Multiple Magma Conduits Model of the Jinchuan Ni-Cu-(PGE) Deposit, Northwestern China: Constraints from the Geochemistry of Platinum-Group Elements

Abstract

The giant Jinchuan nickel-copper-platinum-group element (PGE) deposit is hosted by two individual sub-vertical intrusions, referred to as the western and eastern intrusions (including segment II-W and segment II-E). Exactly how the Jinchuan deposit was formed by a system of sub-vertical magma conduits is still not well understood. This paper reports new major elements, trace elements and PGEs data from the Jinchuan deposit to study the formation mechanism of sulfide ores with different textures and their relationship with the magma conduit system. Our study shows that the PGE tenors of disseminated and net-textured sulfide in segment II-E are significantly lower than segment II-W and the western intrusion, but the Cu/Pd ratios are opposite. In addition, net-textured sulfides in segment II-W show a negative correlation between IPGE (Ir, Ru and Rh) and PPGE (Pt and Pd) in contrast to the positive correlation in segment II-E and the western intrusion. These features indicate the parental magma sources of the western intrusion, segment II-W and segment II-E were originally three different surges of PGE-depleted magma. Modeling of parental magma in the western intrusion, segment II-W and segment II-E suggests that they were formed by the same initial picritic basalt (100 ppm Cu, 1 ppb Ir and 10 ppb Pd) with different prior sulfide segregations (0.0075%, 0.0085% and 0.011%). The three parts of Jinchuan sulfides show that the Pt/Pd and (Pt + Pd)/(Ir + Ru + Rh) ratios decrease from section III-5 toward both sides in the western intrusion and decrease from section II-14 toward all sides, whereas no regular spatial variations occur in segment II-E, showing the different fractionation processes of sulfide melt. The massive sulfides in the western intrusion and segment II-E experienced a ~20% to 40% and ~40% to 60% fractionation of sulfide melt, respectively. We propose that the Jinchuan deposit was generated in a metallogenic system of multiple magma conduits, where sulfides entrained in parental magma experienced different amounts of prior removal.

Deep Mineral Exploration of the Jinchuan Cu–Ni Sulfide Deposit Based on Aeromagnetic, Gravity, and CSAMT Methods

“…For the deep prospecting of the mining area, the previous study shows that the deep part of No.2 ore body in the II mining area has the possibility of a branch compound and a pinch-out reappearance of ore body. The main reason for this is that the extension of No.2 ore body below the 1000 m level is not revealed, and the geological sections of line [28][29][30] show that the ore body below the 1100 m level has not been pinched out [2]. Based on the multiple geophysical data, this paper investigates the deep of No.1 ore body so as to find out whether there is the possibility of new ore body in the deep.…”

“…The ore bearing ultrabasic rock bodies unconformity intrudes into the Baijiazuizi formation of pre-Great Wall system, which is in direct contact with gneiss, marble and banded migmatite, in the form of wall. It is about 6500 m long, 20-527 m wide on the surface (Figure 2a) and has a southwestern downward extension of more than 1000 m from the ground surface ( Figure 2b) [28]. The overall strike is N50 • W, inclined to the SW, with a dip angle of 50 • -80 • .…”

“…The NCC is one of the three major Precambrian blocks in China, which formed from an amalgamation of micro blocks [24][25][26]. The Alxa block is located in the westernmost part of the NCC and is in fault contact with the Tarim craton to the west, bounded by the North Qilian orogenic belt in the south and the Central Asia orogenic belt in the north (Figure 1a) [27,28]. The Longshoushan terrane is a long narrow northwest trending uplift.…”

“…(a) The location of the study area and (b) a simplified geological map of the Longshoushan terrane. Both subfigures are based on[27,28].…”

Basalt Tectonic Discrimination Using Combined Machine Learning Approach

“…The contents of major elements, trace elements and isotopic composition can be obtained by whole-rock geochemical analysis. Thus, it is feasible to discriminate the tectonic settings where magma formed and the chemical properties of magmatic source areas through elemental composition [11]. Basalt, as a typical magmatic rock, is widely distributed in the earth's crust, and its chemical element content can provide sufficient information about mantle sources, mantle partial melting, magma crystallization processes and mantle metasomatism [12,13].…”