Gold–copper–bismuth mineralization in hedenbergitic skarn, Tombstone Mountains, Yukon
Gold mineralization on the Marn property, Yukon, occurs in two pyroxene skarn bodies, which are adjacent to the Mount Brenner Stock in the Ogilvie Mountains. The skarns are separated by a 600 m wide monzonite intrusion and show contrasting mineralogical and geochemical characteristics in addition to quite different metal values. Significant but uneconomic Au, Ag, W, and Cu mineralization is found in skarn on the north side of the intrusion, while very low Au grades (0.052 g/t) occur at the southern contact. The mineral assemblages of both skarns are dominated by iron-rich pyroxenes. The iron content of the pyroxenes varies between Hd 40 and Hd 80 in the northern location and Hd 80 and Hd 100 in the southern skarn. A well-developed sequence of retrograde alteration affected only the northern skarn. This was probably the result of porosity and permeability differences in the early, high-temperature pyroxene skarn, which permitted greater fluid–rock interaction in the northern skarn during cooling. A small volume of diopsidic, aluminous, wollastonite-bearing skarn occurs in both the northern and southern localities. The relationship of this type of skarn to the hedenbergitic skarn is ambiguous, since there is no large-scale mineralogical zoning. The Marn is similar to hedenbergitic, auriferous skarns of Japan, where the oxidation state of the intrusive rocks is believed to be the controlling factor in the development of skarn mineralogy.