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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]

The Laird Lake property, southwest Red Lake greenstone belt, straddles the contact between the Balmer (2.99–2.96 Ga) and the Confederation (2.74–2.73 Ga) assemblages. The property is 10 km along strike from the Madsen and Starrat–Olsen Au mines that are hosted near the contact. The Balmer assemblage consists of fine-grained, aphyric, locally pillowed mafic volcanic rocks, ultramafic intrusive and volcanic rocks with flow breccia textures hosting local spinifex-bearing clasts, and banded-iron formations. In contrast, the Confederation assemblage comprises porphyritic mafic volcanic rocks intercalated with intermediate to felsic volcanic rocks that include crystal lapilli tuffs, crystal tuffs, and tuffs. The Balmer assemblage is composed of tholeiitic mafic volcanic rocks with minor Al-undepleted komatiites, whereas the Confederation assemblage is calc–alkalic. Neodymium isotopes, in conjunction with trace element geochemistry, suggests that parts of the Balmer assemblage were weakly contaminated by an older intermediate basement. Both arc and back-arc volcanism occurs in the Confederation assemblage, with the arc rocks showing a stronger crustal component than the back-arc rocks. A maximum U–Pb age of 2741 ± 19 Ma for a crystal tuff and an age of 2737.68 ± 0.79 Ma for a diorite are consistent with a Confederation assemblage affinity for the intermediate calc–alkaline rocks south of the Au-bearing horizon. The Balmer assemblage represents an oceanic plateau formed by plume magmatism on the margins of the North Caribou Terrane, whereas the Confederation assemblage at Laird Lake formed in an oceanic arc setting where both arc and back-arc volcanism occurred simultaneously. [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]

[Display omitted] • The magmatic-hydrothermal system of Yangla skarn Cu deposit is relatively reduced. • The barren quartz porphyry was also originated from reduced magmas and thus less fertile for porphyry Cu formation. • Intense fluid-carbonate rocks interaction and fluid focusing could be the key to formation of the reduced Yangla Cu deposit. In the Paleo-Tethyan orogenic belt, reduced mineralization systems result in extensive saturation and segregation of magmatic sulfide phases, which in turn leads to the loss of chalcophile elements and hinders the precipitation of large amounts of copper. As a result, porphyry Cu deposits are rare within the belt. In this contribution, we investigate the composition of zircons from the granodiorite, fluid inclusions and hydrothermal scheelite from orebodies of the Yangla Skarn Cu deposit, and assess the magmatic-hydrothermal redox state of the system. The igneous zircons from ore-causative granodiorite have negative Eu anomalies (Eu N /Eu N * = 0.27 ± 0.03, n = 11) and the calculated ΔFMQ values are -0.23 ± 0.31 (n = 11). The ore-forming fluids in the pre-ore stage was rich in CH 4 , and precipitating hydrothermal scheelite in the syn -ore stage with low Mo contents (Mo = 17.4 to 149 ppm, mean 57.7 ppm) and positive Ce anomalies (Ce N /Ce N * = 1.14 ± 0.06, n = 29). The geochemistry suggests that the magmatic-hydrothermal system at Yangla is relatively reduced, consistent with the abundant development of pyrrhotite in the ore body. In addition, we have carried out zircon SIMS U-Pb dating and trace analysis of the barren quartz porphyry in the Yangla ore field, which gives an emplacement age of 227 Ma and is also derived from reduced magmas (ΔFMQ = -1.27 ± 0.55; Eu N /Eu N * = 0.32 ± 0.10, n = 7) that were less fertile for porphyry Cu formation. By comparing with the barren quartz porphyry, we suggest that the key control of the skarn ore body may have been that the low-permeability of carbonate units seal the mineralizing fluids, enhancing fluid-carbonate reactions and concentrating the sulfides in a relatively small area. Without these processes, it can only form barren porphyry system. We highlight that, unlike typical porphyry Cu deposits, intense fluid-carbonate rocks interaction and fluid focusing could result in skarn Cu formation, even though in relatively reduced magmatic-hydrothermal conditions. [ABSTRACT FROM AUTHOR]