Textural and Isotopic Characteristics of Tectonic Hydrothermal Breccias in Fractured Carbonates in Northeast Thailand, a Contrast with the Features of Meteoric Karst Breccias

Abstract

Fractured and cemented Permian carbonates in a quarry in Chumpae region, Khon Kaen district, Northeast Thailand, reveal two fault-related hydrothermal breccia zones with distinctive features that can be used to differentiate fault-related hydrothermal breccias from meteoric karst breccias. Mesoscopically (core-observation relevant scale), hydrothermal breccias possess a set of distinctive features that include: calcite-encased sharp angular edge boundaries, rotated fitted clasts, offsetting calcite-filled fractures and calcites following (folded) bedding planes, with distinct low spectral gamma values; whereas, meteoric karst breccias edges tend to show rounded solution-enlarged boundaries, calcitic speleothems or soils passively penetrating the host rock along planes of pre-existing weakness (solutionenlarged joints and fractures or sub-cropping fault intersections with the land surface). Isotopic data, from the matrix and calcite vein cements in the hydrothermal breccias, exhibit two main burial diagenetic trends, namely; 1) A regional burial trend that is the same as the regional isotopic burial trend seen in Ratburi limestone across central and northern Thailand with increasingly negative values of oxygen and mostly positive, but decreasing values of carbon with time; and 2) Another trend or plot field shows aligned values of both oxygen and carbon that are all negative, defining a burial-related mixing of the rock matrix with a cementing fluid derived from an organic-influenced fluid source. This fluid is considered as related to pressurised fluid crossflows and entry into hydrofractured rock. This distinctive fluid/faulting event is diagenetically later than diagenetic fluids of Indosinian burial; it may relate to the stresses driving the rotation of the Khorat Plateau during the Himalayan Orogeny.