Metallogenetic Epochs

ECONOMIC GEOLOGY •VITH WHICH IS INCORPORATED THE AMERICA.N GEOLOGIST VoL. IV AUGUST, x9o9 No. 5 METALLOGENETIC EPOCHS. 1 WALDEMAR LINDGREN. Thedifficulties of a studentof mineraldeposits are increasing. As in petrographythe first descriptiveperiod of the scienceis drawingto a close. We knowthe principaltypes, and, though much remains to be done in detailed accounts,it seemsthat we are graduallycoming to the same"impasse" which confronts thepetrologist, who is nowturning to theexact experiments of physicalchemistry for the solutionof the riddleswhich con- front him. And as we transfer some of our most important problemsinto the sameable hands for experimentalwork at high temperaturesand pressures, some of us may confessto a slight feelingof disappointmentthat inductivework has failed to fur- nish definite answersto many of our questions. But such a stateof mind shouldonly be like the shadowof a passingcloud; for the study of mineral depositsis not a sciencedistinct from others;it leanson all branchesof geologyas well ason chemistry and physics,and this new developmentmeans only that we have taken a stepin advance;that we have acquireda new and power- ful collaborator; it means also still further specialization. It doesnot mean that the scientificservices of the mining geologist will be unnecessaryor futile. His work will be necessaryto assembleand assortthe facts, to test the resultsof the laboratory, •Read at the meeting of the Canadian Mining Institute at Montreal, March, 1909. 409 4IO I4/,4LDEM•1R LINDGREN. to propoundne•v problems,and to generalizefrom the study of wide areas. This critical examination of mineral depositsover wide areas is a work which now confronts us. ,It is the geographicand historical side of our science. With some hesitation I venture to placebefore this Institute an epitomeof the principalepochs of the segregationof metalsover our continent. Many of the factsare well knownto you and the only featureof this paperwhich can possiblymerit your attentionis the summarizingof thesefacts. This continent of North America is rich in metallic wealth; its total productionexceeds that of any otherequivalent division of the earth. Gold and silver come from the Cordilleran belt and also from the easternmargin of the continent. Copper in ever increasingquantities is derivedfrom the CentralBasin and from depositsin the west extendingfrom Sonora to British Columbia. Lead and zinc are producedfrom our vast Missis- sippi basin and from the rangesof the Rocky Mountains. Half of the world's nickel is drawn from Ontario and a large part of the tungstencomes from Colorado. The total valueof the metallicproduct of the continentin I9o8 wasabout $I,ooo,- 000,000. In some measure we have succeededin classifying the vast numberof deposits,not only with referenceto their minera- logicaland structuralcharacteristics, but alsowith referenceto theirage. We findthat metalliferous deposits have been formed sincethe earliest times of geologicalhistory. .We find also that theyare notequally distributed over the continent,but occur in metallogeneticprovinces of greatlydiffering form and extent. Moreover,these metallic concentrations have not beengoing on at equalrates throughout geologic time, but the formationof each groupcorresponds to a fairly brief epochwhich, with few ex- ceptions,is alsoan epochof vulcanism. No fact is betterestab- lishedthan this,however much we may differ in detailsof genetic history. On theother hand, regions of vulcanismdo not neces- sarilycontain ore deposits. Naturally ,aTe find the ores in regions of uplift and erosion,for herethe rocksare bestexposed, but ME T.4 LL 0 GENE T1C EP 0 CHS. 411 they are not necessarilyconnected with such warpings and cor- rugations,as is illustratedby the oft-cited poverty in metalliferous depositsof the Appalachianranges of sedimentaryrocks. Since the earliest times, then, metallogenetic epochs have recurred on the North American continent, and, comparing it with others, we must concludethat either the original sources from which the ores have been concentrated have been richer than elsewhere or the conditions for their formation have been more favorable than elsewhere. In describing these epochsit will be convenientto separate the eastern and the western halves of the continent for, with exceptionof the earliest part of their history, they have little in common. EPOCHS OF ORE DEPOSITION IN THE EASTERN PART OF THE CONTINENT. •. The Pre-Ca•nbrian Period.--The Pre-Cambrian period em- bracesa very long time and many differing epochsof ore formation; but for our presentpurposes it will be necessaryto consider it as a whole. Ages of sedimentationalternated with violent igneousaction. At many placeslong erosionpreceded the deposition of the Cambrian. The Pre-Cambrian deposits form a belt along the southernAppalachians, extending north•vard into Canada,where they are found over a •vide area. They occupy parts of Michigan, \Visconsin,and Minnesota, and reappearin the west, in South Dakota, Wyoming, Colorado, New Mexico and Arizona; possiblyalso in southwesternCalifornia, but with less of the diversity characterizingthe Lake Superior region. In the latter region there is a great successionof igneousrocks, effusiveas well as intrusive,ranging from granitesto basalts, diabasesand gabbros. In the Cordilleranregion the intrusives are almostexclusively represented and the prevailingtype is the normal red granite contrastingstrangely with the later inter- mediate type of intrusives. Smaller amounts of intrusive diorite, gabbro and diabaseare present. The metalscharacteristic of this periodare iron, copper,nickel, 412 WALDEMAR LINDGREN. gold and silver. Lead and zinc appearto be presentin far smallerquantities than in later periods. Quicksilverand anti- mony are rare. The iron ores.are, as well known, of several types; the ilmenitesand the magnetitesare chieflyof igneous origin; the hematitesof Lake Superior,according to Leith, of partly igneous,partly sedimentaryorigin, and ultimatelyoxidized and concentratedby surface waters. It is believed that this concentrationtook place mainly in Pre-Cambrian time. The copperand nickel ores are connectedwith basic igneousrocks, in part intrusive,in part, as in the Michigan copperregion, of effusive type. The concentrationof the copperof this latter region into workable depositsalso seemsto have taken place chiefly,if not wholly, in Pre-Cambriantime, and probablyfol- lowed the close of the igneousactivity in Keweenawan time. The silver veins of Cobalt, which have provedso rich, are like- wise Pre-Cambrian and some writers are inclined to connect them with Keweenawan basic intrusives into Huronian rocks. In the Southern States the deposition of the gold-bearing quartz veinsin most casesdirectly followed granitic intrusionsin various sedimentaryschists.' The assertionsof earlier writers, that these veins are genetically connectedwith basic intrusives, are not supportedby recentwork. Some silver and copperare associatedwith these gold-bearing veins. It should be added that some writers assign a Cambrian or even later age to the southern granites. Minor copper depositsin the same region are chiefly derived from intrusions of diabase or allied rocks. Pre-Cambrian gold-quartz veins are also known from Western Ontario. 2. Epoch of PaleozoicIntrusives.--From New York and Nexv England northward into Quebec and Nova Scotia granitic in- trusionstook placeat various times during the Paleozoicas late as the Carboniferousand •vere accompaniedby some metalliza- tion. The gold-quartzveins of Quebecand Nova Scotia were in part, at least,formed after theseintrusions, and varioussmall depositsin New England have a similar origin. 3. Epochsof the SedimentaryIron Ores.--Processesof sedi- METALLOGENETIC EPO,CHS. 4x3 menration under favorable conditions lead to concentration of iron ores. During the long period of unbroken Paleozoicsedi- mentation in the Appalachian region there were at least two epochswhich are characterizedby such iron ores. During the Clinton stageof the Silurian, persistentbeds of 1.owgrade hema- tite were formed; and during the Carboniferousless important layers of carbonateblack band ores were deposited. 4. Epochsof the TriassicTraps.--The importantperiod of the history of the igneo-geneticore depositsof the easternpart of the continentclosed in the late Paleozoicand the majority of these depositshad been formed much earlier. A feeble recur- rence of ore formation took place during the early Mesozoic, when the traps of the Eastern States were injected as sheetsor overflowedas lava streams. Smaller massesof iron and copper ores developedalong the igneouscontacts, in part as veins, in part as contactmetamorphic deposits. 5. Cretaceous,Tertiary, or Later Epochsof Zinc and Lead Concentra.tion.--Sincethe Triassic, vulcanismhas rested and in the eastern part of the continent metal depositshave formed only by the concentratingpower of flowing surface waters or of groundwaterin decayingrocks, or of ascendingwaters of atmosphericorigin. In the central valleys, such ore concentrationhas been effected in comparativelylate time, apparentlyindependent of vulcanism, and has resulted in the most important lead and zinc deposits on the continent. Regardingthe modeof origin of thesedeposits opinions differ widely, but most observersbelieve that the lead and zinc has beenleached by surfacewaters from Paleozoiclime- stoneswhich derivedtheir sparselydisseminated content from the pre-Cambriannorthern continent. Some regard the processas a simpleaffair of downwardpercolating

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