The controls of "contact-type" Cu-Pb (Ag) mineralization within the Tsumeb subgroup of the Otavi Valley syncline, northern Namibia

Abstract

Abstract provided by author:

A feature amongst the "contact-type" mineralization is a strong calcite alteration halo which commonly resulted in solution collapse or replacement brecciation. Calcite alteration, however, is also common in areas of no mineralization. Stable (C and O) and radiogenic (Sr) isotope studies on these various calcite phases has aided in determining their relative ages

The association of Fe-Mn oxide/silicate assemblages related to the Kombat deposits has led to conflicting ideas regarding the genesis of these deposits. Major and trace element analysis support a model where migrating hydrothermal fluids have leached-out Fe and Mn from the Askevold volcanics in the northern graben during the final stages of rift tectonics

Otavi Valley sandstone is also intimately associated with "contact-type" mineralization. The sandstone commonly forms karst-fill within karst structures that have developed along the carbonate platform margin, referred to as the Kombat lineament. The sandstone is either well mineralized or barren. Major and trace element analysis suggests that all sandstone originates from a similar source rock, and the various occurrences are probably of a similar age

Regional and local field evidence, supported by the analytical results, has led to the construction of a genetic model for the "contact-type" mineralization. A marine transgression, resulting from late-stage rift tectonism drowned the southern parts of the Otavi Valley basin. This allowed for deep-seated hydrothermal fluids from the northern graben to migrate up the rift structures, enter the Otavi Valley basin, and deposit Fe and Mn as oxide/silicate assemblages in favourable third order structures in the oxidizing zone at the platform margin. A D1-age non-depositional hiatus over the platform margin resulted in pervasive calcite alteration, and local solution collapse. It is probable that the first phase of mineralization was related to these diagenetic processes, resulting in sub-economical mineral deposits such as Gross Otavi, and Guchab

With further basin drowning, the Kombat Formation transgressed over the carbonate platform margin and karsts developed in the subsurface environment at the intersection of favourable structures at Kombat. During the latter part of theD1, or with the onset of the regiona D2 deformation, basement highs became exposed and subsequently eroded with the karsts being infilled with Otavi Valley sandstone. D2 folding of the Otavi Valley basin formed the Otavi Valley syncline with the development of a flexure, the Otavi Valley monocline, along the original platform margin

During the late regional D2 (or local D 3b) deformation the Otavi Valley syncline ruptured forming a complex network of north-northeastern trending faults and shear zones. Orogenic brines expelled from higher temperature metamorphic areas further south, deposited the next phase of Cu mineralization within these karst structures to form the economic Kombat deposits

Various phases of post-Damara faulting resulted in the development of more recent karsts (Baltika and Guchab). This has further concentrated the base metal mineralization as oxide facies assemblages, including vanadium