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Presents a brief discussion of the assumptions, accuracy and precision of 'geological' errors present in physical geochronometry. The five major physical methods of rock and mineral age determinations currently in use, are described.

The isotopic composition of lead in Red Sea chloride brine containing 0.5 ppm Pb is found to be similar to that of some Cenozoic ore leads such as galena at Rabigh in Saudi Arabia that may have formed during mineralization accompanying Tertiary rifting. Bir Ranga galena in Miocene sediments from United Arab Republic (Egypt) is also isotopically similar to lead in Red Sea brine. The chlorine brine must be considered a possible mineralizing fluid. Lead isotopes show promise for use in mineral prospect evaluation in that galena from Samrah is isotopically similar to that from Mahd adh Dhahab, which has been the only ore producer in Saudi Arabia since 1945. Drilling at Samrah does indicate a possible economic mineralization. The lead isotope data coupled with available geologic knowledge and geochronometry are used to tentatively divide the ore prospects of the Kingdom of Saudi Arabia into relative categories of mineralization age. Two Mesozoic and Cenozoic mineralizations are distinguished on the basis of a 207 Pb/ 204 Pb difference; an early Paleozoic mineralization grouping is outlined; and a late Precambrian mineralization period is suggested.

The potassium-argon method of geochronometry may be applied quantitatively to biotite, hornblende and sanidine if these minerals have been kept at low temperature since their formation. Fine-grained whole rocks, particularly of mafic varieties, may also have 100 % retentivity under suitable conditions. Other whole-rock samples may give 75 – 85 % of the true age. Young volcanic glasses may also be used for dating if they have not been devitrified but probably give somewhat low ages even in the best cases.

Samples from petrographic collections were analyzed with the potassium- argon method of geochronometry. The implicattons of the results with respect to the geological history of eastern Greenland are discussed. (D.J.C.)