Search Results (3)

Geochemical overprinting on placer gold deposits produces complex deposits with apparently paradoxical geochemical and morphological attributes. The La Cholla placer gold deposits near Quartzsite, Arizona, USA, contain sharp and often undamaged gold crystals within high-energy gravels as a result of postdepositional recrystallization by localized hot springs. Geothermal fluids emanated from a basin-bounding fault and left a distinct signature that includes recrystallized gold with low Pb and δ⁶⁵Cu > 1.14‰, secondary uraninite and carnotite, travertine deposits, and mounds of highly silicified placer gravels. Surrounding these deposits are placer gravels with angular gold morphology and geochemistry indicative of flash-flood pulse placers with a lode gold source < 5 km and gold particles with low Pb and δ⁶⁵Cu < 0.77‰. The multitude of small local lode gold source veins is likely related to specialized fluid migration along mid-Tertiary detachment faults. The unroofing of these veins at 5 to 17 Ma by high-angle Basin and Range faulting led to extensive placer development, with subsequent localized modification by geothermal fluid migration along the same faults. This study documents geochemical overprinting and modification of placer deposits and bears upon the study of other placers containing crystalline gold and uranium mineralization. Similar deposits may document past geothermal activity and identify structural fluid conduits and steep paleo-topographic gradients. Full article

Travertine spring mounds are common in Slovakia; however, their age and depositional temperature are still poorly known. Our study is the first multimethodological investigation involving stable carbon, oxygen, and clumped isotope (Δ₄₇) analyses and U-Th age determination of travertine mounds from different locations in Slovakia (Santovka, Dudince, Čerin, Bešeňová, Liptovský Ján, Liptovské Sliače, Vyšné Ružbachy, Gánovce, and Sivá Brada) to provide information about their age, origin, precipitation conditions, and temperature. The positive δ¹³C values imply that the parent water was charged with heavy CO₂ of deep origin. The δ¹⁸O values of spring waters range between −11.4‰ and −8.9‰, whereas the δ²H values vary from −80.5‰ to −58.3, indicating a meteoric origin for spring waters. Clumped isotope compositions (Δ₄₇) correspond to a deposition temperature between 4 °C and 32 °C. The U-Th age data of the studied travertines vary from 1.2 (Liptovské Sliače) to 301 ka (Dudince). Our results can serve as a basis for further detailed geochronological and geochemical studies to reconstruct the paleoclimate and paleoenvironment during travertine deposition periods in Slovakia since the mid-Pleistocene transition. Full article

Travertines formed of crystalline crust have been widely reported, but there has not been focus on their geochemical characteristics. We therefore carefully conducted a series of geochemical investigations and U-Th dating on a travertine mound mainly composed of crystalline crust from Sobcha (southwest China) to determine their geochemical features and geological implications. The Sobcha travertines dominantly consist of granular crystals and fan crystals and show δ¹³C from 3.4‰ to 4.9‰ V-PDB, δ¹⁸O from −26.7‰ to −23.7‰ V-PDB, and ⁸⁷Sr/⁸⁶Sr from 0.712458 to 0.712951. When normalized to PASS, the Sobcha travertines exhibit MREE enrichment relative to HREE and LREE, HREE enrichment relative to LREE, and positive Eu anomalies. The δ¹³C signatures and mother CO₂ evaluation of the Sobcha travertines show that the Sobcha travertines were thermogene travertines largely receiving mother CO₂ from (upper) mantle (i.e., magmatic CO₂) or a mixture of soil-derived CO₂ and CO₂ related to carbonate decarbonation. The ⁸⁷Sr/⁸⁶Sr of the Sobcha travertines is out of the ⁸⁷Sr/⁸⁶Sr ranges of local deposits exposed at Sobcha and surrounding areas but is well matched with the mean ⁸⁷Sr/⁸⁶Sr of Nadi Kangri volcanic rocks which cropped out to the northeast of the studied travertines (over 20 km away). This might indicate the important role of the Nadi Kangri volcanic rocks in suppling Sr to the studied travertines, but more studies are required. The LREE depletion compared to MREE and HREE in the Sobcha travertines was interpreted to be caused by the difference in geochemical mobility between LREEs and HREEs during water–rock interaction at depth, while the MREE enrichment compared to HREE was considered to be most likely inherited from reservoir/aquifer rocks. The positive Eu anomalies of the Sobcha travertines may result from very high reservoir temperatures and/or preferential dissolution of Eu-rich minerals/rocks (especially plagioclase). The Sobcha travertine mounds displays no or very slight vertical variations in δ¹³C, ⁸⁷Sr/⁸⁶Sr, and REE patterns, indicating the compositional stability of mother CO₂ and paleo-fluids. However, a significant vertical increase in δ¹⁸O was observed and was explained as the result of gradual water temperature decrease related to climate cooling, self-closure of the vents, or mound vertical growth. The findings in this study might help us better understand the deposition of crystalline crust in Ca²⁺-deficient hot spring systems. Full article