A Cretaceous siliceous sinter in NE China: Sedimentological and geochemical constraints on its genesis

Abstract

Phanerozoic sinter deposits have been reported globally, with their identification mainly based on sedimentological, petrological, and mineralogical studies. In this study, a detailed geochemical investigation, combined with sedimentological characterization, was conducted on the Wugonglilu siliceous deposit, a Cretaceous (106 Ma) sinter in NE China, to examine its genesis. The deposit is inferred as a siliceous hot spring deposit (sinter) owing to its geological setting in an epithermal gold-mining district, its morphologically variable microbial textures typical of subaerial sinters, and nearly pure silica composition. Three lithofacies were recognized in this study, including laminated to thinly bedded sinter, silica-cemented breccia, and epithermal vein, which represent deposits from three contrasting hydrothermal environments. The ⁸⁷Sr/⁸⁶Sr ratios of the sinter are similar to the bedrock through which the geothermal fluids passed, namely the Upper Triassic Dajiahe Formation (T₃dj), a marine siliceous rock unit, and the Lower Cretaceous Datashanlinchang Formation (K₁d), a rhyolite unit. Results indicate that both underlying units are possible Sr sources of the silica in the Wugonglilu sinter. The REE + Y (rare-earth elements and yttrium) patterns of the sinter exhibit significant variability, primarily due to terrestrial detrital contamination. Differential REE + Y contamination by terrestrial detritus was striking in the white and dark laminae/beds of the laminated to thinly bedded sinter. The white laminae/beds, generally less contaminated, were probably formed during the dry season, whereas the dark laminae/beds exhibiting greater detrital contamination were likely formed during the wet season. When excluding the samples strongly contaminated by detritus, the sinter exhibits Y anomalies comparable to Post-Archean Australian Shale (PAAS), and to the adjacent underlying K₁d rhyolite and T₃dj siliceous rock samples, indicating that these anomalies were not inherited from rocks dominating the reservoirs. Furthermore, the sinter shows a (Ce/Ce*)_N range close to that of the underlying marine siliceous rock and rhyolite samples, reflecting that the (Ce/Ce*)_N of the sinter is largely inherited from the reservoir rock(s). The overall REE + Y patterns of the sinter range from nearly flat to LREE-depleted, similar to some samples of the K₁d rhyolite and T₃dj siliceous rock units. However, most samples of the K₁d rhyolite and T₃dj siliceous rock display (Pr/Tb)_N and (Pr/Yb)_N higher than many studied sinter samples. This is likely because of the formation of strong HREE–carbonate complexes during groundwater migration. Based on these results, a generalized formation model of the studied sinter system was constructed. This study suggests that integrated sedimentological and geochemical investigations can aid in interpreting the genesis of sinter deposits, in particular the link between sinter deposits and corresponding reservoir rocks.