RESEARCH· ·1970·

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ECONOMIC GEOLOGY
surface seems to be borne out by the magnetic survey
(fig. 3), which shows a sharp truncation of the ironformation
anmnalies on the east side.
The third fold system involves the Nemo and Estes
Systen1 and all younger Precambrian rocks, although
only the basal part of the younger system, the limestone
and so forth, can be seen in the Estes Creek area.
Because this is the youngest Precambrian folding, the
results are widespread over the region in the youngest
rocks and consequently well documented, though not
wholly understood. The late folding seems to have
taken place in two distinct stages: ( 1) the production
of very tight isoclinal folds with roughly a northwest
trend, probably caused by a north-south compression,
and ( 2) local refolding of the isoclinal folds by massive
lateral translations between very large northwesttrending
blocks, the movement of which apparently
reflects a continuation of the original north-south compression.
Several such zones of translation lie just
beyond the west margin of the Estes Creek area ; the
closest one is probably in a heavily silicified belt just
above the westermnost exposures of limestone; however,
the location of that fault zone is uncertain in the
latitude of the Estes Creek area. The faults to the west
of the Estes Creek area are demonstrably right lateral,
whereas some within the area, in the east, seem to have
moved in the opposite sense.
The folded limestone of the Estes Creek area provides
the only clues to the nature of the third folding.
In general it seems that previous anticlines of old
N emo quartzite and the gneiss basement moved upward
again, thus compressing further the synclines of
Estes conglomerate and infolding the overlying limestone
and conglomerate. Secondarily, continued compression,
after maximum folding, caused differential
movements between 1najor blocks-left lateral east of
the main quartzite block in sections 28 and 33, and
right lateral to the west of that block-which caused
bending of the Estes rocks to the present oxbow configuration
and refolding of the overlying infolded
limestone. The present strong regional cleavage and
north-plunging lineation relate to this last deformation
and are unrelated to any previous deformation.
The faults shown on the Estes Creek area map have
been inferred; none are actually exposed. Of principal
interest is the fault zone which passes northwestward
through sections 11, 3, 34, and 28. The dislocations in
sections 2 and 11 suggest normal or late :val :faulting.
Limy quartzite breccia marks the southwest edge of
. the fault zone. The location of the fault in the south
part _of section 34 is uncertain, but it is thought to be
continuous. Northwest of Nemo, in sections 27 and 28,
the fault apparently turns abruptly east along the contact
of an inverted arm of Nemo quartzite. This east
segment of the fault appears to be a reverse fault or
underthrust, with only north-plunging, talcose metagabbro
exposed on the south side. Only two possible
explanations for the peculiar behavior of this fault
come to mind: (1) the fault changes from left lateral
to a reverse or underthrust fault against a quartzite
buttress which was a feature on the pre-Estes System
topography; and (2) the quartzite arm and the fault
were drag folded because of right-lateral movements
between the Estes rocks and the gneiss basement to
the east, the edge of which is represented at Nemo by
the Nemo iron-formation and quartzite(~) which show
offsets possibly caused by right-lateral faulting.
QUANTITY AND QUALITY OF IRON-FORMATION
The area unde"rlain by iron-for1nation of the Nemo
System is approximately 2,710,000 square feet, 2,000,000
square feet of which are in the area of the main,
easternmost, iron-formation body in section 33. When
10 cubic feet per short ton is used as a rule of thumb,
the eastern deposit alone should produce 200,000 short
tons of iron-formation for every foot of depth. Exploration
by drilling is needed to determine the total recoverable
tonnage. On the basis of the belief that the main
deposit is synclinal and in consideration of the attitude
of the beds and appearance of the magnetic survey, the
minimum depth of the deposit is estimated to be about
1,000 feet. The total tonnage will probably prove too
little to sustain a long tern1 ( 30 years) beneficiation
project; however, other nearby deposits, within 2 miles
hauling distance (sees. 27 and 34), could easily provide
iron-formation enough for a nominal project.
The chemical quality of the main iron-formation at
Estes Creek is indicated by the following average of
partial analyses by the U.S. Bureau of Mines (Harrer,
1966, p. 44-45).
Fe --------------------- 1 29.9
Si02 -------------------- 2 54.9
Ti02 -------------------- 3 Q10
1 Eleven analyses.
2 Eight analyses.
a Three analyses.
p ---------------- 3 0.051
s ---------------- 3 0.021
~n _______________ 3 0.07
A spectrographic analysis of a composite of three
of the samp1es shows : as much a:s 1.0 percent Al ; a.s
much as 0.1 percent Ca, I{, N a, ~1g, Ni, Se, and Ti;
as much as 0.01 percent Ba, Cr, Cu, and Zr; and as
much as 0.001 percent Ga, Li, and Rb.
CONCENTRA.T ABILITY
The iron-formation is very fine grained, and the
minerals are intricately interlocked but not more so
than some iron-formation now being successfully beneficiated.
Laboratory concentration tests carried out by