The Schakalsberg seamount : physical volcanology, structure, alteration and mineralization

Alexandre Aubin, Masters Thesis.

Master's Thesis in Earth Sciences in collaboration with Kumba Resources (formerly known as ISCOR Mining) and under the supervision of Dr Wulf U. Mueller.

Project Description

The Schakalsberg Mountains of the Sperrgebiet in Namibia are considered an integral part of the ca. 740-550 Ma Neoproterozoic Gariep Belt, which is renowned for the ca. 30 million tonne Zn-Pb-Cu-Ag Rosh Pinah base metal deposit. The study area is part of the Marmora terrane and the concession EPL 2757, acquired by Kumba Resources, South Africa, is located at 28°00'00" to 28°07'30" S, 16°30'00" to 16°37'30" E. The mapped area represents a segment of the Schakalsberg Mountains that covers 225 km². Lithofacies mapping revealed a ca. 1.4 km-thick volcanic-dominated succession that displays the salient characteristics of an oceanic seamount. The composite stratigraphy of the Schakalsberg area represents an upward shoaling sequence in which mafic volcanic flows dominate the base and volcaniclastic rocks are prevalent at the top of the succession.

Two principal lithofacies and one subordinate lithofacies are associated with Schakalsberg seamount construction including: 1) a mafic volcanic lithofacies, 2) a volcaniclastic lithofacies, and 3) a sedimentary breccia lithofacies. A carbonate lithofacies recognized in the study area is probably the lateral equivalent of the seamount sequence, but they are now separated by a major thrust fault. The up to 1 km-thick mafic volcanic lithofacies, composed of lava flows and mafic intrusive rocks, has been divided into several facies based on phenocryst content, chemical composition and cross-cutting relationships. The (i) aphanitic, (ii) microporphyritic, (iii) feldspar-phyric, (iv) pyroxene±feldspar-phyric, and (v) phonolitic volcanic facies are submarine massive, pillowed, and pillow breccia flows. Columnar joints, observed in massive portions of flows and large pillow tubes, represent master tubes, and are locally exposed and traceable for 10's of metres. Extensive pillow breccia units are characteristic of the aphanitic volcanic facies, which is located in the upper part of the seamount succession. In contrast, the phenocryst-rich volcanic facies are predominant at the base of the sequence.The intrusive gabbro volcanic facies is considered coeval with effusive submarine volcanism as is borne out by the major, trace element and rare earth element geochemistry.

The volcaniclastic lithofacies is the reworked autoclastic and explosive fragmental counterpart of the subaqueous lava flows and is divided into (i) tuff, (ii) lapilli tuff, and (iii) lapilli tuff breccia facies. The 0.10-90 m-thick tuff facies, composed of thin parallel laminated and graded beds that may be derived from subaqueous explosions or thermal granulation processes, was deposited via dilute turbidity currents and suspension deposits. The 0.20-1.50 m-thick lapilli tuff facies contains both massive to graded beds and planar to cross beds. The former facies resulted from (re)deposition of high-concentration turbidites and mass flows, whereas the latter is considered a primary pyroclastic deposit produced by subaqueous eruption-fed density currents. The lapilli tuff breccia facies, which has a heterolithic clast population, is a reworked volcaniclastic deposit from the summit of the edifice that has been transported downslope via laminar debris flow processes. The sedimentary breccia lithofacies with angular tuff rip-up clasts is only observed in the 90 m-thick tuff facies and is considered a mass flow deposit, related to synsedimentary slumping.

The striking characteristic of the Schakalsberg Mountains is the pervasive hydrothermal carbonate alteration that can be traced for at least 20 km along strike. The alteration is well-developed in the upper part of the stratigraphy where the volcaniclastic lithofacies is predominant.The semi-conformable alteration pattern is common to Archean massive sulphide deposits, referred to as Mattabi-type. The carbonate alteration assemblage consists of calcite (CaCO₃), dolomite [CaMg(CO₃)₂] and Fe-dolomite [Ca(Mg,Fe)(CO₃)₂], with an increase in Mg and Fe content from the northwest to the southeast. This assemblage, considered the equivalent of the distal calcite and dolomite to intermediate calcite-Fe-dolomite carbonate alteration halo found in Mattabi-type deposits, is consistent with modern and ancient seafloor alteration patterns. The carbonate alteration was produced by circulating CO₂-rich hydrothermal fluids within the seamount. The inferred source for (CO₃)^-2 is the carbonate lithofacies, because it is the lateral platformal continuation of the Schakalsberg seamount.

Two types of mineralization were identified: 1) Fe-mineralization consisting of (i) μm-to-mm hematite (Fe₂O₃) needles disseminated in the volcanic rock matrix, and (ii) replacement of primary magnetite (Fe₃O₄) crystals by hematite (Fe₂O₃) along the cleavage planes, and 2) Mn oxyhydroxide mineralization, manjiroite [(Na,K)(Mn⁴⁺, Mn²⁺)₈O₁₆·n(H₂O)], in brecciated or massive zones, 2-6 m-thick and traceable for up to 1 km along strike. The Fe-mineralization is a low-temperature mineralization, which was deposited in an oxidizing environment, with the carbonatized rocks acting as cap rocks. The mineralization precipitated in the porous volcaniclastic units that were already carbonatized. This mineralization could represent a distal, lower temperature phase of a VMS deposit. The Mn mineralization is supergene and was produced by percolation of fluids and leaching of the surrounding rocks after the deformation of the Schakalsberg seamount (Quaternary?).

The structural evolution of the Schakalsberg Mountain segment is consistent with a fold and thrust belt that was subsequently subjected to sinistral transpression. The prominent structure is the Red Dunes thrust, which separates the marine carbonate lithofacies from the volcanic-dominated sequence of the Schakalsberg Mountains. The Red Dunes thrust trends and has a stretching lineation consistent with a south over north movement. The study area was divided into three structural domains according to the principal schistosity (Sp): (i) domain 1 has a mean Sp of 158/84, (ii) domain 2 of 355/65 and (iii) domain 3 of 149/40.

The Schakalsberg study area is consistent with evolution of a Precambrian seamount that formed during the opening of the proto-Atlantic Adamastor Sea. The pervasive hydrothermal carbonate alteration zone with local intense Fe-mineralization is primarily restricted to the porous volcaniclastic lithofacies that presumably forms the shoaling part of the seamount sequence. The study area is the first detailed account of a volcanic-volcaniclastic sequence in the Sperrgebiet of Namibia.

Résumé francais. Géologie de la propriété EPL 2757, Schakalsberg Mountains, Sperrgebiet, Namibie.

For questions or comments, contact me at: etchouta@hotmail.com.