Fluid and source magma evolution of the Questa porphyry Mo deposit, New Mexico, USA

Abstract

Combined fluid inclusion microthermometry and microanalysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) are used to constrain the hydrothermal processes forming a typical Climax-type porphyry Mo deposit. Molybdenum mineralisation at Questa occurred in two superimposed hydrothermal stages, a magmatic-hydrothermal breccia and later stockwork veining. In both stages, texturally earliest fluids were single-phase, of low salinity (~7 wt.% NaCl equiv. ) and intermediate-density. Upon decompression to~300 bar, they boiled off a vapour phase, leaving behind a residual brine (up to 45 wt.% NaCl equiv ) at temperatures of~420°C. The highest average Mo concentrations in this hot brine were~500 μg/g, exceeding the Mo content of the intermediate-density input fluid by about an order of magnitude and reflecting pre-concentration of Mo by fluid phase separation prior to MoS 2 deposition from the brine. Molybdenum concentrations in brine inclusions, then, decrease down to 5 μg/g, recording Mo precipitation in response to cooling of the saline liquid to~360°C. Molybdenite precipitation from a dense, residual and probably sulphide-depleted brine is proposed to explain the tabular shape of the ore body and the absence of CuFe sulphides in contrast to the more common Cu-Mo deposits related to porphyry stocks. Cesium and Rb concentrations in the single-phase fluids of the breccia range from 2 to 8 and from 40 to 65 μg/g, respectively. In the stockwork veins, Cs and Rb concentrations are significantly higher (45-90 and 110-230 μg/g, respectively). Because Cs and Rb are incompatible and hydrothermally non-reactive elements, the systematic increase in their concentration requires two distinct pulses of fluid exsolution from a progressively more fractionated magma. By contrast, major element and ore metal concentrations of these two fluid pulses remain essentially constant. Mass balance calculations using fluid chemical data from LA-ICPMS suggest that at least 25 km 3 of melt and 7 Gt of deep input fluid were necessary to provide the amount of Mo contained in the stockwork vein stage alone. While the absolute amounts of fluid and melt are uncertain, the wellconstrained element ratios in the fluids together with empirical fluid/melt partition coefficients derived from the inclusion analyses suggest a high water content of the source melt of~10%. In line with other circumstantial evidence, these results suggest that initial fluid exsolution may have occurred at a confining pressure exceeding 5 kbar. The source of the molybdenum-mineralising fluids probably was a particularly large magma chamber that crystallised and fractionated in the lower crust or at midcrustal level, well below the shallow intrusions immediately underlying Questa and other porphyry molybdenum deposits.

Fluid Inclusion and Oxygen Isotope Constraints on the Origin and Hydrothermal Evolution of the Haisugou Porphyry Mo Deposit in the Northern Xilamulun District, NE China

“…As for fluid boiling, however, it has been recently suggested that it is not a major cause for Mo precipitation in many porphyry Mo deposits [13]. This is because the initial supercritical (single-phase) fluid has a relatively low salinity (e.g., ∼7 wt% NaCl equiv [2,56,59]), and the two-phase field is intersected on the vapor limb once boiling occurs; that is, brine will condense out of a vapor-like bulk fluid, rather than the other way around. In this case the condensation of small amounts of brine out of a vapor-like bulk fluid should not significantly influence the properties of the bulk system, and therefore fluid boiling in the porphyry Mo system is unlikely associated with metal precipitation [13].…”

“…In some of these deposits Mo is recovered as byproduct (i.e., porphyry Cu-Mo (±Au) deposits), whereas, in others, the deposits are characterized by high abundances of Mo but virtually no Cu or Au (i.e., purely porphyry Mo deposits) such as the Climax-type deposits along the North American Cordillera [1][2][3]. China has more than half of the world's molybdenum metal [4], and many of the Mo deposits are similar to the Climax-type Mo deposits where molybdenite is almost the only sulfide [5].…”

“…The nature and evolution of the fluids associated with porphyry Mo mineralization have been well documented through mineralogical, stable isotopic, melt and fluid inclusion studies [2,3,7,10,[12][13][14]. It is generally accepted that both the Climax-and the Dabie-type Mo deposits are related to hydrothermal fluids with high contents of F and H 2 O but variable CO 2 [7,13].…”

Fluid inclusion and H–O–C isotope geochemistry of the Yaochong porphyry Mo deposit in Dabie Shan, China: a case study of porphyry systems in continental collision orogens

“…In Climax-type deposits, the earliest fluids were singlephase, CO 2 -bearing aqueous fluids, were of low salinity and intermediate density, and were released from the evolving magma from mid-crustal to lower crustal level; by decompression from near-lithostatic to near-hydrostatic conditions, they boiled off a vapor phase, leaving behind a residual brine that ponded in the roof region of the granite porphyry; Mo precipitated from a dense, residual and probably sulfide-depleted brine, in response to cooling of the saline liquid from 420 °C to 350 °C (Klemm et al 2008). By contrast, the Yaochong porphyry Mo system was formed with a large number of PC-and C-type inclusions in the minerals in earlier stages (Table 4).…”

“…This alteration feature is shared by all the Mesozoic porphyry Mo systems in the East Qinling and Dabie Shan, such as the deposits at Yuchiling (li et al 2012c), Jinduicheng (Yang et al 2009), nannihu , Tangjiaping (Chen and Wang 2011), and Qian'echong . ); Jingduicheng (Yang et al 2009) Climax (Hall et al 1974) Henderson (Seedorff and Einaudi 2004a, b;Wallace et al 1978) Urad (Wallace et al 1978) Questa (Cline and Bodnar 1994;Klemm et al 2008) 1 3…”