Summary:
The Moss deposit is a steeply dipping (average 70°) quartz-calcite vein and stockwork system, which extends over a strike length of approximately 1,400 m in the project area, but can be traced for more than 5.6 km in total length.
The Moss vein system is considered a high level, low-sulfidation (adularia-sericite) epithermal gold-silver deposit in the classification of Heald et al (1987) and White and Hedenquist (1995). Low sulfidation epithermal deposits form from hydrothermal waters in the relatively near-surface environment, typically within 1.5 km of the earth’s surface (Taylor, 2007). They are commonly found associated with magmatism and volcanism, but are somewhat distal (vertically or laterally) from the actual center of magmatism, in environments where meteroric waters have mixed with and diluted magmatic waters.
Epithermal deposits comprise one of three sub- types: high sulphidation; intermediate sulphidation; and low sulphidation. Each sub-type is identified by characteristic alteration and ore-stage mineral assemblages, occurrences, textures and suites of associated geochemical elements. The designation of high sulfidation vs low sulfidation is based on the sulfidation state of the ore-stage sulfide suite, not the abundance of sulfides in the ore. However, precious metals mineralization at Moss is characterized by a low sulfidation suite of minerals and a very low sulfide content (<1%) as well.
The quartz-calcite vein textures at Moss (massive, breccia, vuggy, colloform), are typical of low sulphidation epithermal veins. Gold occurs as very fine native gold and electrum, and silver typically occurs as electrum and very fine grained acanthite, similar to other low-sulfidation precious metals deposits.
The very low (usually trace) levels of base metals in the Moss ores are also consistent with high- level low-sulfidation gold deposits. Alteration related to main-stage precious metals mineralization is confined to silicification and minor sericitization of wallrock adjacent to the veins.
The Moss mineralization differs from typical low- sulfidation precious metals deposits in its lack of adularia (possibly present, but not yet positively identified) and lack of deleterious elements such as arsenic, antimony, and mercury.
The mineralogy of the Moss vein system is simple and the mineralization is nearly void of all deleterious elements. Key elements are:
- Gangue consists of quartz and calcite with minor fluorite locally occurring as late stage veins and vug fillings.
- Gold mineralization is predominantly in the form of very fine grained native gold and silver-rich native gold grading to electrum (an alloy of gold and silver with Ag:Au >1:5).
- Silver occurs as electrum and within the silver- rich gold. Minor native silver has also been identified. In addition, minor amounts of very fine grained, grey to black sulphides (dominantly acanthite, Ag2S) are present as disseminations and occasionally in very thin grey bands in unoxidized or weakly oxidized parts of the veins. The silver minerals bring the overall Ag:Au ratio of the deposit to approximately 8:1.
- Base metals (Cu, Pb, Zn) are very low, especially in the upper parts of the system, but show a slight increase with depth, consistent with low-sulfidation epithermal veins.
- No arsenic or antimony minerals occur.
- Mercury is negligible.