COUNTRY ROCK MONAZITE RESPONSE TO INTRUSION OF

THE SEARCHLIGHTPLUTON, SOUTHERN NEVADA

John C. Ayers, Scott Crombie, Calvin Miller, Yan Luo,

Miranda Loflin

Vanderbilt University

Objectives

• To study the response of wallrock monazite to contact metamorphism & magmatic fluid infiltration.

• To identify the geologic process associated with monazite ages measured in-situ.

Why study contact metamorphic aureoles

• Have better geologic control than regional metamorphism (small scale, simple geometry)

• Protolith compositions generally available• Transects allow evaluation of effects of

continuous changes in metamorphic grade• Fluid fluxes and peak temperatures vary

systematically in relation to the contact

Searchlight Pluton Panoramic

Copper Mtn.Ireteba PeaksBig Granite Mtn.

Searchlight/Ireteba plutons

• Located in the Eldorado Mountains of Southern Nevada

• Tilted to expose deeper portions of the pluton

• Two lithologies in the wallrock of the SL pluton:– Ireteba

granite– Proterozoi

c gneiss• Lithologies

contain monazite and sericitization

• Metamorphism at 250-400°C and ~0.15-0.4 GPa.

• Focus on transects.

IR1

IR20

Cu, Au, and Ag ore deposits in roof zone

Comparison of country rock and Searchlight granite intrusion

Pluton Age (Ma) δ18OSMOW (‰)

Ireteba (excluding altered pendant samples)

66±2 (Kapp et al., 2002)

8.2-8.8 (Townsend et al., 2000; Kapp et al., 2002)

Proterozoic gneiss ? ?

Searchlight 16.5 ± 1 Ma (Cates et al., 2003)

7.0, 7.1 (Bachl et al. 2001)

Ireteba Whole Rock Stable Isotopes

Distance from Searchlight contact (km)

0.0 0.1 0.2 0.3 0.4 0.5

18

OS

MO

W(‰

)

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

IG-1 IG-2 IG-3 IG-4 IG-5

DS

MO

W(‰

)

-100

-98

-96

-94

-92

-90

-88

-86

18OSMOWDSMOW

SL

Ireteba Granite: Monazite Zoning & Analysis Spots

d18OSMOWIMP208Pb/232Th age

Ireteba granite monazite ages

208Pb/232Th monazite age (Ma)

0 20 40 60 80

Relative probability

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

16.5 Ma

IMP Ages: Townsend et al. (2000) all analysesLA-ICP-MS Ages: This studySearchlight Pluton

Ireteba Monazite Ages

Distance from Searchlight Contact (Ma)

0.0 0.1 0.2 0.3 0.4 0.5

Average

208P

b/232

Th A

ge (Ma)

10

20

30

40

50

60

70

Ireteba Granite: Sample IR1

d18OSMOW208Pb/232Th age

5 6 7 8 9 10 110

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

IR1 Primary n=9

IR1 Secondary n=11

δ18O SMOW

Re

lati

ve

pro

ba

bil

ity

Ireteba Granite: Sample IR20

5 6 7 8 9 10 11 120

0.02

0.04

IR20 Primary n=12

IR20 Secondary n=3

δ18OSMOW

Re

lativ

e p

rob

ab

ility

Gneiss Whole Rock Stable Isotopes

Distance from Searchlight contact (km)

0.0 0.2 0.4 0.6

18O

SM

OW

(‰)

8

9

10

11

12

13

XG-6XG-5 XG-4XG-10

Proterozoic Gneiss Monazite

LA-ICP-MS analysis pits and EMP analysis spots labeled with ages in Ma with 1σ errors.

Gneiss EMP monazite ages

0 200 400 600 800 1000 1200 1400 16000

10

20

30

40

50

60

EMP Age (Ma)

Nu

mb

er

1412

1312

1200

65

Proterozoic Gneiss Monazite

Gneiss wallrock zircon grains

Gneiss Wallrock Monazite and Zircon

Conclusions

• Monazite in deep wallrocks (Ireteba granite) partially recrystallized/reset and developed patchy zoning in response to Searchlight intrusion at 16.5 Ma.

• No good evidence that fluids were responsible for monazite recrystallization – perhaps it was strain?

• Monazite in shallow wallrocks (Proterozoic gneiss) had preexisting patchy zoning and lost some Pb in response to Ireteba intrusion at 65 Ma, but were unaffected by Searchlight intrusion.

• Monazite is absent from Proterozoic Gneiss in roof zone samples, suggesting that high fluid fluxes that formed hydrothermal ore deposits destroyed monazite.

• Focusing of fluids in roof zone prevented development of contact metamorphic aureole and monazite recrystallization on pluton flanks.