EASC 3210: MODERN ANALOGUES OF ORE-FORMING PROCESSES

LECTURE 6, LECTURE 7

growth of a black smoker chimney stage 1

• Hot, Ca-rich vent fluid mixes turbulently with cold, sulphate and Ca-rich seawater

• result: precipitation of a ring of Calcium Sulphate (Anhydrite) –due to the inverse solubility behaviour of Anhydrite

• Metals carried in the hot fluid also rapidly precipitate -as sulphides and oxides -during the mixing process, forming a plume of dark particles above the vent

growth of a black smoker chimney stage 2

• Chalcopyrite begins to precipitate and plate the inner wall of the Anhydrite chimney

• mixing of seawater and hydrothermal fluid components across the porous wall by advection and diffusion

• result: the deposition of Sphalerite, Chalcopyrite and Pyrite in the interstices of the wall

• gradually makes the chimney less porous and more metal-rich

schematic of the active TAG hydrothermal mound

• hydrothermal fluid rises rapidly, exiting the mound at the black smoker complex

• cold Ca and SO4- rich seawater is entrained into the mound and mixes with hydrothermal fluid

• mixing causes anhydrite, pyrite and chalcopyrite to precipitate inside the mound- increasing the acidity of the saline hydrothermal fluid

• Zn dissolve in this acidic fluid and are carried by the white smoker fluid to the edges of the mound. here cooler temperatures within white smoker chimney walls causes precipitation of sphalerite and other minerals

mixing of cold Ca and SO4- rich seawater and hydrothermal fluid causes what minerals to precipitate inside a TAG hydrothermal mound?

anhydrite, pyrite, and chalcopyrite

major features of a submarine hydrothermal field at a mid-ocean ridge

chloritization of basalt is associated with

the underlying feeder zone of the mound

fast spreading centre

• >10cm/yr

• host sequences volcanic-dominated

• more rapid heat dissipation

• smaller, more numerous VMS deposits

• (EPR) east pacific rise

slow spreading centre

• ~1-6cm/yr

• deeper faulting, deeper fluid circulation

• less rapid heat dissipation

• larger VMS deposits

• intrusive rocks in near surface

the “rose bowl” vest site on the Galapagos rift

• concave, ring-shaped, and ~10m in diameter

• a more passive hydrothermal system which formed as a more active system waned

• shows that not all submarine hydrothermal emanations are as dramatic as the “black smoker'“ fields or zones of active eruption

where are the 7 brine pools that contain metalliferous sediments located?

where ENE trending fractures and transform faults intersect the ridge axis

atlantis II deep

• 14 km long and 5km wide at 2000m water depth

• 5km3 of densely stratified brine comprises 2 layers:

  • lower brine 150m thick T>60 deg C

  • upper brine 50m thick T>50 deg C

• below the lower brine are metalliferous sediments resting on basalt

what is below the lower brine of Atlantis II deep?

metalliferous sediments resting on basalt

metalliferous sediments in Atlantis II deep appear to have precipitated from fluid at what temp?

>200 deg C

red sea brine pools

• convective hydrothermal system related to rifting and driven by magmatic heat

• Hydrothermal fluids predominantly derive from very saline connate fluids rather than seawater

• fluids are discharged in an environment where the sulphides and oxides precipitated are stable and stratified over a widely dispersed area

• enhances their potential to create extremely large metallic ore deposits

• modern analogue for SEDEX (Sedimentary Exhalative Massive Sulphide)

how are red sea brine pools different from MOR setting?

• Hydrothermal fluids predominantly derive from very saline connate fluids rather than seawater

• fluids are discharged in an environment where the sulphides and oxides precipitated are stable and stratified over a widely dispersed area

cyprus-type VMS deposits

• host basalts are typically pillowed and tholeiitic

• mineralization comprises pyrite and chalcopyrite with lesser sphalerite

• ore is associated with zones of intense chloritization and silicification of host rock

• overlying pelagic sediments (shales and cherts) are Fe- and Mn- rich (umber)

• some deposits capped by a Mn-poor Fe-rich oxidized layer (ochre)

ophiolites

• a section of MOR subsequently preserved by obduction onto continental crust

• characterized by sheeted gabbroic dykes, pillow basalts, and overlying pelagic sediments

basic structure of the oceanic crust

layer 1- sediments

layer 2- basaltic pillow lavas

sheeted dykes

layer 3- gabbroic rocks, fossilized magma chamber

MOHO

Mantle- peridotites, mantle