In situ multiple sulfur isotope analysis by SIMS of pyrite, chalcopyrite, pyrrhotite, and pentlandite to refine magmatic ore genetic models

Abstract

With growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems, we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic, magmatic-hydrothermal, and hydrothermal ore systems. These materials include Sierra pyrite (FeS2), Nifty-b chalcopyrite (CuFeS2), Alexo pyrrhotite (Fe(1-x)S), and VMSO pentlandite ((Fe,Ni)9S8) that have been chemically characterized by electron microprobe analysis, isotopically characterized for δ 33 S, δ 34 S, and δ 36 S by fluorination gas-source mass spectrometry, and tested for homogeneity at the micro-scale by secondary ion mass spectrometry. Beam-sample interaction as a function of crystallographic orientation is determined to have no effect on δ 34 S and Δ 33 S isotopic measurements of pentlandite. These new findings provided the basis for a case study on the genesis of the Long-Victor nickel-sulfide deposit located in the world class Kambalda nickel camp in the southern Kalgoorlie Terrane of Western Australia. Results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite, pyrrhotite and chalcopyrite can better constrain genetic models related to ore-forming processes. Data indicate that pentlandite, pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar Δ 33 S values +0.2‰. This isotopic equilibrium unequivocally fingerprints the isotopic signature of the 2 magmatic assemblage. The three sulfide phases show slightly variable δ 34 S values (δ 34 Schalcopyrite = 2.9 ± 0.3‰, δ 34 Spentlandite = 3.1 ± 0.2‰, and δ 34 Spyrrhotite = 3.9 ± 0.5‰), which are indicative of natural fractionation. Careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage, which may help resolve the nature of the ore-forming process. Hence, this SIMS-based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic-and alteration-related sulfides, which is not resolved during bulk rock fluorination analysis. The results indicate that, unlike the giant dunite-hosted komatiite systems that thermo-mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative Δ 33 S values, the Kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales. HIGHLIGHTS  Characterisation of four sulfide standards for multiple sulfur isotope analysis: pyrite, chalcopyrite, pyrrhotite, and pentlandite for distribution  Analysis of orientation effect in pentlandite  Natural sulfur isotope fractionation between pentlandite and pyrrhotite  Case study multiple sulfur isotope analysis of three sulfide phases within world-class Long-Victor komatiite-hosted nickel-sulfide deposit KEYWORDS Multiple sulfur isotopes, SIMS, in situ, sulfide minerals, ore genesis 1.

Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging: Extracting local paleo‐environmental information from modern and ancient sediments

“…Iron sulfide minerals, particularly pyrite (FeS 2 ), represent a substantial geologic reservoir of sulfur. Pyrite is a key constituent of many iron sulfide ore deposits, a common accessory phase in an array of igneous and metamorphic rocks, and a nearly ubiquitous mineral in marine sedimentary rocks of all ages . Sedimentary pyrites have diverse morphologies, crystal sizes, and S‐isotope compositions, and these characteristics have proven to be invaluable archives for reconstructing local environmental conditions as well as global‐scale changes in biogeochemistry .…”

“…First, by rastering over grains, scanning ion imaging can generate a continuous record of isotope variations, one that can be interrogated at variable spatial resolution after data collection. Previously, most SIMS studies of pyrites relied on analyses of fairly large (≥10 μm‐diameter) spots within grains . This precludes analyses of many sedimentary pyrites (i.e., those with diameters <10 μm).…”

“…Progress has been made using spot analyses by secondary ion mass spectrometry (SIMS) 1,23,[28][29][30][31][32][33][34][35][36][37][38] or laser ablation (LA)-MC-ICP-MS, 20,39,40 to make spatially resolved δ 34 S measurements. Some have already attributed detectable intra-sample pyrite δ 34 S variability to temporal changes in the S-isotopic composition of the fluids from which the pyrites precipitated.…”

Advances and Opportunities in Ore Mineralogy

“…The growing interest in in-situ analysis of "traditional" stable isotopes notwithstanding (e.g., sulphur [116], development and wider availability of laser-ablation multi-collector (LA-MC)-ICP-MS instrumentation has catalysed a rapid expansion in analysis of non-traditional stable isotopes, including the isotopes of several elements concentrated within ores and measured in ore minerals. These include iron, nickel, copper, zinc, germanium, selenium, and molybdenum.…”

Three‐Dimensional Spatially Constrained Sulfur Isotopes Highlight Processes Controlling Sulfur Cycling in the Near Surface of the Iheya North Hydrothermal System, Okinawa Trough

“…Measurements followed the procedures defined in LaFlamme et al (). Reference material Sierra pyrite (δ 34 S = 2.1‰, Δ 33 S = −0.02‰; LaFlamme et al, ) was used to correct for drift, monitor internal sample repeatability, and calibrate isotope ratios. Secondary reference material Isua pyrite (δ 34 S = 4.3‰, Δ 33 S = +3.1‰; Whitehouse, ) was used to monitor the accuracy of the data.…”