V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia

a_sklr@mail.ru

The occurrence of melilite-bearing high-temperature calcic skarn is relatively rare. In the Tazheran alkaline massif a melilite-bearing calcic skarn is developed as separated bodies and veins in the nepheline syenites. The typical mineral assemblage in this skarn is melilite, wollastonite, grandite garnet, monticellite, calcite, and also pectolite [NaCa₂Si₃O₈(OH)] and cuspidine [Ca₄Si₂O₇F₂]. Idiomorphic graphite, pyrite and such rare minerals as perovskite [CaTiO₃] and djerfisherite (K-Cl-bearing sulfide) are accessory phases.

A melilite-bearing skarn from the Tazheran massif has some unique features that are unusual for high-temperature calcic skarns. Monticellite forms a solid solution between monticellite [CaMgSiO₄] and kirschsteinite [CaFe²⁺SiO₄] with #fe up to 49% (Tab.). Such Fe-montichellite has only few occurrences in the world. In the Tazheran skarns it forms a symplectitic phase in association with grossular-andradite garnet (Grs – 55%, Adr - 45%). Such symplectites can be a result of collaborative growth. Presence of wollastonite and calcite symplectic intergrowth indicate reaction of tilleyite [Ca₅Si₂O₇(CO₃)₂] decomposition. Melilite forms compositional range between the end members Na-melilite [NaCaAlSi₂O₇] (35-50%) and åkermanite [Ca₂MgSi₂O₇] (45-60%) with the contribution of other minals (gehlenite [Ca₂Al₂SiO₇] and Fe-åkermanite) less than 10%. Similar composition was reported for melilite from carbonatite volcano Oldoinyo Lengai and it is among the most Na-rich ever reported [1]. To produce such Na-bearing melilite and djerfisherite there should be alkaline magmatic fluids. But the influence of surrounding nepheline syenites is not so obvious.

Melilite			Garnet								Monticellite
	1	2			1			2					1	2
	n=30	n=20			min	max	n=60	min	max	n=40			n=10	n=30
SiO2	44,66	44,17		SiO2	37,04	37,74	39,52	35,90	37,57	37,80		SiO2	35,28	36,93
Al2O3	7,73	5,27		TiO2	0,52	1,79	0,00	3,45	1,03	1,66		FeO	17,09	9,32
FeO	1,39	1,13		Al2O3	8,21	17,41	18,01	9,54	13,52	12,34		MnO	1,70	0,50
Fe2O3	0,28	0,00		FeO	0,35	1,04	0,13	1,00	0,00	0,60		MgO	13,16	18,88
MnO	0,41	0,00		Fe2O3	19,05	6,40	6,29	14,93	12,34	12,09		CaO	32,84	34,39
MgO	7,92	10,17		MgO	0,00	0,00	0,83	0,58	0,27	0,69		Total	100,06	100,03
CaO	33,19	35,20		CaO	34,66	35,67	35,63	34,34	35,28	35,02		Si	1,000	1,006
Na2O	4,69	3,15		Total	99,83	100,04	100,41	99,74	100,00	100,19		Fe2+	0,405	0,212
Total	100,26	99,09		Si	2,997	2,917	3,011	2,887	2,954	2,972		Mn	0,041	0,012
Si	1,995	2,002		Ti	0,032	0,104	0,000	0,209	0,061	0,098		Mg	0,556	0,766
Al	0,407	0,281		Al	0,783	1,586	1,617	0,904	1,252	1,143		Ca	0,998	1,004
Fe2+	0,052	0,043		Fe2+	0,024	0,067	0,008	0,067	0,000	0,039		#fe	40,44	21,44
Fe3+	0,010	0,000		Fe3+	1,160	0,372	0,361	0,903	0,730	0,715		#mn	4,08	1,16
Mn	0,016	0,000		Mg	0,000	0,000	0,094	0,070	0,031	0,081		#mg	55,48	77,40
Mg	0,527	0,687		Ca	3,005	2,954	2,909	2,959	2,972	2,951
Ca	1,588	1,710		Adr	58,83	18,80	18,24	47,45	36,55	37,08
Na	0,406	0,277		Grs	38,78	73,73	78,36	37,17	59,35	53,92
Na-Ml	39,94	27,85		Schrl	1,61	5,26	0,00	10,96	3,05	5,10
Åk	52,40	67,78		pyralspite	0,78	2,21	3,40	4,41	1,04	3,90
Gel+Fe-Åk	7,66	4,37

Table. Chemical data for melilite, garnet and monticellite (wt.%).

Note: 1 – calcic skarn from the Tazheran massif; 2 - calcic skarn from the Birkhin massif. min – minimum content of FeO_tot, max – maximum content of FeO_tot. n – amount of analysis.

Another high-temperature calcic skarn with melilite was found 20km south-west of the Tazheran in the gabbroid massif Birkhin. This skarn consists of mineral assemblage that has many similar features with those from the Tazheran massif. Garnet-monticellite symplectitic intergrowth was found in the Birkhin skarn. These monticellite has high Fe content (#fe up to 25%). The composition of melilite is also similar to that from the Tazheran massif (Na-Ml ≈ 28%, Åk ≈ 68%). In addition, djerfisherite occurs as accessory phase. A melilite-bearing skarn from Birkhin massif forms linear separated bodies and veins in the gabbro. There is no connection with any source of alkaline magmatic fluid. On the other hand, the Tazheran and the Birkhin massifs are parts of the Olkhon collision system of Western Baikal area. And the generation of high-temperature calcic skarn in those massifs can be caused by the influence of one source of high-temperature alkaline fluids flowing through the zone of fracture.

The work was supported by the Russian Foundation of Basic Research (projects nos. 12-05-00229) and special grant of OPTEC Company.

References:

1. Wiedenmann D., Keller J., Zaitsev A.N. Melilite-group minerals at Oldoinyo Lengai, Tanzania. Lithos 118, 112-118 (2010).