Metallogeny and characterization of late cretaceous superimposed porphyry Cu-Au-Mo and epithermal Au-Ag systems in the Dawson Range, Yukon, Canada: case study on the Klaza deposit

Abstract

The Dawson Range Gold Belt (DRGB; Yukon’s richest mineral district by resource) lies in the Yukon segment of the North American Cordillera and is dominated by Late Cretaceous (77–74 Ma) porphyry-epithermal systems. Mineral occurrences in the DRGB have seen limited exploration due to: (1) poor surface exposure; (2) incoherent classification of intrusive rocks; and (3) outdated exploration models. A multidisciplinary study utilizing: (1) field observations (drill core logging and mapping); (2) geochronology (U-Pb in zircon by LA-ICP-MS and CA-TIMS; Ar-Ar in muscovite; Re-Os in molybdenite); (3) whole-rock geochemistry; (4) zircon trace element geochemistry; (5) petrography (SEM-EDS, optical microscopy); and sulfide mineral geochemistry (LA-ICP-MS element maps) is designed to address the above challenges through a detailed investigation on the well-preserved Klaza deposit using 2011-2020 drilling data. Results suggest the presence of six intrusive phases of mafic to intermediate compositions. Intrusive activity occurs in four pulses spanning the Late Triassic to the Late Cretaceous. The Late Cretaceous magmatic pulse is protracted (80–65 Ma) and displays timedependant compositional changes. The youngest plutonic suites: (1) display enrichments in LREEs relative to the older suites; (2) are related to garnet-bearing sources (depleted HREEs, high La/Yb); (3) are hydrous (presence of hornblende-biotite); and (4) reflect a dynamic magma chamber (magma mixing textures). Two clusters of hydrothermal ages are constrained (ca. 77 Ma and ca. 71 Ma), correlating with Casino suite and Prospector Mt. suite magmatism, respectively. Higher temperature A-, B-and EDM-like veins are related to the 77 Ma event and cut by fault-veins related to the 71 Ma event. The fault-veins consist of four stages. Gold is hosted in early (Stage 2a, 2b) arsenopyrite and pyrite lattices and later liberated through late (Stage 2d) Copper-bearing fluids, whereas silver occurs later (Stage 2c) as native silver and sulfosalts. The Klaza system is best described as a porphyry-related intermediate sulfidation epithermal deposit superimposed on an older, unrelated porphyry system. Similar observations of spatial-temporal overprinting are documented throughout the DRGB (e.g., Casino deposit and Freegold Mt. district), suggesting these Late Cretaceous porphyry systems are linked to a fertile metallogenic event spanning 15 million years. This study is the first detailed characterization of Late Cretaceous porphyry systems in the DRGB and presents the first use of machine learning assisted paragenetic study of sulfide minerals for exploration and improving ore body knowledge.