THE IMPORTANCE OF EARLY QUARTZ CEMENTATION EVENTS IN THE EVOLUTION OF AQUIFER PROPERTIES OF ANCIENT SANDSTONES: ISOTOPIC EVIDENCE FROM ION PROBE ANALYSIS OF SANDSTONES FROM THE MCARTHUR, ATHABASCA, AND THELON BASINS

Quartz cementation plays a major role in controlling the hydrologic properties of sandstones; it effectively occludes pores and decreases permeability. Because sedimentary rock-hosted economic deposits involve fluid flow at critical intervals during their burial history, it is crucial to understand the timing of pore-occluding phases. Most quartz cementation occurs at >2 km burial depth where pressure solution generates silica. We have, however, observed quartz cements in 3 Proterozoic basins (the Athabasca and Thelon basins, Canada, and the McArthur Basin, Australia) which host important mineral deposits (U, Pb, Zn, etc.) that exhibit petrographic characteristics suggesting quartz cementation began at shallow depth before mineralizing fluids were present. To estimate the depth of quartz cementation, we coupled quartz cement stratigraphy (from petrography) with high precision in situ ion probe-generated d¹⁸O data (n=81) from 10 µm areas. Conventional analysis of quartz overgrowths is difficult due to the small size of the overgrowths, and usually mixes detrital and authigenic phases that results in burial depth estimates ranging from 3-5 km. Isotopic data presented here is from specific, small-scale individual phases including isopachous, fringing cement, and blocky pore-filling quartz that preserves pre-compaction grain-to-grain spatial relationships and sometimes exhibits later overgrowths. d ¹⁸O values from the early isopachous (25 µm thick) fringing phase are as high as 33.2 (SMOW) o/oo, with a mean of 27.1 o/oo. These data along with petrographic relationships indicate that this phase precipitated near the surface. d¹⁸O values from early blocky quartz cement range from 20.1 to 24.5, with a mean of 22.4 o/oo, and are overlain by cements with much lower d¹⁸O values. The d¹⁸O data show that early quartz cements in these Proterozoic basins began at or near the surface at < 1 km burial depth. Reports of such early phases are rare due to the limitations of existing techniques to resolve individual phases. Understanding the timing of pore-occluding quartz cement is important in the case of ancient sandstones because these cement phases can transform aquifers into diagenetic aquicludes and can potentially either stop or redirect mineralizing fluid flow.