Geology and Ore

 

Deposits of the La Plata District Colorado

 

 

By EDWIN B. ECKEL with sections by J. S. WILLIAMS

                                                             F. W. GALBRAITH and others

GEOLOGICAL      SURVEY       PROFESSIONAL     PAPER    219

 

 

 

Prepared in cooperation with the

Colorado State Geological Survey Board

And the Colorado Metal Mining Fund

 

 

 

(RED ARROW SEGMENT)

 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1949

 

 

      Red Arrow (2C) History and Production

 

The Red Arrow mine, the most recent discovery of commercial importance in the La Plata district, is on the west side of Gold Run, 800 feet northwest of its junction with the East Mancos River and at an altitude of about 9,350 feet. It is about 8 miles from Mancos, with which it is connected by a good county road. The discovery, made by Raymond and Charles Starr on June 3, 1933, attracted wide attention in newspaper accounts, and led to a feverish but comparatively short-lived boom in the East Mancos sector. While panning small gravel bars along Gold Run, the Starrs obtained spectacular showings of coarse gold in the creek, just below the crossing of a small fissure. Their effort to trace the gold to its source resulted in the discovery of a comparatively small outcrop of oxidized vein material that contained coarse gold. This outcrop was explored by a small open-cut, and a drift adit was subsequently opened just below it.

 

Soon after the discovery, the Red Arrow Gold Corp., composed largely of local owners, was formed. The company was almost immediately involved in a suit brought by the Starrs' former partner. This litigation, which was finally settled in 1936 in favor of the defendants, undoubtedly acted as a check on exploration and production for several years. As shown in the following table, the mine produced nearly $150,000 in gold, silver, and copper in the years 1933-37, thus becoming, in a space of five years, the sixth largest producer in the district. According to local newspapers, the total expense in 1937 amounted to about $42,000, against a production of about $70.000.

 

                                          

                                   Production of Red Arrow mine, 1933-43

 

 

YEAR

ORE

GOLD

SILVER

COPPER

 

(short tons)

(fine ounce

(fine ounce

(lbs., wet assay)

1933

21

169.65

129

---------

1934

124

687.44

772

600

1935

81

270.00

562

438

1936

180

1,058.26

3,183

1,850

1937

215

1,929.15

2,382

1,945

TOTAL

621

4,114.50

6,928

----------

 
Compiled from mine records of the U.S. Bureau of Mines.
No production for years not listed.

 

 

The vein is developed principally by means of two drifts and several raises and stopes. As the eastern part of the Red Arrow vein outcrop lies on the older Gold Run patented claim, where it is worked in the Outwest mine, the lower drift is reached by means of a crosscut on Red Arrow ground, instead of by the shorter course that could be taken from the Gold run claim. By 1939 the Red Arrow and Outwest workings together had explored the vein for more than 1,000 feet along its strike and more than 300 feet vertically.

 

                                                  

                                        GEOLOGIC RELATIONS

 

The rocks in the vicinity of the Red Arrow include all the beds from Entrada sandstone to the top of the Morrison formation and are unmetamorphosed. The beds, which are very poorly exposed, dip 10 to 15 degrees SW.

 

The Red Arrow vein occupies one of a series of discontinuous normal faults that strike nearly due east and dip south. They are believed to represent a westward extension of the Parrott fault system, and there are probably several more in addition to the three shown in plate 2. The displacement along the Red Arrow fault amounts to about 40 feet and the other faults appear to have displacements of 5 to 30 feet. The vein has not been definitely identified east of Gold Run, nor could its extension west of the mine be traced with certainty by members of the writer's party for more than a few hundred feet; the mine management, however, feels confident that it extends several thousand feet in this direction.

 

At the mine the Red Arrow vein strikes N. 82 degrees E. to N. 87 degrees W. and dips 55 degrees to 70 degrees S. The base of the Morrison is about 10 feet below the main drift on the south wall of the vein, and 30 feet above the drift on the north wall. The beds consist in large part of greenish sandstone with many partings of soft green shale. The under lying Junction Creek sandstone is of normal aspect.

 

On the main level, where the footwall is sandstone and the hanging wall is shaly sandstone of the Morrison, the vein occupies a 1 foot to 5 foot breccia zone that averages 3 feet in width. The footwall is well defined and bears vertical slickensides in several places. The hanging wall is nearly as well defined, but locally the wall rock on that side is strongly seamed and fractured. Twenty-five to thirty feet above the floor of the drift, where the shaly beds form both walls, the vein pinches markedly. Its width and character on the lower crosscut level, where both walls are sandstone, have not been disclosed, but in the Outwest mine, at about the same altitude, it is about as wide as in the upper Red Arrow workings.

 

On and above the main level the greater part of the vein is characterized by brecciated and silicified sandstone. The breccia contains a little pyrite and appreciable quantities of finely divided free gold but is not known to have yielded any high-grade ore.

 

The rich ore for which Red Arrow became noted lies almost exclusively on the footwall of the vein, though similar ore occurs in a few places along the hanging wall. The footwall streak ranges from less than an inch to 3 feet in thickness and probably averages 8 or 10 inches. Part of the typical ore is friable and oxidized and consists of interlocking fine to coarse crystals of barite, enclosing some fragments of country rock. The barite crystals are fractured and stained with limonite and with carbonates of copper. Vugs and interstices are filled with limonite and a little clay. Locally, small quantities of pyrite, chalcopyrite, tetrahedrite, and chalcocite are present.

 

                                            

                                                       ORE DEPOSITS

 

Native gold is associated with the limonitic material. It occurs in platy or foliated forms that range in size from tiny flakes to large masses weighing several pounds. The largest "nugget" that has been reported weighed more than five pounds. Most of the masses are rough and irregular in shape. On a few of them wires and small octahedral crystals were seen. Tiny crystals of tetrahedrite are perched on some of the gold crystals. Some of the coarse gold is reported to be 880 fine, but some of it is said to be alloyed with copper and to be only about 500 fine. Some of the ore is very rich. One small sheet, where the vein was 8 inches wide, is said to have yielded 30 pounds of gold nuggets and thirty-six 100 hundred pound sacks of 32 ounce ore from a stope 35 feet high and 10 feet long.

 

In several places a streak of sulfide ore from half an inch to 3 inches wide lies along the footwall side of the gold-bearing barite streak. This ore consists largely of pyrite in a gangue of barite, ankerite, and quartz. Small quantities of tetrahedrite, chalcopyrite, and barite, which appear to be nearly contemporaneous, are associated with the pyrite. Locally, sphalerite and galena are also present. This sulfide ore commonly contains one or more ounces of free gold to the ton. In several high-grade specimens that were examined the gold is intimately inter-grown with tetrahedrite. A little hessite is associated with tetrahedrite in a single specimen, but tellurides are rare or absent throughout the deposit.

 

In mining, the low-grade silicified breccia is first removed, after which the high-grade streak and, if present, the sulfide streak are scaled off by hand. Careful hand-sorting is practiced. Much of the product has been shipped directly to the mint, but some has been amalgamated and retorted in a small mill at the mine. Some of the better-grade breccia and sulfide ore has been shipped crude to the smelter, but the greater part of the production shown in the foregoing table probably represents coarse free gold.

 

The scanty data that is available is not adequate to determine the origin of the coarse gold or to justify a confident statement as to the probable extent of the ore shoot. The close association of gold and tetrahedrite in many places, and the tetrahedrite crystals perched on crystals of gold, are clear evidence that some of the gold is primary. It seems probable, indeed, that all of it is primary, though some �nuggets� may have been slightly enlarged during oxidation of the deposit. Available data on this and other mines in the district seem to indicate that further exploration of the lower part of the Junction Creek sandstone, of the limestone member of the Wanakah, and of the Entrada sandstone would be fully justified. If any ore bodies exist in the marl member of the Wanakah, they are likely to be small and erratic, but the base of this member may well have formed a structural trap for the ore-bearing solutions.

 

 

OUTWEST

 

The Outwest mine, operated by the Outwest Mining Co., is on the eastward extension of the Red Arrow vein and traverses the patented Gold Run placer claim. It was first opened shortly after the Red Arrow discovery in 1933. When the mine was examined in 1936 the only opening on the property was a 145-foot drift adit, 115 feet below the main Red Arrow adit. A 20-foot winze had been sunk 10 feet from the breast of the adit. Forty tons of ore recovered from development workings and from a small stope at the bottom of the winze yielded 387.02 ounces of gold, 189 ounces of silver, and 205 pounds of copper. The mine was idle in 1937 but was active in 1938-39. A lower adit was then driven on the vein, and stopes were opened both above and below the upper level. The vein is similar in all essential respects to that in the Red Arrow mine and is characterized by coarse free gold, chalcopyrite, tetrahedrite, and barite. The wall rocks belong to the Junction Creek sandstone.