SUSTAINABLE MINERAL PRODUCTION
oidal textures of marcasite often remain visible to the trained eye, indicating that it was present. The highest gold grades are often in this pseudomorph zone. Pyrite generally forms gold colored euhedral cubic or rarely dodecahedral crystals, often complexly embayed, intergrown and banded. Marcasite is silvery or greenish in color and forms complex crystal struc- tures, reniform or fibrous habits. It forms bands with pyrite and remnant textures are often visible after conversion to pyrite. In the absence of microscope and analytical identifica- tion of fine-grained sulfides, banded veins and areas that lack a pyrite habit could be remnant marcasite. In recent decades, areas of marcasite were targeted for exploration with mixed results. It must be remembered that, at least hypothetically, a large area of marcasite near favorable host rocks may be a kilometer up-gradient from a major gold deposit (Figure 1).
The exploration importance of marcasite and its pseudo-
morphs gives the explorer a general idea of the depths and temperatures of formation. The upper end of marcasite stabil- ity overlaps with the optimum temperature of gold deposition, around 240-220°C. Marcasite is unstable in much of the gold depositional environment, so the importance of recognizing pseudomorphs becomes readily apparent. Marcasite is only stable in lower temperature environments moderately distal from the optimum gold depositional conditions. Therefore, a small high-level gold deposit with abundant marcasite can be a good indication of a larger gold deposit at depth. It should be remembered that fluid pathways in this environment are rarely vertical, and a new discovery may be a kilometer down- gradient and lateral from a surface target (Figure 1). Favorable
stratigraphy may only be mineralized near feeder structures and within fold or breccia structures that have trapped min- eralizing fluids. Temperatures of deposition and mineralogy constraints will change with pressure and depth, so Figure 1 is only a general guide meant to give the exploration geologist a general idea of geologic process and a mineralogical vector orientation to a Carlin-like gold deposit.
References
Emsbro P., Hofstra, A.H., Lauha, E. A., Griffin, G.L. and Hutchinson, R.W., 2003, Economic Geology v.98, p. 1069-1105.
Hofstra, A.H., 1997, Isotopic composition of sulfur in Carlin-type gold deposits: implications for genetic models, in Vikre, P., Thompson, T.B., Bettles, K., Christensen, O., Parratt, R., eds., Carlin-Type Au Deposits Field Conference, Economic Geology Guidebook Series v. 28, p.119-129.
Murowchick, J.B., 1992, Marcasite inversion and determination of pyrite ancestry: Economic Geology, v. 87, p. 1141-1152.
About the Author John Wood worked 24 years for Amax Exploration Inc., 8 years
as Chief Geologist Americas for Newcrest Resources Inc. and 6 years as Director of Exploration Latin America and U.S.A. for Goldcorp USA Inc. (Managed the Mount Tolman molybdenum deposit resource expansion 1978-79, discovered Sleeper gold deposit 1982 and man- aged many successful exploration and mine expansion projects since that time.) Renders, P.J.N., and Seward, T.M., 1989, The stability of hydrosulphido- and sulphido-complexes of Au(I) and Ag(I) at 25°C: Geochimica et CosmochimicaActa, v. 53, p.245-253.
Figure 1 - Hypothetical Marcasite Distribution within a Carlin-like Gold District.
Key: Medium Gray: environments favorable for Gold. Dark Gray: Marcasite-bearing zones. Wide grey band: zone of marcasite destruction and occurrence of pseudomorphs after marcasite
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