Sr-isotopic composition of siderite for assessing the origin of mineralizing fluids: the case study from the Jebel Awam deposit (Central Morocco)

The Sr-isotope ratios were determined for 17 samples of siderite from the p olymetallic deposit of Jebel Awam, formed because of a granitic intrusion t hrough metasedimentary Palaeozoic rocks about 286 Ma in the eastern part of the Moroccan Meseta. The isotopic ratios range from 0.712829 to 0.713960. As siderite contains negligible Rb, the measured isotopic ratios are the sa me as its parent fluids. In this view, the isotopic ratios of the carbonate rule out that the mineralizing solutions were composed of either magmatic fluids, as the local granites show a range of initial ratios between 0.706 and 0.709, or seawater, whose isotopic ratio about 286 Ma was 0.7075. On th e contrary, the mineralizing fluids were composed of meteoric water circula ting mainly through the schists of the Palaeozoic basement, whose isotopic range was between 0.712 and 0.718 at the time of the ore deposit formation. Moreover, the Sr-isotope ratios of siderite rule out any possible recrysta llization of the carbonate during the Alpine orogenesis. As a whole, this s tudy shows the possibility of using siderite for determining the type of fl uids involved in ore deposits, provided that the age and the Sr-isotope rat ios of the rocks of the mineralized area are known. This may be useful to a ssess also the provenance of the metals carried by the fluids. (C) 2000 Pub lished by Elsevier Science B.V.