Monday, November 9, 2015

Skarn and Polymetallic Carbonate

Replacement deposits are related genetically to magmas that intrude into sedimentary rocks (fig. 1). The deposits form when magmatic-hydrothermal fluids expelled from cooling magmas react chemically with carbonate-rich sedimentary rocks. Skarn and Polymetallic Carbonate replacement deposits are associated with many other types of magmatic-hydrothermal deposits in mineral districts. In fact, distinction between skarn and other deposit types is not always easy. In many districts, skarns form an intermediate "zone" of deposits between porphyry deposits in the center of mining districts and outer zones of polymetallic vein and replacement, and distal-disseminated, deposits.

Carbonate-hosted mineral deposits generally are less likely than other deposit types to generate acid mine drainage because of the availability of acid- neutralizing minerals, such as carbonates and reactive silicates, the coarse mineral grain size, and the high metal to sulfur ratios of many sulfide minerals in skarns (Kwong, 1993). However, these deposits locally can include massive concentrations of sulfide minerals as well as potentially toxic elements that remain in solution at high pH (e.g., Zn, Mn) and therefore are difficult to remove in order to meet water-quality standards. We know of three cases where significant environmental problems associated with processed waste from carbonate-hosted mineral deposits led to U.S. Environmental Protection Agency (EPA) National Priority listing for site clean-up under the Superfund program. The Cleveland mill in Grant County, southwestern New Mexico (U.S. EPA, 2000a; Boulet and Larocque, 1997) processed ores from copper and zinc skarns.

Mill tailings washed into a creek resulting in locally acidic surface waters (pH as low as 2.2) and elevated dissolved-metal concentrations (As, Pb, Cd, Zn, Be). In western Colorado, a century of mining of polymetallic carbonate-replacement deposits in the Leadville and Gilman mining districts generated large volumes of mining wastes that impacted area soils and rivers. In the 1980s, ecological impacts and human health threats associated with mine waste led to Superfund site designations at California Gulch in the Leadville mining district, Colorado and at the Eagle mine in the historic Gilman mining district near Vail, Colorado (U.S. EPA 2000b, 2000c; Engineering Science, Inc., 1985).

Geologic setting of high temperature model
Figure 1. Generalized conceptual model for the geologic setting of high-temperature, carbonate-hosted and related deposits associated with igneous intrusions. (From Plumlee and others, 1999, figure 19.18).

Preliminary geoenvironmental models (duBray, 1995) were developed for skarns and polymetallic replacement deposits (Plumlee and others, 1995; Hammarstrom and others, 1995a, b, c). The most important geological characteristics shared by these types of deposits are listed in table 1. In the following discussion, we synthesize data included in the preliminary models, incorporate new data, and focus on salient differences between skarns and polymetallic carbonate-replacement deposits and differences among skarn types, with particular emphasis on differences that bear on their environmental signatures (table 1).

Geological characteristics of skarn and polymetallic carbonate
Table 1. Most important geological characteristics of skarn and polymetallic carbonate- replacement deposits (based largely on Meinert, 1998, 1992).

Deposit Geology Skarn and Polymetallic Carbonate
1. Skarns
Skarns (tactites) are coarsely-crystalline metamorphic rocks composed of calcium-iron-magnesium- manganese-aluminum silicate minerals (commonly referred to as "calcsilicate" minerals) that form by replacement mainly of carbonate-bearing rocks during contact or regional metamorphism and metasomatism. The majority of the world's major skarn deposits are thought to be related to hydrothermal systems (Einaudi and others, 1981). Skarns can be barren or may contain metals or other minerals with economic value. Skarn deposits are important sources of base and precious metals as well as tin, tungsten, and iron. Skarns are relatively high-temperature mineral deposits resulting from magmatic-hydrothermal activity associated with granitoid plutons in orogenic tectonic settings. Skarns generally form where a granitoid pluton has intruded sedimentary strata that include limestone or other carbonate-rich rocks.

The processes that lead to formation of all types of skarn deposits include the following stages: (1) isochemical contact metamorphism during pluton emplacement, (2) prograde metasomatic skarn formation as the pluton cools and an ore fluid develops, and (3) retrograde alteration of earlier- formed mineral assemblages. Deposition of ore minerals accompanies stages 2 and 3. Skarn deposit mineralogy is spatially zoned with respect to pluton contacts, host rock lithology, and (or) fluid pathways. Later petrogenetic processes may partly or completely obliterate earlier stages of skarn development. Skarns are classified as calcic if the protolith was limestone, and as magnesian if the protolith was dolostone. Skarns also are classified by the most economically important metal present, including copper, iron, zinc-lead, gold, tin, tungsten, and molybdenum (table 2). In some cases, subdivisions also are made on the basis of oxidation state (oxidized versus reduced) as reflected by mineralogy. A variety of different skarn types can occur in a relatively small geographic area. In the Winnemucca area of north-central Nevada, for example, all skarn types except tin skarns are observed (fig. 2). In this part of Nevada, iron, tungsten, and base-metal skarns are associated with Jurassic and Cretaceous intrusions whereas gold skarns are associated with Tertiary intrusions.

Each class of skarn deposit has a characteristic, though not necessarily unique, size, grade, tectonic setting, granitoid association, and mineralogy (Einaudi and Burt, 1982; Einaudi and others, 1981; Meinert, 1983; Ray and Webster, 1990). Not surprisingly, therefore, the various classes of skarn deposits have different geochemical signatures and oxidation-sulfidation states. Some skarn deposits are best described as polymetallic and are difficult to classify by existing schemes. The Nabesna Fe-Au skarn in Alaska, for example, has been classified as an iron skarn with by-product gold and as a gold-rich copper skarn by different workers (Eppinger and others, 2000, 1999, 1997).

In addition, some skarns contain unusual concentrations of rare-earth minerals and uranium or platinum group elements (Meinert, 1998). Many skarns mined in the past for iron, copper, or tungsten have been reexamined and developed for gold in the past 30 years. The Bonfim W-Au deposit in northeastern Brazil , for example, has proven total reserves of 70 tons of scheelite, but discovery of gold in abandoned dumps in the 1990s led to further development and production of 100 kilos of gold (Souza Neto and others, 1998). Most economic skarn ore is present as exoskarn, which forms in carbonate host rocks proximal to an intrusion. The parts of the intrusion that are altered and can host ore are referred to as endoskarn. Endoskarn is variably developed on the intrusion side of intrusion-wallrock contacts. Endoskarn may include high-grade ore zones or may be part of the waste rock. For a recent review of skarn classification, geology, mineralogy, genesis, exploration strategies, and case examples, see Meinert (1992).

2. Polymetallic carbonate-replacement deposits
Polymetallic carbonate-replacement deposits are massive lenses, pods, and (or) pipes (mantos or chimneys) of sulfide minerals that comprise Pb-Zn-Ag-Cu-Au ores hosted by, and replacing, limestone, dolostone, and other sedimentary rocks. Most massive replacement ores contain more than 50 percent sulfide minerals (Plumlee and others, 1995; Megaw, 1998). Some polymetallic carbonate-replacement deposits occur with, or grade into, skarn deposits at the contact between sediments and intrusion. In both polymetallic carbonate-replacement deposits and base-metal-rich skarn deposits, ore metals typically are zoned. Copper and gold are enriched proximal to the intrusion and lead-zinc-silver ores occur in distal parts of the mineralized system.

Some types of igneous rocks potentially provide long-term acid-neutralizing capacity because mafic silicate minerals such as biotite, chlorite, pyroxene, and calcic plagioclase consume acid upon weathering. Many of these same minerals also are present in retrograde skarn mineral assemblages. Calcite (or dolomite) is ubiquitous in skarn and replacement deposits. Calcite in ores, along with carbonate host rocks, typically provides enough neutralization capacity to mitigate any acid generated by weathering of iron sulfide minerals. Endoskarn minerals or altered igneous rocks may neutralize acid, albeit at a much slower rate than the highly reactive carbonate minerals. 

Andrews, R.D., 1975, Tailings-Environmental consequences and a review of control strategies, i n International conference on heavy metals in the environment, Symposium proceedings, v. 2, Toronto, 1975, p. 645-675. Anstett, T.F., Bleiwas, D.I., and Hurdelbrink, R.J., 1985, Tungsten availability-Market economy countries: U.S. Bureau of Mines Information Circular 9025.

Brooks, J.W., Meinert, L.D., Kuyper, B.A., and Lane, M.L., 1991, Petrology and geochemistry of the McCoy gold skarn, Lander County, Nevada, in Raines, G.L. and others, eds., Geology and ore deposits of the Great Basin, Symposium Proceedings, Geological Society of Nevada, Reno, p. 419-442.

Castro-Larrogoitia, J., Kramar, U., and Puchelt, H., 1997, 200 years of mining activities at La Paz/San Luis Potosi/Mexico – Consequences for environment and geochemical exploration: Journal of Geochemical Exploration v. 58, p. 81-91.

Cox, D.P., 1986a, Descriptive model of W skarn deposits, in Cox, D.P. and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 55.

Einaudi, M.T., 1982, Description of skarns associated with porphyry copper plutons, in Titley, S.R., ed. Advances in geology of the porphyry copper deposits: The University of Arizona Press, Tucson, AZ, p. 139-183.

Einaudi, M.T., and Burt, D.M., 1982, Introduction: terminology, classification, and composition of skarn deposits: Economic Geology v. 77, p. 745-754.

Written by: Jane M. Hammarstrom, with contributions from Brad Van Gosen and Bob Eppinger, Editor by: Flyshgeost.

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