- Geology and Exploration Criteria -

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

Classification of gold deposits

1. Intrusion-related gold deposits

2. Carlin-type gold deposits in Nevada, USA

3. Epithermal gold deposits in volcanic terranes

“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

Fig 1. Stages in the development of skarn deposits

1. Gold type2. Iron type3. Copper type

: nonpophyritic, small-size, high- gold grade in high sulfide content, intense retrograde alteration

4. Porphyry copper type: Large-scale and low-gold grade

5. Lead-zinc type : very little gold contain

The type of skarn deposits

(Enaudi et al., 1981)

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)

Skarn gold deposits

1. Related to copper-mineralized porphyritic intrusions

2. Usually clastic or volcanoclastic component

3. Age : Cambrian or older ~ Miocene4. Arsenopyrite and pyrrhotite are

abundant5. Typically contain As, Bi, Te

Fig. 3. Electron microprobe analysis (mole%) of garnet and pyroxene from Fortitude, Nevada ; Hedley, British Columbia; Mccoy, Nevada: west central Montana gold skarn.

Table 1. Gold Distribution in Gold Skarn DepositsLocalty 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 <0.1 17,280 240

Beal 9,200,000 1.5 1.3 0.0 37.7 13,883 11,960 3,680

Brown’sCreek(u.g) 450,000 5.2 9.0 0.4 11.8 2,340 4,050 1,980

Brown’sCreek(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

Deposit ContainedAu, tones Age, Ma Ore-related

intrusion Host rocks MetalAssociation

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 EarlyJurassic

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

>100 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)

Table 2. Geologic Characteristic of Gold skarn Deposits

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

4. Most gold skarns are associated with relatively mafic reduced plutons and contain anomalous of As, Bi, and Te.

5. 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.

6. Economic gold skarns should be sought in the distal part of under reducing conditions.