| Popis: |
The Newton Au-Ag deposit is located in the Chilcotin Plateau of central British Columbia, Canada. The deposit lacks vein textures typical of magmatic-hydrothermal deposits and its genesis is poorly understood. Investigations focused on drill core logging, petrography, short wave infrared spectroscopy, X-ray diffraction, lithogeochemistry and stable isotopic analyses. The mineralization is hosted within Late Cretaceous age felsic volcanic rocks (U-Pb age of 72.1 ±0.6 Ma). These rocks are interbedded with an intra-arc sedimentary rock sequence that overlies a mafic rock sequence. Porphyritic dykes of felsic to intermediate composition intrude this stratigraphy. The quartz feldspar porphyry yielded a U-Pb age of 70.9±0.5 Ma. Re-Os dating of molybdenite yields an age of 72.1±0.3 Ma and a minimum age for mineralization. The mineralization is stratabound and characterized by pervasive quartz-sericite alteration and disseminated open-space filling sulfides providing strong evidence that the felsic volcanic rocks had high permeability relative to adjacent lithologies. Gold-silver mineralization is associated with two sulfide assemblages 1) pyrite-dominant (pyrite-arsenopyrite) and 2) marcasite–base metal dominant (marcasite-pyrite-chalcopyrite-sphalerite) and rare polymetallic (pyrite-chalcopyrite-sphalerite-arsenopyrite) veins. Siderite has replaced marcasite and chalcopyrite. Variations in Zn-Ag concentrations define a paleothermal gradient that developed in the hydrothermal system. The poorly mineralized felsic porphyry stocks and dykes have quartz-sericite-pyrite-carbonate-kaolinite alteration and minor quartz-calcite-pyrite-molybdenite veinlets. The mafic rocks have propylitic alteration (chlorite-epidote-pyrite-calcite-albite-magnetite). Whole rock lithogeochemistry suggests a calc-alkaline magmatic affinity for the felsic volcanic rocks and the felsic porphyritic intrusions. Alteration intensity is indicated by relative addition of SiO₂, K, C, Fe and removal of Ca and Na. Major rock types show distinct Y-Zr ratios. Hydrothermal fluids in equilibrium with sericite have δ¹⁸O values from 2.5 to 6.8‰ and δD values from -63 to -46.5‰ suggesting a magmatic fluid source that intermixed with meteoric waters. Sulfides have δ³⁴S values from -1.1 to 3.5‰ that indicate a magmatic origin for the sulfur. The results suggest that Newton is a disseminated strata-bound intermediate-sulfidation type epithermal deposit. The Newton deposit is comparable to the Capoose and Blackwater deposits but lacks garnet alteration. The felsic volcanic host rocks at Newton are contemporaneous with the youngest stratigraphy of the Kasalka Group rhyolites (68-71Ma). |
