Geochronology and Structure of the Lupa Goldfield, Tanzania

Abstract

Gold occurrences in the Lupa goldfield, SW Tanzania, are typical of the orogenic Au deposit type. Auriferous quartz veins and mylonitic shear zones cross cut Archean granitoids (ca. 2740 Ma) and Paleoproterozoic felsic-mafic intrusions (1960–1880 Ma) comprising a Paleoproterozoic magmatic arc at the Tanzanian cratonic margin. Lu-Hf zircon results provide evidence for ca. 3.1 Ga basement beneath the Lupa goldfield and imply significant portions of the Ubendian Belt represent re-worked Archean crust. A greenschist facies metamorphic mineral assemblage overprints all of the magmatic phases and U-Pb dating of syn-metamorphic titanite hosted by a foliated Archean granitoid at ca. 1950 Ma suggests the onset of this tectonothermal episode occurred during the Paleoproterozoic.

The majority of mineralization is hosted by a network of brittle-ductile and mylonitic shear zones, which record evidence for transpressional deformation of triclinic, or lower order, symmetry. Relacement of igneous feldspar (plagioclase and K feldspar) with intrinsically weaker phyllosilicates, during sericitization of the granitic wall rock, created the ideal conditions for strain localization and locally may have led to the onset of crystal plastic deformation processes. Continued feedback between fluid, rock and deformation generated interconnected networks of weak mylonitic shear zones that are subject to reactivation. Quartz veins are the other significant host for Au and possess geometries that imply mineralization occurred concurrently with episodic fluid pressure fluctuations.

Re-Os molybdenite, pyrite and chalcopyrite geochronology ages record a protracted metallogenic history and provide evidence for at least two mineralizing events at ca. 1940 and 1885 Ma. Each metallogenic event is represented in detail by a hydrothermal history that occurred at a time scale less than the resolution of the Re-Os method. High precision U-Pb zircon ages for the Saza Granodiorite overlap with Re-Os ages and provide unequivocal evidence for magmatism concomitant with sulphidation, however the wide range of Re-Os sulphide ages precludes a genetic relationship between any individual intrusion and Au. The goldfield-wide metallogenic event at ca. 1885 Ma occurred concurrently with eclogite facies metamorphism elsewhere in the Ubendian Belt and provides one of Earth’s earliest examples of subduction processes temporally linked to orogenic Au deposit formation. Mesoproterozoic Re-Os ages (ca. 1125 Ma) correlated to the Kibaran/Irumide Orogenies and compatible with inferred Pb loss events provide evidence for sulphidation during at least two discrete orogenic cycles and suggest mid-crustal mylonitic shear zones represent long-lived zones of structural weakness.