Cathodoluminescence, fluid inclusions, and trace element data for the syntaxial quartz cementation in the sandstones of the Ora Formation, northern Iraq

Abstract

Quartz cements of the quartz arenitic sandstones from the Chalky Nasara and Ora sections of the (Devonian-Carboniferous)Ora Formation in northern Iraq have been studied. A combination of hot cathodoluminescence, LA-ICP-MS, and fluid inclusionmicrothermometry revealed three syntaxial quartz cement generations (Q1, Q2, and Q3). The early Q1 cementation has gray to slightlybrown luminescences, postdated compaction, and reduced intergranular porosity associated with illite formed during eogenesis. Q2 ischaracterized by dark brown luminescence overgrowths and is more voluminous in the thinly bedded sandstones than in the thicklybedded sandstones filling most of the remaining pore space during mesogenesis. Q3 was formed during the early telogenesis stage fullycementing the sandstones and the fractures were filled by hydrothermal chlorite and sulfides. Significant amounts of trace elements Al,Li, Ge, and Fe have been detected in quartz overgrowths. Al varies consistently between each cement with averages of 7125, 4044, and2036 ppm for the Q1, Q2, and Q3 generations, respectively. A strong linear correlation between Al and Li in the three quartz cementswith an average Li/Al of ~0.02 in Q1 and Q2 indicates sufficient availability of both Al and Li where Li is most likely to be found in highsaline pore waters. Illite is the most probable origin of Li since high salinities favor the mobilization of Li during diagenesis. Germaniumconcentrations in quartz cements are slightly less than that in the detrital quartz of the Ora Formation, indicating that the pressuredissolutions of quartz and feldspar are the dominant sources of cementation in the Ora Formation. Homogenization temperatures offluid inclusions indicate precipitation of the Q1, Q2, and Q3 cement generations at temperature ranges of 155–160 °C, 160–166 °C,and 168–178 °C, respectively, with salinities ranging between 5.0 and 6.4 wt.% NaCl equiv., as an indication of hydrothermal burialconditions for Q3 cement, which was affected by the major Zagros Thrust Zone faulting.