UNCONVENTIONAL CONSTRAINTS ON THE TIMING OF ORE FORMATION: 40AR-39AR GEOCHRONOLOGY OF TOURMALINE ASSOCIATED WITH THE CASINO DEPOSIT, YUKON, CANADA

Tourmaline (tur) is a chemically refractory mineral responsive to the geological environment in which it forms, but it can also provide important geochronological information, if applied appropriately. The small amounts of K in the tourmaline structure are typically enough to generate measurable time-dependent ⁴⁰Ar-³⁹Ar ratios useful in temporally constraining geological processes. Previously, ⁴⁰Ar-³⁹Ar closure temperatures (T) of tur were determined to be ~580ºC i.e., a relatively high-T thermochronometer. As such, in settings where tur forms during geologic events below this T, the age of the event can be determined. An ideal application is to ore deposits where tur develops at relatively low T as an associated gangue mineral to ore minerals.

The mineralized calc-alkaline porphyry Cu-Mo-Au system of the Casino deposit (Yukon, Canada) provides a good test case for this approach. In this locality tur crystallizes as an early hydrothermal mineral within breccias and is interpreted as synchronous to or predating mineralization. Thermal conditions of mineralization are well below the tur ⁴⁰Ar-³⁹Ar closure T. Overall, the average composition of the Casino tur is (Na_0.77[ ]_0.17Ca_0.06) (Mg_1.95Fe_0.61Al_0.37) (Al_5.98Mg_0.02) Si₆O₁₈ (BO₃)₃ (OH)₄ but with substantial Fe-Mg and Fe³⁺-Al variation. Significantly, tur has 87-671 ppm K. Two samples of disseminated tur (CEBB-042 and -045) and one sample of tur breccia (CEBB-039) were investigated. Sample CEBB-039 gave a robust ⁴⁰Ar-³⁹Ar plateau age of 74.45±0.73 Ma. This age compares well with earlier geochronological measurements of U-Pb zircon ages of 74.7±0.6 and 74.3±0.5 Ma, and hydrothermal K-feldspar ⁴⁰Ar-³⁹Ar age of 71.8 ± 0.6. Further, based on low-T thermochronometers the Casino deposit cooled quickly to ambient T at the 1.5 km emplacement depth over of period of 1.9-4.2 Ma. In contrast, samples CEBB-042 and -045 have disturbed age spectra. Examination of tur from the disturbed samples reveal significant chemical zoning and replacement of primary tur by later generations of tur. Consequently, it is important to examine samples for tur replacement features prior to choosing tur samples for ⁴⁰Ar-³⁹Ar analysis. With these precautions, ⁴⁰Ar-³⁹Ar geochronology using tur can be extremely effective for determining the age of low-T geological events.