Word to the Wise - Miami Universityrakovajf/WTTW Gangue.pdf · 2006-08-18 · Word to the Wise JOHN...
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Word to the Wise JOHN RAKOVAN Department of Geology Miami University Oxford, Ohio 45056 [email protected] G angue (pronounced "gang") is the term used to col- lectively describe the valueless minerals in an ore deposit. This view of gangue equates value with ore min- erals. The table gives the most common gangue minerals (Rimstidt 1997). As one reads through this list, however, it becomes obvious that many gangue minerals may be of great value in the mineral-specimen market. Also, inclu- sion in the table does not preclude a mineral from being economically important in some deposits; it only indicates that it is commonly found in uneconomic concentrations associated with other minerals that are ores. For example, fluorite (our major source of fluorine) is a primary ore min- eral in many deposits, such as the manto fluorite deposits of the Buenavista-Encantada and El Tule mining districts, northern Coahuila, Mexico (Temple and Grogan 1963; Rakovan 2003), but it is a common gangue mineral in most Mississippi Valley-type (MVT) lead-zinc deposits (Misra 1999). An important exception to this last example are anomalously fluorite-rich MVTs such as those found in the Kentucky-Illinois fluorite district (once the world's largest source of fluorite ore; Park and MacDiarmid 1975). In the previous Word to the Wise column (Rakovan 2005), we explored the metasomatic alteration of rocks, a natural process that is often associated with hydrothermal activity and ore deposit formation. Gangue can consist of the altered and unaltered host rocks of a deposit as well as non-ore minerals that were transported to the deposit in solution. Although gangue is not economically valuable, it can prove very useful. Specifically, gangue formed by both metasomatic alteration and precipitation during ore forma- tion can yield a wealth of information about the origins and physical conditions of the ore-forming fluids (Barnes 1997). For example, because gangue minerals such as quartz and calcite are transparent in thin section, they are most often used for fluid inclusion studies (Roedder 1984) rather than the sulfide ore minerals that are commonly opaque. Fluid inclusion studies give us information such as the temperatures at which the minerals formed and the salin- ity of the fluids from which they precipitated. This in turn Dr. John Rakovan, an executive editor o/Rocks & Minerals, is a professor of mineralogy and geochemistry at Miami University in Oxford, Ohio. He is currently a visiting professor at the Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Japan, Figure 1. Apatite and magnetite (gangue and ore respectively) from the Dashkesan (Dashkezan) Co-Fe deposit, Dashkesan, Rayonu, Azerbaijan. Specimen measures 7 x 5.5 cm. can Increase our understanding of ore formation and guide future exploration. Separation of gangue from ore minerals can be one of the major obstacles in the successful development of an ore deposit. Because the two are usually intimately inter- grown (fig. 1), crushing of ores to a fine grain size is usually required. Once the various minerals are mechanically liber- Common gangue minerals. silicates chalcedony opal quartz carbonates ankerite aragonite calcite cerrusite dolomite kutnohorite magnesite rhodochrosite siderite smith son ite strontianite witherite Source; Modified from Rimstidt (1997). sulfates alunite anglesite anhydrite celestine gypsum halides fluorite sulfides pyrite Volume 80, September/October 2005 365
Transcript of Word to the Wise - Miami Universityrakovajf/WTTW Gangue.pdf · 2006-08-18 · Word to the Wise JOHN...
Word to the Wise
JOHN RAKOVANDepartment of Geology
Miami UniversityOxford, Ohio 45056
G angue (pronounced "gang") is the term used to col-lectively describe the valueless minerals in an ore
deposit. This view of gangue equates value with ore min-erals. The table gives the most common gangue minerals(Rimstidt 1997). As one reads through this list, however,it becomes obvious that many gangue minerals may be ofgreat value in the mineral-specimen market. Also, inclu-sion in the table does not preclude a mineral from beingeconomically important in some deposits; it only indicatesthat it is commonly found in uneconomic concentrationsassociated with other minerals that are ores. For example,fluorite (our major source of fluorine) is a primary ore min-eral in many deposits, such as the manto fluorite depositsof the Buenavista-Encantada and El Tule mining districts,northern Coahuila, Mexico (Temple and Grogan 1963;Rakovan 2003), but it is a common gangue mineral in mostMississippi Valley-type (MVT) lead-zinc deposits (Misra1999). An important exception to this last example areanomalously fluorite-rich MVTs such as those found in theKentucky-Illinois fluorite district (once the world's largestsource of fluorite ore; Park and MacDiarmid 1975).
In the previous Word to the Wise column (Rakovan2005), we explored the metasomatic alteration of rocks, anatural process that is often associated with hydrothermalactivity and ore deposit formation. Gangue can consist ofthe altered and unaltered host rocks of a deposit as well asnon-ore minerals that were transported to the deposit insolution. Although gangue is not economically valuable, itcan prove very useful. Specifically, gangue formed by bothmetasomatic alteration and precipitation during ore forma-tion can yield a wealth of information about the originsand physical conditions of the ore-forming fluids (Barnes1997). For example, because gangue minerals such as quartzand calcite are transparent in thin section, they are mostoften used for fluid inclusion studies (Roedder 1984) ratherthan the sulfide ore minerals that are commonly opaque.Fluid inclusion studies give us information such as thetemperatures at which the minerals formed and the salin-ity of the fluids from which they precipitated. This in turn
Dr. John Rakovan, an executive editor o/Rocks & Minerals,is a professor of mineralogy and geochemistry at MiamiUniversity in Oxford, Ohio. He is currently a visiting professorat the Graduate School of Human and Environmental Studies,Kyoto University, Kyoto, Japan,
Figure 1. Apatite and magnetite (gangue and ore respectively)from the Dashkesan (Dashkezan) Co-Fe deposit, Dashkesan,Rayonu, Azerbaijan. Specimen measures 7 x 5.5 cm.
can Increase our understanding of ore formation and guidefuture exploration.
Separation of gangue from ore minerals can be one ofthe major obstacles in the successful development of anore deposit. Because the two are usually intimately inter-grown (fig. 1), crushing of ores to a fine grain size is usuallyrequired. Once the various minerals are mechanically liber-
Common gangue minerals.
silicateschalcedonyopalquartz
carbonatesankeritearagonitecalcitecerrusitedolomitekutnohoritemagnesiterhodochrositesideritesmith son itestrontianitewitherite
Source; Modified from Rimstidt (1997).
sulfatesaluniteanglesiteanhydritecelestinegypsum
halidesfluorite
sulfidespyrite
Volume 80, September/October 2005 365
Figure 2. Acid mine drainage with typical yellow-boy precipita-tion (amorphous iron oxide) that forms from the iron releasedby pyrite during oxidation, Gossan Lead, Galax, Virginia.
ated from one another, differences in physical and chemicalproperties (i.e., density, magnetism, wetting abilities, solu-bility, and so on) can be exploited to separate them and toconcentrate the ore minerals.
The majority of metal ore minerals are sulfides (i.e.,chakocite, Cu,S; sphalerite, ZnS; and molybdenite, MoS^).Because of the abundance of iron in the earth's crust (rough-ly 6 percent) pyrite, FeS,, is associated with almost all sulfideore deposits. However, it is rarely of economic importanceand thus is often a gangue mineral. Pyrite is also a commongangue mineral in coal deposits. In mine development mostgangue minerals end up on the waste pile. When it comes topyrite, this leads to one of the major environmental prob-lems associated with mining: acid mine drainage (AMD).AMD is formed by inorganic and microbially mediatedoxidation that occurs when oxygen-rich surface water comesin contact with pyrite. The resulting water is usually highin acidity (low pH) and dissolved heavy metals that stay insolution (and are thus bioavailable) until the pH is increased(fig. 2). Acidic, metal-rich waters can also form deep withinmines that allow the entry of oxygen and access to buriedpyrite and other sulfides. Problems associated with AMDinclude contaminated drinking water, disrupted growth andreproduction of aquatic plants and animals, and acceler-ated corrosion of steel in structures such as bridges (Azcue1999).
ACKNOWLEDGMENTSI would like to thank Kendall Hauer for his review of this column
and for his helpful suggestions.
REFERENCES
Azcue, J. M., ed. 1999. Environmental impacts of mining activi-ties: Emphasis on mitigation and remedial measures. New York:Springer.
Barnes, H. B. 1997. Geochemistry of hydrothermal ore deposits. NewYork: John Wiley & Sons.
Misra, K. C. 1999. Understanding mineral deposits. Boston: KluwerAcademic Publishers.
Park, C. R, Jr., and R. A. MacDiarmid. 1975. Ore deposits. San Fran-cisco: W. H. Freeman and Company.
Rakovan, I. 2003. A word to the wise: Manto. Rocks & Minerals78:351-53.
. 2005. A word to the wise: Metasomatism. Rocks & Minerals80:63-64.
Rimstidt, J. D. 1997. Gangue mineral transport and deposition.In Geochemistry of hydrothermal ore deposits, ed. H. L. Barnes,487-516. New York: John Wiley & Sons.
Roedder, E. 1984. Fluid inclusions. Reviews in Mineralogy, vol. 12.Washington DC: Mineralogical Society of America.
Temple, A. K., and R. M. Grogan. 1963. Manto deposits of fluorspar,northern Coahuila, Mexico. Economic Geology and the Bulletin ofthe Society of Economic Geologists 58:1037-53, •
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