Significance of Trace Element Composition of Quartz Cement as A Key to Reveal the Origin of Silica in Sandstones: An Example from the Cretaceous of the Barrow Sub-Basin, Western Australia

Summary

Trace elements have been used to constrain the origin of silica in quartz cement: one of the most abundant authigenic minerals in marine sandstones of the Lower Cretaceous Barrow Group, Barrow Sub-basin, Australia. Petrographic, fluid inclusion, electron microprobe and cathodoluminescence data from quartz cements in these sandstones indicate multiple stages of cementation at different temperatures. Three quartz cementation phases were detected by using the cathodoluminescence imaging. Homogenization temperature calculations from fluid inclusions indicate precipitation temperature in a range of 60–140°C; phase I between 60 and 90°C; phase II between 90 and 105°C; phase III between 105 and 140°C. Oil stains and bitumen coat the second phase of quartz overgrowths and were trapped between the middle and the late phases, implying that the late phase formed after oil migration commenced. Trace element concentrations and mass balance calculations suggest two predominant sources for silica: feldspar alteration and pressure dissolution. Aluminium varies consistently between each cement phase with an average 245 p.p.m. for phase I, 805 p.p.m. for phase II and 89 p.p.m. for phase III. Textural analysis and depth-related patterns suggest that silica in the two first phases was probably a by-product of feldspar alteration reactions whilst the last phase was probably a result of pressure dissolution. The lack of correlation between aluminium content and temperature implies that the primary source of the silica is the main control on the aluminium content of quartz cement.