UBC Undergraduate Research

Nature and Origin of Gold-Rich Carbonate Replacement Deposits at the Rau Occurrence, Central Yukon Kingston, Scott P.

Abstract

The Rau occurrence is an unusual gold-rich carbonate replacement style deposit located in the Keno Hill District of central Yukon. It is most similar to the Ketza River deposit in southeastern Yukon, which is one of a small number of known gold-rich and base metal-poor carbonate replacement deposits in the world. The Rau occurrence itself hosts economically significant gold grades, and new information on the occurrence provides constraints on the genesis of this poorly understood type of deposit. A detailed petrographic study, utilizing both reflected light and a scanning electron microscopy, was completed using polished thin sections prepared from samples of drill core from two 2008 diamond drill holes on the Rau property. The petrographic analyses identified a simple hydrothermal mineralogy consisting of pyrite, pyrrhotite, and arsenopyrite as the dominant sulphides and a suite of accessory minerals that includes bismuthenite, sphalerite and chalcopyrite. No free gold was identified in the samples, and it remains unclear whether any of the gold in the deposit occurs as isolated grains or it occurs entirely in a more refractory form, perhaps in solid solution in arsenian pyrite. The mineralogy and paragenetic sequence observed at the Rau occurrence show a number of differences from the Ketza River deposit, including an abundance of talc and early stage marcasite in the Rau occurrence. Major and trace element geochemistry suggests that felsic dykes at the Rau occurrence appear to be of similar age and composition to the McQuesten Plutonic Suite to the south; however, the Rau dykes were likely derived from different source rocks and this may explain why their metallogenic signature differs from that of the McQuesten Suite. This suggestion is supported by Pb isotopic compositions of feldspars from the McQuesten Suite and Rau dykes. A Pb isotopic study was undertaken to constrain the source of metals in the sulphide mineralization at the Rau occurrence. The Pb isotopic signature of the sulphides is consistent with much of the Pb having been derived from the Rau intrusions themselves, with a variable contribution of Pb from the host carbonate rocks. Although the new work has shed some light on the nature and origin of gold-bearing carbonate replacement mineralization at the Rau occurrence, many questions regarding the genesis of the mineralization remain unanswered.

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