Geological Fluid Mapping in the Tongling Area: Implications for the Paleozoic Submarine Hydrothermal System in the Middle‐Lower Yangtze Metallogenic Belt, East China

Abstract

Abstract: The Tongling area is one of the 7 ore‐cluster areas in the Middle‐Lower Yangtze metallogenic belt, East China, and has tectonically undergone a long‐term geologic history from the late Paleozoic continental rifting, through the Middle Triassic continent‐continent collision to the Jurassic‐Cretaceous intracontinental tectono‐magmatic activation. The Carboniferous sedimentary‐exhalative processes in the area produced widespread massive sulfides with ages of 303–321 Ma, which partly formed massive pyrite‐Cu deposits, but mostly provided significant sulfur and metals to the skarn Cu mineralization associated with the Yanshanian felsic intrusions.To understand the Carboniferous submarine hydrothermal system, an area of about 1046 km2 was chosen to carry out the geological fluid mapping. Associated with massive sulfide formation, footwall sequences 948 m to 1146 m thick, composed of the Lower Silurian‐Upper Devonian sandstone, siltstone and thin‐layered shale, were widely altered. This hydrothermal alteration is interpreted to reflect large‐scale hydrothermal fluid flow associated with the late Paleozoic crustal rifting and subsidence. Three hydrothermal alteration types, i.e., deep‐level semiconformable silicification (S1), fracture‐controlled quartz‐sericite‐pyrite alteration (S2–3), and upper‐level sub‐discordant quartz‐sericite‐chlorite alteration (D3), were developed to form distinct zones in the mapped area. About 50‐m thick semiconformable silicification zones are located at ∼1‐km depth below massive sulfides and developed between an impermeable shale caprock (S1) and the underlying Ordovician unaltered limestone. Comparisons with modern geothermal systems suggest that the alteration zones record a sub‐seafloor aquifer with the most productive hydrothermal fluid flow. Fracture‐controlled quartz‐sericite‐pyrite alteration formed transgressive zones, which downward crosscut the semiconformable alteration zones, and upwards grade into sub‐discordant alteration zones that enveloped no economic stringer‐stockwork zones beneath massive sulfides. This transgressive zone likely marks an upflow path of high‐flux fluids from the hydrothermal aquifer. Lateral zonation of the sub‐discordant alteration zones and their relationship to overlying massive sulfide lenses suggest lateral flows and diffusive discharging of the hydrothermal fluids in a permeable sandstone sequence. Three large‐sized, 14 middle‐small massive sulfide deposits, and 40 massive sulfide sites have been mapped in detail. They show regional strata‐bound characters and two major styles, i.e., the layered sheet plus strata‐bound stringer‐style and the mound‐style. Associated exhalite and chemical sedimentary rock suites include (1) anhydrite‐barite, (2) jasper‐chert, (3) Mg‐rich mudstone‐pyrite shale, (4) barite lens, (5) siderite‐Fe‐bearing dolomite, and (6) Mn‐rich shale‐mudstone, which usually comprise three sulfide‐exhalite cyclic units in the area.The spatial distribution of these alteration zones (minerals) and associated massive sulfides and exhalites, and regional variation in δ34S of hydrothermal pyrite and in δ18O‐δ34C of hanging wall carbonates, suggest three WNW‐extending domains of fluid flow, controlled by the basement faults and syn‐depositional faults. Each fluid domain appears to have at least two upflow zones, with estimated even spacing of about 5–8 km in the mapped area. The repeated appearance of sulfide‐sulfate or sulfide‐carbonate rhythmic units in the area suggests episodically venting of fluids through the upflow conduits by breaking the overlying seals of the hydrothermal aquifer.