T7 Iron Formations, Ferruginous Sediments, and Redox through Time and Space

Sunday, 22 September 2019
8:00 AM - 12:00 PM
Room 229A, North Building

GEOCHEMICAL FACIES OF NEOPROTEROZOIC ‘SNOWBALL EARTH’ DIAMICTITE SEQUENCES AT THE KANSANSHI MINE, ZAMBIA: IMPLICATIONS FOR SEAWATER CHEMISTRY

Paper number: T7-9
Program: Technical Sessions
Time: 10:05 AM – 10:20 AM

Authors

Abstract

The return of ironstones to the geologic record in the Cryogenian after a nearly 1.0 Ga hiatus has been interpreted as an indication of extreme fluctuations in seawater chemistry related to ‘snowball Earth’ episodes. However, relatively little published work considers other elements in adjacent strata that may be useful in constraining depositional conditions during this controversial time in Earth history. In the Neoproterozoic Katangan Basin, Sturtian diamictite within the ‘Grand Conglomérat’ locally contains highly ferruginous (and sulfidic) intervals near the base of the sequence, including a well-developed ironstone at the Kansanshi Cu(-Au) Mine. In this study 26,437 m of metadiamictite from diamond core drilling at the mine was analyzed by 48-element bulk rock geochemistry (4-acid digest, ICP-MS) and categorized into 4 geochemical facies based on Fe, Mn, S, P, and Ti concentrations. A typical sequence consists of a 10-20 m thick basal Fe+P-rich sequence containing two 1-5 m ironstones, a 25-40 m S-rich sequence, a 5-15 m Mn-rich sequence, and >45 m of overlying ‘background’ composition strata. These variations suggest that anoxic, reducing conditions prevailed during deposition of the oldest diamictite, and that ironstones are likely related to local mixing between Fe-rich brines and oxidized fluids. Oxygenation of the water column occurred near the middle of the diamictite sequence, marked by a thin but remarkably consistent progression from S to P to Fe to Mn-rich strata capping the S-rich sequence. The geochemical facies are laterally persistent, with horizons <2 m thick traceable for kilometers. These internal subdivisions in what is typically a homogeneous metadiamictite sequence provide a new method of subdividing the ‘Grand Conglomérat’ and have been useful in deciphering the complex ductile deformation in the Kansanshi area.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-337634

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