T. Netshidzivhe, S. Tappe, A. H. Wilson, S. Burness, J. F. Wotzlaw, H. Strauss, K. A. Smart, B. M. Guy, K. S. Viljoen
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.
{"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 (&lt;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}
Jorge Crespo, Carolina Muñoz-Saez, Simon M. Jowitt, Randal Burns
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.
锂（Li）和稀土元素（ree）是现代技术和能源转型的关键矿物，因为它们在能源储存、运输和低二氧化碳发电方面发挥着重要作用，而且它们的供应链本身就不安全。内华达州和俄勒冈州的麦克德米特火山口拥有重要的锂资源，包括内华达州的Thacker Pass矿床，这是全球已知最大的锂资源之一，矿产资源估计为6610万吨碳酸锂当量（LCE），以及俄勒冈州的麦克德米特矿床（含21.5亿吨LCE）。虽然这些矿床在勘探过程中发现了一定数量的稀土元素，但这些关键商品并不是本区李粘土勘探的主要目标。然而，稀土元素的临界性和它们在这些锂系统中的矿化性质尚不清楚，因此很难评估它们作为锂萃取副产物的潜在回收率。本文研究了萨克山口黎系粘土中含稀土矿物的赋存状态。对岩心地球化学电感耦合等离子体质谱（ICP-MS）数据库的统计分析表明，Ce和La之间存在较强的相关性（r = 0.9）。扫描电镜观察证实，在富锂蒙脱石和伊利石矿化带中，存在bastnäsite、合土石和独居石等微米级颗粒。半定量分析确定Ce、La、Nd和Pr是这些矿物的主要成分。尽管Thacker Pass大量样品的单个REE浓度较低（Ce <300 ppm, La <150 ppm），但REE作为离散矿物的存在表明它们可能作为Li提取的副产物被回收。这是对锂粘土矿床稀土元素潜力的首次评估，强调了对内华达州和全球类似矿床进行进一步评估的必要性。这些发现强调了从锂粘土资源中回收稀土元素的潜在重要性，有助于实现更多样化和安全的供应链，并支持全球能源转型。
{"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 &lt;300 ppm, La &lt;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}
Ying Zhou Li, William D. Smith, M. Christopher Jenkins, Zhuosen Yao, James E. Mungall
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.
岩浆硫化物矿石的Pd/Pt比值通常高于其母岩浆的类地幔值。这种差异没有简单的解释，因为这两种元素在硫化物和硅酸盐液体之间的分配行为非常相似。基性和超基性岩浆对富含硫和碳的岩石的同化作用被认为是岩浆贱金属硫化物矿床形成的关键步骤，即使不是必不可少的步骤。虽然外界硫和碳的同化作用促进了硫化物的饱和，但碳同化作用对天然含硫硅酸盐熔体中铂族元素溶解度的影响一直被忽视。在本研究中，我们通过热力学模拟研究了石墨和甲烷同化过程中铂和钯溶解度的变化，并与一系列极具特色的岩浆硫化物矿石系统的数据进行了比较，这些数据代表了从太古代到古生代的年龄，从科马地岩到玄武岩的熔融成分，以及岩浆环境，包括熔岩，浅成岩侵入体，深部大陆弧根和深部层状侵入体，即：Raglan, Norilsk-Talnakh， Lac des Iles和jm Reef of Stillwater Complex。我们模拟了同化-分数结晶过程，以估计由于石墨和甲烷的掺入，熔体的氧逸度（fO2）的减少。模拟结果表明，尽管Pd在还原碳的逐渐同化过程中保持高可溶性，但Pt的溶解度随着硅酸盐熔体的减少而显著降低。在沉积物同化率低于8%的情况下，Pt合金可能会饱和，然后偏离硫化物-不饱和硅酸盐熔体，从而增加Raglan和Norilsk-Talnakh体系剩余熔体的Pd/Pt值。对于Lac des Iles和Stillwater系统，由于地壳下部岩石相对碳含量较低，需要更高程度的同化才能达到Pt饱和度。甲烷挥发物的同化被证明比石墨的同化更有效，并且在没有可检测到的大块宿主岩同化的情况下，它为Pt合金分馏提供了一条途径。在许多世界级的岩浆硫化物矿床中发现了高Pd/Pt值，这些矿床的母岩浆明显经历了地壳污染。我们的模型表明，尽管异常的Pd/Pt值可以用其他机制来解释，例如先前存在的硫化物的不一致熔化或金属在达到平衡时的扩散率差异，但石墨或甲烷的同化可能在全球出现具有高Pd/Pt值的岩浆硫化物矿石中发挥重要作用。
{"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}
Sarah-Jane Barnes, Wolfgang D. Maier, Dany Savard, Stephen A. Prevec
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.
{"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}
Shuling Song, Yu Zhang, Robert A. Creaser, Huayong Chen, Changzhou Deng, Matthew J. Brzozowski, Runsheng Yin, Jonathan Toma, Yongjun Shao, Hongbin Li, Xu Wang
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
{"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}
E. A. Cortes-Calderon, B. S. Ellis, L. Tavazzani, T. Magna, C. Harris, T. R. Benson
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.
{"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 (&gt;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}
M. A. MacDonald, G. Bat-Erdene, M. X. Gillis, P. J. Dalton, I. Kavalieris, B.-E. Khashgerel, A. Kloppenburg, A. Coote, J. W. Hedenquist
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
{"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}
Hidaya Hassan, Jacob Kaavera, Akira Imai, Kotaro Yonezu, Thomas Tindell, Kenzo Sanematsu, Koichiro Watanabe
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.
{"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 δ<sup>34</sup>S 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 CO<sub>2</sub> 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}
Marina D. Schofield, Bruno Lafrance, Harold L. Gibson, K. Howard Poulsen, Christophe Scheffer, Benoît Quesnel, Georges Beaudoin, Michael A. Hamilton
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 δ18Oquartz values of 8.5 ± 0.8‰, reflecting δ18Ofluid 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 δ18Oquartz values of 11.3 ± 0.8‰, reflecting δ18Ofluid 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.
{"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 δ<sup>18</sup>O<sub>quartz</sub> values of 8.5 ± 0.8‰, reflecting δ<sup>18</sup>O<sub>fluid</sub> 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 δ<sup>18</sup>O 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 δ<sup>18</sup>O<sub>quartz</sub> values of 11.3 ± 0.8‰, reflecting δ<sup>18</sup>O<sub>fluid</sub> compositions of 2.4 to 5.9‰ (250°–350°C), typical of a metamorphic fluid, possibly mixed with a lower δ<sup>18</sup>O upper crustal fluid. They are associated with ankerite, calcite, muscovite, chlorite, albite, and quartz ± hematite alteration with whole-rock δ<sup>18</sup>O 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 δ<sup>18</sup>O 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 δ<sup>18</sup>O 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}
Robert R. Loucks, Gonzalo J. Henríquez, Marco L. Fiorentini
To meet surging requirements of copper for the green energy revolution, minable resources subequal to all copper production in history must be found in the next two decades. We show that trace elements in zircon and whole-rock samples that are diagnostic of unusually high-pressure magmatic differentiation and high hydration state and oxidation state of their parent silicate melt are effective for discriminating copper sulfide-ore-productive arc magmas from infertile arc magmas. Tests on our database of 5,777 zircons from 80 igneous complexes, including 2,220 zircons from ore-generative intrusions in 37 major porphyry and high-sulfidation epithermal Cu(-Au-Mo) deposits worldwide, demonstrate that our magmatic copper fertility discriminants apparently perform equally well in intraoceanic arcs, continental margin arcs, and continental collision orogens of Ordovician to Quaternary age. That performance consistency means that the tectono-magmatic controls on development of magmatic-hydrothermal copper ore-forming fertility are essentially the same in all those plate-convergence settings. The ratio Ce/√(U × Ti) in zircon is a quantitative indicator of the relative oxygen fugacity of the silicate melt and its sulfur-carrying capacity. The ratio of the europium anomaly to ytterbium in granitoid melts and zircon is an uncalibrated but empirically useful indicator of the melt’s hydration state and ability to provide chloride-complexed metals to exsolving hydrothermal fluids. Plots of (EuN/Eu*)/YbN vs. Ce/√(U × Ti) in zircon are remarkably effective for discriminating igneous complexes, arc segments, and time intervals within them that can generate and are likely to host magmatic-hydrothermal Cu(-Au-Mo) ore deposits. Arrays of cognate zircons on such plots have slopes that vary with pressure-dependent chlorinity of exsolving fluid and its efficacy in scavenging CuCl from the melt. Our zircon indicators of Cu metallogenic fertility are applicable to detrital as well as in situ zircons and can assist with ore discovery in watersheds upstream from a sediment sampling site. We formulated a composite zircon copper fertility index (ZCFI) that can be applied to each microbeam spot analysis—ZCFI = 104 (EuN/EuN*)/YbN + 5 Ce/√(Ui × Ti)—and substantially decreases the number of zircon analyses needed for reliable prospectivity assessment in a detrital grain population, thereby making this watershed-scale exploration tool cost-competitive with other methods of geochemical exploration.
{"title":"Zircon and Whole-Rock Trace Element Indicators of Magmatic Hydration State and Oxidation State Discriminate Copper Ore-Forming from Barren Arc Magmas","authors":"Robert R. Loucks, Gonzalo J. Henríquez, Marco L. Fiorentini","doi":"10.5382/econgeo.5071","DOIUrl":"https://doi.org/10.5382/econgeo.5071","url":null,"abstract":"To meet surging requirements of copper for the green energy revolution, minable resources subequal to all copper production in history must be found in the next two decades. We show that trace elements in zircon and whole-rock samples that are diagnostic of unusually high-pressure magmatic differentiation and high hydration state and oxidation state of their parent silicate melt are effective for discriminating copper sulfide-ore-productive arc magmas from infertile arc magmas. Tests on our database of 5,777 zircons from 80 igneous complexes, including 2,220 zircons from ore-generative intrusions in 37 major porphyry and high-sulfidation epithermal Cu(-Au-Mo) deposits worldwide, demonstrate that our magmatic copper fertility discriminants apparently perform equally well in intraoceanic arcs, continental margin arcs, and continental collision orogens of Ordovician to Quaternary age. That performance consistency means that the tectono-magmatic controls on development of magmatic-hydrothermal copper ore-forming fertility are essentially the same in all those plate-convergence settings. The ratio Ce/√(U × Ti) in zircon is a quantitative indicator of the relative oxygen fugacity of the silicate melt and its sulfur-carrying capacity. The ratio of the europium anomaly to ytterbium in granitoid melts and zircon is an uncalibrated but empirically useful indicator of the melt’s hydration state and ability to provide chloride-complexed metals to exsolving hydrothermal fluids. Plots of (Eu<sub>N</sub>/Eu*)/Yb<sub>N</sub> vs. Ce/√(U × Ti) in zircon are remarkably effective for discriminating igneous complexes, arc segments, and time intervals within them that can generate and are likely to host magmatic-hydrothermal Cu(-Au-Mo) ore deposits. Arrays of cognate zircons on such plots have slopes that vary with pressure-dependent chlorinity of exsolving fluid and its efficacy in scavenging CuCl from the melt. Our zircon indicators of Cu metallogenic fertility are applicable to detrital as well as in situ zircons and can assist with ore discovery in watersheds upstream from a sediment sampling site. We formulated a composite zircon copper fertility index (ZCFI) that can be applied to each microbeam spot analysis—ZCFI = 10<sup>4</sup> (Eu<sub>N</sub>/Eu<sub>N</sub>*)/Yb<sub>N</sub> + 5 Ce/√(U<sub>i</sub> × Ti)—and substantially decreases the number of zircon analyses needed for reliable prospectivity assessment in a detrital grain population, thereby making this watershed-scale exploration tool cost-competitive with other methods of geochemical exploration.","PeriodicalId":11469,"journal":{"name":"Economic Geology","volume":"10 1","pages":""},"PeriodicalIF":5.8,"publicationDate":"2024-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140817872","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}
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