Geochemistry and fluid evolution of a Carboniferous-hosted sphalerite breccia deposit, Isle of Man

Base-metal-sulfide deposits were mined historically on the Isle of Man, principally from quartz-vein deposits in Ordovician-Silurian age metasedimentary rocks. During a study of dolomitization of overlying Carboniferous sedimentary rocks, an extensive (140 m long, 1-10 m wide) sphalerite-bearing breccia deposit (~7 wt.% Zn) was found. Mineralization is comprised of four stages, marked by episodes of deformation and abrupt changes in fluid temperature and chemistry. In Stage I, high-temperature (T_h = 300-400˚C), high-salinity (20-45 wt.% equiv. NaCl) fluids of likely basement origin deposited a discontinuous quartz vein, which was dismembered subsequently during a major brecciation event. Stages II-IV represent open-space-filling sphalerite, quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105-180˚C and salinities of ~15-20 wt.% equiv. NaCl. The d³⁴S values of sphalerite and galena (3.0-6.5‰) have the same signature as ore sulfides from mines in the metasedimentary basement.

Calculated d¹⁸O_water values for minerals in the breccia indicate two main fluid sources, local Carboniferous-hosted brines (~2.0-6.0‰) and basement-derived fluids (~5.0-9.0‰), whose relative importance varied in space and time. The fluid history of the breccia is compatible with introduction of a basement-derived fluid that displaced local sedimentary brines. As the hydrothermal system waned, the relative importance of brines within the Carboniferous section re-emerged, resulting in Stage IV breccia-cementing dolomite.

A hydrologic numerical simulation suggests that basement-derived (~180˚C) fluids with upward flow rates of ≥ 1 m/yr could have migrated vertically along a fault and penetrated into the surrounding sedimentary units, raising the temperature in the area of the breccia by ~50˚C. Cooling of basement-derived fluids as they flowed across the resultant thermal gradient was a likely mechanism of sphalerite precipitation.

Recognition of this type of mineralization within the Carboniferous sedimentary rocks suggests that similar deposits may be present elsewhere in the carbonate section of the Isle of Man.