Friday, July 27, 2012

Centennial Ridge Gold & Diamond District

In the Medicine Bow Mountains of southeastern Wyoming, Proterozoic metasedimentary rocks to the north from Proterozoic volcanogenic rocksto the south.  To the north, the rocks represent part of an early continental core partially overlain by thick successions of younger (2.5-1.7 billion years old) metamorphosed sedimentary and minor volcanic rocks deposited in a basin near the margin of the ancient continent.  In total, more than 42,000 feet of quartzite, conglomerate, phyllite, and lesser basalt and dolomite were deposited in rivers, braided streams & along the shore of the ancient sea.
To the south younger (2.0-1.8 billion year old) metamorphosed sedimentary and volcanic rock form schist and gneiss.  These have been intruded by granite and mafic complexes 1.8-1.4 billion years ago, and were deposited on an ocean basin and in volcanic islands.

More recently (~100 million years ago), a major mountain-forming event resulted in the uplift of the Medicine Bows above the nearby basins.
During the Pleistocene (10,000-2 million years ago), the high peaks of the Snowy Range were periodically covered by alpine glaciers.  Boulders carried in ice, known as erratics, were transported down to the lower valleys and were deposited in extensive out wash plains. 
Panning for gold along the Middle Fork of the Little Laramie River. Following instructions on how to pan for gold, attendees of the field trip discovered hundreds of pyrope garnets but no gold. Were they sad? Not at all. Pyrope garnets are a mineral pathfinder for diamond deposits. So where are the diamond deposits? Somewhere upstream from this location at the Mother Lode prospect.

History
The district gained national attention following the discovery of the Centennial Lode in 1875.  The lode was described as an N45°E-trending quartz vein in a shear zone hosted by iron-stained hornblende schist. 
 
The Centennial ore was reported to average 1.5 opt Au (ounces per ton of gold)  Sporadic ore shoots were intersected that yielded some highly prized specimens.  One of these won first prize as a ore specimen at the 1878 Paris Mining Exposition; a gold-bearing garnet schist was placed in the Smithsonian collection; and another was sent to the Denver Mint in 1876 that assayed 2,263 opt Au and 210 opt Ag!  At today’s prices, a ton of this material would be worth more than $600,000.

Mining continued on the vein, and the operator ordered a 10 stamp mill.  The tunnel was located on the hillside 260 ft above the valley floor & a 425-foot tramway was constructed to lower the ore down-slope to the mill.  At the mill, the ore was crushed and gold was recovered on amalgamation plates.  Since a large portion of the gold occurred in pyrite; significant gold values were undoubtedly lost to the tailings.

Everything seemed to be very favorable until the summer of 1877, when the worst fears of the miners were realized.  The mine face was drilled and blasted.  When the dust cleared, instead of the rich vein; barren, hornblende schist was exposed.  The vein had terminated in fault gouge!  Several more blasts into the face revealed no clues to where the vein went or how it was offset.  Drifting to the north and south did not reveal any clues, and it is said that the mine owner not only lost the vein, but also lost his mind.  Residents of historic Centennial claimed that the owner could be heard any time of the day or night blasting along the hillside, trying to find the offset extension of the vein.

The offset vein was never found.  Possibly one day, someone will reopen the Centennial adit and search for clues as to which way the vein was offset, and use modern geophysical techniques to search for the offset.  Production was reported at 4,500 ounces.

In the early 1900s, some renewed interest followed the discovery of platinum, but the reports on the amount of platinum was exaggerated.

Geology

The Centennial district is dominated by hornblende schist with a strong northeasterly foliation.  Two types of lode deposits were described: (1) Au-bearing quartz veins parallel to foliation; and (2) gold-platinum veins occurring as fracture fillings & replacements in shear zones cutting the gneiss and schist.  The gold & platinum are associated with pyrite & arsenopyrite. 

Placer gold & platinum generally occur as small flakes (or colors) along Fall Creek and the Middle Fork and also the Queen Mill Run drainage.


The Paul Boden diamonds. Two excellent, gem-quality octahedral diamonds
recovered by Saratoga gold prospector Paul Boden from Cortez Creek. The
source of these diamonds remains unknown, but the later discovery of several
diamond indicator mineral anomalies and several cryptovolcanic structures
strongly suggest the Medicine Bow Mountains contain several undiscovered
kimberlite and/or lamprophyres.
The presence of pyrope is significant.  Several years ago, the Wyoming Geological Survey recovered several pyrope garnets to the west and northeast of this locality.  The source of the garnets has never been found.  Diamonds were also discovered in 1977, from a gold placer 14 miles to the northwest of the district, in the Cortez Creek drainage.


Mother Lode. The Mother Lode lies in pyrite-rich eluvial quartz along the bank of the Middle Fork of the Little Laramie River.  Several years ago, I collected some samples of the pyrite-rich quartz for gold.  The assays showed no detectable gold.  However, in August 2000, some attendees on one of my field trips panned several gold colors from the river, and also recovered pyrope garnets in their pans.

Platinum City: In 1930, minor amounts of platinum were found in some ores in the district.  Several mines reopened.  The platinum values were greatly exaggerated by a local promoter who made plans for a town called Platinum City 2 miles south of Centennial along the Railroad.  The promoter was indicated in mail fraud related to the exaggerated stories of unusually rich platinum ore.

STOP 1 INDEPENDENCE MINE: (1) Note Shear zones in gossan, (2) point out contact of gossan with country rock, (3) Note Graphite (4) Note quartz boudins with potassic alteration in boulder on mine dump (5) examine mylonites in hornblende gneiss.
The Independence mine was driven on a N35°W trend intersecting 5 small shear zones within 80 ft of the portal.  A 110-ft drift was driven on one of these.  It is interesting to note that samples collected in the shears contained only trace Au, and some samples in the altered wallrock contained anomalous Au.  This suggests that the primary mineralizing event occurred prior to shearing and later remobilization of mineralization was relatively insignificant.
Some early assays indicated anomalous PGMs were identified.  One reported 0.3 opt Pt, 0.5 opt Ir & 0.1 opt Os. 
STOP 2 CLIFF MINE: (1) Note boulder on concrete w/folds (2) point out mafic igneous country rock, (3) Note mine dump segregation.
NORTH (1) Point out cabin sites (at least 7 structures) and outhouse (2) examine garbage dump and age of cans (3) examine prospect pits in river gravel.
SOUTH (1) Drainage tunnel (2) mill site (3) large boulder with quartz vein, (4) recent mining claim cut in tree, (5) Middle Fork placers.
Queen mine headframe as it appeared before it was cut down.
The Cliff mine was developed by a 775-ft tunnel that followed schistosity.  Near the end of the main tunnel, a 325-ft crosscut ran NE intersecting 4 quartz- and sulfide-bearing veins hosted by mafic schist and submylonite.
The 1st mineralized zone was intersected a few feet from the main drift and was a quartz-sulfide vein that yielded gold values of 0.6 opt Au over 15-ft.  The #3 vein, located 140 feet from the main drift, was reported to be anomalous in Pt.
Tin Cans: Hole in the top soldered cans – these were phased out in 1904.  (Side seam cans with solder, phased out in 1887, without solder, dated after 1887).

Middle Fork Placers.  A large flat area located a short distance downstream from the Independence and Cliff gold mines occurs along the Middle Fork of the Little Laramie.  The drainage cuts across country rock foliation, several veins & shear zones.  Because it lies immediately downstream from several mineralized veins, this placer should contain some Au.  But there is little evidence of much past activity and this might be a good placer to prospect.

STOP 3 QUEEN MILL RUN PLACER: This lies high on Centennial Ridge between 9,400 feet to 9,600 ft.  Portions of the placer were probably mined in the 1930s.  The placer lies at the top of a ridge, paralleling country rock foliation.  Historic reports indicate that the placer yielded several nuggets weighing about 0.1 oz with others up to 0.25 oz.  Since the placer was mined to the top of the ridge paralleling foliation, this may suggest that it lies on top of a hidden gold vein.  Unmined ground can be found between the mined areas.
STOP 4 QUEEN MINE: (1) Examine chloritized specimen of amphibolite on winch (2) Examine chloritized and sulfide stained prospect pit to the east of the shaft, (3) Mention boulders in the road.

Sunk to a depth of 90 ft in fault gouge in amphibolitized metapyroxenite.  Exposed several small faults and veins of calcite and pegmatite.  Some of the faults contained sulfides and calcite.  A sample of fault gouge collected in 1926 assayed 0.03 opt Pt, 0.05 opt Ir and a trace Au.
Folds in French Creek slate
STOP 5 - FRENCH SLATE (Upper Proterozoic): (1) Examine fold closure w/qtz vein (2) Examine refolded isoclinal fold, (3) Note eratics.

Immediately north of the Mullen Creek-Nash Fork shear zone.  Movement along the MC-NF shear zone occurred at about 1.6 to 1.7 Ga.
Note the stromatalite erratic from the Nash Fork Formation that was formed in a reef in an ancient ocean more than 1.7 Ga.  Stomatalites were deposited by the influence of blue-green agae.  At least three reefs have been mapped in the Sugarloaf Recreation area.
STOP 6 - WEST LAKE MARIE PARKING LOT: (1) Examine Sugarloaf Quartzite and Lookout Schist (2) Examine fold closure w/quartz vein near stonehouse, (3) Examine quartz pebble conglomerate boulders of Medicine Peak Quartzite.

Lookout Schist (Early Proterozoic). Medicine Peak Quartzite contains minor sericite, kyanite, and tourmaline.  Some boulders have cross-beds and occasion quartz pebbles indicating that it was deposited in a river or subtital portions of a delta or estuary.    Paleocurrent directions indicate transport was from the northeast.  Small-scale oscillation ripples indicate portions were deposited as sand in shallow water along the edge of the sea.

The high ridge to the north is composed of Sugarloaf Quartzite and the skyline by Medicine Peak Quartzite