Sulfur and Metal Fertilization of the Lower Continental Crust

Mantle-derived melts and metasomatic fluids are considered to be important in the transport and distribution of trace elements in the subcontinental lithospheric mantle. However, the mechanisms that facilitate sulfur and metal transfer from the upper mantle into the lower continental crust are poorly constrained. This study addresses this knowledge gap by examining a series of sulfide- and hydrous mineral-rich alkaline mafic-ultramafic pipes that intruded the lower continental crust of the Ivrea-Verbano Zone in the Italian Western Alps. The pipes are relatively small (<300 m diameter) and primarily composed of a matrix of subhedral to anhedral amphibole (pargasite), phlogopite and orthopyroxene that enclose sub-centimeter-sized grains of olivine. The 1 to 5 m wide rim portions of the pipes locally contain significant blebby and disseminated Fe-Ni-Cu-PGE sulfide mineralization.Stratigraphic relationships, mineral chemistry, geochemical modeling and phase equilibria suggest that the pipes represent open-ended conduits within a large magmatic plumbing system. The earliest formed pipe rocks were olivine-rich cumulates that reacted with hydrous melts to produce orthopyroxene, amphibole and phlogopite.Sulfides precipitated as immiscible liquid droplets that were retained within a matrix of silicate crystals and scavenged metals from the percolating hydrous melt. New high-precision chemical abrasion TIMS-UPb dating of zircons from one of the pipes indicates that these pipes were emplaced at 249.1+/-0.2 Ma, following partial melting of lithospheric mantle pods that were metasomatized during the Eo-Variscan oceanic to continental subduction (approx. 420-310 Ma). The thermal energy required to generate partial melting of the metasomatized mantle was most likely derived from crustal extension, lithospheric decompression and subsequent asthenospheric rise during the orogenic collapse of the Variscan belt (<300 Ma). Unlike previous models, outcomes from this study suggest a significant temporal gap between the occurrence of mantle metasomatism, subsequent partial melting and emplacement of the pipes.We argue that this multi-stage process is a very effective mechanism to fertilize the commonly dry and refractory lower continental crust in metals and volatiles. During the four-dimensional evolution of the thermo-tectonic architecture of any given terrain, metals and volatiles stored in the lower continental crust may become available as sources for subsequent ore-forming processes, thus enhancing the prospectivity of continental block margins for a wide range of mineral systems

The Shepherd Mountain Iron Ore Deposit in Southeast Missouri, USA – an Extension of the Pilot Knob Magmatic-Hydrothermal Ore System: Evidence from Iron Oxide Chemistry

The Southeast Missouri Iron Metallogenic Province in the Midcontinent USA contains seven major and several minor IOA/IOCG-type deposits and a series of shallow vein-type deposits/prospects, all of which are spatially and temporally associated with early Mesoproterozoic (1500–1440 Ma) magmatism in the St. Francois Mountains terrane. One of the vein-type deposits is the Shepherd Mountain deposit, which consists of two northeast-trending ore veins dominated by magnetite and lesser amounts of hematite. Here we report the findings of a study that investigates the origin of the Shepherd Mountain deposit and a possible genetic link to the nearby (i.e., away) magmatic to magmatic-hydrothermal Pilot Knob ore system that comprises the massive-to-disseminated Pilot Knob Magnetite deposit and the overlying bedded and brecciated Pilot Knob Hematite deposit. Petrographic observations, whole-rock data and the trace element and Fe isotope composition of magnetite and hematite show that the Shepherd Mountain deposit formed from at least five pulses of magmatic-hydrothermal fluids with different compositions and physicochemical parameters. Integration of the data for the Shepherd Mountain deposit with new and published data from the Pilot Knob Magnetite and Pilot Knob Hematite deposits shows that the three deposits are genetically linked through two local faults. The Ironton and Pilot Knob faults provided fluid pathways that connected the Pilot Knob Magnetite deposit to the shallower Shepherd Mountain and Pilot Knob Hematite deposits. Consequently, we argue that the Shepherd Mountain and Pilot Knob Hematite deposits are near-surface extensions of the same magmatic to hydrothermal plumbing system that formed the Pilot Knob Magnetite deposit at depth

Genesis Of The 1.45 Ga Kratz Spring Iron Oxide-Apatite Deposit Complex In Southeast Missouri, USA: Constraints From Oxide Mineral Chemistry

Seven major and numerous lesser Fe oxide occurrences within the 1.47 Ga St. Francois Mountains terrane in Missouri (USA) have previously been described as iron oxide-apatite (IOA) and iron oxide-copper-gold (IOCG) deposits. Researchers speculate that these contain significant amounts of critical minerals, most notably rare earth elements and cobalt. One of the less-studied deposits in the region is the 1.455 Ga Kratz Spring deposit. The deposit consists of two steeply dipping magnetite bodies beneath 450 m of sedimentary cover. The genesis of the Kratz Spring deposit and its relationship to nearby IOA-IOCG deposits remains poorly constrained. To better understand the formation of the Kratz Spring deposit, the authors integrated stratigraphic, petrographic, and bulk rock studies within situ trace element and Fe isotope chemistry of magnetite and hematite. These data show that the Kratz Spring deposit is hydrothermal in origin but is divided into two sub deposits according to different fluid sources and formation conditions: (1) a deep but cooler hydrothermal Kratz Spring South deposit with a juvenile fluid source and (2) a shallow but hotter magmatic-hydrothermal Kratz Spring North deposit with variable fluid sources. Our genetic model suggests the two Kratz Spring deposits are local expressions of the same mineralization system, i.e., the Kratz Spring South deposit is a distal, lower-temperature offshoot of the feeder system that formed the Kratz Spring North deposit. Understanding the magmatic-hydrothermal plumbing system that formed Missouri\u27s IOA-IOCG deposits is important to guiding critical mineral exploration efforts in the region

Mechanisms of platinum-group element fractionation in ultramafic melts and implications for the exploration for magmatic nickel sulphide deposits

Thesis (PhD)--Macquarie University, Faculty of Science, Dept. of Earth and Planetary Sciences, The ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), 2010.Bibliography: p. 225-241.1. Introduction -- 2. Komatiites, komatiitic basalts and ferro-picrites: petrogenesis and geochemistry -- 3. Localities and sample settings -- 4. Analytical methods -- 5. Petrography and mineral chemistry -- 6. Whole-rock chemistry -- 7. In-situ laser ablation ICP-MS analysis of ruthenium in chromite -- 8. Ruthenium in chromite from komatiites, komatiitic basalts, and ferro-picrites -- 9. Anomalous sulphur-poor platinum-group element mineralisation in komatiitic cumulates, Mount Clifford, Western Australia -- 10. The role of chromite, olivine and platinum-group minerals in the fractionation and concentration of platinum-group elements -- 11. Ruthenium content of chromite: implications for the exploration for magmatic nickel-sulphide deposits -- 12. Conclusions: the petrogenesis of komatiites and komatiite-derived melts - new insights from high accuracy and precision platinum-group element analysis.Platinum-group elements (PGE) are important as petrogenetic tracers, but owing to their low abundances and complex behaviour they are among the least understood elements in geochemistry. This study investigates the mechanisms of PGE fractionation in ultramafic systems (komatiites, komatiitic basalts, ferro-picrites) and focuses on the role of chromite. Samples from a range of occurrences have been analysed to assess potential controls on PGE behaviour, such as geochemical affinities (Munro-type and Karasjok-type), age (2.0 and 2.7 Ga), emplacement styles, metamorphic grade and nickel-sulphide mineralisation endowment and style. -- Data obtained by in-situ laser ablation ICP-MS analysis provide the first direct evidence that Ru can exist in solid solution in chromite with concentrations up to several hundred ppb. The data show that the behaviour of Ru is dominantly controlled by the sulphide-saturation state. In systems that did not equilibrate with a sulphide liquid, chromites have distinctly higher Ru concentrations than chromites from systems that interacted with a sulphur-source during crystallisation. Carius tube digestion isotope dilution ICP-MS analyses of chromite separates confirm the accuracy of the in-situ study and also show that Ir is weakly compatible in chromite. Anomalously high Pt and Pd concentrations in chromite separates reflect the presence of platinum-group minerals (PGM) and suggest that PGM are common accessory phases in komatiites. A study of the PGE-mineralogy shows that PGM in komatiites can be of magmatic and post-magmatic origin and that they often remain undetected due to grain sizes less than 5 urn. As a consequence, the presence of PGE minerals has to be taken into account when whole-rock PGE signatures are interpreted. -- The association of Ru-poor chromites with Ni mineralisation and Ru-rich chromites with barren systems provides a new tool for the exploration for nickel-sulphide deposits. This model applies to all magma types and is independent of the age, the geochemical affinity, and other sample characteristics.Mode of access: World Wide Web.398 p. ill. (some col.), map

Erratum to Platinum-Group Element Distribution in the Oxidized Main Sulfide Zone, Great Dyke, Zimbabwe

The online version of the original article can be found at http://dx.doi.org/10.1007/s00126-009-0258-y In the original version of this article unfortunately the first name of Frank Melcher was written incorrectly

Platinum-Group Element Distribution in the Oxidized Main Sulfide Zone, Great Dyke, Zimbabwe

In the Great Dyke mafic/ultramafic layered intrusion of Zimbabwe, economic concentrations of platinum-group elements (PGE) are restricted to sulfide disseminations in pyroxenites of the Main Sulfide Zone (MSZ). Oxidized ores near the surface constitute a resource of ca. 400 Mt. Mining of this ore type has so far been hampered due to insufficient recovery rates. During the oxidation/weathering of the pristine ores, most notably, S and Pd are depleted, whereas Cu and Au are enriched. The concentrations of most other elements (including the other PGE) remain quite constant. In the oxidized MSZ, PGE occur in different modes: (1) as relict primary PGM (mainly sperrylite, cooperite, and braggite), (2) in solid solution in relict sulfides (dominantly Pd in pentlandite, up to 6,500 ppm Pd and 450 ppm Pt), (3) as secondary PGM neoformations (i. e., Pt-Fe alloy and zvyagintsevite), (4) as PGE oxides/hydroxides that replace primary PGM as the result of oxidation, (5) hosted in weathering products, i. e., iron oxides/hydroxides (up to 3,600 ppm Pt and 3,100 ppm Pd), manganese oxides/hydroxides (up to 1.6 wt.% Pt and 1,150 ppm Pd), and in secondary phyllosilicates (up to a few hundred ppm Pt and Pd). In the oxidized MSZ, most of the Pt and Pd are hosted by relict primary and secondary PGM; subordinate amounts are found in iron and manganese oxides/hydroxides. The amount of PGE hosted in solid solution in sulfides is negligible. Considerable local variations in the distribution of PGE in the oxidized ores complicate a mineralogical balance. Experiments to evaluate the PGE recovery from oxidized MSZ ore show that using physical concentration techniques (i. e., electric pulse disaggregation, hydroseparation, and magnetic separation), the PGE are preferentially concentrated into smaller grain size fractions by a factor of 2. Highest PGE concentrations occur in the volumetrically insignificant magnetic fraction. This indicates that a physical preconcentration of PGE is not feasible and that chemical, bulk-leaching methods need to be developed in order to successfully recover PGE from oxidized MSZ ore

The Oxidized Ores of the Main Sulphide Zone, Great Dyke, Zimbabwe: Turning Resources into Minable Reserves-Mineralogy Is the Key

The Great Dyke of Zimbabwe constitutes the world\u27s second largest reserve of platinum group elements (PGE) after the Bushveld Complex in neighbouring South Africa. Within the Great Dyke, economic concentrations of PGE are restricted to sulphide disseminations of the Main Sulphide Zone (MSZ), which are currently mined at the Ngezi, Unki, and Mimosa mines. Near-surface oxidized MSZ ores have a large potential. Their total resources are in the range of 160-250 Mt; however, all previous attempts to extract the PGE from this ore type have proved uneconomic due to low PGE recoveries (\u3c\u3c 50 per cent) achieved by conventional metallurgical methods. Within the ores of pristine, sulphide-bearing MSZ, the PGE are bimodally distributed. Platinum occurs mainly in the form of discrete platinum group mineral (PGM) grains (mainly bismuthotellurides, sulphides, and arsenides), whereas approximately 80 per cent of the Pd (and some Rh) is hosted in pentlandite. Within the oxidized MSZ ores, the PGE are polymodally distributed. Whereas the arsenide- and sulphide-PGMs that make up approximately 25 per cent of the original Pt content of the ore largely remain stable (relict PGMs), the remaining PGMs are disintegrated. The base metal sulphides are destroyed, partly releasing their base metal and PGE contents, and are replaced by iron oxides or hydroxides. Unspecified amounts of the PGE are redistributed and either form secondary PGMs, are found in chemically and mineralogically ill-defined (Pt/Pd)-oxides or hydroxides, or in iron-hydroxides, Mn-Co-hydroxides, and in secondary silicates. The problematic processing of oxidized MSZ ores is attributable to their complex nature and polymodal distribution of the PGE, prohibiting a significant upgrading of the ores by conventional metallurgical methods. Therefore, only bulk leaching methods are viable for ore treatment, and novel metallurgical methods have to be developed for the processing of these ores. Our ongoing work aims at locating the PGE in their mineralogical form in order to understand the mineralogical balance of the PGE in the ores and thereby facilitate the evaluation of metallurgical options for their recovery. A short overview on options and recent advances regarding the recovery of the PGE from oxidized ores is given

Improved Precision and Accuracy of Quantification of Rare Earth Element Abundances via Medium-Resolution LA-ICP-MS

Laser ablation ICP-MS enables streamlined, high-sensitivity measurements of rare earth element (REE) abundances in geological materials. However, many REE isotope mass stations are plagued by isobaric interferences, particularly from diatomic oxides and argides. In this study, we compare REE abundances quantitated from mass spectra collected with low-resolution (m/Δm = 300 at 5% peak height) and medium-resolution (m/Δm = 2500) mass discrimination. A wide array of geological samples was analyzed, including USGS and NIST glasses ranging from mafic to felsic in composition, with NIST 610 employed as the bracketing calibrating reference material. The medium-resolution REE analyses are shown to be significantly more accurate and precise (at the 95% confidence level) than low-resolution analyses, particularly in samples characterized by low (\u3cμg/g levels) REE abundances. A list of preferred mass stations that are least susceptible to isobaric interferences is reported. These findings impact the reliability of REE abundances derived from LA-ICP-MS methods, particularly those relying on mass analyzers that do not offer tuneable mass-resolution and/or collision cell technologies that can reduce oxide and/or argide formation

Anomalous Sulfur-Poor Platinum Group Element Mineralization in Komatiitic Cumulates, Mount Clifford, Western Australia

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