2
AGE OF MINING DISTRICTS AND
ASSOCIATED PLUTONS
Previous geochronology data for Laramide or potentially
Laramide plutons and associated mining districts in New
Mexico suggests two to three pulses of magmatism (Table 1,
Figure 2). The first initial pulse is from ~75 to 71 Ma, as
seen by ages for Black Hawk, Hillsboro, Piños Altos, Little
Hatchet Mountains, and Georgetown. The next pulse is
from ~58–54 Ma, possibly extending into ~50 Ma, which
includes ages for Lordsburg, Santa Rita, Fierro-Hanover,
Copper Flat (Grant County), Burro Mountains, Lone
Mountain, and potentially Tres Hermanas. Ages younger
than ~45 Ma are likely mid-Tertiary and are a result of
extensional magmatism, such as those for Cookes Peak and
Old Hadley, Camel Mountain, Tres Hermanas, San Simon,
McGhee Peak, Apache No. 2, and Fremont.
Ages of plutons that produced mineralized deposits
are correlated with episodes of arc magmatism in south-
west New Mexico. Both McMillan (2004) [4] and Amato
et al. (2017) [5] report three episodes of arc magmatism
in southwest New Mexico. A lack of mineralized deposits
exists between 70–60 Ma in New Mexico, but exists else-
where such as in Arizona [2]. The lack of magmatism and
mineralized deposits in New Mexico could be due to a lack
of outcropping plutons as compared to Arizona or a quies-
cence of magmatism in the specific area.
Table 1. Mining districts with current estimated age and
dating method. “?” indicates age may be incorrect
District
Age
(Ma) Method Reference
Hillsboro 75 40 Ar/39Ar McLemore et al.
(1999)
Black Hawk 75.7,
72.5
U-Pb, K-Ar Amato et al. (2017),
Hedlund (1980)
Piños Altos 74.4 K-Ar McDowell (1971)
Eureka and
Sylvanite (Little
Hatchet Mtns)
71.4 40Ar/39Ar Lawton et al. (1993)
Georgetown 71 40 Ar/39Ar McLemore (1998)
Lordsburg 58.5 40Ar/39Ar McLemore et al.
(2000)
Santa Rita
(Chino mine)
58.3 40Ar/39Ar Heizler et al.
unpublished
Fierro-Hanover 57.55 40Ar/39Ar McLemore et al.
(1995)
Copper Flat
(Grant County)
55.4 K-Ar NMBGMR
unpublished data
Burro
Mountains
(Tyrone mine)
54.5 40Ar/39Ar Heizler et al.
unpublished
Lone Mountain 51.5 K-Ar P.B. Hubbard
and P.G. Dunn
Unpublished
Cooks Peak and
Old Hadley
38.95 40Ar/39Ar Schwenk
unpublished
Camel
Mountain –
Eagle’s Nest
36.8,
86.3?
40 Ar/39Ar McLemore et al.
(2001), NMBGMR
unpublished data
Tres Hermanas 34.65,
50.3?
40Ar/39Ar,
K-Ar
McLemore et al.
(2001), Leonard
(1982)
San Simon 33.2 40Ar/39Ar McLemore et al.
(1995)
McGhee Peak 32.5 K-Ar Hoggat et al. (1977)
Apache No. 2
and Fremont
30.66 K-Ar Deal et al. (1978)
SAMPLING AND METHODS
New ages for some districts were obtained using mod-
ern, high-precision 40 Ar/39Ar geochronology performed
at the New Mexico Geochronology Research Laboratory
(NMGRL). Samples were collected at the Camel Mountain
– Eagle’s Nest, Hillsboro, Tres Hermanas, Black Hawk,
Eureka, Sylvanite, McGhee Peak, Lordsburg, and Piños
Altos districts. Samples of fresh, least altered intrusive
rocks were collected for geochronology, as well as for whole
rock and trace element geochemistry and thin section
Figure 1. Map of suspected Laramide-age mining districts
and plutons in SW NM. Districts are from McLemore (2017)
AGE OF MINING DISTRICTS AND
ASSOCIATED PLUTONS
Previous geochronology data for Laramide or potentially
Laramide plutons and associated mining districts in New
Mexico suggests two to three pulses of magmatism (Table 1,
Figure 2). The first initial pulse is from ~75 to 71 Ma, as
seen by ages for Black Hawk, Hillsboro, Piños Altos, Little
Hatchet Mountains, and Georgetown. The next pulse is
from ~58–54 Ma, possibly extending into ~50 Ma, which
includes ages for Lordsburg, Santa Rita, Fierro-Hanover,
Copper Flat (Grant County), Burro Mountains, Lone
Mountain, and potentially Tres Hermanas. Ages younger
than ~45 Ma are likely mid-Tertiary and are a result of
extensional magmatism, such as those for Cookes Peak and
Old Hadley, Camel Mountain, Tres Hermanas, San Simon,
McGhee Peak, Apache No. 2, and Fremont.
Ages of plutons that produced mineralized deposits
are correlated with episodes of arc magmatism in south-
west New Mexico. Both McMillan (2004) [4] and Amato
et al. (2017) [5] report three episodes of arc magmatism
in southwest New Mexico. A lack of mineralized deposits
exists between 70–60 Ma in New Mexico, but exists else-
where such as in Arizona [2]. The lack of magmatism and
mineralized deposits in New Mexico could be due to a lack
of outcropping plutons as compared to Arizona or a quies-
cence of magmatism in the specific area.
Table 1. Mining districts with current estimated age and
dating method. “?” indicates age may be incorrect
District
Age
(Ma) Method Reference
Hillsboro 75 40 Ar/39Ar McLemore et al.
(1999)
Black Hawk 75.7,
72.5
U-Pb, K-Ar Amato et al. (2017),
Hedlund (1980)
Piños Altos 74.4 K-Ar McDowell (1971)
Eureka and
Sylvanite (Little
Hatchet Mtns)
71.4 40Ar/39Ar Lawton et al. (1993)
Georgetown 71 40 Ar/39Ar McLemore (1998)
Lordsburg 58.5 40Ar/39Ar McLemore et al.
(2000)
Santa Rita
(Chino mine)
58.3 40Ar/39Ar Heizler et al.
unpublished
Fierro-Hanover 57.55 40Ar/39Ar McLemore et al.
(1995)
Copper Flat
(Grant County)
55.4 K-Ar NMBGMR
unpublished data
Burro
Mountains
(Tyrone mine)
54.5 40Ar/39Ar Heizler et al.
unpublished
Lone Mountain 51.5 K-Ar P.B. Hubbard
and P.G. Dunn
Unpublished
Cooks Peak and
Old Hadley
38.95 40Ar/39Ar Schwenk
unpublished
Camel
Mountain –
Eagle’s Nest
36.8,
86.3?
40 Ar/39Ar McLemore et al.
(2001), NMBGMR
unpublished data
Tres Hermanas 34.65,
50.3?
40Ar/39Ar,
K-Ar
McLemore et al.
(2001), Leonard
(1982)
San Simon 33.2 40Ar/39Ar McLemore et al.
(1995)
McGhee Peak 32.5 K-Ar Hoggat et al. (1977)
Apache No. 2
and Fremont
30.66 K-Ar Deal et al. (1978)
SAMPLING AND METHODS
New ages for some districts were obtained using mod-
ern, high-precision 40 Ar/39Ar geochronology performed
at the New Mexico Geochronology Research Laboratory
(NMGRL). Samples were collected at the Camel Mountain
– Eagle’s Nest, Hillsboro, Tres Hermanas, Black Hawk,
Eureka, Sylvanite, McGhee Peak, Lordsburg, and Piños
Altos districts. Samples of fresh, least altered intrusive
rocks were collected for geochronology, as well as for whole
rock and trace element geochemistry and thin section
Figure 1. Map of suspected Laramide-age mining districts
and plutons in SW NM. Districts are from McLemore (2017)