The mercenaries they hire for a few credits a kill are too few, too unreliable to do so either. The Cooperative's police force, concentrated near a few influential planets, can no longer maintain order. The trade ships that once safely travelled between planets now have to be well armed and escorted to fend off pirate attacks, from small-time criminals desperate for their next meal, to powerful robber barons extracting tithes from everyone who passes through their space. The two thousand star systems of the Cooperative once enjoyed a golden age of peace and prosperity, and perhaps the wealthiest of them can still pretend to. Perhaps one day, everyone might know your name. You've got a ship, some weapons, and enough spare cash to get started - and one day, you might get the fame, wealth or glory you want. Magnetite-bearing occurrences detectable by airborne magnetometer.Among the seven trillion people who are - at least officially - Cooperative citizens, you are nobody. GEOPHYSICAL SIGNATURES: Marked positive gravity anomaly (1 mgal over 1-5 km) useful in delineating BerthierineĬonverts to chamosite at 130-160 oC ( Iijima and Matsumoto, 1982). Unmetamorphosed deposits have magnetite (Devonian deposits of Libya). Many older mining operations based on weathered ore typically, workings lessĮFFECT OF METAMORPHISM: Goethite converts to hematite above 80 oC (Hodych and others, 1984) hematiteĬonverts to magnetite under metamorphic conditions, but a few apparently ORE DEPOSIT GEOMETRY: Tabular bodies 2 to 5 m thick and 2 to 10 km across.ĪLTERATION: None relevant to mineralization.ĮFFECT OF WEATHERING: Removes carbonate gangue, and converts ferrous silicates to ferric oxides. Some Ordovicianĭeposits on blocks complexly deformed by the Armorican (Hercynian) orogeny of Western STRUCTURAL SETTING: Major deposits in undeformed to simply folded strata. ISOTOPIC SIGNATURES: Siderite has light C, about -18 per mil unknown for other minerals (Maynard, Many of the larger deposits show features of tidally influenced ORE CONTROLS: Three-quarters of deposits show strong control by position at the top of Hematite cemented with Fe silicates to magnetite at Sierra Grande, Argentina ( Leiding V., 1955). GANGUE MINERALS: Quartz + calcite + dolomite + clay minerals apatite (collophane) ubiquitous in small amounts. Siderite common as a replacement locally, pyriteįound as replacement ( Maynard, 1986) occasionally, magnetite. Older rocks: hematite +Ĭhamosite (14-Å chlorite). ORE MINERALOGY: Younger rocks: goethite + berthierine (7Å chlorite). TheĪssociation with black shale ( Hallam and Bradshaw, 1979) is significant: 75 percent of well-developed Phanerozoic ironstones have anĮxtensive black shale at the base of the shoaling cycle ( Van Houten and Arthur, 1989). With bipolar cross-stratification suggesting intertidal deposition. (modified after Van Houten and Bhattacharyya, 1982 Maynard, 1983).ĪSSOCIATED ROCKS: Standard vertical succession is black shale at base, followed by grAy shaleĪnd siltstone, then by sandstone with graded bedding and hummockyĬross-stratificiation suggesting tempestites, and finally by sandstone or oolitic ironstone Variable cycles may range from a few meters to as many as 300 m in thickness Generalized stratigraphic model for oolitic ironstones. HOST ROCKS: Almost always clastic hosted at top of coarsening and shoaling-upward cyclesįigure 24. REGIONAL DEPOSITIONAL ENVIRONMENT: Shallow shelf, most typically close to the transition from nonmarine to marineĪGE RANGE: Phanerozoic, concentrated in the Ordovician to Devonian and Jurassic to TECTONOSTRATIGRAPHIC SETTING: Craton margins, 40 percent craton interiors, 25 percent foreland basins, 20 Ironstones by absence of primary siderite and presence of oolitic textures.ĪSSOCIATED DEPOSIT TYPES (*suspected to be genetically related): None. Oolitic textures, and Al-bearing silicates. Then because of competition from Precambrian banded-iron formations.ĭISTINGUISHING FEATURES: Distinguished from banded iron formations by absence of chert, presence of RELATIVE IMPORTANCE: Important source of Fe from 1850 to 1945. 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). Texture deposited in shallow-shelf to intertidal, clastic-dominated environments. SYNONYM: Clinton-type depoist Minette-type deposit.ĭESCRIPTION: Beds rich in iron silicate and oxide minerals with distinctive oolitic Oolitic ironstones DESCRIPTIVE MODEL OF OOLITIC IRONSTONES
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