GEOLOGIC AND GEOCHEMICAL ASSOCIATION OF ALKALIC AND CALCALKALIC PORPHYRY DEPOSITS OF THE WOODJAM DISTRICT, HORSEFLY BC

The Woodjam Cu±Au±Mo porphyry camp is located in the Quesnellia terrane, south-central British Columbia. Porphyry centers include the large Cu-Mo±Au Southeast Zone, hosted in the calc-alkalic Takomkane batholith, and the Au-Cu Deerhorn, Megabuck and Takom deposits. The latter are peripheral to the margin of the batholith and are associated with smaller monzonitic bodies of more alkalic affinity that intrude and mineralize Nicola strata.

Local Nicola rocks record a succession of subalkalic andesitic and dacitic volcaniclastic facies represented by a series of subaqueously emplaced mafic lavas and shallow intrusions, including an andesite flow (203.9±0.4 Ma¹). These are overlain by a thick succession of subaerially erupted units, including a dacite (200.18±0.87 Ma), which grades into a variably reworked volcaniclastic succession of breccia and sandstone facies (197.78±0.13 Ma), that are capped by andesite flows. A vertical accumulation of at least ~1.5 km over ~6 million years is recorded.

The Takomkane batholith, host to the Southeast Zone, intrudes and underplates this stratigraphy. The main phases include coarse quartz monzonite (197.48±0.44 Ma) and a later, finer-grained quartz monzonite. Both are mineralized and cross-cut by post-mineralization dykes (196.52±0.23 Ma), tightly constraining the age of mineralization.

The more shallowly emplaced monzonites at Deerhorn (196.34±0.19 to 196.48±0.21 Ma) and Megabuck (198.58±0.27) are similar in age to both the Takomkane batholith and Nicola stratigraphy.

Southeast Zone and Deerhorn intrusive units also share geochemical characteristics. The early Takomkane units have higher SiO₂and incompatible element concentrations, enriched HREEs, negative Eu anomalies, and less radiogenic Nd isotope ratios than the Deerhorn monzonites. The latter are themselves similar to the late Southeast Zone fine-grained quartz monzonite and late dykes, characteristics more comparable to the host Nicola stratigraphy.

The Woodjam property presents unique spatial, temporal and geochemical relationships between regional volcanic rocks and mineralized porphyry systems of alkalic (Au-Cu) and calc-alkalic (Cu-Mo) affinities.

¹Schiarizza et al., 2013