Extreme sulfur isotope fractionation of hydrothermal auriferous pyrites from the SW fringe of the Taupo Volcanic Zone, New Zealand: Implications for epithermal gold exploration
Bibliographic record
Abstract
The Taupo Volcanic Zone and its environs, New Zealand, represent an ideal place for epithermal gold exploration. We have documented structurally-bound gold in low temperature hydrothermal pyrite veins from the Erua Basin area on the outside fringe of the Taupo Volcanic Zone. Extreme 34S/32S fractionation of 122.2‰ from auriferous pyrite is documented in hydrothermal veins hosted in alluvial deposits from the Erua Basin. We document three generations of pyrite based on petrographic and Nano-SIMS microstructural analysis. First generation pyrite (Py assemblage -1) has extremely high δ34S values of +70.3 to +100.0‰ probably resulting from sequential-cyclic bacterial reduction and Rayleigh fractionation of limited sulfate. Second generation pyrite (Py assemblage -2) has relatively lower δ34S values of -39.8 to -3.3‰, interpreted to reflect the influx of magmatic-related oxidized fluids mixed with remobilized, reduced sedimentary sulfides that then formed the host calcite-pyrite veins. Third generation pyrite (Py assemblage -3) incorporated much heavier 34S resulting in positive δ34S values ranging from +6.8 to +72.2‰, indicating greater reduction of sulfate to H2S in an anoxic (reducing) environment. One single composite ~ 10 um pyrite grain encompassing all three types of subgrains has a large δ34S range from -22.2‰ along its rim and +100.0‰ from its core (122.2‰ for a single grain, and 139.8‰ for the system). This is the largest known variation in δ34S ever reported in a single pyrite grain from any system on Earth, but is comparable to δ34S ranges in pyrite systems of up to 186‰ reported from microbial sulfate reduction in deep bedrock fracture systems in Sweden. The growth stages from Py-2 to Py-3 indicate a transformation of the environment from an open to a closed (highly reducing) system, and it is these late-stage vein pyrites that are slightly auriferous. This process is interpreted to be induced by the precipitation of sulfides perhaps near the margin of the hydrothermal system or closed growth of pyrite in hydrothermal veins, which both restrict circulation and convection of sulfate within the veins. Our results suggest that the Erua Basin may sit on the fringe of a low-temperature epithermal gold system; other “low-temperature” areas around the TVZ may also preserve similar, potentially enriched zones on the edge of an epithermal Au belt.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".