Stratiform Orebodies Research Articles

Origin of siderite deposits from the Lombardy Valleys, northern Italy: a carbon, oxygen and strontium isotope study

In the Lombardy Valleys, siderite mineralization is widespread as stratiform orebodies in Lower Triassic terrigenous-carbonate rocks as well as vein orebodies in Permian volcanics and continental sequences and in the metamorphic basement. A number of these orebodies and host limestone/dolomite rocks have been analysed for C, O and Sr isotope compositions, in order to constrain geochemically the origin of the siderite. The intermediate δ 13C-, δ 18O- and 87 Sr 86 Sr-values of stratiform siderite relative to vein siderite and host limestone and the very good linear correlations between δ-δ and δ- 87 Sr 86 Sr data points concur to support a metasomatic origin for stratiform siderite after calcite. In addition, stratiform and vein siderite should have formed from the same kind of fluids, and possibly are the products of the same mineralizing event. The δ-values of carbonate rocks are discussed in terms of recrystallization/dolomitization effects during diagenesis. These processes occurred earlier than siderite mineralization.

Lead-isotope study of the sulphide ore and alteration zone, Bleikvassli zinc-lead deposit, northern Norway

The Bleikvassli Zn-Pb deposit is located in the Uppermost Allochthon of the northern Norwegian Caledonides and is enclosed in amphibolite facies, multiply deformed supracrustal rocks. The stratiform orebody occurs stratigraphically above a sequence of gneiss and amphibolite and below a thick carbonate unit. The orebody, spatially associated with a footwall microcline gneiss that contains as much as 12wt‰ K2O, occurs in the lower part of the Mine Sequence which also comprises (kyanite-) mica schist and quartzo-feldspathic to siliceous rocks. The host rock lithology and the metal content of the Bleikvassli orebody are consistent with a SEDEX origin of the deposit. Field relationships and chemistry suggest that the microcline gneiss represents a potassic alteration of pelitic sediments related to the ore-forming process. A 464 ± 22 Ma Rb-Sr isochron for the microcline gneiss is interpreted to be a metamorphic age resulting from resetting of the Rb-Sr isotopic system during the Caledonian orogeny. The U-Pb in the whole rock shows evidence of recent mobilization of uranium and a partial or total resetting of the system during peak metamorphism. As with most SEDEX deposits, the lead isotope composition of the Bleikvassli ore plots close to the orogen growth curve. The geological setting of the ore and the lead — isotope compositions of the galenas indicate a Cambrian age of mineralization. However, the slope of the lead isotope data indicate an age of about 1000 Ma, which is also a maximum age of ore deposition. The lead isotope data for the galena, in conjunction with the compositions of the microcline gneiss during peak metamorphism, support a model whereby the microcline rock was formed as an alteration product by the ore forming fluid and the initial lead isotope composition of the microcline rock was similar to that of the galenas during ore deposition.

Stable isotope geochemistry of carbonates from the Apuane Alps mining district, northern Tuscany, Italy

rock, vein filling and ore-gangue carbonates associated with the main stratabound and largely stratiform barite-iron oxides-pyrite deposits of the Apuane Alps were analysed for carbon and oxygen isotpe compositions. The orebodies occur within a metamorphic phyllitic complex with carbonate-bearing intercalations (Scisti di Fornovolasco), and at the contact with the overlying metamorphic dolostones (Grezzoni). Carbonates from the tectonically underlying Nucleo Metamorfico Apuano unit were also analysed. The δ 13 C PDB and δ 18 O SMOW values of dolomite and calcite are in the ranges -0.9 to 2.8‰ and 23.1 to 31.4‰, respectively, for country rocks, -1.9 to 0.0‰ and 22.2 to 26.0‰ for vein carbonates, and -29.3 to -20.3‰ and 16.6 to 19.4‰ for gangue calcites. The main isotopic features are (1) the lowest δ 11 8O values for carbonate rocks are found at the contacts with either schist or orebody, (2) vein dolomite is depleted in both 13 C and 18 O relative to host rock dolomite, and their δ 18 O values very well correlate positively, (3) the δ 13 C of gangue calcites matches with that of carbonates associated to sulphate-reducing bacterial activity. The isotopic data and their spatial distribution can satisfactorily be interpreted either in terms of variable alteration of the protolith isotopic signatures by interaction with metamorphic fluids, or in terms of crystallization at metamorphic temperatures from fluids riven out of the schists. Water to rock mole ratios are tentatively estimated. The present results are compatible with a sedimentary-diagenetic origin for stratiform orebodies, followed by metamorphic recrystallization and mobilization processes

Application of integer planning and simulation to production planning for a stratiform ore body : Chanda, E C K Min Sci TechnolV11, N2, Sept 1990, P165–172

Application of integer planning and simulation to production planning for a stratiform ore body : Chanda, E C K Min Sci TechnolV11, N2, Sept 1990, P165–172

Pb isotopes of the Hualgayoc area, northern Peru; implications for metal provenance and genesis of a Cordilleran polymetallic mining district

The Hualgayoc district of northern Peru is typical of several Pb-Zn-Ag-Cu deposits in the Peruvian Cordillera in which veins are closely associated with stratiform (manto) orebodies. Steeply dipping veins containing mainly galena, sphalerite, pyrite, chalcopyrite, tetrahedrite, and rhodochrosite are hosted by Miocene andesitic plutons and by the gently folded and faulted Cretaceous miogeosynclinal sediments they intrude. The sediments also host concordant pyrite-rich orebodies that are crosscut by the veins.A comprehensive survey of the isotopic composition of lead in crustal rocks of the Hualgayoc region demonstrates that the Tertiary plutons, which partly host the veins, contain less radiogenic lead than either the Cretaceous sedimentary rocks that host the ores or the regional metamorphic basement rocks believed to underlie the Hualgayoc district. Sedimentary and metamorphic rocks from the Hualgayoc region contain no component of the highly nonradiogenic lead found in ancient high-grade rocks like those of the southern Peru coast or inferred to exist in eastern Colombia and also differ from basement rocks of the Guyana shield in Venezuela.Lead in vein and manto galenas from Hualgayoc is isotopically indistinguishable and coincides with the composition of the Tertiary plutons. The vein and the manto ores thus probably formed from the same solutions. Ion microprobe traverses across the growth zones of large, euhedral vein galena crystals reveal little isotopic variability of Pb during their growth. These data indicate that the ore-forming solutions had essentially a single source of lead, that this source was the Tertiary plutons in the district, and that local mixing of lead from several sources was not important. Although the plutons may have assimilated some lead from crustal rocks at depth, these inputs were homogenized by the time of ore formation.The manto ores are interpreted to be metasomatic bodies formed during the same event that produced the veins, rather than syngenetic massive sulfide ores overprinted by a later episode of vein mineralization as proposed by other workers. This interpretation is supported by the lead isotope evidence, by the overlap of paragenetic sequences in the vein and manto ores and the orderly progression of fluid inclusion homogenization temperatures and salinities from the manto to the vein minerals, by the absence of local igneous activity during the interval of sedimentation, and by the abundance of replacement textures in the manto ore sufides.The absence in the ores of lead leached from supracrustal rocks indicates (1) that processes within the magma determined the supply of metals to the ore-forming system, (2) that supracrustal host rocks enriched in ore metals are not necessary for the formation of this type of deposit, and (3) that meteorically derived waters, though probably comprising part of the ore solutions, did not scavenge important amounts of crustal lead.

An application of integer programming and simulation to production planning for a stratiform ore body

Planning of underground mines poses considerable difficulties in the areas of safety, environment, ground control and production scheduling. As the industry is faced with more and more marginal reserves, it is becoming imperative to generate mine plans which will provide optimal operating strategies and make the industry more competitive. The problem of production planning in underground mines can be formulated using available operations research techniques and computing power. In this paper a computerized model for short-term production scheduling in a typical block caving mine with a stratiform ore body is presented. By combining simulation with mixed integer programming, it has been possible to model realistically the problem of scheduling draw points for production at the Chingola Mine of Zambia Consolidated Copper Mines Limited. The model is based on a given layout of the mining block which is considered fixed during the planning period. The model is intended for use by planning engineers or mine captains working with personal computers. In comparison with manual scheduling, the computerized model has been found to be faster and generates near-optimal schedules.

Macrotextures, microtextures, and carbonate-sulfide relationships in stratiform, carbonate-hosted Zn-Pb orebodies of Silvermines, Ireland

Ore-stage carbonates of the Silvermines stratiform orebodies have increased Zn and Pb and high Fe and Mn content. A major microtexture observed is produced by the replacement of carbonate allochems by dolomite, Fe dolomite, and siderite followed by the replacement of carbonates by sulfides. Generally, the process of replacement took place while the former fabric of the host carbonates was preserved. The reactivity of metal carbonates with sulfur favored the concentration of Pb and Zn in the sulfide and of Fe and Mn in the carbonate form. Sulfide-carbonate reactions probably maintained a high permeability in the ore zone over an extended period of time eventually producing a high-grade ore from solutions rich in Fe and Mn but poor in Zn and Pb. Initial porosity and permeability created by gravity flows controlled the mineralizing process at the beginning, but later stages were probably controlled by replacements, i.e., sulfide-carbonate reactions.

The Occurrence and Gemological Properties of Wessels Mine Sugilite

Examination of jewelry-quality siigilite shows that i t consists o f manganese-bearing sugilite and other minerals i n a polycrystalline aggregate. The material occurs in a large stratiform manganese orebody at the Wessels m ine near Kuruman, South Africa. Some 12 to 1.5 tons o f sugilite of varying quality lire estimated to occur at the mine. The attractive purple color is due to the presence of about 1-3 wt.% n~anganese oxide. The research reported here revealed that there are actually two types of gem materials that have heretofore been called sugilite: one that is predominantly manganoan sugilite with minor impurity minerals, and the other that is chalcedony mixed wi th (and colored by ) sugjlite.

Geochemistry and structure of stratiform deposits with a portable microcomputer

Mineral exploration generally involves the synthesis of a large amount of quantitative and qualitative data. Data types are highly variable, ranging from chemical analyses to colors, and their relative importance is difficult to rank-order prior to data collection. Statistics and visual examination of plotted data help with rank-ordering and with identification of geographic areas of interest. A portable microcomputer offers great assistance in the plotting and statistical analysis of data in the field so that exploration strategy may respond rapidly. Data synthesis is particularly important for stratiform ore bodies because they commonly grade laterally into extensive subore beds. Trends in the subore may point to an ore deposit. Relevant microcomputer programs are listed and described herein. Detailed descriptions of these programs are provided to exemplify programming techniques.

Les gisements de spinelles chromifères

Chrome spinels are common accessory minerals in most mantle peridotites, mafic-ultramafic magmas and cumulates. In a Cr-spinel compositional diagram these various rock-types may be separated. From an economic point of view, three main types of chromite orebodies are described : stratiform orebodies from mafic-ultramafic stratiform complexes (Bushveld, Great Dyke) ; podiform orebodies from mantle peridotites in ophiolite complexes (New Caledonia) and orebodies from archean ultramafic magmas (Selukwe). The most important ore potential lies in the first type, whereas the best ore grades are found in the two others. The geological setting, the orebody and the ore textures are described ; geochemical data on major and trace elements are given ; then the genetic models are discussed. At last, some non-economic chromite bodies are also found in mafic-ultramafic ring complexes (Ural), ultramafic ophiolite cumulates, anorthosite archean stratiform complexes and some pyroxenite dikes from lherzolite massifs. In conclusion, the most important chromite potential lies in precambrian magmatic complexes and the Cr-rich ores are related to ultramafic or Mg-rich tholeïtic magmas.

十和田湖東南地区の中新統中の雲母／スメクタイト混合層鉱物

Core samples from 18 boreholes in the area southeast of the Lake Towada were investigated to determine expandability of mica/smectite in the Miocene sediments. The expandability decreases with depth from about 80 to 0% over intervals of 100 to 400m in the western part of the area, while intervals of more than 600m in the eastern part. The percentage of expandable layers in mica/smectite decrease from east to west. The expandable layer isopleth map at the top of the Yotsuzawa Formation shows a decrease in expandable layers from 80 to 10% at a distance of about 500m. The conversion of smectite to mica/smectite mixed layers begins at 0.6-0.8% vitrinite reflectivity, and the conversion to the 10% of expandable layers occurs in 1.2-2.8% vitrinite reflectivity. Stratiform ore bodies of Kuroko and copper-lead-zinc veins are restricted to the zone less than 20% of expandable layers at the top of the Yotsuzawa Formation. The expandability of mica/smectite mixed layer in sediments is useful as a guide to mineral exploration.

Sulphur isotopes in metamorphosed Precambrian Fe-Pb-Zn-Cu sulphides and baryte at Aggeneys and Gamsberg, South Africa

AbstractSulphur isotope ratios in sulphides and baryte from stratabound and stratiform orebodies in a metavolcanic-sedimentary sequence in Namaqualand were found, in part, to be extreme for Precambrian sulphur. Black Mountain, Aggeneys, in the west gave an average for the sulphides of δ34S = +8.9±3.7‰ (9 samples), an average for barytes of +20.6±4.3‰ (3 samples). Broken Hill, Aggeneys, in the centre gave an average for the sulphides of δ34S = +19.8±3.1‰ (19 samples). Gamsberg, in the east, gave an average for the sulphides of δ34S = +29.2±1.8‰ (24 samples), and an average for barytes of +35.4±0.2‰ (2 samples). The δ34S values increase eastward. Their range is strongly on the positive side and does not centre around zero. The Gamsberg barytes and most Gamsberg sulphides have more positive δ34S values than those reported for other Precambrian sulphides and sulphates. We interpret the above sulphur isotope range as being mainly due to the varying contributions of submarine-exhalative sulphide sulphur with δ34S close to zero and bacterially(?) reduced sulphate with strongly positive δ34S, apparently from evaporites in the east. Metamorphism of amphibolite facies grade has partly isotopically re-equilibrated the ore minerals, as indicated by galena-pyrrhotine and sulphide-baryte isotope temperatures from single specimens, but has not destroyed the primary sulphur isotope range indicating pre-existing sulphate concentrations.

Lead isotopes as a tool for gossan assessment in base metal exploration

Lead isotope measurements on gossans may be used as an exploration tool for Pb-Zn-Cu deposits of the “stratiform” type. The method is based on the homogeneous isotopic composition of stratiform orebodies and the close fit of their ratios to the so-called “growth curve”. These features also characterize oxidized outcrops. (gossans) of the primary sulphide ore. The retention of homogeneous Pb isotope ratios during oxidation was tested and proved for true gossans derived from known mineralization in which there was a variation in vertical depth from gossan to primary sulphide, different ages and different geological environments. The deposits included Pb-Zn-Cu (Woodlawn), Pb-Zn (Broken Hill), Zn-Pb (Dugald River), Zn-Cu-Pb (Currawang). The method has been extended to distinguish true gossans from false (pseudo) gossans where the base metals and associated trace elements have been scavenged from the surrounding rocks. It has also been applied to a company exploration program in which a number of ironstone cappings had been drilled and other geochemical information was available. Diamond drilling indicated that five prospects were barren, i.e. underlain by iron sulphides devoid of significant base metal mineralization. On the Pb isotope data, three of the prospects would be rejected; a fourth shows a particularly high potential and is worthy of further exploration, whilst the fifth initially showed high potential but a comparison of gossan and sulphide Pb isotope data after drilling suggests that the drill hole was sited on the fringes of economic mineralization. The Pb isotope method may be utilized further in exploration at the drilling stage. If sulphides are intersected, Pb isotopes may be used to distinguish barren sulphides from those with economic potential, even though they both may contain similar base metal concentrations.

Exploration Planning for Sandur Manganese Ores

Abstract Three prevailing ore types viz., stratiform, podiform and tabular deposits occur at the incidence ratio of 75:6:19. The stratiform orebodies can be designated as plenemensurate and regular; the podiform types as partimensurate and complicated; and the tabular ones as extramensurate and irregular. Based on the study presented here, a stratiform body may be explored on 100m × 100m square grid by core-drilling for the results to remain within the confidence level of ±20%; while pediform bodies may be explored with jack hammer-cum-diamond drilling at 50m × 50m square grid and the tabular deposits at 20 m × 20m square grid with deep pits-cum-drilling for the results to remain at the same level of assurance.

Kuroko deposits of the Kosaka Mine, Northeast Honshu, Japan; products of submarine hot springs on Miocene sea floor

The Kosaka mine is a productive Cu-Pb-Zn-Au-Ag mine in Japan and is located at the eastern margin of the Hokuroku mining district in northeast Honshu Island. The ore deposits of the mine are divided into three main groups from north to south, namely, the Motoyama, Uchinotai, and Uwamuki ore deposits. Each ore deposit has both massive stratiform and underlying network or disseminated, siliceous orebodies. The geologic features of each deposit are described.The iron content of sphalerite coexisting with pyrite from the Uchinotai-nishi and Uwamuki No. 4 ore deposits never exceeds 5 mole percent FeS. The iron content of sphalerite from the latter and two other Kuroko deposits decreases gradually upward from stockwork to stratiform orebodies. This vertical zoning may be a common feature of the Kuroko deposits, although those from the Uchinotai-nishi ore deposit show a minimum iron content at the boundary between the black and yellow ore zones.Mixing of sea water with the ascending hydrothermal solution causes an increase in oxygen fugacity and a simultaneous temperature decrease in the ore-forming environment. They are the main causes of ore deposition. Deposition of barite is likely to be the result of an increase in activity of metastable sulfate ions which are added from sea water to the reducing ore-forming system.A magmatic origin of the ore-forming fluid is also proposed on the basis of fluid inclusion data and geologic evidence.

Volcanogenic mineralization and a rhyolite dome in the Arabian Shield

The district of Mahawiyah in the Proterozoic shield of Arabia contains a group of Zn-Cu-Au-Ag-Ba mineral prospects in folded meta-sedimentary, volcanoclastic and volcanic rocks, ranging in composition from basalt to rhyolite. The mineralization occurs in veins and as strata-bound, disseminated orebodies associated with intense argillic alteration of adjacent rocks. An intrusive rhyolite dome or laccolith is situated at the centre of an eight square kilometre area of slight but pervasive alteration whose outline can be traced from aerial photographs and within which many of the ore mineral occurrences lie. A model is proposed to explain the pattern of alteration in the volcano-sedimentary pile and formation of the volcanogenic mineralization, based on a concept of the dome acting as a heat source to drive a geothermal "cell". Circulating connate-hydrothermal fluids could have caused alteration and redistribution of trace metals within the volcanics and sediments which mantle the sub-volcanic, rhyolite intrusion. The ore genetic model implies that clusters of veins, disseminated strata-bound and stratiform massive sulphide orebodies occur in distinct areas of the shield, marked by tracts of pervasive alteration which can be identified in aerial photographs and satellite images.

Volcanogenic mineralization at Avoca, Co. Wicklow, Ireland, and its regional implications

Abstract A description is given to the geological setting of the stratiform pyritic orebodies at Avoca in Ireland and a volcanogenic origin is proposed on the basis of comparison with stratiform orebodies located in similar volcanic host rocks in other parts of the Caledonian orogen and elsewhere. The Ordovician volcanic sequence in southeast Ireland is interpreted as a fossil island arc complex, which was formed above a contemporary margin of southeasterly subduction, which locally defined the boundary of the evolving Caledonian orogenic belt in southeast Ireland. The Avoca ore deposits are located within a sequence of calc-alkaline lavas, pyroclastics and sediments at a level marked by the predominance of felsic eruptives. The stratigraphy of the host rocks is described in some detail and related to the general geology of the Avoca belt. Massive pyritic ores and disseminated sulphide ores of ‘stringer type’ can both be recognized at Avoca. The massive ore shows a compositional gradation suggestive of a primary stratigraphy; a Cu-rich pyritic zone is overlain by a zone of Pb-Zn-Cu pyritic ore. The interpretation of the genesis of the orebody is complicated by its deformed state, which is typical of other volcanogenic orebodies within the various Caledonian provinces. A genetic model can be proposed, however, by using the descriptions of the undeformed ‘Kuroko’ deposits in Japan as a standard for comparison. The individual orebodies of Avoca type contain 5 000 000–6 000 000 tons of ore, and these are associated with extensive zones of low-grade disseminated ore in acid pyroclastic host rocks. Sequences of manganiferous and dolomitized sediments are stratigraphically closely related to the ores. The principal geological features of the Avoca orebodies having been defined, it is deduced that there is a strong possibility that similar orebodies lie hidden in the volcanic rocks of the southeast Irish Caledonides and that an aggressive exploration programme could lead to new discoveries.

Sulphur and sulphate-oxygen isotopes and the origin of the silvermines deposits, Ireland

New sulphur and sulphate-oxygen isotope measurements for the main discordant and stratiform lead-zinc-barite orebodies at Silvermines Co. Tipperary, allow reappraisal of previously offered differing interpretations (Graham, 1970; Greig et al., 1971) of the bearing of sulphur isotopes on the genesis of this important Irish deposit. The following aspects of the data are confirmed: barite δ34 S-values range from 17–21‰, similar to lower Carboniferous seawater sulphate: stratiform sulphide lens pyrites have δ34 S-values ranging from −13 to −36‰; vein sulphide δ34 S-values range from −8 to 4‰; sulphide δ34 S-values increase upwards and outwards respectively in the related discordant and stratiform “G” orebodies; galena-sphalerite isotope palaeotemperatures are not too consistent, ranging from 40 to 430°C (using the calibration of Czamanske and Rye (1974). New facts are as follows: barite δ18O-values range from −13 to −17‰, stratiform barites ranging from 13 to 14.5‰; sulphides separated from a single stratiform ore lens hand specimen usually have δ34 Ssl > δ34 Sga > δ34 Spy; the outward decrease in δ34 S-values in the stratiform “G” orebody is confined to the first few hundred feet only; pyrite δ34 S-values progressively increase downwards through one stratiform sulphide orebody; yet variations of 13‰ occur within a single colloform pyrite structure from another stratiform orebody. It is concluded that there were at least two sources of sulphur, seawater sulphate and deep-seated sulphur. The former was the dominant source of all sulphate and, via biogenic reduction, of the sulphur in the bulk of the stratiform sulphide. The latter was the source of the sulphur in the vein sulphides. There was minimal isotopic interaction between the cool seawater sulphate and the warm unwelling ore fluid sulphur species, even though the latter precipitated under near isotopic equilibrium conditions when the temperature dropped and/or the pH and Eh increased. The lack of isotopic equilibrium between pyrite and ore sulphides in the stratiform ore lenses may result from the latter having precipitated slightly later than the former because of solubility relationships. Overall the present isotopic evidence supports considerable geological evidence favoring a syngenetic origin for the stratiform Silvermines orebodies.

Origins of some massive pyritic ore deposits of western Europe

The geologic environments of formation of three types of massive, pyritic, largely stratiform orebodies of western Europe are compared. The largest concentration of such ores, the pyrite belt of Spain and Portugal, seems to have resulted from volcanic-hydrothermal precipitation on the sea floor largely in or adjacent to quartz keratophyre eruptive centers. The smaller deposits of northwestern Germany, important for their content of zinc and lead and with abundant barite, are in pelites close to reef limestones. They apparently are shallow-water accumulations, largely organically precipitated. The third and economically least important pyritic ores are those of southern Tuscany. The original sulfide accumulations appear to have formed in stagnant deep-water basins associated with calcium sulfate evaporites. The Triassic ores of Tuscany have been strongly modified and mobilized during a near-surface thermal event accompanied by a few intrusions during the Pliocene.

The tetrahedrite-freibergite series, with reference to the Mount Isa Pb-Zn-Ag orebody

Twenty eight electron microprobe analyses of freibergite from the Mount Isa (Queensland) Pb-Zn-Ag stratiform orebody, range in silver content from 18.4 to 42.5 wt. % Ag. These values significantly extend the tetrahedrite-freibergite series. The compositional range based on twenty-one complete analyses is indicated by the formula (Ag,Cu)9.21–11.44(Fe,Zn)1.59–2.31(Sb,As)3.87–4.43S13.0. As far as could be determined, Mount Isa freibergite is homogeneous and no marked compositional changes were detected either across individual grains, or in different grains of the same electron microprobe sample. The linear, atom for atom, replacement of copper by silver reported for lower silver bearing tetrahedrites continues in Mount Isa freibergite. A maximum silver content of about 51 wt. % Ag is predicted. X-ray investigations indicate however that in contrast to the structural expansion with increasing silver content reported for argentian tetrahedrite, Mount Isa freibergite contracts with increase in silver. The extrapolated lattice parameter for the theoretical freibergite (Ag10(Fe,Zn)2Sb4S13) end member is of the same order as tetrahedrite.