2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 247-9
Presentation Time: 3:30 PM

WILL OVERSIGHT CORRECT AN 'OVERLOOKED' STRUCTURAL QUESTION AND ECONOMIC POTENTIAL AT REPUBLIC, WA?


RASMUSSEN, Michael G., Nine Mile Falls, WA 99026, mgrasmussen@comcast.net

The Overlook gold deposit on the southern slopes of Cooke Mountain near Republic is in the Quesnel terrane, which accreted to ancestral North America in the mid-Jurassic. Overlook was mined by Echo Bay Mines, Ltd. from 1990-1992, producing 280,000 oz of gold at an average grade of approximately 0.15 opt. Eric Cheney first recognized that the deposit consisted of an overturned section of felsic volcaniclastic rocks of the Triassic Brooklyn Fm. separated from underlying, younger limestones by a zone of auriferous massive magnetite –pyrhhotite. Overlook was in the upper plate of the (Eocene) Lambert Creek detachment fault, on which it had been translated eastward 7 km from equivalent rocks in the lower plate containing the similar but larger Lamefoot gold deposit.

At Lamefoot, the mineralized horizon was also a contact between Brooklyn limestone and felsic volcaniclastic rocks, and the proposal was made independently by Eric Cheney and Keith Glover that the Lamefoot mineralized horizon was on the nose of an isoclinal antiformal syncline, which carried the mineralized horizon from one side of the fold to the other. The problem with this interpretation was that the limestone in the core of the fold was notably uncleaved, unfolded and only weakly recrystallized. The Cheney-Glover structural model was never fully accepted, nor was it ever applied to Overlook, which when restored to its original contiguity with Lamefoot, would have to conform to the same structural domain as Lamefoot. You might say the entire issue was ‘overlooked’.

Forward 20 years. Adamera Minerals Corp has recently drilled 6 m of massive magnetite-pyrite and anomalous gold beneath Brooklyn limestone at their Oversight prospect, 600 m SE of Overlook but 150 m higher. Overlook is overturned yet the Oversight section appears to be upright. Is Oversight on the lower limb of a recumbent isoclinal fold, on which an overturned section comprises the upper limb, and the lower limb is an upright section of the same horizon? It fits with the Lamefoot structure only if one accepts the Cheney-Glover isoclinal antiformal syncline configuration for Lamefoot and applies it to Overlook.

We wait for further news from Adamera, and possible clarification of Cooke Mountain’s overlooked structural issue and its overlooked economic implications.

Handouts
  • GSA Cheney Session MGR.pptx (11.3 MB)