The occurrence of ca. 360 Ma greisen-related molybdenite mineralization in the Clayton Hill biotite–muscovite leucomonzongranite (<1 km2) is unusual for the southwestern Meguma terrane of the Maritime Appalachians, where 380 Ma granite-related Sn mineralization dominates. The presence of extensive hornfelsing and occurrence of mineralized (Sn–W–Cu–Zn) skarns in the area suggests a larger intrusion occurs at depth. The intrusion and mineralization are constrained to 361 Ma based on concordant Re–Os (molybdenite) and 40Ar/39Ar (muscovite) ages, which equates to other ca. 360 Ma granites in the area, hence magmatism of this age is more widespread than previously considered. The intrusion is highly evolved (74 wt.% SiO2), metaluminous (A/CNK = 1–1.05; A/CNK = molar ratio [Al2O3/(CaO + Na2O + K2O)]) and depleted in cafemic and transition elements. Rare-earth elements (∑REE = 106–23 ppm) are strongly fractionated with light rare-earth elements (LREE) enriched (La/LuN = 10), and negative Eu anomalies. Biotite is Fe-rich (Fe/(Fe + Mg) = 0.75), and both biotite and muscovite are F deficient. These geochemical features indicate affinity to the other 360 Ma granites and suggest a similar petrogenesis with some A-type affinity. Mineralization occurs as molybdenite-bearing greisens and rusty miaroles lined with pyrite and enrichment in Cu–As–Bi–Au. Stable isotopes (O, S) indicate greisen formation at 450 °C from fluids with δ18OH2O = 7‰ and δ34SH2S = 2‰–3‰, which suggests an A- or I-type magmatic reservoir rather than a peraluminous magma. These data indicate the presence of a widespread magmatic event that is 20 Ma younger than the 380 Ma Sn-base metal event in southern Nova Scotia (e.g., East Kemptville). That this event is the same age as the world class Mount Pleasant Mo–Sn–W–Zn–In deposit of southern New Brunswick indicates additional potential for significant granite-related mineralization in this part of Atlantic Canada.

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