DESCRIPTIVE MODEL OF OOLITIC IRONSTONES

By J.B. Maynardand F.B. Van Houten

BRIEF DESCRIPTION

SYNONYM: Clinton-type depoist; Minette-type deposit.

DESCRIPTION: Beds rich in iron silicate and oxide minerals with distinctive oolitic texture deposited in shallow-shelf to intertidal, clastic-dominated environments.

TYPICAL DEPOSITS: Wabana, Newfoundland (Ranger and others, 1984); Birmingham, Alabama (Simpson and Gray, 1968); Lorraine, France and Luxembourg (Teyssen, 1984); southern Algeria (Guerrak, 1987); Cleveland, northeast England (Hallimond, 1925); Northampton Sand, England (Taylor, 1949).

RELATIVE IMPORTANCE: Important source of Fe from 1850 to 1945. Declining world importance since then because of competition from Precambrian banded-iron formations.

DISTINGUISHING FEATURES: Distinguished from banded iron formations by absence of chert, presence of oolitic textures, and Al-bearing silicates. Distinguished from blackband ironstones by absence of primary siderite and presence of oolitic textures.

COMMODITIES: Fe.

OTHER COMMODITIES: Ocher.

ASSOCIATED DEPOSIT TYPES (*suspected to be genetically related): None.

REGIONAL GEOLOGIC ATTRIBUTES

TECTONOSTRATIGRAPHIC SETTING: Craton margins, 40 percent; craton interiors, 25 percent; foreland basins, 20 percent; exotic terranes, 15 percent.

REGIONAL DEPOSITIONAL ENVIRONMENT: Shallow shelf, most typically close to the transition from nonmarine to marine environments.

AGE RANGE: Phanerozoic, concentrated in the Ordovician to Devonian and Jurassic to Paleogene. A few Proterozoic examples.

LOCAL GEOLOGIC ATTRIBUTES

HOST ROCKS: Almost always clastic hosted at top of coarsening and shoaling-upward cycles (fig. 24).

Figure 24. Generalized stratigraphic model for oolitic ironstones. Vertical scale is variable; cycles may range from a few meters to as many as 300 m in thickness (modified after Van Houten and Bhattacharyya, 1982; Maynard, 1983).

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ASSOCIATED ROCKS: Standard vertical succession is black shale at base, followed by grAy shale and siltstone, then by sandstone with graded bedding and hummocky cross-stratificiation suggesting tempestites, and finally by sandstone or oolitic ironstone with bipolar cross-stratification suggesting intertidal deposition. The association with black shale (Hallam and Bradshaw, 1979) is significant: 75 percent of well-developed Phanerozoic ironstones have an extensive black shale at the base of the shoaling cycle (Van Houten and Arthur, 1989).

ORE MINERALOGY: Younger rocks: goethite + berthierine (7Å chlorite). Older rocks: hematite + chamosite (14-Å chlorite). Siderite common as a replacement; locally, pyrite found as replacement (Maynard, 1986); occasionally, magnetite.

GANGUE MINERALS: Quartz + calcite + dolomite + clay minerals; apatite (collophane) ubiquitous in small amounts.

STRUCTURE AND ZONING: Rarely reported. Hematite cemented with Fe silicates to magnetite at Sierra Grande, Argentina (Leiding V., 1955).

ORE CONTROLS: Three-quarters of deposits show strong control by position at the top of sedimentary cycle. Many of the larger deposits show features of tidally influenced deposition.

ISOTOPIC SIGNATURES: Siderite has light C, about -18 per mil; unknown for other minerals (Maynard, 1983).

STRUCTURAL SETTING: Major deposits in undeformed to simply folded strata. Some Ordovician deposits on blocks complexly deformed by the Armorican (Hercynian) orogeny of Western Europe.

ORE DEPOSIT GEOMETRY: Tabular bodies 2 to 5 m thick and 2 to 10 km across.

ALTERATION: None relevant to mineralization.

EFFECT OF WEATHERING: Removes carbonate gangue, and converts ferrous silicates to ferric oxides. Many older mining operations based on weathered ore; typically, workings less than 30 m into outcrops.

EFFECT OF METAMORPHISM: Goethite converts to hematite above 80 oC (Hodych and others, 1984); hematite converts to magnetite under metamorphic conditions, but a few apparently unmetamorphosed deposits have magnetite (Devonian deposits of Libya). Berthierine converts to chamosite at 130-160 oC (Iijima and Matsumoto, 1982). Most deposits unmetamorphosed.

GEOCHEMICAL SIGNATURES: Only Fe.

GEOPHYSICAL SIGNATURES: Marked positive gravity anomaly (1 mgal over 1-5 km) useful in delineating orebodies (Miller, 1983). Magnetite-bearing occurrences detectable by airborne magnetometer.

OVERBURDEN: Most commonly clastic sedimentary rocks, from 0 to 500 m in recently active mines.