Geochronology, geochemistry, and fluid inclusion study of the Newton epithermal gold deposit, British Columbia
L. Liu, J. P. Richards, J. Huebert, B. Abbassi, B. Lee, M. J. Unsworth, LZ. Cheng
Newton is an epithermal gold deposit located 105 km southwest of Williams Lake, British
Colombia. Disseminated Au mineralization is mainly associated with quartz-sericite alteration, in which gold is mostly present as electrum and gold-silver telluride inclusions in sulfides. The mineralization can be divided into 3 stages: (1) quartz-sericite-pyrite-gold; (2) quartz-sericitemarcasite-gold-base metal-sulfide; and (3) polymetallic veins (pyrite-chalcopyrite-sphaleritegalena-arsenopyrite). Re-Os dating of molybdenite from stage 1 yielded an age of 72.1 ± 0.3 Ma
(McClenaghan, 2013). The mineralization is mainly hosted by Late Cretaceous felsic volcanic rocks, granodiorite porphyry, and quartz-feldspar porphyry, which yielded U-Pb ages of 72.1 ±
0.6 Ma (Amarc Resources Ltd., unpublished data), 72.1 ± 0.48 Ma (new data), 70.9 ± 0.5 Ma
(Amarc Resources Ltd., unpublished data), respectively. These mineralized rocks are intruded by a barren diorite, which yielded a U-Pb age of 69.07 ± 0.52 Ma.
The narrow age range for all the volcanic and intrusive rocks (~3 m.y.) and their major and trace element compositions suggest that they constitute a comagmatic volcanoplutonic complex. All the rocks are enriched in incompatible large-ion lithophile elements, and depleted in Nb, Ta, Ti, and P. These igneous rocks are interpreted to be part of a major Late Cretaceous suite of continental-arc magmatism in central British Columbia.
A series of magmatic events in central British Columbia records the tectonic evolution from an island arc during the Late Triassic–Early Jurassic to an Andean-type continental margin magmatic arc which started in the Middle Jurassic and continued to the Eocene. A major phase of arc volcanism and plutonism occurred during the Late Cretaceous, and is associated with several large porphyry deposits in British Columbia, such as Prosperity (79 Ma; Cooke and Hollings,
2005).
Fluid inclusions were found in mineralized quartz veins hosted by quartz-sericite-altered felsic volcanic rocks and quartz-feldspar porphyry. The fluid inclusion data indicate that a relatively hot and saline fluid (average Th = 294 ± 2°C, 5.16 ± 0.2 % NaCl equiv., n = 18) of probable magmatic origin was dominantly responsible for the first stage of mineralization. Cooler and less saline fluids were also present in the veins, which may represent groundwaters that locally mixed with the magmatic fluids. Some evidence for boiling was also observed in the veins, although the veins do not host significant amounts of gold. It is thought that fluid-wall rock reactions and cooling were the main controls on gold precipitation in the quartz-sericite altered wallrocks.