Deposits related to

submarine volcanism

and sedimentation –

SedEx –

Genetic Models

- GLY 361 -

Lecture 13

Background

• Historic debate:

Replacement Syngenesis

• Concepts:

Syngenetic, Diagenetic, Epigenetic

• Syngenetic: ore formed at the same time as wall rocks.

• Diagenetic: formed during the time when sediments converted to

sedimentary rocks.

• Epigenetic: ore introduced from external sources after rock

formation.

GENERAL

MODEL

GENERAL MODEL

• The simplest model recognizes SEDEX type deposits

form as:

• hydrothermal discharge onto the seafloor

• mostly located at areas of complex transform and

extensional fault interference.

• Sea water ingresses into the fault system (the source of

energy) which dissolves metals from fault rocks into the

hydrothermal fluids which are eventually released onto the

sea floor.

A simple model

= Deep formational brines.

• Metal ions are found in trace amounts in all sediments - weakly

bound to hydrous clay and phyllosilicate minerals.

• Metal ions (in solution in seawater) are adsorpted by sediments; few freshwater sediments are considered to have as much metal carrying capacity as saline waters.

GENERAL MODEL

Source of Mineralizing Fluids

GENERAL MODEL

Source of Mineralizing Fluids

• Diagenesis - sedimentary pile dehydrates in response to

heat and pressure

highly saline formational brine liberated, carries metal ions within

the solution.

• Metals, liberated from clay and carbonate minerals as

they are changed, enter the remaining pore fluid, which by

this time has become concentrated into what is known as

a deep formation brine.

What is a brine?

Water salinity based on dissolved salts (e.g., NaCl) in parts per

thousand (ppt)

Fresh Water Brackish Water Saline Water Brine

< 0.5 0.5 - 35 35 - 50 > 50

Brine is water saturated or nearly saturated

with salt (e.g., NaCl).

GENERAL MODEL

Metal source

The source of metals that form SEDEX-type deposits are

oxidized (H2S poor) fluids. These originate from:

• geopressured, hydrothermal reservoirs

• in syn-rift clastic sediments

– e.g. fault breccias, evaporites

• sealed by fine-grained marine sediments

– e.g. shales/carbonates

GENERAL MODEL

Metal source

GENERAL MODEL

Mineralizing Fluids

• 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 100 ppm 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.

GENERAL MODEL

• As the sea water traverses through the crust - dissolves base metals from

host rock - collection and precipitation near the surface.

• In early stages of convection, leaching of Fe, Mn and Si would occur.

• With deeper circulation, T would increase leading to the leaching of Pb and

Zn.

• Finally, deep seated circulation would leach Pb, Zn and Cu but this is not

always the case which is why some of these deposit types are without

copper.

GENERAL MODEL

~350°C

~250°C

~150°C

Mineralizing Fluid Transport

Conducted upwards (due to thermal ascent and pressure of the underlying reservoir) within sedimentary units toward basin-bounding faults.

Brines percolate up the faults and are released into the overlying oceanic water.

Faults which host the hydrothermal flow can show evidence of this flow due to development of massive sulphide veins, hydrothermal breccias, quartz and carbonate veining and pervasive ankerite-siderite-chlorite-sericite alteration.

GENERAL MODEL

Fluid Trap Sites

Lower/depressed areas of the ocean topography where the heavy,

hot brines flow and mix with cooler sea water

dissolved metal and sulphur in the brine precipitates from

solution as solid metal sulphide ore, deposited as layers of

sulphide sediment.

GENERAL MODEL

However:

• Mechanism for metal precipitation?

– It is well understood that metal precipitation requires a supply

of reduced S.

– Hydrothermal fluids would be depleted in reduced S.

What is the source of reduced S?

The reduced S source

Generally accepted source of reduced S to precipitate hydrothermal metals:

• bacteriogenic H2S – generated in anoxic water columns.

– availability of this will control metal precipitation.

• During a consistent period of hydrothermal activity, metals are not consistently precipitated.

• Fluctations in bacteriogenic activity, and therefore anoxic conditions and reduced S supply, neatly explain periods of precipitation and no precipitation despite consistent hydrothermal activity at the time.

The reduced S source

The reduced S source

anorogenic and continental

basin metal deposits

(Barley Groves, 1992)

The reduced S source

The low number of SEDEX-type deposits outside the Meso-Palaeoproterozoic is also explained by a bacteriogenic source of reduced S for metal precipitation:

• Archean ocean was Fe dominated.

• Proterozoic and Phanerozic trended towards an S dominated ocean.

• Change in ocean chemistry resulted in a build up of sulphate, a source for bacterial sulphate reduction.

• Bacterial sulphate reduction produced anoxic water columns (H2S rich).

• which finally allowed for precipitation of metals from SEDEX-type hydrothermal fluid sources.

REPLACEMENT

THEORY

Replacement Theory - two processes:

1. Chemically reducing, commonly carbonaceous and pyritic, grey sediment is initially enriched in sulphur (iron sulphide and/or gypsum/anhydrite) by primary diagenetic processes.

2. Subsequently, Cu +/- associated metals zonally overprint the S-rich host during a post-sedimentary influx of dissolved base metals from adjacent coarse-grained, highly porous and permeable, continental red bed sediments.

REPLACEMENT THEORY

Replacement Theory

• Cu-source: soluble metal chloride complexes dissolved in oxidized, saline pore fluids of red beds.

• Metal influx from infiltration or diffusion.

• Infiltration requires adequate host permeability – becomes less important as the fine-grained grey beds are compacted and cemented – remaining interconnected pore spaces of the grey beds may still allow for metal diffusion over short distances along chemical potential gradients towards the chemical sink of pyrite-bearing grey beds, when infiltration is restricted.

REPLACEMENT THEORY

Deposition of Metals

• Low temperature chemical reaction between abundant reduced S in the host and base metals from the solution.

• The reduced sediment forms a chemical trap for the metals contained in the solution - distinctive contact between the red beds and the grey beds therefore represents a fossil redoxcline.

• This feature is an obvious exploration target – “metallotect”.

REPLACEMENT THEORY

REPLACEMENT THEORY

SYNGENETIC

THEORY

Syngenetic Theory

• Related to hydrothermal activity in areas of continental

rifting.

• Source of base metals:

– magmatic fluids from subseafloor magma chambers and

– hydrothermal fluids generated by the heat of a magma

chamber intruding into saturated sediments fumaroles

(“black smokers”) in mid-ocean ridge and island arc

environments.

SYNGENETIC THEORY

Syngenetic Theory

• Form on shelf - need chemical difference, i.e. redoxcline, otherwise would find them on entire ocean floor.

• Interplay of biochemical agents (bacteria, algae etc. to form redoxcline – SEDEX not found in Archaean rocks).

• Metals precipitate out of solution with sediments.

SYNGENETIC THEORY

Timing of Mineralization

Difficult to establish actual timing of mineralization esp. in deposits formed prior to lithification as critical evidence is obscured by the transformation of the soft sediment to hard rock.

This is progressively more intense when comparing deposits formed in sandstone to those in clays, carbonates, and sulphates.

These changes may happen over long time periods and stratigraphic intervals.

Effects include: large volume reductions, loss of porosity, loss of pore fluids, recrystallization, and or replacement of unstable initial minerals in chemical systems open to exchange with changing groundwater.

Syngenetic Theory

• Many interpreted the textures of SEDEX deposits as indicative of syngenetic deposition.

• The Kupferschiefer was proposed to represent the prototype of sedimentary-syngenetic deposition of sulphide mineralization. Evidence included:

1. The extraordinary continuity of the 1m thick mineralization over an area of 6 x 105 km2.

2. S – isotope data that suggested bacterial sulphate reduction and geochemical similarities with other marine black shales.

• Vertical zonation in this unit was ascribed to increasing sulphide concentrations in seawater through time due to bacterial activity and the source of the metals was believed to be the underlying red beds. Metals were mobilized into the Zechstein Sea during marine transgression over the red bed terrane.

TIMING OF MINERALIZATION

Syngenetic Theory

• Similarly the Central Africa Copperbelt was regarded as a type example of syngenetic copper mineralization. Evidence included:

1. Extreme continuity of conformable mineralization especially in the “shale ores”.

2. Abundant evidence of control of sulphide distribution by detailed sedimentological structures.

3. Mineral zonation apparently closely related to paleogeography during deposition of the sediments

4. Discordant veinlets, previously regarded as evidence for an epigenetic origin, were reinterpreted as the result of local remobilization of the sulphides during or after metamorphism.

TIMING OF MINERALIZATION

Syngenetic Theory

• A syngenetic sedimentary origin was also proposed for the lead-zinc deposits of Mt. Isa, McArthur River, Meggen, and Rammelsberg where similar lines of evidence were sited together with aspects such as:

1. a) The finely banded nature of these ores.

2. b) Abundant evidence of soft sediment deformation.

3. c) The lack of apparent permeability control.

• The syngenetic sedimentary theory has also been strongly influenced by the exhalative model for volcanogenic massive sulphide deposits and similarities were identified with Meggen, Rammelsberg, and Sullivan.

TIMING OF MINERALIZATION

Challenges to the syngenetic sedimentary

theory:

• Contradicting evidence to the syngenetic sedimentary origin came from observations at White Pine where it was shown that:

1. The sharp transition zone at the top of the cupriferous zone is regionally discordant to the bedding.

2. Textural evidence that py is replaced by cc.

TIMING OF MINERALIZATION

• This led to the proposal that cupriferous mineralizing fluids from deeper in the sedimentary basin reached the host sediments sometime after their deposition and produced copper mineralization by py replacement along a sharp reaction front that moved upward through the sediments.

• It is also suggested that lateral flow is more likely and that the transition zone marks a solution interface in a gravity-stratification model.

TIMING OF MINERALIZATION

Deformation

• Deformation is needed to mobilize and concentrate metals.

• Base metals mobile under tectonic activity – move to low strain areas.

• Wilson cycle:

– Break-up: MOR – fumaroles – potential to form ore fluids and SEDEX.

– Amalgamation: deform and remobilise and concentrate metals.

ALL THEORIES

METAL / ORE

MINERAL

ZONING

Zonation

• Caused by chemical conditions during deposition.

• Zoning upward and outward.

• Zoning appears to show relationship to palaegeography.

• Proceeding basinwards:

– Cu + Ag Pb Zn.

• Solubilities (in increasing order):

– CuS – PbS – ZnS

Zonation

• Precipitation dependant on:

– changes in pH,

– decrease in temperature with distance from source,

– reaction with wall rock compositions,

– mixing with meteoric waters, etc.

Zonation: Cu-Type

• Sulphide zoning, both laterally and vertically:

Barren (no sulphide, often with haematite) cc

bn cpy py

• Control of zonation: uncertain. – primary feature that formed at the time of deposition of the host

sediments, or

– secondary effect that influenced the distribution of the organic reluctant or permeability much later.

– originally, it was suggested that this zonation pattern as witnessed in the Central Africa Copperbelt resulted from sedimentary action down a paleo-slope away from the shoreline.

– more likely that sulphide zonation reflects postdepositional gradients either during early diagenesis or later.

Zonation: Cu-Type

• cc, bn, cpy and other base metal sulphides (gn, sp) deposited downstream from the redoxcline.

• cc – least soluble – precipitated close to redoxcline.

• bn and cpy more soluble – deposited progressively further downstream towards original pyritic zone.

• If a haematite zone was introduced – forms immediately upstream of the cupriferous grey beds.

Zonation: Pb-Zn-Type

Variation in proportions of galena and sphalerite.

E.g. Mt Isa:

Cu Pb Zn Fe

Association with Iron Formations

• Many of the sediment-hosted, stratiform Pb-Zn deposits are spatially associated to sulphide or oxide facies iron-formation.

• Mt Isa, Lady Loretta and McArthur River are associated with well-developed sulphide facies iron-formation, whereas Gamsberg, Aggeneys and the Tynagh deposits are associated with oxide facies iron-formation.

• A geochemical Pb Zn Mn halo is typically developed within the iron-formation.

Classification of Sediment Hosted Sulphide

Deposits

Based on the environment of deposition of the

enclosing sedimentary pile.

Classes

1) Intracontinental Basin Deposits (Epicratonic):

Kupferschiefer, Central African Copperbelt, Largentiere,

White Pine.

2) Flysch Basin Deposits: Rammelsberg, Tom Deposit,

Sullivan.

3) Platform-Marginal Deposits: McArthur River, Howard Pass.

SEDEX-Type Deposit Formation Model

A more complete model (after Lydon, 1983; Large, 1986):

• A continental rift basin

• Aproximately 2-5 km of coarse grained clastics (permeable)

• A related volcanics syn-rift phase

• These units are then overlain by deep marine deposits (impermeable)

forming a seal

• The syn-rift clastic sedimentary units are the source of the metals,

which dissolve into sea water which has ingressed into fault systems.

Model of the sedimentary basin architecture of productive basins hosting SEDEX

deposits. (Goodfellow Lydon, 2007)

SEDEX-Type Deposit Targeting

It is possible to use the following features of SEDEX-Type deposits

for targeting:

• The contact type

– Sharp basal contact

– Gradational upper contacts into turbidite sequences

• Discrete vent sites

• Hydrothermal alteration of local sediments (km scale)

SEDEX-Type Deposit Targeting

• Small fault bounded basins within a large basin

• Faults with evidence of fluid movement

• mineralised (feeder) veins

• sulphur-rich clastic (shales) interbedded with chemical rocks (chert,

carbonate, barite)

• Large sulphur rich horizons which show up with geophysical gravity

surveys.

• Geochemical surveys for lead, zinc, barium to pick up geochemical

halos of vent regions