The contribution of the RSCM geothermometry to detect the thermal anomalies in metasediments, induced by hidden intrusions and fluid circulation
Résumé
The study of the maturity of CM provides a relatively precise tool to estimate the thermal evolution of rocks and therefore to provide relevant constraints on the burial/exhumation history of geological formations. Both the CM structure and chemistry give information about the degree of transformation during the geological history that involves carbonization during advanced diagenesis, low-grade metamorphism and graphitization under more intense metamorphism. The characterization of the CM structure by Raman Spectroscopy can be used to estimate temperature maxima. Consequently, a new geothermometer has been developed, namely the Raman Spectroscopy of Carbonaceous Material (RSCM). This tool measures the structural evolution of the organic matter that is present in rocks. A version of the RSCM, proposed to accurately measure temperatures ranging from 330° to 650°C, was first developed by Beyssac et al (2002). Its application range was later expanded to lower temperatures between 200° and 350°C by Lahfid et al (2010).
The objective of this study is to show the accuracy of the of the RSCM geothermometer to 1) delimit the metamorphic area due to hidden intrusions and 2) to elucidate the thermal anomalies induced by fluid-flow.
For these purposes, we respectively have chosen the Guemassa massif (morocco), which includes the Hajjar Zn-Pb-Cu mine and the Lucia subterrane in the Franciscan Complex (California, USA), which includes gold-bearing quartz veins.
In the Guemassa massif, our peak temperature estimated show values superior to 500°C. These temperatures differ from the ones obtained by other classical methods, which are not higher than 450°C. Nevertheless, fluid inclusion homogenization temperatures of 450°C represent minimum trapping temperature conditions, since the fluids were trapped above boiling conditions. Also, 450°C represents minimum thermic condition for the biotite isograd (Hibti, 2001). Higher Raman temperatures obtained in this work confirm the hypothesis of a late heat flow related to a deep granitic intrusion (Hibti, 2001, Watanabe, 2002). This intrusion could be closer to the Hajjar deposit which would explain the higher Raman temperature around the mineralization.
The use of the RSCM geothermometry in the Lucia subterrane, reveals anomalous values of maximum temperature, recorded in Los Burros area (gold district). These anomalous values seem to be caused by a hydrothermal overprint in the Los Burros district.
These results suggest that RSCM could be used as a reliable tool to geothermometer to determine the thermal effect of hidden intrusions and fluid circulations in metasediments.