Saturday, October 17, 2015

RARE EARTH ELEMENTS: COLORADO TROUT CREEK FIELD TRIP



Colorado has seen some absolutely gorgeous weather in Fall 2015.  Although it seems unusually dry, especially after the local weather forecasters promised us a Godzilla El Nino wet fall, the days have allowed rockhounds to scour the countryside for treasures.  However, not all rockhounding is created “equal.”  For example, some of my friends spent the second weekend in October near Moab, Utah, (Yellowcat) collecting petrified woods and agates.  I spent this same weekend pretty much confined to my chair or using a cane to hobble around!  In early May I had my right knee replaced and in late September I was the recipient of a brand spanking new right hip.  It appears that as I age the body parts begin to wear out; this may reflect the many hills, mountains and streams I crossed during my active geology field days, or it simply may be genetics!  Of course I have absolutely no regrets about that career, and was unable to really do much with my parental genetics.  The good news is that I am healing and hopeful for a complete recovery.

This time at home allowed me to read a voluminous amount of material, including a rereading of one of my favorite authors, Bernd Heinrich, an ecologist from the University of Vermont.  In A Year in the Maine Woods Heinrich sort of summed up my philosophy for this summer: We all have the capacity to wonder. We all use it.  But most of us must restrict it to the immediate things that affect our well-being.  I am currently living a life of luxury (maybe intellectually but not physically) in that I can spend hours per day wondering about useless things, like: why is Mr. Rockhounding the Rockies fascinated with smoky quartz, how do the hummingbirds at my feeders find their way to Central America, how loud was the “roar” of a dinosaur, how was stereoscopic sight developed in mammals, etc.?  

So, in order to get in a single leaf peeping trip before the recent hip replacement, I decided to take a little day trip to the west just enjoying the scenery, the fall colors, and maybe pounding on a few rocks.  I grabbed a large coffee, a couple of sweet rolls, water, and made it all the way to Leadville before retreating toward home.

One of the places that I wanted to explore was the geology in the Trout Creek Pegmatite District (TCPA) situated along US 24/285 east of Buena Vista in the Mosquito Range. I had visited the area perhaps nine years ago hunting for uncommon minerals and always vowed to return.  Well, the years just sort of moseyed along and I decided that visiting the area now would be an easy chore for an old guy limping along.

Location map showing the Trout Creek Pegmatite Area situated east of Buena Vista.  Map from Hanson and others (1992).

The TCPA, at first glance, seems nothing spectacular with much of the area exposing sections of a large granodiorite (granite-like rock with large amounts of plagioclase feldspar) pluton(s).  Hanson and others (1992), in a major description of the pluton mineralogy, termed the unit the Denny Creek Granodiorite.  However, the Colorado Geological Survey (Wallace and Keeler, 2003) chose not to formalize a name for the plutonic rocks and mapped them as Early Proterozoic (Precambrian) Xgd dated at ~1.7+- Ga.  Whatever the case, there are several intrusive pegmatites and these are the source of numerous Rare Earth Minerals (REM) containing a variety of Rare Earth Elements (REE).  The TCPA is perhaps the most accessible area in Colorado where rockhounds can hunt for, and collect, some fairly uncommon minerals containing REE.


Los Alamos National Laboratory's Chemistry Division Presents a
Periodic Table of the Elements
Period
Group
















1
IA
1A
















18
V
IIIA
8A
1
1
H

1.008
2
IIA
2A










13
IIIA
3A
14
IVA
4A
15
VA
5A
16
VIA
6A
17
VIIA
7A
2
He
4.003
2
3
Li
6.94
4
Be

9.012










5
B
10.81
6
C
12.01
7
N
14.01
8
O
16.00
9
F
19.00
10
Ne
20.18
3
11
Na

22.99
12
Mg

24.31
3
IIIB
3B
4
IVB
4B
5
VB
5B
6
VIB
6B
7
VIIB
7B
8
9
10
11
IB
1B
12
IIB
2B
13
Al
26.98
14
Si
28.09
15
P
30.97
16
S
32.06
17
Cl
35.45
18
Ar
39.95
------- VIII -------
------- 8 -------
4
19
K
39.10
20
Ca
40.08
21
Sc
44.96
22
Ti
47.88
23
V
50.94
24
Cr
52.00
25
Mn
54.94
26
Fe
55.85
27
Co
58.93
28
Ni
58.69
29
Cu
63.55
30
Zn
65.39
31
Ga
69.72
32
Ge
72.64
33
As
74.92
34
Se
78.96
35
Br
79.90
36
Kr
83.79
5
37
Rb
85.47
38
Sr
87.62
39
Y
88.91
40
Zr
91.22
41
Nb
92.91
42
Mo
95.94
43
Tc
(98)
44
Ru
101.1
45
Rh
102.9
46
Pd
106.4
47
Ag
107.9
48
Cd
112.4
49
In
114.8
50
Sn
118.7
51
Sb
121.8
52
Te
127.6
53
I
126.9
54
Xe
131.3
6
55
Cs
132.9
56
Ba
137.3
*
72
Hf
178.5
73
Ta
180.9
74
W
183.9
75
Re
186.2
76
Os
190.2
77
Ir
192.2
78
Pt
195.1
79
Au
197.0
80
Hg
200.5
81
Tl
204.4
82
Pb
207.2
83
Bi
209.0
84
Po
(209)
85
At
(210)
86
Rn
(222)
7
87
Fr
(223)
88
Ra
(226)
**
104
Rf
(265)
105
Db
(268)
106
Sg
(271)
107
Bh
(270)
108
Hs
(277)
109
Mt
(276)
110
Ds
(281)
111
Rg
(280)
112
Cn
(285)
113
Uut
(284)
114
Fl
(289)
115
Uup
(288)
116
Lv
(293)
117
Uus
(294)
118
Uuo
(294)



















Lanthanide Series*

57
La
138.9
58
Ce
140.1
59
Pr
140.9
60
Nd
144.2
61
Pm
(145)
62
Sm
150.4
63
Eu
152.0
64
Gd
157.2
65
Tb
158.9
66
Dy
162.5
67
Ho
164.9
68
Er
167.3
69
Tm
168.9
70
Yb
173.0
71
Lu
175.0


Actinide Series**

89
Ac
(227)
90
Th
232
91
Pa
231
92
U
238
93
Np
(237)
94
Pu
(242)
95
Am
(243)
96
Cm
(247)
97
Bk
(247)
98
Cf
(251)
99
Es
(252)
100
Fm
(257)
101
Md
(258)
102
No
(259)
103
Lr
(262)





The rare earth elements (REE) are a group of ~18 elements that are not really rare, but have strange sounding names that are tough for many people to pronounce. On a periodic table, the REEs occupy a special line with atomic numbers from 57-71 (the lanthanides: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium) plus #21scandium and #39 yttrium. Together these elements are relatively abundant and may “make up” about 200 parts per million (PPM) in rocks of the earth’s crust.  In contrast, tin is a little over 2 PPM and silver is approximately .07 PPM.  Yet in 2012 the world mined about 230,000 tons of tin while ~9000 tons of yttrium were produced.  In fact, the individual crustal abundances of cerium, yttrium, lanthanum, and neodymium are about the same as crustal abundances of lead, nickel, zinc and tin.  What gives?  It appears the answer is concentration!  Many valuable metallic elements are concentrated in rocks of the earth’s crust and companies have developed the ability to mine elements with only a few PPM in the ore.  The REE, although “common,” are scattered in the rocks and are rarely concentrated in economically mineable amounts.  However, the REEs are a valuable commodity and US companies are rapidly trying to ramp up mining permit applications and develop new and better methods for extraction.  If the country is not successful in this endeavor, then the nation will continue to be held hostage by other successful countries. How will we make electric/hybrid car batteries, or even cell phone batteries, or night vision goggles? 

The REM then are minerals that contain one or more REE.  The USGS (2015) noted the principal economic sources of rare earths are the minerals bastn√§site (carbonate containing cerium, yttrium, lanthanum), monazite (phosphate mineral containing cerium, lanthanum, niobium, samarium, gadolinium), loparite (oxide containing cerium, niobium) and the lateritic ion-adsorption clays. The first three minerals are commonly found in igneous rocks of varied composition. The ion- absorption clays are found in tropical or subtropical environments where there has been intensive weathering of bedrock.  Over 250 lateritic deposits containing REE are known and in fact these clays are an important and critical source of commercially mined REE (Cocker, 2012).

Although Colorado has a large number of sites across the state containing REM/REE, the TCPA has open access (I think) collecting areas close to a major highway.  There are several pegmatites intruding into the granodiorite (the best exposed are the Yard, Crystal No. 8, Tie Gulch, and the Clora May); many contain REM, especially aeschynite. 
One of the small pegmatites exposed in the TCPA.
Actually aeschynite is referred to as the Aeschynite Group and contains minerals that are a combination of various REE plus oxygen (O), or oxygen and the hydroxide radical (OH).  One of the reasons that REE are “rare” and difficult to extract is that the REM contain a variety of REE and lack the concentration of just a single REE.  At least that is my understanding, and it may be a little thin!  One of the small pegmatites exposed in the TCPA.


Aeschynite-Y collected from the Clora May Mine, TCPA.  Width ~2 cm.

At the TCPA the major REM is aeschynite-Y with a complex formula of [(Y,Ca,Fe,Th)(Ti,Nb)2(O,OH)6]; yttrium is the dominant REE.  The mineral would be somewhat difficult for me to identify if I had not known the area was a major producer.  It sort of looks like slag or some sort of a shiny metal (it is a metal).  My specimen has a metallic to submetallic luster although some pieces may have a waxy or resinous luster. The color of aeschynite-Y is all over the place with mine being black; however, Webmineral.com lists other colors as brownish-black, brown, brownish-yellow, and yellow.  There are some neat photos on the web showing very yellow specimens.  It is hard to measure the hardness but somewhere between 5 and 6 (Mohs).  Crystals are indistinguishable and the mineral usually appears as massive.  My specimen is covered by small conchoidal fractures and because of the mineral’s brittleness, a small fragment has spalled off.

As a softrocker/paleo person I am sort of baffled by the entire REE thing, but especially their formation.  For an explanation I refer the readers to Hanson and others (1992) who noted that at the TCPA “the formation of …aeschynite-Y as a late-stage pegmatite mineral requires the rare combination of Ti, Nb, Y ...enrichment coupled with low concentration of fluorine in the final stages of pegmatite formation.”

There are several small mines in the TCPA pegmatites where evidently feldspar (and perhaps other minerals) was mined several decades ago. Of these mines, the Clora May is probably the best known.  Nine years ago I hiked up to the Clora May and found the specimen of aeschynite-Y.  On this trip I was limited to about 10 steps from my pickup and was unable to make the hike.

Thanks to the Los Alamos National Laboratory for the use of the Periodic Table. 

REFERENCES CITED

Cocker, M. D., 2012, Lateritic, supergene rare earth element (REE) deposits: Arizona Geological Survey, Special Paper 9, Chapter 4. 

Hanson, S.L., W.B. Simmons, K.L. Webber and A.U. Falster, 1992, Rare-earth-element mineralogy of granitic pegmatites in the Trout Creek Pass District, Chaffee County, Colorado: Canadian Mineralogist, v. 30.

United States Geological Survey, 2015:  www.minerals.usgs.gov/minerals/pubs/commodity/rare_earths

Wallace, C.A. and J.W. Keller, 2003, Geologic Map of the Castle Rock Gulch Quadrangle, Chaffee and Park Counties, Colorado: Colorado Geological Survey Open-File Report 01-1.
 
And finally in closing, back to Bernd Heinrich: I may not have noticed it, but since “growing up” I’d been constantly taking on more and more responsibilities and projects.  To have any chance of getting them done, I’d gradually been speeding up and finally I arrived in the fast lane.  The landscape had become a blur; after a while, I did not really see it any more.  I may still have known it was there, but only because I recognized the cues from previous experience.  Perhaps life was flashing by like a tape player speeded up.  I recognized the sound, but I did not hear the music.  Perhaps that is why I walked off a job I dearly loved the day I turned 63.  Perhaps that is why I spent so many hours this summer doing “nothing” but looking at the mountains, thinking about rocks (and sometimes mortality) drinking coffee, reading and sometimes doing nothing.  I do not think it has been wasted.

No comments:

Post a Comment