Fluid inclusions in quartz from oceanic hydrothermal specimens and the Bingham, Utah porphyry-Cu deposit: a study with PIXE and SXRF

摘要

Individual natural fluid inclusions in quartz were selected for non-destructive microprobe analysis by synchrotron X-ray fluorescence (SXRF) and proton-induced X-ray emission (PIXE) in order to compare and contrast the compositions of porphyry-type ore forming brines and two types of ocean crustal brines. The inclusions contained brines with high salt concentrations (typically 20-40 wt.% total), and one or more daughter crystals. The X-ray and proton beams produced detectable characteristic X-rays for many elements and allowed the quantification of concentrations for many elements with atomic number Z > 14. Fluid inclusions associated with the core of the Bingham, UT, porphyry-Cu(Mo) deposit contain NaCl brines that also contain (element, typical range in ppm): K, 12,000-100,000; Ca, less than or equal to 17,000; Mn, less than or equal to 8000; Fe, less than or equal to 120,000; Cu, less than or equal to 8000; Zn, less than or equal to 5000; Br, 12000. These values are in good agreement with published estimates, and results from SXRF and PIXE experiments are generally consistent. The Bingham fluid compositions closely resemble brines from the porphyry-Mo(Cu) deposit at Questa, NM. Inclusions from two suites of oceanic plutonic rock samples gave PIXE results consistent with published SXRF analyses from other inclusions in the same samples. New inclusion analyses from the Mathematician Ridge locality indicate the NaCl brines also contain, on average (element, ppm): K, 3500; Ca, 35,000; Mn, 4900; Cu, 400; Zn, 200; Br, 1000. New inclusion analyses from the Oceanographer Transform locality indicate NaCl brines that also contain, on average: K, 37,000; Ca, 35,000; Mn, 10,000; Cu, less than or equal to 400; Zn, 2200; Br, 1000. These values are calculated on the assumption that Fe in the inclusions is 50,000 ppm. Compared to continental ore forming brines, the oceanic brines have higher Ca and lower K and Cu concentrations. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 31]