SONORA PALEOCHANNEL
Estimate of Te and Au in the Sonora Paleochannel - Discussion
The Te/Au ratio of tetradymite nuggets
from the Sonora paleochannel is known from assays reported by Schulz (2009; 1,949/1) and sample 100730.01 (851). Assuming a hydraulic advantage for accumulation of 6/1 for Au over tetradymite, and an average Au grade in the gravels of 1 gpt Au, there should therefore be approximately233 ppm of placer and diagenetic Te in the Sonora paleochannel. These estimations of course assume reducing groundwater conditions prevailed ever since more than 30 meters of black swamp muck was deposited on top of and adjacent to the paleochannel gravels. Any prolonged oxidation of the groundwa- ters would dissolve tetradymite, but that does not appear to have happened. As long as the black swamp muck enclosed the grav- els, we can safely assume the preservation of a reducing chemical environment within the paleochannel gravels.
Economic potential of pay gravel in the
Sonora paleochannel (Figures 5, 7 and 8) can be estimated. Assuming an average thick- ness of 2 meters, an average width of 350 meters, and a strike length of 4,000 meters, the volume of pay gravel is 3,500,000 m3. Assuming gravel density is 3,000 lbs/cubic yard (LeBarge and Welsh, 2007, page 232), gravels in the Sonora paleochannel total 6,230,000 tonnes. Assuming 1 gpt Au in the gravel, total gold in the Sonora paleochan- nel could be 6,230,000 g Au, or 181,750 oz Au. Assuming 233 ppm Te/1 gpt Au, the gravels of the paleochannel should contain 1,453 tonnes of Te metal. An increase in thickness or density (highly likely) will cor- respondingly increase the estimate of metal abundance.
Testing the Paleochannel Hypothesis - Discussion
The hypothesis that 1,453 tonnes of Te metal are contained in the Sonora
paleochannel, along with significant Au, must be tested. The obvious method is grid drilling from the permafrosted top of the enclosing black mud. Best time for such operations in this part of the Yukon is March and April. The drill would have to pass through from 15 to 30 meters of black mud and then another five to perhaps 25 meters of gravel before drilling through at least five meters of schistose bedrock placer-miner- alized at the unconformity. In other words, the depth of vertical drill holes developing Te and Au in the Sonora paleochannel will range from 25 to 130 meters.
Tetradymite and elemental Te may prove
to be extremely volatile in oxidizing fluids. Care should be taken to ensure the drilling fluids do not appreciably dissolve tetrady- mite during the sampling process.
Conclusions The well-mineralized, late Cretaceous,
Sonora quartz monzonite stock and sheared and veined wallrock has been the mechani- cal and chemical source for Te and Au deposited in the paleochannel gravels. Mechanically deposited tetradymite and Au, and perhaps other metallic minerals in the gravels, appear to have been catalysts for diagenetic increases in Te and Au grades. The Sonora placer deposit has been aug- mented by long-term soilwater and ground- water flow from the Sonora Cu-Au porphyry deposits through the chemically reduced, black swamp mudstones and gravels of the Sonora paleochannel.
Diagenetic processes that produced the
Te-Au deposit in the Sonora paleochan- nel operated for at least the past 50,000 years. Preservation of the reduced nature of the gravel, and the included deposit of tetradymite and gold, was due to the longstanding chemical influence of enclos- ing black, organic, swamp muds exceeding 30 meters in thickness. The size and grade of this potentially economic Te-Au deposit depends upon the thickness and horizontal
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extent of the paleochannel gravels. A con- servative estimate (see discussion above) suggests the Sonora paleochannel contains at least 1,453 tonnes of placer Te metal and 181,750 oz of placer Au. These hypothetical numbers will increase if pay gravel thickness and density of the pay gravels increase as suspected.
References
Bennett, V. and C. Schulze, D. Ouellette, and B. Pollries, 2010, Deconstructing com- plex Au-Ag-Cu mineralization, Sonora Gulch project, Dawson Range – a Late Cretaceous evolution in the epithermal environment, in Yukon Exploration and Geology 2009, MacFarlane, K.E. et al (eds): Yukon Geological Survey, page 23-45.
LeBarge, W.P. 1996, Placer deposits of the Yukon. Overview and potential for new discoveries, in LeBarge, W.P. (ed.), Yukon Quaternary Geology Volume 1, Exploration and Geologic Services Division, Northern Affairs Program, Yukon Region, page 1-12.
LeBarge W.P. and C.S. Welsh, 2007, Yukon Placer Mining Industry 2003-2006: Yukon Geological Survey, 235 pages.
Schultz, C.M., 2004 April 14, Summary and preliminary exploration report on the Sonora Gulch property, Dawson Range, Yukon: technical report for Firestone Ventures Inc., 59 pages, 10 figures, 5 tables, 4 appendices.
Schultz, C.M., 2009 October 1, Draft assess- ment report on the 2009 diamond drill- ing program, Sonora property, Dawson Range, Yukon: technical report for Northern Tiger Resources Inc., 57 pages, 5 figures, 4 tables, 6 appendices.
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