Presentation on theme: "- Geology and Exploration Criteria -. Object To understand and classify Skarn deposits To understand geology,mineralogy, characteristics, and distributions."— Presentation transcript:
- 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
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 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 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 LocaltySize(t) Au(g/t)Ag(g/t)Cu(%)Au/CuAu(kg)Ag(kg)Cu(tons) Bau2,400,0007.20.1<0.117,280240 Beal9,200,0001.51.30.037.713,88311,9603,680 Brown sCreek(u.g)450,0005.29.00.411.82,3404,0501,980 Brown sCreek(0.p)1,382,3804.59.00.410.26,22112,4416,082 Cable1,000,0006.05.03.02.06,0005,00030,000 Fortitude10,300,0006.9220.127.116.111,379254,4108,240 Golden Curry930,0008.54.20.325.87,9053,9063,069 Hedley(o.p)8,900,0004.61.40.145.640,58412,4608,900 Hedle yu.g)3,604,84913.53.00.1224.948,64210,8152,163 La Luz16,000,0004.11.20.49.365,60019,20070,400 Marn100,0001.42.80.34.7140280 Maura Sipongi113,0005.62.50.228.0633283226 McCoy Creek14,500,0001.50.1 15.021,7501,45014,500 Minnie-Tomboy3,900,0002.89.00.39.310,92035,10011,700 Red Dome15,000,0002.64.60.55.739,00069,000 Silver Starsmall50.04.012.5 Southern Cross400,00013.016.00.1147.75,2006,400352 Suian530,00013.04.96,8902,597 Thanksgiving1,700,0006.440.60.417.810,89768,9356,120 Tillicum126,00020.62,596 Tul Mi Chung400,00012.04,800
Deposit Contained Au, tones Age, Ma Ore-related intrusion Host rocks Metal Association Skarn Mineralogy Fortitude, Nevada, USA 9637.2 Porphyritic granodiorite stock and dykes Carboniferous -Permian limestone Au-Ag-Cu- Zn-Pb-As- (Bi-Te) Gar,pyx,ep,act, chl McCoy, Nevada, USA 3039.7 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,axinit e,qtz,ep,tr,cz,c hl,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 135.5 Diorite porphyry stock and dykes Miocene limestone Au-Ag-Zn- (Pb-Cu-As- Te) Gar,act- tr,ep,cz,id,chl Suan, North Korea >100JurassicGrinite pluton Late Protetozoic- Cambrian limestone and dolomite Cu-Au-(Zn- Pb-As-Bi) Navachab, Namibia 22Cambro- Ordovician Leucognite dykesLate Proterozoic dolomitic mable Au-(Cu-Pb- Zn-W-Bi-Te- As-Mo) Table 2. Geologic Characteristic of Gold skarn Deposits
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.