The Endako batholith: Episodic plutonism culminating in formation of the Endako porphyry molybdenite deposit, north-central British Columbia

The Endako batholith is a composite batholith that consists of early foliat ed hornblende +/- biotite diorites, intermediate-age unfoliated hornblende +/- biotite diorites, and late granodiorites to monzogranites. These latter plutons host the Endako molybdenite deposit, a major porphyry deposit loca ted in the central Canadian Cordillera. Extensive K-Ar dating on the bathol ith had given relatively reproducible ages centered on 140 Ma, indicating a short life span for magmatic activity However, a combined U-Pb and Ar-40/A r-39 study has resulted in a substantial reappraisal of the ape and time sp an of intrusive activity and its association with molybdenite mineralizatio n. The new data show that the batholith had a protracted emplacement histor y covering similar to 75 m.y., with clear evidence for periods of magmatic quiescence. The oldest magmatic suite of the Endako batholith, the Stern Cr eek suite, is dated at similar to 220 Ma and comprises foliated gabbros and diorites. Mafic to intermediate plutons of the Stal: Lake suite range in a ge from 180 Ma to 161 Ma. The Francois Lake suite is divided into two subsu ites: the Glenannan subsuite dated at 157 Ma to similar to 155 Ma and the 1 49 to 145 Ma Endako subsuite that hosts the Endako molybdenite deposit. Ar- 40/Ar-39 dating of hydrothermal biotite indicates molybdenite mineralizatio n is genetically associated with terminal stages of magmatic activity. Biot ite within the mine had its Ar-40/Ar-39 systematics reset at similar to 145 Ma by hydrothermal fluids with temperatures in excess of 300 degreesC. Thi s resetting is associated with intrusion of the 145.1 +/- 0.2 Ma Casey phas e monzogranite, the youngest phase within die Endako subsuite. A further pa rtial resetting of biotite is interpreted to be coeval with Eocene intrusio n of the high-level, miarolitic Sam Ross Creek phase and numerous mafic dik es that cut through the region. Hydrothermal fluids associated with this la tter event were below biotite closure temperature and do not appear to have any relationship to molybdenite mineralization within the Endako deposit. The new geochronologic data, coupled with detailed mapping, show a general age progression from mafic plutonism at the margins to a central core of hi gh-level granodiorite to monzogranite. The data document the existence of a long-lived, relatively stationary magmatic center. Pulsed magmatism, evide nt even within individual magmatic suites, may play a major role in concent rating and ultimately depositing molybdenite in some porphyry systems. At E ndako, molybdenite would have formed within the higher level portions of th e batholith, a position that would normally have rendered the deposit prone to erosional removal. However, subsequent Eocene Extensional block faultin g may explain the preservation of this Jurassic deposit, which is the oldes t economic molybdenite deposit in the Cordillera of North America.