3
Von Bargen (1979) examined the silver- antimony-
mercury system using laboratory experiments, as well as
the mineralogy of the Black Hawk district. Much more
detailed mineralogy was conducted, with descriptions of
four primary and two secondary mineral bodies. These
include descriptions of native elements, arsenides, sulfo-
salts, sulfides, and many secondary phases. The same phases
as described in Gillerman and Whitebread (1953) are pres-
ent, including the aforementioned smaltite and chloan-
thite, another discredited phase bravoite, and gersdorffite,
pearceite, mckinstryite, siegenite, and more.
Gerwe (1986) examined the overall geology of the
deposit and used mineralogy and fluid inclusion data to
estimate depth of emplacement. Whole rock geochemistry
was analyzed, showing anomalously high values for cobalt
and nickel in the host rock. K/Ar dating was used to date the
argillic alteration assemblage to about 65 ma.. Coupling the
geology, fluid inclusions, and geochemistry, Gerwe (1986)
proposed that the deposit was associated with a Laramide
epithermal system that was deeper and hotter than typical
epithermal systems. Gerwe (1986) describes similarities to
other five-element systems as similar carbonate vein occur-
rences in Precambrian plutonic rocks, as well as early sul-
fide phases and similar silver mineralization temperatures.
Differences included variable ages of mineralization, depth
of emplacement, and composition of host rock.
SIGNIFICANCE
This study is significant due to the growing interest in
critical minerals worldwide, and the unusual nature of this
deposit type. The Black Hawk district is one of a handful
of five-element deposits, and of nickel and cobalt miner-
alization, in the United States. The nature of these depos-
its being pods of mineralization in carbonate veins makes
discovering continuing trends of mineralization difficult.
However, ore fluid evolution and precipitation conditions
will lead to a better understanding of where and why these
deposits occur and add constraints to the extent of their
mineralization.
Nickel, cobalt, bismuth, zinc, and arsenic are all listed
as critical minerals according to the 2022 USGS listing,
Burton (2022), and are known to occur in the Black Hawk
district. Uranium is also present, though it is no longer
listed as a critical mineral. Many elements reported to occur
in these deposit styles are rarely concentrated to this magni-
tude in other deposits styles. A greater understanding of the
geochemical relationships between the unusual mineralogy
and the fluids that precipitated them will help facilitate a
better understanding of this deposit style, as well as benefit
the search for critical minerals in New Mexico and around
the world.
METHODS
Mineralogical samples will be collected from dumps, core,
and any other available sources. We also have obtained a
suite of mineralized samples from Santa Fe Gold from the
Black Hawk mine. Available drill core will be examined and
sampled. Samples from my personal collection are available
from Black Hawk and other worldwide localities. These
samples will be examined petrographically in polished
section as well as by X-ray powder diffraction, scanning
electron microscope, and electron microprobe for mineral
identification and mineral chemistry. Mineral chemistry
will be correlated with mineral paragenesis to identify rela-
tionships to deposit formation. Mineralogy will be corre-
lated between deposits to improve understanding of ore
forming fluids.
PRELIMINARY RESULTS
The mineralogy of the native metal-arsenide systems of
the Black Hawk District of New Mexico and the Cobalt-
Gowganda Region of Ontario, Canada, are explored below
using reflected light petrography and electron microprobe
analysis. Both deposits show similar overall mineralogies
and primary mineral textures. This is characterized by early
precipitation of native metals as dendritic, skeletal, and
massive aggregates, followed by Ni-Co-Fe arsenides as co-
precipitates and rims in and around the native metals.
Overall, both deposits share a remarkably similar min-
eral assemblage, while some major elements are abundant
in the Cobalt district, they are not observed at Black Hawk.
Early observations show the predominance of native silver
and Ni-Co-Fe arsenides in both deposits. These arsenides
include nickeline, skutterudite, and several that need fur-
ther characterization to accurately identify. As its name sug-
gests, the Cobalt deposit is dominated by cobalt, though
there is still a significant nickel presence. Also observed is
native bismuth, and several antimony rich phases, dyscra-
site and breithauptite. This enrichment of bismuth and
antimony is not observed at Black Hawk, however. Black
Hawk shows a greater abundance of nickel than cobalt, as
seen in the arsenides.
Below are select reflected light and back- scattered
electron (BSE) images of samples from Black Hawk and
Cobalt, Ontario.
Von Bargen (1979) examined the silver- antimony-
mercury system using laboratory experiments, as well as
the mineralogy of the Black Hawk district. Much more
detailed mineralogy was conducted, with descriptions of
four primary and two secondary mineral bodies. These
include descriptions of native elements, arsenides, sulfo-
salts, sulfides, and many secondary phases. The same phases
as described in Gillerman and Whitebread (1953) are pres-
ent, including the aforementioned smaltite and chloan-
thite, another discredited phase bravoite, and gersdorffite,
pearceite, mckinstryite, siegenite, and more.
Gerwe (1986) examined the overall geology of the
deposit and used mineralogy and fluid inclusion data to
estimate depth of emplacement. Whole rock geochemistry
was analyzed, showing anomalously high values for cobalt
and nickel in the host rock. K/Ar dating was used to date the
argillic alteration assemblage to about 65 ma.. Coupling the
geology, fluid inclusions, and geochemistry, Gerwe (1986)
proposed that the deposit was associated with a Laramide
epithermal system that was deeper and hotter than typical
epithermal systems. Gerwe (1986) describes similarities to
other five-element systems as similar carbonate vein occur-
rences in Precambrian plutonic rocks, as well as early sul-
fide phases and similar silver mineralization temperatures.
Differences included variable ages of mineralization, depth
of emplacement, and composition of host rock.
SIGNIFICANCE
This study is significant due to the growing interest in
critical minerals worldwide, and the unusual nature of this
deposit type. The Black Hawk district is one of a handful
of five-element deposits, and of nickel and cobalt miner-
alization, in the United States. The nature of these depos-
its being pods of mineralization in carbonate veins makes
discovering continuing trends of mineralization difficult.
However, ore fluid evolution and precipitation conditions
will lead to a better understanding of where and why these
deposits occur and add constraints to the extent of their
mineralization.
Nickel, cobalt, bismuth, zinc, and arsenic are all listed
as critical minerals according to the 2022 USGS listing,
Burton (2022), and are known to occur in the Black Hawk
district. Uranium is also present, though it is no longer
listed as a critical mineral. Many elements reported to occur
in these deposit styles are rarely concentrated to this magni-
tude in other deposits styles. A greater understanding of the
geochemical relationships between the unusual mineralogy
and the fluids that precipitated them will help facilitate a
better understanding of this deposit style, as well as benefit
the search for critical minerals in New Mexico and around
the world.
METHODS
Mineralogical samples will be collected from dumps, core,
and any other available sources. We also have obtained a
suite of mineralized samples from Santa Fe Gold from the
Black Hawk mine. Available drill core will be examined and
sampled. Samples from my personal collection are available
from Black Hawk and other worldwide localities. These
samples will be examined petrographically in polished
section as well as by X-ray powder diffraction, scanning
electron microscope, and electron microprobe for mineral
identification and mineral chemistry. Mineral chemistry
will be correlated with mineral paragenesis to identify rela-
tionships to deposit formation. Mineralogy will be corre-
lated between deposits to improve understanding of ore
forming fluids.
PRELIMINARY RESULTS
The mineralogy of the native metal-arsenide systems of
the Black Hawk District of New Mexico and the Cobalt-
Gowganda Region of Ontario, Canada, are explored below
using reflected light petrography and electron microprobe
analysis. Both deposits show similar overall mineralogies
and primary mineral textures. This is characterized by early
precipitation of native metals as dendritic, skeletal, and
massive aggregates, followed by Ni-Co-Fe arsenides as co-
precipitates and rims in and around the native metals.
Overall, both deposits share a remarkably similar min-
eral assemblage, while some major elements are abundant
in the Cobalt district, they are not observed at Black Hawk.
Early observations show the predominance of native silver
and Ni-Co-Fe arsenides in both deposits. These arsenides
include nickeline, skutterudite, and several that need fur-
ther characterization to accurately identify. As its name sug-
gests, the Cobalt deposit is dominated by cobalt, though
there is still a significant nickel presence. Also observed is
native bismuth, and several antimony rich phases, dyscra-
site and breithauptite. This enrichment of bismuth and
antimony is not observed at Black Hawk, however. Black
Hawk shows a greater abundance of nickel than cobalt, as
seen in the arsenides.
Below are select reflected light and back- scattered
electron (BSE) images of samples from Black Hawk and
Cobalt, Ontario.