During my time in Tucson at the 2015 shows I took a
break from looking for treasures and headed off on a road trip north toward the
community of Hayden, about 70 miles.
That particular area is well known for a variety of metal-producing
mines and spectacular collector minerals. Today, Hayden is the home of smelting
operations for ASARCO (American Smelting and Refining Company), a major mining,
smelting and refining company and a subsidiary of Grupo Mexico. ASARCO is a major copper producer in the U.S.
and has three open pit mines near Tucson:
Mission Mine, Ray Mine, and the Silver Belle Mine. It also has ~20 Superfund sites scattered across
the country. I have collected copper
nuggets and chrysocolla from the Ray mines---on a sponsored field trip, and
visited the Mission Mine from a vantage point.
I could only observe the Silver Bell mine from a distance—no rockhounds
allowed :)
The ASARCO web site noted the Hayden Operations
consists of a 27,400 ton/day concentrator and a 720,000 ton/year copper smelter,
and processes ore from the nearby Ray Mine. Anodes produced at the smelter are then
shipped to the Amarillo, Texas, Copper Refinery. The sulfuric acid produced at
the acid plant is used in the leaching operations or sold into the market. I believe that the Hayden smelter is one of
only three? operating copper smelters left in the U.S.
One of the more famous mines in the Hayden area is
the 79 Mine in the Dripping Springs Mountains.
A former underground Pb-Zn-Cu-Ag-Au-Mo-Sb-V-Fe mine, the operation
produced ore (starting ~1879) until about the middle of the 20th
century. Since then the mine has
produced collector specimens---on a sporadic basis. At almost any rock show in Arizona, and in
virtually every rock shop, specimens from the79 Mine may be found “for sale.” Evidently thousands of mineral specimens have
survived from the mine. MinDat lists 74
valid minerals collected from the 79 Mine including classic butterscotch
wulfenite and blue hemimorphite. I had
hoped that perhaps interested rockhounds could get into the area and examine
the dumps, but alas, locked gates.
The 79 Mine includes numerous surface works, the
main incline, and in excess of 3000 m of tunnels and stopes. The oldest rock exposed in the Hayden area is
the Proterozoic Precambrian Mescal Formation of the Apache Group. Above
this unit are several thousand feet of Paleozoic rocks (Cambrian to
Pennsylvanian). The major ore body is over 300 meters long and a dozen
meters wide and occurs as replacements in the Naco Limestone (Pennsylvanian)
and a dike of rhyolite porphyry. The mineralization is most likely Laramide
(late Cretaceous and early Tertiary) in age (Keith, 1972).
In examining trays of minerals at one of the Tucson
show venues I came across a single specimen containing, what appeared to the
naked eye, as a clutch of metallic looking tiny fish eggs! It was labeled murdochite (unknown to me at
the time) from the 79 Mine so I scooped it up for $3.
The original discovery of murdochite, in the 1950s,
was in rocks of the Mammoth-St. Anthony Mine not many miles south of the 79
Mine where “tiny black octahedra of murdochite are found on the surface of and
embedded within plates of wulfenite and on the surface of crystals of fluorite”
(Fahey, 1955). The mineral seems interesting to me since it contains both
chlorine and bromine. MinDat lists the
chemical formula as PbCu6O8-x(Cl,Br)2x where
x<=0.5).
Mass of murdochite appearing as a druze on a limonite
matrix. However, it is not a druze but
hundreds of tiny octahedral crystals.
Width of specimens ~1.3 cm.
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Murdochite is usually black in color, a metallic
black, with a metallic to submetallic luster; however, the crystal faces
reflect light quite nicely and appear adamantine. The mass of “fish eggs” is
actually a mass of tiny octahedral or cubic octahedral crystals. Hardness is rated at ~4 (Mohs) and when
rubbed on an unglazed porcelain plate, murdochite gives off a black streak. Like other metallic luster minerals,
murdochite is opaque.
Murdochite is a secondary mineral found in the
oxidized zones of copper-lead deposits. According to MinDat the primary
hypozone lead mineral at the 79 Mine seems to be galena and the oxidized zone
includes secondary lead minerals such as cerussite and anglesite (Eastlick,
1968). The copper primary minerals
include the sulfide chalcopyrite and perhaps it provided copper for the several
secondary minerals. Since secondary murdochite
includes both copper and lead I suppose the metals must/might have oxidized
from solutions passing through this sulfide.
But again, ore mineralogy is certainly not my forte!
And one final note, synthetic metallic oxides whose
compositions are inspired by the structure of murdochite “exhibit interesting
resistive properties which sound for the possible onset of superconductivity
near room temperature. (Djurek and others, 1990).
REFERENCES CITED
Djurek, D., V. Manojlovic,
Z. Medunic, N. Martinic, Paljevic, 1990,
Cu-Pb-Ag-O system as a possible superconductor at T>200 K: Journal of
the Less Common Metals, v.164-165, pt. 2.
Fahey, J.J., 1955, Murdochite, a new copper lead
oxide mineral: American Mineralogist: v. 40.
Keith, S.B., 1972, Mineralogy and paragenesis of the
79 Mine lead-zinc-copper deposit: Mineralogical Record v. 3.
Synthetic and natural PbCu6O8 has a clathrat-like structure, I red in a pdf-file which I found in the WWW. The structure consists of CuO4 plaquettes building cages.
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