Ore minerals at peak include native gold, electrum, chalcopyrite, pyrargyrite
(Ag3SbS3), acanthite (Ag2S), argentopyrite (AgFe2S3), and hessite (Ag2Te). Additional
metals present at Peak include Bi, Te, As, Sb, Zn, Cd, Ni, Co, and W (Table 3.1). This contrasts to the typical Au-Bi-Te (As) metallogeny of intrusive related deposits of interior Alaska (e.g. Fort Knox and Pogo; Flanigan et al., 2000). Copper rich skam at Peak suggests instead an affinity with porphyry copper and associated deposits of easternmost Interior Alaska and western Yukon (e.g. Casino).
Elemental correlations at Peak are consistent with petrographic associations of various ore, sulfide, telluride and sulfosalt minerals. For example Au and Bi show a weak correlation (R=0.49) because although the two are associated, Cu skam (lacking Au) also contains native bismuth. Exsolution textures between gold and native bismuth suggest that a small fraction of Au and Bi originally precipitated as maldonite (e.g. Figure 3.7A). The vast majority (>99%) of Au in thin section is (Au,Ag) suggesting a complex recrystallization history, possibly starting with a maldonite phase.
Many plutonic related gold systems possess strong Au:Bi correlation coefficients. Mid-Cretaceous deposits of Alaska yield correlation coefficients (R2) between gold and bismuth of 0.71 to 0.89 for Fort Knox, Pogo and Dolphin (Flanigan et al., 2000). Late Cretaceous plutonic related gold deposits of Alaska yield lower correlation coefficients for gold and bismuth (0.24 to 0.74) for Nixon Fork, Shotgun, Golden Zone and Donlin Creek (Flanigan et al., 2000).
Bi: Au ratios for the Chief Danny exploration area range between approximately 9 to greater than 10,000. The majority of analyses from the Peak deposit yield Bi:Au ratios of 9-30, and the 450 highest Au samples yield an average Bi:Au of 21. These ratios are typical of plutonic related gold deposits (e.g. Fort Knox Bi:Au ~20; Pogo Bi:Au ~5; Nixon Fork Bi:Au 15; Flanigan et al., 2000). The maximum range in values is similar to other Au skam deposits such as the Buckhom (Bi:Au of 1 to >500; Deal, 2012).
Myers (1994) demonstrated Cu-Au (Pb-Zn-Ag) zoning (Figure 3.18) in the vicinity of the Fortitude skam deposit, Nevada, with proximal Cu and distal Au (Ag-Zn-Pb). The Peak deposit is more complicated because all known outcrops and drill intercepts of intrusions are greater than 1 kilometer from the Peak deposit and are in separate fault blocks (Figure 2.1). These faults postdate mineralization and the original position of any intrusive body relative to the Peak deposit is poorly constrained. However, in comparison with Fortitude, high Cu:Au at Peak is present on the east side of the deposit (Figure 3.15), suggesting that the causative intrusion is east of Peak. This is the opposite direction from the current location of the Mohawk pluton (Figure 2.1). Although the Mohawk intrusion has a porphyritic texture and is in the general vicinity of the Mohawk skam (Figure 2.1) it is implausible to make the fluid source for Peak simultaneously to the east and to the west of Peak. If the Fortitude model can be applied, the causative pluton lies below and east of the Peak deposit.
3.6.2 Ore Body Morphology
I interpret the Peak deposit as seen in cross-section (Figure 3.16) to consist of two bifurcating, sub-parallel horizons complexly deformed into 10 meter scale recumbent folds. Other interpretations are possible, but this is consistent with broader-scale folding present in the area. In particular meter-scale recumbent folding is present in marble layers near the SE edge of the map area (Figure 2.1) and 10 meter scale folding identified in amphibolite layers through a drill hole cross section of the Discovery zone (Figure 2.22) is likely recumbent. A bifurcation in amphibolite layer similar to the skam bifurcation is also present (Figure 3.16). This folding occurred before metasomatism as the amphiboles in the skam are randomly oriented and display no sign of alignment. That is, the original host rock (micaceous marble?) was complexly folded and then replaced during the hydrothermal skam-forming event.
I have similarly interpreted the long section through the Peak deposit (Figure 3.17) as containing folded skam horizons. The more-flat-lying aspect of the skam in the long section seemingly indicates that this direction (WNW-ESE) is the strike of the layers. Folding in this section would imply two different stages of folding.
Mineralogical zoning as expressed in the long section and cross section is problematic. The long section shows a greater abundance of skam horizons to the east and greater abundance of calc-silicate homfels to the west. Such—in conjunction with the E to W zoning from Cu to Cu-Au to Au—suggests a fluid source to the east and thus, westward flowing fluids. The apparent interfingering between skam and calc-silicate homfels suggests that the amphibole-rich skam may be a complex replacement of homfels as well as marble. However, this same section shows that massive to semi-massive sulfides, which
are typically seen as marble replacements away from skam—are restricted to the eastern part of the long section.
Fortitude Au + Ag ^W e s t C o p p er C anyon Porphyry meters U pper Fortitude NE E x ten sio n I East Copperl C anyon ^ Underground^ S Minnie f e 500
Plan Map
Tom boy / A*VCross-section through skarn
Fortitude, Nv
Skarn zoning
GAR:PYX:ACT j 6 :2 :2 5 :3 :2 5:5:0 4 :6 :0 1:8:1 NO C A LC - S lL IC A T E S Cu:Au x 1000 4:1 1:1 1:2 1:3■
1:28 Representative skam metal valuesn W®st. Fortitude Orebody Au{opt) 0.025 0.15 Cu {%) 0.8 <0.1 ppm As 337 1425 Bi 9 68 Co 100 17 Mn 350 2800 Mo 63 8 Ni 79 28 Pb 96 2316 Sb 4 34 Zn 280 2525
Figure 3.17: Mineral and metal zoning in and near the Fortitude skam deposit, Nevada. Modified from Myers (1994).
The cross-section also poses difficulties with regards to metal zoning. A broad northeast to southwest zoning from Cu to Cu+Au suggests fluid flow from the north. Late Pb-Sb(Zn) bearing veins cross cut skam, sulfide replacement and schist. Both the orientation of late Pb-rich veins and Cu:Au trends (high Cu:Au easterly and downdip) suggest an intrusive source easterly and at depth. However, the west to east change from skam to massive- and semi-massive sulfide + quartz indicates lower temperature fluids to the east. Similarly, the only marble horizon encountered in either section is in the easternmost drill hole of the cross-section. One would expect to find unreplaced marble at the distal edge of mineralization. However, this isolated lens of marble is well above the skam and may have simply escaped hydrothermal fluids.
I could not find a contact between skam and marble in any drill hole or any surface exposure in the area. In two cases the skam apparently terminates into coarse grained quartz + sulfide. In most cases it abruptly terminates against quartz schist. However, massive sulfide does contain variable amounts of calcite, which is compatible with direct sulfide replacement of marble.
One possible explanation for the mineralogical and metal zoning is that fluids did move upward from a source to the east of and below the Peak zone, outside of current drilling, but in a complex manner. As steeply dipping dip-slip faults are younger than— and offset—the volcanic rocks, it is likely that some post-ore tilting has occurred in the Peak zone. That is, the current geometry is likely tilted from the geometry at the time of ore formation. Skam layers that are currently flat-lying or west-dipping in the eastern part of the cross-section could have been east-dipping at that time. In any event, it is not clear whether the amphibole-rich skam represents a replacement of marble, of calc-silicate
homfels, of a pyroxene-rich skam, or of all three. Later, lower-temperature fluids using the same fluid conduits could have replaced amphibole skam, but more likely replaced
marble layers. These hypothetical marble layers were bypassed during the earlier
Chapter 4 Structure