In the Lyell Stream-Bald Hill area, Northwest Nelson, South Island, New Zealand, molybdenite mineralization occurs within brecciated and hornfels-textured Greenland Group low-grade metasediments in contact with minor quartz trondhjemite porphyry intrusions (Lyell porphyry). Potassium-argon ages of mineralized country rocks with hornfels texture, porphyry intrusions, and several granites of the adjacent Karamea granite batholith at Bald Hill range from 103 to 120 m.y. (mid-Cretaceous) and are grouped as follows. 1) Three biotite ages of the Lyell porphyry, 112 to 116 m.y., and three total rock ages of associated biotite hornfels, 103 to 120 m.y., indicate that the porphyry intrusion and probably the molybdenite mineralization occurred about 116 m.y. ago (Aptian). 2) Phyllitic Greenland Group country rocks (Lower Ordovician) immediately outside the thermal aureole, yield four total rock ages, 112 to 226 m.y., indicating partial argon outgassing during younger thermal events. 3) From the Bald Hill granites (informal name), biotite and muscovite ages of a pink gneissic micro-granite, 106 to 110 m.y., gneissic biotite granite, 102 to 109 m.y., and gneissic muscovite leucogranite, 105 to 110 m.y., are not significantly different; they probably reflect the time of tectonic uplift and cooling of the granites or possibly a later thermal metamorphism associated with the Lyell porphyry (informal name). The geological setting of the Lyell porphyry and Bald Hill granites is similar to that of the Berlins Porphyry and Buckland Granite of the Lower Buller Gorge. The molybdenite mineralization parallels that of other Northwest Nelson occurrences, being associated with minor mid-Cretaceous granitic porphyry intrusions, emplaced in Paleozoic metasediments, close to the margins of the Karamea and Separation Point granite batholiths.

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