Iron Ores Of The Pre-penokean, Superior-type Riverton Iron Formation, Michigan: Keweenawan Supergene Oxidation, Iron Enrichment, And Paucity Of Other Metals

The Riverton Iron Formation is chert-siderite facies of Proterozoic Superior-Type Iron Formation within the Paint River Group, Marquette Range Supergroup, of the Upper Penninsula, Michigan. It is 90 to 250 m thick, underlain by the 150 to 450 m thick Dunn Creek Slate Formation, and overlain successively by the 15 to 150 m thick Hiawatha Greywacke Formation, the 30 to 60 m thick Stambaugh Formation of siderite-and magnetite-bearing slate, and the 1200 m thick Fortune Lakes Slate Formation. The Dunn, Hiawatha, Stambaugh, and Fortune Formations are variably pyritic and sideritic shale, chert, siltstone, and greywacke. The Paint River Group is in the greenschist facies of regional metamorphism and in tight, steeply northwest-plunging folds and open to tight, gently east-plunging folds. Granitoid intrusions which are syn- to post-deformation and metamorphism are ca. 1.85 Ga, the age of the Penokean orogeny.;There are minor discordant quartz veins with chlorite, stilpnomelane, pyrite, and trace chalcopyrite in all formations of the Paint River Group. These veins are both parallel and discordant to faults and fold axes attributed to the Penokean orogeny. Veins are interpreted as syn- to post-orogenic. There are no significant concentrations of metals in these veins.;Chert-siderite facies of the Riverton Iron Formation has 20 to 30% iron. However, locally iron ore bodies with more than 50% iron are (i) predominantly hematite and goethite, (ii) in axes of plunging synclines with footwall aquicludes, (iii) undeformed, and (iv) of lower metamorphic grade than the regional greenschist facies.;Formation of iron ore is interpreted as a two stage process of (i) widespread pseudomorphism of siderite by hematite and goethite and (ii) dissolution of chert and replacement by hematite and goethite. Because quartz veins of Penokean age are pseudomorphed by hematite and goethite, the iron orebodies are younger. They most likely formed when the water table was lowered because of uplift of the Paint River Group on the south ridge and flank of the Keweenawan Rift at about 1.1 Ga and, meteoric water was acidified by pyrite oxidation and ferrolysis in pyrite-beating slate and iron formation of the hanging wall. Hence they represent supergene oxidation of primary chert-siderite iron formation.;Primary iron formation and iron ores do not contain significant concentrations of metals other than iron because (i) the primary iron formation is essentially a chemogenic, ferriferous, siliceous precipitate from seawater, devoid of metals other than iron (ii) there were no hypogene metal concentrations superimposed on the iron formation during the Penokean orogeny, which caused only low grade metamorphism and minor to nil plutonism and hydrothermalism in the Paint River Group