• journal_title：Geology
• Contributor：Birger Rasmussen ; Ian R. Fletcher ; Janet R. Muhling ; Andreas G. Mueller ; Greg C. Hall
• Publisher：Geological Society of America
• Date：2007-
• Format：text/html
• Language：en
• Identifier：10.1130/G23588A.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：35
• issue：10
• firstpage：931

In situ U-Pb dating of monazite and xenotime in gold reefs and unmineralized greenschist facies sedimentary rocks from the Witwatersrand basin, South Africa, reveals two episodes of tectonothermal activity. A major event between 2.06 and 2.03 Ga is recorded in the Wit-watersrand and Transvaal Supergroups in the northwestern and central basin, and broadly coincides with the ca. 2.06 Ga Bushveld event. In the central and southern basin, a previously unrecognized event has been dated between 2.14 and 2.12 Ga. The widespread geographic and stratigraphic occurrence of Bushveld-aged monazite and xenotime, including both auriferous reefs and unmineralized strata, indicates that metamorphism and fluid flow associated with magmatism was pervasive, affecting most of the succession (>10 km thick) in the central and northern parts of the basin. The metamorphic phosphate dates, which are younger away from the complex, indicate a lag of 20–30 m.y. between emplacement and phosphate growth in the central basin (∼100 km south), suggesting that heat related to magmatism was transferred southward at an average rate of 3–5 mm yr⁻¹. The absence of 2.06–2.03 Ga phosphates in the Welkom goldfield at the southern end of the basin implies that Bushveld-related heating and fluid flow did not affect this part of the basin. The intergrowth of ca. 2.045 Ga monazite with gold in quartz-pebble conglomerate from the West Rand goldfield indicates that fluid flow related to the Bushveld event caused mobilization of gold in the Witwatersrand basin.