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

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Base-metal sulfide deposits have been mined historically on the Isle of Man, principally from large quartz-vein deposits hosted in Ordovician-Silurian age metasedimentary rocks. During a regional study of dolomitization of the overlying Carboniferous sedimentary section, an extensive sphalerite-bearing breccia (7.4 wt. % Zn) was found. This study investigates the relationship of this unusual deposit to deposits in the underlying metamorphic basement rocks, through a combination of petrographic, cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modeling techniques. Mineralization of the breccia deposit is comprised of four discrete paragenetic stages, which are marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In Stage I, high-temperature (T[subscript h] = 300[degree sign] to 450[degree sign]C), high-salinity (20.0 to 45.0 wt. % equiv. NaCl) fluids of likely basement origin deposited a discontinuous quartz vein. This vein was subsequently dismembered during the major breccia event. Stages II through IV represent open-space-filling sphalerite, quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105 [degree sign] to 180[degree sign]C and salinities of ~15 to 20 wt. % equiv. NaCl. The c34S values of sphalerite and galena (~3.0 to 6.5[per mille sign]) have the same signature as ore sulfides from mines in the metasedimentary basement rocks. Calculated [lowercase delta][superscript 18]O[subscript water] values, based on analyses of minerals in the breccia, indicate two main fluid sources, local Carboniferous-hosted brines (~2.0 to 6.0[per mille sign]) and basement-derived fluids (~5.0 to 9.0[per mille sign]), whose relative importance varied in space and time. The fluid history of the breccia is compatible with the introduction of a hot, basement-derived fluid that displaced local sedimentary brines. As the hydrothermal system waned, the relative importance of local brines within the Carboniferous section re-emerged, resulting in Stage IV breccia-cementing dolomite. A hydrologic simulation was employed to test this conceptual model. Results suggest a moderate-temperature (~180[degree sign]C), basement-derived fluid at upward flow rates of [greater than or equal to] 1 m/yr could have migrated vertically along the fault and penetrated into the surrounding sedimentary units, raising the temperature in the area of the breccia by ~45[degree sign]C. Cooling of the basement-derived fluid as it flowed across the resultant thermal gradient was the 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.