Hw. Carr et al., The petrogenesis of Merensky Reef potholes at the Western Platinum Mine, Bushveld Complex: Sr-isotopic evidence for synmagmatic deformation, MIN DEPOSIT, 34(4), 1999, pp. 335-347
Potholes represent areas where the normally planar PGE-rich Merensky Reef o
f the upper Critical Zone of the Bushveld Complex transgresses its footwall
, such geometric relationships being unusual in layered intrusions. The rec
ognition of vertical dykes of Merensky pyroxenite in the footwall suggests
downward collapse of crystal mush into pull-apart sites resulting from tens
ional deformation due to the loading effects of major new magma additions.
In contrast, crosscutting anorthosite veins display physical and isotopic e
vidence of upward emplacement. The Merensky Reef and its footwall have dist
inct initial Sr-isotope ratios (R-0 > 0.7066 and < 0.7066, respectively), w
hich may be used to constrain these processes related to pothole formation.
Merensky Reef in potholes (R-0 = 0.7069-0.7078) shows no isotopic evidence
of assimilation of, or reaction with, footwall material. Discrete, discord
ant replacement bodies of anorthosite extend from the footwall lithologies
to cross-cut the Merensky Reef and its hanging wall. The initial Sr-isotope
ratio in these replaced rocks is totally reset to footwall values (R-0 = 0
.7066), and immediately adjacent stratiform lithologies are slightly modifi
ed towards footwall values. In contrast, Neptunian pyroxenitic (Merensky) d
ykes cross-cutting the footwall lithologies: with a large surface area to v
olume ratio, and low Sr content, do not display footwall-like Sr-isotope in
itial-ratios (R-0 = 0.7077), and thus show no evidence for assimilation of
or reaction with footwall material. Furthermore, pegmatoidal replacement py
roxenite ("replacement pegmatoid"), at the base of the Merensky Reef within
potholes, has a high initial-ratio (R-0 > 0.7071), and so models of pervas
ive metasomatism by footwall material are not applicable. This isotopic evi
dence indicates that there was no active interaction of footwall material w
ith the overlying magma during, or after, the formation of Merensky Reef po
tholes, a basic tenet of existing pothole formation hypotheses involving fo
otwall mass-transfer. In contrast, the isotopic data are entirely consisten
t with an extensional model for pothole formation, with the more radiogenic
Merensky magma migrating laterally to fill extensional zones in the footwa
ll layers.