EVOLUTION OF TRACE ELEMENT COMPOSITIONS IN SPHALERITE AND PYRITE THROUGH TIME IN THE ZN-PB-AG SHALE-HOSTED RED DOG DEPOSITS, WESTERN BROOKS RANGE, ALASKA

In-situ trace element geochemical data (LA-ICP-MS) for sulfides from the Red Dog deposits provide a tool for unraveling stages of sulfide deposition. At least four texturally and compositionally distinct types of sphalerite are recognized. From oldest to youngest these are: (1) early brown sphalerite (EBS); (2) yellow to reddish-brown sphalerite (YBS); (3) coarse-grained, strongly zoned red-brown sphalerite (RBS); and (4) late tan sphalerite (LTS). EBS and YBS largely overlapped barite deposition, whereas RBS and LTS formed later. Galena and pyrite/marcasite precipitated with the first three types. Distinct chemical differences exist with regards to Fe, Co, Mn, Tl, and Ge contents. From EBS to later YBS and RBS, there is a trend of increasing Fe and Co and decreasing Mn, Tl, and Ge. The last two types display more compositional variation and higher Fe and Co (up to 8.3 wt % and 400 ppm, respectively in RBS), and lower Tl (<37 ppm) and Ge (mostly <300 ppm) than EBS. LTS has distinctly lower Fe (<0.5 wt %) and higher Tl (up to 350 ppm), Mn (up to 180 ppm), and Ge (430 ppm), relative to earlier sphalerites. Wide ranges in concentrations of Ag, Cu, Pb, and Sb characterize all types, particularly YBS and RBS (Ag=5-750 ppm in YBS; Cu=16-2,800 ppm in YBS; Pb=30-2,000 ppm in RBS; Sb=0.4 to 2,400 ppm in YBS). The observed changes in trace element composition of sphalerite through the paragenesis could reflect changes in the metal content of the ore fluid, the involvement of multiple metal-bearing fluids, and/or the changes in the availability of reduced sulfur at the depositional site. Trace elements in pyrite lack distinct variations among ore stages. Contents of Ag, As, Co, Sb, and Tl overlap and display wide ranges (As=16-10,749 ppm; Ag=6-900 ppm; Co=1.8–5576 ppm; Sb=7- 6717 ppm; Tl=36-2332 ppm). However, pyrite from the Red Dog deposits is distinct from pyrite in unmineralized shale distant from the deposits; the latter has uniformly lower Ag (<20 ppm) and Tl (<40 ppm), and higher Se (up to 709 ppm) relative to Red Dog pyrites (<40 ppm Se). The chemical variations may reflect different temperatures of pyrite formation, or hydrothermal fluids with significantly different trace element/sulfur ratios.