Abstract

The 2.7 Ga old Helen Iron Formation (HIF) with its uniquely large siderite–Pyrite orebody conformably overlies chemically altered rhyolite–dacite pyroclastic rocks, including an ottrelite-bearing alteration pipe, the product of exhalative venting in freshly accumulated pyroclastics. HIF, in turn, is conformably overlain by pillowed mafic lava flows.HIF internal stratigraphy is conformable from lower massive siderite–sulfide through middle sulfidic, carbonaceous chert, to upper magnetitic chert. Major- and trace-element and carbon- and sulfur-isotope data demonstrate cyclic fluctuations in chemical precipitation from early chemically complex to later chemically "simple" components.HIF development is attributed to seawater-charged volcanic exhalations in a transitory subaqueous cauldron subsidence basin. Volcanic exhalations contained (1) the main chemical components of HIF; (2) nutrients that triggered intense bacterial activity, the principal low-Eh-inducing agent; and (3) acidic components (e.g., HCI, HF), the principal low-pH-inducing agent. HIF carbonates (predominantly siderite) and sulfides (mainly pyrite) are of marine carbonate and marine sulfate origin, respectively. Organic activity was a vital catalyst in carbonate–sulfide precipitation. HIF is notably deficient in base and noble metals, thereby indicating low circulation–exhalation temperatures, in sharp contrast to contemporary hot, metal-rich geothermal brines at modern spreading ridge axes. This transient subaqueous late Archean volcanic environment, then, was unique in scale but not in kind.

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