Evidence for the origin and age of some mineralized Laramide intrusives in the southwestern United States from strontium isotope and rubidium-strontium measurements

Initial Sr ⁸⁷ /Sr ⁸⁶ values in the range 0.706-0.708 are reported for i) Laramide intrusive porphyries associated with major copper deposits in Arizona, New Mexico and Utah, ii) an intrusive Laramide stock associated with lead-silver mineralization in the Sawatch range, Colorado, iii) the Cripple Creek gold-silver deposit. Rb-Sr ages of micas from these rocks fall in the range 53-73 million years. The whole rock Sr ⁸⁷ /Sr ⁸⁶ ratios in Laramide times of five representative samples of Precambrian (ca. 1,600-1,700 million years) granite-gneiss country rock in the Sawatch Range, Colorado, fall in the range 0.76-0.80, which is probably close to the average range for the whole basement complex. It is concluded that the intrusive rocks did not originate by fusion of ancient, sialic basement, but must have developed in an environment with Rb/Sr and Sr ⁸⁷ /Sr ⁸⁶ ratios as low, or nearly as low, as the source regions of basalt.