Field, geochemical and U-Pb isotopic constraints from hypabyssal felsic intrusions within the Barberton greenstone belt, South Africa: Implications for tectonics and the timing of gold mineralization

Abstract

Field mapping, geochemical and U-Pb zircon isotopic data are reported for two undeformed feldspar-quartz porphyries from within the central region of the Barberton greenstone belt (∼ 3450 Ma) and from a porphyry at the deepest level of one of the largest gold mines in the Barberton area, Fairview Mine. One porphyry, the Mudpools porphyry, has an age of circa 3230 Ma and provides a minimum age on unusual BIF/ferruginous shale-hosted mudpool structures. Field mapping constraints and similar geochemical signatures between the Mudpools porphyry and a second porphyry, the Ironstone pod porphyry, suggest that 3230 Ma can also be considered the minimum age of the equally unusual Ironstone pods. The two porphyries cross-cut Fig Tree sediments which are regionally deformed (D 2 ), and intrude up a thrust fault(s) suggesting pre-3230 Ma ages for D 2 deformation and thrusting. Geochemical data (trace elements, REE) indicate that these two porphyries are high-level, co-magmatic equivalents of the nearby 3227 Ma Kaap Valley tonalite, which in part intrudes the lower sequences of the greenstone bell. Since the edge of the Kaap Valley pluton is also deformed by the D 2 deformational event, either D 2 is diachronous, or D 2 in the center of the belt is not equal to D 2 at the edge of the greenstone belt. Zircon from the Fairview Mine porphyry indicates a maximum age of 3126 ± 21 Ma (2σ). A gold-mineralized shear which cross-cuts the Fairview porphyryl is responsible for hydrothermal alteration of the porphyry. U-Pb analysis of hydrothermal rutile (associated with gold) from the porphyry gives an age of 3084 ± 18 Ma (2σ)_and may represent the age of gold mineralization associated with this late shearing. It appears that ∼ 3230 Ma is a important age in early Barberton greenstone belt history, signifying intrusion of the Kaap Valley tonalite adjacent to the greenstone belt, together with the emplacement of high-level, co-magmatic equivalents (porphyries) of the Kaap Valley tonalite along regional thrust faults (shear zones) within the greenstone belt. Gold-bearing hydrothermal fluids have utilized similar shear zones ∼ 150 Ma after emplacement of both the porphyries in the central region of the greenstone belt, and the Kaap Valley tonalite.