41 D26. Recent Advances in Precambrian Geology (Posters)

Sunday, 22 September 2019
9:00 AM - 5:30 PM
Hall AB, North Building

DECIPHERING PRIMARY AND DIAGENETIC CONTROLS ON MINERALOGY IN THE ~1.9 GA BIWABIK IRON FORMATION, MN USING PAIRED TEXTURAL AND GEOCHEMICAL ANALYSES

Paper number: 41-2
Program: Technical Sessions
Time: 9:00 AM – 5:30 PM

Authors

Abstract

The primary mineral inventory of Precambrian Iron Formations (IF) records the evolving composition of seawater. However, post-depositional changes during diagenesis and later metamorphism often obscure the original mineralogy. Here, we investigate texturally early silicate and carbonate minerals of the well-preserved (sub-greenschist) ~1.9 Ga Biwabik IF in Minnesota to infer potential primary phases present in Paleoproterozoic oceans. We examined 53 samples from drill core LWD-99-1 for sedimentary characteristics as well as textural and mineralogical data using transmitted and reflected light microscopy. We focused sampling for detailed scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA) work at contacts between granular and banded horizons, screening samples for geochemical analyses based on textural indicators. Here, we report novel mineral data for greenalite granules and a range of carbonate phases reported as average compositions. Based on SEM and EPMA imaging, greenalite granules display cuspate fractures interpreted to result from dewatering of a precursor Fe-Si gel composition. Such grains appear as an intergrowth of 2 phases at the sub-μm scale with an average composition of (Fe5.22Mn0.03Mg0.56)Al0.14Si4.06O10(OH)8 (9 points on 3 granules). The same sample contains a thin layer of fine-grained (~5-10 μm) siderite ((Ca0.04Mg0.16Mn0.06Fe0.75)CO3; 23 points) displaying a mottled texture. Within other carbonate horizons, we identify 2 potentially primary textures: (1) a ~9 mm putative crystal fan of calcite composition ((Ca0.99(Mg0.01)CO3); 17 points) associated with fine-grained calcite of the same composition (40 points); and (2) a separate fine-grained (~10-30 μm) ferroan-dolomite horizon (Ca1.06(Mg0.63Fe0.29Mn0.02)(CO3)2 averaged from 17 points) containing a putative microbial mat morphology. Both features are distinct from secondary euhedral crystals (~1-5mm) which possess thick rims of ankerite composition (Ca1.08(Mg0.38Fe0.51Mn0.03)(CO3)2; 5 points). Broadly, our results identify a range of near-primary to diagenetic textures and compositions from different horizons of the Biwabik IF, and highlight the importance of pairing quantitative geochemical analyses with spatial relationships.

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

doi: 10.1130/abs/2019AM-339482

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