Meinert (cica 2000) prepared a workshop on the exploration of skarn deposits. This is notes from that workshop. He reported in his Introduction that skarn deposits have been mined for a variety of metals, including Fe, W, Cu, Pb, Zn, Mo, Ag, Au, U, REE, F, B, and Sn. Skarns occur on all continents and in rocks of almost all ages. Although the majority are found in lithologies containing at least some limestone, they can form in almost any rock type including shale, sandstone, granite, basalt, and komatiite. Skarns can form during regional or contact metamorphism and from a variety of metasomatic processes involving fluids of magmatic, metamorphic, meteoric, and/or marine origin.
They are found adjacent to plutons, along faults and major shear zones, in shallow geothermal systems, on the bottom of the seafloor, and at lower crustal depths in deeply buried metamorphic terrains. What links these diverse environments, and what defines a rock as skarn, is the mineralogy which includes a wide variety of calc-silicate and associated minerals but is usually dominated by garnet and pyroxene. Thus, the presence of skarn does not necessarily indicate a particular geologic setting, protolith composition, or metasomatic process. Rather, its development indicates that the combination of temperature, pressure, fluid and host rock composition was within the stability range of the identified skarn minerals.
Just as mineralogy is the key to recognizing and defining skarns, it is also critical in understanding their origin and in distinguishing economically important deposits from interesting but uneconomic mineral localities. Skarn mineralogy is mappable in the field and serves as the broader "alteration envelope" around a potential ore body. Because most skarn deposits are zoned, recognition of distal alteration features can be critically important in the early exploration stages. Details of skarn mineralogy and zonation can be used to construct deposit-specific exploration models as well as more general models useful in developing grass roots exploration programs or regional syntheses. In addition, because most economic skarn deposits are related to magmatism, investigations of igneous petrogenesis and tectonic setting can form a framework for regional exploration or classification.
Economic skarn deposits can be subdivided into several main types based upon the dominant contained metal (e.g. W, Fe, Cu, Au etc.). This is similar to the classification of porphyry deposits into porphyry copper, porphyry molybdenum and porphyry tin types; deposits which share many alteration and geochemical features but are, nevertheless, easily distinguishable. In order to explore for economically viable skarn deposits, it is necessary to understand the typical characteristics of each skarn group and the differences among them. Thus, the purpose of this workshop is to develop an understanding of skarns in general with particular emphasis on field identifiable characteristics that areuseful in exploration. There have been numerous general review papers on skarn deposits in the past few decades (e.g. Watanabe, 1960; Phan, 1969; Zharikov, 1970; Smirnov, 1976; Burt, 1977; Einaudi et al., 1981; Meinert, 1983; Ray and Webster, 1991a; Meinert 1992, 1997; and the skarn Internet site at: www.wsu.edu/~meinert/skarnHP.html) and the reader is referred to these sources for more detailed considerations of skarn terminology and genesis.
Dr. Meinert concluded that whether one thinks of skarn as a rock type or as an alteration of previously existing rocks, skarns are mappable in the field and the basic map unit is skarn mineralogy. A detailed map showing the distribution of skarn mineral phases will yield important information about the overall size, characteristics, and genesis of a skarn system. Of course, the skarn mineralogy needs to be interpreted in terms of lithology, structure, and timing. For example, a given mineral may develop under different P-T-xCO2 conditions in limestone relative to shale or where fluid flow is enhanced by faulting or brecciation. Comparison of the field characteristics of a given occurrence with the general skarn features outlined in this paper provides a predictive basis for evaluating economic potential and overall genesis. Models of skarn zonation are particularly useful in evaluating incompletely exposed or explored skarn systems.
Skarns are one of the more variable classes of ore deposits and the references cited in this manual will lead the reader to many variations upon the general theme. The purpose of this skarn workshop has been to develop the general theme that skarn deposits are understandable, that they are amenable to mapping and simple field observations, and that modern geochemical techniques yield a fairly clear picture of their evolution. Skarns are distinct from many other types of ore deposits. Almost all large skarn deposits are directly related to igneous rocks. One of the fundamental controls of skarn mineralogy and metal content is the genesis and crystallization history of the associated pluton. The late-stage hydrothermal evolution of the pluton is mirrored by alteration and mineralization in the surrounding rocks. The reactivity of sedimentary rocks, particularly carbonate rocks, accounts for the abundance of skarns in most localities where magma is emplaced in the upper crust.
All skarn deposits form over a wide temperature range but it is the high temperature part of that range (>350°C) which distinguishes skarns from ore types, such as MVT Pb-Zn deposits, which may contain similar metals and form in similar rocks. The high temperatures and concentrated metasomatic fluids stabilize a variety of skarn minerals such as garnet and pyroxene, which are the defining characteristic of skarns. The waning stages of hydrothermal activity invariably are cooler and result in retrograde alteration of the early skarn minerals. This pattern of early metamorphism followed by prograde and retrograde metasomatism is common to most skarns. Variations on this general pattern are many and include depth of formation, oxidation state, petrochemistry, and wall rock composition.
In the world of economic geology where fluctuating commodity prices and world demand can change an obscure skarn occurrence into a producing mine (and vice versa) in a short period of time, skarn deposits remain interesting economic and academic challenges. It is likely that skarn deposits will remain an important source of many metals in the next century. The challenge will be to predict which ones will be mined and for what commodities.
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