Presentation on theme: "Gold skarn Deposits - Geology and Exploration Criteria -"— Presentation transcript:
1Gold skarn Deposits- Geology and Exploration Criteria -
2Object To understand and classify Skarn deposits To understand geology,mineralogy, characteristics, and distributions from main Skarn gold depositsSuggest relevant exploration criteria for economic skarn gold deposits
3Classification of gold deposits Intrusion-related gold depositsCarlin-type gold deposits in Nevada, USAEpithermal gold deposits in volcanic terranes
4“Skarn”Originally a term applied to coarse-grained calc-silicate gangue associated with the iron ore deposits of SwedenIt 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
5Fig 1. Stages in the development of skarn deposits
6The type of skarn deposits (Enaudi et al., 1981) Gold typeIron typeCopper type: nonpophyritic, small-size, high- gold grade in high sulfide content, intense retrograde alterationPorphyry copper type: Large-scale and low-gold gradeLead-zinc type: very little gold contain
7Fig.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)
8Skarn gold depositsRelated to copper-mineralized porphyritic intrusionsUsually clastic or volcanoclastic componentAge : Cambrian or older ~ MioceneArsenopyrite and pyrrhotite are abundantTypically contain As, Bi, Te
9Fig. 3. Electron microprobe analysis (mole%) of garnet and pyroxene from Fortitude, Nevada ; Hedley, British Columbia; Mccoy, Nevada: west central Montana gold skarn.
10Table 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<0.117,280240Beal9,200,0001.51.30.037.713,88311,9603,680Brown’sCreek(u.g)450,0005.29.00.411.82,3404,0501,980Brown’sCreek(0.p)1,382,3804.510.26,22112,4416,082Cable1,000,0006.05.03.02.06,0005,00030,000Fortitude10,300,0006.924.786.671,379254,4108,240Golden Curry930,0008.54.20.325.87,9053,9063,069Hedley(o.p)8,900,0004.61.445.640,58412,4608,900Hedle yu.g)3,604,84913.5224.948,64210,8152,163La Luz16,000,0004.11.29.365,60019,20070,400Marn100,0002.84.7140280Maura Sipongi113,0005.62.50.228.0633283226McCoy Creek14,500,00015.021,7501,45014,500Minnie-Tomboy3,900,00010,92035,10011,700Red Dome15,000,0002.60.55.739,00069,000Silver Starsmall50.04.012.5Southern Cross400,00013.016.0147.75,2006,400352Suian530,0004.96,8902,597Thanksgiving1,700,0006.440.617.810,89768,9356,120Tillicum126,00020.62,596Tul Mi Chung12.04,800
11Table 2. Geologic Characteristic of Gold skarn Deposits ContainedAu, tonesAge, MaOre-relatedintrusionHost rocksMetalAssociationSkarnMineralogyFortitude,Nevada,USA9637.2Porphyritic granodiorite stock and dykesCarboniferous-Permian limestoneAu-Ag-Cu-Zn-Pb-As-(Bi-Te)Gar,pyx,ep,act,chlMcCoy,3039.7Triassic limestoneAu-Ag-(Cu)Gar,pyx,ep,wo,scp,adulariaHedley,British Columbia,Canada83.5EarlyJurassicDiorite and quartz diorite porphyry sills and dykesTriassic calcareous and tuffaceous siltstoneAu-Cu-As-Zn-(Co-Ni-Bi-Mo-Te)Ad,di,scp,axinite,qtz,ep,tr,cz,chl,ksparRed Dome,Queensland,Australia39Caboni-ferousRhyolite porphyry dykeSilurian limestoneAu-Cu-As-Zn-(W-Sn-Bi-Te)Wo,gar,pyx,act,epThanksgiving,Philippines135.5Diorite porphyry stock and dykesMiocene limestoneAu-Ag-Zn-(Pb-Cu-As-Te)Gar,act-tr,ep,cz,id,chlSuan,North Korea>100Grinite plutonLate Protetozoic-Cambrian limestone and dolomiteCu-Au-(Zn-Pb-As-Bi)Navachab,Namibia22Cambro-OrdovicianLeucognite dykesLate Proterozoic dolomitic mableAu-(Cu-Pb-Zn-W-Bi-Te-As-Mo)
12Summary 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
13Most 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.