Grew Creek epithermal gold-silver deposit, Tintina Trench, Yukon, 105 K/2

The Grew Creek epithermal gold-silver deposit in southeast Yukon (MINFILE 105 K 009) is hosted by Eocene volcanic and sedimentary rocks deposited in a pull-apart basin within the Tintina Fault Zone. Flow rhyolites forming a dome in the Tarn Zone area, 1.5 km east of Grew Creek, pass westward into a succession of rhyolitic ignimbrites and air fall tuffs, exposed along Grew Creek and in the Main Zone, 500 m west of Grew Creek. These rhyolitic rocks are faulted against fluvial sediments to the north, along the W-E Fault, and basaltic rocks to the west. In the Main Zone, the volcanics, sediments, and the W-E Fault all dip steeply to the north.

The gold-silver mineralization forms an elongate tabular zone within the rhyolitic tuffs. The zone strikes parallel to the W-E Fault and dips vertically or steeply to the north. The eastern end of the mineralized zone is defined by a decrease in grade, whereas the western end is faulted off against the basaltic rocks. Within the zone, stockwork veins and hydrothermal breccias contain assemblages which include quartz, adularia, carbonates, quartz peudomorphous after calcite, pyrite, marcasite, and traces of arsenopyrite, chalcopyrite, acanthite, electrum, silver selenides, galena, and sphalerite.

There is good correlation between gold and silver in drill core assays, with a gold::silver ratio of around 1::4 for the ore grade mineralization. The mineralization is strongly anomalous in arsenic and mercury, but there is only a weak correlation of mercury with gold and silver, with most high values for mercury lying above the gold-silver zone and associated with the W-E Fault. Arsenic conccentrations are elevated over much of the area but there is no statistical correlation with the locally high concentrations of gold or silver.

Outcropping rhyolitic rocks are hydrothermally altered to intermediate argillic and advanced argillic assemblages, whereas subsurface rhyolitic rocks are altered to quartz-adularia or illite-quartz assemblages adjacent to veins, and to illite-quartz-adularia ± carbonate elsewhere. Advanced illite-quartz-adularia ± carbonate alteration is accompanied by an increase in Na2O and decreases in TiO2, CaO and Al2O3. Basalts are altered to carbonate-chlorite (propylitic) assemblages, accompanied at an advanced stage by a slight increase in CaO and decreases in K2O, Na2O, SiO2, and Al2O3.

Mineralization postdated tilting of the host pyroclastic and sedimentary rocks. Episodic fault movements in the Tintina Fault Zone structurally focused the hydrothermal fluids by providing locally high secondary permeability, whereas the high primary permeability of the rhyolitic tuffs promoted the development of stockwork veins and breccias. The absence of significant alteration and mineralization in the sediments suggests that a partly welded and intensely altered tuff unit, along the footwall of the W-E Fault, acted as an aquiclude, confining the hydrothermal fluid within the rhyolitic tuffs. Intense pyritic alteration of this unit and high concentrations of mercury in the vicinity of the W-E Fault form pyrite and mercury zones north of the mineralization.

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