Travertine deposition and diagenesis in Ca-deficiency perched hot spring systems: A case from Shihuadong, Tengchong, China

Travertines are generally considered as carbonate deposits related to Ca-rich hot springs. Ca-deficiency hot springs, however, can also produce travertines under suitable conditions and little is known about the travertine deposition and diagenesis in these systems. Therefore, we studied a fossil perched Ca-deficiency spring system at Shihuadong, China to discuss its genesis and diagenesis. This systemis a platform-like build-up with travertines developing mainly in the steep frontal area. The travertines were deposited from high-temperature fluids (dominantly 45 degrees C to 55 degrees C) with strong interaction mostly with underground Gaoligong Group metamorphic rocks. Their parent CO2 was originated from magmatic CO2 and a little soil CO2. The travertine-depositing paleofluid was predominantly Ca-deficiency hot spring water, but stream water possibly from cold springs on surrounding hills might make a small contribution. The low calcium levels and flat substrate on the top area of the studied system handicapped the travertine deposition, only developing a small perched travertine channel entirely composed of crystalline crust. On the contrary, progressive CO2 degassing in the top area and increasing CO2 degassing and water flow rates in the frontal area induced the formation of abundant travertines. However, water scattering/dispersion in the frontal area might not heavily increase the water flow rate and only induced boundstone deposition. After the travertine deposition, underground thermal water, meteoric water, and microbial activity progressively altered the primary fabrics, forming mosaic and need fiber calcite cements, dissolution pores, microborings, and dissolution-induced and microbe-induced micritization. The studied perched spring system shows limited scales, notable progradation but slight aggradation, and the dearth of distal autochthonous travertines, unlike those formed by Ca-rich hot springs, which might extend widely and produce striking distal travertine deposits away from the frontal slope/waterfall. These findings suggest the significant roles of water chemistry and local topography in travertine deposition and the influence of slowly underground thermal water seeping on the diagenesis of travertines in hilly areas and might be used to recognize fossil Ca-deficiency and Ca-rich hot spring systems. (C) 2020 Elsevier B.V. All rights reserved.