ABSTRACT

We analyzed the major and trace element chemical compositions of 66 granitic rocks from 15 different areas in Japan. The intrusions from which the samples were collected were associated with Pb-Zn, Mo, Cu-Fe, Sn, or W mineralization and, for comparison, samples were also collected from intrusions not associated with any metal mineralization. The analyses indicated that the granitic rocks associated with Pb-Zn, Mo, or Cu-Fe mineralization were granites, granodiorites, or diorites, and that they were all I-type and formed in a volcanic arc tectonic setting. The granitic rocks associated with Sn or W mineralization and barren granitic rocks were classified as granites and as I-type with the exception of a few S-type granitic rocks. Most of the Sn- or W-associated granitic rocks and barren granitic rocks are thought to have formed in a volcanic arc tectonic setting. The Pb-Zn-, Mo-, or Cu-Fe-asso- ciated granitic rocks rarely shows negative Eu anomalies and a few of them are adakitic rocks, whereas all of the Sn- or W-associated granitic rocks and barren granitic rocks show negative Eu anomalies. For these Japanese granitic rocks, the contents of K₂O, La, Y, Rb, Ta, Pb, Th, U, and REEs other than Eu increase with increasing SiO₂. Conversely, the contents of major components other than Na₂O and K₂O and the trace components V, Zn, Sr, Eu, and Sc decrease with increasing SiO₂. The Zr, Sn, and Hf abundances increase with increasing SiO₂ up to 70 wt%, but their abundances decrease when the SiO₂ exceeds 70 wt%. This suggests that granitic magma is saturated with these elements at 70 wt% of SiO₂, approximately.

Keywords:

Granitic Rock, Hydrothermal Mineralization, Chemical Composition, Tectonic Setting, Japan

1. Introduction

Uchida et al. (2007) investigated the relationship between the chemical composition of biotite and mineralization type associated with representative granitic rocks in Japan. It was found that the average total Al content of biotite, listed by mineralization type, was Pb-Zn = Mo < Cu-Fe < Sn < W < no mineralization. In addition, the study found that the total Al content in biotite increased with the granite’s solidification pressure. However, Uchida et al. (2007) only measured the major elements in the granitic rocks; trace elements were not analyzed. In this paper, we determined the whole rock chemical compositions, including trace elements and rare earth elements, for samples of granitic rocks from the same intrusions studied by Uchida et al. (2007). With these analyses, we aim to confirm and clarify the relationships between the chemical composition of the granitic rocks and 1) the type of metal mineralization and 2) the tectonic settings in which the granitic rocks were emplaced (e.g., [1] [2] ).

2. Granitic Intrusions Investigated

The granitic rocks analyzed for this study are from the same 15 areas studied by Uchida et al. [3] (Figure 1). Table 1 lists the names of the areas and mining districts from which the samples were collected, rock type, age, associated metal type, and sample number for the samples.

Intrusions in the Taishu [4] , Obira (granite porphyry) [5] and Chichibu mining districts were designated as granitic rocks related to Pb-Zn mineralization. We studied samples from the Ohkawame [6] and Daito-Yamasa [7] mining districts as granitic rocks related to Mo mineralization. As for Cu-Fe mineralization, granitic rocks in the Kamaishi [8] [9] and Yaguki mining districts and granitic rocks in the Tanzawa area [10] were selected for study. The granitic rocks from the Yaguki mining district were the Eastern granodiorite (the Ohisa granodiorite [11] ) and the Central granodiorite. For intrusions related to Sn mineralization, samples from biotite granite in the Obira mining district [5] and granitic rocks in the Osuzu [12] and Suzuyama mining districts were selected. As for granitic rocks related to W mineralization, we studied granitic rocks in the Yakushima and the Fujigatani-Kiwida (the Habu granodiorite and the Osogoe complex), Ohtani (the Gyojayama granite [7] ) and Yaguki mining districts as well as the Inada coarse-grained granite [13] in the Tsukuba area. In the Yaguki mining district, samples were collected from intrusions of the Western granodiorite (the Yokokawa granodiorite [11] ). Barren granitic rocks were collected from the Hidaka metamorphic belt (the Toyonidake cordierite tonalite and hornblende tonalite [14] ), the Tsukuba area other than the Inada coarse-grained granite [13] and the Fujigatani-Kiwada mining district (the Nakayamagawa complex and the Shimokuhara granite).

The granitic rocks associated with Pb-Zn, Mo, or Cu-Fe mineralization are almost magnetite-series but some are ilmenite-series whereas the granitic rocks with Sn or W mineralization and those without mineralization are all ilmenite- series (Table 1) [1] .

3. Sample Preparation, Analysis, and Results

Whole-rock chemical analyses were carried out on 66 samples from the granitic rocks mentioned above (Table 1). Broken down by metal type, there were 10 samples related to Pb-Zn mineralization, 12 samples related to Mo mineralization, 11 samples for Cu-Fe mineralization, 9 samples for Sn mineralization, 13 samples for W mineralization, and 11 samples for barren granitic rocks. Samples

were pulverized using a tungsten carbide rod mill. About 5 g of the pulverized samples were sent to Activation Laboratories Ltd. (Ancaster, Canada) for analysis by their code “4 Litho” lithogeochemistry package. For those analyses, the granitic rock powders were fused using lithium metaborate/tetraborate and digested in dilute nitric acid. Analyses for a total of 55 elements were then obtained by analyzing the aqueous solutions thus prepared using inductively coupled plasma optical emission spectrometer (ICP-OES) and inductively coupled plasma mass spectrometer (ICP-MS). The analytical results are shown in Table 2. Because the samples were contaminated with Co and W by the tungsten carbide rod mill during grinding, Co and W values are not listed in Table 2.

4. Discussion

4.1. Chemical Compositions and Granitic Rock Classification

According to the classification of plutonic rocks based on the SiO₂ vs. (Na₂O + K₂O) diagram (Figure 2) [15] [16] , most of the granitic rocks associated with Sn or W mineralizetion and barren granitic rocks are classified as granites except some of them deviate into granodiorite. The samples associated with Pb-Zn or Mo mineralization fall into granite, diorite and diorite fields, whereas the samples associated with Cu-Fe mineralization are mainly in the diorite to granodiorite fields except one sample fall into granite field.

According to the A/NK vs. A/CNK diagram (Figure 3), A/CNK values for the granitic rocks associated with Pb-Zn, Mo or Cu-Fe mineralization range from 0.7 to 1.1; this means that they are metaluminous to slightly peraluminous. They were all classified as I-type granitic rocks [17] . The A/CNK values for the granitic rocks associated with Sn or W mineralization and barren granitic rocks range from 1.0 to 1.3, which corresponds to peraluminous. Most of these rocks are I-type granitic rocks but some are S-type granitic rocks.

4.2. Tectonic Setting

Based on the Rb vs. (Yb + Ta) diagram (Figure 4) [18] , most of the granitic

rocks analyzed were classified as volcanic arc granitic rocks. However, some granitic rocks associated with Sn or W mineralization (some Obira and some Fujigatani-Kiwada mining districts samples) were plotted in syn-collision granitic rock field. Because the Rb content of the Tanzawa granitic rocks is extremely low, they were classified as M-type granitic rocks [19] . As the Chichibu granitic rocks are also depleted in Rb, although not as depleted as the Tanzawa granitic rocks, they were also classified as M-type granitic rocks.

According to the Sr/Y vs. Y diagram by Defant and Drummond [20] (Figure 5), most of the granitic rocks analyzed in this study are non-adakitic, but a few adakitic rocks are present in the granitic rocks in the Ohkawame and Kamaishi mining districts [21] . For the adakitic rocks analyzed, all the Al₂O₃ content are greater than 15 wt% except for one sample (OK11 at 14.59 wt%). On the SiO₂ vs. Zr/TiO₂ diagram (Figure 6) [22] , most of these adakitic rocks were classified as true adakitic rocks which were produced by the partial melting of young subducting oceanic crust.

4.3. REE Patterns

Chondrite-normalized REE patterns for the granitic rocks organized by type of associated metal are shown in Figure 7.

All of the granitic rocks associated with Sn or W mineralization show negative Eu anomalies. The Eu anomalies for the biotite granites associated with Sn mineralization in the Obira mining district are particularly large. Barren granitic rocks also show negative Eu anomalies except for the hornblende granodiorite in

the Toyonidake area in the southern part of the Hidaka metamorphic belt. This indicates that the Sn- or W-associated granitic rocks and barren ones were formed by the fractional crystallization of plagioclase from granitic magma under reducing conditions where divalent Eu was stable. The hornblende granodiorite in the Toyonidake area shows a pronounced positive Eu anomaly and it is relatively depleted in REEs. These observations suggest that plagioclase may have accumulated during hornblende granodiorite formation.

Conversely, the majority of the granitic rocks with Pb-Zn, Mo, or Cu-Fe mineralization do not show pronounced negative Eu anomalies. This suggests that these rocks formed under oxidizing conditions where trivalent Eu was stable. The samples associated with Pb-Zn, Mo, or Cu-Fe mineralization that do show significant negative Eu anomalies, the Uchiyama sample TS05 (Pb-Zn) and the Yamasa sample DY04 (Mo), are rich in SiO₂ (>74%) and relatively enriched in REEs. This suggests that they formed from differentiated magmas. The REE patterns for the granitic rocks from the Tanzawa area (TZ samples, Cu-Fe) are almost flat, consistent with the features of M-type granitic rocks [20] [23] . The REE patterns for the granitic rocks from the Chichibu mining district (CC samples, Pb-Zn) are also relatively flat and show the features of M-type granitic rocks.

Figure 8 shows the relationship between the magnitude of the negative Eu anomaly and the differentiation index. The vertical axis in Figure 9 shows the distance (Eu/Eu*) from the line connecting Sm and Gd on the chondrite-norma- lized REE pattern to Eu in Figure 8. The Eu/Eu* is defined by the following equation:

,

where REE with subscript of cn indicates each REE concentration of chondrite.

Figure 8 indicates that the magnitude of the negative Eu anomalies increase as the differentiation index increases.

4.4. Interelement Correlations and SiO₂ Content

Figure 9 shows graphs of SiO₂ vs. the major element oxides and graphs of SiO₂

vs. trace elements. The relationships illustrated in Figure 9 are discussed below.

The Al₂O₃, Fe₂O₃(T), MgO, MnO, CaO, TiO₂, and P₂O₅ show a tendency to decrease with increasing SiO₂, whereas, despite a lot of scatter in the data, K₂O seems to increase. The granitic rocks in the Tanzawa area and the Chichibu mining district have low K₂O and are high in CaO (Table 2), suggesting that they are M-type granitic rocks. It seems that Na₂O content increase slightly or is almost constant with increasing SiO₂.

The elements V, Zn, Sr, Eu, and Sc decrease as SiO₂ increases. Vanadium is commonly incorporated in magnetite but is also partitioned into mafic minerals like pyroxenes, amphiboles, and biotite [24] . Because Sc³⁺ is close to Fe²⁺ and Mg²⁺ in ionic radius [25] , it is thought that Sc³⁺ is incorporated in mafic minerals in a similar way to Zn and V. In contrast, Sr is incorporated in plagioclase by substituting Ca.

Yttrium, Nb, Rb, Ta, Pb, Th, U, and REEs other than Eu are elements that increase as SiO₂ increases. Among the REEs, Ce, Pr, Nd, and Sm behave like La, and Ga, Tb, Dy, Ho, Er, Tm, Yb, and Lu behave like Y. The granitic rocks associated with Sn or W mineralization and barren granitic rocks have higher SiO₂ contents compared to the granitic rocks associated with Pb-Zn, Mo, or Cu-Fe mineralization, so there is a tendency for La, Y, Nb, Rb, Ta, Pb, Th, U, and REEs other than Eu to be enriched in the Sn- or W-associated granitic rocks and barren granitic rocks (e.g., [1] [26] [27] [28] ).

Zirconium, Sn, and Hf increase with increasing SiO₂, but start to decrease when the SiO₂ content exceeds 70 wt%. Zirconium concentrates in the magma during crystal differentiation. However, when the SiO₂ content reaches around 70 wt%, it is likely that magma will be saturated with Zr and zircons will precipitate [29] . Thus above 70 wt% SiO₂, the Zr content in the remaining magma tends to decrease. As shown in Figure 9, the distribution of Hf data points mimics the distribution of the Zr points, indicating that Hf is concentrated in zircon. Tin behaves similarly to Zr and Hf and it is concentrated in the magma as crystal differentiation progresses. Magmas will be saturated with Sn when the SiO₂ content reaches about 70 wt%. A granite porphyry accompanied by Pb-Zn mineralization in the Obira mining district is enriched in Zr, Hf, and Sn compared with other granitic rocks.

The behavior of Ba on Figure 9 is complicated. On the whole, Ba shows a similar behavior to the REEs including La with a larger ionic radius, and Ba content increases as SiO₂ increases. At the same time, however, the Ba content tends to decrease similarly to the alkali earth elements such as Ca and Sr as SiO₂ increases.

5. Conclusions

Major and trace element contents were determined for 66 samples of granitic rocks from 15 different areas in Japan. The samples were from intrusions that were either associated with Pb-Zn, Mo, Cu-Fe, Sn, or W mineralization or were barren. Examination of the analytical results for the studied granitic rocks allows the following conclusions to be drawn.

The studied granitic rocks associated with Pb-Zn, Mo, or Cu-Fe mineralization were classified as granite to diorite and were magnetite-series and I-type granitic rocks. However, the granitic rocks in the Tanzawa area and the Chichibu mining district were classified as M-type granitic rocks. The granitic rocks in the Ohkawame and Kamaishi mining districts are adakitic rocks and were generated from the partial melting of a subducting oceanic plate. It is thought that all the granitic rocks formed in a volcanic arc tectonic setting. Only a few REE analyses for rocks of this type show negative Eu anomalies.

Most of the studied granitic rocks associated with Sn or W mineralization and the barren granitic rocks were classified as granite and were ilmenite-series and I-type granitic rocks. A few of these rocks are S-type granitic rocks. It is likely that most of the granitic rocks formed in a volcanic arc tectonic setting. None of these granitic rocks is adakitic and most of their chondrite-normalized REE pattern show negative Eu anomalies.

The contents of K₂O, La, Y, Nb, Rb, Ta, Pb, Th, U, and REEs other than Eu in the granitic rocks increase with increasing SiO₂. These elements tend to be enriched in the high SiO₂ granitic rocks associated with Sn or W mineralization and the high SiO₂ barren granitic rocks.

The major components other than Na₂O and K₂O, Sr, Eu, and Sc in the granitic rocks tend to decrease as SiO₂ increases.

Barium has two different trends. For some sets of analyses, Ba tends to increase with increasing SiO₂, for other sets Ba decreases with increasing SiO₂.

The contents of Zr, Sn, and Hf increase with increasing SiO₂ up to approximately 70 wt%, but then decrease when the SiO₂ content exceeds 70 wt%. This phenomenon is thought to be related to the melt becoming saturated with these elements.

Acknowledgements

This research was supported in part by a Grant-in-Aid for Scientific Research of the Japan Society for the Promotion of Science (Grant No. 16K06931: E. Uchida). The granitic rocks analyzed in this study were collected together with members of our laboratory: T. Hosono, K. Hirose, Y. Katagiri, S. Katsuno, N. Matsushima, M. Miyoshi, S. Nishi, S. Ozawa, F. Sato, T. Shibayama, and S. Takeda.

Cite this paper

Uchida, E., Osada, K. and Nakao, K. (2017) Major and Trace Element Chemical Compositional Signatures of Some Granitic Rocks Related to Metal Mineralization in Japan. Open Journal of Geology, 7, 559-576. https://doi.org/10.4236/ojg.2017.74038

References