The role of evaporites in the genesis of base metal sulphide mineralisation in the Northern Platform of the Pan-African Damara Belt, Namibia: geochemical and fluid inclusion evidence from carbonate wall rock alteration |
| |
Authors: | D Chetty H E Frimmel |
| |
Institution: | (1) Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7701, South Africa e-mail: hef@geology.uct.ac.za, ZA |
| |
Abstract: | The Otavi Mountain Land is a base metal sulphide ore province in northern Namibia where deposits are hosted by platform carbonates
of the Otavi Group in a foreland fold-and-thrust belt on the northern edge of the Pan-African Damara Belt. Deposits have been
classified as the Berg Aukas- or Tsumeb-types, based on differences in ore association, stratigraphic position and geochemistry
of ores and gangue carbonates. Mineralisation at these deposits is accompanied by carbonate alteration in the form of dolomite
and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Based on cathodoluminescence imaging,
trace and rare earth element (REE), O and C isotope, and fluid inclusion data, a series of carbonate generations, constituting
wall rock alteration around the Tsumeb and Kombat (Tsumeb-type) and Berg Aukas (Berg Aukas-type) deposits, was established.
Similar data obtained on the recently discovered Khusib Springs deposit indicate a strong affinity to Tsumeb-type deposits.
Tsumeb-type deposits are distinguished from Berg Aukas-type deposits by having trace element and REE concentrations that are
significantly higher in the alteration products compared to the carbonate host rocks. Only around Tsumeb-type deposits a relative
enrichment in light REE is noted for the hydrothermal carbonate generations that are cogenetic with the main stage of mineralisation.
Microthermometric results from fluid inclusions in carbonate alteration phases and associated quartz indicate relatively high
salinity (17–23 wt% NaCl equivalent) for the main mineralising and subsequent sulphide remobilisation stages at the deposits
investigated. Estimated mineralisation temperatures are significantly higher for Tsumeb-type deposits (370–405 °C) with early
sulphide remobilisation in Tsumeb at 275 °C, whereas they are lower at Berg Aukas (up to 255 °C). Fluid inclusion leachate
analysis suggests that most of the observed salinity can be ascribed to dissolved, predominantly Ca- and Mg-carbonates and
chlorides with subordinate NaCl. Na-Cl-Br leachate systematics indicate a derivation of the fluid salinity from the interaction
with evaporitic rocks en route. Tsumeb-type mineralisation is interpreted to be derived from fluids expelled during Pan-African
orogeny in the more intensely deformed internal zones of the Damara Belt further south. When the high salinity fluids reached
the carbonate platform after having scavenged high concentrations of base metals, base metal sulphide precipitation occurred
in zones of high porosity, provided by karst features in the carbonate sequence. Results obtained for the Berg Aukas-type
deposits emphasise their derivation from basinal brines, similar to Mississippi Valley-type deposits, and confirm that mineralisation
of the Berg Aukas- and Tsumeb-types are both spatially and temporally distinct.
Received: 5 May 1999 / Accepted: 10 November 1999 |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|