Abstract

A complex hydrothermal system is described in a 2.3-Ga sedimentary succession. It comprises an epigenetic part developed just below an uncomformity, characterized by intense desilicification of cherty dolostone and by mineralization forming the matrix of a breccia, probably representing a collapsed paleocave horizon. Two facies can be identified in this desilicification zone: a blue facies that is characterized by replacement of chert by coarsely crystallized Mn-Fe-rich dolomite and a breccia matrix filled in by the same dolomite plus some galena and sphalerite, and a red facies, that is characterized by chert replacement by a similar dolomite, which is only Mn-rich, rubefaction of dolostone, and deposition of primary Mn,Fe oxides (+Zn, Pb). The syngenetic part immediately overlies the uncomformity and forms Zn shale interpreted as former metalliferous mud deposited in a paleosinkhole flooded by hydrothermal solutions.Theoretical calculations suggest that the desilicification of the dolostone can be best explained by mixing of warm, relatively dilute hydrothermal solutions with cold alkaline surficial brines. It is proposed that the latter resulted from surface evaporation of deep bicarbonated solutions, which then descended into the substratum where mixing occurred.The Genadendal occurrence is part of a local group of Pb-Zn deposits in a cherty dolostone sequence. These deposits have not been controlled by igneous intrusions and may be related to the Mississippi Valley type. Comparisons are made with other mineral deposits such as Kombat (Namibia) and Franklin Furnace (New Jersey) which also may have formed under alkaline conditions.

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