Geochronology, petrogenesis, and mineralization potential of the syenogranites in the Yama fluorite deposit, Tataleng granitic batholith, Qilian Orogen, NW China

Abstract

The Yama fluorite deposit in the Qilian Orogen in NW China is characterized by predominant granitic magmatism and fluorite mineralization. The wall rocks hosting the fluorite–quartz veins in the Yama area consist of the Yama porphyritic syenogranite and the Wulandawu syenogranite. In this study, the major and trace element compositions, zircon U–Pb ages, and zircon Hf isotopes and trace element compositions were investigated. Two samples for the Yama porphyritic syenogranite yielded weighted mean 206Pb/238U zircon ages of 440.7±4.8 Ma and 447.9±6.0 Ma, and one sample for the Wulandawu syenogranite yielded a weighted mean 206Pb/238U zircon age of 441.2±5.0 Ma, all of which indicate a Late Ordovician to Early Silurian crystallization age. Geochemically, both syenogranites are peraluminous, with high alkali contents, low Zr+Nb+Ce+Y contents, relatively low (K2O+Na2O)/CaO ratios, and high Rb/Sr and Rb/Ba ratios, suggesting that they are fractionated S-type granites. They have zircon εHf(t) values of –14.9 to –4.4 and −12.2 to −4.5, respectively, suggesting that they were derived from the partial melting of pelitic-rich sources within the relatively shallow Paleoproterozoic reworked crust. The detailed geochronological and geochemical data suggest that the studied syenogranites were emplaced in a post-collisional setting related to the closure of the South Qilian Ocean. Finally, they have relatively low quantitative oxygen fugacity (fO2) values, indicating relatively reduced redox states, but their very low zircon Ce4+/Ce3+ ratios, low whole-rock K/Rb ratios, relatively high Nb/Ta ratios, and low Zr/Hf ratios suggest that they are not likely to form important W-Sn deposits and related fluorite deposits.

Geochemical and Isotopic Compositions of Fluorites from the Yama Fluorite Deposit in the Qilian Orogen in Northwest China, and Their Metallogenic Implications

“…The QLO, the northernmost orogenic collage of the Tethyan domain, has an excellent record of the Early to Middle Paleozoic subduction-accretion that resulted from the convergence of the Alax and Qaidam blocks [1,6,9,25]. The North Qilian and South Qilian suture zones normally split the OLO into three tectonic belts, i.e., the NQB, CQB, and SQB (Figure 1a) [1,3,4,6,7,9,10,26].…”

“…The SQB is generally considered to be an accretionary belt comprising microcontinents, ophiolite complexes, and Early Paleozoic arc-related magmatic rocks [1,5,6,20,25]. The Early Paleozoic arc-related magmatic rocks in the SQB are mainly composed of dominant granitic rocks including several large-scale granitic batholiths, such as the Tataleng granitic batholith [6,10,11,20,[32][33][34][35][36][37], the Delingha pluton [18], and the Huangyuan pluton [38]. These granitic rocks from the Early Paleozoic era are typically believed to have formed due to subduction and collisional events related to the South Qilian Ocean, which was a southern branch of the Proto-Tethys Ocean in the Qilian Orogen [14,[17][18][19][20].…”

“…The Qilian Orogen (QLO) is traditionally subdivided from north to south into the North Qilian Belt (NQB), the Central Qilian Block (CQB), and the South Qilian Belt (SQB) (Figure 1a) [1][2][3][4][5][6]. It is generally believed that the QLO has developed as a result of the closure of the Proto-Tethys Ocean [1,2,[6][7][8][9][10]. Because numerous granitic rocks in the QLO are thought to be associated with subducted, collisional, or post-collisional magmatism that was connected to the Proto-Tethys Ocean, these rocks have been studied in the past [1,4,6,[10][11][12][13][14][15][16][17][18][19][20].…”

“…It is generally believed that the QLO has developed as a result of the closure of the Proto-Tethys Ocean [1,2,[6][7][8][9][10]. Because numerous granitic rocks in the QLO are thought to be associated with subducted, collisional, or post-collisional magmatism that was connected to the Proto-Tethys Ocean, these rocks have been studied in the past [1,4,6,[10][11][12][13][14][15][16][17][18][19][20]. Following the widespread Early Paleozoic granitic magmatism, numerous hydrothermal mineralizations of W, Mo, Nb, Ta, Cu, Pb, Zn, Au, and F have been developed throughout the NQB and CQB [6,[21][22][23].…”

“…However, in the SQB, there are apparently less granitic-magmatism-related mineralizations than those in the NQB and CQB. Currently, the SQB has only a limited number of significant mineral deposits, including a few gold deposits in the Danghe Nanshan mountain in the northern margin of the SQB [6,21], a few W deposits in the Tianjun area in the central segment of the SQB [6,23], and the Yama fluorite deposit and the Yansenhada (i.e., Yamatu) quartz vein deposit in the Tataleng granitic batholith in the western segment of the SQB [6,10,23,24]. Any detailed reports of geological and geochemical research on the mineralization in the SQB are lacking, leaving the ore genesis unresolved for the time being.…”