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

Abstract

The Sr-isotope ratios were determined for 17 samples of siderite from the polymetallic deposit of Jebel Awam, formed because of a granitic intrusion through 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 same 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 the contrary, the mineralizing fluids were composed of meteoric water circulating 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 recrystallization of the carbonate during the Alpine orogenesis. As a whole, this study shows the possibility of using siderite for determining the type of fluids involved in ore deposits, provided that the age and the Sr-isotope ratios of the rocks of the mineralized area are known. This may be useful to assess also the provenance of the metals carried by the fluids.