Mésoprotérozoïque

Banded fine-grained pyrite in shale from the Precambrian of Australia. (public display, Leadville Mining Museum, Leadville, Colorado, USA) A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates. The sulfide minerals contain one or more sulfide anions (S-2). The sulfides are usually considered together with the arsenide minerals, the sulfarsenide minerals, and the telluride minerals. Many sulfides are economically significant, as they occur commonly in ores. The metals that combine with S-2 are mainly Fe, Cu, Ni, Ag, etc. Most sulfides have a metallic luster, are moderately soft, and are noticeably heavy for their size. These minerals will not form in the presence of free oxygen. Under an oxygen-rich atmosphere, sulfide minerals tend to chemically weather to various oxide and hydroxide minerals. Pyrite is a common iron sulfide mineral (FeS2). It’s nickname is “fool's gold”. Pyrite has a metallic luster, brassy gold color (in contrast to the deep rich yellow gold color of true gold - www.flickr.com/photos/jsjgeology/sets/72157651325153769/), dark gray to black streak, is hard (H=6 to 6.5), has no cleavage, and is moderately heavy for its size. It often forms cubic crystals or pyritohedrons (crystals having pentagonal faces). Pyrite is common in many hydrothermal veins, shales, coals, various metamorphic rocks, and massive sulfide deposits. The rock shown above consists of numerous bands of fine-grained pyrite interbedded with dark shale. Published research has shown that the pyrite is diagenetic, formed by sulfate reduction from sulfate-bearing groundwater that moved along bedding planes of the Urquhart Shale host rocks (see Painter et al., 1999). The sulfate source was evaporitic gypsum-anhydrite-barite in the same stratigraphic unit. Stratigraphy: Urquhart Shale, Mount Isa Group, Mesoproterozoic, ~1655 Ma Age of metamorphism: peak greenschist-facies metamorphism at ~1505 Ma during the Isan Orogeny Locality: Mount Isa Mines, northwestern Queensland, northeastern Australia Some info. from: Kawasaki & Symons (2010) - Dating of Mesoproterozoic metamorphism in the Mount Isa and George Fisher Zn-Pb-Cu-Ag deposits, Australia, by paleomagnetism. American Geophysical Union, Fall Meeting 2010, Abstract GP33C-0953. Painter et al. (1999) - Sedimentologic, petrographic, and sulfur isotope constraints on fine-grained pyrite formation at Mount Isa Mine and environs, northwest Queensland, Australia. Economic Geology 94: 883-912. Photo gallery of pyrite: www.mindat.org/gallery.php?min=3314

Banded iron formation from the Precambrian of Wyoming, USA. (~10.9 cm across at its widest) Banded iron formations, or BIFs, are unusual, dense sedimentary rocks consisting of alternating layers of iron-rich oxides and iron-rich silicates. Most BIFs are Proterozoic in age (although some are Late Archean), and do not form today - they're “extinct”! Many specific varieties of iron formation are known, and some are given special rock names. For example, jaspilite is an attractive reddish & silvery gray banded rock consisting of hematite, red chert (“jasper”), and specular hematite or magnetite. Because of their age, most BIFs have been around long enough to have been subjected to one or more orogenic (mountain-building) events. As such, most BIFs are folded and/or metamorphosed to varying degrees. BIFs are known from around the world, but some of the most famous & extensive BIF deposits are found in the vicinity of North America’s Lake Superior Basin. Many BIFs have economic concentrations of iron and are mined. BIFs are the most important variety of iron ore on Earth. Some iron mines in west-central Wyoming exploit BIFs in the Goldman Meadows Formation, a Mesoarchean unit exposed in the Wind River Range. These rocks have been multiply metamorphosed during the Precambrian. The result of this metamorphism is highly contorted folding and fracturing. The rock shown above is a folded quartz-hematite-limonite meta-BIF. Stratigraphy: iron formation member (probably the upper iron formation member) of the Goldman Meadows Formation, upper Mesoarchean, 2.87 Ga (metamorphosed in the Archean at 2.8 Ga and in the Mesoproterozoic at 1.4 Ga) Geologic context: northwestern flank of the South Pass Greenstone Belt, southern Wind River Range Locality: Atlantic City Iron Mine (open-pit mine; sample possibly collected from tailings piles around the now-flooded pit) (E1/2 of section 26, T30N, R100W, Miners Delight 7.5' USGS topographic quadrangle), South Pass-Atlantic City Mining District, along Rt. 28, southwestern side of South Pass, north of Atlantic City, southwestern Fremont County, west-central Wyoming, USA (mine is at 42° 32' 45" North latitude, 108° 44' 33" West longitude)