SONORA PALEOCHANNEL A factor common to the Dawson Range
porphyry districts appears to be the regional spatial association of the Big Creek Fault with numerous mineralized Cretaceous intrusions (Figures 2 and 3). At Sonora, a late Cretaceous quartz monzonite porphyry (Kqm) intrudes the northern flank of the mid-Cretaceous Dawson granite batholith (Kg) and adjacent Devono-Mississippian schist and gneiss wallrocks (Pzm). The Sonora stock and the wall rocks are tran- sected by a northwest-striking, district-scale splay of the Big Creek Fault (Figures 5 and 7). U-Pb age dating indicates the Sonora quartz monzonite porphyry has a late Cretaceous age of 75 Ma, the same intrusive age and lithology as the Casino gold-copper-molyb- denum porphyry (Schulze, 2009).
District Hardrock Geology of Te - Discussion
Cu-Au mineralization at Sonora is cen-
tered on the late Cretaceous quartz mon- zonite porphyry stock (Kqm) and consists of numerous bodies drilled from Sonora Camp eastwards to the east side of Hayes Creek (Figure 5). The drilled mineralized zones seem to represent numerous small “bonanza-style” mesothermal quartz-arse- nopyrite veins to the west and lenticular bodies of semi-massive copper-gold-silver skarn to the east. The regionally active, long-term, northwest-southeast striking Big Creek Fault also appears to have been the conduit for many of the hydrothermal pulses originating from the metalliferous quartz monzonite (Kqm) stock at Sonora. Cu-Au mineralization occurs on the fault, in splays between the fault and the ultramafics (gnst), and in reactive metamorphic strata south of the ultramafic horizon (Bennett 2010).
High grade Te has been found in soils,
rock chips, and drill hole intervals in the mineralized Paleozoic metamorphics (Pzm) and Cretaceous quartz monzonite porphyry (Kqm). All of these Te-rich samples appear to have been in epithermal veins near the trace of the Big Creek fault splay. Some may be in the hanging wall, and some may be in the footwall of the fault splay, but in either case they are in the headwaters of Sonora Gulch. The same mineralization is also in the headwaters of Kline’s Gulch, draining to the east (outside of the southeast boundary of Figure 5).
Geology of Te in Sonora Paleochannel - Discussion
George Wilson has produced placer gold along a 700-meter stretch of lower Sonora Gulch since 1995. His pay ground has been Pleistocene gravels (the Sonora paleochan- nel) subcropping from beneath the lower, steep banks of Sonora Gulch. The Sonora
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paleochannel is covered by 25 meters or more of black, permafrosted swamp mud. This capping mud has been a major produc- tion problem for George Wilson, constantly thawing and slumping into the streambed (Photo 1). He has not undertaken to strip the capping mud back from the gulch axis and has therefore not accessed all of the basal gravel or the bedrock contact.
High grade Te has also been assayed in placer nuggets recovered from near the center of the Sonora paleochannel bench. This is where the upper gravel strata are exposed by the deeply down-cutting Sonora Gulch. The lower and basal strata of the gravel paleochannel do not appear to have been mined by George Wilson yet.
Placer tetradymite and most likely large
volumes of diagenetic tetradymite are pres- ent in unoxidized portions of the Sonora paleochannel, along with gold. Gold and tetradymite often occur together in the same nugget in these situations, where tetradymite seems to have catalyzed from high Te+Bi+S groundwater onto pre-existing gold and tetradymite grains.
The syn-metallic relationship between
Au and tetradymite has been observed by the author in samples from the Gow placers at Nucleus-Revenue in the Big Creek placer district and in photographs of grains from Miller Creek in the Sixtymile District west of Dawson City (Photos #4 and #5). At the Gow placers, newly re-flocculated tetradymite was observed by this author in 14-year-old, water-filled tailings buckets of tetradymite- magnetite-pyrite black sand.
At Miller Creek, tetradymite had precipi-
tated on old primer cord left underground and below the water table in reducing
Jul.Aug.Sep 2024 • TPG 17
conditions for several decades (Photo #5). The re-precipitated nature of crystalline tetradymite may be discerned, along with the same annealed textures visible in gold nuggets collected and photographed in the 1980’s by Jim McFaull directly from the underground placer workings at Miller Creek (Photos #4 and #5).
Geologic observations (Figures 1, 5 and 7)
and topography (Figures 6 through 8) indi- cate the Pleistocene gravels of the Sonora paleochannel, capped by black, organic, swamp mudstones, were preserved from pre-Reid (LeBarge, 1996) glacial erosion. The Sonora paleochannel then appears to have been preserved from complete isostatic dissection and erosion by later Quaternary incorporation into a triangular, downthrown block between the local Big Creek fault splay and the Hoocheekoo Fault (Figures 5 and 7).
Groundwater under reducing conditions,
with Te, Bi, and S derived from erosion of the Sonora porphyries, has precipitated dia- genetic tetradymite and Au down-gradient in the Sonora paleochannel over the last few tens of thousands of years. Tetradymite content of the Sonora paleochannel’s grav- els may therefore be quite high, judging by the tetradymite nodules caught by George Wilson’s 11/2” screen deck.
Caution: Tetradymite is extremely vola-
tile in oxidizing fluids. Care will have to be taken to ensure the drilling fluids do not appreciably dissolve the tetradymite and that long-term oxidizing solutions are not introduced into the gravels through the drill holes.
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