hydrothermal ores and skarn - portland state universityweb.pdx.edu/~jjackson/23jan17.pdf ·...

Magmatic Processes

•  Within the magma chamber•  During magmatic ascent•  During volcanic venting•  The role of water

– Magmatic water– Ground water

Old Faithful Geyser, Yellowston NP, WY

Mt St Helens, May 1982

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https://pubs.usgs.gov/fs/1997/fs113-97/

Mount Pinatubo, The Philippines, June 12, 1991

https://pubs.usgs.gov/fs/2005/3036/fs2005-3036.html

Mt St Helens Ash and Steam Eruption, March 8, 2005

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Water: polar, high dialectric constant, high heat capacity, high surface tension, maximum density above 32°F

PLUTON

3. Hydrothermal settings- as magmas crystallize, the water present in the liquid magma is driven off into fractures or permeable beds within the host rock. Minerals species that are water-soluble will thus be carried away from the parent magma body. As the water encounters lower temperatures and pressures, it may boil. Sulfide ores will precipitate in the order shown in this diagram, beginning with zinc or lead if present, and ending with a sinter of silica and or carbonate. The host rock may thus become host to hydrothermal deposits. Hydrothermal waters may also be derived from the circulation of ground water in deep basins. If the resulting brines ascend to shallow depths, the same order of precipitation of dissolved sulfides may occur.

Hydrothermal settings Sinter (Silica/Carbonate) Arsenic

Antimony

Mercury

Gold

Silver

Copper

Zinc-Lead

Order of precipitation ofhydrothermal mineralsbetween pluton andsurface

Contact metamorphic aureole settingSkarns-formed at high temperatures with both addition and

subtraction of material within contact aureole. Most economic skarnsoccur in carbonate country rocks, which are converted to marble. Oresinclude copper, iron, tungsten, zinc-lead, molybdenum, tin, talc, Au, Ag.

AUREOLE

CarbonateCountryRock

Intrusive rock

Robb (2005) An Introduction to Ore Forming Processes

Model of three stage skarn deposition

Copper porphery and skarn ores,Bingham Copper Mine, Utah

South Sister Volcano: the surface expression of magmatism

Solfaterra Caldera, Napoli

Solfaterra Hot Spring & Sulfur

Broken Top: shallow volcanic plumbing

Phantom Ship, Crater Lake, Or: shallow plumbing below vent

Beacon Rock: shallow plumbing below volcanic vent

River Mountains Lake Mead Mount Wilson Pluton

River Mountains, Henderson, Las Vegas

12 Ma Wilson Ridge Pluton (near Las Vegas) shallow plumbing

12 Ma Wilson Ridge Pluton: quartz monzonite shallow plumbing

12 Ma Wilson Ridge Pluton with riebeckite hydrothermal alteration

Mt Davidson,18 Ma granodiorite plutonComstock Lode,Virginia City, NV

Plutonism: aka deep volcanic plumbing Cretaceous Granite Dome, Texas

Coarse grained plutonic rock���Cretaceous Granite Dome, Texas

Mono Lake

Tioga Pass

Yosemite National Park, Sierra Nevada, Mono Lake

Gaylor Lakes

Gaylor Lakes Basin, Sierra Nevada, CA

Toulumne Intrusive Suite

Gaylor PeakU Triassic-JurassicMetamorphicRocks

U Triassic-JurassicMetamorphic RocksDana, CA

Fracture filled with quartz at Dana, CA

Plutonic Plumbing:���

1.7 Ga���Granitic veins���

cutting schist & gneiss ���

Black Canyon of the Gunnison

River