Geology topics

M.J.K. Flohr

Publications and source records attributed to M.J.K. Flohr.

6 recordsLinked to original sources

USGS1994

Titanium, vanadium, and niobium mineralization and alkali metasomatism from the Magnet Cove complex, Arkansas

The Christy Ti-V-Nb deposit is hosted by the Arkansas Novaculite, adjacent to the carbonatite-alkaline Magnet Cove Complex. Mineralized rock contains up to 8 wt percent Ti, 2.68 wt percent V, and 0.124 wt percent Nb. Minor amounts of Li and Mo are present, and there is evidence for the preferential concentration of the middle to heavy rare earth elements (REE) in altered clay-rich dikes. Mineralized rock consists of (1) thermally recrystallized novaculite that is partly replaced by actinolite, sodic amphibole, taeniolite (KLiMg 2 Si 4 O 10 F 2 ), pyrite,and siderite, (2) brookite-bearing amphibole- and taeniolite-rich rock that contains siderite and pyrite, (3) quartz-brookite aggregates, and (4) goethite-rich rocks that form the oxidized zone of the deposit. Unaltered rock is scarce at the Christy deposit. Comparison of mineralized lithologies from the Christy deposit with those from two other areas of mineralization associated with the Magnet Cove Complex, the Magnet Cove Titanium Corporation deposit ("Rutile deposit") and the Hardy-Walsh prospect, provides mineralogical and chemical evidence for the source of the mineralizing fluids at the Christy deposit.Massive clay-rich (kaolinite + or - smectite) dikes are in contact with the mineralized zones of the Christy deposit. The presence of relict K feldspar and pyrite in the least altered dikes and certain characteristic trace elements (e.g., Ba, Rb) suggest that the Christy dikes are related to less pervasively altered rutile-mineralized, pyrite-bearing carbonate-feldspar dikes fromthe Rutile deposit. The feldspar-carbonate dikes from the Rutile deposit may be the products of intense alkali metasomatism of alkaline igneous silicate rocks that were emplaced prior to the intrusion of the Magnet Cove carbonatite.The Christy deposit formed through a series of complex processes. The initial phase of mineralization is directly related to the infiltration of novaculite by alkali-rich fluids that were probably derived from carbonatite magma. Introduction of the metasomatic fluids preceded emplacement of massive feldspar- and pyrite-bearing dikes that were subsequently altered to the clay-rich masses Titanium, V, Nb, and Li were introduced by the alkali-rich fluids at temperatures that were, as indicated by fluid inclusion data (Willis et al., 1991), as high as 600 degrees C. During the initial stage of mineralization, V was concentrated in aegirine and sodic amphibole, Li was concentrated in taeniolite, minor amounts of Ti were concentrated in aegirine, and pyrite formed. The replacement of novaculite by the aforementioned minerals yielded excess silica, which precipitated as quartz in veins and as clear overgrowths on recrystallized, inclusion-rich (partly replaced) quartz. Niobium- and V-bearing brookite precipitated with the quartz. Minerals formed during the first stage reacted with a second fluid at temperatures of 100 degrees to 300 degrees C, as indicated by fluid inclusion data (Willis et al., 1991), and V was then concentrated in smectite and goethite. The isotopic composition of siderite [delta 18 O SMOW = 23.8-25.0ppm, delta 13 C PDB = -7.1 to -8.9ppm suggests that the second fluid was a mixture of low-temperature metasomatic fluid and ground water. Vanadium was further concentrated in clay minerals (Breit et al., 1992), in goethite, and in vug minerals as low-temperature alteration proceeded. Brookite is not totally resistant to weathering and it is at least partly replaced by other oxides in the oxidized zone.

Economic Geology↗

USGS1992

Geochemistry and origin of the Bald Knob manganese deposit, North Carolina

Geochemical and textural evidence for the submarine hydrothermal origin of the Bald Knob Mn deposit, North Carolina, is preserved in a mineralogically diverse suite of rocks that have experienced amphibolite facies metamorphism. Manganese- and Mn-Fe-rich lithologies have high Mn/Fe ratios and low concentrations of Ni, Cu, Zn, U, Th, and rare earth elements that are similar to concentrations reported from other ancient manganiferous deposits and from crusts and sediments which are forming in hydrothermally active regions of the present sea floor. Manganese- and Ca-rich carbonate minerals, a variety of Mn-rich silicates (including tephroite, the Mn humite-group minerals, spessartine, rhodonite, and pyroxmangite), and the oxides galaxite and jacobsite form fine laminations that represent different bulk compositions. No chemical or textural evidence for replacement of Mn-poor by Mn-rich minerals was found and, in some samples, the same lithologic units are repeated on the scale of a thin section, consistent with a primary sedimentary origin for the layering. Chert, which is one of the country rocks at Bald Knob, and Mn- and Ca-rich carbonate rocks represent original sedimentary protoliths. More than one detrital component may be present in the Mn-rich layers. Biogenic detritus was the probable source of at least some of the siliceous material that allowed for the formation of the Mn-rich silicates. An aluminous detrital component, which probably was derived from a continental or andesitic source, is reflected in spessartine-rich layers. Minor Mn and Co sulfide mineralization (alabandite, cattierite, cobaltite) was contemporaneous with deposition of the Mn-rich sediments. The association of the manganiferous lithologies with chert and meta-igneous rocks (amphibolite) is consistent with deposition of the sediments in a rifting environment, such as a back-arc basin, or possibly, a midocean ridge.

Economic Geology↗

USGS1992

Chemical fluxes and origin of a manganese carbonate-oxide-silicate deposit in bedded chert

Lens-like rhodochrosite-rich bodies within interbedded chert and shale are associated with basalt and/or graywacke in ophiolitic and orogenic zones. The Buckeye manganese mine in the Franciscan Complex of the California Coast Ranges is associated with metagraywacke. Despite blueschist-facies metamorphism, this deposit preserves the compositions and some textural features of its sedimentary protoliths. For this reason, it is a suitable deposit with which to compare more intensely altered deposits, or deposits originating in different paleoenvironments. Six Mn-rich and three Mn-poor minerals form monomineralic layers and mixtures: rhodochrosite, gageite, Mn-oxides (hausmannite, braunite), divalent Mn-silicates (caryopilite, taneyamalite), chlorite, quartz (metachert) and aegirine-augite. The Mn-rich protoliths have high Mn/Fe combined with relatively low concentrations of Ca, Al, Ti, Co, Ni, Cu, Th and REE. REE patterns of various protoliths are distinct. Rhodochrosite and gageite layers are depleted (seawater ?? 5 ?? 104) and flat, whereas patterns of metachert and the Mn-silicate-rich layers mimic the patterns of metashale and metagraywacke (seawater ?? 106). Hausmannite layers have flat patterns (seawater ?? 7 ?? 104) whereas braunite-rich layers are more enriched (seawater ?? 2 ?? 105) and show a distinct positive Ce anomaly. Factor analysis reveals components and fluxes attributed to sub-seafloor fluids (Ni, As, Zn, Sb, W, Mn), seawater (Mg, Au, V, Mo), detritus and veins (Ca, Ba, Sr). Silica is negatively correlated with the sub-seafloor factor. The observed variances indicate that water from the sediment column mixed with seawater, that deposition occurred near the sediment-seawater interface before mixtures of subsurface fluid and seawater homogenized, and that the system was not entirely closed during metamorphism. The variations in REE enrichment can be related to kinetics of deposition: rhodochrosite and gageite were precipitated most rapidly, and therefore were the protoliths that most effectively diluted the REE-rich background resulting from fine clastic material (derived from distal turbidites). The variation of the Ce anomaly and U/Th among diverse lithologies and the differences in Mn oxidation states are consistent with progressive dilution of reduced subsurface fluids with oxidized seawater. By this scheme, rhodochrosite, gageite and hausmannite were deposited from the most reduced fluids, braunite from intermediate mixtures, and Mn-silicates from the sub-seafloor fluids most diluted with fresh seawater. Comparison of the Buckeye with other lens-like and sheet-like deposits having high Mn/Fe and containing Mn3+ and/or Mn2+ suggests that each had three essential fluxes: a sub-seafloor source of Mn, a local source of very soluble silica and a source of relatively fresh, oxygenated water. Additional fluxes, such as clastics, appear to be more characteristic of the paleoenvironment than the three essential fluxes. ?? 1992.

Chemical Geology↗

USGS1992

Mineralogy and geochemistry of two metamorphosed sedimentary manganese deposits, Sierra Nevada, California, USA

Laminated to massive rhodochrosite, hausmannite, and Mn-silicates from the Smith prospect and Manga-Chrome mine, Sierra Nevada, California were deposited as ocean floor sediments associated with chert and shale. The principal lithologies at Smith are chert, argillite, rhodochrosite-, hausmannite- and chlorite-rich layers, and relatively uncommon layers of jacobsite. The Manga-Chrome mine also contains layers rich in manganoan calcite and caryopilite. Tephroite, rhodonite, spessartine, and accessory alleghanyite and sonolite formed during metamorphism. Volcaniclastic components are present at Manga-Chrome as metavolcanic clasts and as Mn-poor, red, garnet- and hematite-rich layers. There is no evidence, such as relict lithologies, that Mn was introduced into Mn-poor lithologies such as chert, limestone or mudstone. Replacement of Mn-poor phases by Mn-rich phases is observed only in the groundmass of volcanic clasts that appear to have fallen into soft Mn-rich mud. Manganiferous samples from the Smith prospect and Manga-Chrome mine have high Mn Fe and low concentrations of Ni, Cu, Zn, Co, U, Th and the rare-earth elements that are similar to concentrations reported from other ancient Mn deposits found in chert-greenstone complexes and from manganiferous sediments and crusts that are forming near modern sea floor vents. The Sierra Nevada deposits formed as precipitates of Mn-rich sediments on the sea floor, probably from mixtures of circulating hydrothermal fluids and seawater. The composition of a metabasalt from the Smith prospect is consistent with those of island-arc tholeiites. Metavolcanic clasts from the Manga-Chrome mine are compositionally distinct from the Smith metabasalt and have alkaline to calc-alkaline affinities. A back-arc basin is considered to be the most likely paleoenvironment for the formation of the Mn-rich lenses at the Manga-Chrome mine and, by association, the Smith prospect. Layers of rhodochrosite, hausmannite and chert preserve the composition and some textures of the sedimentary protoliths at both Sierra Nevada deposits. Jacobsite-rich layers probably represent a Fe-rich protolith. Caryopilite and manganoan calcite represent additional protoliths at the Manga-Chrome mine. The metamorphic assemblage prehnite-chlorite-epidote-calcite in a metabasalt from the Smith prospect constrains regional metamorphic conditions to a maximum temperature of 325??C and a pressure of 2 kbar. Slightly higher temperatures are indicated by the presence of actinolite in another metabasalt. Compositions of Mn-rich minerals in Smith samples are consistent with these metamorphic conditions. ?? 1992.

LITHOS↗

USGS1992

Crystalline solution series and order-disorder within the natrolite mineral group

Electron microprobe and X-ray analyses were made of natrolite, tetranatrolite, gonnardite, and thomsonite from the Magnet Cove alkaline igneous complex, Arkansas, and of selected specimens from the U.S. National Museum. This information and data from the literature indicate that natrolite, mesolite, scolecite, edingtonite, and tetraedingtonite show only small deviations from the ideal stoichiometry. In contrast, gonnardite, tetranatrolite, and thomsonite show large deviations from the ideal end-member compositions and compose three crystalline series. The structures of the natrolite minerals are defined by combining each of the three types of framework structures with various combinations of channel-occupying polyhedra. Various polysomatic series can be constructed by combining slices of two basic structures to form new hybrid structures. -from Authors

American Mineralogist↗

USGS1990

Alkaline igneous rocks of Magnet Cove, Arkansas: Mineralogy and geochemistry of syenites

Syenites from the Magnet Cove alkaline igneous complex form a diverse mineralogical and geochemical suite. Compositional zoning in primary and late-stage minerals indicates complex, multi-stage crystallization and replacement histories. Residual magmatic fluids, rich in F, Cl, CO2 and H2O, reacted with primary minerals to form complex intergrowths of minerals such as rinkite, fluorite, V-bearing magnetite, F-bearing garnet and aegirine. Abundant sodalite and natrolite formed in pegmatitic segregations within nepheline syenite where Cl- and Na-rich fluids were trapped. During autometasomatism compatible elements such as Mn, Ti, V and Zr were redistributed on a local scale and concentrated in late-stage minerals. Early crystallization of apatite and perovskite controlled the compatible behavior of P and Ti, respectively. The formation of melanite garnet also affected the behaviour of Ti, as well as Zr, Hf and the heavy rare-earth elements. Pseudoleucite syenite and garnet-nepheline syenite differentiated along separate trends, but the two groups are related to the same parental magma by early fractionation of leucite, the presumed precursor of intergrowths of K-feldspar and nepheline. The Diamond Jo nepheline syenite group defines a different differentiation trend. Sphene-nepheline syenite, alkali syenite and several miscellaneous nepheline syenites do not consistently plot with the other syenite groups or each other on element and oxide variation diagrams, indicating that they were derived from still other parental syenite magmas. Mineral assemblages indicate that relatively high f{hook};O2, at or above the fayalite-magnetite-quartz buffer, prevailed throughout the crystallization history of the syenites. ?? 1990.

LITHOS↗