In the early stages of prospecting for economically viable skarn deposits, what is generally used as the indication of potential deposits is not an individual clue but the combination of several clues, namely intrusion geology, alteration features, pathfinder geochemistry, and geophysics. Since skarns tend to be zoned, the most productive method of prospecting would be to trace alterations distally and then follow the zoning closer to the intrusion-contact zone.
The first factor of significance for economically valuable skarn deposits is good geology. That would be an intrusive rock formation, ideally a calc-alkaline or metalliferous pluton, in contact with highly reactive carbonate formations, such as limestone and dolostone. The key here is that skarn mineralization is formed through the process of interaction between heat and fluids from the intrusion with the wall-rock material; hence, contact zones, faults, and other areas with increased permeability are significant controls on ore deposits. The presence of marble, skarnoids, hornfels, or other reaction structures indicates great prospectivity.
The second sign is the existence of skarn minerals and zoning. Big skarn deposits tend to have a near zone that is rich in garnets, a far zone with pyroxenes, and other types of minerals such as wollastonite, vesuvianite, or rhodonite at the edge of the marble. Practically, finding coarse garnets, clinopyroxenes, magnetite, epidotes, actinolites, and wollastonite in altered limestones is better evidence than minor and sporadic mineralization because they show a true metasomatic deposit instead of contact metamorphism. Also, distal signs such as bleaching, escape features, and isotopic haloes are helpful clues to trace ore deposits.
The third sign relates to geochemistry. Specific metal signatures depend on the deposit type; however, elements like Cu, Au, Fe, W, Zn, Pb, Ag, Mn, As, Bi, Te, Co, and Ni may be relevant. Metal zoning ratios are important in defining the center of ore deposits from their peripheries. Therefore, a strong multi-element anomaly in soils, rock chips, float, and channel samples is more promising when it occurs with the existence of skarn mineralization and the intruding body contact. In gold skarn deposits, for instance, elements like As, Bi, Te, Co, Cu, Zn, or Ni can be especially relevant.
Fourthly, we have geophysics. Some of the skarn deposits, particularly those containing high amounts of iron and copper, can generate magnetic, gravitational, resistivity, and IP signatures that distinguish them from other rocks in the area. Magnetic responses are an indication of magnetite-rich skarn, whereas a chargeable signature would denote a zone of sulfides; however, low and medium resistivity might imply altered or mineralized contact zones. The application of geophysics should be accompanied by geological and geochemical data, since a single anomaly on its own may not be enough to rank a target.
Lastly, we need to look for scale and continuity. An economic skarn deposit will usually contain alteration halos, a number of mineralized lenses or zones, and/or a number of structures controlling mineralization. Indicators of continuity would include linear exposure along the contact, stacked zones of replacement, excellent dip continuity, and repeat development of skarn zones along favorable horizons. Clearly, a skarn showing well-defined zonation over meters and even hundreds of meters will be better than a random occurrence of garnets and magnetites.
From an exploration point of view, the best skarn prospects in their early stage of development are the ones where everything aligns together; these include altered carbonate country rocks, a favorable intrusion, skarn minerals, metal anomalies, and geophysical indicators. It is a good principle that if a skarn prospect shows strong alteration, good zoning, consistent chemistry, and geophysical continuity, then the skarn prospect becomes more economical.
