Formation Of Ore Deposits Research Articles

Biogeochemistry. Sulfate reducers--dominant players in a low-oxygen world?

Sulfate reducing bacteria can adapt to extreme physical and chemical conditions and play an important role in global geochemical cycles, but their role in the formation of ore deposits has remained controversial. Strong support for such a role is provided by Labrenz et al., who have discovered sulfate-reducing bacteria that can tolerate low levels of oxygen and can precipitate zinc sulfide minerals. The results may have implications for bioremediation and may provide clues to processes that may have been more widespread in the geologic past.

Two series of copper-gold deposits in the middle and lower reaches of the Yangtze River area (MLYRA) and the hydrogen, oxygen, sulfur and lead isotopes of their ore-forming hydrothermal systems

Based on studies on the geological characteristics of the copper-gold deposits in the middle and lower reaches of the Yangtze River area (MLYRA) and their hydrogen, oxygen, sulfur and lead isotope compositions, it is concluded that there existed two series of copper-gold deposits. They are evolutional products of two ore-forming hydrothermal systems in different geodynamic settings and geological era. Series I is stratiform or stratabound copper-gold deposits. These deposits were formed by submarine exhalation and sedimentation of hydrothermal solutions in Her-cynian tensional tectonic environment after bot brine ascending along contemporaneous faults and exhaled into the sea-floor. Series II consists of copper-gold deposits related to medium and acidic magmatic intrusions. Their mineralizations took place in Yanshanian in a tensional or a transitional period to the tensional tectonic environment from the composite of the tethys tectonic regime and the Paleo-Pacific ocean tectonic regime, as well as in the upper mantle doming and crustal thinning environment. Copper-gold deposits were formed from the hydrothermal fluids, mixtures of magmatic water and part of meteoric water, by complex water-rock interactions and coupling dynamic processes of transport-chemical reactions. Superposition is an important condition for the formation of the large-scale copper-gold ore deposits.

Genetic links between ash-flow calderas and associated ore deposits as revealed by large-scale thermo-mechanical modeling

In addition to a number of epithermal ore deposits, some porphyry-type ore deposits are also hosted by ash-flow calderas. Spatial correlation between ash-flow calderas and these deposits is observed in several parts of the world, but temporal correlation between the mineralization and the episode of caldera formation remains unclear, especially because geochronological data indicate that the mineralization may date to several million years before or after caldera formation. Because caldera collapse represents only a very short event in a long time-scale evolution between a silicic magma chamber and its host rocks, we have developed a thermo-mechanical model of upper crustal and magmatic system behavior during the pre- and post-caldera formation stages. Major petrophysical properties of ash-flow caldera related rocks are incorporated into the model, which accounts for brittle–elasto-ductile rheology and heat transfer, and reproduces fault initiation and various caldera collapse scenarios. Large-scale mechanisms of initiation of thermal and structural traps for epithermal ore deposits are described. In particular, we found that the outer sides of caldera border faults are associated with both anomalous heat-transfer and a high fracture density. The finite size of the insulating intracaldera units forces the accumulated heat excess to be laterally transferred across the border faults. In most cases, the model predicts “Y-shaped” fault geometry, which is surprisingly similar to that of mineralized faults in the field. Variations in lithostatic pressure on the magma chamber roof and in the vicinity of the border faults can reach 10 to 20MPa, with deep overpressured zones close to the inner edge of the chamber corners. Despite the fact that hydrothermal convection and fluid geochemistry are not included in the model, these thermal and mechanical results are believed to constrain the conditions for mineralization processes. Our results show that in the presence of regional extension, the development of caldera border faults may be suppressed to the advantage of deep-seated fractures, clustered above the central part of the magma chamber, thus favoring conditions for the formation of porphyry-type ore deposits. Finally, we demonstrate that well before the eruption, the gradual emplacement of a hot and large silicic magma chamber may create significant extensional stress in the brittle upper crust, resulting in breakup and pre-eruptive normal faulting and thus allowing for conditions of pre-caldera mineralization.

Sr-isotopic composition of siderite for assessing the origin of mineralizing fluids: the case study from the Jebel Awam deposit (Central Morocco)

The Sr-isotope ratios were determined for 17 samples of siderite from the polymetallic deposit of Jebel Awam, formed because of a granitic intrusion through metasedimentary Palaeozoic rocks about 286 Ma in the eastern part of the Moroccan Meseta. The isotopic ratios range from 0.712829 to 0.713960. As siderite contains negligible Rb, the measured isotopic ratios are the same as its parent fluids. In this view, the isotopic ratios of the carbonate rule out that the mineralizing solutions were composed of either magmatic fluids, as the local granites show a range of initial ratios between 0.706 and 0.709, or seawater, whose isotopic ratio about 286 Ma was 0.7075. On the contrary, the mineralizing fluids were composed of meteoric water circulating mainly through the schists of the Palaeozoic basement, whose isotopic range was between 0.712 and 0.718 at the time of the ore deposit formation. Moreover, the Sr-isotope ratios of siderite rule out any possible recrystallization of the carbonate during the Alpine orogenesis. As a whole, this study shows the possibility of using siderite for determining the type of fluids involved in ore deposits, provided that the age and the Sr-isotope ratios of the rocks of the mineralized area are known. This may be useful to assess also the provenance of the metals carried by the fluids.

SHRIMP measurements of U and Pb isotopes in the Koongarra secondary ore deposit, Northern Australia.

SHRIMP analyses have been conducted for rock samples from the Koongarra secondary ore deposit in Northern Australia to obtain activity ratios of 234U/238U and isotopic ratios of 207Pb/206Pb and 204Pb/206Pb and to understand their migration behavior within the secondary ore deposit. Main target minerals for the analyses were iron minerals and kaolinite, which are the main weathering products in this area. The activity ratios of 234U/238U were based on SHRIMP counts at the mass of uranium metal. The 234U/238U activity ratios based on counts of uranium oxides were not satisfactory, because the count rates of 234U16O were affected by interference from the 238U12C fragment. The activity ratios of 234U/238U were approximately unity for crystalline iron minerals, whereas the amorphous iron minerals (precursors of the crystalline iron minerals) had also values above unity. The mean residence time of uranium within the iron nodules was estimated to be up to approximately 2–3 million years. Results of lead isotopes, represented by a diagram of 207Pb/206Pb vs. 204Pb/206Pb, indicated that the present three samples contained radiogenic lead, common lead and mixture of both components. In addition, the recent radiogenic lead component within the primary ore is dominant in the area closer to the primary ore. The radiogenic lead isotope compositions of samples at greater distances from the primary ore have been affected only by the earlier mobilization event of radiogenic lead. All these results indicate that some geologic event causing migration of the radiogenic lead had occurred before formation of the secondary ore deposit due to the weathering. After the weathering commenced at least 2–3 million years ago, uranium and lead have migrated from the primary ore. While the uranium has dispersed throughout the secondary ore deposit, the lead reached only the area closer to the primary ore probably due to its immobility compared to the uranium.

Assessment of prospecting potentiality for superlarge continental volcanic rock-type uranium deposits in China

The superlarge continental volcanic rock-type uranium deposits, which were discovered abroad long ago, have not yet been reported up to now in China. This is an important problem that needs to be urgently solved by uranium geologists at present. In this paper, on the basis of analyzing the metallogenic settings and geological conditions of the superlarge continental volcanic rock-type uranium deposits discovered in the world along with the metallogenic characteristics of those of the same type in China, the space-time distribution patterns of continental volcanics and the metallogenic potential of main tectono-volcanic belts in China are discussed, and a synthetic conclusion has been drawn that there is a possibility to discover the superlarge continental volcanic rock-type uranium deposits in China. Moreover, it is evidenced that the Ganhang, Nanling, Yanliao, Da Hinggan Ling and other tectono-volcanic belts possess favorable geological conditions for the formation of ssuperlarge ore deposits of the continental volcanic rock type. The intersecting and overlapping locations of the aforementioned main belts with other tectono-volcanic (-intrusive) belts are the most potential areas where the superlarge continental volcanic rock-type uranium deposits would be found.

Minerogenetic mechanism of the Songxi silver-antimony deposit of northeastern Guangdong—Ore-controlling role of organic matter

Organic geochemistry and comparisons of characteristics of the organic matter in wall rocks of the ore-controlling strata and ores of the Lower Jurassic Songling black shale formation and the related Songxi silver-antimony deposit of northeastern Guangdong have been studied in this paper. The results show that the Lower Jurassic Songling shale formation is a suite of biologic-rich and organic-rich ore-bearing marine sedimentary rocks. Micro-components of the organic matter in the Songling black shale formation consists primarily of algae, amorphous marine kerogen, solid bitumen, and pyrobitumen. The thermal evolution of organic matter is at the over-maturity stage. There is a general positive correlation between total organic carbon (TOC) and metallogenetic elements such as Ag and Sb in the black shale formation. Organic matter in the host rocks in the Songxi ore deposit played a role in controlling the silver-antimony depositing environment during the forming process of the black shale ore-bearing formation. In the absence of vitrinite, the relative level of thermal maturity calculated by solid bitumen reflectance indicates that the ore-forming temperature of the Songxi silver-antimony deposit was about 150–170°C, which was considered as an epithermally reworked ore deposit. The roles of organic matter in the formation of the Songxi ore deposit are a primitive accumulation of the metallogenetic elements (Ag, Sb) in the sea-water cycle system for ore source and a concentration of metals by ion exchange or chelation as well as reduction of the oxidized metals.

Growth of granite–greenstone terranes at convergent margins, and stabilization of Archean cratons

Archean granite–greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events. Most Archean granite greenstone terranes appear to have acquired their first-order structural and metamorphic characteristics at convergent plate margins, where large accretionary wedges similar in aspect to the Chugach, Makran, and Altaids grew through offscraping and accretion of oceanic plateaux, oceanic crustal fragments, juvenile island arcs, rifted continental margins, and pelagic and terrigenous sediments. Buoyant slabs of parts of Archean oceanic lithosphere may have been underplated beneath these orogens, forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere. Back-stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges. Dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas (sanukitoid suite) in the mantle wedge, whereas other magmas (tonalite, trondhjemite, granodiorite or TTG suite) appear to have been generated by melting of hot young subducted slabs. Eventual collision of these juvenile orogens with other continental blocks formed anatectic granites, then thickened the crust beyond its ability to support its own mass, which initiated gravitational collapse and decompressional release of syn- to late-tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs, and aided in the cratonization of the granite–greenstone terranes. Deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid- and upper-crust, forming ore deposits and plutons. Most preserved granite–greenstone terranes have been tectonically stable since the Archean, and form the cratonic interiors of many continents.

A currently forming petroleum associated with hydrothermal mineralization in a submarine caldera, Kagoshima Bay, Japan.

Petroleum-like products have been found in hydrothermally altered recent sediments of the submarine Wakamiko Caldera (depth = 200 m) in the northern Kagoshima Bay, southern Kyushu, Japan. High concentrations of polynuclear aromatic hydrocarbons (PAHs) of pyrolytic origin and biomarkers signatures representing diverse degrees of maturation have indicated that the associated bitumen is a typical hydrothermal petroleum. This is the first report on a significant content of hydrothermal petroleum related with a submarine volcanism of island-arc. The petroleum was generated by pyrolysis of organic matter in recent sediments deposited in the caldera. The hydrothermal petroleum and massive sulfide deposits are concluded to be currently forming within the caldera by hydrothermal activities below the seafloor where presently forming Kuroko-type sulfide deposits were also reported. Considering simultaneous formation of the hydrothermal petroleum and the Kuroko-type deposits in the Wakamiko Caldera, we propose a hypothesis that, during the middle Miocene, the same phenomena happened extensively in northeast Japan in association with the large-scale marine volcanisms, resulting in the formation of both petroleum and sulfide ore deposits (Kuroko) in the same geological horizons.

Boudinage; a key to an organizing principle for the formation of ore deposits

A number of ore deposits are juxtaposed to illustrate a common boudinage signature. Examples are (1) Kalgoorlie (an epigenetic vein-type gold deposit from the Eastern gold fields of Western Australia; (2) Broken Hill (a stratiform sediment-hosted lead-zinc-silver deposit from New South Wales, Australia); (3) three hematite enrichment iron ore deposits, one from the Hamersley region of Western Australia, one from the Krivoi Rog region of Ukraine, and one from Nimba Range, Liberia; (4) Kambalda (an Archcan stratiform and strata-bound ultramafic-hosted nickel deposit in the Eastern Goldfields of Western Australia, with the same tectonic environment as Kalgoorlie); (5) the diamondiferous Argyle kimberlite pipe in the Kimberley region of Western Australia; and (6) typical oil and gas plays of western Europe. The first four of these deposits are the type, or close to the type for their class, emphasizing the importance of the linkage. A further boudinage emphasis lies in the historical perspective for these first enumerated deposits, for which a boudinage control was either explicitly or implicitly described by others. It is concluded that an important organizing principal in ore emplacement is probably being addressed, one which operates regardless of mineralization type or crustal level (tectonic environment), which remains unrecognized in the popular consensus on ore controls, which could usefully be a framework for other popular consensual models, and which could substantially advance understanding of controls on emplacement of mineralization and related tectonics generally.

Seamount formation and associated caldera complex and hydrothermal mineralization in ancient oceanic crust, Troodos ophiolite (Cyprus)

The inter-graben zone between the Solea and Mitsero grabens in the northern part of the Troodos ophiolite (Cyprus) displays igneous, tectonic, and physiographic features that are characteristic of seamount volcanism at modern oceanic spreading centers. In this zone the 200-m-high Alestos hill volcanic edifice consisting of pillow lavas with preserved lava tubes and cone sheet intrusions is interpreted to have developed as a seamount fed by a late-stage, off-axis shallow-level gabbroic pluton that was intrusive into young oceanic crust. Concentric cone sheet intrusions and associated faults and shear zones in and around the Alestos hill seamount collectively form a seafloor caldera complex, which probably formed during the extrusion of lavas and subsequent cauldron subsidence over the gabbroic pluton. The ENE-trending elongated massive sulfide ore deposit formation in the Alestos–Memi area represents a focused upflow zone for hydrothermal fluids driven by the same off-axis gabbroic intrusion responsible for formation of the Alestos hill seamount. The ENE-trending and up to 150-m-wide Alestos–Memi fault zone dissects the elongated ore body in a dextral sense and might have provided a high permeability conduit in the upper crust for upwelling hydrothermal flow at the time of magmatism. The geometry of this fault zone and its kinematic relations with a N–S-striking Alestos Forest normal fault in the west are suggestive of its transfer fault origin, reminiscent of mineralized transfer faults found at the Mid-Atlantic Ridge. The active TAG hydrothermal mound with massive sulfide deposits that is located on a small volcanic dome 3 km off-axis and along a transfer fault in the Trans-Atlantic Geotraverse (TAG) area at 26°N provides a modern analogue for the Alestos hill seamount and associated faulting and mineralization in the ancient oceanic crust of the Troodos ophiolite. These findings indicate that off-axis magmatism played a major role in crustal accretion, hydrothermal alteration, and ore mineralization during the late-stage evolution of the Troodos ophiolite.

Anoxic environment and formation of superlarge ore deposits

The formation of many metallic and non-metallic ore deposits is a result of coupling of mineralization, related to anoxic environment and regional, or global background. Inter act~on, concordance and continual development of the mineralization and environmental background control the large to superlarge scale of ore deposits. Three kinds of ore-forming models related to anoxic environment and three important background patterns are suggested. The turning period of geological history is favorable to the formation of large to superlarge ore deposits.

Geo-tectonic conditions of the formation of Proterozoic large and superlarge ore deposits along northwestern margin of North China Plate

Four large and superlarge copper-multimetal ore deposits hosted in the Mid-Proterozoic Zhartaishan Group along northwestern margin of North China Plate are stratabound hydrothermal sedimentary deposits, and these deposits formed in mid-late stage of the Mid-Proterozoic Langshan-Zhartaishan rifting system. Four necessary conditions are proposed for the formation of these large and superlarge ore deposits: (i) favorable geological background and tectonic environment; (ii) abundant sources of metallogenic material; (iii) favorable sedimentary lithofacies and paleogeographic conditions; (iv) sudden change events during metallogenic processes.

Synchronous structure control of superlarge ore deposits

The control of synchronous structures on formation of superlarge stratabound ore deposits is immense. Based on studies of ore deposits in South Qiniing, northern Guangdong, Langshan and other areas, three new ideas in comparison with examples of ore control of synchronous structures both in China and abroad are proposed: (i) multiorder ore control of synchronous structures, which means that synchronous structures of different orders display different controls on ore deposits; (ii) synchronous structures in different stages of basin evolution display different controls on basin fluid system and ore-forming system; (iii) synchronous strurture accompanying hydrothermal mineralization as a preexisting weak surface in earth crust often reactivate in later tcctono-thermal event to be a channelway for magma or thermal fluids which superimposed on and reformed preexisting ore beds to form large or superlarge composite ore deposits.

Formation of a magmatic-hydrothermal ore deposit: insights with LA-ICP-MS analysis of fluid inclusions

The physical and chemical mechanism of ore precipitation in the Yankee Lode tin deposit (Mole Granite, Australia) was quantified by direct trace-element microanalysis of fluid inclusions. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used to measure element concentrations in a series of fluid inclusions representing the fluid before, during, and after the deposition of cassiterite (SnO2). Tin precipitation was driven by mixing of hot magmatic brine with cooler meteoric water. At the same time, a separate magmatic vapor phase selectively transported copper and boron into the liquid mixture.

Origins of Hydrothermal Ores

The formation of industrially important ore deposits has been studied for over a century, but much remains to be learned about the detailed chemical and physical processes involved. In their research commentary, Barnes and Rose discuss results reported in the same issue by [ AudA©tat et al .][1] in which a laser was used to drill into fluid inclusions in rock; the fluid was then analyzed with a mass spectrometer. The method yields unique experimental data that can be used to evaluate current models of ore deposit formation. [1]: http://www.sciencemag.org/cgi/content/short/279/5359/2091

High form of pentlandite and its thermal stability

The high-temperature form of pentlandite (Fe4.5Ni4.5S8) was found to be stable between 584 6 3 and 865 6 3 8C, breaking down into monosulfide solid solution and liquid at the later temperature. The phase is unquenchable and always displays the X-ray pattern of pentlandite (low form) at room temperature. High-temperature X-ray diffraction demonstrated that the high form has a primitive cubic cell with a 5 5.189 A ˚ (620 8C) corresponding to a/2 of pentlandite. The high-low inversion is reversible, accompanied by a large latent heat. It is thought to be order-disorder in character. The transition temperature falls with decreasing S content. The high form of pentlandite has a limited solid solution from Fe5.07Ni3.93S7.85 to Fe3.61Ni5.39S7.85 at 850 8C. However its solid solution extends rapidly toward Ni3 6 XS2 in the Ni-S join with decreasing temperature. High-form pentlandite with Fe 5 Ni in atomic percent crystallizes first by a pseudoperitectic reaction between monosulfide solid solution and liquid. The high form (Fe 5 Ni) crystallized from the liquid always has the metal-rich (S-poor) composition in the solid solution at each temperature and coexists with taenite g (Fe,Ni) below 746 6 3 8C. This metal-rich high-form Fe4.5Ni4.5S7.4 breaks down into pentlandite and g (Fe,Ni) at 584 6 3 8C (pseudoeutectoid). These results suggest that in geological processes, such as the formation of Ni-Cu ore deposits, pentlandite can crystallize as the high form from liquid (sulfide magma) at the comparatively high temperatures around 800 8C.

Drilling of sediment-hosted massive sulphide deposits at the Middle Valley and Escanaba Trough spreading centres: ODP Leg 169

Abstract Massive sulphide deposits actively forming from hydrothermal systems within sedimented environments have been drilled during Ocean Drilling Program Leg 169 at two locations along the Juan de Fuca/Gorda spreading centres. The Bent Hill Massive Sulphide and Ore Drilling Program deposits, Middle Valley, include iron- and zinc-rich massive and semi-massive sulphides underlain by a well-developed feeder zone characterized by sulphide impregnations and cross-cutting copper-rich veins. Ridge-parallel normal faulting is probably involved in providing high-permeability pathways for focused discharge at the seafloor, and this is a key element in creating these large ore deposits. In strong contrast, massive sulphide recovered from the Central Hill hydrothermal site, Escanaba Trough, suggests mineralization forms only a thin (5–15 m) veneer over the sediment sequence. Interstitial waters recovered from this area have chlorinities both significantly higher and lower than seawater. The only way to explain this variation is that the fluids contain a hydrothermal component which has undergone supercritical phase separation at depth. Diffuse discharge of hydrothermal fluids through the sediments evidently precludes the formation of a large ore deposit in this area.

A Review of Shallow, Ore-related Granites: Textures, Volatiles, and Ore Metals

Shallow, texturally complex granites can be associated, spatially and temporally, with hydrothermal alternation and mineralization. The formation of a granite-related ore deposit represents the confluence of many chemical and physical geological factors, including: magma composition (e.g. concentrations of ore metals, chlorine , and water); magma oxidation state; the relative timing of crystallization, magma ascent, magmatic volatile phase exsolution; and the depth of emplacement. Early magmatic crystallization of ore–metal-rich minerals coupled with the exsolution of the magmatic volatile phase is promoted by deeper emplacement levels and lower magmatic water concentrations, and leads to low efficiencies of removal of compatible elements from melts into the magmatic volatile phase(s). Water concentrations in the melt and pressures of crystallization can be estimated from phase equilibria and textural relationships of hornblende in some felsic systems. Physical factors affect the dynamics of fluid (magma, melt and volatile phase) movement, which in turn affect the size of zones of alternation and mineralization; granite textures can record the partial history of some dynamic processes. Miarolitic cavities are good evidence for magmatic volatile phase exsolution; both miarolitic cavities and pegmatitic texture exhibit evidence for external nucleation of crystals. Nucleation-controlled phenomena, expressed as variations in granite textures, can result from a multiplicity of causes including undercooling of the melt; textural elements such as pegmatite, aplite, skeletal crystals, and graphic textures can form as a result of undercooling. Interconnected volumes of the magmatic volatile phase allow for advection of hydrothermal ore fluids through the magma. In shallow granitic systems, miarolitic cavities can be interconnected, consistent with this hypothesis.

A Review of Shallow, Ore-related Granites: Textures, Volatiles, and Ore Metals

Shallow, texturally complex granites can be associated, spatially and temporally, with hydrothermal alternation and mineralization. The formation of a granite-related ore deposit represents the confluence of many chemical and physical geological factors, including: magma composition (e.g. concentrations of ore metals, chlorine , and water); magma oxidation state; the relative timing of crystallization, magma ascent, magmatic volatile phase exsolution; and the depth of emplacement. Early magmatic crystallization of ore–metal-rich minerals coupled with the exsolution of the magmatic volatile phase is promoted by deeper emplacement levels and lower magmatic water concentrations, and leads to low efficiencies of removal of compatible elements from melts into the magmatic volatile phase(s). Water concentrations in the melt and pressures of crystallization can be estimated from phase equilibria and textural relationships of hornblende in some felsic systems. Physical factors affect the dynamics of fluid (magma, melt and volatile phase) movement, which in turn affect the size of zones of alternation and mineralization; granite textures can record the partial history of some dynamic processes. Miarolitic cavities are good evidence for magmatic volatile phase exsolution; both miarolitic cavities and pegmatitic texture exhibit evidence for external nucleation of crystals. Nucleation-controlled phenomena, expressed as variations in granite textures, can result from a multiplicity of causes including undercooling of the melt; textural elements such as pegmatite, aplite, skeletal crystals, and graphic textures can form as a result of undercooling. Interconnected volumes of the magmatic volatile phase allow for advection of hydrothermal ore fluids through the magma. In shallow granitic systems, miarolitic cavities can be interconnected, consistent with this hypothesis.