Geochemistry and U-Pb geochronology of columbite of the Cachoeirinha Deposit, Rondônia Tin Province, Brazil

Silva, Raquel Guimarães da; Souza, Valmir da Silva; Gonçalves, Guilherme de Oliveira; Ferreira, Luiza dos Santos; Santos Júnior, Paulo Sérgio Mendes dos

doi:10.1590/2317-4889202420230054

Acessibilidade / Reportar erro

Brasil

Brazilian Journal of Geology

Español English

Brasil

Español English

sumário « anterior atual seguinte »

Sumário

Article • Braz. J. Geol. 54 (1) • 2024 • https://doi.org/10.1590/2317-4889202420230054 copy

Geochemistry and U-Pb geochronology of columbite of the Cachoeirinha Deposit, Rondônia Tin Province, Brazil

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

The Cachoeirinha Sn ± (Nb-Ta) deposit is an important producer of the Rondônia Tin Province. It is inserted in the regional unit named Rondônia Intrusive Suite (995–956 Ma) and hosts granular accumulations of columbite group minerals (CGM) embedded in thick Cenozoic colluvial-alluvial covers. Cachoeirinha CGM have subrounded to angular grains, with irregular to regular internal zoning, micro-inclusion, and exsolution features. They have columbite-(Fe) as the dominant mineral phase, with variation in the Ta/(Ta+Nb) ratio and with Sn, Ti, W, U, Ba, Cr, REE, and V as chemical impurities. It has high total REE content (LREE_N depleted, HREE_N enriched, Ce and Eu anomalies), δ¹⁸O = 2.78‰, and with two U-Pb age groups: 1052 ± 6 and 909 ± 7 Ma. This information indicates that physicochemical oscillations favored the ionic substitutions and resorption mechanisms between Nb ↔ Ta, as well as coupled substitutions with the entry of geochemical impurities, according to the equation: 3(Sn, Ti, U, Si)⁴⁺ + (W, Cr)⁶⁺ ↔ 2(V, Nb, Ta)⁵⁺ + 2(Al, REE)³⁺ + (Fe, Mn, Ba)²⁺, at the temperature range between 400° and 500°C. Finally, the Cachoeirinha Sn ± (Nb-Ta) alluvial deposit host ore is related to at least two Neo-Mesoproterozoic magmatic pulses.

KEYWORDS:
Cachoeirinha Sn ± (Nb-Ta) deposit; Rondônia Tin Province; mineral chemistry; oxygen isotope; columbite U-Pb geochronology

Spots number	1	5	6	7	8	9	10	12	13	15	17	18	19	22	24	25	27	28	29	30	31	32	35	36	37	38	40	41
Nb₂O₅ (wt%)	71.16	53.72	61.09	51.63	53.44	63.72	69.17	72.85	57.05	70.93	67.20	48.77	73.45	64.05	71.07	60.21	54.54	70.56	68.86	59.83	64.19	47.38	61.95	68.80	73.12	49.02	52.07	64.08
Ta₂O₅	3.09	22.00	16.41	24.51	23.28	10.68	3.37	2.96	19.16	4.06	6.47	26.38	2.76	9.19	4.00	13.26	22.26	3.68	5.55	14.85	9.93	24.64	12.70	7.21	5.01	22.21	21.89	7.70
TiO₂	1.44	0.27	0.58	0.42	0.68	2.47	1.83	0.35	0.60	0.66	2.38	2.97	1.63	2.96	1.79	2.50	2.42	1.48	1.54	1.75	2.18	3.74	2.72	1.90	0.68	3.77	4.52	4.62
UO₂	0.33	0.36	0.34	0.32	0.34	0.28	0.28	0.37	0.30	0.31	0.43	0.30	0.34	0.38	0.23	0.32	0.36	0.37	0.38	0.40	0.36	0.40	0.33	0.34	0.27	0.31	0.36	0.45
SnO₂	0.00	1.22	0.00	0.45	0.00	0.86	0.70	0.00	0.30	0.87	0.27	0.00	0.36	0.68	0.00	0.64	0.12	0.36	0.00	0.31	0.49	1.76	1.02	0.73	0.00	1.54	1.26	0.25
SiO₂	0.06	0.38	0.28	0.53	0.43	0.24	0.07	0.09	0.36	0.04	0.14	0.43	0.06	0.14	0.05	0.23	0.36	0.05	0.07	0.27	0.14	0.52	0.24	0.14	0.10	0.31	0.41	0.10
FeO	19.30	18.29	18.65	17.99	17.59	18.32	19.35	19.99	18.58	19.38	19.01	17.16	19.50	18.78	19.41	17.66	17.53	19.40	18.86	17.85	18.39	17.21	18.34	18.63	19.51	17.63	17.02	18.74
MnO	2.33	2.38	2.39	2.41	2.57	2.67	2.43	2.35	2.41	2.38	2.78	2.48	2.16	2.41	2.30	2.77	2.67	2.34	2.18	2.44	2.63	2.42	2.30	2.25	2.45	2.28	2.52	2.77
Al₂O₃	0.00	0.00	0.01	0.00	0.01	0.00	0.02	0.00	0.00	0.01	0.03	0.00	0.00	0.00	0.01	0.03	0.03	0.00	0.01	0.00	0.01	0.00	0.00	0.02	0.00	0.01	0.00	0.01
BaO	0.08	0.00	0.00	0.00	0.08	0.07	0.12	0.05	0.00	0.03	0.00	0.00	0.02	0.20	0.06	0.12	0.00	0.08	0.00	0.13	0.08	0.06	0.07	0.01	0.19	0.09	0.09	0.01
Cr₂O₃	0.17	0.06	0.00	0.00	0.04	0.04	0.03	0.01	0.12	0.00	0.06	0.00	0.00	0.00	0.00	0.06	0.01	0.07	0.07	0.00	0.00	0.00	0.00	0.05	0.02	0.01	0.03	0.08
V₂O₅	0.07	0.01	0.00	0.04	0.07	0.09	0.07	0.00	0.04	0.04	0.08	0.08	0.10	0.03	0.00	0.03	0.00	0.05	0.04	0.06	0.00	0.12	0.04	0.06	0.08	0.12	0.06	0.12
WO₃	0.67	0.40	0.20	0.71	0.49	1.18	0.57	0.53	1.11	0.67	0.85	0.87	0.39	1.08	0.68	1.70	0.65	0.39	0.93	1.32	1.16	0.61	0.86	1.08	0.45	1.82	1.23	2.26
Total	98.71	99.10	99.96	99.01	99.03	100.61	98.01	99.55	100.03	99.38	99.70	99.44	100.77	99.90	99.59	99.51	100.94	98.83	98.48	99.20	99.54	98.85	100.57	101.21	101.88	99.11	101.46	101.18
Cation formula based on six atoms of oxygen
Fe (apfu)	0.931	0.959	0.939	0.951	0.922	0.893	0.943	0.961	0.950	0.938	0.919	0.899	0.919	0.917	0.930	0.881	0.889	0.938	0.921	0.901	0.905	0.902	0.900	0.890	0.920	0.917	0.854	0.893
Mn	0.114	0.127	0.122	0.129	0.137	0.132	0.120	0.114	0.125	0.117	0.136	0.131	0.103	0.119	0.112	0.140	0.137	0.115	0.108	0.125	0.131	0.129	0.114	0.109	0.117	0.120	0.128	0.134
Al	0.000	0.000	0.001	0.000	0.001	0.000	0.001	0.000	0.000	0.001	0.002	0.000	0.000	0.000	0.001	0.002	0.002	0.000	0.000	0.000	0.000	0.000	0.000	0.001	0.000	0.001	0.000	0.001
Ba	0.002	0.000	0.000	0.000	0.002	0.002	0.003	0.001	0.000	0.001	0.000	0.000	0.000	0.005	0.001	0.003	0.000	0.002	0.000	0.003	0.002	0.001	0.002	0.000	0.004	0.002	0.002	0.000
A-site total	1.047	1.086	1.062	1.070	1.060	1.025	1.067	1.076	1.075	1.067	1.067	1.030	1.022	1.031	1.044	1.026	1.028	1.055	1.029	1.029	1.038	1.032	1.016	1.000	1.041	1.040	0.982	1.028
Nb (apfu)	1.857	1.523	1.664	1.476	1.514	1.678	1.822	1.893	1.577	1.856	1.756	1.381	1.871	1.690	1.842	1.624	1.496	1.845	1.818	1.631	1.708	1.343	1.644	1.776	1.865	1.378	1.412	1.651
Ta	0.049	0.375	0.269	0.421	0.397	0.169	0.053	0.046	0.319	0.064	0.102	0.449	0.042	0.146	0.062	0.215	0.367	0.058	0.088	0.244	0.159	0.420	0.203	0.112	0.077	0.376	0.357	0.119
Ti	0.063	0.013	0.026	0.020	0.032	0.108	0.080	0.015	0.027	0.029	0.104	0.140	0.069	0.130	0.077	0.112	0.110	0.064	0.068	0.079	0.096	0.176	0.120	0.082	0.029	0.176	0.204	0.198
Sn	0.000	0.031	0.000	0.011	0.000	0.020	0.016	0.000	0.007	0.020	0.006	0.000	0.008	0.016	0.000	0.015	0.003	0.008	0.000	0.007	0.011	0.044	0.024	0.017	0.000	0.038	0.030	0.006
Si	0.004	0.024	0.017	0.033	0.027	0.014	0.004	0.005	0.022	0.002	0.008	0.027	0.003	0.008	0.003	0.014	0.022	0.003	0.004	0.016	0.008	0.033	0.014	0.008	0.006	0.019	0.025	0.006
W	0.010	0.007	0.003	0.012	0.008	0.018	0.009	0.008	0.018	0.010	0.013	0.014	0.006	0.016	0.010	0.026	0.010	0.006	0.014	0.021	0.018	0.010	0.013	0.016	0.007	0.029	0.019	0.033
Cr	0.008	0.003	0.000	0.000	0.002	0.002	0.001	0.000	0.006	0.000	0.003	0.000	0.000	0.000	0.000	0.003	0.000	0.003	0.003	0.000	0.000	0.000	0.000	0.002	0.001	0.000	0.001	0.003
V	0.003	0.000	0.000	0.002	0.003	0.004	0.003	0.000	0.002	0.001	0.003	0.003	0.004	0.001	0.000	0.001	0.000	0.002	0.002	0.003	0.000	0.005	0.001	0.002	0.003	0.005	0.002	0.005
U	0.004	0.005	0.005	0.005	0.005	0.004	0.004	0.005	0.004	0.004	0.005	0.004	0.004	0.005	0.003	0.004	0.005	0.005	0.005	0.005	0.005	0.006	0.004	0.004	0.003	0.004	0.005	0.006
B-site total	1.998	1.981	1.984	1.980	1.988	2.017	1.992	1.972	1.982	1.986	2.000	2.018	2.007	2.012	1.997	2.014	2.013	1.994	2.002	2.006	2.005	2.037	2.023	2.019	1.991	2.026	2.056	2.026
A+B total	3.045	3.067	3.046	3.060	3.048	3.042	3.059	3.048	3.057	3.043	3.067	3.048	3.029	3.043	3.041	3.040	3.041	3.049	3.031	3.035	3.043	3.069	3.039	3.019	3.032	3.066	3.038	3.054
Mn/(Mn+Fe)	0.109	0.117	0.115	0.119	0.129	0.129	0.113	0.106	0.116	0.111	0.129	0.128	0.101	0.115	0.107	0.137	0.133	0.109	0.105	0.122	0.126	0.125	0.109	0.109	0.113	0.116	0.130	0.130
Ta/(Nb+Ta)	0.025	0.198	0.139	0.222	0.208	0.092	0.028	0.024	0.168	0.033	0.055	0.245	0.022	0.079	0.033	0.117	0.197	0.030	0.046	0.130	0.085	0.238	0.025	0.059	0.040	0.214	0.202	0.067
Spots number	42	43	44	45	46	47	48	49	50	51	52	54	55	56	58	59	60	61	62	63	65	66	67	69	70	73	76
Nb₂O₅ (wt%)	57.87	54.09	67.47	47.84	68.87	47.97	58.84	71.73	64.61	47.23	60.18	47.63	50.99	72.31	53.80	60.23	45.98	70.94	72.74	48.78	62.61	53.50	46.37	66.23	58.80	58.95	69.19
Ta₂O₅	15.69	19.72	6.97	26.73	7.41	26.69	16.64	3.09	10.56	30.33	14.64	30.83	24.96	3.54	22.80	10.38	29.44	4.80	4.00	23.77	13.85	25.16	31.43	9.34	15.66	13.03	6.08
TiO₂	3.54	3.49	1.63	2.65	2.27	3.05	2.69	1.53	2.13	2.15	2.16	1.34	2.12	2.03	2.40	4.07	2.74	2.10	2.11	2.36	0.44	0.62	0.63	2.03	1.08	2.95	2.02
UO₂	0.42	0.38	0.38	0.31	0.34	0.34	0.41	0.33	0.38	0.38	0.45	0.41	0.38	0.31	0.35	0.35	0.39	0.32	0.39	0.42	0.39	0.35	0.26	0.37	0.45	1.57	0.33
SnO₂	1.01	0.00	0.00	0.00	0.11	0.00	0.30	0.00	0.00	0.20	0.00	0.87	0.00	0.52	0.62	0.67	1.11	0.00	0.00	1.34	0.00	0.82	0.00	0.70	0.33	0.23	0.65
SiO₂	0.27	0.32	0.07	0.50	0.08	0.44	0.31	0.04	0.15	0.59	0.21	0.62	0.48	0.09	0.44	0.21	0.56	0.09	0.03	0.37	0.23	0.39	0.56	0.20	0.35	0.29	0.10
FeO	18.33	17.93	19.28	17.29	18.85	17.43	18.15	19.50	18.52	17.00	18.42	17.33	17.39	19.67	17.60	18.06	17.05	19.29	19.54	17.93	18.81	18.24	17.86	18.77	18.73	12.01	11.63
MnO	2.73	2.92	2.40	2.50	2.43	2.33	2.34	2.46	2.58	2.58	3.16	2.16	2.92	2.31	2.51	2.83	2.64	2.31	2.30	2.63	2.67	2.14	2.18	2.57	1.93	8.10	9.74
Al₂O₃	0.02	0.00	0.01	0.00	0.01	0.00	0.00	0.01	0.01	0.00	0.01	0.00	0.02	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00
BaO	0.07	0.08	0.16	0.00	0.16	0.17	0.11	0.10	0.00	0.12	0.07	0.03	0.00	0.03	0.03	0.04	0.18	0.08	0.01	0.16	0.03	0.05	0.00	0.00	0.00	0.00	0.00
Cr₂O₃	0.00	0.00	0.00	0.00	0.06	0.06	0.08	0.00	0.13	0.02	0.02	0.00	0.00	0.10	0.08	0.03	0.02	0.00	0.06	0.05	0.02	0.12	0.00	0.00	0.00	0.09	0.01
V₂O₅	0.06	0.11	0.00	0.05	0.01	0.06	0.08	0.00	0.05	0.07	0.09	0.04	0.11	0.07	0.04	0.08	0.11	0.02	0.08	0.01	0.00	0.00	0.05	0.07	0.11	0.04	0.00
WO₃	1.52	1.55	1.17	0.84	0.64	0.73	1.38	0.57	1.00	0.70	1.29	0.33	0.78	0.77	0.93	2.37	0.67	1.08	0.50	1.44	0.45	0.28	0.33	1.20	0.93	1.24	1.98
Total	101.50	100.59	99.53	98.72	101.25	99.26	101.32	99.36	100.11	101.36	100.70	101.59	100.13	101.72	101.61	99.33	100.89	101.04	101.76	99.26	99.50	101.67	99.67	101.47	98.37	98.50	101.73
Cation formula based on six atoms of oxygen
Fe (apfu)	0.902	0.904	0.940	0.917	0.899	0.917	0.898	0.936	0.906	0.890	0.914	0.910	0.902	0.921	0.890	0.889	0.895	0.914	0.914	0.944	0.945	0.938	0.963	0.903	0.958	0.606	0.552
Mn	0.136	0.149	0.119	0.134	0.117	0.124	0.117	0.120	0.128	0.137	0.159	0.115	0.153	0.109	0.129	0.141	0.140	0.111	0.109	0.140	0.136	0.112	0.119	0.125	0.100	0.414	0.468
Al	0.001	0.000	0.000	0.000	0.001	0.000	0.000	0.001	0.001	0.000	0.001	0.000	0.001	0.000	0.000	0.000	0.000	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.001	0.000
Ba	0.002	0.002	0.004	0.000	0.004	0.004	0.003	0.002	0.000	0.003	0.002	0.001	0.000	0.001	0.001	0.001	0.004	0.002	0.000	0.004	0.001	0.001	0.000	0.000	0.000	0.000	0.000
A-site total	1.041	1.053	1.063	1.050	1.021	1.045	1.018	1.059	1.035	1.030	1.076	1.026	1.056	1.031	1.020	1.031	1.039	1.028	1.023	1.088	1.082	1.051	1.082	1.028	1.058	1.001	1.020
Nb (apfu)	1.541	1.475	1.779	1.370	1.776	1.365	1.574	1.860	1.709	1.336	1.614	1.352	1.430	1.831	1.471	1.603	1.305	1.816	1.840	1.388	1.701	1.488	1.351	1.723	1.626	1.608	1.774
Ta	0.251	0.323	0.110	0.461	0.115	0.457	0.268	0.048	0.168	0.516	0.236	0.526	0.421	0.054	0.375	0.166	0.503	0.074	0.061	0.407	0.226	0.421	0.551	0.146	0.260	0.214	0.094
Ti	0.157	0.158	0.071	0.126	0.097	0.145	0.120	0.066	0.094	0.101	0.096	0.063	0.099	0.085	0.109	0.180	0.129	0.089	0.089	0.112	0.020	0.029	0.031	0.088	0.050	0.134	0.086
Sn	0.024	0.000	0.000	0.000	0.002	0.000	0.007	0.000	0.000	0.005	0.000	0.022	0.000	0.012	0.015	0.016	0.028	0.000	0.000	0.034	0.000	0.020	0.000	0.016	0.008	0.005	0.015
Si	0.016	0.019	0.004	0.031	0.005	0.028	0.018	0.002	0.009	0.037	0.012	0.039	0.029	0.005	0.026	0.012	0.035	0.005	0.002	0.023	0.014	0.024	0.036	0.012	0.022	0.018	0.006
W	0.023	0.024	0.018	0.014	0.010	0.012	0.021	0.008	0.015	0.011	0.020	0.005	0.012	0.011	0.015	0.036	0.011	0.016	0.007	0.023	0.007	0.004	0.005	0.018	0.015	0.019	0.029
Cr	0.000	0.000	0.000	0.000	0.003	0.003	0.004	0.000	0.006	0.001	0.001	0.000	0.000	0.004	0.004	0.002	0.001	0.000	0.003	0.002	0.001	0.006	0.000	0.000	0.000	0.004	0.001
V	0.002	0.004	0.000	0.002	0.001	0.002	0.003	0.000	0.002	0.003	0.003	0.001	0.004	0.002	0.002	0.003	0.005	0.001	0.003	0.000	0.000	0.000	0.002	0.003	0.004	0.002	0.000
U	0.005	0.005	0.005	0.004	0.004	0.005	0.005	0.004	0.005	0.005	0.006	0.006	0.005	0.004	0.005	0.005	0.006	0.004	0.005	0.006	0.005	0.005	0.004	0.005	0.006	0.021	0.004
B-site total	2.027	2.010	1.987	2.009	2.003	2.016	2.020	1.989	2.006	2.016	1.988	2.008	2.000	2.008	2.022	2.024	2.023	2.005	2.010	1.996	1.974	1.997	1.980	2.009	1.991	2.045	2.008
A+B total	3.068	3.063	3.050	3.059	3.024	3.061	3.038	3.048	3.041	3.046	3.064	3.040	3.056	3.039	3.042	3.055	3.062	3.033	3.033	3.084	3.056	3.048	3.062	3.037	3.049	3.046	3.028
Mn/(Mn+Fe)	0.131	0.142	0.112	0.128	0.116	0.119	0.115	0.113	0.124	0.133	0.148	0.112	0.145	0.106	0.126	0.137	0.136	0.108	0.107	0.129	0.126	0.106	0.110	0.122	0.094	0.406	0.459
Ta/(Nb+Ta)	0.140	0.180	0.058	0.252	0.061	0.251	0.145	0.025	0.089	0.279	0.128	0.280	0.227	0.029	0.203	0.094	0.278	0.039	0.032	0.227	0.117	0.220	0.290	0.078	0.138	0.117	0.050

Types	CGM1						CGM2
Spots	1	2	4	6	7	9	3	5	8
¹³⁹La (ppm)	285	203	6	3.6	14	14	16	7	1.4
¹⁴⁰Ce	256	1323	13	104	39	375	21	110	36
¹⁴¹Pr	34	16	3.6	4.3	5.1	5.6	2.5	1.05	0.28
¹⁴⁶Nd	59	113	26	24	23	63	10	5.2	2.1
¹⁴⁷Sm	10	41	26	36	21	29	3.4	9	3.2
¹⁵³Eu	0.29	2.4	1.14	0.56	0.41	0.31	0.02	0.35	0.06
¹⁵⁷Gd	20	58	45	62	39	61	5.8	10	6.3
¹⁵⁹Tb	10	41	25	28	21	45	3.3	4.4	4.1
¹⁶³Dy	161	400	343	417	332	519	37	34	54
¹⁶⁵Ho	56	155	108	161	132	158	15	11	15
¹⁶⁶Er	341	853	642	898	664	984	91	49	67
¹⁶⁹Tm	112	257	236	259	193	327	27	12	30
¹⁷²Yb	1,253	3,272	3,008	3,317	2,454	3,563	269	170	385
¹⁷⁵Lu	244	741	504	745	549	629	56	35	59
Eu/Eu*	0.06	0.15	0.10	0.04	0.04	0.02	0.01	0.11	0.04
(Gd/Lu)_N	0.01	0.01	0.01	0.01	0.04	0.01	0.01	0.01	0.01
MREE_N	2,067	5,904	4,294	5,734	4,557	6,543	550	506	640
HREE_N	24,369	66,201	52,749	67,016	49,155	67,074	5,632	3,275	6,417
(HRRE/MREE)_N	11.78	11.21	12.28	11.68	10.78	10.25	10.23	6.47	10.01

Spots CCGM							Data for Tera-Wasserburg plot ^b b data not corrected for common-Pb				Data for Wetherill plot ^c c data corrected/not corrected for common-Pb					Dates ^c c data corrected/not corrected for common-Pb									% conc ^d d concordance calculated as (206Pb-238U age/207Pb-206Pb age)*100
Older age group	f206c	²⁰⁶Pb (cps)	U (μg g-1) ^a	Th/U	²⁰⁶Pb/²⁰⁴Pb	1s(%)	²³⁸U/²⁰⁶Pb	1s(%)	²⁰⁷Pb/²⁰⁶Pb	1s(%)	²⁰⁷Pb/²³⁵U	1s(%)	²⁰⁶Pb/²³⁸U	1s(%)	Rho	²⁰⁷Pb/²⁰⁶Pb	2s abs	2s_sysabs	²⁰⁶Pb/²³⁸U	2s abs	2s_sysabs	²⁰⁷Pb/²³⁵U	2s abs	2s_sysabs
9	5.79	239,391	235	0.34	-244	-89.7	4.88	1.7	0.11883	2.94	3.4917	2.8	0.20647	1.5	0.00	1919	110	110	1,216	35	35	1,508	43	44	63.3
11	0.62	162,868	202	0.26	-2,627	-6.48	5.66	0.9	0.07256	1.37	1.7947	1.1	0.17811	0.9	0.15	972	53	53	1,056	17	17	1,039	14	14	108.6
12	0.50	267,461	335	0.06	-3,754	-8.72	5.65	0.8	0.07270	1.29	1.7933	1.2	0.17811	1.0	0.59	992	44	44	1,055	20	20	1,038	15	15	106.4
13	0.29	198,987	251	0.12	-3,784	-6.53	5.69	1.1	0.07460	1.73	1.8234	1.6	0.17854	1.1	0.30	1003	66	66	1,058	22	22	1,047	19	19	105.4
14	0.32	171,217	222	0.16	-3,004	-6.25	5.65	1.0	0.07440	1.94	1.8258	1.6	0.17894	0.9	0.09	962	54	54	1,060	18	18	1,046	18	19	110.1
15	0.10	207,121	262	0.30	-3,517	-5.37	5.67	0.9	0.07359	2.09	1.8308	2.1	0.17828	0.9	0.08	955	48	48	1,057	17	17	1,036	17	17	110.7
16	9.48	123,363	102	0.36	8.3	764	3.83	1.0	0.16224	1.52	5.9354	1.3	0.26383	1.0	0.05	2462	46	46	1,508	27	27	1,959	21	22	61.2
17	0.65	113,639	144	0.16	-1,689	-6.03	5.69	1.2	0.07478	2.17	1.8319	2.3	0.17576	1.3	0.13	986	71	71	1,042	25	25	1,040	21	21	105.6
18	0.83	161,442	187	0.18	-1,965	-10.0	5.54	1.5	0.08024	1.97	2.0196	2.0	0.18281	1.4	0.30	1166	73	73	1,081	27	27	1,113	25	25	92.7
19	3.20	150,327	169	0.25	-695	-16.7	5.16	1.1	0.09842	2.09	2.7179	2.5	0.19728	1.1	0.32	1509	72	72	1,159	23	23	1,308	32	33	76.8
20	2.19	159,255	190	0.19	-1,082	-13.2	5.42	0.9	0.08594	1.87	2.2251	1.7	0.18621	0.9	0.00	1279	67	67	1,100	19	19	1,178	22	23	86.0
Younger age group	Spots CCGM						Data for Tera-Wasserburg plot ^b b data not corrected for common-Pb				Data for Wetherill plot ^c c data corrected/not corrected for common-Pb					Dates ^c c data corrected/not corrected for common-Pb									% conc ^d d concordance calculated as (206Pb-238U age/207Pb-206Pb age)*100
Younger age group	f206c	²⁰⁶Pb (cps)	U (ppm)	Th/U	²⁰⁶Pb/²⁰⁴Pb	1s (%)	²³⁸U/²⁰⁶Pb	1s (%)	²⁰⁷Pb/²⁰⁶Pb	1s (%)	²⁰⁷Pb/²³⁵U	1s (%)	²⁰⁶Pb/²³⁸U	1s (%)	Rho	²⁰⁷Pb/²⁰⁶Pb	2s abs	2s_sysabs	²⁰⁶Pb/²³⁸U	2s abs	2s_sysabs	²⁰⁷Pb/²³⁵U	2s abs	2s_sysabs
1	1.33	340,612	485	0.11	-3,339	-11.9	6.58	0.6	0.08103	1.08	1.7386	1.0	0.15327	0.6	1,202	39	39	919	11	11	1,019	12	12	1,202	76.5
2	15.08	198,719	207	0.30	162	6.5	5.19	1.2	0.18150	1.78	4.9409	1.7	0.19387	1.2	2,658	54	54	1141	25	25	1,801	28	28	2,658	42.9
3	0.60	315,921	428	0.09	-6,451	-6.0	6.56	0.8	0.07292	1.50	1.5457	1.2	0.15348	0.8	960	55	55	920	13	13	945	14	15	960	95.8
4^* * unused data	0.68	399,123	587	0.09	-5,705	-9.2	6.84	0.8	0.07457	1.29	1.5217	1.1	0.14698	0.8	1,022	51	51	883	13	13	936	13	14	1,022	86.4
5	0.26	309,216	411	0.09	-6,287	-7.0	6.48	0.9	0.07299	1.54	1.5714	1.4	0.15526	0.9	999	69	69	930	16	16	955	16	17	999	93.0
6	1.00	278,358	407	0.21	-3,409	-14.9	6.48	0.8	0.07594	1.43	1.6295	1.4	0.15461	0.9	1,072	55	55	929	15	15	979	17	17	1,072	86.7
7	1.25	320,941	441	0.33	-2,503	-14.4	6.51	0.7	0.08323	1.53	1.8292	1.9	0.15477	0.7	1,222	46	46	927	12	12	1,034	14	15	1,222	75.8
8	0.61	480,505	693	0.12	-5,318	-13.6	6.54	0.9	0.07571	1.37	1.6242	1.2	0.15431	0.9	1,060	53	53	925	15	15	977	14	14	1,060	87.2
10	0.25	301,173	442	0.14	-5,403	-8.7	6.52	1.0	0.07475	1.84	1.5803	1.6	0.15348	1.1	1,038	74	74	920	18	18	959	19	19	1,038	88.6

Sociedade Brasileira de Geologia R. do Lago, 562 - Cidade Universitária, 05466-040 São Paulo SP Brasil, Tel.: (55 11) 3459-5940 - São Paulo - SP - Brazil
E-mail: sbgeol@uol.com.br

Acompanhe os números deste periódico no seu leitor de RSS

[1] * Corresponding author.