Alderley edge minerals
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The area around Alderley Edge in Cheshire is a classic geological site in the UK. The mineral assemblage there is unique and has since the early 1800's attracted the attention of geologists from around the world.
This is Part 2 of a two part article
see also Alderley Edge Geology
Contents |
[edit] MINERALIZATION
The mineralization of Alderley Edge has been a matter of speculation for centuries. We do not know what ideas the Bronze Age or Roman miners had about the deposition and disposition of minerals but it is fair to assume that they would recognise the link between minerals and faults, even if they had no idea what a fault was. Certainly the mediaeval miners would have recognised that there were veins at Alderley, resembling the veins in Derbyshire. In the nineteenth century, geology was developing as a science and geologists were drawing up ideas about the deposition at Alderley. Bakewell writing at the beginning of the 19C described Alderley as
“an object of more interest to the mineralogist that the picturesque tourist. In the space of a few acres he may be presented with ores of most of the metals found in England, but placed in such situations, and presenting such appearances as are rarely met elsewhere”.
Early theories, based on the malachite (copper carbonate [CuCo3]) deposits being worked at the time, said that the ores had been deposited "syngenetically", in other words at the same time as the sandstone. This theory was based on the way in which ore was found and extracted in bed. However, by the twentieth century, most geologists accepted that the ore was epigenetic that is introduced after the sandstone was deposited. New theories and research have shown that there have been several stages in the mineralization and that a complex sequence of both epigenetic {change in the mineral content of a rock because of outside influences} and syngenetic {at the same time as the sediments were laid down} deposition coupled with complex chemical reactions is much closer to the truth as we understand it now.
The fluvial and aeolian sediments of the Wilmslow formation are host to a number of mineral deposits and are of the type termed U-V-Cu sandstone deposits, referred to in short as red bed deposits. The mineralization at Alderley comprises copper (Cu) as carbonates and lead (Pb) as sulphides together with a number of hydrated and oxygenated species with minor amounts of zinc (Zn), silver (Ag), cobalt (Co), vanadium (V), nickel (Ni), arsenic (As), cadmium (Cd), antimony (Sb), and manganese (Mn). Mineralization is concentrated along the basin margin faults and as cements in the sandstone down dip from the faults. In the northeast of the Cheshire Basin, where Alderley lies, lead and zinc sulphides are found in and adjacent to fault zones whereas, the copper minerals are more widespread. The Wilmslow Sandstone Formation is cemented by authigenic {Pertaining to minerals or materials that grow in place with a rock, rather than having been transported and deposited} quartz overgrowths enclosing iron sulphides, such as tennantite [(Cu,Fe)12As4S13], bornite [Cu5FeS4] and sulpharsenides which predate the main sulphide cements..
The origin of the mineralization at Alderley has received extensive discussion in literature. Early workers such as Hull (1860's) and later Dewey & Eastwood (1925) favoured the theory that detrital lead and copper sulphides were deposited contemporaneously with the Sandstone. Dewey & Eastwood suggested that detrital grains from an easterly or southerly source were incorporated in the sediments during deposition and were subsequently altered to carbonates and oxides by the action of meteoric water. Subsequently authors such as Taylor (1963) and Warrington (1965) suggested that the minerals were deposited from fluids introduced long after the deposition of the sandstone. Warrington summarizes the evidence for such an epigenetic model where metals were introduced by late hydrothermal fluids moving up the faults and proposed that the fluids may have originated from concealed acid igneous sandstone at depth during the Jurassic Period. A model proposed by King (1968) stressed the importance of precipitation from downward percolating metal bearing groundwater. A diagenetic {the process of chemical & physical change in deposited sediment during its conversion to rock} origin for the mineralising fluids was discussed by Holmes et al (1983) who proposed that the diagenesis of the red bed within the basin was responsible for the release of trace metals from detrital minerals into solution. The mineralising fluids were thought to be oxidising basinal brines of moderate salinity.
The ores are mainly developed in the topmost 7m of the Upper Mottled Sandstone and at 3 levels in the Conglomerates. The disseminations are richest immediately above the clay bands in the sequence. In this position at the base of the arenaceous units, the concentrations follow the general dip of the beds. Higher in the units they tend to follow structures in the host rocks such as current bedding. Malachite [Cu2(CO3)(OH)2] is concentrated around clay pellets in conglomeratic portions of the Conglomerate series and is also concentrated along some fault zones and in joint fissures. Azurite [Cu3(CO3)2(OH)2] and chrysocolla [(Cu,Al)2H2Si2O5(OH)4·n(H2O)] occur in lesser amounts and have a more restricted occurrence. Copper sulphides are rare but occur in fault breccia at Engine Vein and Stormy Point. At Engine Vein Cu and Fe sulphides occur sporadically as small flecks in masses of galena [PbS] which is in the fault breccia. A peculiar occurrence of azurite is seen at two locations Engine Vein and at the openworks 300m to the north. It occurs as small spheres scattered through grey clay. This occurrence appears to be restricted to the immediate neighbourhood of faults and to a grey clay lithology. The grey clay in which the spheres are found passes into a red clay at a short distance from the fault at Engine Vein. Lead ores galena and cerrusite [PbCO3] both occur in masses in the fault breccia zones in Wood and at Engine Vein and Stormy Point. The ores appear to be restricted almost entirely to the Conglomerate series. pyromorphite [Pb5(PO4)3Cl] is moderately common associated with galena and cerrussite in fault breccias. The lead ores are always associated with barites [BaSO4] which form crystalline masses in the fault breccias and veins, impregnations and crystal rosettes and the adjacent country rocks. Manganese, cobalt and nickel occur exclusively in the form of Wad, a complex secondary mineral consisting of oxides and hydroxy-oxides of the metals. Asbolite [(Co,Ni)(MnO2)2(OH)2(H2O)] is a cobalt rich variety of Wad and forms large segregations, vein like masses and spots in the host rocks. The rocks in which it occurs are typically free from iron staining.
Other minerals have been found as rare or trace minerals mainly at Engine Vein, these include, Anglesite [PbSO4], Chalcocite [Cu2S], Pisanite (or Melanterite) [FeSO4·7(H2O)], Enargite [Cu3AsS4], Gypsum [CaSO4·2(H2O)], Minium [Pb,2PbO4], Massicot [PbS}, sparry iron ore [FeO}, Wulfenite [PbMoO4] , Brochantite [Cu4(SO4)(OH)6], Langite [Cu4(SO4)(OH)6·2(H2O)], Native Sulphur [S], and Plancheite [Cu8Si8O22(OH)4·(H2O)].
The bulk of the minerals are secondary, and the small amounts of sulphides found are probably the remnants of primary ores. Many of the rarer minerals assuming correct identification are those which characterize the zone of secondary enrichment of copper sulphide lodes. Anglesite, linarite [PbCu(SO4)(OH)2] and vanadinite [Pb5(VO4)3Cl] are minerals which are typical of the oxidised zones of lead ore bodies.
The galena which is probably a primary ore shows a cobalt to nickel ratio of 50:1, and amongst the secondary minerals that have been analysed, asbolite also has a high Co:Ni ratio. In malachite, the nickel predominates over cobalt. This high Co:Ni ratio in the galena appears to indicate that the minerals are syngenetic, but other minerals appear to show that there has been migration of mineral bearing fluids through the sediments after deposition. In the case of the copper ores, only the carbonates are seen accentuating the sedimentary structures in the manner shown, however the situation where cobalt predominates over nickel has never been recorded in sedimentary ore deposits. It is concluded that the high ratio of cobalt to nickel in galena at Alderley indicates an epigenetic origin. The reverse situation in malachite is probably attributable to the secondary nature of the malachite in which the ratios of the minor elements need not reflect in any way their initial relations in the primary ore. Current bedding in the ore bearing horizons indicates that the sediments were derived from the south and south east. Derivation from this direction suggests that the Derbyshire lead field where the lead is important but insignificant as the source of the copper ore minerals. Lead age determinations by Moorbath suggest that the Derbyshire mineralization did not occur until the period between the Mid Trias and The Upper Jurassic. The deposit is therefore unlikely to have been in existence to act as the primary source during the early Triassic.
Work by Ixer & Vaughan concluded that the ore minerals at Alderley Edge show that there were several phases of mineralization and some of the mineralization was syngenetic or at least contemporary. The detrital and authigenic oxide component of the opaque mineral association is dominated by anatase [TiO2] minerals and shows a distinct depletion in detrital hematite compared to normal red bed sediments. This is however similar to the associations observed in other bleached red sandstones. The early authigenic assemblage comprises not only titanium dioxide minerals but also minor sulphides containing iron, nickel and copper. It is very minor in terms of the amounts of metals involved but may be of genetic interest. This phase of mineralization was succeeded by the main sulphide mineralization in which the consistent paragenetic sequence of iron-nickel-cobalt sulphides and sulpharsenides followed the copper-iron then zinc and finally lead sulphides are observed. Associated with this major phase of mineralization was the introduction of minor amounts of silver, cadmium and antimony.
Sulphides of the main phase were later altered and replaced by copper sulphides which are so abundant as to suggest that additional copper may have been introduced at this stage. Processes of oxidation and weathering of the sulphide ores accompanied by episodes of shattering, have produced an extensive suite of carbonates, sulphates and associated minerals. Analysis of the major ore minerals has shown pyrite to contain significant nickel and to be remarkably high in copper content.
The sphalerite [(Zn,Fe)S] contains some copper but is very poor in iron and significantly enriched in Cadmium (Cd). Altered varieties of sphalerite show further enrichment in copper and cadmium but the galena analysis is notable for the absence of elements other than lead and sulphur (sphalerite) in particular no enrichment in silver has been observed. Silver is found in substantial amounts in tetahedrite although this is a rare mineral at Alderley, and in trace amounts in sphalerite and in more significant minor concentrations in the chalcopyrite [CuFeS2]. However much of the silver has been concentrated in the secondary copper sulphide, djurleite [Cu31S16] which may hold up to 2.35 wt % silver. In some cases the alteration of chalcopyrite to djurleite takes place via an intermediate altered chalcopyrite phase which has optical properties similar to those reported for idaite [Cu5FeS6] and nukundamite [(Cu,Fe)4S4] but which is too small to characterize further other than to provide analyses which show copper enrichment and iron depletion. Using the relatively simple primary ore mineral assemblages observed at Alderley it is difficult to deduce much regarding the physio-chemical conditions of mineralization.
Liquid and vapour filled inclusions in the late calcite cements at Alderley were used for microthermometric study. It was accepted that inclusions in calcite are prone to leakage and post trapping alteration however the results from the studies are encouraging as they are both consistent and geologically reasonable. Fluid inclusions in the calcite range from 2-20µm in diameter with a mean of 13µm.. The mean depression of freezing point for these inclusions was found to be -13.3°C which corresponds to a salinity of approx 17wt% NaCl equivalent. Homogenization temps obtained for these inclusions range from 56.6°C to 79.9°C with a mean of 61.3°C. The few clearly primary inclusions that were isolated yielded similar results to those controlled by cleavage direction which were indisputably of secondary origin. These data indicate that the late aqueous fluids with salinities of 9–22 wt% equivalent NaCl passing up the fault and through the porous beds of the Wilmslow Sandstone is consistent with mineralizing fluids which were warm and saline.
Whilst the fluid inclusion studies show temperatures in the range 56.6°C to 79.9°C further consideration of the basin history suggests that fluid temperatures in the Permo-Triassic sequence may have reached 150°C at the time of mineralization. However, the exact temperature of the mineralisation remains poorly constrained as a result of the fine grained nature of the gangue phases. This new data significantly constrains the genetic hypothesis for the deposits, refuting a magmatic origin. A basin brine expulsion model is favoured.
Sulphur Isotope [δ34S] ratios obtained for barite from Alderly Edge have a narrow range (+13.8‰ to +17.6‰) with a mean value of 16‰. The values for barite are generally heavier than the values for galena and chalcopyrite The sulphur isotope data from the barite from different stratigraphic horizons, from coarsely crystalline barite on fault planes and from barite cements within the sandstone were not significantly different (mean δ34S=+17‰) suggesting that the bulk of the barite sulphur was derived directly from the Upper Triassic evaporites (mean δ34S =+19‰), whilst the distribution of δ34S values for sulphides (-1.8‰ to +16.2‰) is consistent with ultimate derivation of sulphur from the evaporites by closed system reduction of a sulphate bearing brine... A discrepancy in the δ values from Alderley to other areas may be due to different transport and depositional mechanisms operating in the NE part of the basin. The δ values for galena at Alderley range from (1.3‰ to +15.7‰) with a mean of 8.2‰.
Ixer and Vaughan found that there was no significant stratigraphical or spatial control on the δ values for galena but there is a significant difference in the isotopic values obtained for different textural varieties of galena at Engine Vein. The fine intergrowths of chalcopyrite and galena at Alderley prevented their separation for isotopic analysis hence it was not possible to establish whether these co-existing sulphides have achieved isotopic equilibrium. This is considered unlikely as isotopic equilibrium between sulphides is not a common feature of low temperature red bed deposits. The isotopic disequilibrium exhibited between the sulphides and barite in these deposits is also common in low temp ore forming bodies where the temperature may not have exceeded 200°C
Researchers have put forward other theories for the reduction processes and say that it seems likely that the reducing agent was organic material (methane) derived from the black shales of the underlying Coal Measures, but it is uncertain at present whether the principal mechanism was thermochemical or bacterial reduction. The pattern of sulphide δ34S favours the former process. The potential use of methane by sulphate reducing bacteria was investigated by Davis (1967) and bacteria and mineral cycling processes by Fenchel and Blackburn (1979). Others have looked at co-metabolism involving Pseudomonas or methanobacteria that produce the organic donor for sulphate reduction and further work may use carbon isotopes for origin determination (Berner).
The proposed mineralisation temperatures of 56-150°C for the basin are considerably higher than those generally accepted for the activities of sulphate reducing bacteria which typically operate at 45-50°C. Reports of species with active temperatures up to 80°C are known but no continental species has been found that reduces above these temperatures. In addition there is no evidence of a nutrient supply (in the form of organic matter) for the bacteria to metabolise. Trapped methane such as may have accumulated in the Basin has not yet been proven to be a significant nutrient supply for sulphate reducing bacteria. It is accepted that whilst bacterial reduction may have played a part the physiological constraints make it a not very significant mechanism of sulphide precipitation in the Cheshire Basin.
Other possible sulphate reduction mechanisms include chemical reduction by inorganic material such as Fe2+, bacteriogenic reduction or chemical reduction by organic matter. Inorganic reduction of sulphate by iron minerals fixed in the sediment has been shown to be a important mechanism at temperatures above 250-300°C circulating fluids in the Cheshire Basin are unlikely to have attained these temperatures as burial in the Permo-Triassic reached a maximum of 4.5km before the Tertiary. Even allowing for high geothermal gradients associated with tectonic activity fluids circulating at a depth of 4.5km may only have reached 130-150°C. It is also unlikely that the clean fluvial and Aeolian sands hosting the minerals ever contained sufficient ferrous iron minerals to make this type of reduction significant. Bacterial sulphate reduction is thought to be a major mechanism whereby sulphide is produced in a number of sediment hosted ore deposits.
Temperature Isotope fractionation factors are very sensitive to temperature this property of stable isotopes can be used as a thermometer in geologic studies. In general, the isotope fractionation factor for a particular reaction decreases with increasing temperature. The magnitude of this isotope fractionation factor is temperature dependent, and for this particular reaction it varies from about 1.002 at 500°C to about 1.010 at 25°C. In other words, the S34/S32 ratio of the sphalerite is higher than that of the galena. We are frequently interested in the isotope fractionation factor between two species. We can write an expression for the equilibrium constant (i.e., the isotope fractionation factor) between the two species; and express the stable isotope ratios in terms of their delta values (often abbreviated to "del-values"). This del-value has the units of per mil, ‰. The relationship between del-values and the isotope fractionation factor is can then be used to determine the temperature at which the mineral composition was formed. Source of the minerals
Possible sources for the Cheshire basin deposits that have been proposed include detrital sulphide minerals (Dewey & Eastwood 1925) or hydrothermal fluids of magmatic origin (Warrington 1965), groundwater model (King 1968) and diagenetic (Holmes at al 1983). Thompson suggests that they may have originated in the black shales of the underlying coal measures.
Objections to any theory involving contemporary or syngenetic deposition of detrital grains with the host sandstone include the fact that the composition of the mineral assemblage is also not what might be expected if the minerals were derived from pre-existing mineral veins by weathering and erosion. While the copper ores are readily converted into the carbonate state in which they are easily transportable, the alteration of galena is hampered by the formation of cerrusite as a surface coating. Therefore in a deposit derived from lead and copper lodes by weathering, only negligible amounts of lead ore would be expected, contrary to the situation at Alderley. Other objections put forward include
i) The close association of sulphide ores with faults
ii) The distribution of carbonates only in the faulted areas
iii) The restriction of mineralization to arenaceous rocks except near faults
iv) The richer occurrences down dip from certain faults
v) The lack of concentrations in old stream channels
vi) The irregular occurrence of the ores both stratagraphically and geographically
vii) The high Co:Ni ratio found in the sulphide ore
viii) The absence of suitable source material in the areas to the south and east of the basin
ix) the distal sedimentological setting of the mineralization would make transport and continued survival of such material unlikely
x) there is no evidence of mineralised calcite or detrital sulphides
A source involving hydrothermal fluids from magmatic sources where the presence of a deep body cannot be totally discounted but appears unlikely in the light of recent geophysical data (Gale 1984) sulphur isotope studies and fluid inclusion studies. Geophysical studies in the basin indicate that there is no igneous mass at depth and widespread hydrothermal alteration of a type often associated with the passage of hydrothermal fluids has not been recorded in Permo - Triassic rocks.
Mineral precipitation of ores from downward percolating ground water (King 1968) is not supported as field surveys show where primary sulphides are seen to concentrate beneath as well as along the upper surfaces of impermeable mudstones horizons, it appears that ascending not descending fluids were dominant during deposition.
A possible sedimentary source of metals is from the dissolution of detrital grains introduced into the basin notably potassium feldspar and ferromagnesian minerals which it has been argued can produce substantial amounts of metals (Holmes et al 1983). In his study of red bed diagenesis (Walker 1976) has shown that during burial ferromagnesian silicates, feldspar and detrital oxides are often completely dissolved releasing both major and minor elements into solution. Studies of trace metal concentrations in detrital phases have shown that copper contents in pyroxenes may reach 1 000ppm and potassium feldspar (K spar) may contain up to 11 000ppm lead (Weddepohl 1978). Diagenetic studies of the Sherwood Sandstone Group of the Cheshire Basin by Thompson 1983 revealed evidence of substantial alteration and dissolution of K spar and ferromagnesian minerals similar to that described by Walker (1976). Thus the potential exists to derive metals from detrital minerals during diagenesis via dissolution processes.
The black organic rich shales of the Carboniferous constitute an alternative source of base metals. Black shales are known to be enriched in metals including Pb, Zn, Cu and Ag and have been invoked as the source of metals for the carbonate hosted "Mississippi Valley-type" ore deposits by numerous authors since Jackson & Beales (1967). Possible sulphur sources include the magmatic/hydrothermal source proposed by several authors including Warrington and the detrital source proposed by earlier authors such as Dewey and Eastwood. The later hypothesis has been shown to be invalid from the evidence outlined above and the sulphur isotope data from the present studies which does not support a magmatic origin for the sulphur.
Ore deposits known to have a clear magmatic source typically exhibit a narrow range of sulphur isotope values approx 0±4‰. The sulphur isotope ratios for the Cheshire basin range from -1.8‰ to +16.2‰ and support the geological evidence that the sulphur was not derived from an ultimately magmatic source. Potential sedimentary sulphur sources are found in the form of Carboniferous shales and the evaporites within the Permo Triassic basin fill. Sulphur isotope data from the present study support the hypothesis that barite was derived directly from solutions passing though or derived from the gypsum and anhydrite horizons within the Mercia Mudstones. In addition the absence of inclusions of anhydrite and other evaporites in the barite crystals suggests that direct replacement of evaporite minerals in the mineralised horizons was not a widespread phenomenon. Partial dissolution of gypsum and anhydrite with δ34S of +19‰ would produce a fluid with a δ34S value of c+17.5‰. Thus a fluid responsible for evaporite dissolution would precipitate barite with a sulphur isotopic value of 17.5‰ because barite incorporates sulphur without significant isotope fractionation. In addition preliminary oxygen isotope data were recently obtained from Alderley Edge and gave δ18O values of 17.1‰. These data also correspond closely to values obtained for δ18O of Triassic seawater sulphate (Claypool et al 1980) and are constant with the hypothesis that barite was derived from the Triassic evaporites of the Cheshire Basin.
Almost without exception red-bed copper deposits are hosted by clastic successions that contain evaporites and this has lead to suggestions by several authors that a genetic link exists between evaporites, chloride brines and copper mineralisation (Rose 1976). The sulphur (and oxygen) data emphasize the importance of the evaporites as a source for the barite mineralisation and the range of δ34S values for the sulphides and their distribution relative to the barite values suggests that the evaporites were also the ultimate source of sulphur for the sulphides. A recent study illustrates that even if the brine were initially reducing, interaction with a thick red bed evaporite sequence would result in the evolution of the fluid into a more oxidising state envisaged as being capable of transporting metals such as copper, lead and zinc as chloride complexes. An indication of the importance of the Carboniferous source may be the contribution of isotopically light sulphate to the barite at Alderley
Conclusions
The data outlined above suggests that the metals and sulphur were derived from a sedimentary source and that in common with other red beds the ores were precipitated after deposition of the host rocks by subsurface fluids in equilibrium with the mineral assemblage in the red beds. Despite the fact that copper solubility is low on oxidizing ground waters of the type that would be in equilibrium with the assemblage in red beds, Rose has shown that at temperatures as low as 75ºC, chloride solutions containing more that 0.01m Chlorine [Cl] (350ppm) can be effective solvents for copper due to the formation of copper chloride [CuCl] and copper chloride complexes. These complexes allow solubility’s of 100ppm in 0.5m Chloride solutions at intermediate Eh in the stability field of hematite at neutral Ph. In Basin Cheshire the evaporites of the Mercia Mudstones could have provided the high Chloride fluids for the transportation of copper and possibly of other trace metals. Alternatively the fluids may have originated in the Carboniferous strata where they may have been initially reducing and have evolved into a more oxidised state after migration through a red bed sequence. Given that the dominant source of dissolved sulphur in these fluids is likely to be leached diagenetic sulphide fixed within carbonate sediments, the evolved sulphate would be isotopically lighter than sulphur derived from Triassic sediments. The chemistry of the reduction mechanisms becomes complex and is outlined below.
[edit] THE CURRENT BELIEF
In 1989, Dr Helen Naylor published the the present idea that the deposits are not hydrothermal (i.e. from a magmatic source) as the chemistry does not match that from other hydrothermal sources. The work by Naylor looked at sulphur isotopes and inclusions in the rock and deduced that the mineral had been deposited from cool solutions. The process by which the ores reached Alderley as malachite and azurite dispersions is considered in four stages:
(a) Origin, (b) Transport, (c) Precipitation and d) Redeposition.
Origin: The source of the copper, lead, zinc, barium and iron found at Alderley Edge is believed to be the black shales of the Carboniferous rocks underlying the Cheshire Basin. These shales have been proved just to the north of Alderley in Poynton where due to faulting the Carboniferous coal deposits were thrown upwards and enabled them to be worked commercially at shallow depths just below the Triassic sandstones.
Transport: Fluids from the Triassic beds are responsible for dissolving the metal ions from the shales and transporting them through the Cheshire Basin. Such fluids would contain the chloride and sulphate ions capable of making lead, copper, barium etc. soluble. The chlorine and sulphur source is the evaporite (salt and gypsum) beds in Cheshire.
Precipitation: In order for the mineral veins to form at Alderley, the solutions need firstly to be trapped and then to form insoluble sulphide compounds. The entrapment would arise from the presence of faults and clay bands in the same manner as an oil-field or gas-field trap. In the absence of organic matter, and by simple mixing and cooling, the barium sulphate would precipitate as barite. It is believed that this event occurred separately from the other metal deposition explaining why barite veins are far more common. With organic material present, the sulphate ions in the transport solutions could be reduced to sulphide leaving minerals such as galena, bornite and chalcopyrite. Where did the organic materials come from? The most likely source is methane [CH4] from the subjacent coal measures; this is borne out by the discovery of methane in similar deposits elsewhere in the world. The product of the reduction of the sulphates would be oxidation of the methane into weak organic acids which would be capable of oxidising adjacent iron rich beds, hence the decolouration of the sandstone in the proximity of veins.
Redeposition: The fourth stage to the process is the redeposition of copper and (to a smaller extent) lead as carbonate ores in the sandstone adjacent to the faults. This process would arise from invasion of the faults by weak carbonic acid from the land surface; carbonic acid is formed from rainfall onto organic matter on the surface. The location of these deposits is controlled by the porosity of the sandstone and the barriers formed by clay bands.
[edit] REDUCTION CHEMISTRY
Possible sulphate reduction mechanisms include chemical reduction by inorganic material such as Fe2+, bacteriogenic reduction or chemical reduction by organic matter. Inorganic reduction of sulphate by iron minerals fixed in the sediment has been shown to be a important mechanism at temperatures above 250-300°C circulating fluids in the Cheshire Basin are unlikely to have attained these temperatures as burial in the Permo – Trias reached a maximum of 4.5km before the Tertiary. Even allowing for high geothermal gradients associated with tectonic activity fluids circulating at a depth of 4.5km may only have reached 130-150°C. It is also unlikely that the clean fluvial aeolian sands hosting the minerals ever contained sufficient ferrous iron minerals to make this type of reduction significant. Bacterial sulphate reduction is thought to be a major mechanism whereby sulphide is produced in a number of sediment hosted ore deposits. Bacteriogenic sulphate reduction may have been significant in the genesis of the Cheshire deposits. The proposed mineralization temperatures of 70-150°C for the basin are considerably higher than those generally accepted for the activities of sulphate reducing bacteria which typically operate at 45-50°C. Reports of species with active temperatures up to 80°C are known but no species has been found that reduces above these temperatures. In addition there is no evidence of a nutrient supply (in the form of organic matter) for the bacteria to metabolise. Trapped methane such as may have accumulated in the Basin has not yet been proven to be a significant nutrient supply for sulphate reducing bacteria. It is accepted that whilst bacterial reduction may have played a part the physiological constraints make it a not very significant mechanism of sulphide precipitation in the Cheshire Basin. Thermo-chemical reduction of sulphate by hydrocarbons has been suggested to be significant for H2S formation and the precipitation of metals (Pine Point, Canada) also gaseous hydrocarbons may have been important in the genesis of the basin deposits. Against this theory is the fact that the host rocks are aeolian and fluvial deposited in a continental setting in which there is no evidence of organic debris. However potential methane source rocks in the Lower and Upper Carboniferous are found below the Basin. The geological setting and the isotope data favour two possible closed system sulphate reduction mechanisms where the sulphate was derived from the Triassic sequence evaporites. the first involves thermo-chemical sulphate reduction of a chloride and sulphate bearing aqueous fluid carrying metals via oxidation of trapped hydrocarbons beneath the impermeable Tarporley Siltstone Formation, alternatively bacteriogenic sulphate reduction may have been significant despite the limiting factors of relatively high temperatures for sulphide deposition and the lack of organic matter at the site of ore deposition. In view of the uncertainty surrounding the temperature neither mechanism can be proven. However the pattern of δ34S sulphide from the basin does not favour the bacterial reduction model.
It is suggested that the sulphide ores were also ultimately derived from the Triassic evaporites as a result of a closed system reduction of sulphate bearing fluids. The limited fluid inclusion data from calcite gangue suggests minimum temperatures for ore deposition in the range 65-80°C. Mineralising fluids are considered to have been saline (17wt% NaCl equivalent), oxidising brines with a near neutral pH. Taking into account the maximum burial temperature fluids derived from the Permo - Trias sediments or originating in the Carboniferous may have reached temperatures of 150°C or higher. The exact temperature of the mineralization remains poorly constrained as a result of the fine grained nature of the gangue phases. Metals such as Cu, Pb and Zn are envisaged as having been transported as chloride complexes. A recent study illustrates that even if the brine were initially reducing, interaction with a thick red bed evaporite sequence would result in the evolution of the fluid into a more oxidising state capable of transporting Cu, Pb and Zn and sulphate. An indication of the importance of this Carboniferous source may be the contribution of isotopically light sulphate to the barite at Alderley.
The earliest phases are associated with the diagenesis of the sandstone and include anatase, bravoite [(Fe,Ni,Co)S2], pyrite [PbS] and chalcopyrite within authigenic quartz overgrowths. This is followed by the formation of intergrown bravoite pyrite, chalcopyrite, sphalerite and galena accompanied by small amounts of Ni-Co-Fe sulpharsenides, marcasite and tetrahedrite [(Cu,Fe)12Sb4S13] cementing the clastic grains. This primary assemblage has undergone extensive alteration resulting in the formation of covelline [CuS] accompanied by Pb sulphates, limonite [FeO(OH)] and others.
Later extensive shattering of the cemented grains has been in-filled by supergene Cu, Pb and Zn carbonates, sulphates, limonite and others. Many of the mineralised sediments show an extensive phase of shattering in which the brittle quartz grains have been extensively fractured these have been in-filled with malachite, azurite and other carbonates and sulphates but no sulphides. Limonite occurs in much oxidised specimens and replaces all phases. All secondary minerals have been extensively altered to secondary Cu sulphides or to carbonates and sulphates. The secondary Cu sulphides form volumetrically important cement for these and show a number of important fabrics;
1. Fine intergrowths of two or more sulphides 2. Box-works of acicular sulphides orientated along cleavages 3. The Cu sulphides form void filling textures in the pore spaces including botryoidal and rhythmic crustification textures and rosette and plumose structures.
Malachite and azurite form botryoidal masses of fibrous crystals or euhedral rhombic crystals growing into voids (Tucker 1981), other non sulphides include anglesite, cerrussite and smithsonite ZNCO3]. Smithsonite replaces sphalerite whilst cerrusite replaces galena.
The geochemistry of [[bromine [Br]]], Sr, K and Mg in the salt beds provides definite arguments for very frequent marine intrusions in order to account for the lower Northwich Halite Formation. He even demonstrates that small amounts of Potassium and Manganese salts which only develop when circa 98% of the original seawater has evaporated are present at the top of many of the individual cycles of salt deposition in the Winsford bodies.
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