Explained by Douglas B. Stoeser
On the choice of deposit models
Theodore and others (1991) recognize a class of skarn deposits as gold-bearing skarn if they have an average gold grade of at least 1 g/t and typical skarn mineralogy. They recognize two subtypes of gold-bearing skarns: gold-skarn and byproduct gold-skarn. They define gold-skarns as skarn deposits where gold is the principal commodity, and byproduct gold-skarns as deposits where gold had been or is being recovered as a byproduct. Only the gold-skarn subtype is considered here, because byproduct gold-skarns are primarily Cu skarn and Pb-Zn skarn and these deposit types are considered elsewhere in the present study. Western Utah has been judged to be permissive for this deposit type by (Reid, 1991).
On the delineation of permissive tracts
Gold skarns are found in the same areas as skarns enriched in other metals, where epizonal calc-alkaline granitoid stocks intrude carbonate rocks. Theodore and others (1991) indicated that the associated intrusions are typically "compositionally expanded I-type felsic and intermediate plutons, dikes, sills, or stocks that may or may not be porphyritic." They also state that host rocks include a wide variety of sedimentary and igneous rocks, including limestone, dolomite, clastic sedimentary rocks, volcanic rocks, and granitoids, with a calcareous component typically being present. The Tertiary magmatic belts of western Utah contain appropriate intrusions, and because of the approximately seventy percent alluvial cover within the magmatic belts, the unknown distribution of rock types beneath that cover, and the fact that the bulk of the these terranes are underlain by a sedimentary shelf sequence dominated by carbonate sedimentary rocks, we use the same permissive terranes for gold skarns as for porphyry copper deposits.
Important examples of this type of deposit
There are no significant gold skarn deposits in Utah and the only known occurrence is the Midas deposit in the Gold Hill district that produced 600 metric tons of ore with a grade of 25 g/t Au (Theodore and others, 1991).
On the numerical estimates made
This deposit type has not been vigorously sought in Utah until recently. The median deposit for this model (Theodore and others, 1991) is quite small (213,000 metric tons at 8.6 g/t equaling 59,000 oz Au). We judged ourselves unable to estimate numbers of deposits as small as some of those in the grade and tonnage model, and we made our estimate with reference to a truncated model consisting only of those deposits that are larger than 25,000 metric tons. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 2, and 4 or more deposits consistent with the grade and tonnage model of Theodore and others (1991), truncated as described above.
Reid, J.E., 1991, Skarn occurrences in Utah and the potential for associated gold mineralization: Utah Geological Survey Contract Report 91-13, 49 p.
Theodore, T.G., Orris, G.J., Hammarstrom, J.M., and Bliss, J.D., 1991, Gold-bearing skarns: U.S. Geological Survey Bulletin 1930, 61 p.