1 - Mines Magazine - Colorado School of Mines

ni(;kel, molybdenum and uranium concenti'ations.Some comparisons of the metal content of plant ashfrom non-mineralized and mineralized areas as shownby Ginzburg (3) are given below:Noji-Mineralizeti Mineralized Degree ofiRiemeiit area area AccumulationppmppmH) (2) (2-1)C,r 5 100 20Mn 100 10,000 1000Co 4 50 12^N'i 10 100 10Cu 50 1000 20Zn 100 10,000 iOOMo 5 100 20Pb 1 100 100U 0.01 JOO 10,000In this country, the search for uranuun in the ColoradoPlateau was materially aided by mapping thedistribution of the indicator plant commonly knownas the "loco weed" (Astragalus pattersoni). Becausethis perennial thrives in selenium rich soils known tobe associated with uranium, theh location has oftenled to ore bodies of uranium. Four ore bodies out of10 discovered in the Yellow Cat area, Utah, for example,would have been missed but for the additional informationfurnished by the plant. In the Thompson(Ustrict of Utah, of 190 holes that were drilled in areas•supporting the growth of this plant, 90 were in mineralizedground.The choice between using plants or soil samples,depends largely on the field problem. However, it isfar easier to collect and process soils than plants. Thelatter need be identified,- dried, ground and ashed,usually below 550° C. before analysis is bcg-un, a processwliich is time consuming.Stream WatersMetals may enter thc drainage as detritai particles,suspensions, colloids or ions. The distance they traveldepends upon a number of factors: as ions, they aredependent upon the pH of the stream waters, whichusually run between 6 and 8. When the pld of hydrolysisof a given metal is exceeded, the metal is removedfrom solution: lead hydrolyzes at 6.0, cupric copperat 5.3, and zinc above 7.0. Despite this laboratory fact,copper and zinc are found in waters whose pH is abovethen- respective hydrolysis points. In a semi-arid areamolybdenum sulphide is believed to be first oxidizedto a soluble MoO^SO., whereupon it reacts with calcareousrocks to form CaMo04. The latter is found tobe quite insoluble between pH 3 to 9, and yet molybdenumis foimd widely distributed in waters, soils andplants whose pH environment is about 7 (7). Thesuggestion that metals may travel as complex inorganicand organic ions, with diiferent chemical propertiesseems valid.Changes in water volume, caused by seasonal anddaily rainfall may frequently dilute the metal concentrationto below the sensitivity of many water tests.Or alternatively, raising the level of the water table,may sometimes result in the flushing out of oxidizingore metals into the drainage with subsequent highmetal readings. The problem of interpretation of metalvalues in waters may thus become exceedingly difficult.The accumulation of metals by ground water inareas of mineralization and in areas of background(non-mincralization) are shown in the following tablefrom Ginzbtu'g (3).f Concentration fin Piirts Per Billion) of Surfaceand Underground Dj'ainageA'fetal Baekg]-ound MineralizedXi 1 to 10 10 to iOOO(]o 0.1 to 10 10 to 1000Zn 0.1 to to 10 to 1000Cu 1 to iO 10 to 1000\J 0.01 to 10 10 to 1000Mo 0.1 tn 1 100Pb 0.1 to 1 10 to 1,000Stream SedimentsClosely alfied to water sampling is the use of streamsediments. Metals carried by waters are lost to sedimentsby CO precipitation, sorption or ion exchangewith clays. Once removed, they are found to be looselyheld by the sediments and can be removed by coldextraction with acetate, citrate or dilute acid solutions.This has served as the Ijasis for the simple chemicalprocedtu'os described under "(chemical Procedures"below.Sediment sampling techniques have certain advanfagesover water sampling (5): (a) local dilution eft'ectsare minimized, (b) dry stream channels may be tested,(c) thc chemical procedures arc simpler, and (d) samplesmay be stored for future reference.Chemical ProceduresChemical procedures used in geochemical prospectingmay be broadly classified on thc basis of the completenessof sample solution, into three groups; (A) Solutionof primary rock-forming minerals usually silicates, bycausti