Fluid Inclusion and Isotopic Constraints on the Origin of the Paleoproterozoic Yinachang Fe-Cu-(REE) Deposit, Southwest China

Abstract

The Yinachang deposit in the Kangdian Fe-Cu metallogenic province, Southwest China, contains approximately 20 Mt ore @ 41.9 to 44.5 wt % Fe and 15 Mt ore @ 0.85 to 0.97 wt % Cu, with potentially significant REEs. Orebodies are hosted in the late Paleoproterozoic Dongchuan Group, and consist mainly of massive and banded replacement ores. The paragenetic sequence of this deposit includes pre-ore Na-(Fe) alteration (stage I), Fe-(REE) mineralization dominated by magnetite and siderite with subsidiary apatite and fluorite (stage II), and Cu-(REE) mineralization with chalcopyrite, ankerite, biotite, and subordinate apatite, fluorite, allanite, and synchysite (stage III). This sequence is similar to those of many iron oxide copper-gold (IOCG) deposits elsewhere. There are two distinct types of fluid inclusions within apatite of both stages II and III. Type 1 fluid inclusions with liquid and vapor phases have relatively low homogenization temperatures (98°–345°C) and salinities (4.7–16.2 wt % NaCl equiv), whereas type 2 fluid inclusions with one or more solid phases, a liquid phase, and a vapor phase have higher homogenization temperatures (273°–>500°C) and salinities (36.2–>59.8 wt % NaCl equiv). Fluids in sulfur isotope equilibrium with chalcopyrite of stages III have δ 34 S CDT values ranging from −2.8 to +2.7‰, consistent with a magmatic origin for the sulfur. Thus, sulfur isotopie compositions imply that magmatic-hydrothermal fluids, probably represented by the hot and saline type 2 fluid inclusions, were involved in the ore-forming process, although δ 34 S close to 0 might also mean that sulfur was leached from igneous rocks by nonmagmatic fluids. Siderite of stage II and ankerite of stage III have δ 13 C PDB values ranging from −12.4 to −7.5‰ and from −8.5 to −4.4‰, respectively. The large variation of δ 13 C PDB values reveals a strong interaction between the magmatic-hydrothermal fluids and the dolostone in the ore-hosting sequence. Calculated δ 18 O SMOW values of the fluids in stages II and III are broadly similar, and vary from 11.1 to 17.1‰ (avg = 13.5‰). Such values are higher than that of magmatic-hydrothermal fluids and thus, may indicate extensive fluid–wall-rock interaction and/or mixing of magmatic-hydrothermal fluids with shallow-level nonmagmatic fluids. This interpretation is consistent with the radiogenic Sr isotope compositions of bulk ores (( 87 Sr/ 86 Sr) i = 0.710404–0.734034) and fluorite separated from ores (( 87 Sr/ 86 Sr) i = 0.709851–0.723658). These nonmagmatic fluids are probably representative of the type 1 fluid inclusions. Bulk ores and mineral separates have ɛ Nd(t) values ranging from −8.1 to +0.4 (mostly −0.9 to +0.4), similar to the coeval mafic intrusions in the region (+2.0 to +2.8). This similarity suggests that REEs in the Yinachang deposit might have been derived dominantly from mantle-derived magmas. On the other hand, relatively low ɛ Nd(t) values (−5.5 and −8.1) of some samples indicate insignificant REE contributions from wall rocks. The Yinachang deposit is temporally associated with regional mantle-derived mafic magmatism, and may have a genetic relationship with it. Fluids derived from deep-seated magmas may have supplied abundant sulfur and carbon as well as some of the ore metals in this deposit. The mafic magmatism could also have provided heat to induce the shallow level nonmagmatic fluids to interact with the sedimentary-volcanic sequence, thus leaching additional ore metals into the ore-forming system. Fluid mixing can effectively trigger saturation and deposition of ore minerals. Our work highlights that nonmagmatic fluids are not a prerequisite for Cu-(Au-REE) mineralization, but may facilitate metal deposition in magmatic-hydrothermal IOCG systems.