The Merlin deposit, located in the Cloncurry district, Mt. Isa Inlier, is the world’s highest-grade Mo-Re deposit. Although the district is renowned for iron oxide copper-gold (IOCG)–type mineralization, Merlin and other Mo-rich occurrences in the district cannot be classified as IOCG-type deposits. The Merlin deposit is hosted by the Kuridala Group (1690–1650 Ma), which comprises interbedded phyllite and carbonaceous slate, calcsilicate rocks, and silicified metasiltstone. The Mount Dore Granite, part of the Williams-Naraku batholith (ca. 1540–1490 Ma), is locally thrust over the metasedimentary rock package. An early Cu-Pb-Zn sulfide stage (Mount Dore deposit) mostly overlies, with only partial spatial overlap, the younger Mo-Re mineralization (Merlin deposit). Cu-Pb-Zn is mainly hosted by carbonaceous slate as infill of hydrothermal clast-supported carbonate breccias; the younger Mo-Re mineralization (Merlin deposit) occurs as molybdenite infill of hydrothermal matrix-supported breccias along or near the contact between carbonaceous slate and calc-silicate rocks, and locally cuts the Cu-Pb-Zn mineralization.

Based on detailed petrography and radiometric age dating (U-Pb zircon and titanite ages, Re-Os molybdenite ages), we define four main stages of hydrothermal alteration. Stage 1 represents the earliest hydrothermal stage, dated here by U-Pb titanite geochronology at 1551 ± 12 Ma, and is characterized by Na-(Ca) alteration with the formation of hematite-dusted albite + amphibole + carbonates + titanite + apatite + quartz ± magnetite ± epidote. Stage 2 is characterized by the formation of K-feldspar + tourmaline + carbonates + quartz ± apatite and Cu-Pb-Zn mineralization with chalcopyrite + sphalerite + pyrite ± galena. Stage 3 is dated at 1535 ± 6 Ma (Re-Os molybdenite) and consists of the main Mo-Re event that formed intensely mineralized, matrix-supported breccias accompanied by K-feldspar ± chlorite alteration. Stage 4, dated here at 1521 ± 3 Ma (Re-Os molybdenite), is of an age similar to that of the emplacement of the Mount Dore Granite (1517 ± 7 Ma; U-Pb zircon), and is interpreted as either a remobilization event or a later addition of Mo-Re in veins and disseminations that extends several meters below the Mo-Re breccias, with formation of chlorite + K-feldspar + apatite ± rutile. The Mount Dore Granite was thrust above the metasedimentary package at ca. 1500 Ma, which coincided with the waning of the hydrothermal activity in the Cloncurry district. This event is interpreted to be manifested at the Merlin deposit as minor, millimeter-scale Mo-Re vein formation. Collectively, our data indicate that the Merlin and the Mount Dore mineralization formed via a complex and protracted hydrothermal history that includes a time period (ca. 1560–1540) that has previously been regarded as unprospective for ore deposit formation in the region.

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