4
GEOCHRONOLOGY AND
GEOCHEMISTRY
Previous geochronology studies suggest further work is
needed on the ages and characteristics of the Laramide and
Paleogene mineral deposits in southwestern New Mexico.
Previous studies suggest that there are spatial and temporal
trends between these three pulses of ore producing magma-
tism (Figure 2 Stafford, 2024).
The most recent geochronology work has been the
preparation and dispatch of samples from the Little Hatchet
Mountains (Eureka and Sylvanite district). Drill core from
the Victorio Mountains and beryllium and tungsten vein
and skarn deposits in the Tres Hermanas Mountains are cur-
rently being analysed for geochronology and geochemistry.
Drill core from McGhee Peak (Steins Pass) has been
logged and will be sampled. From the geological logs per-
vasive hydrothermal alteration within the andesite (phyllic
and propylitic) is present and Cu in the form of chalcoc-
ite has been found. The quartz monzonite at the bottom
of the hole was also pyritized in spots. McGhee Peak has
Pb-Zn-Cu-Ag CRDs and skarns (Elston et al., 1979) and
thin section samples from the drill core hole 1221 have
recently been prepared for geochemical thin section and
petrography analysis in the future. Samples from the same
core are also currently being mineral separated in the lab for
40Ar/39Ar age dating. Samples of altered and mineralised
andesite groundmass, a latite porphyritic dyke and pyritized
quartz monzonite samples are currently being prepared for
40Ar/39Ar age dating from this hole. It has been suggested
that there is a Mo-W deposit in the area (McLemore, 2024).
A porphyry deposit was drilled in the 1970s in the north-
west part of the district (McLemore, 2008), and there is
some evidence for a mid-Tertiary porphyry Mo-W deposit
as well (David Brown, written communication Stafford,
2024).
Recent geochemical data also suggest that these Mo-W
deposits have geochemical trends of anomalous levels of Re
(compared to the Laramide Cu porphyry deposits). Re con-
tent varies from about 100 to 3,000 ppm in porphyry Cu
deposits, ≤150 ppm in arc-related porphyry Mo deposits,
and ≤35 ppm in alkali-feldspar rhyolite-granite (Climax-
type) porphyry Mo deposits (John and Taylor, 2016). New
and historic whole-rock geochemical data have also high-
lighted areas with critical minerals potential (e.g., Zn 1%,
Bi 1000 ppm, Co to 1026 ppm, and Te to 13 ppm in drill
core from Piños Altos, Grant Co., NM).
New and published geochemical analyses of ores,
mineralized rocks, and mine wastes provide insights into
potential critical minerals in these deposits and hence the
importance of further research. Although most samples are
low in TREE (Figure 3), a few deposits contain elevated
TREE, including Piños Altos (409 ppm TREE), Santa
Rita (717 ppm TREE), Fierro-Hanover (937 ppm TREE),
and Oro (Eureka, 2272 ppm TREE). High Co concen-
trations are found in some of the Piños Altos samples (as
much as 1026 ppm Co), whereas the Ground Hog mine in
the Central district has as much as 64 ppm Te (Figures 4,
5). Many of these critical minerals are found in the skarns
adjacent to the porphyry copper deposits. Although, these
values are not normally economic, the large volumes of
porphyry copper deposits mined enhance the economics of
critical minerals in these deposits.
Hence further geochemical and geochronological char-
acterization of these trends is critical as it could become
targeting criteriums for these select Mo-W deposits in
southern New Mexico.
PRELIMINARY CONCLUSIONS
New 40Ar/39Ar dating and prior age determinations have
constrained the southwestern New Mexico deposits into
three periods: (1) ~78–71 Ma (Piños Altos, 78.55±1.75 Ma,
Copper Flat, Hillsboro district, 75.9±0.66 Ma, and Oro,
Eureka district 71.4±0.19 Ma), (2) ~59–50 Ma (Lordsburg,
59.25–57 Ma, Santa Rita, 59.05±0.36 Ma, Hanover-
Hermosa Mountain, Fierro-Hanover district, 58.3±0.7
Ma, McGhee Peak, Peloncillo Mountains, 57.28±0.65 Ma,
Tyrone, 55.2±0.6 Ma, Lone Mountain, 50.6±1.9 Ma) and
(3) ~40–30 Ma Mid-Tertiary pulse (Mo-W deposits, maybe
Figure 2. Above shows the most recent geochronological
timeline of these ore producing pulses of magmatism and the
current area of study. Modified from (Stafford, 2024)
GEOCHRONOLOGY AND
GEOCHEMISTRY
Previous geochronology studies suggest further work is
needed on the ages and characteristics of the Laramide and
Paleogene mineral deposits in southwestern New Mexico.
Previous studies suggest that there are spatial and temporal
trends between these three pulses of ore producing magma-
tism (Figure 2 Stafford, 2024).
The most recent geochronology work has been the
preparation and dispatch of samples from the Little Hatchet
Mountains (Eureka and Sylvanite district). Drill core from
the Victorio Mountains and beryllium and tungsten vein
and skarn deposits in the Tres Hermanas Mountains are cur-
rently being analysed for geochronology and geochemistry.
Drill core from McGhee Peak (Steins Pass) has been
logged and will be sampled. From the geological logs per-
vasive hydrothermal alteration within the andesite (phyllic
and propylitic) is present and Cu in the form of chalcoc-
ite has been found. The quartz monzonite at the bottom
of the hole was also pyritized in spots. McGhee Peak has
Pb-Zn-Cu-Ag CRDs and skarns (Elston et al., 1979) and
thin section samples from the drill core hole 1221 have
recently been prepared for geochemical thin section and
petrography analysis in the future. Samples from the same
core are also currently being mineral separated in the lab for
40Ar/39Ar age dating. Samples of altered and mineralised
andesite groundmass, a latite porphyritic dyke and pyritized
quartz monzonite samples are currently being prepared for
40Ar/39Ar age dating from this hole. It has been suggested
that there is a Mo-W deposit in the area (McLemore, 2024).
A porphyry deposit was drilled in the 1970s in the north-
west part of the district (McLemore, 2008), and there is
some evidence for a mid-Tertiary porphyry Mo-W deposit
as well (David Brown, written communication Stafford,
2024).
Recent geochemical data also suggest that these Mo-W
deposits have geochemical trends of anomalous levels of Re
(compared to the Laramide Cu porphyry deposits). Re con-
tent varies from about 100 to 3,000 ppm in porphyry Cu
deposits, ≤150 ppm in arc-related porphyry Mo deposits,
and ≤35 ppm in alkali-feldspar rhyolite-granite (Climax-
type) porphyry Mo deposits (John and Taylor, 2016). New
and historic whole-rock geochemical data have also high-
lighted areas with critical minerals potential (e.g., Zn 1%,
Bi 1000 ppm, Co to 1026 ppm, and Te to 13 ppm in drill
core from Piños Altos, Grant Co., NM).
New and published geochemical analyses of ores,
mineralized rocks, and mine wastes provide insights into
potential critical minerals in these deposits and hence the
importance of further research. Although most samples are
low in TREE (Figure 3), a few deposits contain elevated
TREE, including Piños Altos (409 ppm TREE), Santa
Rita (717 ppm TREE), Fierro-Hanover (937 ppm TREE),
and Oro (Eureka, 2272 ppm TREE). High Co concen-
trations are found in some of the Piños Altos samples (as
much as 1026 ppm Co), whereas the Ground Hog mine in
the Central district has as much as 64 ppm Te (Figures 4,
5). Many of these critical minerals are found in the skarns
adjacent to the porphyry copper deposits. Although, these
values are not normally economic, the large volumes of
porphyry copper deposits mined enhance the economics of
critical minerals in these deposits.
Hence further geochemical and geochronological char-
acterization of these trends is critical as it could become
targeting criteriums for these select Mo-W deposits in
southern New Mexico.
PRELIMINARY CONCLUSIONS
New 40Ar/39Ar dating and prior age determinations have
constrained the southwestern New Mexico deposits into
three periods: (1) ~78–71 Ma (Piños Altos, 78.55±1.75 Ma,
Copper Flat, Hillsboro district, 75.9±0.66 Ma, and Oro,
Eureka district 71.4±0.19 Ma), (2) ~59–50 Ma (Lordsburg,
59.25–57 Ma, Santa Rita, 59.05±0.36 Ma, Hanover-
Hermosa Mountain, Fierro-Hanover district, 58.3±0.7
Ma, McGhee Peak, Peloncillo Mountains, 57.28±0.65 Ma,
Tyrone, 55.2±0.6 Ma, Lone Mountain, 50.6±1.9 Ma) and
(3) ~40–30 Ma Mid-Tertiary pulse (Mo-W deposits, maybe
Figure 2. Above shows the most recent geochronological
timeline of these ore producing pulses of magmatism and the
current area of study. Modified from (Stafford, 2024)