3
preparation. Additional samples of mineralized material
were collected where applicable.
Samples for 40 Ar/39Ar geochronology were crushed and
sieved to several size fractions to determine the best min-
eral separates for each sample. Samples were washed with
hydrochloric acid in an ultrasonic bath, with heavy liquid
density separation or magnetic separation being performed
as necessary. Mineral separates were picked using a bin-
ocular microscope, with biotite, hornblende, groundmass
concentrate, and plagioclase being used for the emplace-
ment or eruptive ages, and K-feldspar being used for the
alteration ages.
Samples were sent to the USGS TRIGA reactor in
Denver, Colorado for irradiation. After irradiation, samples
were loaded in trays for analysis. Samples were step-heated
using a Photon Machines 810 nm diode, and the extracted
gas was cleaned using a cold trap at -198 °C and a D50
getter. Gas was analyzed using a Thermo-Fisher Helix MC
mass spectrometer. Gas from the sample, atmospheric air
samples with an 40 Ar/36Ar ratio of 295.5, and a gas “cock-
tail” with an 40 Ar/39Ar ratio of 6.71 were run to monitor
detector drift and intercalibration, with blanks run for the
air, cocktail, and unknowns.
DATA
New dates (Table 2) from the Hillsboro and Camel
Mountain – Eagle’s Nest districts show that the intrusion
at Granite Hill is 40.19 ± 0.041 Ma, and the Eagle’s Nest
andesite is 39.31 ± 0.5 Ma. A pegmatite dike at Prospect
Hills at the south of the district was dated at 35.96 ± 0.84
Ma for the hornblende and 31.26 ± 0.56 Ma for K-feldspar.
The intrusion at Prospect Hills was dated at 35.2 ± 3.4 Ma,
although the mineral dated contained very little potas-
sium and the date is very poor. A coarse-grained biotite
from Hillsboro was dated at 83.65 ± 2.2 Ma using inverse
isochron, and also yielded a very complex and discordant
age spectrum.
Figure 2. Legacy geochronology data for Laramide porphyry and related deposits in SW NM.
Districts younger than ~45 Ma are likely related to mid-Tertiary extensional magmatism.
Supergene dates are from Gerwe (1986), Cook (1994), and Leveille and Stegen (2012). “?”
indicates age may be incorrect
preparation. Additional samples of mineralized material
were collected where applicable.
Samples for 40 Ar/39Ar geochronology were crushed and
sieved to several size fractions to determine the best min-
eral separates for each sample. Samples were washed with
hydrochloric acid in an ultrasonic bath, with heavy liquid
density separation or magnetic separation being performed
as necessary. Mineral separates were picked using a bin-
ocular microscope, with biotite, hornblende, groundmass
concentrate, and plagioclase being used for the emplace-
ment or eruptive ages, and K-feldspar being used for the
alteration ages.
Samples were sent to the USGS TRIGA reactor in
Denver, Colorado for irradiation. After irradiation, samples
were loaded in trays for analysis. Samples were step-heated
using a Photon Machines 810 nm diode, and the extracted
gas was cleaned using a cold trap at -198 °C and a D50
getter. Gas was analyzed using a Thermo-Fisher Helix MC
mass spectrometer. Gas from the sample, atmospheric air
samples with an 40 Ar/36Ar ratio of 295.5, and a gas “cock-
tail” with an 40 Ar/39Ar ratio of 6.71 were run to monitor
detector drift and intercalibration, with blanks run for the
air, cocktail, and unknowns.
DATA
New dates (Table 2) from the Hillsboro and Camel
Mountain – Eagle’s Nest districts show that the intrusion
at Granite Hill is 40.19 ± 0.041 Ma, and the Eagle’s Nest
andesite is 39.31 ± 0.5 Ma. A pegmatite dike at Prospect
Hills at the south of the district was dated at 35.96 ± 0.84
Ma for the hornblende and 31.26 ± 0.56 Ma for K-feldspar.
The intrusion at Prospect Hills was dated at 35.2 ± 3.4 Ma,
although the mineral dated contained very little potas-
sium and the date is very poor. A coarse-grained biotite
from Hillsboro was dated at 83.65 ± 2.2 Ma using inverse
isochron, and also yielded a very complex and discordant
age spectrum.
Figure 2. Legacy geochronology data for Laramide porphyry and related deposits in SW NM.
Districts younger than ~45 Ma are likely related to mid-Tertiary extensional magmatism.
Supergene dates are from Gerwe (1986), Cook (1994), and Leveille and Stegen (2012). “?”
indicates age may be incorrect