GMAT–RC结论说明类1
AmyHou
;OG上结论说明文章范例;结论说明文章特征;Accordingtoarecenttheory,Archean-agegold-quartzveinsystemswereformedmorethantwobillionyearsagofrommagmatic?uidsthatoriginatedfrommoltengranite-likebodiesdeepbeneaththesurfaceoftheEarth.Thistheoryiscontrarytothewidelyheldviewthatthesystemsweredepositedfrommetamorphic?uids,thatis,from?uidsthatformedduringthedehydrationofwetsedimentaryrocks.
Therecentlydevelopedtheoryhasconsiderablepracticalimportance.MostofthegolddepositsdiscoveredduringtheoriginalgoldrusheswereexposedattheEarth’ssurfaceandwerefoundbecausetheyhadshedtrailsofalluvialgoldthatwereeasilytracedbysimpleprospectingmethods.Althoughthesesamemethodsstillleadtoanoccasionaldiscovery,mostdepositsnotyetdiscoveredhavegoneundetectedbecausetheyareburiedandhavenosurfaceexpression.;Thechallengeinexplorationisthereforetounravelthesubsurfacegeologyofanareaandpinpointthepositionofburiedminerals.Methodswidelyusedtodayincludeanalysisofaerialimagesthatyieldabroadgeologicaloverview;geophysicaltechniquesthatprovidedataonthemagnetic,electrical,andmineralogicalpropertiesoftherocksbeinginvestigated;andsensitivechemicalteststhatareabletodetectthesubtlechemicalhalosthatoftenenvelopmineralization.However,noneofthesehigh-technologymethodsareofanyvalueifthesitestowhichtheyareappliedhavenevermineralized,andtomaximizethechancesofdiscoverytheexplorermustthereforepayparticularattentiontoselectingthegroundformationsmostlikelytobemineralized.Suchgroundselectionreliestovaryingdegreesonconceptualmodels,whichtakeintoaccounttheoreticalstudiesofrelevantfactors.
Thesemodelsareconstructedprimarilyfromempiricalobservationsofknownmineraldepositsandfromtheoriesofore-formingprocesses.Theexplorerusesthemodelst