Papers by Anthony Fallick
Minerals, Feb 13, 2019
Although emerald deposits are relatively rare, they can be formed in several different, but speci... more Although emerald deposits are relatively rare, they can be formed in several different, but specific geologic settings and the classification systems and models currently used to describe emerald precipitation and predict its occurrence are too restrictive, leading to confusion as to the exact mode of formation for some emerald deposits. Generally speaking, emerald is beryl with sufficient concentrations of the chromophores, chromium and vanadium, to result in green and sometimes bluish green or yellowish green crystals. The limiting factor in the formation of emerald is geological conditions resulting in an environment rich in both beryllium and chromium or vanadium. Historically, emerald deposits have been classified into three broad types. The first and most abundant deposit type, in terms of production, is the desilicated pegmatite related type that formed via the interaction of metasomatic fluids with beryllium-rich pegmatites, or similar granitic bodies, that intruded into chromium-or vanadium-rich rocks, such as ultramafic and volcanic rocks, or shales derived from those rocks. A second deposit type, accounting for most of the emerald of gem quality, is the sedimentary type, which generally involves the interaction, along faults and fractures, of upper level crustal brines rich in Be from evaporite interaction with shales and other Cr-and/or V-bearing sedimentary rocks. The third, and comparatively most rare, deposit type is the metamorphic-metasomatic deposit. In this deposit model, deeper crustal fluids circulate along faults or shear zones and interact with metamorphosed shales, carbonates, and ultramafic rocks, and Be and Cr (±V) may either be transported to the deposition site via the fluids or already be present in the host metamorphic rocks intersected by the faults or shear zones. All three emerald deposit models require some level of tectonic activity and often continued tectonic activity can result in the metamorphism of an existing sedimentary or magmatic type deposit. In the extreme, at deeper crustal levels, high-grade metamorphism can result in the partial melting of metamorphic rocks, blurring the distinction between metamorphic and magmatic deposit types. In the present paper, we propose an enhanced classification for emerald deposits based on the geological environment, i.e., magmatic or metamorphic; host-rocks type, i.e., mafic-ultramafic rocks, sedimentary rocks, and granitoids; degree of metamorphism; styles of minerlization, i.e., veins, pods, metasomatites, shear zone; type of fluids and their temperature, pressure, composition. The new classification accounts for multi-stage formation of the deposits and ages of formation, as well as probable remobilization of previous beryllium mineralization, such as pegmatite intrusions in mafic-ultramafic rocks. Such new considerations use the concept of genetic models based on studies employing chemical, geochemical, radiogenic, and stable isotope, and fluid and solid inclusion
The ca. 2000 Ma Tulomozerskaya Formation, Russian Karelia, is composed of an 800 m-thick magnesit... more The ca. 2000 Ma Tulomozerskaya Formation, Russian Karelia, is composed of an 800 m-thick magnesite-stromatolite-{!olostone 'red bed' succession with the most 13 C-rich dolostones (up to +18%o V-PDB) that have ever been reporte d. Terrigenous 'red beds' are developed throughout the sequence and represent three main depositional settings: (l) a braided fluvial system over a lower energy, river-dominated coastal plain, (2) a low-energy, barred lagoon or bight, and (3) a non-marine, playa lake. A significant component of the sequence consists of biostromal and biohermal colurnn ar stromatolites accreted in shallow-water, low-energy, intertidal zones, barred evaporitic lagoons and peritidal evaporitic environments. Only a small portion of stromatolites might have been accreted in relatively 'open' marine environments. The red, ftat-larninated, dolomitic and magnesite stromatolites formed in evaporative ephemeral ponds, coastal sabkhas and playa lakes. Tepees, mudcracks, pseudomorphs after calcium sulphate, halite casts, and abundant 'red beds' in the sequence suggest that (l) terrestrial environments dominated over aqueous, and (2) partial or total decoupling took place between the stromatolite-dominated depositional systems and the bordering sea. The greatest enrichment in 13 C occurs in the playa magnesite (up to +17.2%.) and in the larninated dolomitic stromatolites accreted in ephemeral ponds (up to + 16.8%o), whereas the dolostones from more open environments are less rich in 13 C (+5.6 to + 10.7%.). The isotopic shift (ca. 5%o) induced by global factors (i.e. accelerated accumulation of organic material in an external basin) was augmented by that driven by a series of local factors (restriction, evaporation, biological photosynthesis). The latter enhanced a global 8 13 C value due to an isotopic disequilibrium between atrnospheric C� and dissolved inorganic carbon in the local aquatic reservoirs precipitating the carbonate minerals. The interplay between global and Iocal factors should be taken into account when interpreting the Palaeoproterozoic carbon isotope excursion and its implications.
Arabian Journal of Geosciences, 2018
At the southern margin of the Tethys, the Es Souabaa area recorded traces of Oceanic Anoxic Event... more At the southern margin of the Tethys, the Es Souabaa area recorded traces of Oceanic Anoxic Event 2 (OAE2) around the Cenomanian-Turonian boundary (C/Tb). The dark, laminated, filament-and pyrite-bearing limestones represent the typical facies of this event. In terms of sedimentary environment, these features reflect a transgressive drowning that had induced hypoxia in these sedimentary environments. Such conditions favored the deposition and preservation of organic matter of marine origin, the distribution of which was controlled by paleogeography and halokinetic tectonics at that period. The OAE2 reached a climax between the last upper Cenomanian occurrence of Rotalipora cushmani and the lower Turonian occurrence of Whiteinella praehelvetica. Positive shift of the δ 13 C excursion along with relatively high total organic carbon (TOC) contents during OAE2 both indicate palaeo-environmental modifications enhanced by a significant change in primary marine productivity. Meanwhile, negative δ 18 O peaks in carbonates reflect increasing temperatures. Comparison of the data from this study with those from the neighboring Kalaat Senan section (Tunisia) suggests close similarities of events, although OAE2 is much more enhanced in Algeria.
Journal of African Earth Sciences, 2016
In the Saharan Atlas (NE Algeria), the Triassic evaporitic formation was brought to the surface t... more In the Saharan Atlas (NE Algeria), the Triassic evaporitic formation was brought to the surface through the thick Cretaceous and Tertiary sedimentary cover as diapirs due to the effect of Atlasic tectonic events. The diapir piercing began in the Jurassic and has continued through present day. Many outcrops of several square kilometres are distributed in a large area (approximately 80 km wide) that extends northeasterly over 300 km towards Tunisia. The diapiric evaporitic formation is often accompanied by the emplacement of Pb-Zn-Ba-F mineralization. The Mesloula massif is an example of these deposits. Fluid inclusion and sulphur, carbon and oxygen isotope studies were carried out on Pb-Zn-Ba mineralization and associated gangue carbonates. Gypsum of the Triassic formation was also analysed for its sulphur isotope composition to show the role of evaporates in the generation of this typical peridiapiric deposit. Gypsum from the Triassic formation showed a narrow range of δ 34 S VCDT values, ranging from +14.6 to +15.5‰ (n=8). This range is comparable to that of Triassic seawater sulphates. Sulphide minerals yielded δ 34 S VCDT values between 0 and +11.7‰ (n=15), indicating that sulphide sulphur was likely derived from Triassic sulphates through thermochemical sulphate reduction (TSR) because fluid inclusion microthermometric measurements yielded a mean temperature of 150°C. Residual sulphate in such a system would have been enriched in 34 S; this is reflected in the barite δ 34 S VCDT values, which range from +21.1 to +33.5‰ (n=5). The δ 13 C VPDB values of calcite minerals, ranging from +2.1 to +6.3‰ (n=4), indicate an inorganic carbon origin, likely from the host carbonate rocks. δ 18 O VSMOW values were between +21.9 and +24.9‰, indicating that the most likely source of mineralising fluids was formation water.
Chemical Geology: Isotope Geoscience section, 1991
ABSTRACT
Madagascar is one of the principal producers of gem corundums recovered from continental basaltic... more Madagascar is one of the principal producers of gem corundums recovered from continental basaltic fields. The main deposits are the secondary deposits of Ambondromifehy from the Antsiranana Province in the northern part of the country (sapphire-bearing palaeoplacer and placer) Soamiakatra - Mandrosohasina from the Antananarivo Province in the central part (primary ruby deposit, and ruby and/or sapphire-bearing placer and palaeoplacer) and Vatomandry from the Toamasina Province in the eastern part of the island (ruby and sapphire-bearing paleoplacer). Soamiakatra is the only known primary deposit where ruby is found in metagabbro and pyroxenite xenoliths, which were entrained and brought up to the upper crust by the Ankaratra volcanic event. Petrographic studies have demonstrated the existence of two different conditions of ruby formation at the boundary of the eclogite domain (HT ~ 1100°C, HP ~ 20Kb) and granulite facies (HT ~ 1100°C, BP less than 15Kb). In contrast, the sapphires o...
The Late Archaean-Early Palaeoproterozoic transition (2500–2000 Ma) represents a hallmark period ... more The Late Archaean-Early Palaeoproterozoic transition (2500–2000 Ma) represents a hallmark period when the Earth System experienced a series of fundamental upheavals. Among them, the most important was the establishment of an oxygen-rich atmosphere (sometimes referred to as the Great Oxidation Event) and the emergence of an aerobic biosphere. Associated with this, either incidentally or causally, was a cascade of other prominent, global-scale events that considerably modified Earth’s surface environments, either temporarily or permanently; these are reviewed in Parts 1 and 8 in full, and detailed in Part 7. Briefly mentioned here, these include: the severe and global climatic event known as the Huronian glaciation; an unprecedented perturbation of the global carbon cycle, the large-magnitude Lomagundi-Jatuli positive excursion of δ13Ccarb, lasted over 160 Ma; radical changes in the phosphorus and sulphur cycles resulting in accumulation of the first-known massive sulphates and sedimentary phosphates; a radical modification in recycling of organic matter leading to the emergence of a new 13C-depleted carbon reservoir in the form of carbonate concretions; and an unprecedented accumulation of organic-rich sediments and formation of the earliest supergiant petroleum deposits.
Chemical Geology (Isotope Geoscience section), 1992
A laser, especially when focused through a petrographic microscope, can provide a localised heat ... more A laser, especially when focused through a petrographic microscope, can provide a localised heat source for extraction of light elements from solids prior to stable isotope ratio measurements by gas-source mass spectrometry. In certain cases, such as conversion of sulphides to SO2 by combustion in an oxygen atmosphere, it may be necessary to choose operating characteristics which allow the chemistry at the solid target surface to be controlled. Experimental considerations such as wavelength, laser mode and irradiance are discussed and it is shown that several successful systems work at very similar irradiances of ~ 109 W m-'. It is suggested, for those circumstances where high spatial resolution is required and where deep penetration into the surface cannot be tolerated, that a suitable modus operandi is to use a relatively low power but narrowly focused beam to excavate trenches perpendicular to the direction in which resolution is demanded. This allows a spatial resolution of < 100/zm even for porous pyrite with a poor quality of surface.
Transactions of the Royal Society of Edinburgh: Earth Sciences, 1997
The causes of hydrothermal alteration in dolerite dykes intruding Caledonian rocks of W Connacht ... more The causes of hydrothermal alteration in dolerite dykes intruding Caledonian rocks of W Connacht are investigated using stable isotope, water content and K–Ar data for whole rocks and mineral separates. Using an isochron approach the Logmór dyke in the north is re-dated to 308±4 Ma; previously determined older whole-rock ages reflect excess 40Ar. The ∼ 305 Ma age previously proposed for the Teach Dóite suite in the south is reinforced by a 305 Ma age on a pyroxene separate, although the severe resetting of most samples is emphasised by other pyroxene and plagioclase ages of ∼210 Ma. Pyroxene δ18O values for these Upper Carboniferous dykes are mostly 5·5 to 6·1%, indicating negligible crustal contamination. Logmór whole-rock samples have water contents of 1·7–2·1 wt.%, δ5D= 59 to –47‰ and δ18O = 9·4 to 9·6‰; plagioclase shows little mineralogical alteration but its δ18O is 9·7‰. Hydrothermal alteration involving a local formation or metamorphic water took place at high fluid/rock rat...
Mineralium Deposita, 2011
The combination of oxygen isotope composition with VCr -Mn trace element concentrations of V-bear... more The combination of oxygen isotope composition with VCr -Mn trace element concentrations of V-bearing garnets (tsavorites) originating from the main deposits of the Neoproterozoic Mozambique Metamorphic Belt is reported for the first time. The database enables the identification of the geological and geographical sources of the main productive areas from northern and southern Tanzania, Kenya, and Madagascar. Three consistent sets of δ 18 O values between 9.5‰ and 11.0‰, 11.6‰ and 14.5‰, and 15.5‰ and 21.1‰ have been recognized for primary deposits hosted in graphitic gneisses related to the Neoproterozic metasedimentary series. The δ 18 O value of tsavorite is a good tracer of the environment of its formation; the δ 18 O of the fluid in equilibrium with tsavorite was buffered by the host rock during metamorphism and fluid-rock interaction. This study is a first step in characterizing the geochemistry of gem tsavorite from most of the deposits and occurrences worldwide.
Marine and Petroleum Geology, 1994
The Lower Permian Rotliegend Sandstone in the Leman Field today contains formation waters which a... more The Lower Permian Rotliegend Sandstone in the Leman Field today contains formation waters which are extremely saline (165 530-238 531 mg/I), with cation/chloride ratios indicating mixing of Zechstein marine evaporite brines with meteoric water. Ancient Rotliegend formation waters, inferred from the analyses of cements, were similarly a mixture of marine brine and meteoric water (6180 = +0.3%o SMOW; 6D =-16%o SMOW). The strontium isotopic compositions of present day formation waters (87Sr/86Sr = 0.7107) are also consistent with mixing between late Permian marine brines (87Sr/86Sr = 0.707) and radiogenic strontium from a detrital source (87Sr/86Sr = 0.720-0.730). Isotopic analyses of texturally late intergranular and fracture-filling anhydrite suggest waters very similar to the present day formation waters. 634S values of anhydrite cements (+10%o CDT) indicate that sulphate in the Rotliegend originated from the stratigraphically overlying Zechstein evaporites. The (87Sr/86Sr)15o ratios of anhydrites (0.7080-0.7113), however, suggest variable mixing of strontium sourced from the overlying Zechstein evaporites with more radiogenic strontium derived from diagenetic reactions in the underlying Carboniferous shales. The oxygen isotope compositions of anhydrite reflect precipitation temperatures between 120 and 140°C from waters with a 6180 composition similar to the present day formation waters (approximately 0%0 SMOW). These are the maximum inferred burial temperatures for the Rotliegend Sandstone in the Leman Field and would have occurred at maximum burial depths of 3.5-4 km (11 500-13 100 ft) during the Late Jurassic-Early Cretaceous at the start of basin inversion. These data indicate an unusual, short-lived, catastrophic event of cross-formational flow, through fractures which were tectonically induced by basin inversion. Pore fluids may have escaped from the overlying and underlying formations into the lower pressure Rotliegend Sandstone regional aquifer to 'drain' this part of the basin.
Journal of African Earth Sciences, 2002
The petrogenesis of igneous, metamorphic and mineralized rocks in the Edough massif, NE Algeria, ... more The petrogenesis of igneous, metamorphic and mineralized rocks in the Edough massif, NE Algeria, indicates an interplay between crustal and magmatic sources, and magmatic and surface fluids, as determined by sulphur and oxygen isotopic analyses. The Tertiary ...
Journal of African Earth Sciences, 2005
... The geological, geochemical and isotopic data obtained so far in northeastern Algeria (eg Hil... more ... The geological, geochemical and isotopic data obtained so far in northeastern Algeria (eg Hilly, 1962, Fougnot, 1990, Ouabadi et al., 1992, Ouabadi, 1994, Ahmed-Said et al., 1993, Semroud et al., 1994 and Laouar et al., 2002) indicate two types of magmas: a mantle derived ...
Revista Brasileira de …, 2007
Geological Society, London, Special Publications, 2005
This study of four well characterized and adjacent terranes in Northern Britain outlines the sulp... more This study of four well characterized and adjacent terranes in Northern Britain outlines the sulphur isotope variations, assesses the overall importance of crustal and mantle sulphur, and presents a model that can be applied to terrane distinction throughout the North Atlantic Caledonides. The characteristics of metal components within the mineralization provide additional information that can be related to the nature of underlying basement and events from the onset of sedimentation to the cessation of mineralization within stratigraphically linked packages of rock.The δ34S data show that the dominant crustal units in each terrane, whether upper crustal sediments or cratonic basement, provide the main alternative sulphur source to the mantle and act also as the main contaminant of subcrustal melts. The δ34S values of granitoid-related mineralization are either within the subcrustal melt-range of −3‰ to +3‰ or deviate toward the values of major crustal units in the terrane, i.e. towa...
Geological Magazine, 2007
ABSTRACT
Geological Journal, 1990
... Page 8. 366 R. LAOUAR ET AL. ... It is significant that the Cashel microgranite analysed is d... more ... Page 8. 366 R. LAOUAR ET AL. ... It is significant that the Cashel microgranite analysed is definitely established as a melt product formed when 490 Ma gabbroic magma injected Dalradian metasediments and is therefore a confirmed S-type granite (Ahmed-Said and Leake 1990). ...
Geofluids, 2004
Metalliferous (Fe–Cu–Pb–Zn) quartz–carbonate–sulphide veins cut greenschist to epidote–amphibolit... more Metalliferous (Fe–Cu–Pb–Zn) quartz–carbonate–sulphide veins cut greenschist to epidote–amphibolite facies metamorphic rocks of the Dalradian, SW Scottish Highlands, with NE–SW to NW–SE trends, approximately parallel or perpendicular to regional structures. Early quartz was followed by pyrite, chalcopyrite, sphalerite, galena, barite, late dolomite–ankerite and clays. Both quartz–sulphide and carbonate vein mineralisation is associated with brecciation, indicating rapid release of fluid overpressure and hydraulic fracturing.Two distinct mineralising fluids were identified from fluid inclusion and stable isotope studies. High temperature (>350°C) quartz‐precipitating fluids were moderately saline (4.0–12.7 wt.% NaCl equivalent) with low (approximately 0.05). Quartz δ18O (+11.7 to +16.5‰) and sulphide δ34S (−13.6 to −1.1‰) indicate isotopic equilibrium with host metasediments (rock buffering) and a local metasedimentary source of sulphur. Later, low‐temperature (TH = 120–200°C) flui...
Geochimica et Cosmochimica Acta, 2000
The evolution, paragenetic context and origin of remarkably low δD hydrothermal fluids from the H... more The evolution, paragenetic context and origin of remarkably low δD hydrothermal fluids from the Hercynian Panasqueira W-Sn-Cu(Ag) deposit have been investigated through a combined microthermometric, stable isotope, halogen and noble gas fluid inclusion study. Large variations in δD between −60 ‰ and −134‰ have been observed in primary fluid inclusions from growth zones in a quartz crystal (Pa66) paragenetically constrained
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Papers by Anthony Fallick