par Eberhardt, Peter;Geiss, Johannes;Houtermans, F.G. F.G.;Signer, Peter;Jedwab, Jacques 
;Cahen, Lucien
Référence Geochimica et cosmochimica acta, 14, 1-2, page (134-149)
Publication Publié, 1958-08
;Cahen, Lucien Référence Geochimica et cosmochimica acta, 14, 1-2, page (134-149)
Publication Publié, 1958-08
Article révisé par les pairs
| Résumé : | It is now well known that the isotopic composition of common lead varies in a significant way with the age of the lead occurrence. Providing certain assumptions are made, this isotopic composition can be used for an estimate of the age of the mineral. This "model" or "conventional" age is often in accordance with other evidence. However there exist a certain number of leads which do not conform to the systematic pattern generally observed. The purpose of this study is to investigate certain aspects of the geology and geochemistry of those leads, the model ages of which are older than the geological age. Their isotopic composition usually appears to be perfectly normal and if the age of the host formation or of the emplacement of the lead occurrence is unknown it may be impossible to detect which leads belong to this type, so that the search for criteria enabling to recognize them is of some importance. Twenty leads from the Alps and twenty-four from North Africa have been studied. In each case the model age, derived from the isotopic composition, has been compared to the age of the host formation or that of the emplacement of the galena. The silver content (and that of some trace elements) of each of these galenas has been measured and compared to the abovementioned ages. Practically all galenas we have investigated that have model ages comparable to the age of the host formation are found in veins, often with more or less obvious magmatic connexions and they are silver rich. On the other hand the leads of those occurrences, located in sedimentary rocks, and apparently independent of igneous activity, are poor in silver and have model ages which are older than the host formation. The origin of the latter, "telethermal", type of occurrence has often been debated and it has been suggested that these occurrences are "rejuvenated" from an older source of ore. This seems to be borne out by the fact that it is precisely occurrences of this type which contain leads with model ages older than the age of the host formation. It thus seems that when the age of the host formation is known, isotope analyses of lead minerals and the model age derived therefrom, constitute objective criteria enabling us to distinguish which occurrences are emplaced for the first time and which are derived or rejuvenated. In formations of unknown age, a similar criterion which might furnish the same information appears to be the silver content of the galena. In each of the areas studied, we have found that, as regards silver content, there is a sharp distinction, with little or no overlap, between leads with model ages in agreement with the age of the host formation and those with model ages older than that of the host formation. © 1958. |
