Sedimentary exhalative deposits
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Sedimentary exhalative deposits (SedEx deposits) are ore deposits which are interpreted to have been formed by release of ore-bearing hydrothermal fluids into a water reservoir (usually the ocean), resulting in the precipitation of stratiform ore.
The palaeoenvironmental and palaeogeologic setting of these ore deposits sets them apart from other lead, zinc or tungsten deposits which generally do not share the same source or trap morphologies as SedEx deposits.
SedEx deposits are distinctive in that it can be shown that the ore minerals were deposited in a marine second-order basin environment, related to discharge of metal-bearing brines into the seawater. This is distinct from other Pb-Zn-Ag and other deposits which are more intimately associated with intrusive or metamorphic processes or which are trapped within a rock matrix and are not exhalative.
The process of ore genesis of SedEx mineralization is varied, depending on the type of ore which is deposited by sedimentary exhalative processes.
- Source of metals is sedimentary strata which carry metal ions trapped within clay and phyllosilicate minerals and electrochemically adsorbed to their surfaces. During diagenesis, the sedimentary pile dehydrates in response to heat and pressure, liberating a highly saline formational brine, which carries the metal ions within the solution.
Alternately, SedEx deposits may be sourced from magmatic fluids from subseafloor magma chambers and hydrothermal fluids generated by the heat of a magma chamber intruding into saturated sediments. This scenario is relevant to mid-ocean ridge environments and volcanic island arcs where black smokers are formed by discharging hydrothermal fluids.
- Transport of these brines follows stratigraphic reservoir pathways toward faults, which isolate the buried stratigraphy into recognisable sedimentary basins. The brines percolate up the basin bounding faults and are released into the overlying oceanic water.
- Trap sites are lower or depressed areas of the ocean topography where the heavy, hot brines flow and mix with cooler sea water, causing the dissolved metal and sulfur in the brine to precipitate from solution as a solid metal sulfide ore, deposited as layers of sulfide sediment.
Upon mixing of the ore fluids with the seawater, dispersed across the seafloor, the ore constituents and gangue are precipitated onto the seafloor to form an orebody and mineralization halo which are congruent with the underlying stratigraphy and are generally fine grained, finely laminated and can be recognized as chemically deposited from solution.
Arkose-hosted SedEx deposits are known in some cases, associated with arkosic strata adjacent to faults which feed heavy brines into the porous sands, filling the matrix with sulfides, or deposited within a predominantly arkosic layer as a distinct chemical sediment layer usually associated with a shale interbed or at the lowermost levels of a shale formation directly overlying arkosic sands (for example, copper deposits near Maun, Botswana).
Occasionally, mineralization is developed in faults and feeder conduits which fed the mineralizing system. For instance, the Sullivan orebody in south-eastern British Columbia was developed within an interformational diatreme, caused by overpressuring of a lower sedimentary unit and eruption of the fluids through another unit en route to the seafloor.
Within disturbed and tectonized sequences, SedEx mineralization behaves similarly to other massive sulfide deposits, being a low-competence low shear strength layer within more rigid silicate sedimentary rocks. As such, boudinage structures, dikes of sulfides, vein sulfides and hydrothermally remobilized and enriched portions or peripheries of SedEx deposits are individually known from amongst the various examples worldwide.
SedEx mineralization is best known in lead-zinc ore deposit classification schemes as the vast majority of the largest and most important deposits of this type are formed by sedimentary-exhalative processes.
However, other forms of SedEx mineralization are known;
- The supergiant deposits of the Zambian Copperbelt are considered to be SedEx-style copper mineralization formed at arkose-shale interfaces within sedimentary sequences. Within the Botswanan extent of the Damaran Supergroup, the SedEx nature is confirmed by chemical sediment limestones.
- The vast majority of the world's barite deposits are considered to have been formed by SedEx mineralization processes
- The scheelite (tungsten) deposits of the Erzgebirge in the Czech Republic are considered to be formed by SedEx processes
- Some gold associated with Carlin-type deposits of Nevada is interpreted to be stratiform chert or spillite formed by SedEx processes on the seafloor. This concept is controversial because most gold is clearly of later epigenetic origin.
The source of metals and mineralizing solutions for SedEx deposits is deep formational brines in contact with sedimentary rocks.
Metals such as lead and copper and zinc are found in a trace amount in all sediments. These metals are bound weakly to the hydrous clay minerals on the edges of the crystals and are held by weak bonds with hydroxyl groups. Zinc is found within carbonate minerals bound within the carbonate crystal lattice at vertices and along crystal twin planes and crystal boundaries. These metals enter the sedimentary minerals due to adsorption from the seawater which deposited them; few freshwater sediments are considered to have as much metal carrying capacity as saline waters.
Salt is also bound within the matrix of the sediments, generally in pore waters, trapped during deposition. In a typical mud on the seafloor up to 90% of the sediment volume and mass is represented by hydrogen and oxygen either trapped in pore space as water or attached to phyllite minerals (clays) as hydroxyl bonds.
During diagenesis, pore water is squeezed out of the sediments and, as burial continues and heat increases, water is liberated from clay minerals as the peripheral hydroxyl bonds are broken. As the rock enters the submetamorphic field, generally Zeolite facies metamorphism, clay minerals begin to recrystallize into low-temperature metamorphic phyllite minerals such as chlorite, prehnite, pumpellyite, glauconite and so forth. This liberates not only water but incompatible elements attached to the mineral and trapped within crystal lattices.
Metals liberated from clay and carbonate minerals as they are changed from clays and low-pressure disordered carbonate forms enters the remaining pore fluid which by this time has become concentrated into what is known as a deep formation brine. The solution of metal, salts and water produced by diagenesis is produced at temperatures between 150 - 350°C. Hydrothermal fluid compositions are estimated to have a salinity of up to 35% NaCl with metal concentrations of 5-15 ppm Zn, Cu, Pb and up to 100ppm Ba and Fe. High metal concentrations are able to be carried in solution because of the high salinity. Generally these formational brines also carry considerable sulfur.
The mineralizing fluids are conducted upwards within sedimentary units toward basin-bounding faults. The fluids move upwards due to thermal ascent and pressure of the underlying reservoir. Faults which host the hydrothermal flow can show evidence of this flow due to development of massive sulfide veins, hydrothermal breccias, quartz and carbonate veining and pervasive ankerite-siderite-chlorite-sericite alteration.
Fluids eventually discharge onto the seafloor, forming areally extensive, stratiform deposits of chemical precipitates. Discharge zones can be breccia diatremes, or simple fumarole conduits. Black smoker chimneys are also common, as are seepage mounds of chert, jaspilite and sulfides.
Problems of classification
One of the major problems in classifying SedEx deposits is in identifying whether or not the ore was definitively exhaled into the ocean and whether the source was formational brines from sedimentary basins.
In the majority of cases the overprint of metamorphism and faulting, generally thrust faulting, deforms and disturbs the sediments and obscures sedimentary features, although this is generally patchy so that the original configuration will be seen within the deposit.
Most deposits fit the model of having been formed late in the basin history and in most cases feeder systems and metal zonation support exhalative models. However, in the case of diatreme related deposits, such as the giant low-grade Abra deposit, the mineralization is intra-formational, lacks sedimentary textures (is epigenetic and replacement type) and is too low in the basin profile (i.e. in the basal formation).
Following the discovery of hydrothermal vents, deposits similar to those of oceanic vents and fossilized vent life forms have been found in some SedEx deposits, which leads to a potential degree of overlap between Sedex and Volcanogenic massive sulfide ore deposits.
Specific examples of deposits
Sullivan Pb-Zn mine
The Sullivan Pb-Zn Mine in British Columbia was worked for 105 years and produced 16,000,000 tonnes of lead and zinc, as well as 9,000 tonnes of silver. It was Canada's longest lived continuous mining operation and produced metals worth over $20 billion in terms of 2005 metal prices. Grading was in excess of 5% Pb and 6% Zn.
The ore genesis of the Sullivan ore body is summarized by the following process:
- Sediments were deposited in an extensional second-order sedimentary basin during extension
- Earlier, deeply buried sediments devolved fluids into a deep reservoir of sandy siltstones and sandstones
- Intrusion of dolerite sills into the sedimentary basin raised the geothermal gradient locally
- Raised temperatures prompted overpressuring of the lower sedimentary reservoir which breached overlying sediments, forming a breccia diatreme
- Mineralizing fluid flowed upwards through the concave feeder zone of the breccia diatreme, discharging onto the seafloor
- Ore fluids debouched onto the seafloor and pooled in a second-order sub-basin's depocentre, precipitating a stratiform massive sulfide layer from 3 to 8 m thick, with exhalative chert, manganese and barite.