highlighted the need for (i) innovative mineral processing technologies, including more environmentally friendly chemicals/solvents, and (ii) automated recycling technologies for appliances and e-waste. Participants also highlighted the need for a centralized and simplified way to collect recyclable materials, and incentives for the public to participate in recycling.3. Long permitting processes for mining and mineral processing operations, with often unpredictable outcomes. To address this issue, workshop participants suggested the development of new critical mineral focused policies with faster processing times and more transparent / predictable decision-making processes.4. The negative public image of mining and mineral processing operations. To address this issue, workshop participants suggested to design public outreach / education initiatives and to include local communities into decision-making processes.5. Limited availability of a critical mineral workforce. To address this issue, workshop participants suggested an increased focus on critical mineral specific skill training in higher education institutions, and advanced training of the existing workforce

Fluxing of mantle carbon as a physical agent for metallogenic fertilization of the crust

Magmatic systems play a crucial role in enriching the crust with volatiles and elements that reside primarily within the Earth’s mantle, including economically important metals like nickel, copper and platinum-group elements. However, transport of these metals within silicate magmas primarily occurs within dense sulfide liquids, which tend to coalesce, settle and not be efficiently transported in ascending magmas. Here we show textural observations, backed up with carbon and oxygen isotope data, which indicate an intimate association between mantle-derived carbonates and sulfides in some mafic-ultramafic magmatic systems emplaced at the base of the continental crust. We propose that carbon, as a buoyant supercritical CO2 fluid, might be a covert agent aiding and promoting the physical transport of sulfides across the mantle-crust transition. This may be a common but cryptic mechanism that facilitates cycling of volatiles and metals from the mantle to the lower-to-mid continental crust, which leaves little footprint behind by the time magmas reach the Earth’s surface.NERC Minerals Security of Supply (SOS) NE/M010848/1Australian Research Council CE11E0070Consolidated Nickel MinesUniversity of Leiceste