Age of the emerald mineralization from the Itabira-Nova Era District, Minas Gerais, Brazil, based on LA-ICP-MS geochronology of cogenetic titanite

Jordt-Evangelista, Hanna; Lana, Cristiano; Delgado, Carlos Eduardo Reinaldo; Viana, Deiwys José

doi:10.1590/2317-4889201620150074

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Brazilian Journal of Geology

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ARTICLES • Braz. J. Geol. 46 (03) • Jul-Sep 2016 • https://doi.org/10.1590/2317-4889201620150074 copy

Age of the emerald mineralization from the Itabira-Nova Era District, Minas Gerais, Brazil, based on LA-ICP-MS geochronology of cogenetic titanite

Idade da mineralização de esmeralda do distrito de Itabira-Nova Era, Minas Gerais, Brasil, com base em geocronologia LA-ICP-MS de titanita cogenética

Authorship SCIMAGO INSTITUTIONS RANKINGS

ABSTRACT:

In the Itabira-Nova Era Emerald District, southeast Brazil, gemological emerald is extracted from underground mines found in schist-type deposits at the contact zone of the Archean Metavolcanosedimentary Sequence of the Guanhães Complex and Paleoproterozoic anorogenic granites of the Borrachudos Suite. Schist-type deposits are commonly generated by reactions enhanced by deformation and heat during regional metamorphism. The age of the mineralization in the region has been a matter of debate for decades: Ages ranging from the Archean to the Neoproterozoic are mentioned in the literature. In the mineralized zone from the Piteiras mine fluorine-aluminum-bearing titanite is found in metamafic rocks. The fluorine content was probably derived from the Borrachudos granites and pegmatites like the beryllium for emerald, thus both minerals could have been generated during the same event. U-Pb titanite geochronology via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was performed on a thin section of a phlogopite-plagioclase-hornblende schist from the Piteiras mine. The determined age of 576 ± 7 Ma is also the probable age for emerald generation during the Brasiliano cycle, which was the only tectonometamorphic event postdating the intrusion of the granites. This event provided heat and fluids necessary for reactions between the Be- and the Cr-bearing rocks, thus enabling the formation of emeralds.

KEYWORDS:
Titanite; U/Pb geochronology; Neoproterozoic; Emerald; Brazil

	SiO₂	TiO₂	Al₂O₃	FeO	MnO	CaO	F	Sum
PI5-Ca-Tit-1	31.48	28.76	8.01	0.75	0.08	28.60	2.39	100.07
PI5-Ca-Tit-2	31.42	27.59	8.96	0.55	0.13	29.13	2.86	100.64
PI5-Ca-Tit-3	31.32	31.14	6.45	0.69	0.11	29.02	2.38	101.11
PI5-Ca-Tit-4	31.18	28.28	8.35	0.67	0.11	29.06	2.84	100.49
PI5-Ca-Tit-5	31.21	29.64	7.45	0.82	0.15	28.80	2.21	100.28
PI5-Ca-Tit-6	30.86	29.27	7.47	0.72	0.15	28.53	2.64	99.64
PI5-Ca-Tit-7	30.68	27.87	8.38	0.56	0.11	28.69	2.09	98.38
PI5Ca-C1-Tit-8	30.34	27.80	8.34	0.63	0.05	28.30	1.84	97.30
PI5Ca-C1-Tit-9	30.67	28.93	7.47	0.71	0.15	28.11	2.16	98.20
PI5Ca-C1-Tit-10	31.15	28.16	7.89	0.68	0.09	28.35	2.49	98.81
PI5Ca-C1-Tit-11	30.87	29.10	7.26	0.70	0.10	28.37	2.66	99.06
PI5Ca-C1-Tit-12	30.56	29.95	7.19	0.66	0.06	28.54	1.67	98.63

Spot	^a207 Pb (cps)	^bU (ppm)	^bPb (ppm)	^bTh U	^c206 Pb (%)	^d207 Pb 235U	±2s (%)	^d206 Pb 238U	±2s (%)	rhoe	^d207 Pb 206 Pb	±2s (%)	206 Pb 238 U	±2s (Ma)	207 Pb 235 U	±2s (Ma)	207 Pb 206 Pb	±2s (Ma)	Conc %
Sample PI-5						^d207 Pb 235U	±2s (%)	^d206 Pb 238U	±2s (%)	rhoe	^d207 Pb 206 Pb	±2s (%)	206 Pb 238 U	±2s (Ma)	207 Pb 235 U	±2s (Ma)	207 Pb 206 Pb	±2s (Ma)	Conc %
4.sSMPABC056	4910	19.00	1.96	0.06	0.255	0.731	2.770	0.091	2.279	0.823	0.059	1.574	559	12	557	12	550	34	101.65
4.sSMPABC055	4893	19.62	2.06	0.06	0.074	0.746	3.036	0.092	2.520	0.830	0.059	1.693	568	14	566	13	558	37	101.85
4.sSMPABC030	466	2.50	0.26	0.01	0.954	0.844	6.658	0.104	4.507	0.677	0.059	4.901	637	27	621	31	566	107	112.59
4.sSMPABC016	486	3.41	0.31	0.01	1.128	0.730	6.987	0.090	4.776	0.684	0.059	5.099	554	25	556	30	568	111	97.56
4.sSMPABC078	883	2.20	0.20	0.01	2.045	0.740	5.216	0.090	3.363	0.645	0.060	3.988	556	18	562	23	587	87	94.84
4.sSMPABC026	500	2.26	0.21	0.01	1.553	0.749	6.495	0.091	4.019	0.619	0.060	5.102	562	22	568	29	591	111	95.19
4.sSMPABC013	496	4.29	0.41	0.01	0.118	0.801	6.101	0.097	3.760	0.616	0.060	4.805	598	22	597	28	595	104	100.42
4.sSMPABC082	849	1.84	0.17	0.01	0.572	0.754	4.551	0.091	3.209	0.705	0.060	3.226	563	17	570	20	598	70	94.15
4.sSMPABC034	707	2.82	0.26	0.01	0.797	0.783	4.908	0.095	3.205	0.653	0.060	3.717	583	18	587	22	605	80	96.35
4.sSMPABC032	463	1.96	0.19	0.01	1.986	0.794	6.064	0.096	3.506	0.578	0.060	4.948	590	20	593	28	605	107	97.55
4.sSMPABC011	723	5.88	0.56	0.01	1.524	0.778	5.175	0.094	3.600	0.696	0.060	3.718	579	20	584	23	606	80	95.48
4.sSMPABC070	1198	2.92	0.26	0.01	2.526	0.745	4.427	0.090	2.660	0.601	0.060	3.539	555	14	565	19	607	77	91.52
4.sSMPABC081	675	1.30	0.12	0.01	1.940	0.753	6.115	0.091	3.547	0.580	0.060	4.981	560	19	570	27	608	108	92.20
4.sSMPABC084	27029	35.25	3.77	0.03	1.995	0.751	3.559	0.091	2.849	0.801	0.060	2.133	559	15	569	16	608	46	92.01
4.sSMPABC017	687	3.89	0.36	0.01	2.403	0.767	5.206	0.092	3.012	0.579	0.060	4.246	570	16	578	23	611	92	93.35
4.sSMPABC079	617	1.12	0.10	0.01	2.846	0.746	6.715	0.090	4.664	0.695	0.060	4.831	555	25	566	30	611	104	90.71
4.sSMPABC053	27553	63.13	6.82	0.03	2.105	0.772	3.328	0.093	2.469	0.742	0.060	2.231	572	14	581	15	615	48	93.10
4.sSMPABC075	1037	2.63	0.25	0.01	1.591	0.787	4.639	0.095	2.680	0.578	0.060	3.786	583	15	590	21	615	82	94.81
4.sSMPABC033	653	2.08	0.20	0.01	3.521	0.760	6.130	0.091	3.272	0.534	0.060	5.184	563	18	574	27	615	112	91.60
4.sSMPABC052	27073	61.98	6.78	0.03	2.123	0.779	3.568	0.094	2.757	0.773	0.060	2.266	577	15	585	16	616	49	93.67
4.sSMPABC083	801	1.41	0.13	0.01	2.043	0.751	5.885	0.090	3.782	0.643	0.060	4.509	557	20	569	26	618	97	90.16
4.sSMPABC022	760	3.77	0.36	0.01	1.282	0.780	4.898	0.094	3.430	0.700	0.060	3.496	577	19	586	22	620	75	93.05
4.sSMPABC031	703	2.59	0.24	0.01	2.487	0.786	5.089	0.094	2.895	0.569	0.060	4.186	581	16	589	23	607	90	95.71
4.sSMPABC080	684	1.29	0.12	0.01	0.707	0.775	5.325	0.093	4.081	0.566	0.058	3.421	572	22	583	24	532	74	107.46
4.sSMPABC054	28288	63.08	6.96	0.03	2.034	0.802	3.624	0.096	2.912	0.803	0.060	2.158	589	16	598	17	594	46	99.20
4.sSMPABC050	27572	64.55	7.33	0.03	2.180	0.809	3.541	0.096	2.684	0.758	0.059	2.310	594	15	602	16	556	50	106.73
4.sSMPABC096	710	0.89	0.08	0.01	3.097	0.765	5.942	0.091	3.491	0.588	0.061	4.808	562	19	577	26	633	104	88.84
4.sSMPABC051	27430	62.79	7.13	0.03	1.972	0.814	3.412	0.097	2.679	0.585	0.060	2.112	596	15	605	16	607	45	98.25
4.sSMPABC014	632	3.87	0.37	0.01	2.482	0.800	6.095	0.095	4.238	0.695	0.061	4.381	586	24	597	28	638	94	91.85
4.sSMPABC025	632	2.58	0.25	0.01	2.469	0.802	6.090	0.095	4.238	0.696	0.060	4.374	586	24	598	28	607	94	96.49
4.sSMPABC095	608	0.66	0.06	0.01	3.756	0.762	6.166	0.090	3.783	0.583	0.059	4.869	557	20	575	27	578	105	96.26
4.sSMPABC015	510	2.49	0.26	0.01	3.866	0.804	6.671	0.095	4.615	0.592	0.060	4.818	584	26	599	31	588	103	99.31
4.sSMPABC029	510	1.61	0.17	0.01	3.853	0.806	6.666	0.095	4.615	0.692	0.059	4.810	584	26	600	31	556	103	104.92
4.sSMPABC073	583	0.95	0.10	0.01	4.340	0.788	5.845	0.092	4.257	0.728	0.058	4.005	567	23	590	26	534	86	106.24
4.sSMPABC012	529	2.77	0.29	0.01	4.408	0.834	6.198	0.097	4.484	0.723	0.059	4.279	595	26	616	29	550	91	108.35
4.sSMPABC097	32324	36.12	4.34	0.03	2.601	0.897	3.431	0.104	2.382	0.694	0.060	2.470	636	14	650	17	593	53	107.26
Khan Accepted age 522 Ma
Within run standard points normalized with BLR 1
4.sSMPABC094	4852	10.38	1.00	0.06	0.000	0.668	2.986	0.084	2.512	0.841	0.058	1.614	521	13	520	12	516	35	100.90
4.sSMPABC093	4891	9.93	0.96	0.06	0.410	0.674	2.784	0.084	2.318	0.833	0.058	1.542	522	12	523	11	529	34	98.60
4.sSMPABC092	4827	10.47	1.03	0.06	0.000	0.689	2.649	0.086	2.252	0.850	0.058	1.395	532	12	532	11	535	31	99.32
4.sSMPABC091	4933	10.56	1.03	0.06	0.352	0.684	2.985	0.085	2.487	0.833	0.058	1.652	527	13	529	12	539	36	97.70
4.sSMPABC090	4950	10.74	1.04	0.06	0.000	0.668	2.499	0.084	2.047	0.819	0.058	1.435	518	10	519	10	523	31	99.13
4.sSMPABC089	25842	28.78	2.88	0.03	2.114	0.688	3.652	0.084	2.862	0.784	0.059	2.269	519	14	531	15	585	49	88.74
4.sSMPABC086	30209	35.11	3.61	0.03	2.875	0.700	3.862	0.085	2.684	0.695	0.060	2.777	526	14	539	16	594	60	88.57
4.sSMPABC084	799	1.21	0.11	0.01	3.453	0.678	5.969	0.082	3.855	0.646	0.060	4.557	510	19	525	25	593	99	86.06
4.sSMPABC067	989	1.60	0.14	0.01	4.463	0.705	6.089	0.084	3.603	0.592	0.061	4.909	522	18	542	26	629	106	82.98
4.sSMPABC066	934	2.02	0.18	0.01	1.712	0.684	5.093	0.084	2.945	0.578	0.059	4.154	517	15	529	21	582	90	88.85
4.sSMPABC062	738	1.78	0.16	0.01	2.813	0.691	6.186	0.085	4.156	0.672	0.059	4.582	525	21	533	26	570	100	92.00
4.sSMPABC068	5148	16.60	1.54	0.06	0.000	0.660	2.486	0.081	2.089	0.840	0.059	1.348	505	10	515	10	560	29	90.14
4.sSMPABC067	5439	20.51	2.03	0.06	0.000	0.690	2.726	0.086	2.353	0.863	0.058	1.376	534	12	533	11	527	30	101.27
4.sSMPABC021	6311	43.67	4.33	0.06	0.000	0.685	3.212	0.086	2.865	0.892	0.058	1.452	531	15	530	13	525	32	101.21
4.sSMPABC020	6258	45.30	4.48	0.06	0.764	0.680	3.091	0.085	2.486	0.804	0.058	1.837	529	13	527	13	518	40	102.04
4.sSMPABC019	6036	45.36	4.53	0.06	0.000	0.680	3.094	0.086	2.745	0.887	0.058	1.428	529	14	527	13	518	31	102.10
4.sSMPABC018	6737	52.01	5.12	0.06	0.000	0.681	3.430	0.085	3.190	0.930	0.058	1.260	528	16	527	14	524	28	100.72
4.sSMPABC010	6244	73.33	7.35	0.06	0.000	0.696	3.228	0.087	2.872	0.890	0.058	1.473	537	15	536	14	531	32	101.25
4.sSMPABC009	6258	78.25	7.74	0.06	0.764	0.680	3.091	0.085	2.486	0.804	0.058	1.837	529	13	527	13	518	40	102.04
4.sSMPABC008	6092	80.12	7.94	0.06	0.000	0.673	3.075	0.085	2.728	0.887	0.058	1.421	524	14	522	13	514	31	101.99
4.sSMPABC007	6737	92.46	9.11	0.06	0.000	0.681	3.430	0.085	3.190	0.930	0.058	1.260	528	16	527	14	524	28	100.69
4.sSMPABC006	6406	94.36	9.38	0.06	0.000	0.687	3.437	0.086	3.143	0.914	0.058	1.391	532	16	531	14	524	31	101.52
4.sSMPABC004	6418	107.70	10.67	0.06	0.000	0.676	3.037	0.085	2.772	0.913	0.058	1.241	526	14	524	13	517	27	101.83
4.sSMPABC003	6216	114.69	11.20	0.06	0.000	0.665	3.270	0.084	2.989	0.914	0.057	1.325	519	15	518	13	510	29	101.93

BLR 1 Accepted age 1049 Ma
Within run standard points normalized with BLR 1
4.sSMPABC001	44702	382.30	75.32	0.03	1.012062	1.868	2.405	0.180	2.001	0.832	0.075	1.334	1065	20	1070	16	1080	27	98.66
4.sSMPABC002	41961	514.18	99.92	0.03	0.756406	1.839	2.048	0.178	1.804	0.881	0.075	0.969	1057	18	1059	14	1064	19	99.35
4.sSMPABC003	42130	409.02	79.60	0.03	0.694349	1.837	2.051	0.178	1.809	0.882	0.075	0.968	1057	18	1059	14	1062	19	99.52
4.sSMPABC004	42274	446.58	86.57	0.03	0.722678	1.810	2.112	0.177	1.815	0.860	0.074	1.079	1053	18	1049	14	1040	22	101.24
4.sSMPABC005	42275	474.94	90.49	0.03	0.675464	1.769	2.004	0.174	1.759	0.878	0.074	0.961	1032	17	1034	13	1039	19	99.35

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[TFN2] ^aWithin run background-corrected mean ²⁰⁷Pb signal in cps (counts per second); ^bU and Pb content and Th/U ratio were calculated relative to BLR reference STD; ^cpercentage of the common Pb on the ²⁰⁶Pb. b.d. = below dectection limit;^dcorrected for background. within-run Pb/U fractionation (in case of 206Pb/238U) and common Pb using Stacy and Kramers (1975) model Pb composition and subsequently normalised to GJ-1 (ID-TIMS value/measured value); ²⁰⁷Pb/²³⁵U calculated using ²⁰⁷Pb/²⁰⁶Pb/⁽²³⁸U/²⁰⁶Pb*1/137.88); ^erho is the ²⁰⁶Pb/²³⁸U/²⁰⁷Pb/²³⁵U error correlation coefficient.