Mineral chemistry in fenites of the Kalkfeld carbonatite complex and its bearing on the composition of the fenitising fluid

Abstract

Porphyry-style mineralization is characterized by superposition of multiple generations of quartz growth, punctuated by dissolution and juxtaposed within a single vein. Such superposition inevitably obscures temporal relationships between quartz formation, alteration events, and fluid inclusion assemblages (FIAs), and consequently complicates the interpretation of fluid origin and evolution. Very detailed information of these relationships recorded within hydrothermal quartz from the Taiyangshan porphyry Cu-Mo deposit (NW China) is obtained using integrated cathodoluminescence (CL) imaging, trace element analyses, and fluid inclusion petrography.The CL characteristics range from bright blue-CL quartz (Q1) associated with early high-temperature potassic alteration, to dark blue-CL quartz (Q2) broadly coinciding with metal precipitation, to post-ore dull red-CL fine-grained, anhedral, and recrystallized quartz (Q3) subgrains. Early bright blue-CL quartz (Q1) crystals, generally at the edges of stockwork veinlets close to wallrock, are characterized by oscillatory growth zoning, and exhibit alignment of primary, equant-shaped, brine-bearing FIAs along the banded growth zones. Later dark blue-CL quartz (Q2) crystals are generally euhedral in shape and grow with c-axes pointing inward to the vein center. They exhibit well-defined, but rarely preserved, growth zones truncating Q1 and crystal terminations projecting into pyrite and chalcopyrite in the vein center although they are not spatially associated with sulfides. These vein features clearly indicate that Q2 crystals grow into an open cavity or fracture during the ore precipitation. This is also evident by coexisting, secondary, irregular-shaped, liquid-rich inclusions and vapor-rich inclusions along fractures transecting Q1 oscillatory growth bands. The coexistence of hypersaline and vapor-rich inclusions moreover provides unequivocal evidence for phase separation during mineralization. Dull red-CL quartz (Q3) crystals appear to coincide with or follow sericitic alteration, and cut and encompass preexisting Q1 and Q2 crystals. They spatially associated with sulfides. The Q3 grains contain rare well-defined FIAs, but show scattered, secondary, irregular-shaped, low-salinity aqueous inclusions with small- to medium-sized vapor bubbles (10–30 vol%), as well as complex fluid inclusions that have migrated to the margins of anhedral Q3 subgrains due to quartz dissolution and recrystallization.Formation of early blue-CL Q1 and Q2 crystals at moderate temperatures are consistent with enrichment of Ti in the quartz lattice compared to neighboring younger Q3. The Q2 crystals are typified by elevated concentrations of Al and K concentrations when compared to Q1 and Q3. The substitution of Ti4+ for Si4+ in the tetrahedral sites of quartz leads to an increased CL intensity from Q1, to Q2, and then to Q3 corresponding to a decrease in quartz precipitation temperature. Ore deposition, recorded by integration of CL textures, FIAs petrography, and trace element variations in Q2, in the Taiyangshan porphyry Cu-Mo deposit must have occurred over a significant change in physicochemical conditions. This suggests that phase separation due to fluid cooling/pressure drops during the transition from lithostatic to hydrostatic regimes is critical to porphyry mineralization. We further propose a complex hydrothermal quartz vein formation history involving multiple fluid inclusions under varying pressure and temperature conditions.