Gold Skarn Deposits -Geology and Exploration Criteria-

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Gold skarn Deposits
- Geology and Exploration Criteria -
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Object

1. To understand and classify Skarn deposits
2. To understand geology,mineralogy,
   characteristics, and distributions from main
   Skarn gold deposits
3. Suggest relevant exploration criteria for
   economic skarn gold deposits

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Classification of gold deposits

1. Intrusion-related gold deposits
2. Carlin-type gold deposits in Nevada, USA
3. Epithermal gold deposits in volcanic terranes

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Skarn

1. Originally a term applied to coarse-grained
   calc-silicate gangue associated with the iron ore
   deposits of Sweden
2. It include a variety of calc-silicate rocks
   rich in calcium, iron, magnesium, aluminium,
   manganese that formed by replacement of
   originally carbonate-rich rocks.
3. Skarn deposits result from the hydrothermal
   interaction of hot silicate magmas and cooler
   sedimentary rocks

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Fig 1. Stages in the development of skarn deposits
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The type of skarn deposits(Enaudi et al., 1981)

• Gold type
• Iron type
• Copper type
• nonpophyritic, small-size, high- gold grade in
  high sulfide content, intense retrograde
  alteration
• Porphyry copper type
• Large-scale and low-gold grade
• Lead-zinc type
• very little gold contain

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Fig.2 Distribution of Au, Ag, and Cu in skarn,
pluton, and protolith of a large
copper skarn system, Whitehorse copper belt,
Canada (from Meinert, 1986)
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Skarn gold deposits

1. Related to copper-mineralized porphyritic
   intrusions
2. Usually clastic or volcanoclastic component
3. Age Cambrian or older Miocene
4. Arsenopyrite and pyrrhotite are abundant
5. Typically contain As, Bi, Te

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Fig. 3. Electron microprobe analysis (mole) of
garnet and pyroxene from Fortitude, Nevada
Hedley, British Columbia Mccoy, Nevada west
central Montana gold skarn.
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Table 1. Gold Distribution in Gold Skarn Deposits
Localty Size(t) Au(g/t) Ag(g/t) Cu() Au/Cu Au(kg) Ag(kg) Cu(tons)
Bau 2,400,000 7.2 0.1 lt0.1 17,280 240
Beal 9,200,000 1.5 1.3 0.0 37.7 13,883 11,960 3,680
BrownsCreek(u.g) 450,000 5.2 9.0 0.4 11.8 2,340 4,050 1,980
BrownsCreek(0.p) 1,382,380 4.5 9.0 0.4 10.2 6,221 12,441 6,082
Cable 1,000,000 6.0 5.0 3.0 2.0 6,000 5,000 30,000
Fortitude 10,300,000 6.9 24.7 0.1 86.6 71,379 254,410 8,240
Golden Curry 930,000 8.5 4.2 0.3 25.8 7,905 3,906 3,069
Hedley(o.p) 8,900,000 4.6 1.4 0.1 45.6 40,584 12,460 8,900
Hedle yu.g) 3,604,849 13.5 3.0 0.1 224.9 48,642 10,815 2,163
La Luz 16,000,000 4.1 1.2 0.4 9.3 65,600 19,200 70,400
Marn 100,000 1.4 2.8 0.3 4.7 140 280
Maura Sipongi 113,000 5.6 2.5 0.2 28.0 633 283 226
McCoy Creek 14,500,000 1.5 0.1 0.1 15.0 21,750 1,450 14,500
Minnie-Tomboy 3,900,000 2.8 9.0 0.3 9.3 10,920 35,100 11,700
Red Dome 15,000,000 2.6 4.6 0.5 5.7 39,000 69,000 69,000
Silver Star small 50.0 4.0 12.5
Southern Cross 400,000 13.0 16.0 0.1 147.7 5,200 6,400 352
Suian 530,000 13.0 4.9 6,890 2,597
Thanksgiving 1,700,000 6.4 40.6 0.4 17.8 10,897 68,935 6,120
Tillicum 126,000 20.6 2,596
Tul Mi Chung 400,000 12.0 4,800
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Table 2. Geologic Characteristic of Gold skarn
Deposits
Deposit Contained Au, tones Age, Ma Ore-related intrusion Host rocks Metal Association Skarn Mineralogy
Fortitude, Nevada, USA 96 37.2 Porphyritic granodiorite stock and dykes Carboniferous-Permian limestone Au-Ag-Cu-Zn-Pb-As-(Bi-Te) Gar,pyx,ep,act,chl
McCoy, Nevada, USA 30 39.7 Porphyritic granodiorite stock and dykes Triassic limestone Au-Ag-(Cu) Gar,pyx,ep,wo,scp,adularia
Hedley, British Columbia, Canada 83.5 Early Jurassic Diorite and quartz diorite porphyry sills and dykes Triassic calcareous and tuffaceous siltstone Au-Cu-As-Zn-(Co-Ni-Bi-Mo-Te) Ad,di,scp,axinite,qtz,ep,tr,cz,chl,kspar
Red Dome, Queensland, Australia 39 Caboni-ferous Rhyolite porphyry dyke Silurian limestone Au-Cu-As-Zn-(W-Sn-Bi-Te) Wo,gar,pyx,act,ep
Thanksgiving, Philippines 13 5.5 Diorite porphyry stock and dykes Miocene limestone Au-Ag-Zn-(Pb-Cu-As-Te) Gar,act-tr,ep,cz,id,chl
Suan, North Korea gt100 Jurassic Grinite pluton Late Protetozoic-Cambrian limestone and dolomite Cu-Au-(Zn-Pb-As-Bi)
Navachab, Namibia 22 Cambro- Ordovician Leucognite dykes Late Proterozoic dolomitic mable Au-(Cu-Pb-Zn-W-Bi-Te-As-Mo)
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Summary and Exploration Criteria

1. Skarn mineralogy (especially, garnet and
   pyroxene) is the key to explore skarn deposits.
2. The abundance of pyroxene relative to garnet and
   the high ferrous/ferric ratios reflect the
   generally reducing environment of gold skarns.
3. Reducing conditions and subsequent oxidation is
   important in skarn gold deposition

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• Most gold skarns are associated with relatively
  mafic reduced plutons and contain anomalous of
  As, Bi, and Te.
• Explorationists should noted that the more
  proximal gold-poor, garnet-rich part of a skarn
  deposit is likely to be more resistant to erosion
  and thus more likely to crop out and be sampled
  than the more distal gold and pyroxene-rich
  skarn.
• Economic gold skarns should be sought in the
  distal part of under reducing conditions.