Journal of Economic Geology (Nov 2021)
Saveh-Nain-Jiroft Magmatic Belt replaces Urumieh-Dokhtar Magmatic Belt: Investigation of genetic relationship between porphyry copper deposits and adakitic and non-adakitic granitoids
Abstract
Introduction About 75% of world copper, 50% of molybdenum, and 20% of gold are produced from porphyry copper deposits (Sillitoe, 2010) with an average ore grade of 0.45–1.5% Cu, 0.007–0.04% Mo and up to 1.5 ppm Au. Porphyry copper deposits are commonly associated with intermediate composition arc-related igneous rocks with high Sr/Y and La/Yb ratios (Richards, 2011). Igneous rocks having ratios of Sr/ Y > 25 and Y < 10 ppm are considered adakitic type. The aim of this work is to modify the name of Urumieh-Dokhtar magmatic belt (UDMB), petrological studies of granitoids from Saveh to Jiroft, determination of the genetic relationship between porphyry copper deposits and adakitic and non-adakitic granitoids, and comparison of Miocene-Pliocene adakitic volcanic rock in different parts of Iran with barren adakitic granitoids. The role of a thermal gradient, depth of dehydration, water content, source rock, partial melting percentage, and oxygen fugacity in the formation or non-formation of mineralization, grade, and reserve of porphyry copper deposits are also investigated. Materials and methods The information used can be divided into three parts: 1) data related to I-type magnetite series granitoids related to porphyry copper deposits of Miocene age in Saveh-Nain-Jiroft magmatic belt (SNJMB) which in Table 2 are presented, 2) data related to barren I-type magnetite series granitoids of Miocene age of SNJBM are reported in Table 3. In addition, radiogenic isotope information of barren and fertile SNJMB granitoids and volcanic rocks is presented in Table 4. 3) Information related to Miocene-Pliocene adakitic volcanic rocks, which is shown in Table 5. Result Granitoids show the characteristics of subduction zone magmas. So that the enrichment of LILE elements and the depletion of HFSE elements can be seen. Also, enrichment of LILE elements and depletion of HFSE elements of fertile granitoids is more than barren units. Dalli deposit samples show a moderate pattern between barren and fertile granitoids (Fig. 3). All the evidence presented shows that all granitoids are I-type and magnetite series. In the fertile granitoids, the ratio of (La/Yb)n is between 15 and 38. However, this is between 2 and 14 (mostly below 10) in barren granitoids (Tables 2 and 3, Figs. 5A and 5B). Negative anomalous values of Eu are seen in Miocene barren granitoids (Eu /Eu* value between 0.43 and 1 with an average of 0.65) (Table 3). While fertile granitoids have positive to slightly negative Eu anomalies (Eu / Eu* value between 0.82 and 1.3 with an average of 1.2). The initial values of 87Sr/86Sr of Miocene fertile granitoids vary between 0.704253 and 0.704702; while in barren granitoids, it is between 0.705 and 0.7085. Fertile Miocene granitoids have positive εNd (i) (0.29 to 3.39 mean 2.15) and in barren units is between 3- and 2.6 (Table 4). The value of the ratio (La/Yb)n of all volcanic units is between 13 and 78 and mostly above 20. They have positive to slightly negative Eu anomalies (Eu/Eu * values between 0.89 and 1.72) (Table 5). Figure 9 shows the fertile granitoids of the SNJMB similar to adakite volcanic rocks are located in the adakite field. In addition, Figure 11 show that all samples are high silica adakitic type. However, barren granitoids, which are mainly located between Saveh and Nain, are non-adakitic and are plotted within the normal arc range (Fig. 9). Discussion and Conclusion In this paper, the name of UDMB was changed to Saveh-Nain-Jiroft magmatic belt (SNJMB) based on the evidence of lack of magmatism between Saveh to the extent of Takab and absence of air magnet anomaly. Magmatism of Urumieh to Takab is a continuation of the western Alborz magmatic belt. Based on the characteristics of magmatism and mineralization, SNJMB can be divided into two distinct belts: 1) Saveh-Nain Magmatic Belt (SNMB), which mainly consists of non-adakitic barren I-type magnetic granitoids. Based on the ratio (La/Yb)n, these granitoids originate from a depth of 60 to 80 km, and a mantle wedge and based on the amount of Eu/Eu*, conditions were oxidant. The initial 87Sr / 86Sr ratio indicates that they had a lot of contamination with the continental crust. The crustal thickness in SNMB is less than 48 km, 2) Nain-Jiroft Magmatic Belt (NJMB) which hosts porphyry copper deposits. The Miocene granitoids of this belt are magnetite series and I-type adakite. Based on the ratio (La/Yb)n, these granitoids originate from the depth of garnet stability (more than 90 km) and partial melting of slabs and are based on Eu/Eu* Oxidizing conditions have been established at the place of origin. The initial 87Sr / 86Sr ratio indicates slight contamination with the continental crust. The crustal thickness in NJMB is 48 to more than 52 km. Geochemically, adakitic volcanic rocks are similar to the fertile adakitic granitoids of NJMB, but these units do not contain any mineralization. The characteristics of the oceanic slabs of Neo-Tethys varied considerably during the SNJMB, leading to various magmatism and mineralization. The thermal gradient, depth of dehydration, amount of water, source rock, and the percentage of partial melting along the belt control the type of magmatism and the formation of mineralization. Note Figure 15. References Richards, J.P., 2011. High Sr/Y arc magmas and porphyry Cu ± Mo ± Au deposits: Just add water. Economic Geology, 106(7): 1075–1081. https://doi.org/10.2113/econgeo.106.7.1075 Sillitoe, R.H., 2010. Porphyry copper systems. Economic Geology, 105(1): 3–41. https://doi.org/10.2113/gsecongeo.105.1.3
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