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The Khetri Copper Belt (KCB) in northwestern India contains several economically important hydrothermal Cu–(Au) deposits. Orebodies in these deposits are locally rich in monazite-(Ce). In this study, in situ U–Pb ages and Sm–Nd isotopic compositions of monazite-(Ce) are used to constrain the timing of mineralization and to better understand the genesis of the Cu–(Au) deposits. Monazite-(Ce) crystals were identified in the Madhan-Kudhan and Kolihan deposits in the KCB. Most monazite-(Ce) crystals in the two deposits show close textural associations with hydrothermal minerals (biotite, chlorite, and sulfide) and have low contents of ThO2 (mostly < 2 wt.%), indicating a hydrothermal origin. The hydrothermal monazite-(Ce) crystals have U–Pb ages of 833 ± 5 to 840 ± 6 Ma, which are interpreted to represent the timing of the mineralization in the KCB. Another set of monazite-(Ce) crystals in the Kolihan deposit has relatively high contents of ThO2 (4.72 wt.% on average) and commonly shows concentric zonation with respect to Th. They have a weighted average 207Pb/206Pb age of 1362 ± 29 Ma, which may record a pre-ore metamorphic event in the KCB. Hydrothermal monazite-(Ce) crystals from the Madhan-Kudhan deposit have εNd(t = 835 Ma) values ranging from − 4.3 to − 16.8 (− 9.6 on average), indicating that isotopically heterogeneous materials were in the metal sources or along the flow paths of hydrothermal fluids. The ages of Cu–(Au) deposits in the KCB fall within the age range of the regional Ca–Na metasomatism (~ 830 to ~ 850 Ma). Combining this temporal association with relevant geochemical and isotopic data, we propose that circulation of hydrothermal fluids caused widespread Ca–Na metasomatism, mobilized certain ore-metals, and thus facilitated the formation of Cu–(Au) deposits. [ABSTRACT FROM AUTHOR]

The Longbohe Fe-Cu-(LREE) deposit in the Longbohe–Sin Quyen (LSQ) Fe-Cu belt in the border zone of South China and northern Vietnam has a resource of > 1 million tonnes Cu, and was once speculated to be a potentially important subduction-related deposit of Precambrian age, but with poor constraints. Our study reveals that the major Fe-, Cu-, and LREE mineralization comprises three stages and are comparable to typical IOCG deposits worldwide. Stage I is characterized by pervasive Na alteration, followed by Fe mineralization stage (Stage II) consisting of Ti-poor magnetite associated with Ca-Na alteration forming amphibole and albite. Stage III, the Cu-LREE mineralization stage, is composed mainly of chalcopyrite and REE-rich allanite with minor pyrrhotite and pyrite. Stages I and II formed at broadly comparable temperatures of 515 to 670 ºC, and have δ18O fluid values of 4.3 to 11.7 ‰, indicating dominantly magmatic fluids. Furthermore, they have hydrothermal zircon with εHf(t) values (7.5 to 12.7) similar to coeval mafic intrusions in the region, likely indicating a mafic magma-source for the fluids. Stage III fluids have lower temperatures (332 to 402 ºC) but exhibit magmatic-like δ34S values (1.2 to 3.7 ‰) similarly suggestive of a major magmatic source. However, they have distinctly lower εHf(t) and εNd(t) values comparable to those of coeval regional felsic intrusions, suggesting that an additional component of REE-rich external fluid from felsic magmas was involved during the Cu-REE mineralization. Hydrothermal zircons from Stages II and III have similar U–Pb ages of ~ 850 Ma, providing tight constraints on the timing of Fe-Cu mineralization. The new U–Pb ages indicate that the Longbohe deposit is coeval with the Sin Quyen IOCG deposit in northern Vietnam, and with the regional subduction-related magmatism, indicating that the Longbohe–Sin Quyen belt is of Neoproterozoic age and formed in a convergent setting. [ABSTRACT FROM AUTHOR]

The Sin Quyen deposit in northwestern Vietnam contains economic concentrations of Cu, Au and LREE, and sub-economic concentration of U. In this deposit, massive and banded replacement ores are hosted in Neoproterozoic metapelite. The paragenetic sequence includes sodic alteration (stage I), calcic-potassic alteration and associated Fe-REE-(U) mineralization (stage II), Cu-Au mineralization (stage III), and sulfide-(quartz-carbonate) veins (stage IV). The Sin Quyen deposit experienced an extensive post-ore metamorphic overprint, which makes it difficult to precisely determine the mineralization age. In this study, zircon and monazite U-Pb geochronometers and the Rb-Sr isochron method are used to constrain the timing of mineralization. Zircon grains in the ore are closely intergrown or texturally associated with hydrothermal minerals of stage II (e.g., garnet, allanite, and hedenbergite). They may contain primary fluid inclusions and display irregular zoning in cathodoluminescence (CL) images. Zircon grains are rich in U (688 to 2902 ppm) and poor in Th (0.2 to 2.9 ppm). Their δ18OV-SMOW values range from 11.9 to 14.0‰, higher than those of typical magmatic zircon. These textural and compositional features imply that zircon precipitated from 18O- and U-rich hydrothermal fluids, coeval with the minerals of stage II. Monazite occurs in close association with stage II magnetite and allanite and has low contents of Th (<2700 ppm), indicative of a hydrothermal origin. Hydrothermal zircon and monazite have indistinguishable U-Pb ages of 841 ± 12 and 836 ± 18 Ma, respectively, representing the timing of Fe-REE mineralization. There is no direct isotopic constraint on the timing of the Cu-Au mineralization, but geological observations suggest that the Cu-Au and Fe-REE ores most likely formed within a single evolved hydrothermal process. In the plot of 87Rb/86Sr vs. 87Sr/86Sr, the composition of bulk-ore and biotite separates from ore lie along a reference line for 30 Ma, which is consistent with the timing of metamorphism in the region. The mineralization age of the Sin Quyen deposit falls within the overall age range (740 to 860 Ma) of the regional Neoproterozoic igneous rocks. This temporal linkage, in combination with the magmatic-like sulfur isotopes of sulfide minerals (δ34SV-CDT = −0.8 to 3.1), indicates that the mineralization may have a close genetic association with the Neoproterozoic igneous activity. [ABSTRACT FROM AUTHOR]

Apatite is a ubiquitous accessory mineral in a variety of rocks and hydrothermal ores. Strontium isotopes of apatite are well known to retain petrogenetic information and have been widely used to investigate the origin of igneous rocks, but such attempts have rarely been made to constrain ore-forming processes of hydrothermal systems. We here report in situ LA-MC-ICPMS Sr isotope data of apatite from the ~1660-Ma Yinachang Fe-Cu-REE deposit, Southwest China. The formation of this deposit was coeval to the emplacement of regionally distributed doleritic intrusions within a continental-rift setting. The deposit has a paragenetic sequence consisting of sodic alteration (stage I), magnetite mineralization (stage II), Cu sulfide and REE mineralization (stage III), and final barren calcite veining (stage IV). The stage II and III assemblages contain abundant apatite, allowing to investigate the temporal evolution of the Sr isotopic composition of the ore fluids. Apatite of stage II (Apt II) is associated with fluorite, magnetite, and siderite, whereas apatite from stage III (Apt III) occurs intimately intergrown with ankerite and Cu sulfides. Apt II has Sr/Sr ratios varying from 0.70377 to 0.71074, broadly compatible with the coeval doleritic intrusions (0.70592 to 0.70692), indicating that ore-forming fluids responsible for stage II magnetite mineralization were largely equilibrated with mantle-derived mafic rocks. In contrast, Apt III has distinctly higher Sr/Sr ratios from 0.71021 to 0.72114, which are interpreted to reflect external radiogenic Sr, likely derived from the Paleoproterozoic strata. Some Apt III crystals have undergone extensive metasomatism indicated by abundant monazite inclusions. The metasomatized apatite has much higher Sr/Sr ratios up to 0.73721, which is consistent with bulk-rock Rb-Sr isotope analyses of Cu ores with Sr/Sr from 0.71906 to 0.74632. The elevated Sr/Sr values of metasomatized apatite and bulk Cu ores indicate that later fluids were dominated by highly radiogenic Sr equilibrated with the Paleoproterozoic country rocks. Results of this study highlight the utilization of in situ Sr isotope analysis of apatite in unraveling the evolution of hydrothermal systems. [ABSTRACT FROM AUTHOR]