Mineralium Deposita最新文献

Cerros Taguas is one of the high-sulfidation epithermal Au-Ag deposits (131.35 Mt @ 0.29 g/t Au, 8.8 g/t Ag, 0.11% Cu) of the Taguas project, located in the northern sector of the El Indio belt (~ 29°S), Central Andes, Argentina. Zircon LA-ICP-MS U-Pb dating of a rhyolitic tuff that hosts Au-Ag mineralization constrain the timing of volcanism to the middle Miocene (12.14 ± 0.14 to 11.85 ± 0.26 Ma). Above 3800 m.a.s.l., the rocks display advanced argillic alteration, characterized by alunite + pyrophyllite + dickite ± diaspore ± topaz. Hypogene alunite associated with epithermal Au-Ag mineralization yielded a ⁴⁰Ar/³⁹Ar plateau date of 9.5 ± 0.4 Ma. Below 3800 m.a.s.l., the presence of sericitic alteration (muscovite + illite + quartz) and a molybdenum halo associated with molybdenite-bearing B-type and pyrite ± quartz D-type veins suggests a transition from epithermal to porphyry-style mineralization. Molybdenite in quartz-dominated B-type veins and molybdenite ± quartz veins cross-cutting the rhyolitic tuff yielded ID-NTIMS Re-Os dates of 10.60 ± 0.06 and 10.48 ± 0.05 Ma. A nominally older molybdenite ID-NTIMS Re-Os date (11.10 ± 0.11 Ma) was obtained for the hydrothermal cement of a breccia. The timing of molybdenum mineralization at Cerros Taguas was broadly coeval with the emplacement of inter-mineralization porphyritic stocks and slightly older than molybdenite mineralization in the nearby Valeriano and El Encierro porphyry deposits. The occurrence of a porphyry-style mineralization at Cerros Taguas reflects the prospectivity for porphyry deposits beneath Miocene-age volcanic rocks and advanced argillic alteration zones in the northern sector of the El Indio belt.

MVT-related elements show a specific enrichment pattern in a geothermal scale.

Comparison with fluid parameters suggests pressure reduction and CO₂ degassing as basis for mineral formation and element enrichment.

Exchange processes at the contact surface mineral-fluid and changes in the redox potential allow for element-specific concentration changes.

Methane oxidation and sulfate reduction cause major chemical changes.

Isotope data track the observed chemical variations.

The Paleo-Tethys tectonic zone has been recognized as a world-class rare-metal (Li-Rb-Be-Nb-Ta) pegmatite belt. Previous studies indicate that the rare-metal pegmatite mineralization is related to the Late Triassic–Early Jurassic granitoids. However, it remains debated which granites, among the various coeval I-, A- and S-type granitoids in the tectonic belt, are responsible for the rare-metal pegmatite mineralization. We address these questions through a systematic geochemical study of the Bailongshan granite complex, which is composed of both biotite granites and two-mica granites and is related to the largest Li deposit in this zone. The similarities in Sr–Nd–Hf–O isotopic compositions between the two-mica granites (I_Sr=0.7176 to 0.7183, εNd(t)= − 10.7 to − 10.1, εHf(t)= − 14.12 to − 4.58, δ¹⁸O = 10.11 to 13.46‰) and rare-metal pegmatites (I_Sr=0.7181 to 0.7189, εNd(t)= − 11.72 to − 10.68, εHf(t)= − 12.15 to − 5.37, δ¹⁸O = 10.37 to 12.37‰), both showing affinity with sedimentary source, provide convincing evidence that the rare-metal pegmatites were derived from the two-mica granites. The differences in these parameters between the two-mica granites and the biotite granites (I_Sr=0.7083 to 0.7086, εNd(t)= − 5.9 to − 5.7, εHf(t)= − 6.64 to − 1.50, δ¹⁸O = 7.27 to 9.36‰, characteristic of I-type granites) indicate that they were derived from different sources. Trace element modeling indicates that the pegmatites were produced via extremely high fractional crystallization (> 90%) of the two-mica granites, which is also supported by the difference in δ⁷Li values between the two-mica granites (-0.6 to 0.5‰) and pegmatites (2.04 to 4.94‰). Comparison of the geochemical data between the two-mica granites and metasedimentary rocks in the area suggests that the rare metals in the mineralizing magmas were most likely derived from the partial melting of metapelites of the Triassic Bayanharshan Group. The relatively high temperatures (771 to 830 °C) estimated from the Ti-in-zircon thermometer for the two-mica granites favor extraction of rare metals from both biotite and muscovite in the source rocks during the partial melting. The results of this study, together with published data of Late Triassic to Early Jurassic granitoids in the Paleo-Tethys tectonic zone, indicate that the rare-metal pegmatite mineralization is related to S-type granites, but not all S-type granites are fertile. The combination of rare-metal-rich source rocks (metapelites), high temperatures due to an external heat source favoring the release of rare metals from the source rocks, and high degrees of fractional crystallization facilitating further enrichment of rare-metals in the pegmatite magmas, is critical for the rare-metal mineralization.

Li–Cs–Ta (LCT)-type granitic pegmatites commonly occur adjacent to granitic bodies. For many pegmatite fields, it is not obvious whether the pegmatitic melt originated from an evolved granitic magma or from low-degree partial melting of metasedimentary rocks. The Ke’eryin granitic pegmatites in the Songpan–Ganze orogenic belt, China, which hosts large Li₂O reserves around large-volume granitic intrusions, including biotite granite (BG), two-mica granite (TG), and muscovite granite (MG), present an excellent location to investigate the petrogenesis of granitic pegmatites and associated Li mineralization. Our results suggest that these granites were generated from a common magma source and emplaced in pulses, coupled with fractional crystallization. These granites and associated pegmatites show discordant trends in the bulk-rock Zr/Hf and Nb/Ta ratios and apatite Y/Ho and Sr/Y ratios, which reflect an evolution from granitic magma to flux-rich pegmatite melts. Pegmatitic melts might have derived from TG magma during evolution from BG to TG and MG magma. Initial ⁸⁷Sr/⁸⁶Sr ratios of BG apatite (0.7161–0.7188) and low bulk-rock Fe₂O₃/FeO ratios (0.04–0.22) imply that the Xikang Group at depth might have undergone high-degree partial melting to produce the original granitic magma. This melting of metasedimentary rocks, resulting in a large-volume magma with low flux and rare-metal contents, was followed by protracted fractionation during multiple pulses of magma emplacement. This process resulted in the formation of flux- and rare metal-rich pegmatite melts from granitic magma. This mechanism may be applicable to many LCT-type pegmatites associated with large granitic complexes worldwide.

The amethyst and agate geodes from the Los Catalanes Gemmological District in Uruguay represent one of the main deposits of its kind worldwide. The geometry of the deposit is horizontal, with an irregular distribution of amethyst geodes within the upper level of the basalt lava flows and shows strong variations in their abundance, as well as quality, geometry, and shape. Reliable exploration guides are scarce, and the limited knowledge of the geological parameters controlling its occurrence makes exploration unpredictable, leading to inaccurate reserve estimation. Based on cutting-edge methods including nucleation-assisted microthermometry of one-phase fluid inclusions and determination of triple oxygen isotope in silicates and carbonates, as well as analysis of geode-hosted water and groundwater, we estimate the crystallisation temperatures in the range between 15 and 60 °C. These low temperatures point to amethyst crystallisation after the emplacement of the complete basalt pile. The mineralising fluid shows isotopic signatures consistent with meteoric water and very low salinities from pure water up to rarely over 3 wt% NaCl-eq., likely sourced from the groundwater hosted in the aquifers in the basaltic sequence and underlying units. Based on the insights provided by the new data, we propose the combination of open- and closed-system crystallisation inside pre-existing cavities due to the episodic infiltration of meteoric water in a rather stable geological context.

The diamond potential of kimberlites is generally determined using indicator minerals (i.e., xenocrysts), entrained by the parent magma while ascending through the sub-continental lithospheric mantle (SCLM). It is becoming increasingly apparent that olivine can also be used to understand mantle sampling depth, using the Al-in-olivine thermometer, and to constrain the extent of diamond-destructive metasomatism in the SCLM. To further current understanding of vertical sampling and diamond preservation in the SCLM, we present geochemical results for kimberlitic olivine of the Koidu mine (Sierra Leone). We combine our olivine data with pressure-temperature estimates from available olivine diamond inclusions, clinopyroxene xenocrysts, and eclogite xenoliths to visualise the vertical distribution of lithologies in the SCLM beneath Koidu. In agreement with the absence of peridotitic olivine and low abundance of olivine diamond inclusions in the lower SCLM, megacrysts appear to dominate the material sampled from the lowermost lithosphere. At shallower levels a distinct eclogite-dominated region is observed (160-180 km) whereas the SCLM at depths of 110-150 km is heterogeneous comprising depleted harzburgite/dunite, lherzolite, and eclogite. Diamonds are predominantly eclogitic with pressure-temperature estimates for diamondiferous eclogites of 150-190 km within the eclogite-dominated region. The near absence of diamonds sampled from near the lithosphere-asthenosphere boundary is attributed to diamond destruction by extensive infiltration of proto-kimberlite melts leading to metasomatism of the eclogite and peridotite substrate and megacrysts formation. Widespread metasomatism of the deepest reaches of the SCLM sampled by kimberlites elsewhere suggests that Koidu does not represent an isolated case and the extent of diamond-destructive metasomatism can be constrained using olivine xenocrysts.

The Rajapalot area of Finnish Lapland hosts an unusually high-grade association of cobalt-only and gold–cobalt deposits (10.91 Mt @ 2.5 g/t Au + 0.44% Co total inferred resources) within multiply folded metasedimentary rocks of the Paleoproterozoic Svecofennian collisional orogeny. Through the integration of X-ray computed micro-tomography and micro-X-ray fluorescence raster imaging of drill-core samples, we produce a model of cobalt-bearing ore mineralisation, which reveals primary fluid transportation mechanisms and precipitation pathways. When combined with the deposit-scale, high-resolution ground-based magnetic geophysical data, we show that cobalt-bearing ores in the Rajapalot region occur mostly as saddle reefs located in dilated fold hinges, which formed by simultaneous synthetic and antithetic shearing along developing crenulation-cleavage planes and incompetent bedding layers, respectively. We suggest that multi-layered rock complexes with alternations of competent and incompetent layers deformed and metamorphosed to upper greenschist-lower amphibolite facies should represent focus regions for cobalt exploration targeting campaigns in orogenic belts. The non-destructive workflow presented in this study could be an integral part of an early stage of cobalt mineral processing and traceability before metallurgical treatment.

The iron (Fe)-oxide deposits of the Lahn-Dill-type are composed of haematite-quartz and rare siderite-haematite ores. These ores formed as marine chemical sediments on top of volcaniclastic rocks near the Middle to Late Devonian boundary (∼ 380 Ma). As such, their trace element fractionation patterns provide key information on venting style, ocean chemistry, particle-solution interaction, and depositional environment at the time of ore formation. This study combines WDXRF and ICP-MS/OES whole-rock geochemistry with complementary in-situ LA-ICP-MS analysis, and TEM element mapping of ore samples from the Fortuna Mine (Rhenish Massif, Germany). In-situ measurments were conducted on quartz-haematite, haematite, and siderite-haematite microdomains. Bulk major element contents of the ores indicate (volcani)clastic contamination and post-depositional hydrothermal alteration. Microdomain trace element distributions reveal four different trace element signatures, which are related to: (1) syngenetic apatite formation due to sorption of P and REY from seawater; (2) Fe-(oxyhydr)oxide-specific trace element scavenging and fractionation within the seawater column; (3) diagenetic Fe(III) reduction and trace element mobilisation in pore water; and (4) simultaneous deposition of (volcani)clastic material and Fe-(oxyhydr)oxides. These results show that Lahn-Dill-type iron ore formation resulted from mixing of a low-temperature vent fluid with ambient seawater at high seawater to vent fluid ratios. This likely was related to an environment in which diffuse venting dominated over focused venting, and in which quick Fe-particle precipitation led to formation of haematite-quartz ores. Local diagenetic Fe(III) reduction resulted in post-depositional siderite-haematite ore formation during which trace elements were partially remobilised in pore water.

The Augmitto-Bouzan deposit is a 12 km long segment of the Larder Lake-Cadillac Deformation Zone (LLCDz) south of Rouyn-Noranda (Québec, Canada) that is characterized by an uneven gold distribution hosted in quartz-carbonate ± tourmaline veins within Piché Group ultramafic rocks. This study compares the fluid flow conditions between the variable gold-endowed sectors to identify deposit-scale processes responsible for gold endowment. Stable isotopes indicate that quartz and tourmaline have equilibrium temperatures (228–⁠420 °C) that likely define a high vertical thermal gradient (~ 30 °C/100 m) along the LLCDz. Covariation between temperature and computed δ¹⁸O_H2O and δD_H2O is interpreted to result from mixing between a high temperature (> 420 °C), high δ¹⁸O (> 10.8‰), and low δD (< –29‰) deep-seated metamorphic fluid, and a low temperature (< 230 °C), low δ¹⁸O (< 4‰) and high δD (~ 0‰) upper crustal pore fluid. Local upwelling of auriferous deep-seated fluid, shown by interpolation of δ¹⁸O_H2O in the gold-endowed Augmitto-Cinderella and Astoria segments, was likely focused along higher permeability deformation-related pathways. Sectors of low gold endowment have lower δ¹⁸O_H2O and fluid/rock ratios, likely reflecting a larger proportion of upper crustal fluid and differences in fluid-flow behavior. Modeling of fluid flow shows that this is due to 1) weaker metamorphic fluid flux in the thinner band of Piché Group rocks and 2) more porous volcanic rocks north of the LLCDz, drawing more pore fluid into the fault. We suggest that most of the variation of gold endowment is related to variations in advection of auriferous metamorphic fluid along the segment, whereby a weaker metamorphic fluid flux or increased admixture of upper crustal fluids decrease the gold potential along the LLCDz.