Economic Geology最新文献

{"title":"Clay Chemistry of the Thacker Pass Deposit, Nevada: Implications for the Formation of High-Grade Volcano-Sedimentary Lithium Resources","authors":"Christopher Emproto, Thomas R. Benson, Catherine A. Gagnon, Woohyeon Baek, Daniel Ibarra, Adam C. Simon","doi":"10.5382/econgeo.5155","DOIUrl":"https://doi.org/10.5382/econgeo.5155","url":null,"abstract":"Volcano-sedimentary lithium (Li) deposits are a potential source of battery-grade Li, although the important factors controlling Li enrichment in these systems remain unclear. At Thacker Pass in Nevada, high-grade mineralization overprinted intracaldera lacustrine claystone made of authigenic Li-rich smectite with bulk grades of ~3,000 ppm Li, converting it to illitic claystone with grades of ~6,000 ppm Li. Some attribute this enrichment to burial diagenesis, whereas others propose lacustrine Li enrichment through leaching and climate-driven evapoconcentration enhanced by postdepositional hydrothermal alteration. To better understand Li enrichment in volcano-sedimentary systems, claystones from throughout Thacker Pass were analyzed using powder X-ray diffraction (PXRD), electron microprobe (EPMA), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and stable isotope (clay δ18O, δ17O, and δ2H and carbonate δ13C and δ18O) methods. Compositional data suggest that illitization is required to achieve clay Li grades above ~0.9 wt % in Mg silicate clays because of a charge-coupled substitution that requires filling interlayer vacancies with K. Clay chemical trends and computational modeling exercises also suggest that F may be important in the formation of Li-rich clays by lowering kinetic barriers to clay precursor growth and illitization. The results are incompatible with diagenetic smectite/illite formation but are consistent with a model wherein authigenic smectite was subjected to hydrothermal alteration in the presence of a K-, Li-, and F-rich fluid that permeated the stratigraphy through a network of normal faults associated with caldera resurgence. These results also enhance our understanding of Li clay formation in other volcano-sedimentary systems.","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"648 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144533192","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The 3.53 Ga St James Volcanogenic Massive Sulfide Deposit, Kaapvaal Craton: Links to Submarine Rhyolites, Not to Komatiites","authors":"T. Netshidzivhe, S. Tappe, A. H. Wilson, S. Burness, J. F. Wotzlaw, H. Strauss, K. A. Smart, B. M. Guy, K. S. Viljoen","doi":"10.5382/econgeo.5162","DOIUrl":"https://doi.org/10.5382/econgeo.5162","url":null,"abstract":"The Nondweni greenstone belt is located in the southeastern region of the Kaapvaal craton in South Africa and contains significant volumes of 3.53 to 3.45 Ga mafic-ultramafic lava flows (komatiitic and tholeiitic basalts). Minor felsic volcanic rock units, such as massive rhyolites and deformed quartz-feldspar-mica schists (felsic schists), also occur and are demonstrably linked to small base metal sulfide orebodies, interpreted here as volcanogenic massive sulfide (VMS)-type mineralization (e.g., sphalerite, chalcopyrite, pyrrhotite, pyrite, galena, acanthite). Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon analysis yields a rhyolite eruption age of 3531.91 ± 0.46 Ma for the felsic volcanic rock unit that hosts VMS-type mineralization at the St James deposit, which renders this Zn-Cu-Pb-Ag mineralization among the oldest preserved of its kind, offering insights into ore-forming processes that took place on and below the Paleoarchean sea floor. Rare earth element geochemical modeling suggests that the felsic volcanic rocks formed by moderate degrees of partial melting of hydrothermally altered basalts similar in composition to those from the Nondweni greenstone belt. Regarding ore formation, we envisage a scenario where basaltic ocean floor, in close proximity to a back-arc spreading ridge, had been intensively altered and subjected to elevated temperatures, which facilitated localized melting at low pressures (<2 kbar), resulting in the production of rhyolitic magmas accompanied by hydrothermal sulfide deposition. This model is supported by evidence from multiple sulfur isotope data (δ34S and Δ33S), which demonstrates that the basaltic rocks contain unfractionated magmatic sulfur and the rhyolitic rocks contain sulfur sourced from altered oceanic basalts. In contrast, the rhyolite-associated VMS-type mineralization records even more complex sulfur interactions, including contributions from surficial mass independent fractionated sulfur isotopic components; that is, these base metal sulfide ores exhibit a negative sulfur mass-independent fractionation signature of –0.53‰ Δ33S.","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"58 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-06-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144520517","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"RARE EARTH MINERALS IN LITHIUM CLAY DEPOSITS: INSIGHTS FROM THE THACKER PASS DEPOSIT, NORTHERN NEVADA, USA","authors":"Jorge Crespo, Carolina Muñoz-Saez, Simon M. Jowitt, Randal Burns","doi":"10.5382/econgeo.5167","DOIUrl":"https://doi.org/10.5382/econgeo.5167","url":null,"abstract":"Lithium (Li) and the rare earth elements (REEs) are critical minerals for modern technologies and the energy transition due to their roles in energy storage, transport, and low-CO2 power generation and their inherently insecure supply chains. The McDermitt caldera in Nevada and Oregon hosts significant lithium resources, including the Thacker Pass deposit in Nevada—one of the largest known Li resources globally, with a mineral resource estimate of 66.1 million tonnes (Mt) of contained lithium carbonate equivalent (LCE)—as well as the McDermitt deposit in Oregon (215 Mt contained LCE). Although these deposits are known to contain some amount of the REEs as identified during exploration, these critical commodities were not the primary target during the Li clay exploration in this area. However, the criticality of the REEs and the poorly known nature of their mineralization within these lithium systems the evaluation of their potential recovery as by-products of Li extraction. This study investigates the occurrence of REE-bearing minerals within the Li clays at Thacker Pass. Statistical analysis of a drill core geochemical inductively coupled plasma-mass spectrometry (ICP-MS) database shows a strong correlation between Ce and La (r = 0.9). Scanning electron microscopy observations confirmed the presence of bastnäsite, synchysite, and monazite as micrometer-sized grains within Li-rich smectite and illite mineralized zones. Semiquantitative analyses identified Ce, La, Nd, and Pr as major components of these minerals. Despite the low individual REE concentrations (Ce <300 ppm, La <150 ppm) of bulk samples from Thacker Pass, the presence of the REEs as discrete minerals suggests they could potentially be recovered as by-products of Li extraction. This is the first assessment of the REE potential of Li clay deposits, highlighting the need for further evaluation of similar deposits in Nevada and globally. These findings emphasize the potential importance of REE recovery from Li clay resources, contributing to more diverse and secure supply chains and supporting the global energy transition.","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"64 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-06-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144370907","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"ASSIMILATION OF REDUCED CARBON TRIGGERS PLATINUM ALLOY SATURATION IN MAFIC AND ULTRAMAFIC MAGMAS","authors":"Ying Zhou Li, William D. Smith, M. Christopher Jenkins, Zhuosen Yao, James E. Mungall","doi":"10.5382/econgeo.5165","DOIUrl":"https://doi.org/10.5382/econgeo.5165","url":null,"abstract":"It is generally observed that magmatic sulfide ores have higher ratios of Pd/Pt than the mantle-like values of their parental magmas. This discrepancy has defied simple explanation because the partitioning behavior of both elements between sulfide and silicate liquids is very similar. Assimilation of sulfur- and carbon-rich country rocks by mafic and ultramafic magmas is considered a critical, if not essential, step in the formation of magmatic base metal sulfide deposits. Although there is general consensus that the assimilation of external sulfur and carbon promotes sulfide saturation, the effect of carbon assimilation on the solubilities of platinum-group elements in natural S-bearing silicate melt has been overlooked. In this study, we investigate the variations of platinum and palladium solubilities during assimilation of graphite and methane through thermodynamic modeling, in comparison with data from an array of highly distinctive magmatic sulfide ore systems representing ages from Archean to Paleozoic, melt compositions from komatiite to basalt, and magmatic settings including lavas, hypabyssal intrusions, plutonic continental arc roots, and plutonic layered intrusions, namely: Raglan, Norilsk-Talnakh, Lac des Iles, and the J-M Reef of the Stillwater Complex. We model assimilation-fractional crystallization processes to estimate the reduction of oxygen fugacity (fO2) of the melt due to incorporation of graphite and methane. The simulations show that although Pd remains highly soluble during the progressive assimilation of reduced carbon, Pt solubility decreases significantly as the silicate melt becomes increasingly reduced. With less than 8% of sediment assimilation, Pt alloy may saturate and then deviate from sulfide-undersaturated silicate melts, concomitantly increasing the Pd/Pt value of the remaining melts of the Raglan and Norilsk-Talnakh systems. For the Lac des Iles and Stillwater systems, a higher extent of assimilation is needed to reach Pt saturation because of the relatively carbon-poor nature of the lower crustal rocks. The assimilation of methane volatiles is shown to be more effective than graphite assimilation, and it provides a pathway to Pt alloy fractionation in the absence of detectable amounts of bulk host-rock assimilation. High Pd/Pt values have been documented in many world-class magmatic sulfide deposits whose parental magmas have demonstrably experienced crustal contamination. Our model suggests that although anomalous Pd/Pt values may be explained by other mechanisms such as incongruent melting of preexisting sulfide or differences in the diffusivities of the metals within achieving equilibration, the assimilation of graphite or methane may play an important role in the global occurrence of magmatic sulfide ores with elevated Pd/Pt values.","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"37 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144311483","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Major and Trace Element Concentrations in Chromite and Silicate Minerals of the Critical Zone of the Bushveld Complex, South Africa: Effects of Reequilibration and Crystal Fractionation on Chromite Composition","authors":"Sarah-Jane Barnes, Wolfgang D. Maier, Dany Savard, Stephen A. Prevec","doi":"10.5382/econgeo.5156","DOIUrl":"https://doi.org/10.5382/econgeo.5156","url":null,"abstract":"The Rustenburg Layered Suite of the Bushveld Complex, South Africa, contains the world’s largest resource of chrome and platinum group elements (PGEs). Both Cr and PGEs are found in chromitite layers within an approximately 1,000-m-thick section of ultramafic to mafic rocks known as the Critical zone. Neither the process of how the chromitite layers form nor the role that chromite plays in collecting the PGEs is clear. Major and trace element contents of chromite and silicate minerals from each of the 13 chromitite layers, and from chromite in the adjacent peridotites and norites, have been determined. The concentrations of PGEs in both chromite and silicates are less than detection levels (10–20 ppb). Thus, neither are the host of the PGEs in these rocks. The Cr# and Fe# of the chromites from chromitite layers are similar to those found in experiments carried out to model the crystallization of the initial magma (B1) of the Bushveld, with the same decrease in Cr# with increase in Fe#. The fO2 of the experiments Δ 0 FMQ (where FMQ = fayalite-magnetite-quartz buffer) and those of the chromitite chromite calculated from the Fe3+/FeTotal ratios and the V contents of the chromite are similar. Variations in trace element contents of the chromitite chromite can also be modeled using the B1 composition and allowing for ~40% crystal fractionation across the stratigraphy.","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"42 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-06-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144229123","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Multiple Isotopic Constraints on the Origin of the High-Grade Chaxi Gold Deposit in the Jiangnan Orogenic Belt, South China","authors":"Shuling Song, Yu Zhang, Robert A. Creaser, Huayong Chen, Changzhou Deng, Matthew J. Brzozowski, Runsheng Yin, Jonathan Toma, Yongjun Shao, Hongbin Li, Xu Wang","doi":"10.5382/econgeo.5153","DOIUrl":"https://doi.org/10.5382/econgeo.5153","url":null,"abstract":"The Jiangnan orogenic belt in South China is known for its numerous gold deposits that are hosted by Precambrian low-grade metamorphic rocks that were reworked during Paleozoic and Mesozoic orogenic events, which resulted in multiple epochs of gold mineralization. The Chaxi gold deposit, located in southwestern Hunan in the central portion of the Jiangnan orogenic belt, is characterized by localized ultrahigh-grade gold mineralization (visually estimated to be up to several tens of percent). Despite its economic significance, the timing of formation, gold enrichment processes, and origin remain ambiguous; yet these are critical for both exploration and our understanding of gold metallogenesis in the Jiangnan orogenic belt. In addition to the pre-ore quartz-sericite-pyrite stage (stage I), several vein stages associated with primary gold mineralization and alteration have been identified at Chaxi: stage II quartz-chalcopyrite-electrum veins with pyrite alteration, stage III quartz-dolomite-polymetallic sulfides-native gold veins with sericite alteration, and stage IV quartz-dolomite-polymetallic sulfides-native gold veins with chlorite alteration. A supergene stage (stage VI) containing secondary gold mineralization is also present. The Ar-Ar age of sericite in stage I and the Re-Os age of molybdenite-galena intergrowths in stage II are 430.4 ± 2.7 Ma (plateau age; mean square of weighted deviates [MSWD] = 0.82) and 430.6 ± 1.1 Ma (weighted mean model age; MSWD = 0.38), respectively, demonstrating that the gold mineralization is related to the Paleozoic intracontinental orogeny and occurred ~10 m.y. after the metamorphic peak age. Ore-related sulfides from the primary mineralization stages and native gold from superenriched gold ores show negative to zero Δ199Hg values (–0.34 to 0‰), indicating that the ore-forming metals were sourced from the Precambrian metamorphosed volcanic-sedimentary rocks. The restricted Pb isotope signature of galena (207Pb/206Pb = 0.909–0.925) and Sr-Nd isotope compositions of apatite (Sri: 0.710215–0.710392; ɛNd(t): –6.0 to –3.6) further suggest that the ore-forming fluid likely originated from the Neoproterozoic metamorphic basement rocks, with no evidence of contributions from magmatic-hydrothermal fluids. This is consistent with the absence of igneous rocks at Chaxi and the distinct age and Pb isotope compositions between the gold mineralization and diabase in southwestern Hunan. During the mineralizing process, intense sulfidation of the wall rocks controlled the precipitation of compositionally homogeneous electrum (gold fineness of 738–774) in stage II. The continuously decreasing δ34S values of sulfides from stage II to stage IV (stage III: 7.94–18.78‰, stage IV: 2.03–10.90‰) may be a result of phase separation triggered by a fault valve cycle, an interpretation that is supported by the presence of hydrothermal breccias in stage III and stage IV, and by the fact that stage III veins were reopened and refilled by s","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"54 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144229125","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Lithium Inventory of the Cerro Galán Volcanic System (Argentina): The Role of Magmatism as a Source for Li-Bearing Brine Deposits","authors":"E. A. Cortes-Calderon, B. S. Ellis, L. Tavazzani, T. Magna, C. Harris, T. R. Benson","doi":"10.5382/econgeo.5154","DOIUrl":"https://doi.org/10.5382/econgeo.5154","url":null,"abstract":"Lithium-rich brines in South America’s Li triangle host Earth’s largest Li reserves, crucial for the global energy transition. Although Cenozoic magmatism near salars in the Li triangle often is considered as a major potential Li source, there is limited characterization of Li behavior in these magmatic systems. To address this, we present the first detailed Li study of the voluminous ignimbrites within the Cerro Galán volcanic system, a potential Li source for the Salar del Hombre Muerto, which is actively producing Li for batteries. Although most Cerro Galán volcanic system units exhibit normal Li concentrations in groundmass glass (30–50 ppm) relative to rhyolitic centers globally, the ~630-km3 Cerro Galán ignimbrite contains glass with significantly higher Li contents (>90 ppm), reflecting increased melt differentiation. Throughout the volcanic system, plagioclase and quartz display varied Li contents influenced by syneruptive degassing, and additionally for plagioclase, posteruptive modifications. Biotites in the Cerro Galán volcanic system are magmatic and range from 1 to 689 ppm Li, with biotites returning low analytical totals (low total biotites) enriched in Li, Pb, and Cs, consistent with the entrapment of an Li-rich magmatic volatile phase during biotite crystallization. Such a magmatic volatile phase is isotopically light (δ7Li as low as –23‰) and may reach 10,000 ppm Li according to binary mixing modeling. We propose that large magmatic centers, like the Cerro Galán volcanic system, may sustain exsolution of such a magmatic volatile phase and its transport through caldera-hosted hydrothermal systems. When such volcanic centers overlap with closed-basin watershed, magmatic Li-rich fluids could be selectively transported into basins, representing a source for Li-bearing salars, such as in the Salar del Hombre Muerto situated near the Cerro Galán volcanic system.","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"20 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2025-05-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144229130","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Epithermal Gold Discoveries in the Emerging Khundii Metallogenic Province, Southwest Mongolia","authors":"M. A. MacDonald, G. Bat-Erdene, M. X. Gillis, P. J. Dalton, I. Kavalieris, B.-E. Khashgerel, A. Kloppenburg, A. Coote, J. W. Hedenquist","doi":"10.5382/econgeo.5070","DOIUrl":"https://doi.org/10.5382/econgeo.5070","url":null,"abstract":"Mineral exploration since 2005 in a previously underexplored region of southwestern Mongolia resulted in the definition of the Zuun Mod porphyry Mo-Cu deposit, followed by discovery of the Altan Nar and Bayan Khundii epithermal gold deposits along with several prospects and advanced exploration projects. These discoveries form the core of the emerging Khundii (“Valley”) metallogenic province, ~50 × 100 km in size, located within a single island-arc terrane of Middle Carboniferous to early Permian age and predominantly within an individual mapped subterrane. The province is situated ~700 km west-northwest of the late Devonian Oyu Tolgoi porphyry Cu-Au deposit in a belt of mid-Paleozoic island arcs that are part of the Central Asian orogenic belt, host to world-class porphyry Cu-Au and epithermal gold deposits that stretch from southern Mongolia to the west, into China, Kazakhstan, and beyond.The Zuun Mod porphyry Mo-Cu deposit (297 ± 4.8 Ma) is hosted by a granodiorite intrusion cut by B-type quartz-molybdenite-chalcopyrite veins with K-feldspar alteration selvages plus disseminated biotite and magnetite. After definition of this deposit, a regional exploration program was initiated in 2009 over 110,000 km<sup>2</sup>, based on the underexplored nature of the region. Exploration included compilation of existing geologic, geochemical, and geophysical data and interpretation of satellite imagery followed by ground exploration that included stream, soil, and rock-chip sampling and geologic and alteration mapping. The Nomin Tal Cu-Au prospect was discovered in early 2011, and based on the indications from initial soil sampling, a 400- × 400-m soil survey was conducted over the southern part of the exploration license, which identified a Pb-, Zn-, and Au-in-soil anomaly over an area of ~1.5 × ~5.5 km. The first drill hole within the soil anomaly in late 2011 resulted in the discovery of the Altan Nar Au-polymetallic epithermal deposit with veins of coarsely crystalline quartz-adularia (309.7 ± 0.5 Ma) and Ca-, Mg-, Mn-, and Fe-carbonate gangue that host the base metal sulfides.The Bayan Khundii gold deposit was discovered in 2015 as the result of prospecting, ~16 km southeast of Altan Nar. Subsequent discovery of the Khar Mori gold project was announced in early 2021, ~3 km north of Bayan Khundii along a structural trend, and later in 2021 drilling discovered wide zones of disseminated gold at Ulaan Southeast, ~800 m west of Bayan Khundii. The epithermal quartz-adularia-gold veins (336.8 ± 0.5 Ma) at Bayan Khundii have colloform bands with minor pyrite and are enveloped by proximal illite alteration. The epithermal veins and alteration overprint an earlier, unrelated alteration style of residual quartz and pyrophyllite ± dickite ± diaspore-kaolinite. Similarly, residual quartz and pyrophyllite-dickite at Khar Mori are overprinted by epithermal mineralization, including arsenopyrite. At the central Ulaan project, ~3 km northwest of Bayan Khundii, intens","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"7 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2024-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140817834","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Cu-Au-Platinum Group Element Mineralization in the Mbesa Prospect, Southern Tanzania: Unconventional Magmatic Sulfides","authors":"Hidaya Hassan, Jacob Kaavera, Akira Imai, Kotaro Yonezu, Thomas Tindell, Kenzo Sanematsu, Koichiro Watanabe","doi":"10.5382/econgeo.5068","DOIUrl":"https://doi.org/10.5382/econgeo.5068","url":null,"abstract":"High-grade Cu mineralization was recently discovered in the Mbesa prospect in southern Tanzania, but its origin is poorly constrained. Herein, we present new major element, trace element, rare earth element (REE), and platinum group element (PGE) geochemistry and petrographic data to constrain the origin of the mineralization. The sulfide mineralization is dominated by chalcopyrite and bornite and characterized by high Cu/Ni over 100 in the massive and disseminated ores. The mineralization lacks pyrrhotite and pentlandite, which are found commonly in magmatic Ni-Cu-PGE sulfide deposits. Platinum group minerals present are michenerite (PdBiTe) and sudburyite (PdSb). PGE concentrations expressed as Pd + Pt + Au are up to 1.31 and 1.04 ppm in the massive and disseminated ores, respectively. The δ³⁴S of chalcopyrite separates range from –3.9 to –0.6‰, bornite separates vary from –1.8 to 1.1‰, and bornite-chalcopyrite mixtures yield –1.3 and 0.6‰, mostly consistent with a magmatic sulfur origin. The sulfides are hosted by coarse-grained plagioclase amphibolite and hornblende gneiss with recrystallization textures suggestive of metamorphism. Both rocks are characterized by enrichment in large ion lithophile elements (LILEs), with negative anomalies of Zr and Nb, consistent with crustal contamination of magmas derived from partial melting of subcontinental lithospheric mantle (SCLM). An SCLM origin is further suggested by the close association of Cu sulfides with carbonates and apatite. It is suggested that the sulfides crystallized from the fractionated Cu-rich sulfide melt enriched in intermediate solid solution (iss) at shallow crustal levels. Sulfide were likely mobilized as droplets attached to CO₂ vapor phase as expressed by close association between carbonates and apatite with the Cu-rich sulfides. Nickel-rich sulfides may remain undiscovered at deeper portions around the Mbesa prospect.","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"27 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2024-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140817772","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Discriminating Superimposed Alteration Associated with Epigenetic Base and Precious Metal Vein Systems in the Rouyn-Noranda Mining District, Quebec; Implications for Exploration in Ancient Volcanic Districts","authors":"Marina D. Schofield, Bruno Lafrance, Harold L. Gibson, K. Howard Poulsen, Christophe Scheffer, Benoît Quesnel, Georges Beaudoin, Michael A. Hamilton","doi":"10.5382/econgeo.5063","DOIUrl":"https://doi.org/10.5382/econgeo.5063","url":null,"abstract":"The Rouyn-Noranda mining district of Quebec contains 20 Cu-Zn (±Au ±Ag) volcanogenic massive sulfide (VMS) deposits, including the giant and gold-rich Quemont and Horne deposits. Mineralized epigenetic veins are also present, but their origin and relative timing remain enigmatic. The nature and extent of their alteration signatures and the effect of their superposition on district-scale alteration patterns is unknown. The VMS-related quartz-sulfide Cu-Zn-Ag veins have δ¹⁸O_quartz values of 8.5 ± 0.8‰, reflecting δ¹⁸O_fluid compositions of –0.4 to 3.1‰ (250°–350°C) that are typical of Archean seawater. They are associated with a proximal Fe-rich chlorite alteration and marginal spotted sericite-chlorite alteration with whole-rock δ¹⁸O values of 2.9 to 5.9‰ and are interpreted to have formed within the structurally controlled discordant upflow zones of a VMS hydrothermal system. Younger gold-bearing quartz-carbonate veins were emplaced along mechanical anisotropies created by mafic dikes during north-south compression and the formation of regional E-trending faults, folds, and cleavage. They are characterized by δ¹⁸O_quartz values of 11.3 ± 0.8‰, reflecting δ¹⁸O_fluid compositions of 2.4 to 5.9‰ (250°–350°C), typical of a metamorphic fluid, possibly mixed with a lower δ¹⁸O upper crustal fluid. They are associated with ankerite, calcite, muscovite, chlorite, albite, and quartz ± hematite alteration with whole-rock δ¹⁸O values of 5.8 to 10.3‰. Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon ages for two tonalite intrusions constrain the maximum age of the Cu-Zn-Ag veins to 2697.6 ± 0.7 Ma and the minimum age to 2695.3 ± 1.0 Ma, which is also the maximum age of the gold quartz-carbonate veins. Superposition of alteration related to the gold quartz-carbonate veins on previously chlorite- and sericite-altered rocks has resulted in mixed alteration signals with whole-rock δ¹⁸O values of ~6 to 8‰ that have perturbed and masked regional alteration patterns related to older VMS mineralization, such as those found in the Quemont and Horne deposits. These results indicate that defining alteration vectors in camps that have superimposed hydrothermal systems requires full consideration of the hydrothermal history of the camp, and if such constraints are lacking, whole-rock δ¹⁸O values should not be used as a stand-alone exploration method.","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"28 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2024-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140817838","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}