Large Ore Deposits Research Articles

The flow of hot brines in cracks and the formation of ore deposits

We examined the thermal evolution of solutions transported vertically by turbulent flow in fractures and its implications for Mississippi Valley-type ore deposition. The main parameter governing the flow is fracture width, which determines the flow rate, volume, and thermal evolution of the transported fluid. We assumed local equilibrium and traced the chemical evolution of the transported solutions, the mineral deposits associated with the cooling of these solutions, and the limits of fracture clogging. The volume of fluid expelled and the fracture width determine the location of the deposit and, hence, the deposit type, i.e., carbonate hosted or vein. We show that a network of thin cracks can indeed transport the volume of brines needed for the genesis of large Mississippi Valley-type ore deposits, such as the Southeast Missouri lead district.

Volatilization, transport and sublimation of metallic and non-metallic elements in high temperature gases at Merapi Volcano, Indonesia

Condensates, silica tube sublimates and incrustations were sampled from 500–800°C fumaroles and lava samples were collected at Merapi Volcano, Indonesia in Jan.–Feb., 1984. With respect to the magma, Merapi gases are enriched by factors greater than 10 5 in Se, Re, Bi and Cd; 10 4–10 5 in Au, Br, In, Pb and W; 10 3–10 4 in Mo, Cl, Cs, S, Sn and Ag; 10 2–10 3 in As, Zn, F and Rb; and 1–10 2 in Cu, K, Na, Sb, Ni, Ga, V, Fe, Mn and Li. The fumaroles are transporting more than 10 6 grams/day ( g d ) of S, Cl and F; 10 4–10 6 g/d of Al, Br, Zn, Fe, K and Mg; 10 3–10 4 g d of Pb, As, Mo, Mn, V, W and Sr; and less than 10 3 g d of Ni, Cu, Cr, Ga, Sb, Bi, Cd, Li, Co and U. With decreasing temperature (800-500°C) there were five sublimate zones found in silica tubes: 1) cristobalite and magnetite (first deposition of Si, Fe and Al); 2) K-Ca sulfate, acmite, halite, sylvite and pyrite (maximum deposition of Cl, Na, K, Si, S, Fe, Mo, Br, Al, Rb, Cs, Mn, W, P, Ca, Re, Ag, Au and Co); 3) aphthitalite (K-Na sulfate), sphalerite, galena and Cs-K. sulfate (maximum deposition of Zn, Bi, Cd, Se and In; higher deposition of Pb and Sn); 4) Pb-K chloride and Na-K-Fe sulfate (maximum deposition of Pb, Sn and Cu); and 5) Zn, Cu and K-Pb sulfates (maximum deposition of Pb, Sn, Ti, As and Sb). The incrustations surrounding the fumaroles are also chemically zoned. Bi, Cd, Pb, W, Mo, Zn, Cu, K, Na, V, Fe and Mn are concentrated most in or very close to the vent as expected with cooling, atmospheric contamination and dispersion. The highly volatile elements Br, Cl, As and Sb are transported primarily away from high temperature vents. Ba, Si, P, Al, Ca and Cr are derived from wall rock reactions. Incomplete degassing of shallow magma at 915°C is the origin of most of the elements in the Merapi volcanic gas, although it is partly contaminated by particles or wall rock reactions. The metals are transported predominantly as chloride species. As the gas cools in the fumarolic environment, it becomes saturated with sublimate phases that fractionate from the gas in the order of their equilibrium saturation temperatures. Devolatilization of a cooling batholith could transport enough acids and metals to a hydrothermal system to play a significant role in forming an ore deposit. However, sublimation from a high temperature, high velocity carrier gas is not efficient enough to form a large ore deposit. Re, Se, Cd and Bi could be used as supporting evidence for magmatic fluid transport in an ore deposit.

Metal-depleted root zones of the Troodos ore-forming hydrothermal systems, Cyprus

The cupriferous pyrite deposits of Cyprus were precipitated from hydrothermal solutions derived by interaction of contemporaneous seawater with hot mafic rock at the ancient Troodos spreading centre. Here we identify the zones in which this interaction took place. The zones occur in the lower part of the sheeted dyke complex, and within them 30–50% of the rock is made up of epidosite, an epidote-quartz rock, replacing the dykes as sheets and pipes. The epidosites contain abundant fluid inclusions, which give trapping temperatures of 350–400°C or even higher, and contain water normally near seawater in salinity. Zones of epidosite are elongate parallel to the strike of the sheeted dykes, and are up to 1 km wide. The rocks throughout these zones are strongly depleted in Cu and Zn, and the metals removed are sufficient to supply the ore deposits. In fact several large ore deposits lie along strike from zones of epidosite. All of these features support the identification of the epidosites as the hydrothermal reaction zones. The location of the epidosite zones immediately above the gabbros of the plutonic complex supports the hypothesis that the heat to drive the ore-forming systems came from the underlying magma, as is also likely for modern black smoker springs.

Migration of radium from the thorium ore deposit of Morro do Ferro, Poços de Caldas, Brazil

A large thorium ore deposit is located in Morro do Ferro, a hill in the Poços de Caldas Plateau, Minas Gerais, Brazil, which contains an estimated 30 000 t of Th and 100 t of U in a highly weathered matrix exposed to erosion and leaching. 228Ra and 226Ra were analyzed in surface waters collected at various points in the drainage basin and in groundwaters from wells drilled through and around the ore body. The concentrations in groundwater demonstrated that radium is markedly leached by rainwater percolating through the ore body. In its transit underground, radium is removed from groundwater by sorption on soil particles and this natural process greatly reduces the radium discharged to the environment. In dry weather, the concentration of dissolved 228Ra in the main stream draining the Morro do Ferro is 7·0 ± 1·1 mBq litre −1 and in a control stream 1·6 ± 0·3 mBq litre −1. The estimated 228Ra mobilization rate by solubilizations is of the order of 10 −7 y −1.

Hydrothermal models for the generation of massive sulfide ore deposits

The discovery of massive sulfide ore deposits at certain sites at active seafloor spreading centers provides a basis for considering the energetics of processes that have concentrated similar large ore deposits that have been preserved in volcanic rocks on land. In this paper we construct transport models based on hydrothermal convection systems in the oceanic crust. We investigate models in which the circulation is driven by heat extracted from the permeable crustal rocks themselves as well as by heat conducted through the roof of a replenished, vigorously convecting crustal magma chamber. If the hydrothermal fluids carry 100 ppm dissolved iron as found in black smokers on the East Pacific Rise and exit at 350°C or greater, calculations show that the heat content of the oceanic crust is insufficient to account for ore deposits of 3 Mt or greater. Heat extraction through the roof of a magma chamber, in conjunction with the formation of a thin (∼1 km) layer of plated gabbro, may account for ore bodies of the order of 3 Mt, provided the permeability of the oceanic crust in the magma‐hydrothermal fluid contact zone is 10−15 m2 or less. If the conducting boundary layer were only 1 m thick, locally, for 103years, the permeability could be as high as 10−13 m2 The permeability in the discharge zone could, conceivably, be two orders of magnitude greater than the permeability at depth. More work is needed on the details of the effects of magma chamber replenishment and off‐axis dike injection and on the role of magmatic fluids. Very large ore deposits (i.e., ∼100 Mt) are probably formed in multiple hydrothermal cycles.

Evaluation of Ilmenite as Weight Material in Drilling Fluids

Summary This paper is based on field experiences from two wells and laboratory experiments. Flow-induced abrasion is a considerable problem in the field when ilmenite is used as weight material. In general, field observations are confirmed by laboratory experiments. However, the abrasiveness is highly dependent on the particle size distribution, and experiments clearly show that the abrasiveness of ilmenite can be reduced to that of standard barite by removing the largest ilmenite particles. From our experimental results, we recommend that the size distribution of ilmenite contain fewer than 3% particles greater than 45 microns [45 ]. The use of ilmenite also demonstrated problems with dust. dispersion of ilmenite in water, air entrainment, and foaming, which, as found by laboratory investigations, can be eliminated by reducing the concentration of flotation chemicals. On the basis of field experience and subsequent laboratory studies, we conclude that ilmenite is well suited for use as weight material in drilling fluids since the presently observed disadvantages to a large extent can be diminished. Introduction The demand for high-quality barite as drilling fluid weight material is expected to be increasingly difficult to meet. In addition, the supply of barite is geographically limited, with high transportation costs as a consequence in many cases. Because of the expected shortage and possible cost savings, a research program based on Norwegian resources was initiated in Norway in 1975 to investigate possible alternative weight materials. In their study, Haaland et al. found iron-base materials such as ilmenite and hematite to have a good potential for use as weight material. The advantages of ilmenite and hematite as weight material include high specific gravity, solubility in acid, reaction with sour gases, similar or superior fluid properties compared with barite, and availability of large ore deposits. On the basis of this information, Statoil initiated two field evaluations and a subsequent laboratory study during 1979–81 to evaluate ilmenite as weight material. The purposes of the field evaluation were (1) to investigate the overall performance of ilmenite as weight material and (2) to identify problem areas that occur when ilmenite is used. The purposes of the laboratory study were (1) to find, and possibly eliminate, the cause of the observed hydrophobic character of the ilmenite used in the field tests, (2) to evaluate the abrasiveness of ilmenite when used as weight material, and (3) to develop a simple and fast abrasion test applicable for field use. The results of this program give a better basis for evaluating ilmenite for use as weight material in drilling fluids. Field Experience Two wells with ilmenite as weight material were drilled in the Norwegian sector of the North Sea in 1979. Both wells were drilled with the same drilling rig, and the total depths and maximum mud weights for Wells A and B were 12,192 ft [3716 m] and 11.7 lbm/gal [1.40 g/cm3], and 9,121 ft [2780 m] and 15.3 lbm/gal [1.83 g/cm3], respectively. Seawater-gel/lignosulfonate mud systems were used in both cases. Typical mud properties at maximum mud weights are given in Table 1. The drilling contractor reported improved rheological properties when ilmenite was used as weight material. Less fine particles, very low attrition rate, and no appreciable settling resulted in a stable rheology with limited need for water dilution and chemical conditioning. Excessive wear was observed in the mud mixing and storage system, whereas no excessive wear or corrosion was found on corrosion rings run in the low-velocity parts of the drillstring. The contractor reported increased change out of pump parts and noticeable wear in the centrifugal pumps. A washout, which could be caused by the use of ilmenite, appeared in a failsafe valve. Fig. 1 is a sketch of the circulating system, with calculated circulation velocities indicated. Visual inspections were carried out throughout the drilling period, and the parts corresponding to 6, 8 and 9 in Fig. 1 were reported to show excessive wear. As can be seen from Fig. 1, this corresponds to high-velocity parts of the circulation system. According to the drilling contractor's report, ilmenite was also difficult to water-wet and disperse, and air entrainment and foaming occurred. In addition, the reports claimed problems with excessive dust in the mud room. The dust adhered to pipes and walls, causing cleaning problems. JPT P. 969^

Mineralogical and geochemical indicators of large ore deposits

Mineralogical and geochemical indicators of large ore deposits

A model study of the transient electromagnetic coincident loop technique

A free-space parametric model study of the Transient Electromagnetic (TEM) coincident loop technique has been undertaken to provide a basic scheme for the interpretation of coincident loop anomalies and to provide insight on the important survey design parameters. Experiments have been carried out to investigate the effects of changing geometrical relationships, target conductance, lateral resolution in the case of multiple thin dykes, and the masking influence of conductive overburden. The results confirm the advantages of the coincident loop technique in prospecting for large deep-seated ore deposits.

Parameters Controlling the Distribution of large Ore Deposits, Ore Clusters, Mineral Belts and Metallogenic Provinces

Parameters Controlling the Distribution of large Ore Deposits, Ore Clusters, Mineral Belts and Metallogenic Provinces

"Parameters controlling the distribution of large ore deposits, ore clusters, mineral belts and metallogenic provinces"—a four year international project

"Parameters controlling the distribution of large ore deposits, ore clusters, mineral belts and metallogenic provinces"—a four year international project

Deep-seated iron ores from banded-iron formation

Very large iron ore deposits have formed in many parts of the world, evidently by the supergene alteration of Precambrian banded-iron formation (BIF). Some of these deposits extend to great depths, ranging to 2,400 m at Krivoyrog1, beyond the likely reach of oxygenated water. We propose here, on the basis of studies in the Hamersley Ranges, Western Australia, a mechanism for deep-seated ore formation in which electronic conduction through the magnetite layers in BIF connects a cathodic region near the surface, where oxygen is reduced, to an anode at depth, where iron (II) from magnetite, carbonates and silicates is oxidized and precipitated as iron (III) hydroxyoxides. The electrical circuit is completed by ionic conduction through groundwaters. The model is based broadly on a mechanism suggested by Sato and Mooney2 to explain self-potentials associted with sulphide ore bodies. It is based more specifically on a model demonstrated by Thornber3 for the weathering of massive nickel–iron sulphide deposits at Kambalda, Western Australia, but with substantial differecnes in the physical situation and the reactions at depth.

Evolution of the ore formation in the history of the earth

The formation of the earth's crust proceeded unevenly, mainly during the first 2 milliards of years of the earth (4.6-2.6 mld.). This has led to the inclusion into the early protocrust of the first undifferentiated blocks of the oceanic crust containing effusives of ultrabasic composition of the type of comatiites, which were attended by unique nickel deposits. The presence of large gold ore deposits (Yellowknife, Colar) is characteristic of the same old protocrust. The further geological history of the earth's crust is connected with an intensive erosion of the oldest crust which occurred under conditions of a changing earth atmosphere, of an increase of free oxygen in it and of new conditions of sedimentation. One of the most striking examples of this was the appearance and huge development during the lower Proterozoic (2.6-1.9 mld. of years ago) of big deposits of ferruginous quartzites, jaspilites (Hammersli, Krivoi Rog, KMA, etc.). From this time dates the appearance of the first large uranium deposits (2.6-1.9 mld. of years ago) (Witwatersrand, Atabaska, Singhbum, Alligator River). The following middle and upper Proterozoic (1.9-0.6 mld. of years ago) was characterized by a similar global appearance of large polymetallic deposits being mainly of a stratiform type. The genesis of the deposits was connected with the accumulation of thick carbonate strata enriched in C org (MacArthur, Mount-Isa, Zavar, Sullivan, Sardana, Kholodnenskoye) appearing at that time. The geological history of the earth's crust during the Phanerozoic is distinguished by a further complication of rock composition. Owing to the appearance of numerous linear structures and connected with the latter sediments having been subject to unprecedented sedimentary differentiation, enriched in organic materials, which in their turn were distinguished by accumulation, sorption of rare elements and metals—metallogenic provinces of most different type arose in the earth's crust: tin-bearing provinces (Pacific Ocean zone), tungsten provinces (Middle Asia), germanium provinces (Donbass), uranium ones (Hercynides of Europe), etc. Much favourable to this was the locally displayed magmatism in the same mobile zones which favoured the redeposition of the ore load of sediments and the creation of vein deposits. The irreversible and strictly directed character of the evolution of chemical distribution of elements in the structures of the earth's crust, its relatively slight relation to mantle sources of metals and submission to life development and the laws of organic geochemistry—such are the features which determine the contemporaneous distribution of elements in the earth's crust.

Mid-proterozoic sulphate evaporites at Mount Isa mine, Queensland, Australia

PSEUDOMORPHED sulphate evaporites have been discovered in the Middle Proterozoic (1,600–1,500 Myr BP1) silica–dolomite sequence2,3 at the Mount Isa Mine, Queensland, Australia (20°44′ S, 139°29′ E). The silica–dolomites are host to the copper mineralisation at the Mount Isa mine2,3 and interdigitate with the fine-grained silver–lead–zinc-bearing pyritic and dolomitic Urquhart Shales of the Mount Isa Group1. A close association of evaporites with copper4,5 and lead–zinc6 has been reported for many ore deposits. The discovery of replaced sulphate evaporites at Mount Isa reported here raises the possibility that this large ore deposit may have been formed from metal-rich brines of the kind forming today in evaporitic basins7 rather than from volcanically derived hydrothermal solutions8,9.

Metalliferous potential of the United Kingdom

The total estimated value of past non-ferrous metalliferous production, as well as that of fluorspar, barytes and witherite, in the United Kingdom, as a whole, is £5.95 x 10 9 at assumed average 1973 metal and mineral prices. Southwest England produced roughly 70% of this value. The outlook for future production appears to be best in Wales and Scotland. At least one large unmined ore deposit of disseminated type is known, although vein-type ore bodies are more likely to be found. The effects of the development of high-tonnage mechanical equipment favour opencast mines and imply that vein deposits must be rich in order to compete economically. The estimated value of potential production from England and Wales is put in the range of £6.3 x 10 8 to £1.3 x 10 9 at assumed average 1973 prices. No quantified estimate is considered possible for Scotland.

The Oceans; A Possible Source of Iron in Iron-Formations

Recent work on one of the major banded iron-formations of the world--that of the Hamersley Basin of Australia--has defined the total amount of iron and the probable time interval of accumulation of the large iron ore deposits in the basin. The inferred rate of influx of iron into the basin is so large that a derivation of the iron from weathering and transport into the basin by rivers is unlikely. For the same reason derivation of the iron from volcanic sources is also unlikely. Derivation of the iron from the ocean floor is quantitatively possible if deep ocean water containing a few mg/1 of dissolved iron welled up, was oxidized in near surface environments, and deposited its iron in the basin. Iron concentrations of a few mg/1 in deep ocean water are quite reasonable if these water masses were saturated with respect to siderite and calcite.

Analysis of results of lithochemical exploration in metallogenic-prospecting

A truer expression of ore potential of an area, as in the two essentially polymetallic ore districts in Kazakhstan, and a statistically greater probability of discovery of large ore deposits therein are obtainable by a) certain corrections for the landscape-geochemical variables in processing the results of metallometric surveys; b) introduction of the “specific territorial ore potential,” the concept here developed, into pre-estimations of geological reserve of the ore in the given area; c) consideration of geochemical spectra of the mineralization (as in fig. 2). -- V.P. Sokoloff

MANGANESE ORE OF SOUTH‐INDIA AND ITS MAGNETIC PROPERTIES *)

AbstractThe Kodur Manganese belt has been surveyed with a vertical magnetometer. It was generally observed that both high positive and negative anomalies were associated with some of the existing ore bodies. Other large manganese ore deposits gave little or no magnetic anomalies. Further it was observed that both types of manganese ore deposits giving rise to large and little magnetic anomalies, contained similar percentages of manganese and iron. In view of these discrepancies, an extensive study of the magnetic properties of these manganese ores was taken up. The results of these measurements showed that the susceptibilities of the manganese ore samples collected vary over a wide range.It was observed that the high susceptibilities are associated with samples containing manganmagnetite and vredenburgite, whereas samples rich in psilomelane and pyrolusite possess low susceptibilities. From the chemical analysis, normative values of manganmagnetite and vredenburgite–with a general formula of (Mn.Fe)3O4– were calculated and a linear relationship could be established between these normative values and susceptibilities.