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EAGLE'S NEST MINE: Placer County California, THE
Mineralogical Record, Jan/Feb 2004 by Leicht, Wayne C, Cook, Robert
Rock units within the immediate mine area most likely correlate with the Blue Canyon Formation and consist of variably graphitic and pyritic slate, metaconglomerate, gritty quartzite, and metagraywacke. Minor amounts of the Tightner Formation have also been mapped near the mine and consist of distinctive intermediate to mafic metavolcanics including amphibolite and amphibole schist. The thickness of individual units or "beds" is quite variable and progressive changes in unit thickness within the mine suggest the presence of one or more isoclinal folds. Steeply dipping, variably altered lamprophyre dikes up to 1.5 meters thick cut units of both formations and are obvious throughout the mine workings. Dominant east-west-striking and north-south-striking dikes intersect within the mine and have been used for location reference on all levels. Petrographie and geochemical examinations of major rock unit samples collected from within the mine indicate that alteration includes silicification of dramatically variable intensity and pervasive though generally less intense carbonate alteration proximal to quartz veins. Local bands of mylonitic recrystallization are common, particularly near boundaries between units of differing competencies. all major rock units, with the exception of dikes, are foliated at some scale, although the more equigranular grits retain a somewhat massive bedded appearance in outcrop. Foliations most commonly strike steeply to the north and exhibit a variable though typically steep dip. Graded bedding is locally preserved.
The structural setting is somewhat complex. The Blue Canyon and Tightner formations are folded together into relatively tight north-northeast-trending, northwest-vergent antiforms and synforms. Chandra (1961) has mapped a dominant open, northeast-plunging antiform across the southeast portion of the claim block, implying that the majority of the mine workings lie within its west limb. East of this fold, a north-striking, very steeply dipping fault of unknown displacement has carried rocks of the Blue Canyon Formation west of the fault upward against what is interpreted to be a structurally higher part of the Blue Canyon-Tightner stratigraphy. Within the mine proper, the productive workings appear to terminate to the west against a fault of unknown displacement. Elsewhere, very minor normal and reverse displacement is exhibited along widely spaced faults that appear to represent joints along which sporadic local movement has taken place.
Gold occurs within small, very irregularly distributed zones in a set of narrow, subparallel, gently dipping quartz veins. This vein set typically strikes north-northwesterly and dips gently to the northeast. At least four productive veins, separated from each other by several tens of meters, have been identified, along with numerous similar though barren veins. Veins are typically quite thin, averaging about 5 cm in thickness. Although they are generally persistent throughout the workings, they can be seen to pinch and swell locally and in some places have thinned to the point of disappearing completely. Although gone, their traces are marked by joint-like seams which continue on in structural continuity and which gradually become quartz-filled again down dip or along strike. Joints that contain no quartz veins but are subparallel with those serving as the locus of auriferous veins are common throughout the mine.
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