Geochemical methods of mineral exploration are based on the premise that diagnostic disturbances in the normal distribution pattern of chemical elements may exist in accessible material in the vicinity of concealed core deposits. Work on Tin-Tungsten deposits was initiated after it was realized that techniques used for magnetite could be adapted to cassiterite and wolframite solubility determinations. According to Taylor, Sn-W deposits are associated with granites or rhyolites derived by partial melting of continental crust and that the tectonic setting is either convergent plate margins or intra-continental rift zones (1979). In addition, Sn and W occur nearly as cassiterite, wolframite represented by the ferberitehuebnerite solid solution series and scheelite. Pyrite-pyrrhotite groups are common, as are chalcopyrite, arsenopyrite, sphalerite, and galena. Each deposit has its own set of mineral assemblages and within a particular deposit assemblages may be distributed in a regular fashion.
The most commonly occurring sequence is represented by cassiterite-wolframite-sulphides. However a wide range of tectonic style is unique and not duplicated elsewhere. These deposits have low acid drainage because of their low sulfide mineral content (Cox & Singer 1986, p. 379). However, some tin and tungsten deposits are satiated in sulfide mineral rich zones and may have acid generation potential. Tin may occur together with fluorine and beryllium in some skam deposits. These deposits are hosted by carbonate rocks such as dolomite, limestone, argillite, shale and marble. These rocks are highly calcareous sedimentary rocks or metamorphosed calcareous sedimentary rocks with extensive buffering capacity. Cassiterite in tin skarns is commonly very fine grained. Tin concentrations range from 0.1 to 1 weight percent in deposits that include 10 to 90 million tonnes of ore. Tin also may be contained in silicate minerals, including garnet and hornblende. A range of geochemical signatures have been associated with other rock such as Sn skarns that include Sn, W, F, Be, Zn, Cu, Ag, Li, Rb, Cs, Re, B elements (Pollard et al. 1983, pp. 543-5).
Geothermal cycle The use of tin dates back in the 3,500 B.C. and is one of the oldest metals that was used by man. More than 35 countries worldwide mine tin today. However, it is scarcely found in the earths crust and is approximated to be 2ppm compared to 63 ppm of copper elements. Cut and fill stop methods are used at Renison Bell replacement tin deposit. The sulfide rich and sulfide poor minerals are selectively stockpiled on the surface and blended ore is fed into a three stage open crushing circuit that reduces ore from 750mm to 15mm. Ore is processed by flotation to remove sulfide minerals prior to gravity concentration of cassiterite (Morland, 1986). Staged grinding is used to liberate fine-grained cassiterite. Residual sulfide minerals in the gravity concentrate are removed by floatation. Cassiterite concentrates a re leached wit h sulfuric acid to remove siderite, magnetic material is removed using magnetic separators; refined concentrates are then shipped to smelters. Tailings a re combined with lime to adjust pH to 8.5 before being pumped to impoundments. The environment disturbance is associated with open pits, tailing piles and subsidence in areas of underground mining. T he mobility of tin in cassiterite is generally low because cassiterite is very stable in t he surface environment.
Analytical methods to determine the element Tin Tin is usually determined as the total metal but it may also be measured as a specific organotin compound. Flame atomic absorption analysis is the most widely used and straightforward method for determining tin. In addition, furnace atomic absorption analysis is commonly used for very low analyte levels whereas inductively coupled plasma atomic emission analysis is used for multi-analyte analyses that include tin. Gas chromatography has a very high resolution and detector versatility and therefore is the most preferred separation technique for organotin compounds. Analysis of organotin compounds consist of extraction, formation of volatile derivative, separation and detection and quantification. The organotin compounds are extracted from the sample using organic solvents, ion exchange resins or adsorption onto a solid support.
A general clean-up step is required if the samples are biological. The extracted organotin compounds the undergo derivatization to a volatile form to be able to separate them by GC technique. Derivatization involves formation of alkyl derivatives using Grignard reagent, formation of ethyl derivatives using sodium tetraethylborate or by formation of hydrides using sodium borohydride. Separation of these derivatives may be done using differences in their boiling or by GC. Detection is then performed using a flame photometric detector, atomic absorption spectroscopy (AAS), or mass spectroscopy (MS). High performance liquid chromatography (HPLC) has also been used in the analysis of organotin compounds. The advantage of HPLC over GC is that no derivatization step is needed after extraction. Most separations are based on ion exchange or reversed phase separations using gradient elution. AAS, inductively coupled plasma mass spectrometry (ICP-MS), and fluorometric detection can be used. HPLC coupled with AAS is commonly used for speciation of organotin compounds (Beus & Grigorian 1977).
Tin and its compound can enter the human body through ingestion, inhalation or penetration through a skin. To determine tin in biological samples, the sample is first digested in an oxidizing acid mixture followed by atomic spectrometric determination. In addition, blood or urine may be used to determine the amount of tin in the body. Blood samples are typically analyzed by spectrophotometry and photometry methods. Urine samples are first acid-digested to destroy organic matter and to oxidize Sn to Sn (IV) state. Tin is also prevalent in the environment. It is measured in multi-element analysis of air, water and solid waste samples by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). However those samples that are liquid in nature such as drinking water, direct aspiration atomic absorption spectroscopy, such as EPA method 7870, may be used. Other samples such as industrial water, solid, soils or sediments require acid digestion to determine total and acid leachable metal.
Simple Pourbaix diagram The pourbaix diagrams plot of redox potential versus pH are used to describe the mobility of tin in geochemical systems.
The mobility of Sn from cassiterite is typically very low because cassiterite is very stable in the surface environment. At pH above 12, tin may be stabilized in a cassiterite solution as HSnO2– and as SnO32– in the presence of an oxidant and Tin may be removed from the solution at this region (Cox & Singer 1986, p. 379).
The Sn element is soft, pliable and silvery-white metal. It is not reactive to oxygen and water. However, it dissolves in both acids and bases. These characteristics have led to a wide range of industry use such as in solders, alloys, tinplate, among others (Anstett, Bleiwas & Hurdelbrink 1985).
References
Anstett, T. Bleiwas, D. & Hurdelbrink, R. J. 1985, Tungsten availability-Market economy countries, U.S. Bureau of Mines Information Circular 9025.
Beus, A. A. & Grigorian, S.V. 1977, Geochemical exploration methods for mineral deposits: Wilmette, Illinois, Applied Publishing Ltd., p. 287.
Cox, D.P. & Singer, D.A. 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 379. Pollard et al. 1983, ‘Metallogeny of tin, magmatic differentiation versus geochemical heritage’, Economic Geology, vol. 78, pp. 543-5. Taylor, R. G. 1979, ‘Geology of tin deposits’, Development Economic of. Geology, vol. 11, pp. 543.