Petrology, geochronometry and economic geology : the Zeta tin-silver prospect, Arsenic Ridge, west-central Yukon (115P/14 and 116A/03)
Creators: name: Abercrombie, Shirley May
Publisher: name: University of British Columbia
Year: 2010

description: Arsenic Ridge is located in the northwestern part of the Lost Horses batholith, Syenite Range, Yukon Territory. This area is within the Omineca Crystalline Belt of the Canadian Cordillera. North American miogeoclinal rocks of the upper Precambrian to Lower Cambrian Grit Unit were northwardly thrust onto Ordovician to Silurian Road River Formation during the Late Triassic to Early Jurassic arc-continent collision. Small mid-Cretaceous (83 to 100 Ma, K-Ar on biotite), epizonal felsic intrusions were emplaced in radiogenic Sr-enriched metasedimentary rocks of the ancient continental margin (pericratonic sedimentary prism). The mid-Cretaceous (syenite phase, 87+3 Ma, K-Ar on biotite; granite phase, 95+3 Ma, K-Ar on biotite) Lost Horses batholith is a circular, S-type, composite pluton within the Selwyn Basin. The Selwyn Basin is an epicontinental trough partially bounded on the west by the Cassiar Platform and on the east and northeast by carbonate rocks of the MacKenzie Platform. S-type plutons are a product of Hercynotype arc-continental collisional tectonics. The zoned intrusive rocks along Arsenic Ridge, from core to rim, have been classified as tourmaline orbicular granite, granite, quartz syenite and syenite. With increasing SiO₂ : (1) the major elements--Al₂O₃ , FeO, MgO, CaO, TiO₂, MnO and P₂O₅--tend to decrease,(2) trace elements--Zr, V, Sr, Ni, and Ba--are characterized by extreme depletions, and (3) the trace element, Rb, is slightly enriched. The syenite is alkaline whereas the quartz syenite, granite, granite dyke, and tourmaline orbicular granite are sub-alkaline. Magmatic differentiation of the pluton is demonstrated by a decreasing trend of (Na₂O + K₂O) and TiO₂ with increasing SiO₂, and by an increasing Rb content with a decrease in Ba and Sr. The estimated partial pressure of water during formation of the Lost Horses granite melt is broadly estimated as &gt;10 kbar. Electron microprobe traverses across orthoclase megacryst cores and rims identified a concentration of albite lamellae in the rim and barium, strontium and calcite rich cores. The latter is indicative of a melt undergoing progressive depletion of barium by fractional crystallization. The age of the batholith is early Late Cretaceous, approximately 97 Ma. This was determined from early Late Cretaceous dates of 95+3 Ma from K-Ar on biotite and 88+4 Ma from K-Ar on a hornblende, and a late Early Cretaceous date of 101+6 Ma from a whole rock-mineral (biotite, hornblende, total feldspar) Rb-Sr isochron. Initial strontium ratios for the granitic rocks along Arsenic Ridge are about 0.712 suggesting that radiogenic strontium was derived mainly from melting and/or assimilation of old sialic crust during magma genesis. The model Rb-Sr age, TUR, for the granitic rocks on Arsenic Ridge is 238 Ma. This indicates that a dominantly upper mantle source is unlikely. Pb-Pb isotope ratios for the zoned plutonic rocks, the surrounding sediments and the ore mineral separates plot between the pericratonic and Bluebell curves (from 0 Ma to 140 Ma mixing lines) indicating that the lead is a mix of upper crust and lower crust sources. Lead sulphide analyses from the Zeta prospect, Tombstone Range and the Keno-Galena Hill areas are indistinguishable from the feldspar rock lead. This shows that the lead source for these vein deposits is the surrounding plutons and not the surrounding sedimentary rocks. The least radiogenic lead has a model age of about 100 Ma. Nd/Sm and Nd analyses indicate that Arsenic Ridge granitic rocks were derived from, or assimilated, old crustal rocks whose Sm/Nd had been lowered at the time of separation from CHUR. Nd ratios for the granite and the feldspar megacrysts are all very close to 0.51210. The model Sm-Nd age, TDM , for a granite along Arsenic Ridge is 1.26 Ga. Approximate percentages of continental crust and mantle incorporated in the melt were calculated. If the contamination is upper crustal in origin then there was a maximum of 30% mantle incorporated in the melt. No mantle component is needed if the contamination source is lower crust. However, since granite ¹⁴³Sm/¹⁴⁴Nd ratios are close to the average continental crust ratio, the origin is upper crust with a small mantle component. ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios for the granitic rocks from the Lost Horses batholith plot in the Phanerozoic quadrant of Faure (1986) and are similar to values from the Sierra Nevada batholith. Epsilon values of Nd and Sr suggest the granite is S-type which agrees with the field, petrographic and chemical evidence. The granite plots within the field for miogeoclines as determined by Farmer and DePaolo (1983 ). The Zeta tin - silver greisen vein prospects lie in both the Ordovician - Silurian metasediments of the Road River Group at the northeastern contact, and in the zoned, mid-Cretaceous Lost Horses batholith. Mineralization on the property occurs in two forms: (1) cassiterite bearing greisen veins in hornfelsed quartzite, and (2) greisen veins (sulphide and quartz with minor tourmaline, and tourmaline and quartz with minor sulphide in granitic rocks). K-Ar muscovite dating of the sericitic cassiterite greisen (87.0+3.0 Ma), indistinguishable from the K-Ar biotite date for the syenite phase of the batholith (86.8+2.7 Ma), establishes a genetic relationship between the two. The following four-stage model describes the evolution of the Lost Horses batholith: stage 1, initial melting, stage II, melt accumulations and assimilation, stage III, diapiric rise and chemical differentiation (fractional crystallization), and stage IV, magmatic hydrothermal. This last stage generated tin-silver vein and greisen mineralization. The source for this lithophile mineralization and associated S-type granitic rock is dominantly from a sialic clastic wedge with upper crustal geochemical characteristics.