A CRYPTICALLY ZONED AMALGAM (Au1.5-1.9Ag1.1-1.4)Σ2.8-3.0Hg1.0-1.2 FROM A PLACER DEPOSIT IN THE TULAMEEN - SIMILKAMEEN RIVER SYSTEM, BRITISH COLUMBIA, CANADA: NATURAL OR MAN-MADE?

We describe an unusual pattern of zoning in an amalgam of Au and Ag; it occurs as peripheral zones in a 0.3 x 0.15 mm grain in a placer deposit associated with the Tulameen - Similkameen river system, British Columbia. This amalgam formed at the expense of primary Au-Ag alloy, which is present as relics in the core: Au54-55Ag45-46 (Hg-free; not zoned). Abundances of Au gradually increase outward and complement decreasing levels of Ag in the amalgam. Values of Au/Ag increase from 1.1-1.2 in the core to 1.8 at the margin of the grain. The observed range of compositions, (Au1.50-1.87Ag1.06-1.40)(Sigma 2.81-3.00)Hg1.00-1.19, is limited. Two possibilities are discussed to explain the formation of the zoned amalgam. (1) This phase could be natural, formed by reaction between primary Au-Ag alloy and a late melt or oxidizing fluid, which may have remobilized and transported Hg as a result of destabilization and oxidation of cinnabar. If so, the observed zonation would reflect an increasing activity of Au species (and decreasing activity of Ag) during crystallization. (2) This phase may have formed under supergene conditions, as a result of anthropogenic introduction of Hg into placer gravels during historic mining operations in this region; we favor this hypothesis. Zoning in the amalgam may have been controlled principally by electrochemical factors related to the process of self-electrorefining. Gold is more noble than silver, so that ions of Au probably migrated and were preferentially deposited closer to the margin of the grain. This mechanism is similar to that responsible for the secondary rim rich in Au (poor in Ag) in placer grains of Au-Ag alloy, as reported from other localities worldwide.