Economic Geology

Published by Society of Economic Geologists
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Article
Several diamond drill cores from formations within the Hamersley Group of Western Australia have been studied for evidence of short-range variations in the isotopic compositions of the carbonates. For a set of 32 adjacent microbands analyzed in a specimen from the Marra Mamba Iron Formation, carbon isotope compositions of individual microbands ranged from -2.8 to -19.8 per mil compared to PDB and oxygen isotope compositions ranged from 10.2 to 20.8 per mil compared to SMOW. A pattern of alternating abundances was present, with the average isotopic contrasts between adjacent microbands being 3.0 per mil for carbon and 3.1 per mil for oxygen. Similar results were obtained for a suite of 34 microbands (in four groups) from the Bruno's Band unit of the Mount Sylvia Formation. Difficulties were experienced in preparing samples of single microbands from the Dales Gorge Member of the Brockman Iron Formation, but overall isotopic compositions were in good agreement with values reported by previous authors. Chemical analyses showed that isotopically light carbon and oxygen were correlated with increased concentrations of iron. The preservation of these millimeter-scale variations in isotopic abundances is interpreted as inconsistent with a metamorphic origin for the isotopically light carbon in the BIF carbonates. A biological origin is favored for the correlated variations in 13C and Fe, and it is suggested that the 13C-depleted carbonates may derive either from fermentative metabolism or from anaerobic respiration. A model is presented in which these processes occur near the sediment-water interface and are coupled with an initial oxidative precipitation of the iron.
 
Article
The transition zone comprises Campbellrand microbialaminated (replacing "cryptalgalaminate') limestone and shale, with minor dolomite, conformably overlain by the Kuruman Iron Formation of which the basal part is characterized by siderite-rich microbanded iron-formation with minor magnetite and some hematite-containing units. The sequence is virtually unaltered with diagenetic mineral assemblages reflecting a temperature interval of about 110° to 170°C and pressures of 2 kbars. Integration of sedimentary, petrographic, geochemical, and isotopic results makes it possible to distinguish between depositional, early diagenetic, deep burial, and metamorphic effects on the isotopic compositions of the carbonate minerals and the kerogen in the sequence. Major conclusions are that deep burial thermal decarboxylation led to 13C depletion in euhedral ferroan sparites and 13C enrichment in kerogen (organic carbon). -from Authors
 
Article
Visible and near-infrared (0.35 to 2.5 mu m) bidirectional reflection spectra were recorded for a suite of well-characterized hydrothermally altered rock samples. The spectra typically display well-defined bands caused by both electronic and vibrational processes in the individual mineral constituents.Electronic transitions in the iron-bearing constituent minerals produce diagnostic minima near 0.43, 0.65, 0.85, and 0.93 mu m. Vibrational transitions in clay and water-bearing mineral constituents typically produce characteristic single or multiple features over limited spectral ranges near 1.4, 1.75, 1.9, 2.2, and 2.35 mu m. The most abundant feature-producing minerals present in these rocks are hematite, goethite, and alunite, while others frequently present are jarosite, kaolinite, potassium micas, pyrophyllite, montmorillonite, diaspore, and gypsum.This study shows that visible-near infrared spectrometry is a reliable and rapid technique for detecting and identifying clay minerals and alunite in rocks. Because these minerals are important constituents of altered rocks, the feasibility of using the visible and near infrared for detecting altered rocks by remote-sensing techniques is indicated. The spectral region near 2.2 mu m is particularly important for this purpose.
 
Article
Unit regional value (URV) and unit regional weight are area standardized measures of the expected value and quantity, respectively, of the mineral resources of a region. Estimation and manipulation of the URV statistic is the basis of an approach to mineral resource evaluation. Estimates of the kind and value of exploitable mineral resources yet to be discovered in the provinces of Canada are used as an illustration of the procedure. The URV statistic is set within a previously developed model wherein geology, as measured by point counting geologic maps, is related to the historical record of mineral resource production of well-developed regions of the world, such as the 50 states of the U.S.A.; these may be considered the training set. The Canadian provinces are related to this training set using geological information obtained in the same way from geologic maps of the provinces. The desired predictions of yet to be discovered mineral resources in the Canadian provinces arise as a consequence. The implicit assumption is that regions of similar geology, if equally well developed, will produce similar weights and values of mineral resources.
 
Article
Digital image-processing techniques have been used to analyze a variety of geophysical and geochemical map data covering southern Missouri, a region with important basement and strata-bound mineral deposits. Gravity and magnetic anomaly patterns, which have been reformatted to image displays, indicate a deep crustal structure cutting northwest-southeast through southern Missouri. In addition, geologic map data, topography, and Landsat multispectral scanner images have been used as base maps for the digital overlay of aerial gamma-ray and stream sediment chemical data for the 1 x 2-deg Rolla quadrangle. Results indicate enrichment of a variety of elements within the clay-rich alluvium covering many of the interfluvial plains, as well as a complicated pattern of enrichment for the sedimentary units close to the Precambrian rhyolites and granites of the St. Francois Mountains.
 
Article
The use of remote sensing in resource exploration is reviewed, with emphasis placed on new developments in high spectral resolution remote-sensing techniques for mineralogic and vegetation mapping. Topics discussed include aerial photography and satellite remote sensing, concepts and principles of spectral data collection, spectral properties of rocks and minerals, spectral properties of vegetation, and botanical aspects of geochemical stress. The discussion also covers applications of Landsat multispectral scanner data to lithologic and geobotanic studies and the future development of data acquisition and data interpretation techniques.
 
Article
A striking feature of the Carajs region, Brazil, is the clustering of a variety of different types of Cu-Au deposits.The most abundant in the belt are the >200 million metric tons (Mt) of Fe oxide Cu-Au-(U-REE) deposits, which, despite the variety of host rocks and different orebody morphologies, share a number of diagnostic features, including (1) intense Fe metasomatism leading to the formation of grunerite, fayalite, and/or Fe oxides (magnetite and/or hematite); (2) intense carbonate alteration (mainly siderite); (3) sulfur-poor oremineralogy (chalcopyrite and bornite); (4) quartz-deficient gangue; (5) extreme low REE enrichment, and (6) enrichment in U and Co. The Igarap Bahia deposit is perhaps the best documented Fe oxide Cu-Au-(U-REE) deposit of the belt, containing about 219 Mt at 1.4 percent Cu and 0.86 g/t Au. The Cu-Au ore consists ofsteeply dipping breccia bodies that are hosted by hydrothermally altered metavolcano-sedimentary rocks. SHRIMP II zircon dating of the host metavolcanic rocks gives a 207Pb/206Pb age of 2748 34 Ma. This suggests a correlation between the Igarap Bahia volcano-sedimentary sequence and the Gro Par volcanic rocks, which have published ages of ca. 2.75 Ga. SHRIMP dating of monazite from the matrix of ore-bearing magnetite breccias gives a 207Pb/206Pb age of 2575 12 Ma, confirming the epigenetic nature of the mineralization and placing it ~175 m.y. after accumulation of the host volcano-sedimentary sequence. The 2575 12 Ma SHRIMP age of hydrothermal monazite from the Igarap Bahia mineralization is indistinguishable from published conventional 207Pb/206Pb ages for zircons from the Archean A-type granites of the Carajs belt, indicating that mineralization processes at Igarap Bahia were temporally related to these A-type Archean granites. The wide range of highly radiogenic 87Sr/86Sr ratios (0.714-0.755) of carbonates from the Igarap Bahia deposit suggests multiple crustal sources, consistent with a magmatic-hydrothermal origin. SHRIMP dating of zircon xenocrysts recovered from crosscutting diabase dikes indicates a maximum 207Pb/206Pb age of ~2670 Ma, consistent with field evidence and the age of host rocks, but does not unequivocably constrain the age of the ores.The styles of hydrothermal alteration, mineralogy, and geochemistry of the Igarap Bahia ore, as well as published fluid inclusion and stable isotope data, support its classification as a member of the world-class Olympic Dam-type Fe oxide Cu-Au-(U-REE) group of deposits, as previously argued by several authors. The SHRIMP age of 2575 12 Ma for hydrothermal monazite indicates that Igarap Bahia is an Archean example of this deposit group.
 
Article
Many gold deposits in the Laverton greenstone belt, in the northeast of the Eastern Goldfields province of the Yilgarn craton, are located adjacent to, or hosted by, granitoids. This has led to controversy over whether the granitoids provided the auriferous fluids from which the deposits formed or were structural traps controlling the siting of the gold deposits. New regional-scale stable isotope data, combined with robust geochronologyon several deposits, resolves this controversy in the Laverton greenstone belt. The median S, C, and O isotope compositions of ore and gangue minerals from nine different gold deposits in the Laverton greenstone belt fall in a very narrow range. The only exceptions to this are the more negatived 34S values of ore sulfides at the Jupiter gold deposit, which were most likely caused by pre existing highly oxidized host rocks and the more negative d13C values of ore carbonate at deposits with reduced black shale hostrocks. Redox conditions and mineralization temperatures for all gold deposits in the Laverton greenstone belt are broadly similar. Therefore, the lack of variation in the isotopic compositions of ore and gangue minerals is consistent with their deposition from a similar ore fluid. There is no convincing evidence to indicate that more than one ore fluid was involved in deposition of gold deposits within the Laverton greenstone belt, although the data do not uniquely define the source of the ore fluid (e.g., whether it was proximal or distal). SHRIMP U-Pb dating of gold-related monazite and xenotime provides a temporal framework for gold mineralization in the Laverton greenstone belt. Synmineralization phosphates have ages of 2650 7 Ma at MountMorgans, 2649 11 Ma at Jubilee, 2657 21 Ma at Jupiter, and probably 2653 6 Ma at Granny Smith. The similarity in age of these four deposits, as well as previously published ages for the Wallaby (2650 5 Ma) and Sunrise Dam and/or Cleo deposits (2654 8 Ma), places three major constraints on the source of auriferous fluids in the Laverton greenstone belt. First, the Wallaby and, most likely, the Granny Smith gold deposits arenot the same age as adjacent granitoids, ruling out the exposed granitic rocks as a proximal magmatic fluid source. Second, the broadly synchronous timing of gold mineralization on a camp scale provides evidence that the deposits have a similar genesis. Third, the range of ages of the gold deposits is not as great as that of the granitoids postulated to be their source. Magmatic activity that has been invoked as the source of ore fluids by various workers is related to several geochemically distinctive granitoid suites that are diachronous over several tens of millions of years in the Laverton greenstone belt and the wider Eastern Goldfields province. In contrast, the consistent age of gold mineralization in the Laverton greenstone belt supports a single fluid source, as implied by the isotope geochemistry. It is concluded that all studied deposits are orogenic gold deposits with a distal and deep source.
 
Cartoon model illustrating mineralization supposedly postdating the full development of the B and T faults, i.e., these faults acted as the main conduits for the metal-bearing fluids (the Mississippi Valley-type paleohydrologic model). ABL = Argillaceous Bioclastic Limestone Group.
Example of a laminated sulphide (mainly sphalerite) clast hosted by the Boulder Conglomerate (DDH U14898), in close proximity to crinoid ossicles and unmineralized Pale Beds clasts. Note that fine-scale replacement (dominantly sphaleritization) of bioclastic debris is a common feature in the underlying Pale Beds ores (Anderson et al., 1998): these ossicles are not replaced by sulfide.
Composite plot showing the distribution of percent Pb (0->4.5%) within slices 1 to 7, equating to the lower part of 5 lens. (Also shown is the location of the profile illustrated in Fig. 9a and b).
Summary of the ranges of sulfur isotope results from different minerals and textures (extracted from Anderson et al., 1998).
Article
Models of genesis for the Navan orebody are of two distinct types. An early hypothesis that mesothermal (though nonmagmatic) deposition of ore began when a supernatant seawater brine still had access to the host sediments (during the early to mid-Mississippian), has been challenged by recent suggestions favoring a later (mid- to late Mississippian to Pennsylvanian) mineralization derived from the south. These models, characterized here as Irish-type and Mississippi Valley-type respectively, are interrogated as to their particular predictions with regard to metal distributions and sulfur isotope patterns associated with various fault geometries. The basal 5 lens of the Navan Zn + Pb deposit contains similar to70 percent of the known tonnage of the similar to90 Mt orebody and, thus, is the focus of this examination. Lead distribution patterns suggest that migration of metal-bearing fluids was principally directed up early to mid-Mississippian, near vertical north-northeast, northeast, and east-northeast minor normal faults. These faults predate or are coeval with the major extensional, partly listric, east- northeast fualts which now control the general disposition of the deposit. Only where these major east-northeast faults cross putative deep-seated northeast (Caledonoid) and northwest structures are they associated with lead enrichments. A systematic delta(34)S survey in the 5 lens across five minor north-northeast-through to east-northeast-trending faults associated with distinct lead enrichments, and one east- northeast-trending, partly listric, major extensional fault adjacent to that trend, revealed positive delta(34)S values (1- 18parts per thousand) for galena, sphalerite, and marcasite sampled within 3m of all the faults on the profile. Sulfides with positive delta(34)S values associated with the deep- seated, metal-bearing fluid generating the Navan deposit have been highlighted by previous workers (Anderson et al., 1998). The evidence reported here strongly suggests that the metal- bearing fluids rose through all the fractures. Conversely, negative delta(34)S values (-1 to -26parts per thousand) were returned in galena and sphalerite sampled 3 m or more from these faults. These negative values indicate that locally derived bacteriogenic sulfide, reduced from sea-water sulfate, dominated away from these faults. Pyrite delta(34)S values suggest a background level of -29 +/- 3.0 per mil across the profile. However, pyrite delta(34)S values as low as -34 +/- 2.7 per mil were recorded in one sample collected from within 1 m of a fault. Thus, fluids containing highly fractionated, bacteriogenic sulfide also gravitated into these faults on at least one occasion. There is also evidence suggesting that the metal-bearing solutions periodically displaced the locally derived bacteriogenic sulfide-bearing fluid in and near the faults. Mineral sulfide petrography is used to contextualize the sampling and to give a qualitative indication of the degree of chemical disequilibrium of the system. Mineral textures demonstrating comminution and dissolution are revealed by this study that, when coupled with evidence of isotopic overprinting, force the conclusion that mineralizing fluids first invaded the host lithologies during mid-Lower Mississippian times, coincident with active faulting. There is no evidence of reactivation of the minor fault sets encountered in the study area during post-Chadian tectonism, though the major, partly listric, east-northeast extensional faults were reactivated at that time. Ore deposition was effected by bacteriogenic sulfide (reduced from Mississippian seawater sulfate) reacting with rising, metal-bearing mesothermal fluids. Thus any model invoking an onset of mineralization later than the mid-Lower Mississippian does not stand up to this scrutiny.
 
Article
Detailed petrographic study, scanning electron microscope imaging, and electron microprobe analyses of tourmalines from the Sullivan Pb-Zn-Ag massive sulfide deposit (British Columbia, Canada) document multiple paragenetic stages and large compositional variations. The tourmalines mainly belong to two common solid-solution series: dravite-schorl and dravite-uvite. Ca- and Fe-rich feruvite and alkali-deficient tourmalines are present locally. Products of tourmaline-forming stages include (from oldest to youngest): (1) rare Fe-rich dravite-schorl within black tourmalinite clasts in footwall fragmental rocks; (2) widespread Mg-rich, very fine grained, felted dravite in the footwall (the main type of tourmaline in the footwall tourmalinite pipe); (3) recrystallized, Fe-rich dravite-schorl (locally Ca-Fe feruvite) in the tourmalinite pipe, which preferentially occurs near postore gabbroic intrusions; (4) Mg-rich dravite or uvite associated with chlorite-pyrrhotite and chlorite-albite-pyrite-altered rocks in the shallow footwall and hanging wall; (5) discrete Mg-rich tourmaline grains associated with chlorite and discordant Mg-rich tourmaline rims which occur on disseminated Fe-rich schorl in the bedded Pb-Zn-Ag ores. The timing of rare Fe-rich schorl in the bedded ores is uncertain, but it most likely occurred during or between stages 2 and 3. The different paragenetic stages and their respective tourmaline compositions are interpreted in terms of a multistage evolution involving contributions from: (1) variable mixtures of synsedimentary, Fe-rich hydrothermal fluids and entrained seawater; (2) postore, Fe-rich, gabbro-related hydrothermal fluids; and (3) postore metamorphic reactions. Early synsedimentary, Fe-rich hydrothermal fluids which contained little or no entrained seawater formed Fe-rich black tourmalinite clasts locally in the footwall. The major type of tourmaline in the footwall tourmalinite pipe is Mg rich, recording seawater entrainment under high water/rock conditions, rather than control by the chemical composition of the original host sediments. Rare Fe-rich schorl within the bedded Pb-Zn-Ag ores is believed to have formed on the sea floor by reaction of an Fe-rich brine pool with detrital aluminous sediments. Postore emplacement of gabbro sills and local dikes in the footwall produced Fe-rich hydrothermal fluids, which were responsible for formation of minor Fe-rich dravite-schorl which overprinted earlier dravite. Postore, but synsedimentary, hydrothermal alteration involving entrained seawater was responsible for deposition of dravite and uvite in the hanging wall and for dravite in the brown tourmalinites of the shallow footwall. Mg-rich dravite-uvite associated with chlorite and in discordant rims on schorl in the bedded ores formed by sulfide-silicate reactions during greenschist facies regional metamorphism.
 
Article
The strata-bound copper-silver-antimony deposit at Copper Hill, New Mexico, developed in a distinctive structural trap, a "brittle-ductile trap', during regional retrograde metamorphism of Early Proterozoic metasedimentary rocks. The deposit is localized near the contact between massive orthoquartzite (Ortega Formation) and the overlying schists (Rinconada Formation). Fracture-controlled and disseminated Cu-Ag-Sb minerals occur within the quartz veins and in quartzite near the veins. A syn- to late metamorphic age for the primary deposit is indicated by: (1) the crosscutting nature of the mineralized veins, (2) local replacements of kyanite and staurolite by economic minerals, and (3) evidence for postmineralization annealing of the veins and quartzite. During retrograde metamorphism, the quartz veins and massive quartzite behaved brittlely whereas the overlying schists deformed ductilely. SiO2-bearing and then Cu-Ag-Sb-bearing metamorphic fluids migrated through the fractured quartzite and ponded below the folded impermeable schists. -from Authors
 
Article
The Alcudia Valley is intensely mineralized, with a large number of Pb-Zn-Ag-Cu deposits hosted by Neo-proterozoic and Paleozoic sedimentary rocks. Five distinct types of deposits have been recognized. The most significant mineralizing events were related to the two major phases of deformation (H-D1, H- D2) and granite emplacement during the Hercynian orogeny. Stable isotope and lithogeochemical data demonstrate that the metals, as well as sulfur and carbon, were sourced from the local sedimentary rocks. Type A deposits are minor occurrences of strata-bound mineralization with Zn gt Pb, occurring as disseminations and small veins in Late Ordovician limestone. A syndiagenetic origin is suggested by the limited stratigraphic distribution of the mineralization and anomalous base metal contents of Late Ordovician-Early Silurian black shales overlying the mineralized limestone. Minor remobilization of disseminated sulfides into joints took place during late diagenesis or early Hercynian deformation. Types B, C, and D are syntectonic deposits with Zn gt Pb. Type B deposits are strongly deformed veins in H-D1 fractures in Ordovician rocks. A process involving local mobilization of metals by a surface- derived fluid is indicated by depleted metal contents of host rocks adjacent to the veins, the similarity of delta(34)S values of ore sulfides with disseminated pyrite in the host rocks, and the delta18O(H2O) values. Type C deposits exhibit a greater variation in morphology and degree of deformation compared to other types and are found in both H-D1 and H-D2 fractures in Late Ordovician and Silurian rocks. The stratigraphic distribution and delta(34)S values of sulfides are similar to type A deposits, suggesting that localized hydrothermal systems either remobilized type A mineralization or derived metals and sulfur from the same source. Type D deposits occur in H-D2 shear fractures in Neoproterozoic rocks. The delta(13)C values of carbonate gangue and the similarity of delta(34)S values of ore sulfides and disseminated pyrite in the host rocks indicate that black shales of the Neoproterozoic sequence are the major source of sulfur and carbon. Mixing of a metamorphic fluid with a surface-derived fluid is suggested by fluid inclusion and oxygen isotope data. Type E deposits, the most abundant and economically most important type, are post- tectonic Ph gt Zn veins occupying H-D2 fractures and are widely distributed through the district in rocks ranging from Neoproterozoic to Late Ordovician. Mineral assemblages and Ag contents of the veins show a zonal relationship with respect to monzogranite outcrops, indicating that late Hercynian magmatism was the source of heat driving fluid migration through the fracture system. Fluid inclusion and oxygen isotope data are typical of basinal. brines. Reaction of the fluid with black shales of the Neoproterozoic sequence is suggested by delta(34)S values of sulfides and delta(13)C values of carbonates of the main stages of the paragenetic sequence. The final paragenetic stage consists of barite, calcite, and pyrite, which may have been deposited during a later hydrothermal event, probably in the Early Triassic.
 
Article
The Early Archean Bamboo Creek gold deposit contrasts with most other orogenic deposits because of its relatively early timing in the tectonic evolution of the Pilbara granitoid-greenstone terrane. The Bamboo Creek deposit is situated in a bedding-parallel, brittle-ductile shear zone (the Bamboo Creek shear zone) within a komatiite sequence. The laminated quartz-carbonate gold lodes occur in carbonate-altered boudins within the Bamboo Creek shear zone and are associated,with early sinistral, northeast-up deformation in the shear zone, whereas dextral reactivation of the zone postdates gold deposition. Gold-related alteration zones reflect an increase in X-CO2 toward the mineralized zone. Variations in original host-rock composition give rise to asymmetric alteration zoning, kith a fuchsite-carbonate zone in the more Mg- and Cr-rich cumulate-textured footwall and a chlorite-quartz zone in the more aluminous spinifex-textured hanging wall. The alteration envelope is enriched in Na2O, K2O, Rb, Pb, As, and Sb. Whereas pyrite and minor chalcopyrite occur in all alteration zones, tetrahedrite, galena, and sphalerite are strongly associated with gold in the lodes. The alteration and metal enrichment of tire Bamboo Creek gold deposit are indistinguishable from those of other orogenic (mesothermal) lode gold deposits in Archean terranes. Carbonate delta(13)C((PDB)) and delta(18)O((SMOW)) isotope signatures are consistent throughout the alteration envelope at 0.2 +/- 0.6 and 14.6 +/- 0.6 per mil, respectively. The delta(13)C value, in particular, is higher than typical values for orogenic gold deposits, implying interaction of auriferous fluids with preexisting marine carbonates that formed during an early sea-floor alteration event. The temperature of deposition, estimated from chlorite thermometry and alteration assemblages, is about 250degreesC, which is within the lower part of the range for orogenic gold deposits. Lead-lead model ages for galena, together with the relationships between the Bamboo Creek shear zone and dated granites, indicate a relatively early age of gold deposition of ca. 3400 Ma. Correlation of structures associated kith gold deposition and regional structural phases shows that gold deposition was most likely related to an extensional tectonic phase. The early timing and association with extension is unlike the tectonic setting of other Archean gold deposits, which tend to form during the final, compressional or strike-slip stages of orogenesis. The Bamboo Creek gold mineralization may have been related to an Early Archean lower crustal delamination event. This may explain the anomalous timing and the low gold endowment of the Pilbara relative to Late Archean greenstones.
 
Article
Chondrite-normalized rare earth element (REE) plots of whole-rock geochemical analyses are commonly used to describe the history of magmatic systems. In the particular case of porphyry copper deposits, which are produced by ore-forming magmatic-hydrothermal systems, whole-rock REE concentrations commonly show an unusual depletion in the final phase of magmatism associated with the main ore-forming stage. Interpretation of REE signatures requires establishing whether they result from magmatic or hydrothermal fractionation processes, or a combination of both. We investigated whole-rock trace-element and REE patterns of samples from the giant Ok Tedi porphyry copper-gold deposit, Papua New Guinea, and found that REEs were most depleted in zones of greatest hydrothermal alteration. REE-rich accessory phases zircon, apatite, and titanite have lower modal abundances and become smaller and more anhedral in these intensely altered zones as compared to less altered rocks. This mineralogical change is accompanied by depletions in P, Ti, Y, and Zr, and deviations of Zr/Hf and Y/Ho from chondritic trace-element ratios-chemical signatures indicative of hydrothermal fractionation. We conclude that destruction of REE-rich accessory minerals, particularly apatite, has led to hydrothermal remobilization and depletion of REEs. This implies that intrusive rocks that experienced high-salinity, oxidized fluid-rock interaction at temperatures >400C may have had their original magmatic REE signature modified. 2010 Society of Economic Geologists.
 
Article
Typescript (photocopy). Vita. Abstract. Includes appendices. Thesis (M.S.)--University of Nevada, Las Vegas, 1999. Includes bibliographical references (leaves 178-183).
 
Article
The Kalgoorlie gold field contains structurally controlled, epigenetic gold deposits hosted by mafic rocks inthe Archean Yilgarn craton of Western Australia. Its giant size has prompted much interest in the processes that led to its formation, particularly of the Golden Mile mineralization, which hosts over 70 percent of the gold in the Kalgoorlie gold field. It is generally agreed that the widespread presence of hematite and the moderately negative sulfur isotope composition of some of the pyrite (d34S of -10 to -2%) in the Golden Mile lodes and associated alteration indicate the presence of a relatively oxidizing ([SO42-] ~ [HS-] + [H2S], with hematite stable) fluid during gold deposition and wall-rock alteration, but the origin and evolution of this fluid are not well constrained. A piece of evidence that has not been fully integrated into interpretations is the low variance of mineral assemblages in the alteration haloes of the Golden Mile lodes (e.g., coexisting magnetite-hematitesiderite-pyrite-ankerite-albite-muscovite-ilmenite rutile-quartz chlorite). Thermodynamic modeling, using HCh and a purpose-built code that facilitates investigation of systems that involve complex mineral solid solutions, CO2-rich fluids, and open-system chemical behavior was used to investigate the constraints that the low variance assemblages place on the source and evolution of mineralizing fluids. Results of the modeling show that fluid-rock reaction with decreasing temperature can drive pyrrhotite magnetite assemblages, in equilibrium with a fluid that contains aqueous sulfide, to hematite-pyrite-magnetite assemblages in equilibrium with a fluid that contains aqueous sulfate. This modeled shift arises from cooling driven oxidation of sulfides and reduced sulfur-bearing aqueous species by ferric iron in magnetite and formation of hematite and siderite from magnetite and CO2; there is no requirement for electron acceptors other than those provided by the rock. The implication of the model results for the Golden Mile mineralization is that hematite growth and sulfate bearing fluids could have resulted from fluid-wall rock interaction without involvement of an externally derived oxidizing fluid. The change from aqueous-sulfide dominated to aqueous-sulfate rich solutions would destabilize gold sulfide complexes in solution and lead to gold precipitation. Formation of aqueous sulfate species would also result in the precipitation of pyrite with negative d34S. Mass balance calculations show that production of hematite in the carbonate zone of the Golden Mile mineralization could have occurred without any requirement for addition of fluid-derived electron acceptors although open-system behavior is not precluded. Overall, the characteristics of the carbonate zone alteration are consistent with electron redistribution caused by interaction between a reduced auriferous fluid and the host dolerite.
 
Article
The late Miocene Farallon Negro volcanics, comprising basaltic to rhyodacitic volcano-sedimentary rocks, host the Bajo de la Alumbrera porphyry copper-gold deposit in northwest Argentina. Early studies of the geology of the district have underpinned the general model for porphyry ore deposits where hydrothermal alteration and mineralization develop in and around porphyritic intrusions emplaced at shallow depths (2.5-3.5 km) into stratovolcanic assemblages. The Farallon Negro succession is dominated by thick sequences of volcano-sedimentary breccias, with lavas forming a minor component volumetrically. These volcaniclastic rocks conformably overlie crystalline basement-derived sedimentary rocks deposited in a developing foreland basin southeast of the Puna-Altiplano plateau. Within the Farallon Negro volcanics, volcanogenic accumulations evolved from early mafic to intermediate and silicic compositions. The younger and more silicic rocks are demonstrably coeval and comagmatic with the earliest group of mineralized porphyritic intrusions at Bajo de la Alumbrera. Our analysis of the volcanic stratigraphy and facies architecture of the Farallon Negro volcanics indicates that volcanic eruptions evolved from effusive to mixed effusive and explosive styles, as magma compositions changed to more intermediate and silicic compositions. Air early phase of mafic to intermediate voleanism was characterized by small synsedimentary intrusions with peperitic contacts, and lesser lava units scattered widely throughout the district, and interbedded with thick and extensive successions of coarse-grained sedimentary breccias. These sedimentary breccias formed from numerous debris- and hyperconcentrated flow events. A later phase of silicic volcanism included both effusive eruptions, forming several areally restricted lavas, and explosive eruptions, producing more widely dispersed (up to 5 kin) tuff units, some tip to 30-m thickness in proximal sections. Four key features of the volcanic stratigraphy suggest that the Farallon Negro volcanics need not simply record the construction of a large steep-sided polygenetic stratovolcano: (1) sheetlike, laterally continuous debris-flow and other coarse-grained sedimentary deposits are dominant, particularly in the lower sections; (2) mafic-intermediate composition lavas are volumetrically minor; (3) peperites are present throughout the sequence; and (4) fine-grained lacustrine sandstone-siltstone sequences occur in areas previously thought to be proximal to the summit region of the stratovolcano. Instead, the nature, distribution, and geometry of volcanic and volcaniclastic facies suggest that volcanism occurred as a relatively low relief, multiple-vent volcanic complex at the eastern edge of a broad, > 200-km-wide late Miocene volcanic belt and oil ail active foreland sedimentary basin to the Puna-Altiplano. Volcanism that occurred synchronously with the earliest stages of porphyry-related mineralization at Bajo de la Alumbrera apparently developed in an alluvial to ring plain setting that was distal to larger volcanic edifices.
 
Article
The central zone of the Miocene Stiavnica stratovolcano (Western Carpathians) hosts an extensive subvolcanic intrusive complex dominated by a granodiorite pluton (14.5-15.5 Ma). The granodiorite intrusion has a bell-jar form and was emplaced mainly into basement rocks, although its central part extends to the base of a subvolcanic and volcanic andesitic complex, 1.5 to 3 km below the paleosurface. in the more uplifted western part of the complex, the margin of the granodiorite includes numerous apophyses with associated magnetite skarns, whereas in the central, apical part it contains an irregular network of fractures hosting base metal stockwork mineralization that extends to a depth of 250 m. The granodiorite intrusion, especially in the marginal facies, is altered. High-temperature subsolidus deuteric (K-feldspar, secondary biotite, and amphibole) and endoskarn (pyroxene) alteration, intimately related to magnetite skarns, resulted in depletion in Fe-total and increase in CaO and MgO in the intrusion. Low-temperature, hydrothermal alteration (epidote, chlorite, K-feldspar, sericite), closely related to the stockwork mineralization, is characterized by enrichment in K2O, Pb, Zn, Cu, and MnO, and depletion in Na2O. The overlying andesites are affected by acid leaching. Fluid inclusions and stable isotopes (O,H) were studied in samples of granodiorite related to the skarn and to the stockwork, from the top of the pluton down to 1,230 in in depth. Magmatic quartz contains hypersaline fluid inclusions (up to 71 wt % NaCl equiv), vapor- rich saline and low-salinity inclusions, as well as moderate-to low-salinity liquid-rich aqueous inclusions with homogenization temperatures (T-h) ranging from 161degrees to gt 600degreesC. Brine inclusions from magmatic quartz in marginal phases of the granodiorite show signs of NaCl saturation during entrapment, with the majority probably having been captured under vapor unsaturated conditions. Most vapor-rich inclusions represent early stages of fluid immiscibility, whereas the low-salinity liquids are products of dilution by meteoric fluid and/or late- stage portions of magmatic fluids. Inclusion-bearing minerals in the skarn assemblage contain moderate- to low-salinity fluid inclusions (T-h = 215degrees-371degreesC) with evidence of late-stage boiling at low hydrostatic pressure. Hydrothermal minerals from base metal stockwork and massive silica-rich rock contains only low-salinity inclusions (up to 5 wt % NaCl equiv, T-h = 191degrees-367degreesC). The isotopic composition of magmatic biotite and hornblende from granodiorite indicates open magmatic degassing (deuterium depletion) during granodiorite crystallization, influenced by reequilibration to variable degrees. Fluids in equilibrium with skarn and stockwork-related minerals show a clear progressive mixing trend with isotopically relatively heavy, possibly delta(18)O- shifted, meteoric water. Magnetite skarns and zones of high- temperature subsolidus deuteric alteration are related to ponding of magmatic brines in the roof of the intrusion, resulting from fluid immiscibility at relatively low pressures. The associated escaping vapor was responsible for acid leaching in andesites above the apical part of the pluton. Stockwork mineralization and associated low-temperature potassic alteration are probably related to the large convective hydrothermal system and upflow of late-stage, deep, exsolved supercritical fluids, using the increased permeability (fracturing) of the central apical part of the intrusion.
 
Article
Typescript. Thesis (M.S.)--University of Wisconsin--Madison, 1951. Includes bibliographical references (leaf 15).
 
Article
A portion of the system Na/sub 3/PO/sub n4/-CaCO/sub 3/)-H/sub 2/0 was investigated at low temperatures. The formation of a carbonate apatite by phosphate replacement of calcite in alkaline solutions and the structural positions of the carbonate groups are discussed. Data indicate calcite replacement by low concentrations of phosphate to be the principal mechanism of formation of marine phosphorite deposits. (auth)
 
Geologic map of the Dyakou emerald occurrence (after Wang et al., 1996). The cross section (A-B) illustrates the tectonic layering in the area.  
Whole-Rock Geochemical Data for the Major Rock Types in the Study Area
Emerald-bearing quartz vein comprised of quartz (Q), emerald (E), and minor potassic feldspar (K). Multiple laminae of host schist within the vein displaying crack-seal−like texture are dominantly recrystallized to quartz, feldspar, and minor phlogopite. Specimen DYK06-zh.  
Five 40 Ar-39 Ar gas release spectra showing plateau ages ranging from Early Cretaceous to early Tertiary. The plateau ages are shown as a black line across each individual spectrum. Laser heating steps used in the calculation of the plateau ages are shown in gray. All errors are shown at the 2 level. Sample numbers (1−5) correspond to the Ar sample locations in Figure 2. The older ages observed in the high-temperature release portions of the spectra for samples 1, 2, and 4 are interpreted as Ar recoil.
Schematic showing the proposed relationships between the emerald-bearing quartz veins, the emerald-bearing quartz-rich pegmatite veins, the local Cretaceous porphyritic to pegmatitic granitic rocks, and the deformed Proterozoic host rocks.  
Article
The Dyakou emerald occurrence is located in Malipo County in the province of Yunnan, southern China. The occurrence lies in the northern part of the Laojunshan-Song Chay metamorphic core complex, which is exposed in an area of approximately 2,000 km2 and extends across the border between China and Vietnam. Emerald mineralization is hosted by pegmatite and associated quartz veins that intrude deformed Proterozoic biotite-muscovite granofels and schist. Hydrogen and oxygen isotope results from the emerald channel waters and emerald, respectively, are consistent with an igneous fluid source. The δ18O fractionation between emerald and quartz yields vein temperatures of 365° to 420°C. Fluid inclusions indicate that the emerald precipitated from saline brines ranging from almost pure water to 10.5 mass percent NaCl equiv. Fluid inclusion isochores intersected with δ18O data yield pressures changing along the geothermal gradient from 1,500 to 3,300 bars. Ar-Ar geochronology of biotite and muscovite from the emerald veins yields consistent ages of 124 ± 1 Ma. These constraints combined with field observations indicate that the Dyakou emerald deposit is consistent with the igneous-related model for emerald formation.
 
Article
Las Guijas tungsten district is located 58 miles southwest of Tucson, Arizona. The area studied has six pre-Quaternary rock units: granite, granite dikes, volcanic breccia, andesite and rhyolite dikes, black biotite minette, and sedimentary rocks of Cretaceous age. The granite and volcanic breccia occupy most of the area and serve as host rocks. The area has a series of vertical faults consisting of a north-south system (post-mineralization) and an east-west system (pre-mineralization). The deposits are associated structurally and genetically with the Las Guijas granite stock, and all the veins encircle the stock.Economic deposits were formerly mined but active mining has now ceased. The mineralogy is simple. Wolframite, huebnerite, and scheelite are the principal ore minerals along with sulfides and oxide minerals. Quartz is the prevalent gangue and sericite---an alteration product. Spectorgraphic analyses indicate the elements present in the ore minerals and confirm mineral identification.On the basis of field observations and in the light of the recent experimental studies, the Las Guijas tungsten deposits have been classified as epigenetic epithermal deposits.
 
Article
There are six copper-bearing supergene mineral assemblages at the Lakeshore porphyry copper deposit. These assemblages form zones which have been mapped throughout the deposit. The zones are named for their principal copper-bearing minerals: the chalcocite zone, the cuprite-native copper zone, the brochantite zone, the chrysocolla zone, the copper wad zone, and the goethite zone. Two distinct episodes of oxidation have affected the Lakeshore deposit. The first resulted in the creation of a chalcocite enrichment blanket and a large volume of chrysocolla mineralization. The chalcocite enrichment blanket was largely destroyed during the second episode and the brochantite zone (Cu 4(SO 4)(OH) 6) was formed. The Lakeshore porphyry copper deposit is a sulfur-poor system without a phyllic, pyrite-rich, alteration halo. There is no leached capping in the classic sense at the Lakeshore deposit. The goethite zone, which contains an average of 0.3% copper, occupies the relative position of a leached capping. -from Author
 
Article
Thesis (Ph. D.)--University of Wisconsin, 1926.
 
Article
Important epithermal tungsten mineralization in the Boulder County district is mainly hosted by quartz-ferberite (FeWO4) veins. Isotopic studies based on an extensive grid (deltaD) and individual samples of wall-rock alteration and mineralization (deltaD, deltaO) have demonstrated the dominance of meteoric fluids during alteration processes. However, the involvement of magmatic fluids during the deposition of the ferberite veins cannot be ruled out. The fluids responsible for alteration exchanged with the country rocks to produce a large deltaD anomaly but no concomitant delta O-18 anomaly, indicating that water/rock ratios were small. The deltaD anomaly coincides closely with those produced by H2O+ and Rb/Sz used as a proxy for hydrothermal alteration, but does not correlate closely with either lithochemical W anomalies or areas of greatest W production. It is concluded that the fluids responsible for district-wide hydrothermal alteration are distinct from those that produced the W mineralization and that ED anomalies, while a sensitive technique for highlighting areas of significant fluid-rock interact ion, must be used with caution for identifying drilling targets. The study has provided an estimate of the stable isotope composition of early Tertiary meteoric water in the Front Range (ca. deltaD = -140%, deltaO = 18.7%).
 
Article
The Zn-Pb (-Ba) deposits of central Ireland are hosted by Lower Carboniferous Waulsortian limestone and Navan Group carbonate sediments. The deposits are located near the margins of subbasins in close association with extensional faults. The source of metals and the flow pathways utilized by the hydrothermal fluids responsible for ore genesis have long been a matter of conjecture. The Devonian to Lower Carboniferous Old Red Sandstone has been suggested as a possible regional aquifer and metals source. Therefore, a petrographic and lead isotope study of this unit was conducted to assess its putative role in ore formation. Hydrothermally unaltered Old Red Sandstone has Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 ratios ranging from 18.71 to 22.45, 15.52 to 15.86, and 38.57 to 42.71, respectively. The sedimentological characteristics of the red beds, coupled with results of isotopic modeling, support the conclusion that the lead isotope signature of the unit is predominantly controlled by radiogenic Caledonian granite detritus that has an Avalonian-like character. The modeled lead isotope composition of the Old Red Sandstone at the time of the Lower Carboniferous ore-forming event is too radiogenic for it to have acted as the main source of lead for the Irish ore field. Examination of drill core, outcrop, and underground exposures reveals that hydrothermal alteration within the Old Red Sandstone forms a halo around faults, indicating that fluid flow in the unit was predominantly structurally focused. These hydrothermally altered sediments have Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 ratios of 18.21 to 21.77, 15.54 to 15.91, and 38.02 to 41.57, respectively. Unlike the unaltered sediments, several altered samples have lead isotope compositions similar to galena from the Lower Carboniferous deposits. Petrographic examination reveals that these samples contain trace hydrothermal sulfides that are probably responsible for the orelike signature. Therefore, it appears that, at least locally, the Old Red Sandstone functioned as a sink for lead during the Lower Carboniferous mineralizing event. The conclusions from this study eliminate the Old Red Sandstone as the main source of lead in the ore field and suggest that deposit lead is derived primarily from lower Paleozoic rocks ( +/- Precambrian crystalline basement). in view of these results, regional gravity-driven fluid flow models calling upon an Old Red Sandstone-focused flow regime and metals source can be discounted. Rather, deep circulation of fluids through the fractured basement underlying central Ireland plays an integral role in the evolution of the ore-forming system.
 
Article
The study of the Santa Rita gold deposit (gold vein mineralization hosted within Proterozoic metasedimentary formations) reveals interesting features such as typical As-bearing growth bands in pyrite grains. The pyrite grains of the gold mineralization stages are free of detectable native gold. Arsenic-rich oscillatory-zoned pyrites are the only ones to contain invisible gold. The covariations in As and Au indicate that they were transported in the same solution and that the same geochemical processes led to the removal from solution of both. -from Authors
 
Article
The Chalice gold deposit, in the Eastern Goldfields province, Yilgarn Craton, Western Australia, is located in a middle to upper amphibolite facies metamorphic domain and is hosted by a mafic-ultramafic rock sequence that has been locally intruded by four generations of monzogranite dikes. Two stages of gold mineralization,identified on the basis of crosscutting relationships, formed under broadly synpeak to postpeak metamorphicconditions. Main stage gold mineralization (95% of the resource) comprises foliation-parallel, quartz-albitediopside-titanite-garnet-gold veins and wall-rock replacement, both within locally developed asymmetric foldsin mafic amphibolite. Second stage gold mineralization (5% of the resource) is temporally associated with a second-generation monzogranite dike that cross cuts the folds and hence is younger than the Main event. It is representedby disseminated gold in the dike as well as by foliation-discordant quartz-gold, quartz-diopside-gold, actinolite-gold, and molybdenite-tellurobismuthite-gold veins. Gold is in textural equilibrium with the hydrothermal alteration assemblages in both mineralization stages and also with primary igneous phases (quartz and feldspar) in the monzogranite dike in the Second stage ore. Magmatic zircons and titanite from second and fourth generation monzogranite dikes and a monzogranite stock, as well as hydrothermal titanite and molybdenite in equilibrium with gold from hydrothermal alteration assemblages, allow dating of the important magmatic and hydrothermal events using the U-Pb and Re-Os isotope systems. Two gold events are identified. Main stage mineralization is coincident with asymmetric fold developmentat 2644 8 Ma (SHRIMP U-Pb on titanite), and Second stage mineralization (2621 10 Ma; Re-Os on molybdenite) is coeval, within the error of the isotopic ages, with the intrusion of the gold-mineralized second-generation monzogranite dike (2626 9 Ma; SHRIMP U-Pb on zircon). Ages of hydrothermal titanitein monzogranite dikes (2631 10 Ma, 2624 7 Ma, 2623 5 Ma, and 2619 6 Ma; SHRIMP U-Pb ages) indicate contemporaneous hydrothermal alteration, gold mineralization, and evolving magmatism during the Second stage event. A fourth generation, flat-lying pegmatite, which truncates all mine rock units, constrains the minimum age of mine-scale gold-bearing alteration and magmatism to 2622 13 Ma.The geologically constrained geochronologic data suggest that the Chalice gold deposit is a product of two independent gold events separated by up to 20 m.y., within an extended period of granitoid magmatism also extending over ~20 m.y. It demonstrates, for the first time in the extensively gold-mineralized Yilgarn craton, a clear interdependence and interplay between ongoing granitoid magmatism, deformation, hydrothermal alteration, and gold mineralization in amphibolite-hosted deposits. However, although integrated field and geochemical research establish a clear chronology of events, the controversy of contribution from magmatic and/or metamorphic fluids for the ore remains unresolved, because these events are broadly coeval within the resolution of the isotopic techniques.
 
Article
The numerous Zn-Pb deposits in the Irish midlands, together with the quantity and grade of the ore, make this a world-class base metal province. Despite significant exploration and research, there is still disagreement on the origin of the mineralizing fluids. In this study, we have measured the composition of fluid inclusions using a crush-leach technique. These data constrain the possible sources of the fluids both during and after mineralization. Chloride and Br concentrations indicate that the main ore fluid at Tynagh and Silvermines was seawater that had evaporated until the salinity was between 12 and 18 wt percent. However, Na, K, and Li data show that water- rock interactions have resulted in the depletion of Na and the enrichment of K and Li relative to the concentrations expected for evaporated seawater. The fluids that produced postore dolomitization were also derived from seawater that had evaporated to higher salinities and had precipitated halite. The concentrations of Na, K, and Li are what would be expected for seawater at this degree of evaporation, and show that these fluids did not interact with the same lithologies as the ore fluids. The major change to their fluid composition was exchange of Mg for Ca during dolomitization. We suggest that the ore fluids could only have reached their high salinity by evaporation of seawater on the extensive shallow water shelf regions that existed over much of the Irish midlands.
 
Article
We report sulfur and lead isotope analyses of mine concentrates from Navan, the largest zinc deposit in Ireland. These samples, each representing up to a million tonnes of ore, show very limited isotopic variation, with mean delta S-34 = -13.6 +/- 2 per mil (n = 20), and galena concentrate mean Pb-206/Pb-264 = 18.19 +/- 0.03 (n = 7). Calculations suggest greater than or equal to 90 percent of the Navan sulfides were derived through bacteriogenic reduction of Mississipian seawater sulfate, whereas metals were acquired from a local, orogenic crustal source beneath the ore-body. Enhanced bacterial activity was fundamental to ore deposition at Navan: no bacteria, no giant ore deposit.
 
Article
The Bor district in Serbia and the Panagyurishte district in Bulgaria are part of the mineralized Late Cretaceous Carpatho-Balkan igneous belt. 40Ar/ 39Ar laser probe radiometric age information from porphyry copper deposits of the two districts in this study provides new constraints on the timing of host-rock emplacement and the hydrothermal alteration that is linked to the porphyry mineralization. In the Bor district, igneous hornblende from the host intrusions has an average plateau age at 84.0 ± 1.5 Ma (2σ . The timing of alteration is constrained by white mica with an average inverse isochron age of 85.4 ± 1.8 Ma and an average plateau age at 86.6 ± 1.0 Ma (elevated by a small excess 40Ar contribution). Together, these analyses indicate that porphyry mineralization occurred around 85 Ma. In the Panagyurishte district previous isotopic studies have produced much older host-rock ages in the northwest than mineralization ages in the southeast. Ages of igneous and alteration mineral separates from Elatsite and Medet in the northwestern and central parts of the district from the present study confirm pluton emplacement at 90.8 ± 0.8 Ma but much younger alteration at 79.5 ± 0.5 Ma. It is proposed that two episodes of magmatic-hydrothermal activity have affected the district, one at ∼91 Ma, with the emplacement of the majority of the plutonic rocks, and one at 79 to 80 Ma during which the alteration micas were reset. Integration of these new ages with existing radiometric age information emphasizes the complex history of magmatic-hydrothermal activity along the immature Late Cretaceous Carpatho-Balkan convergent margin.
 
Article
The Gubong deposit consists of five massive, gold-bearing mesothermal quartz veins that fill fractures oriented northeast and northwest along fault shear zones over an area of 14 km2 in Precambrian metasedimentary rocks of the Gyeonggi massif. The veins are divided into three groups, based on their orientation and location. They have a ribbon texture that is interpreted to result from repeated hydraulic fracturing events. Mineral deposition was associated with hydrothermal fluid overpressuring within and below the active fault shear zones. The vein mineralogy and paragenesis of the veins allow two separate discrete mineralization episodes to be recognized, separated by a major faulting event. The ore minerals are contained within quartz and calcite associated with fracturing and healing of veins that occurred during both mineralization episodes. Wall-rock alteration minerals during stage I, the main ore stage, include sericite, chlorite, and minor pyrite and carbonates. Sulfide minerals deposited along with electrum during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, and argentite. Electrum also was deposited during stage II mineralization in the one group of veins, along with pyrite, sphalerite, chalcopyrite, and galena, but the second stage of deposition in other veins was barren. Petrographic examination of textural relationships among sulfides, fluid inclusions, microfracturing, and quartz shows chronological and genetic relationships between gold deposition and fluid entrapment. Systematic studies of fluid inclusions in stage I vein indicate two contrasting events: a relatively high temperature (203°–432°C) and pressure (943–2,098 bars) event related to early sulfide deposition and associated with H2O-CO2-CH4-NaCl ± N2 fluids (less than about 13.4 wt % NaCl), and a lower temperature (202°–399°C) and pressure (670–850 bars) late sulfide depositional event involving H2O-NaCl fluids (3.9–17.3 wt % NaCl). The H2O-NaCl fluid involved in ore-related stage II mineralization had a salinity of 0.4 to 4.2 wt percent NaCl and a homogenization temperature of 201° to 378°C. The calculated sulfur isotope compositions of hydrothermal fluids from the stage I veins ({delta}34SH2S = 3.5–10.5{per thousand}) indicate that ore sulfur was derived mainly from a magmatic source but also in part from sulfur contained in the host rocks. The calculated and measured oxygen and hydrogen isotope compositions of the ore-forming fluids (stage I: {delta}18OH2O = 1.1–9.0{per thousand}, {delta}D = –92 to –21{per thousand}; stage II: {delta}18OH2O = –0.1 to +0.3{per thousand}, {delta}D = –95 to –93{per thousand}) indicate that the fluids were derived from magmatic and/or deep-seated metasedimentary rocks (stage I) and evolved by mixing with local meteoric water (stage II), by limited water-rock exchange and by degassing during mineralization in uplift zones. The H2O-NaCl fluids involved in the stage I development of these veins represent fluids that evolved either through unmixing of H2O-CO2-CH4-NaCl ± N2 fluids following a decrease in fluid pressure or through mixing with deeply circulating meteoric waters, possibly as a result of uplift and/or unloading during mineralization. The H2O-NaCl fluid involved in stage II was derived from meteoric water. Early deposition of gold in stage I was caused by a decrease in sulfur fugacity (H2S loss) that accompanied the immiscible separation of carbonic vapor from H2O-CO2-CH4-NaCl ± N2 fluids. Gold in late stage I and stage II veins was precipitated from H2O-NaCl fluids by cooling and by dilution caused by mixing with meteoric water.
 
Article
Les données géochronologiques obtenues à partir des échantillons prélevés dans les districts miniers de Milpo-Atacocha et de Chungar mettent en évidence l'importance des épisodes magmatiques et de l'activité métallogénique à l'Oligocène dans le Pérou central
 
Article
Fluid inclusions in vein quartz from 10 granitoid-hosted gold deposits and prospects in the Birimian terrane of Ghana, as well as from the Sansu mine (Ashanti shear zone type) and quartz veins in the nonmineralized Princess Town granodiorite, were studied by microthermometry and Raman microspectrometry. Fluid inclusions from the granitoid-hosted gold deposits are dominated by aqueous H2O-CO2-NaCl type 1 and liquid CO2-N-2 +/- CH4 type 2, with minor (<10%) aqueous H2O-NaCl type 3. Type 1 inclusions show large variations in CO2 phase volume proportions (10-90%) at 25degreesC and have salinities commonly between 0 and 6 wt percent NaCl equiv. Their bulk densities fall in a major range from 0.62 to 1.11 g/cm(3). Type 2 inclusions show no visible H2O phase at room temperature and have bulk densities between 0.30 and 0.92 g/cm(3). Type 3 inclusions, containing 10 to 20 vol percent H2O vapor, have salinities of 1 to 8 wt percent NaCl equiv and bulk densities of 0.77 to 1.03 g/cm(3). In most cases, the three types of fluid inclusions coexist as groups in individual quartz grains of the samples studied. These fluid inclusions are interpreted to be trapped during phase separation of an originally, homogeneous H2O-CO2 fluid, with low salinity, (<6 wt % NaCl equiv) and moderate to high density (0.65-0.95 g/cm(3)). The type 1 fluid inclusions are suggested to be heterogeneous mixtures of the two end members of type 2 and 3 inclusions. Fluid immiscibility is documented by petrographic characteristics and microthermometric results of the three types of inclusions. Trapping temperatures and pressures, estimated from microthermometry of type 3 inclusions, equation of state, and the P-T-X nature of the H2O-CO2-NaCl system, are mainly between 200degrees and 350degrees Cand 1 and 3 kbars for the gold deposits. By contrast, fluid inclusions in the Sansu mine at Ashanti mostly comprise the liquid CO2-N-2 +/- CH4 type 2. The inclusions are considered to represent postentrapment modifications of trapped fluids. In addition, fluid inclusions in barren vein quartz from the Princess Town granodiorite comprise the low-salinity, (commonly <6 wt % NaCl equiv) H2O-NaCl type 3. Raman microspectrometry shows that gaseous compositions of fluid inclusions from both granitoid- and shear zone-hosted gold deposits are mainly, composed of CO2 (80-95 mol %), with significant amounts of N-2 (2-20 mol %) and CH4 (0-10 mol %). The distinct low-salinity, H2O-CO2-rich fluids from the granitoid-hosted gold deposits are comparable in composition to those from the major Ashanti and Tarkwaian types of gold deposits in the Birimian terrane of Ghana. These fluids are most likely, to be metamorphic in origin and associated with the waning stages of the regional Birimian orogeny. Gold deposition within the Birimian granitoids was related to fluid phase separation and sulfidization of host rocks during hydrothermal alteration and mineralization. The present study, together with previous publications, suggests that fluid inclusions are characterized by H2O-CO2-NaCl and/or CO2-N-2 +/- CH4 types in mineralized areas, whereas H2O-NaCl fluid compositions dominate in barren areas. Fluid inclusion characteristics may, therefore, be a useful tool for regional gold exploration in the Birimian terrane of Ghana.
 
Article
Sixty-eight alluvial diamonds from three placer deposits in Brazil (Arenapolis in the State of Mato Grosso, Boa Vista in the State of Roraima, and Canastra in the State of Minas Gerais) are characterized by similar crystal shapes, body colors, and surface textures, which are related to growth or resorption processes. The concentrations and the aggregation states of nitrogen impurities in the diamonds, as well as in their carbon isotope compositions, are also similar. A higher proportion of diamonds with radiation spots distinguishes the diamonds from Boa Vista from the other deposits. The majority of the diamonds from Arenapolis (~70%) exhibit transport-related abrasion textures. In contrast, diamonds from Boa Vista and Canastra are characterized by the absence or a low abundance (<15%) of such abrasion textures, which indicates that the diamonds are derived from nearby kimberlitic sources. At this time, kimberlitic sources have been located only in the proximity of the Canastra placer deposits. The composition of the mineral inclusions is similar for diamonds from all three deposits. The diamonds formed in a strong to moderately depleted peridotitic mantle, with only minor involvement of eclogitic sources. Pressure and temperature estimates for the diamonds from Boa Vista are similar to the estimates for diamonds from other deposits worldwide and are consistent with a geothermal gradient of 40 to 42 m W/m2 surface heat flow.
 
Article
Two typical lateritic weathering profiles (Jacuba and Angiquinho) from the Niquelandia Ni deposits, Brazil, were studied in order to establish the petrological relationship between the supergene Ni products and the parental pyroxenes. From the base to the top of the profiles, pyroxenes are replaced by goethite and kaolinite through a series of transitional Ni-bearing phyllosilicates. The mineralogy and the chemical composition (especially the Ni content) of these clay minerals depends on the degree of fracturing and serpentinization of the pyroxenite and the location of the pyroxenite with respect to neighboring dunite. Within the Jacuba profile, smectite and pimelite pseudomorphs after pyroxene are especially Ni rich, and in fact, are the most Ni-enriched clay minerals now known in lateritic weathering profiles.
 
Article
The temperature profile in magmatic-hydrothermal systems directly affects the chemical behavior and pressure regime of hydrothermal fluids and the resulting diversity of mineralization. We combine textural observations of igneous and hydrothermal minerals using SEM-CL and -BSE images with three independent mineral thermobarometers to better understand the thermal profile at the porphyry-Cu-Mo deposit in Butte, Montana. We apply the two most recent (and controversial) forms of the Ti-in-quartz thermobarometer from Thomas et al. (2010) and Huang and Audétat (2011), the Zr-in-rutile thermobarometer of Tomkins et al. (2007), and the XMg-Ti-in-biotite thermometer of Henry et al. (2005) to estimate the formation temperatures of these magmatic and hydrothermal minerals. In a comparison of isobaric temperature distributions from Ti-in-quartz (Thomas et al., 2010) and Zr-in-rutile we find that the Thomas et al. calibration consistently yields temperatures that are 50 to 200°C lower than those from Zr-in-rutile. These quartz temperatures are unreasonably low for quartz phenocrysts and are considerably lower than previous estimates for vein quartz. Temperature estimates from the Zr-in-rutile and XMg-Ti-in-biotite thermobarometers agree well with each other and with previous temperature estimates. We conclude that application of the Ti-in-quartz thermobarometer of Thomas et al. is not appropriate for this natural system. Quartz temperatures calculated using the calibration of Huang and Audétat (2011) are closer to those from rutile and biotite. Application of the Ti-in-quartz thermobarometer of Huang and Audétat to hydrothermal samples yields maximum temperature estimates, however, and requires evaluation of trace element abundances (e.g., Ti, Al) and other crystal lattice impurities (e.g., fluid inclusions) in growth zones as a means to determine whether growth zones represent slow or fast-growing quartz. Using thermobarometry from rutile, biotite, and quartz (Huang and Audétat, 2011), we estimate that the final dike injection temperature, and hence the initial magmatic-hydrothermal fluid temperature, was ~700°C while the ambient host rock temperature was ≤500°C. There is a magmatic-hydrothermal continuum represented in hydrothermal veins, ranging from ~710 to <440°C. Distinct mineral generations within veins consistently display large temperature ranges, spanning 50 to 250°C, likely recording mineralization from thermally unequilibrated fluids. Mineral precipitation temperatures within veins are indistinguishable from those in accompanying envelopes, indicating contemporaneous formation of veins and envelopes. Magmatic and hydrothermal samples show no systematic relationship between temperature and depth within the deposit, and we observe anomalous cross-cutting relationships indicating that vein formation temperatures fluctuated significantly within a single cm3 parcel of rock. We suggest that the thermal profile does not mimic domical isograds at any given time, but rather evolves dynamically by discrete cycles of transitory, high-temperature hydrofracturing, fluid release, and vein formation that overprints the cooler host rock thermal gradient.
 
Article
Steep Rock Lake, in Western Ontario, was drained in 1943-44 by means of a large pumping operation. As the water line dropped, serious movements of the exposed soil occurred. Study of these, in connection with the development of the open pit from which the valuable iron ore was to be mined, led to an extensive soil investigation. The varved "clays" revealed by the mining operations with a total thickness of over 100 feet were studied first with engineering objectives in view. These "soil mechanics" investigations led to a detailed study of the soil in individual varves. Results confirm several features of the well-known seasonal theory of deposition but raise some questions which suggest the desirability of carrying still further this laboratory soil research.
 
Article
Lisheen is a strata-bound zinc-lead deposit formed during the Mississippian by replacement of hydrothermally dolomitized, grossly stratiform breccia bodies located near the base of the Carboniferous Waulsortian Limestone. It represents one of a number of carbonate-hosted massive sulfide ore deposits in the Irish ore field that, due to several unique features, have been classified as Irish type. Disseminated pyrite occurs in preore dolomite and around the margins of preore dolomite clasts within dolomite breccias. Early fine-grained sphalerite-pyrite mineralization occurs as infill of intergranular dolomite porosity. Locally, massive to semimassive iron sulfide is observed, mainly comprising pyrite with lesser marcasite, A complex polymetallic sulfide assemblage typifies the main ore stage, dominated by fine-grained disseminated, massive or colloform sphalerite and galena, with minor pyrite, chalcopyrite, arsenopyrite, tennantite, nickel- and cobalt- bearing minerals. Silver occurs in solid solution in tennantite, galena, and sphalerite, Dolomite and barite dominate the gangue, with lesser calcite. Main-stage mineralization involved the progressive replacement of preexisting iron sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts. Coarse crystalline sphalerite and euhedral galena crystals are generally restricted to fracture-fill mineralization or vugs within main ore-stage assemblages where they occur with euhedral dolomite and calcite. Barite intergrown with main ore-stage sulfides has delta(34)S values of 14.3 to 18.1 per mil, consistent with the derivation of sulfate from coeval Carboniferous seawater. The delta(34)S values for sulfides range from -44.1 to +11.8 per mil, with a mean value of -13.7 per mil, typical of the Irish ore deposits. The dominant low delta(34)S signature is considered to be the result of bacterial reduction of coeval seawater sulfate. Extremely low delta(34)S values, in the range of -38 to -44 per mil, are only observed in preore disseminated pyrite; such extreme fractionations are thought to be due to low bacterial sulfate reduction rates coupled with oxidative cycles in near-seafloor pore waters. Main ore-stage sulfides have delta(34)S values in the range of -4 to -18 per mil, with a mode of -10 per mil, consistent with a typical bacterial fractionation from coeval seawater sulfate. Isotopic equilibrium between cogenetic sulfides is not observed. The bacteriogenic sulfur component was probably transported from bacterial colonies fringing the ore system by low-temperature brines. The delta(34)S values of late ore-stage sulfides mainly range from -20.2 to +12.0 per mil, with the majority having relatively high values (mean = -3.0 +/- 8.5%, 1 sigma) interpreted as being due to the presence of a hydrothermal sulfur component, leached from the lower Paleozoic basement. For galena and sphalerite there is a general increase in delta(34)S values with depth in the system, with time, and with proximity to east-west- and northwest-trending faults. These relationships suggest that input of hydrothermal sulfur from depth via fractures became increasingly important. Hydrothermal sulfur appears to be more important at Lisheen than the other major Irish deposits. Galena lead isotope analyses gave average Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 values of 18.183, 15.594, and 38.080, respectively. These data do not correlate with ore-stage, galena texture or delta(34)S. The results are comparable to previous data from Lisheen and from Silvermines, 35 km to the west, implying a common lead source in the lower Paleozoic basement. The textural, mineral, chemical, and isotopic evidence suggests that main-stage ore was precipitated as a consequence of rapid supersaturation, caused by fluid mixing within the permeable dolomite breccias. This process involved relatively high temperature (ca. 240 degrees C), metal-bearing solutions derived from a basement- equilibrated fluid reservoir (carrying Zn, Pb, Fe, Cd) and shallow, saline (ca. 25 wt % NaCl equiv.) formation waters rich in bacteriogenic H2S. Minor metals (Cu, As, Ni, Co) are thought to have been stripped from the footwall Old Red Sandstone during hydrothermal alteration around fault conduits. The availability of abundant seawater sulfate, operation of open-system bacterial sulfate reduction, and episodic availability of free oxygen imply that ore formation cannot have occurred at significant depth below the paleoseafloor. Cessation of mineralization was due to a cut-off of the sulfur-rich brine supply, possibly by deposition of impermeable hanging-wall sediments. This process of ore formation is consistent with evidence from the other economic Irish-type deposits in the ore field.
 
Article
Investigations in the northernmost Carlin trend were undertaken to advance understanding of the geochemical signatures and genesis of precious metal deposits in the trend. Two fundamental geologic relationships near the trend significantly affect regional geochemical distributions: a remarkably intact lower Paleozoic stratigraphic sequence of siliceous rocks in the upper plate of the middle Paleozoic Roberts Mountains thrust, and the widespread repetition of rocks high in the upper plate during late Paleozoic thrusting that thickens the cover above mineralized rock in the lower plate. A compilation of previously published chemical analyses of 440 stream sediment samples and 115 rocks from two 7(1)/(2)-minute quadrangles, as well as new chemical analyses of approximately 1,000 drill core samples in a 1,514 in (4,970 ft) hole through the Rodeo Creek deposit were used to construct three-dimensional element distribution models that highlight metal zonation in the mineralized systems. The Rodeo Creek deposit comprises deep Ag base-metal+/-Au-mineralized rock below the Roberts Mountains thrust and contains an unusually high Ag/Au ratio greater than 30. Stacked geochemical halos related to the deposit are confined to the lower plate of the Roberts Mountains thrust and include two horizons of Hg, Cu, and Zn anomalies-as much as 180 m above the deposit-that mostly result from mercurian sphalerite. Extremely subtle indications of mineralization in the upper plate of the Roberts Mountains thrust above the deposit include arsenopyrite overgrowths on small pyrite crystals in 50- to 75-mum-wide clay-carbonate veinlets that lack alteration halos, arsenical rims on small disseminated crystals of recrystallized diagenetic pyrite, and partial replacement of diagenetic pyrite by tennantite. Some of these minerals contain anomalously high An. However, these As-(Au)-bearing rocks most likely represent another locus of largely untested mineralized rock rather than distal halos related to either the Rodeo Creek or the nearby Dee and Storm gold deposits. Application of micromineralogic techniques helped to identify mineral assemblages that are specific to mineralization and provided an empirical foundation for interpretations of geochemical halos in the Carlin trend. District-scale geochemical patterns of several elements in stream sediments and surface rocks coincide with the northernmost Carlin trend and can be used to explore for Carlin-type deposits. Concentrations of elevated As and Sb in stream sediments (as much as 54 ppm As) have northwest-elongate lobate patterns that clearly outline the trend across a width of approximately 4 km. Arsenic contents of exposed rocks (as much as 90 ppm As) strongly correlate with As contents of derivative stream sediments, and rock contents of Sb show a somewhat lesser but nonetheless strong and similar correspondence. Factor analysis of stream-sediment data shows that those factor scores that are correlated with As, Sb, An, and Pb also are high along the trend and suggest that mineralized rocks may be present. Although As was not detected by scanning electron microscope-energy dispersive spectrometer (SEM-EDS) studies in heavy mineral concentrates of high-As stream sediments in the Carlin trend, X-ray absorption near-edge spectra (XANES) of selected light fractions of stream sediment samples indicate that Al-bearing phases, such as gibbsite, amorphous Al oxyhydroxides, or aluminosilicate clay minerals host most of the As(V). The best fit, visually and in terms of the lowest residual, was obtained by a model compound of As(V) sorbed to gibbsite. Thus, most As in stream sediments derived from altered rock within the Carlin trend apparently is contained in light fractions. The geochemical character of young, unconsolidated, postmineral deposits that cover mineralized rocks on the Carlin trend partly results from mineralized sources along the trend. Concentration of As in the Miocene Carlin Formation shows air exceptionally well developed progressive increase to about 30 ppm As as altered rock surrounding the trend is approached. Mineralized and/or altered rock fragments probably have been shed directly into the sedimentary basin of the Carlin Formation, mid migration of As, now fixed as As(V), also may have occurred in the supergene environment after material was recycled out of the Carlin Formation and into present-day gulleys.
 
Article
Thesis--Johns Hopkins University. "Reprinted from Economic geology, vol. XXXV, nos. 1 and 2, Jan.-Feb., Mar.-April, 1940." Includes bibliographical references (p. 185-187). Cover title.
 
Article
Silver Hill is a stratiform deposit 15 m wide, 5 m thick, and at least 550 m long in chloritic argillite in a sequence of Cambrian andesitic to rhyolitic submarine pyroclastic-flow tuffs. The orebody has sharp upper and lower contacts and grades laterally into disseminated ore; chert extends for up to 200 m on either side of the orebody. Quartz-sulfide-siderite veinlets cut the argillite above and below the orebody. The ore consists of beds of fine-grained sulfides; chert, calcite, and dolomite (metamorphosed to tremolite) that precipitated with the sulfides; and argillite. Assays average 40 percent Zn, 19 percent Pb, and 0.6 percent Cu. Primary ore minerals are pyrite, sphalerite, galena, chalcopyrite, pyrrhotite, arsenopyrite, silver, freibergite, pyrargyrite, mackinawite, cubanite, bismuthinite, bismuth, magnetite, and pentlandite(?). The effect of greenschist facies metamorphism on sulfide minerals is minor. A lens of banded sphalerite-pyrite ore precipitated from a late-stage ore solution. Arsenic, bismuth, and silver were concentrated in the residual ore solution; antimony was not. An area of intense quartz mineralization marks a conduit of ore solution onto the sea floor. Other massive sulfide and quartz stringer deposits near Silver Hill are genetically related to the Silver Hill deposit.
 
Article
Thesis (M.S.)--Michigan State University. Dept. of Geology, 1973. Includes bibliographical references (leaves 21-22).
 
Article
Thesis (Ph. D.)--Dept. of Geology, Stanford University. Bibliography: l. 120-123.
 
Article
E1 Romeral iron mine is in the coastal iron province of northern Chile, a north-trending belt of magnetite deposits about 30 km wide and 600 km long. Iron at E1 Romeral is mined from two orebodies in which magnetite is microscopically intergrown with actinolite. The Main orebody is a lenticular, steeply dipping magnetite- rich mass in a reentrant between two lobes of the Romeral diorite pluton. The North orebody consists of conformable pods of magnetite in actinolitized biotite schists with steeply dipping foliation. Both orebodies trend northerly, adjacent and subparallel to the eastern contact of the western lobe of the diorite pluton and within a northtrending network of anastomosings trike-slip faults. Ore-zone boundaries are gradational, and altered rocks within and around the orebodiesc ontainm agnetite,a ctinolite,p lagioclase( An•.2_a•.),d iopside,c linozoisite,s phene, chlorapatite,m arialitic scapolitet, ourmaline,c hlorite, pyrite, calcite,m icas, and clays. The schists, phyllites, and quartzites of E1 Romeral probably are late Paleozoic, whereast he andesitep orphyry, diorite, and magnetited epositsa re early Cretaceousin age. Emplacemenat nd crystallizationo f the Romeral diorite pluton precededo re deposition. During late stageso f diorite crystallizationq, uartzitea nd andesitep orphyryn ear diorite contactsw ere dioritized by metasomatica dditionso f plagioclase( An20_a0),h ornblende, and diopside. Iron in hematite-bandedq uartzite was mobilizeda nd redepositeda s hematite 80 to 145 m from the diorite contact. Cordieritep orphyroblastgsr ew in phyllitea nd schist,a nd diopsidea nd oligoclasep artially replaceda ndesitep orphyryb etweent he east and west lobes of the diorite pluton. Magnetite ore deposition was hydrothermal and was accompanied by pervasive actinolitization,p robablya t temperaturesm ainly in the range 550ø to 475øC. Leftlateral movemenot n the Romeralf ault was concurrenwt ith ore depositiona nd with emplacemenot f intramineral diorite aplite dikes into en echelon,n orthwest-trending, steeplyd ipping gash fractures,w hich openedi n responseto the left-lateral faulting. Ore depositionw as followedb y chloritization,b y emplacemenot f a few minor granitic dikes,a nd by alterationo f previouslya ctinolitizedd iorite and phyllitet o aplitic rocks consistingo f secondaryp lagioclase( Ans_25), quartz, microcline,a nd minor tourmaline. Aplitization was followedb y emplacemenot f many biotitic diorite aplite dikes, by formationo f late actinolite-magnetite-apatviteei ns, by locally intensea rgillization and martitization, by right-lateral movement on the late North-northeast fault, and by emplacement of the postore batholith east of E1 Romeral.
 
Article
A program of geologic mapping and lithogeochemical and geochronological sampling has been carried out over a 745-km(2) area of the Atacama Desert surrounding the porphyry Cu deposits at Escondida, Zaldivar, and Chimborazo (Cordillera de Domeyko, northern Chile). The pur-pose of this study was to examine the regional tectonic and magmatic setting of this preeminent porphyry Cu district for evidence of features or processes that might explain the giant scale of mineralization at Escondida and provide predictive tools for exploration in other areas. The geologic history of this area as recorded by exposed rocks begins with voluminous, intermediate to felsic Permo- Carboniferous volcanism (La Table Formation), and these rocks appear to constitute the crystalline basement throughout much of the porphyry belt of nor-them Chile. Geochemically: they are I-type in character, but the parental magmas were relatively dry, and thus did not generate effective magmatic-hydrothermal systems (few significant ore deposits are known to be associated with them). Andean cycle are magmatism began in the Triassic, centered on the La Negra magmatic are (now located near the Chilean coast). Farther inland, near Escondida, back- are processes led to the eruption of intermediate to felsic lavas and tuffs and the deposition of marine sediments in rift basins. Closure of these basins in the Late Cretaceous resulted in deformation of the volcano-sedimentary sequences and was followed by emplacement of small alkali gabbro stocks and dikes. The axis of arc magmatism moved eastward in the Paleocene (Central Valley are) and produced widespread calc- alkaline intermediate to felsic volcanism through to the Eocene. East- to northeast-directed convergence maintained a dextral transpressive regime during this period, and early movements in the West Fissure zone, a corridor of orogen- parallel faults that runs the length of the Cordillera de Domeyko (over 1,000 km), reflect this couple. At the end of the Eocene, however, stresses in the are appear to have relaxed, and by the late Oligocene, strike-slip movement along the West Fissure zone had reversed to sinistral. This period of stress relaxation at the end of the Eocene period coincided with the voluminous emplacement of dioritic magmas at shallow crustal levels and also with porphyry development, Six samples of hornblende from these diorites field Ar-40/Ar-39 dates between 38.28 +/- 0.32 and 36.94 +/- 0.46 Ma (2 sigma). Porphyry emplacement at Escondida, Zaldivar, and Chimborazo was coeval with this dioritic magmatism at similar to 38 Ma. Where plutonism was intense, the dioritic magma is interpreted to have evolved by processes of assimilation and fractional crystallization to more felsic compositions characteristic of the ore-forming porphyry intrusions. Whole-rock trace element data indicate that hornblende fractionation was an important control on chemical evolution of the diorites and attests to high-magmatic water contents (less than or equal to4 wt % H2O). Volatile saturation would have occurred during further differentiation of these magmas, evidence for which is provided by the porphyry ore deposits, Porphyry emplacement was localized within a broad zone of intersection between die West Fissure zone and a regionally extensive northwest-trending structural corridor (the Archibarca lineament). It is proposed that the geometry of this junction was conducive to the formation of transtensional pull-apart structures during relaxation or reversal of dextral shear on the West Fissure zone. Such dilational structures would have focused the ascent and pooling of magma in the upper crust and maximized de potential for formation of magmatic-hydrothermal ore deposits. The formation of giant porphyry systems such as Escondida is, therefore, considered to be the result of a fortuitous coincidence of processes, including generation of suitable volumes and compositions of magma, appropriate lithospheric stress conditions, and structural focusing of emplacement; in addition, the development of thick supergene enrichment blankets has been critical to the economic value of these deposits, None of these contributory processes are in themselves unusual or rare, but because they are largely independent of one another, their constructive cooperation in ore formation is not necessarily repeatable at different places and at different times, thus explaining the relative rarity of giant porphyry deposits.
 
Article
Unique magnetite-hematite deposits in the Laco area of northern Chile are believed to represent the crystallization of a recent "ore magma" composed almost entirely of iron oxides and abundant gas. The "ore magma" appears to have intruded tuff at shallow depth and in places to have broken out over the surface. Gas tubes, blisters, and ropy surfaces are common features.
 
Article
The Jiaodong gold province, the largest gold-producing district in China, is located in the jiaodong peninsula at the eastern margin of the North China craton and bounded by the continental scale Tan-Lu fault, 40 kin to the west. Previous geochronological studies suggest that pervasive gold deposition took place in the western part of the province between 122 and 119 Ma. Here we report high-quality Ar-40/Ar-39 ages of the Pengjiakuang and Rushan deposits from the eastern part of the jiaodong gold province, placing additional chronological constraints on the timing of regional mineralization. Seven sericite grains extracted from auriferous alteration assemblages at the Pengiiakuang deposit yielded well-defined plateau ages between 120.9 +/- 0.4 and 119.1 +/- 0.2 Ma (2 sigma). Three separates of igneous biotite from a sample of the Queshan gneissic granite, adjacent to the Pengjiakuang deposit, gave reproducible plateau ages of 124.6 +/- 0.6 to 123.9 +/- 0.4 Ma (2 sigma). Six sericite separates front two samples in the Rushan deposit yielded Ar-40/Ar-39 plateau ages at 109.3 +/- 0.3 to 107.7 +/- 0.5 Ma (2 sigma), whereas biotite from the Kunyushan monzogranite that hosts the Rushan deposit had plateau ages ranging from 129.0 +/- 0.6 to 126.9 +/- 0.6 Ma (3 separates front one sample). The apparent age gap between hydrothermal sericite and magmtic biotite from both deposits, together with the similar argon closure temperatures for these mica minerals, suggest that gold mineralization had no direct relationship to the granitoid magmatism. Instead, gold deposition coincided with the emplacement of mafic to intermediate dikes widespread in the jiaodong gold province, which have been dated at ca. 122 to 119 Ma and, less commonly, at 110 to 102 Ma. The new Ar-40/Ar-39 ages from the eastern jiaodong peninsula, when combined with published data from the western part suggest that gold mineralization was broadly contemporaneous throughout the district. The Early Cretaceous gold mineralization also is widely developed in four other major gold districts along the Tan-Lu fault. The temporal and spatial correlation of these gold deposits with mafic to intermediate dikes commonly found in most mineralized areas, the presence of well-documented metamorphic core complexes and half-graben basins along the Tan-Lu fault, and voluminous basalts therein, suggest that the Early Cretaceous was an important period of lithospheric extension, possibly caused by the late Mesozoic lithospheric thinning beneath the eastern block of the North China craton. Lithospheric thinning and extension could have resulted in abnormally high heat and fluid fluxes necessary for large-scaled gold mineralization.
 
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