Gold skarn Deposits - Geology and Exploration Criteria -

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

Classification of gold deposits Intrusion-related gold deposits Carlin-type gold deposits in Nevada, USA Epithermal gold deposits in volcanic terranes

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

Fig 1. Stages in the development of skarn deposits

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

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 Related to copper-mineralized porphyritic intrusions Usually clastic or volcanoclastic component Age : Cambrian or older ~ Miocene Arsenopyrite and pyrrhotite are abundant 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 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 <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 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 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 45.6 40,584 12,460 8,900 Hedle yu.g) 3,604,849 13.5 224.9 48,642 10,815 2,163 La Luz 16,000,000 4.1 1.2 9.3 65,600 19,200 70,400 Marn 100,000 2.8 4.7 140 280 Maura Sipongi 113,000 5.6 2.5 0.2 28.0 633 283 226 McCoy Creek 14,500,000 15.0 21,750 1,450 14,500 Minnie-Tomboy 3,900,000 10,920 35,100 11,700 Red Dome 15,000,000 2.6 0.5 5.7 39,000 69,000 Silver Star small 50.0 4.0 12.5 Southern Cross 400,000 13.0 16.0 147.7 5,200 6,400 352 Suian 530,000 4.9 6,890 2,597 Thanksgiving 1,700,000 6.4 40.6 17.8 10,897 68,935 6,120 Tillicum 126,000 20.6 2,596 Tul Mi Chung 12.0 4,800

Table 2. Geologic Characteristic of Gold skarn Deposits 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, 30 39.7 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 >100 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)

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

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.