Bushveld-aged fluid flow, peak metamorphism, and gold mobilization in the Witwatersrand basin, South Africa: Constraints from in situ SHRIMP U-Pb dating of monazite and xenotime

Abstract

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 Witwatersrand 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 fl uid fl ow 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 goldfi eld at the southern end of the basin implies that Bushveld-related heating and fl uid fl ow 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 goldfi eld indicates that fl uid fl ow related to the Bushveld event caused mobilization of gold in the Witwatersrand basin.

Stable Isotope Composition of Quartz-Calcite Veins in the Witwatersrand Basin, South Africa: Implication for Basin-Scale Fluid Circulation

“…the Bushveld intrusion and Vredefort impact) responsible for heat flux perturbations within the Witwatersrand Basin. Consequently, a large number of ages have been published for fluid circulation events within the basin with events older than 2.78 Ga (syn-sedimentary events; Kositcin et al, 2003), around 2.7 to ~2.6 Ga (Chuniespoort, Frimmel and Minter, 2002), around 2.37 Ga (Transvaal deposition related; Poujol et al, 1999), between ~2.35 and ~2.08 Ga (Frimmel and Minter, 2002;Kositcin et al, 2003;Friese et al, 2003;Rasmussen et al, 2007), around ~2.06 Ga (probably linked to the Bushveld emplacement; Frimmel and Minter, 2002;Kositcin et al, 2003 andRasmussen et al, 2007) or 2.02 Ga (with the Vredefort impact; Gartz and Frimmel, 1999;Poujol et al, 1999;England et al, 2001;Frimmel andMinter, 2002 andFriese et al, 2003).…”

In situ U-Pb dating of xenotime by laser ablation (LA)-ICP-MS

“…This mineral is enriched in rare earth elements (REE), and is an ideal candidate for U-Pb dating because of its high content of U and negligible content of common Pb. Currently, in situ dating of xenotime is commonly conducted by ion microprobe [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], which can yield precise and accurate ages. However, the expense of this instrumentation hinders the wide application of this technique.…”

The Geometry and Emplacement Mechanics of a Bushveld Complex Peridotite Body, South Africa

“…Some contention exists regarding a morphologically similar fabric present throughout the contact aureole that is considered by to predate Bushveld emplacement, although this contention was heavily criticised by Reimold et al (2007), who favoured a syn-Bushveld age for the fabric. Recent work by Rasmussen et al (2007) does, however, suggest that the pre-Bushveld age documented by for this fabric may be valid. Geometrically, however, this fabric differs from the S 2 fabric of the current study since south of the Bushveld Complex it dips to the south.…”