Deposit Models - Characteristics of Gold Skarns

C - THE CHARACTERISTICS OF GOLD SKARNS
by G.E. Ray, B.C. Geological Survey

Gold skarns are defined as skarn deposits in which gold is the primary or dominant economic metal present. The following features should be noted about these deposits:

1. They occur worldwide along destructive plate margins and tend to have a spatial and temporal association with Cu porphyry provinces.

2. They are associated with subduction and arc-related plutonic rocks of largely gabbro-diorite-granodiorite composition. These intrusives tend to be undifferentiated, being relatively depleted in LIL-elements such as Rb, Ce, Nb, and La, and enriched in Cr, Sc, Sr and V.

3. They are mostly developed in calcic skarn with exoskarn envelopes dominated by Ca-silicate assemblages (clinopyroxene and garnet). Magnesian Au skarns (with Mg-silicates such as olivine and serpentine) are very rare; one example however, is the Butte Highlands deposit (Montana).

4. The gold in Au skarns is commonly micron-sized; thus, the ore is visually indistinguishable from waste. It may be associated with Bi-tellurides and arsenopyrite, and in some deposits there is an enrichment in Co.

5. Depending on the mineralogy and garnet-pyroxene chemistry of the prograde exoskarn and ore, Au skarns can be separated into reduced and oxidized types.

6. Reduced Au skarns are marked by low garnet/pyroxene and pyrite/pyrrhotite ratios and the presence of hedenbergitic pyroxene and Fe-rich biotite. The intrusives have low Fe2O3/FeO ratios and the ore bodies are developed distal to the pluton, in the outer parts of the pyroxene-rich exoskarn envelopes. Examples include Nickel Plate (B.C.), Fortitude (Nevada) and Buckhorn Mountain (Washington State).

7. Oxidized Au skarns are characterized by high garnet/pyroxene and pyrite/pyrrhotite ratios, and by the presence of diopsidic pyroxene, pyrite, magnetite and hematite. Ore bodies tend to form more proximal to the intrusions than those in the reduced Au skarns. Examples include Nambija (Ecuador) and McCoy (Nevada).

8. Compared to the ore in Cu, Fe, Mo, W, Pb-Zn and Sn skarns, ore in most reduced and oxidized Au skarns has distinctly low metal ratios (Cu/Au <2000; Cu/Ag <1000; Zn/Au < 100, Ag/Au < 1).

9. There is no correlation between Cu and Au in many Au skarns (unlike in Fe and some Cu skarns where a good correlation exists between these metals). Thus, the gold potential of a skarn can be easily overlooked if copper sulphide-rich outcrops are preferentially sampled and other sulphide-bearing or sulphide-lean assemblages ignored.

10. In some Au skarns (e.g. the Nickel Plate and Fortitude deposits) there is a metal and mineralogical zoning throughout the exoskarn envelope. This zoning consists of proximal garnet-dominant skarn with high Cu/Au ratios and distal pyroxene-dominant skarn with low Cu/Au ratios and the gold ore bodies.

Although most Au skarns have some or most of the above characteristics, individual deposits can have unique features, and it is likely that new types or varieties of Au skarns will be discovered. In exploration, any skarn of any class should be routinely and systematically assayed for gold. Essentially, any calcareous or carbonate rock package intruded by an arc pluton has a potential for hosting Au skarn deposits, although such favorable packages are generally small and relatively rare in typical island arc or back-arc environments. However, primary target areas would include

(a) reefs aprons, which often flanked the original island volcanoes;

(b) carbonate facies formed along the margins of the back-arc basins;

(c) thrust slices of allochthonous platformal carbonates which were subsequently intruded by arc magmatism.