AGE AND ORIGIN OF FLUORITE-BEARING, SNOWBIRD-TYPE VEINS, WESTERN

AGE AND ORIGIN OF FLUORITE-BEARING, SNOWBIRD-TYPE VEINS, WESTERN

AGE AND ORIGIN OF FLUORITE-BEARING, SNOWBIRD-TYPE VEINS, WESTERN MONTANA.
Rosenberg, Philip E., ([email protected]) and Wilkie, Kurtis M., School of the Environment, Washington State University, Pullman. WA, 99164-2812

a magmatic signature (Sawkins per. com., 1972; Norman and
Sawkins., 1987) supporting the conclusion that F-bearing
fluids, derived from the Idaho Batholith, intruded and
metasomatized pre-existing Proterozoic veins, precipitated
fluorite-bearing assemblages and contributed additional REEs
to both calcites and fluorites (Fig. 10).

Introduction

Fig. 1. Map showing locations of principal Snowbird-type deposits..

Fig. 2. Sketch of Cold Creek (Stage I) Vein (maximum vein width ~ 3).

207Pb/206Pb

206Pb/238U

The Snowbird-type deposits are a series of hydrothermal,
quartz-carbonate veins that intrude Belt metasediments, on a
strike line extending NW-SE from the Coeur dAlene Mining
District along the Idaho-Montana border toward the Idaho
Batholith. These include the Snowbird Pod (SBP), Cold C reek
(CC), Spires (SPI). Spar (SP), Snowbird (SB), Florence (FL) and
Swallow (SW) (Fig. 1).
In the northwest they are Proterozoic (Ramos and Rosenberg
2012), open space fillings with pegmatitic textures and similar
parageneses: outer ankerite, peripheral, euhedral quartz up to
20 feet in length at SB and SPI, and massive calcite cores
(Stage I). At least 4 of these deposits originated from breccia
dikes (Fig. 2). SP and all deposits to the SE are characterized by
later fluorite-bearing assemblages (Stage II) . Stage I and II
mineralization are both present only at the SP and SB
deposits.

0.024

0.020

Fig. 3. Massive calcite transected by vein of fluorite, ankerite and
quartz, SB deposit.

0.051

0.016

Conclusions
1. Snowbird type veins were deposited during at least two
time periods; Mesoproterozoic, Stage I and Cretaceous to
Eocene, Stage II. Brecciation preceded both stages.

0.012

0.008
0.0

0.2

0.4

0.045
93

0.6

207Pb/235U

95

97

99

101

103

238U/206Pb

Fig. 4. Concordia diagram for xenotime in calcite associated with purple fluorite,
ankerite and quartz. SB deposit. Age 72 1 Ma (MSWD = 0.57). Data-point error
ellipses are 2s..

Fig. 5. Concordia diagram for xenotime in calcite associated with white to green
fluorite. SB deposit. Age 65.58 0.61 Ma (MSWD = 2.4). Data-point error ellipses
are 2s..

206Pb/238U

Laser ablation studies of xenotime in calcite associated with
fluorite have yielded several concordia ages for the fluoritebearing assemblages. At the SB deposit the massive calcite
core is transected by veins of quartz, ankerite and purple
fluorite (or parisite) (Fig. 3). An age of 721.0 Ma
(MSWD=0.57) for this assemblage (Fig. 4) is in agreement
with the U-Th-Pb parisite age, 71.11.0 Ma, reported by Metz
et al. (1985). An approximate concordia age of 722.7 Ma was
also recorded for a purple fluorite assemblage near SP. At the
SB the green fluorite was found to have an age of 65.580.61
Ma (MSWD=2.4) (Fig. 5) while at SW (Fig. 6) an age of 61.55.1
Ma (MSWD=2.2) (Fig. 7) was obtained. Thus the massive white
to green fluorite, the ore mineral at the SB deposit, is
Cretaceous to Eocene in age. This suggests that fluorite
precipitated from F-bearing fluids derived from the Idaho
Batholith approximately 22 miles south of the SB trend.

Initial 87Sr/ 86Sr ratios of carbonates are <0.80 for Stage I mineralization but are >0.80 where fluorite is present (Stage II)
(Table 2). Thus, early mineralization could be due to Proterozoic
leaching of Belt metasediments but later fluorite-bearing
assemblages require a more recent source of radiogenic Sr,
probably pre-Belt basement.

0.049

0.047

Laser ablation

Strontium isotope ratios

Fig. 6. Breccia intruded by clear to grayish
white fluorite and calcite. SW deposit.

Sample number

2. During the Mesoproterozoic, fluids leached Belt
metasediments, depositing ankerite, quartz and calcite in
open spaces. Stage I veins are predominant to the NW of the
Snowbird deposit.
3. During the Cretaceous , fluids derived from the intrusion of
the Idaho Batholith, which leached pre-Belt basement and
Belt metasediments, deposited ankerite (or calcite), quartz
and fluorite (or parisite) resulting in Stage II mineralization .
Stage II veins are predominant SE of the Snowbird deposit.

0.0112
0.0108
0.0104
0.0100
0.0096
0.0092

4. The Snowbird and Spar deposits display both Stage I and
Stage II mineralization; late fluids partially replaced and
metasomatized the earlier Stage I mineral assemblages.

0.0088
0.0084
0.04

0.06

0.08

0.10

0.12

207Pb/235U
Fig. 7. Concordia diagram for xenoime in calcite associated with green fluorite, SW
deposit. Age, 61.5 5.1 Ma (MSWD = 2.2). Data-point error ellipses are 2s..

Fig. 8 Chondrite-normalized rare-earth element variations in SB-type
veins. Sample numbers from Metz et al., 1985.

Sample number

REE

Table 1. Table of Eu anomalies. Sample
numbers from Metz et al., 1985.

References
METZ, M.C., BROOKINS, D.C., ROSENBERG, P.E. AND ZARTMAN,R.E. (1985) ECON. GEOL., 80,
394-409.

Carbonates are generally LREE depleted, HREE enriched, and
retain negative Eu anomalies (Fig. 8). Early carbonates
Eu/Eu*>0.7, late carbonates and all fluorites Eu/Eu*<0.5. (Table 1). Normalization of SB deposit calcite REE patterns to those of adjacent Belt wall rocks results in a similar REE distribution and virtually no Eu anomaly implying that REEs from Belt metasediments were scavenged by fluids responsible for early carbonates (Ramos and Rosenberg, 2012). Ar/He/N2 ratios of fluid inclusions in late quartz reflect NORMAN. D.I. and SAWKINS, F.J. (1987). Chem. Geol, 61, 1-10. RAMOS, F. C. AND ROSENBERG, P. E. (2012) ECON. GEOL., 107. 1321-1339. Acknowledgement We thank Charles Knaack and Chris Fisher of the GeoAnalytical Lab Washington State University for technical assistance. Note: Sample numbers from Metz et al., 1985 Fig. 10. Variation of total REE (ppm) with Eu/Eu*. Table 2. Initial Sr isotope ratios. Sample numbers from Metz et al., 1985.

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