POTASSIUM-ARGON AGES OF MOLYBDENUM MINERALIZATION AND ASSOCIATED GRANITES AT BALD HILL AND CORRELATION WITH OTHER MOLYBDENUM OCCURRENCES IN THE SOUTH ISLAND, NEW-ZEALAND

In the Lyell Stream-Bald Hill area, Northwest Nelson, South Island, New Zealand, molbdenite mineralization occurs within brecciated and hornfelstextured Greenland Group low-grade metasediments in contact with minor quartz trondhjemite porphyry intrusions (Lyell porphyry). Potassium-argon ages of mineralized country rocks with hornfel's tecture, porphyry intrusions, and several granites of the adjacent Karamea granite batholith at Bald Hill range from 103 to 120 My (mid-Cretaceous) and are grouped as follows. 1) Three biotite ages of the Lyell porphyry, 112 to 116 My, and three total rock ages of associated biotite hornfels, 103 to 120 My, indicate that the porphyry intrusion an dprobably the molybdenite mineralization occurred about 116 My ago (Aptian). 2) Phyllitic Greenland Group country rocks (Lower Ordovician) immediately outside the thermal aureole, yield four total rock ages, 112 to 226 My, indicating partial argon outgassing during younger thermal events. 3) For the Bald Hill granites (informal name), biotite and muscovite ages of a pink gneissic micro-granite, 106 to 110 My, gneissic biotite granite, 102 to 109 My, and gneissic muscovite leucogranite, 105 to 110 My, 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).-Authors