Mineralogy texture and chemistry of fine-grained lepidolite bodies from granitic pegmatites

Journal of Pegmatology

VOLUME 1, NUMBER 1

Mineralogy, texture and chemistry of fine-grained lepidolite bodies from
granitic pegmatites

Tara S. Kirkpatrick

Lepidolite is a silvery-purple, sometimes white or gray, lithium-bearing species within the Li-Al mica series. Lepidolite often forms pods or lenses within the cores or central units of complex, rare-element LCT pegmatites.

There has been a long standing controversy over whether these bodies form as a result of primary crystallization from a melt, or secondary replacement from pre-existing minerals. The objectives of this study was to examine textural relationships of fine-grained lepidolite-bearing assemblages in conjunction with chemistry in an attempt to determine the crystallization history of these lepidolite bodies.

The basic mineral assemblage of 14 hand samples from 12 localities consisted of lepidolite, albite and quartz in varying amounts. Tourmaline and Nb-Ta oxide minerals were also minor constituents of some samples. The lepidolite was generally anhedral and ranged from fine-grained (1 mm) toe medium-grained (5 mm). White albite was typically euhedral and medium-grained, ranging in size from 5-30 mm. When euhedral, albite occurred in needle-like radial masses or aggregates. The relationship between lepidolite, albite and quartz were examined in all samples. The most common relationship was lepidolite occurring within albite. Other common relationships included lepidolite intruding into albite, albite intruding quartz, and quartz and albite intergrown together.

Modal mineralogy of the lepidolite-bearing assemblages was determine by point-counting. Modal quartz values ranged from 0-58.93 %, albite 0-41.99% and lepidolite 41.45-100%.

The lepidolite data clustered into 3 distinct groups: one group of 5 samples between 30-50% lepidolite, another group of three in the 60-70% range, and the last group of 6 samples within the 80-100% range. All samples in field 1 were texturally similar, though the modal abundances of the minerals varied somewhat. The lepidolite was generally fine-grained and anhedral, though it coarsended slightly in the sample from the Tourmaline King and Strickland pegmatites. The albite was medium grained and euhedral and all shared the following relationships: lepidolite occurring in albite, albite within quartz and albite intruding quartz. Field 2 showed large variations in average grain size and crystal form of all minerals. Within field 3, two subgroups (A & B) were noticed. Subgroup A included samples with fine- to medium-grain, subhedral lepidolite, meidum-grained, euhedral albite and little quartz. Subgroup B could be considered anomalous, with extremely high amounts of fine-grained lepidolite and few other minerals.

Examination of the lepidolite bodies showed some textural features that clearly indicate lepidolite replacing albite, while others showed the opposite relationship. Even though I could confidently characterize replacement, as of yet it is still difficult to infer primary crystallization vs secondary replacement from textural data alone. For example, in the samples where lepidolite replaced albite, the lepidolite could have indeed been a secondary metasomatic product formed by late F-rich aqueous fluids replacing earlier-formed K-feldspar and Li-aluminosilicates. However, the lepidolite could have also formed by primary crystallization which was remobilized by later hydrothermal activity to replace the albite. Both of these hypotheses are equally plausible, leaving the primary vs secondary question still unanswered.

Bulk major element chemistry showed lepidolite samples to contain high amounts of Si, Al and K, but with low Na. Li₂O contents varied from 1.46 to 3.53% with all but 1 sample in the 3.06-3.53% range. Fe₂O₃ and MnO contents are low (up to 0.20 and 0.78% respectively). The bulk trace element chemistry indicated most of the samples are highly, and in some cases, extremely fractionated. Trace element concentrations were variable; Rb and Cs contents were very high (up to 3.7% Rb₂O and Cs=472-6819 ppm). Tl, Ga, Ta, Nb, and Sn also show large variations in their concentrations.

SAMPLE	Rb	Cs	Nb	Ta	Ga	Tl	Sn	Zn	K/Rb	K/Cs	Nb/Ta
BLACK MTN	12464	2132	126	61	80	77	330	335	6.76	39.52	2.07
MT MICA 30-31	12418	2384	114	57	121	98	272	69	6.42	33.43	2
MT MICA 81	12731	4740	165	110	82	116	223	3	5.53	14.85	1.5
STRICKLAND	4003	1107	41	150	151	63	216	136	15.64	56.54	0.27
DOUGLAS	7146	3188	83	74	80	60	25	20	11.83	26.53	1.12
TIN MTN	6245	909	89	94	82	79	602	131	8.03	55.16	0.95
BROWN DERBY	11296	1283	67	80	107	73	165	8	4.28	37.72	0.84
TANCO	33674	6819	60	386	487	314	502	212	2.15	10.62	0.155
SILVERLEAF	23817	1706	67	117	156	169	346	124	3.37	47.1	0.57
NWT 166811	11252	1613	140	74	125	94	252	295	5.61	39.17	1.89
1.1 STEWART	2589	941	137	42	66	32	56	216	14.3	39.35	3.26
1.2 STEWART	2535	564	84	34	49	22	25	56	10.61	47.69	2.47
1.3 STEWART	7401	1697	127	50	76	56	89	43	11.99	52.29	2.54
2.1 STEWART	5184	574	55	29	86	45	90	262	7.67	69.28	1.9
2.2 STEWART	2405	637	30	21	31	22	10	18	8.53	32.19	1.43
STEWART 1	7500	970	86	25	75	67	48	12	6.71	51.86	3.44
STEWART 3	6573	845	74	27	79	56	55	45	7.29	56.69	2.74
STEWART 4	13153	2176	188	44	95	115	55	11	6.17	37.27	4.27
STEWART 88528-9	4627	472	45	21	77	42	60	115	7.16	70.18	2.14