Search

The Mines Gaspé area hosts multiple Cu–Mo skarn and porphyry orebodies near the town of Murdochville in the northeastern part of the Gaspé Peninsula, Québec. The orebodies occur within overlapping alteration aureoles in calcareous Lower Devonian sedimentary rocks. The strata are intruded by numerous multiphase porphyry sills, dykes, and plugs of Devonian age. The Porphyry Mountain intrusion and a sill in the Copper Mountain pit have been dated at 378.80 ± 0.37 and 377.60 ± 0.45 Ma, respectively, refining the results of previous studies, and demonstrating Porphyry Mountain intrusion emplacement at least 0.38 m.y. before Copper Mountain. Circa 392 Ma inherited zircon grains at Mines Gaspé suggest an early phase of magmatism that produced the extensive skarn alteration aureoles throughout the Gaspé Peninsula at sites such as Mines Gaspé and the nearby McGerrigle Complex, followed by significantly later (>10 m.y.) porphyritic intrusions and associated mineralization that added to existing skarn resources. Epidote at both Mines Gaspé and Sullipek occur as disseminated/granular crystals within the host groundmass and as larger crystals within veinlets or veinlet halos in metasomatised sedimentary rocks. Epidote ages suggest that there are several different propylitic hydrothermal events within the region at Mines Gaspé and Sullipek, which combined with new zircon U–Pb ages implies a prolonged and complex history of propylitic alteration within Gaspésie. [ABSTRACT FROM AUTHOR]

Exploitable or potentially exploitable deposits of critical metals, such as rare-earth (REE) and high-field-strength elements (HFSE), are commonly associated with alkaline or peralkaline igneous rocks. However, the origin, transport and concentration of these metals in peralkaline systems remains poorly understood. This study presents the results of a mineralogical and geochemical investigation of the Na-metasomatism of alkali amphiboles and clinopyroxenes from a barren peralkaline granite pluton in NE China, to assess the remobilization and redistribution of REE and HFSE during magmatic-hydrothermal evolution. Alkali amphiboles and aegirine-augites from the peralkaline granites show evolutionary trends from sodic-calcic to sodic compositions, with increasing REE and HFSE concentrations as a function of increasing Na-index [Na#, defined as molar Na/(Na + Ca) ratios]. The Na-amphiboles (i.e., arfvedsonite) and aegirine-augites can be subsequently altered, or breakdown, to form hydrothermal aegirine during late- or post-magmatic alteration. Representative compositions analyzed by insitu LA-ICPMS show that the primary aegirine-augites have high and variable REE (2194-3627 ppm) and HFSE (4194-16,862 ppm) contents, suggesting that these critical metals can be scavenged by alkali amphiboles and aegirine-augites. Compared to the primary aegirine-augites, the presentative early replacement aegirine (Aeg-I, Na# = 0.91-0.94) has notably lower REE (1484-1972) and HFSE (4351-5621) contents. In contrast, the late hydrothermal aegirine (Aeg-II, Na# = 0.92-0.96) has significantly lower REE (317-456 ppm) and HFSE (6.44-72.2 ppm) contents. Given that the increasing Na# from aegirine-augites to hydrothermal aegirines likely resulted from Na-metasomatism, a scavenging-release model can explain the remobilization of REE and HFSE in peralkaline granitic systems. The scavenging and release of REE and HFSE by Na-metasomatism provides key insights into the genesis of globally significant REE and HFSE deposits. The high Na-index of the hydrothermal aegirine might be useful as a geochemical indicator in the exploration for these critical-metals. [ABSTRACT FROM AUTHOR]

The question of whether the high U and Th concentrations in zircon are primary or secondary is often difficult to resolve, and a clear understanding of the modification processes of secondary U- and Thrich zircon is lacking. Zircon crystals from two well-studied, highly evolved granites, the Jiangjunshan muscovite granite in the Chinese Altai Mountains and the Cuonadong leucogranite in the Eastern Tethyan Himalaya, have been investigated and classified into two types. Type I exhibits typical igneous growth zoning, and type II has "diffuse" or "spongy" internal structures. These textures, along with compositional data, indicate that the type II zircon crystals formed through hydrothermal modification of magmatic zircon (type I) by infiltrating hydrothermal fluids. During hydrothermal modification, the U and Th concentrations increase from type I to type II in the Jiangjunshan muscovite granite but decrease from type I to type II in the Cuonadong leucogranite. The Raman spectra of type II zircon crystals from Jiangjunshan muscovite granite have broader peaks (i.e., measured as the full width at half maximum, FWHM) with decreased intensities than their type I counterparts, which indicates that the former are affected by significant accumulated radiation damage. However, the preserved radiation damage in both the type I and II zircon measured by Raman spectroscopy is less than that expected from the total dose of alpha particles calculated from the U and Th contents, which indicates variable degrees of annealing. We propose that late magmatic-hydrothermal alteration was responsible for the modification of magmatic zircon grains in highly evolved granites and resulted in the enrichment or redistribution of U and Th. The calculated radiation dose of the Cuonadong leucogranite zircon is far below that required for metamictization, which indicates that metamictization is not always responsible for diffuse and spongy textures. [ABSTRACT FROM AUTHOR]

Widespread crust-derived granitoids in North China provide evidence of Mesozoic lithospheric thinning and destruction of the North China Craton. However, the link between lower crust and lithospheric mantle reactivation remains poorly understood. We present whole-rock geochemistry and zircon O-Hf isotopes for Early Cretaceous peralkaline A-type granites (i.e., Xiwanzi, Xiangshan, and Yansehu) from the Yanshan Mountains of the eastern block of the North China Craton. The Xiwanzi granite has a U-Pb age of 134.3 ± 0.4 Ma, with homogeneous whole-rock Nd isotopes [εNd(t) = -7.7 to -7.5] but highly variable zircon O-Hf isotopes [δ18O = 5.1‰ to 7.2‰ and εHf(t) = -7.7 to +9.6]. Such isotopic variations likely resulted from a hybrid juvenile source consisting of partial melting of newly underplated basaltic rocks derived from both the metasomatized ancient and juvenile sub-continental lithospheric mantle. In contrast, the Xiangshan (117.4 ± 1.0 Ma) and Yansehu (116.3 ± 1.4 Ma) granites have relatively uniform isotopic compositions [whole-rock εNd(t) = -12.2 to -10.8, zircon δ18O = 5.2‰ to 6.4‰, and εHf(t) = -17.1 to -4.7], close to those of the Proterozoic mafic granulite xenoliths from the ancient lower crust. The isotopic variations of the Early Cretaceous A-type granites suggest a continuous change in the melting model from the underplated juvenile lowermost crust to the overlying ancient lower crust at high temperatures in an extensional environment. Our results indicate that A-type granites can be used to probe the nature and reactivation of the lower crust of an Archean craton. Significantly, large-scale continuous thinning of lower continental crust provides key insights into the generation of the widespread Mesozoic crust-derived granitoids in North China. [ABSTRACT FROM AUTHOR]

The Xiaoxi’nancha porphyry Au-Cu deposit is located in Yanbian, Jilin Province, NE China. Gold-Cu mineralization is mainly associated with chlorite-sericite alteration. The 40Ar/39Ar age of pre-mineralization hydrothermal biotite in potassic alteration defines a relatively well-defined cluster at ~ 111 Ma to 114 Ma with a total fusion age of 112.0 ± 0.3 Ma. In-situ secondary-ion mass spectrometry U-Pb dating of hydrothermal titanite occurring with chalcopyrite yielded an intercept age of 109.0 ± 2.4 Ma. The similarity between the biotite and titanite formation ages suggests a mineralization age of ~ 110 Ma. Chlorite, quartz and apatite coexist in equilibrium and are closely related to mineralization. The Al-in-chlorite geothermometer indicates a formation temperature of 236–351℃ (mean 309℃), and the quartz-apatite pair yielded an average formation temperature of 306℃. The in-situ δ34S compositions of sulfide have restricted and slightly positive values (pyrite 2.3 to 3.9‰, chalcopyrite 1.6 to 3.8‰ and molybdenite 2.3 to 3.7‰). The fluid δ18O values, calculated assuming quartz-fluid equilibrium, vary from 2.4 to 5.5‰ (average = 4.0‰). Therefore, the ore-forming hydrothermal fluids were of moderate-temperature with predominantly magmatic characteristics. Apatite exhibits distinct variations in structure and composition, and slight variations in oxygen isotopic composition. The areas in apatite with dark BSE textures are characterized by lower δ18O values, Cl contents and temperatures and higher F contents, consistent with the result of water–rock interaction rather than mixing with meteoric water. The water–rock interaction and its resulting cooling, can reduce the metal solubility, likely triggering mineralization at Xiaoxi’nancha. [ABSTRACT FROM AUTHOR]