Showing papers on "Ankerite published in 2002"

Genesis of high-grade hematite orebodies of the hamersley province, western australia—a discussion

Abstract: Sir: Taylor et al. (2001) have presented a hypogene-supergene model for the genesis of the premium martite-microplaty hematite ores derived from banded iron formation (BIF), to replace the supergene-metamorphic model (Morris, 1980, 1985, 1998; Harmsworth et al., 1990). The new model is based on hydrothermal features in two of ten Tom Price hematite deposits, recorded previously as carbonate metasomatism of BIF (Ewers and Morris, 1981), and as localized postore metasomatism (Harmsworth et al., 1990; Morris, 1998). From new data, Taylor et al. (2001) proposed that hypogene leaching of free silica with addition of further siderite in BIF, followed by compaction, produced a general hematite ore precursor. During deep weathering episodes, or even within Tertiary weathering profiles, oxidation of magnetite to martite, and siderite to microplaty hematite and ankerite, followed by leaching of residual gangue, enriched this modified BIF to martite-microplaty hematite ore. In contrast, in the supergene-metamorphic model, martite-microplaty hematite ore genesis is linked to metamorphism of precursor ores of the Phanerozoic supergene martite-goethite type. These comprise about 90 percent of the Hamersley BIF-hosted resource, and deposits range to over a billion tonnes (Harmsworth et al., 1990) and >250 m deep, with high phosphorus levels (0.07–0.17% P). Although stratigraphically thinned by leaching (~35%) they maintain primary BIF banding and texture, with gangue minerals altered to phosphorus-bearing goethite pseudomorphs. Their martite-microplaty hematite analogues (< 0.06 % P) are attributed to Proterozoic supergene enrichment of BIF, with burial metamorphism partly converting the goethite pseudomorphs to secondary, largely microplaty, hematite, yet preserving the primary banding. Some 20 known deposits of this type range from small pockets of enrichment to the massive Mount Whaleback (1700 Mt). Later re-exposures of the microplaty hematite deposits produced further stratigraphic thinning (total ~45–50 %), with loss of phosphorus by leaching of remnant goethite. However, significant …

Ankerite carbonatite from Swartbooisdrif, Namibia: the first evidence for magmatic ferrocarbonatite

Abstract: Although general accounts of carbonatites usually envisage Ca–Mg carbonate melts evolving by fractional crystallisation to Fe-rich residua, there is longstanding concern that ferrocarbonatites may actually be products of hydrothermal rather than magmatic processes. All previously published examples of ankerite- and/or siderite-carbonatites fail to show one or more of the isotopic criteria (all determined on the same sample) thought to be diagnostic of crystallised magmatic carbonate liquids. Ferrocarbonatite dykes cut Archaean-Proterozoic basement at Swartbooisdrif, adjacent to the NW Namibia-Angola border. Their age is uncertain but probably ~1,100 Ma and their associated fenites are rich in sodalite. Where unaffected by subsequent recrystallisation, their petrographic textures resemble those of silicate layered intrusions; ankerite, magnetite and occasionally calcite are cumulus phases, joined by trace amounts of intercumulus pyrochlore. Ankerite is zoned, from Ca(Mg, Fe2+)(CO3)2 cores towards ferroan dolomite rims. Calcite contains ~1.7% SrO, plus abundant, tiny exsolved strontianite grains. Magnetite is close to pure Fe3O4. Pyrochlore has fine-scale euhedral oscillatory zoning and light-REE-enriched rims. ICP-MS analysis of magnetite and pyrochlore from the carbonatite allows calculation of their modal amounts from mass-balance considerations. Sodalite from the fenite is REE poor. Geothermometry, using either the calcite-dolomite solvus or oxygen isotope fractionation between calcite and magnetite, gives temperatures in the range 420–460 °C. Initial Sr, Nd and Pb isotopic ratios of the ferrocarbonatites (87Sr/86Sr=0.7033; eNd=0.2–1.0; 206Pb/204Pb=16.37; 207Pb/204Pb=15.42; 208Pb/204Pb=36.01) are appropriate for an ~1,100-Ma magmatic carbonatite. Likewise, carbonate δ18O=8.0 and δ13C=–7.36 indicate little or no subsequent shift from magmatic values. It appears that dense ankerite and magnetite dominated crystal accumulation from a melt saturated in these phases, plus calcite and pyrochlore, with prior fractionation of a silicate mineral and apatite. The resulting ferrocarbonatite lacks a silicate mineral (excluding fenite xenocrysts) and apatite. It has unusually low (basalt-like) REE abundances and (La/Lu)n, and low concentrations of Ba, Rb, U, Th, Nb, Ta, Zr and Hf. Very high Nb/Ta and low Zr/Hf imply that the evolution of the parental magma involved immiscible separation of a carbonate from a silicate melt. The sodalite-dominated Swartbooisdrif fenites suggest that the parental melt also had a substantial Na content, in contrast with the ferrocarbonatite rock.

A Typical Alkaline Rock-Carbonatite Complex in Bayan Obo, Inner Mongolia

Abstract: In the Bayan Obo area, there is a unique large rock complex composed of an alkaline-carbonatite complex (H8), carbonatite dykes, sedimento-volcanic series (H9), shallow intermediate-basic dykes and breccia peri-dotite. The carbonatite (H8) , the host rock of the super-large Bayan Obo Nb-Fe-REE Deposit, has long been erroneously assumed to be sedimentary dolomites. A comprehensive comparative study suggests that the car-bonatite is quite similar to the igneous carbonatite dykes in this region and other carbonatites in the rest of the world and quite different from the micrite mound at Sailinhuodong and dolomite in the typical sedimentary sec-tion. Thus, the host carbonatite of the deposit is actually a typical alkaline-carbonatite complex. In the CaO-MgO-(FeO + Fe2O3 + MnO) diagram, the carbonatite falls into two regions, i. e. , a calcic carbonatite repre-sented by calcite carbonatite and a magnesian carbonatite represented by ferrous dolomite or ankerite carbon-atite. The magmatic composition of the former belongs to the Ca-Fe series and that of the latter Mg-Fe series, both having an evolutionary tendency of Fe enrichment.A statistics of main igneous carbonatites (excluding sodic carbonatite) in the world also shows two indepen-dent rock types and two magmatic series. This suggests that carbonatite have an intrinsic lithogenic and metal-logenic specialization, which is independent of locality, age and occurrence. The host carbonatite originally may be subhorizontal sills and later changed into a synformal fold due to syntectonic deformation. It is inferred that there is a deep magmatic passage between the main and east ore bodies.The recognition and description of basalt in H9 put new constraint on its nature of formation and offer a new understanding on the origin of K enrichment in the K-rich slate, that the K enrichment is a fenitization mainly represented by the K-Na metasomatism accompanying the host carbonatite.

Catagenic ankerite replacing biogenic calcite in the Marhøgda Bed (Jurassic), Sassenfjorden, Spitsbergen

Abstract: The Marhogda Bed occurring at base of the Adventdalen Group in Sassen− fjorden, Spitsbergen contains common ankerite-replaced belemnite skeletons. Petrographic, major element geochemical, and stable carbon and oxygen isotopic data indicate that the ankerite originated in a catagenic environment associated with thermal degradation of kerogenand hydrocarbongenerationinthe sequence. It formed at maximum temperature of 150°C under burial of approx. 2 000 m, most probably during Paleogene filling and subsi− dence of the Central Spitsbergen Basin. Dissolution of biogenic calcite and precipitation of ankerite reflect extensive heat flow through the Adventdalen Group sequence related to the Cretaceous and Paleogene magmatic and orogenic activity in Svalbard.

Rift valley sedimentation and diagenesis, Tanzanian Examples - A review

Abstract: The main diagenetic events of polyphase rift valley successions are controlled by provenance geology, tectonic evolution and subsidence, weathering related to different episodes of fresh water flushing, and topographic and climatic effects. Based on the Tanzanian examples as case studies and comparisons with published studies on rift valley sedimentation and diagenesis, the main diagenetic phases of Tanzanian rift valley successions can be presented: 1. Early diagenetic precipitation of evaporites, hematite and calcrete along with feldspar dissolution, and in some cases formation of pore-filling clay minerals, zeolites and feldspar, may take place. 2. Middle diagenetic formation of albite, zeolites, chlorite, recrystallization of calcite and some quartz precipitation have been reported from several rift valley settings. Continuous subsidence results in enhanced recrystallization of calcite, with a trend towards larger crystal size and more poikilotopic configurations, and new generations of quartz and clay minerals such as e.g . illite. The precipitation of dolomite, ankerite and siderite have commonly been observed during this stage. 3. Late diagenetic processes is related to uplift and renewed tectonic activity. Under such conditions new generations of hematite, calcite and clay minerals were formed, partly as a result of feldspar dissolution caused by meteorically-induced fresh water flushing.

Fluidos associados à mineralização da mina de ouro schramm, complexo granulítico luis alves (sc)

Abstract: The gold mineralization at the Schramm mine, in Santa Catarina State, southern Brazil, is associated with quartz and carbonate veins within a ductile-brittle, NNE-striking, subvertical shear zone, of Neoproterozoic age. The shear zone crosscuts Archean basic granulites and gneisses of the Luiz Alves complex, next to the contact with the volcano-sedimentary sequence of the Itajai Group. Within the vein vicinities gneisses have been altered to quartz and muscovite and granulites to chlorite and albite; carbonate alteration is pervasive in all host rocks. Both carbonate (siderite, ankerite and dolomite) and quartz veins in the mineralized zone contain As, Ni, Co, Cu, Zn, Pb and Fe sulfides, but the high grade zones (above 500 ppm Au) are mostly within the former. On the basis of their modes of occurrence and microthermometric behavior, four types of fluid inclusions were identified in carbonates (C1, C2, C3, and C4) and three in quartz (Q1, Q2, and Q3). Primary H 2 O-CO 2 inclusions with salinity values in the range of 0,2 to 14 wt% NaCl eq. and total homogenization temperatures between 280°C and 310°C are represented by the inclusion types C1, C2 and Q1. The inclusion types Q2 and C3 are primary aqueous, with salinities between 4 and 11 wt% NaCl eq. and total homogenization temperatures within the interval of 205°C and 270°C. The inclusion types Q3 and C4 are of secondary origin and contain an aqueous solution with salinities varying from 3 e 13 wt% NaCl eq. and total homogenization between 140oC e 190oC. These inclusions were trapped during the quartz and carbonate recrystallization, as a result of the reactivation of the shear zone and formation of the Itajai Basin. The primary inclusion types C1, C2, C3 and Q1 and Q2 in carbonate and quartz, respectively, are considered as representatives of the gold-mineralizing fluid, which are considered originally a low sulfur, CH 4 – and N 2 - free H 2 O-CO 2 solution, rich in CO 2 , Fe, Mg, Ca, Na, K, Cl, Au, Ag, Ni , As and SiO 2 , with Zn and Co in subordinate amounts. Massive carbonate veins were the first to precipitate from this fluid during the temperature rise of the hydrothermal system, together with the gold and sulfides, due to the reaction of this fluid with iron rocks intercepted by the shear zone. This reaction may have led to the desestabilization of the metal-bearing sulfur complexes, reduction of the gold and precipitation of the sulfides. Sulfide-rich, gold-poor quartz veins crystallized afterwards, in the waning stages of temperature drop due to the mixing of the H 2 O-CO 2 fluid with meteoric waters.

ABSTRACT: Petrography of Ankerite Cement, Grain Replacement, and Fracture Fill in Foreland Sandstones of the Central Rocky Mountains

Abstract: Ferroan dolomite and ankerite (here referred to as ankerite) are widespread late diagenetic precipitates in both sandstones and carbonate rocks in a variety of basinal settings. In foreland basins of the central Rocky Mountains, in rocks ranging in age from Cambrian to Cretaceous, ankerite is sufficiently abundant locally to exert significant control on both matrix and fracture porosity in oil and gas reservoirs. Thus, deciphering the controls on the emplacement of this mineral is interesting from a practical standpoint. Ankerite in these rocks, similar to other volumetrically important authigenic minerals, manifests a spatial distribution at small scales that reflects a difficulty with nucleation. Within sandstones, ankerite post-dates quartz cementation and is widely observed, at the scale of a few micrometers, to be localized either as overgrowths on detrital dolomite grains in sandstones or as replacement of detrital K-feldspar. Detrital dolomite cores are observed to contain abundant intragranular fractures, confirming that ankerite precipitation post-dates at least a portion of the compaction in these rocks. Spatial affiliation with dissolving detrital K-feldspar suggests a possible microscale pH control on precipitation. Controls on ankerite precipitation in fracture porosity are not evident, but elemental compositions of cements, grain replacements, and fracture fills show similar ranges of variation, suggesting that these different petrographic forms of ankerite precipitated from the same fluids and record a common history of fluid/rock interaction.

Dolomite-group ferroan carbonates from kremikovtsi deposit

Abstract: dolomite-group ferroan carbonates (ferroan dolomite, ankerite) are the main non-metallic component in the primary ores of the Kremikovtsi deposit. They (1) formed carbonate assemblages in the transition zones between the siderite ore bodies and the host Middle Triassic dolomitic limestones, (2) accompanied all sulfide assemblages, (3) constituted alteration zones in the carbonate rocks hosting the Pb-Cu sulfide mineralization, and (4) deposited as post-ore rhombohedral crystals in cavities within the dolomitic limestones. In the non-sulfide mineral assemblages these minerals are represented by micro-grained aggregates of ferroan-manganoan dolomite in a coarse-grained ankerite matrix. Zonal manganoan ankerites with decreasing Fe contents toward the rims are characteristic for the sulfide assemblages. In cavities within the host carbonate rock linings from coarse-grained manganoan ankerite are formed with the highest FeCO3 content (up to 23 mol%) in the deposit.