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TitlePalaeoproterozoic orogenic gold style mineralization at the southwestern Archaean Tanzanian cratonic margin, Lupa Goldfield, SW Tanzania:: implications from U-Pb titanite geochronology
AuthorLawley, C J M; Selby, D; Condon, D; Imber, J
SourceGondwana Research 2013 p. 1-18, (Open Access)
Alt SeriesEarth Sciences Sector, Contribution Series 20130046
PublisherElsevier BV
Mediaon-line; digital
File formatpdf
AreaLake Rukwa; Tanzania
Lat/Long WENS 32.9361 33.2542 -8.3222 -8.4139
Subjectsmetallic minerals; geochronology; mineralization; Archean; titanite; uranium lead dates; gold; mylonites; shear zones; metamorphic facies; greenschist facies; mineral deposits; Lupa Goldfield; Ubendian Orogeny; Saza Granodiorite; Proterozoic; Precambrian
Illustrationssketch maps; photographs
ProgramGold Ore Systems, Targeted Geoscience Initiative (TGI-4)
AbstractThe Lupa Goldfield, situated at the southwestern Tanzanian cratonic margin, comprises a network of auriferous quartz veins and greenschist facies mylonitic shear zones cutting a suite of Archaean-Palaeoproterozoic granitic-gabbroic intrusions. The existing geochronological database points to a protracted, but episodic 1.96-1.88 Ga magmatic history that is broadly coincident with the 2.1-1.8 Ga Ubendian Orogeny. Molybdenite, pyrite and chalcopyrite samples from mineralized quartz veins and mylonitic shear zones yield Re-Os model ages that range from 1.95 to 1.88 Ga, whereas ca. 1.88 Ga pyrite with gold bearing inclusions and sampled from the host mylonitic shear zone suggest that gold occurred relatively late in this hydrothermal history. The ca. 1.88 Ga gold event is recorded at all five of the studied prospects, whereas the relationship between gold and the disparately older 1.95 and 1.94 Ga Re-Os molybdenite ages is unclear. New U-Pb metamorphic titanite dating of a foliated Archaean granite sample (ca. 2.76 Ga) suggests that the onset of ductile deformation within the Lupa Goldfield occurred at ca. 1.92 Ga, and some ca. 40 Myr prior to auriferous and brittle-ductile mylonitic shear zones at ca. 1.88 Ga. Early ductile deformation is not associated with gold mineralization, but the ductile deformation fabrics and, in particular the development of rheologically weak chloritic folia, may have acted as zones of pre-existing weakness that localized strain and influenced the geometry of later auriferous mylonitic shear zones. The large age difference between U-Pb zircon and titanite ages for the Archaean granite sample is in contrast to new U-Pb titanite ages for the Saza Granodiorite (1930±3Ma), which are only slightly outside of analytical uncertainty at the level with a previously reported U-Pb zircon age for the same sample (1935 ±1Ma). These new age results, together with previously reported U-Pb and Re-Os ages, highlight the protracted magmatic, hydrothermal and structural evolution of the Lupa Goldfield (1.96-1.88Ga). They are also consistent with other palaeo-convergent margins where orogenic gold style mineralization occurs relatively late in the orogen's tectono-thermal history.
Summary(Plain Language Summary, not published)
The ages of the faults and structures that host gold deposits are typically poorly understood. In this contribution, we synthesize previously published geochronology and report new U-Pb titanite ages for a group of gold concurrences in the Lupa goldfield, southwest Tanzania. The new age constraints suggest that barren structures pre-dated gold-bearing structures by at least 40 million years and thus provide evidence for an extended structural history for the goldfield. Our results highlight the long-lived nature of weak fault networks and suggest that barren and pre-existing structures may have influenced the location of later gold-bearing structures.