Sm-Nd isotopy of the dykes of the Vorontsovskoe gold-ore deposit (Northern Urals)

Fig. 1. Schematic geological map of the Vorontsovskoe deposit, according to [5]:

1 — turinskaya series (S 2 —D 1 ): volcanites oftrahybasalt-trahyandesite formation, rarely — tuffogenic-sedimental varieties and limestones; 2—3 — krasnoturins-kaya series (D 1 ): 2 — andesitic porphyries, tuffs, tuffaceous sandstones and siltstones; 3 — limestones; 4 — Lower Devonian subvolcanic rocks: diorite, andesite basaltic and dolerite porphyrites; 5—8 — Auerbakh gabbro-diorite-granite complex (D 1 ): 5 — granites, 6 — granodiorites, 7 — diorites, quartz diorites, 8 — gabbro, gabbrodiorites; 9 — dykes of diorite porphyries, lamprophyres, dolerites and gabbrodolerites; 10 — facial margins; 11 — faults bordering the structure; 12 — other faults; 13 — skarns; 17 — outlines of the Vorontsovskoe quarries.

Рис. 1. Схематическая геологическая карта района Воронцовского месторождения [5]:

1 — туринская свита (S 2 —D 1 ): вулканиты трахибазальт-трахиандезитовой формации, реже — туфогенно-осадочные разности и известняки; 2—3 — краснотурьинская свита (D 1 ): 2 — порфириты андезитовые, их туфы, туфопесчаники, туфоалевролиты, 3 — известняки; 4 — нижнедевонские субвулканические образования: диоритовые, андезибазальтовые и доле-ритовые порфириты; 5—8 — Ауэрбаховский габбро-диорит-гранитовый комплекс (D 1 ): 5 — граниты, 6 — гранодиориты, 7 — диориты, кварцевые диориты, 8 — габбро, габбро-диориты; 9 — дайки диоритовых порфиритов, лампрофиров, долеритов и габбро-долеритов; 10 — фациальные границы; 11 — разломы, ограничивающие структуру; 12 — прочие тектонические нарушения; 13 — участки скарнирования; 17 — контуры карьеров Воронцовского месторождения

Table 1. The chemical composition of the rocks (wt. %) and REE content (ppm) Таблица 1. Химический состав пород (мас. %) и содержание РЗЭ (ppm)

Author’s No Авторский №	Vor-4/17 Вор-4/17	Vor-14/17 Вор-14/17	Vor-21-1/17 Вор-21-1/17	Vor-21-1/17 Вор-21-1/17	Vor-34-2/17 Вор-34-2/17
SiO 2	50.31	53.60	50.74	49.45	50.80
TiO2	0.74	0.66	0.54	0.67	0.84
Al 2 O 3	16.09	18.50	18.52	18.82	15.85
Fe 2 O 3	4.33	3.88	3.72	3.43	6.06
FeO	6.8	4.9	3.6	3.9	1.0
MnO	0.13	0.16	0.20	0.36	0.09
MgO	6.62	2.86	4.25	3.92	7.43
CaO	7.71	9.40	10.16	10.59	7.75
Na2O	2.25	2.72	3.05	2.87	2.58
K 2 O	0.91	1.53	1.37	2.24	2.29
P 2 O 5	0.24	0.16	0.19	0.26	0.40
POI/ппп	3.6	1.3	3.4	3.2	4.9
Total / Сумма	99.73	99.66	99.73	99.70	100.0
S	2.10	0.07	1.08	0.34	н/о
La	8.220	7.313	6.474	7.415	32.566
Ce	20.959	14.470	14.715	15.839	69.796
Pr	2.840	1.913	2.061	2.136	9.218
Nd	12.744	8.178	9.271	9.360	36.291
Sm	3.341	2.023	2.403	2.345	6.085
Eu	0.867	0.749	0.974	0.910	1.853
Gd	3.731	2.278	2.821	2.641	4.412
Tb	0.626	0.367	0.472	0.423	0.481
Dy	4.294	2.477	3.241	2.906	2.737
Ho	0.944	0.551	0.730	0.642	0.510
Er	2.926	1.694	2.279	1.981	1.432
Tm	0.429	0.253	0.343	0.298	0.203
Yb	2.864	1.720	2.326	2.004	1.360
Lu	0.449	0.278	0.365	0.317	0.208
EREE	65.23	44.26	48.48	49.22	167.15
EuN/EuN*	0.75	1.07	1.15	1.12	1.05

Note: E ^N * = Eu ^N /(Sm ^N +Gd ^N )/2. REE content has done by ICP-MS method by mass spectrometer ELAN 9000 (operators N. V. Cherednichenko, L. K. Deryagina).

Примечание: E ^N * = Eu ^N /(Sm ^N +Gd ^N )/2. Определение РЗЭ проведено методом ICP-MS на масс-спектрометре ELAN 9000 (аналитики Н. В. Чередниченко, Л. К. Дерягина).

The procedure for the chemical preparation of samples for measuring the concentrations and isotopic composition of Sm and Nd consisted of decomposing samples, isolating sum of rare earths, and separating Sm and Nd. A sample was taken into a Savillex vial with a screw cap, a mixture of concentrated acids HF and HNO3 (3: 1) was added and heated in an oven at 120 °C for three days. Then the samples were evaporated to dryness, concentrated HCl was added and kept at 120 °Ñ for one day. At the moment of taking a sample, a mixed tracer 149Sm—150Nd was added to each sample. After dissection, the samples were subjected to chromatographic isolation of Sm, Nd in two stages: with the isolation of rare earth elements on TRU-spec resin and

Fig. 2. REE distribution in dykes (a) and SEM photo of REE minerals, smp. Vor-34-2/17 (b, c).

Minerals: all — allanite, amf — amphibole (hornblende), cl — chlorite, ep — epidote, fsp — K-feldspar, pl — plagioclase, q — quartz

Рис. 2. Распрсдслснис РЗЭ иссёсдова^^ых дайках (a) и при-ìåðû îñíîâíûõ ìèíåðàëîâ-êîíöåíòðàòîðîâ ÐÇÝ, îáð. Âîð-34-2/17 (b, c).

Минералы: all — аёланит, amf — амфибол (роговая обманка), cl — õëîðèò, ep — ýïèäîò, fsp — ÊÏØ, pl — ïëàãèîêëàç, q — êâàðö the final separation on LN-spec resin [7]. The isotope ratios were measured by ICP-MS Neptune Plus MC (Thermo Finnigan) in a static mode from 3 % HNO3 solution. The uncertainty in the 147Sm/144Nd ratios is 0.3 % (2σ) — the average of 13 measurements in the BHVO-2 standard. The error in measuring the Nd isotopic composition in an individual analysis is up to 0.003 %. The isotopic ratios of neodymium are normalized to the ratio 146Nd/144Nd = 0.7219 according to the exponential law. Isochron parameters were calculated using ISOPLOT 3-v3.71_r5 program [6].

Research results and discussion

The results of Sm and Nd isotopic studies in the selected rocks of the dyke complex are shown in Table 2.

147Sm/144Nd values vary from 0.10408 to 0.19441, and 143Nd/144Nd values vary from 0.512660 to 0.512860. For the studied dyke samples, a calculation was made for the age of 340 ± 35 Ma, with an initial 143Nd/144Nd = 0.512436 ± 0.000036. MSWD = 2.3 (Fig. 3).

We also calculated εNd for the age of 340 Ma (Table 2). The εNd values vary in a fairly narrow range, from 4.4 to 5.3. Positive εNd values indicate that the source of Nd could have been abyssal magma chambers.

The currently available data on the ages of intrusive rocks in the part of the Krasnoturinsko-Auerbakhovsky ore region close to the Vorontsovskoye deposit are shown in the diagram (Fig. 4)

Fig. 3. Sm-Nd isotopic relations of the dykes of the Vorontsov-skoe deposit

Рис. 3 . Sm-Nd-изотоп^ыс от^о0с^ия пород даск Bоро^-öîâñêîãî çîëîòîðóäíîãî ìåñòîðîæäåíèÿ

Table 2. Sm-Nd isotope composition of dyke complex rocks of the Vorontsovskoe deposit

Таблица 2. Sm-Nd-изотопный состав пород дайкового комплекса Воронцовского месторождения

Sample / Ïðîáà	147 Sm/ 144 Nd	2SE, abs	(143Nd/144Nd)	2SE, abs	Sm, ppm	Nd, ppm	εNd (t)
Vor-4/17 Âîð-4/17	0.16334	0.00049	0.512835	0.000006	4.16	15.4	5.3
-14/17	0.15724	0.00047	0.512790	0.000010	2.5	9.6	4.7
-21-1/17	0.16114	0.00048	0.512790	0.000010	3.02	11.3	4.5
-22/17	0.19441	0.00058	0.512860	0.000015	1.81	5.6	4.4
-25/17	0.15518	0.00047	0.512790	0.000010	2.88	11.2	4.8
-34-2/17	0.10408	0.00031	0.512660	0.000015	8.2	48	4.5

Fig. 4. The age of magmatic rocks at the area of Vorontsovskoe deposit (blue marks — Murzin et al., 2017):

1 — Sm-Nd, dykes of the Vorontsovskoe deposit (Northern quarry); 2 — Sm-Nd, biotite-pyroxene-amphibole diorite [5]; 3, 4 — U-Pb, zircon, granodiorites (3) and the age of alteration (4) [2, 7]; 5 — U-Pb, zircon , quartz diorite [1]; 6, 7 — U-Pb, zircon, quartz diorite (6) and the age of alteration (7) [2, 7]; 8 — Rb-Sr, different rocks [2, 7]

Ðèñ. 4. Âîçðàñòíûå õàðàêòåðèñòèêè èíòðóçèâíûõ ïîðîä ðàé-îíà Âîðîíöîâñêîãî ìåñòîðîæäåíèÿ (ñèíèå çíàêè — ïî Ìóð-çèíó è äð., 2017):

1 — Sm-Nd, äàéêè Âîðîíöîâñêîãî ìåñòîðîæäåíèÿ (Ñåâåðíûé êàðüåð); 2 — Sm-Nd, áèîòèò-ïèðîêñåí-àìôèáîëîâûå äèîðèòû [5]; 3, 4 — U-Pb, öèðêîíû, ãðàíîäèîðèò (3) è âðåìÿ ïðåîáðàçî-âàíèÿ (4) [2, 7]; 5 — U-Pb, öèðêîí, êâàðöåâûé äèîðèò [1]; 6, 7 — U-Pb, öèðêîíû, êâàðöåâûé äèîðèò (6) è âðåìÿ ïðåîáðàçîâàíèÿ (7) [2, 7]; 8 — Rb-Sr, ðàçëè÷íûå ïîðîäû [2, 7]

Various intrusive formations are known in the area of the deposit. According to [1], the isotopic composition of granodiorites of the Krasnoturinsky ore field indicates the abyssal nature of the fluid, and the U-Pb age of zircon from quartz diorite sampled from an outcrop on the right bank of the Turya River is 407.7 ± 1.6 Ma (MSWD 1.5), which corresponds to the Early Devonian. According to [5], Sm-Nd age of biotite-pyroxene-amphibole diorites, related to the Auerbakh gabbro-granite complex, is 411.7 ± 25 Ma (MSWD 0.36), 143Nd/144Nd = 0.512395 ± 0, 000028. As noted above, the dykes of the Vorontsovskoye deposit traditionally belonged to the Early Devonian subvolcanics and/ or formations of the Auerbakh complex; they have not been dated earlier. The studies, carried out by the authors of the paper, indicate a relatively young (Early Carboniferous) age of the dykes, which may indicate a later post-collisional stage of magmatism in this area.

The work was performed within the theme No. ÀÀÀÀ-À18-118052590028-9 of the State Assignment of the IGG UB RAS.