Cordilleran Vein Type Deposits Examples

Veins 11/3/10 

Cordilleran Vein Type Deposits 

• Base-‐ and precious-‐metal deposits that occur 

as open-‐space filling or replacement 

polymetallic vein deposits which are generally 

not obviously related to immediately adjacent 

igneous ac>vity. 

• ‘Type’ locality is the N/S American cordillera 

but they are found around the world 

Examples 

• Ontario-‐Tin>c Ag mines in Utah 

• Peru: Casapalca, San Cristobal, Cerro de 

Pasco 

• Ag-‐Pb deposits of Coeur d’Alene, ID 

• Au-‐Ag-‐Te deposits of B.C. and the Colorado 

Front Range 

• Ag mines of Leadville, CO 

• Magma copper deposits, AZ 

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Generaliza>ons -‐1 

• Associa>on in space and >me with calc-‐ 

alkaline igneous ac>vity 

• Hydrothermal transport of ore components, 

deposited epigene>cally in faults 

• Ore minerals as typical open-‐space fillings 

or co-‐precipitates in silicate host rocks and 

as replacements in carbonate hosts 

• Deposi>on within 1+ km of the surface 

• Well developed metal zona>on 

Generaliza>ons-‐2 

• Sulfur isotope ra>os close to 0 ‰ 

• Structural control 

• Typically well-‐developed bilaterally 

symmetrical wall-‐rock altera>on 

• Normally contain porphyry-‐Cu suite of 

elements with zona>on from (Sn-‐W-‐)Mo-‐ 

Cu-‐Zn-‐Pb-‐Mn-‐Ag 

• Magma>c-‐hydrothermal fluids mixed (late, 

post-‐ore?) with meteoric fluids 

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Casapalca, Peru 

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Casapalca, Peru 

• Mineraliza>on in a series of veins 

• Veins cut series of N-‐S an>clines and 

synclines in Cretaceous ls; 1500m of 

Ter>ary subaerial Casapalca Fm; 1500m of 

andesi>c tuffs, flows, and volcaniclas>c 

sediments and 1500m of pre-‐Miocene 

sediments and volcanic rocks 

• Principal C vein N45°E, dips 70-‐80 NW 

– Other veins subparallel; or as splays 

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5


Casapalca Structures 

• Miocene folding and diori>c intrusions 

• Major intrusion cut by mineralized fault -‐ 

complicated history 

• Liile skarn in district 

– Found in the 11km Graton Tunnel driven below 

the workings to drain water (up to 66°C), 

ven>late, haulage 

• Veins occupy shear planes -‐ 6 km long, 2 

km high 

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7


Timing, temperature, mineralogy, 

zona>on 

• See figure 

– Py-‐Sphalerite-‐Galena-‐(huebnerite-‐ 

arsenopyrite) early at 350-‐400° C 

– Chalcopyrite, then Tetrahedrite follow, ZnS-‐PbS 

begin to wane 

– Pyrite and quartz gangue 

– Late sulfides strongly zoned: both 

mineralogically and composi>onally 

– Altera>on is extensive centrally becoming less 

intense and localized outward 

8


Timing, temperature, mineralogy, 

zona>on (cont.) 

• Chalcopyrite abundant in center, Sb-‐As-‐Bi 

sulfosalts take over peripherally 

• 4-‐40% NaCl, varied erra>cally, minor boil 

• Average ore grade: 

– 2% Pb 3% Zn 

– 0.3% Cu 10 oz/ton (310 ppm) Ag 

• Veins up to 1 meter wide 

– Important small adjacent parallel fractures 

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Cerro de Pasco, Peru 

Buie, Montana 

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Colorado Telluride Belt 

• NE end of the Colorado Mineral Belt 

• Placer gold discovered in 1859 

• Au-‐Te ores discovered in 1872 

• Small, erra>cally distributed, rich ore bodies 

• Mined un>l about 1960 

• Te not recovered from these ores -‐ Te 

recovered during Cu refining (porphyries) 

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Geologic Seong 

• Precambrian schist and gneiss intruded by 

Ter>ary stocks & dikes 

• Te mineraliza>on related to dikes and 

intrusion breccias 

• Localized where NW ‘breccia reefs’ intersect 

NE veins 

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Ore Deposits -‐ Mul>ple Events 

• Pb-‐Ag deposits: widely scaiered veins (NE) 

– (Pb,Ag)S, tetrahedrite, ± sph, cpy, py 

• Fluorite deposits: Jamestown 

– 1-‐20’ wide veins, 100-‐1000’ long 

– Adjacent to sodic grani>c stocks 

– Deep violet color: up to 0.05% U 

• Pyri>c Au deposits: NE trending veins 

– Coarse pyrite, cpy, sph, gal, tetrahedrite 

– Earlier than Au-‐Te event 

Mul>ple Events -‐ con>nued 

• Au-‐Te deposits: NE trending fissures 

– Inches to feet thick, mined to 3-‐700’ depth 

– Ore con>nues at depth but water and Au price 

made non-‐economic 

– Best ore in breccia zones 

– Petzite, hessite, sylvanite, calaverite, … 

• Tungsten deposits: E-‐NE belt overlapping 

end of Te belt 

– Fe-‐Mn tungstates (ferberite) 

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Coeur D’Alene, Idaho 

• One of the largest Ag districts in the world 

• $3+ billion of ore since 1879 

– 1 billion oz of Ag 

– 10 million tonnes of Pb 

– 3 million tonnes of Zn 

– 200,000 tonnes of Cu 

– Significant Sb 

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• 50 x 20 km area 

• District-‐wide zoning 

– Au in north 

– Pb-‐Zn in center 

– Cu-‐Ag in south 

• Underlain by: 

General Seong 

– Proterozoic Belt Group 

– Cretaceous igneous intrusions 

– Ter>ary and Quaternary unconsolidated units 

Belt Group 

• Quartzites, argillites and calcareous rocks 

up to 6km thick 

• Shallow water origin 

– Mud cracks, ripples, stromatolites 

• Folded, faulted and minor metamorphism 

in Proterozoic and during Laramide 

– Monzonite stocks and lamprophyre dikes and 

sills were intruded aver Mesozoic deforma>on 

– Minor diabase sills appear to be Precambrian 

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Faul>ng 

• WNW faults, the major being: 

• Osburn Fault can be traced for hundreds of 

kms outside of district 

– Dips 55-‐65° south 

– 4.5 km normal dip-‐slip offset 

– 25 km right lateral strike slip displacement 

– Intermiiently ac>ve since Precambrian 

– 30-‐60 m wide fault zone 

– Many parallel and oblique 2nd order faults 

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Ore deposits 

• Most in fractures and shears in Belt rocks 

– Minor structures more favorable than major 

• Some ore in monzonites, lamprophyres 

unmineralized 

• Briile rocks more highly mineralized 

• Galena, sphalerite, tetrahedrite, 

chalcopyrite, pyrrho>te, magne>te, 

arsenopyrite 

– Minor bornite, chalcocite, s>bnite, Cu-‐Sb-‐S 

sulfosalts, scheelite, uraninite 

Ores (cont) 

• Arsenopyrite forms envelopes around ore 

• Qtz, siderite, pyrite, (barite) as gangue 

– Typical ‘mesothermal’ assemblage 

• 10% combined Pb-‐Zn; 1 oz/tonne Ag 

• Ag in tetrahedrite, not galena 

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Timing or Mineraliza>on 

• Very confusing and s>ll debated 

– Some may be Precambrian 

– Most probably syn-‐ post-‐Laramide 

– Some stocks seem directly implicated 

– Pb age is Precambrian -‐ remobilized from Belt 

basement by Laramide hydrothermal systems? 

– No single paragenesis for district as whole 

– Altera>on variable -‐ sericite most common 

Cobalt, Ont. Loca>on Map 

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Cobalt, Ontario -‐ Summary 1 

• 445 million oz Ag since 1903 

• Na>ve Ag w/ cobalt arsenides and sulfosalts 

• Near ver>cal carbonate veins cuong 

– Huronian Gowganda 

– Archean metavolcanics 

– Nipissing diabase 

• All major deposits within a few hundred 

meters of the Archean unconformity and in 

proximity to diabase and VMS sulfide 

mounds in the Archean metavolcanics 

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Cobalt, Ontario -‐ Summary 2 

• Ag mobilized from country rocks by 

hypersaline brines and deposited by mixing 

with cooler meteoric water 

• Chloride complexing dominant 

• Halite-‐bearing fluid inclusions 

• 300-‐350° C 

• 600-‐1360 bars (2-‐4 km lithosta>c depth) 

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Cordilleran Vein Type Deposits Examples

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