4
DISCUSSION
Age spectra and inverse isochrons can be used to inter-
pret 40Ar/39Ar geochronology data beyond just the age of
the intrusion. Age spectra are obtained by step heating a
sample, with each step correlating to an apparent age. An
inverse isochron is obtained by plotting the 36Ar/40Ar ratio
over the 39Ar/40Ar ratio, which can help identify potential
outlying steps.
Sample EAG 101 is likely affected by excess argon,
which can show an anomalously old age. This is shown
by the 40Ar/36Ar ratio being 312.6 (Figure 3), whereas the
normal 40Ar/36Ar ratio corrected for atmosphere is 295.5.
Sample EAG 103 shows two closely agreeing isochrons
(Figure 4) for the plagioclase and groundmass concentrate,
when outlying steps are eliminated. The groundmass shows
a more precise age, while the plagioclase shows a lower
MSWD.
Sample PROS 100 shows a highly imprecise spectrum,
with a high initial step followed by a rising age spectrum
(Figure 5). The K/Ca values and imprecise age infer that the
mineral dated contained little potassium, and could pos-
sibly be a pyroxene or hornblende altered to actinolite. The
weighted mean could possibly be interpreted as the young-
est possible age for the intrusion.
Samples PROS 100-1 and 1004, a hornblende and
K-spar from a pegmatite dike, show plateaus in the age
spectra. PROS 1004 (K-spar) shows low steps at the begin-
ning, followed by high steps before a plateau (Figure 6).
This is likely a result of argon loss for the low steps and
excess argon for the higher steps [6]. Due to the differ-
ence in closure temperature between hornblende (~550
°C) and K-spar (lower, around ~250 °C) [7], the age of
Table 2. Geochronology results for Hillsboro and the Camel Mountain – Eagle’s Nest districts. GMC:
groundmass concentrate, Plag: plagioclase, Hbld: Hornblende
Sample Area Material Mineral Age (Ma) Age Method
Hill 2000 Copper Flat Pit Coarse grained phlogopite Biotite 83.65 ± 2.2 Inverse Isochron
EAG 101 Eagle’s Nest Granitoid Biotite 40.19 ± 0.041 Inverse Isochron
EAG 103 Eagle’s Nest Andesite Plag 39.31 ± 0.5 Inverse Isochron
EAG 103 Eagle’s Nest Andesite GMC 39.35 ± 0.23 Inverse Isochron
PROS 100 Prospect Hills Granitoid Hbld 35.2 ± 3.4 Age spectrum
PROS 100-1 Prospect Hills Pegmatite dike Hbld 35.96 ± 0.84 Age Spectrum
PROS 1004 Prospect Hills Pegmatite dike K-spar 31.26 ± 0.56 Age Spectrum
Figure 3. Inverse isochron diagram for EAG 101 Figure 4. Inverse isochron diagram for EAG 103, showing
isochrons for both plagioclase (black) and groundmass
(green)
DISCUSSION
Age spectra and inverse isochrons can be used to inter-
pret 40Ar/39Ar geochronology data beyond just the age of
the intrusion. Age spectra are obtained by step heating a
sample, with each step correlating to an apparent age. An
inverse isochron is obtained by plotting the 36Ar/40Ar ratio
over the 39Ar/40Ar ratio, which can help identify potential
outlying steps.
Sample EAG 101 is likely affected by excess argon,
which can show an anomalously old age. This is shown
by the 40Ar/36Ar ratio being 312.6 (Figure 3), whereas the
normal 40Ar/36Ar ratio corrected for atmosphere is 295.5.
Sample EAG 103 shows two closely agreeing isochrons
(Figure 4) for the plagioclase and groundmass concentrate,
when outlying steps are eliminated. The groundmass shows
a more precise age, while the plagioclase shows a lower
MSWD.
Sample PROS 100 shows a highly imprecise spectrum,
with a high initial step followed by a rising age spectrum
(Figure 5). The K/Ca values and imprecise age infer that the
mineral dated contained little potassium, and could pos-
sibly be a pyroxene or hornblende altered to actinolite. The
weighted mean could possibly be interpreted as the young-
est possible age for the intrusion.
Samples PROS 100-1 and 1004, a hornblende and
K-spar from a pegmatite dike, show plateaus in the age
spectra. PROS 1004 (K-spar) shows low steps at the begin-
ning, followed by high steps before a plateau (Figure 6).
This is likely a result of argon loss for the low steps and
excess argon for the higher steps [6]. Due to the differ-
ence in closure temperature between hornblende (~550
°C) and K-spar (lower, around ~250 °C) [7], the age of
Table 2. Geochronology results for Hillsboro and the Camel Mountain – Eagle’s Nest districts. GMC:
groundmass concentrate, Plag: plagioclase, Hbld: Hornblende
Sample Area Material Mineral Age (Ma) Age Method
Hill 2000 Copper Flat Pit Coarse grained phlogopite Biotite 83.65 ± 2.2 Inverse Isochron
EAG 101 Eagle’s Nest Granitoid Biotite 40.19 ± 0.041 Inverse Isochron
EAG 103 Eagle’s Nest Andesite Plag 39.31 ± 0.5 Inverse Isochron
EAG 103 Eagle’s Nest Andesite GMC 39.35 ± 0.23 Inverse Isochron
PROS 100 Prospect Hills Granitoid Hbld 35.2 ± 3.4 Age spectrum
PROS 100-1 Prospect Hills Pegmatite dike Hbld 35.96 ± 0.84 Age Spectrum
PROS 1004 Prospect Hills Pegmatite dike K-spar 31.26 ± 0.56 Age Spectrum
Figure 3. Inverse isochron diagram for EAG 101 Figure 4. Inverse isochron diagram for EAG 103, showing
isochrons for both plagioclase (black) and groundmass
(green)