GENERAL33464

• Permit No: M1983141
• IBM Index Class Name: General Documents
• Doc Date: 7/12/1994
• From: COM INC
• To: MLRD
• Media Type: D
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SAMESTOWN DISTRICT
Finely disseminated pyrite is common in silicified wall
rock and in sericitized wall rock bordering the ore bodies
or the wider parts of veins. Ore bodies are usually bor-
dered by relatively wide zones of strongly sericitized
wall rock or by silicified wall rock, both being com-
monly accompanied by disseminated pyrite, but these
conditions do not invariably indicate proximity to ore.
In the fluorspar deposits, clay minerals, chiefly hydrous
mica and nontronite(?) as well as sericite, are mixed
with the ore and were apparently derived from alter~-
tion of the crushed and brecciated granite.
Supergcne enrichment.-The greater part of the ore
mined in the district has been primary ore, but in sonce
of the mines a noteworthy amount of secondary ora
formed by supergene enrichment has been mined. Most
of the enrichment has been limited to the oxidized
zone and is due to the leaching of soluble material, such
as pyrite, chalcopyrite, and tellurimn, the gold being
left behind. By this process of residual enrichment
many of the low-grade pyritic gold veins became of
cmnmercial grade near the surface. The oxidized parts
of the telluride veins have been somewhat enriched, bnt
in many of the telluride veins primary minerals remain
within a few feet of the surface. Complete oxidation
along [he veins extends to depths ranging from 5 to GO
feet beneath the surface, and partial oxidation has ex-
tended to depths of GO and even 120 feet, The depth is
largely dependent on thr openness of the vein and mr
the topography and is greatest beneath flat areas that
are remnants of old erosion surfaces. In the lead-silver
deposits evidence of supergene enrichment is conspicu-
ous only for 10 to 20 feet below the surface, but even
there oxidation is not complete. In this zone galena
has been altered to argentiferous cerussite and the copper
minerals to azurite and malachite. Some sooty chalco-
cite of supergene origin is found at n depth of 50 feet
in the Argo mine but has not been noted more than 100
feet below the surface anywhere in the district.
Structural control of are bodies.-1'he general distri-
bution of ore deposits in the district seems dependent
on a variety of factors. The fissures and breccia zones
occupied by the fluorspar and early lend-silver deposits
are believed to be related to forces accompanying the
intrusion of the sodic granite-quartz monzonite por-
phyry stock a' Many of the nortlreastw'ard-trending
fissures occupied by later veins appear to have been
first opened by forces accompanying the intrusion of
the granodiorite stock,though they were later reopened
by other forces. lfany of the productive veins are
grouped close to the strong breccia-reef faults, and it
seems probable that these faults served as deep circu-
lating channels for some of the ore-forming solutions.
Large solid areas of strong rocks, such as granite and
granodiorite, seem to have resisted the forces that
Goddnrd, E. N., op. ci [.. DP, 950-385.
265
formed vein Sssures, as almost no veins of importance
are found in such areas. On the other hand, in large
schist areas, the sclrists and gneisses yielded readily to
the stresses, but irI most places the faulting was parallel
to the foliation, and tight gongy faults were fot•med
rather than open fissures. However, in areas where the
schist and gneiss 1ral-e been intimately injected by gran-
ite the rock is more competent, and open fractures ten•1
to form, especially in the granite layers. Thus we find
that nearly all the important ore [leposits in the district
are limited to areas of mixed schist and granite.
The local distribution of ore within the veins depends
largely mr three factors. Listed in the order of their
importance, they are: The presence of vein jmrctious,
the irregularity of the veins, and the physical character
of the wall rock. These factors are illustrated in fig-
ure 79. Vein jmrctions are by far the most important,
and fully 75 percent of the output in the district has
come from vein junctions of one type or another. The
most effective type of junction is tlrnt in which one vein
cuts across arr enr•]y fault or vein; the principal ore
bodies in the Smuggler and $uena mines occur at s•rch
junctions. Splits or junctions of contemporaneous
veins have also been favorable places, for example, in
the John Jay and Golden Age mines. In many of the
veins or•e bodies ar•e found at places where the vein
takes an abrupt change in dip or strike, as discussed on
Page 95, and numerous ore bodies nre also found Ivhere
the wall rock changes from schist to granite, or from
either schist or• granite to porphyry. All these factors
tended to produce openings that were available for the
deposition of ore, and the most ideal conditions were
combinations of three factors, as is well illustrated irr
the "big stope" ore body of the $uena mine (fig. 80).
B UENA ]FINE (COLD-TELLUnIDE)
The $uena mine, the most productive in the district,
differs from dre other mines in having a large number
of veins striking in various directions and in having ore
in large stockwork bodies at vein jmrrtions.
The mine is on the south side of a steep gulch, 2,000
feet east of the top of Buena \lountain and about a mile
N. 70° lI'. of Jamestown. The workings range in alti-
tude from i,500 to 5,050 feet and comprise seven levels,
with two extensive crosscut tunnels, one small crosscut,
and numerous rai=_es, winzes, open cuts,:uul small shafts,
totaling about G,000 feet. The mine was discovered in
September 1579 and is generally credited with a total
output valued at about X2,000,000. The output from
1901 to 19.12 has amounted to 3G,794 ounces ofo°ld, 2,725
ounces of silver, and 205 pounds of copper, having a
total value of about ~ 1,154,000.
The Buena workings are in coarse-grained Sih•er
Plume granite, which contains roughly rectangular
blocks of black fine-grained biotite schist a few feet to