Petrography and geochemistry of tourmaline breccia in the Longtoushan Au deposit, South China: genesis and its exploration significance
Bibliographic record
Abstract
The Longtoushan Au deposit, located in the Qin-Hang Metallogenic Belt, South China, is a breccia-hosted epithermal Au deposit. It is characterized by the distinctive occurrence of magmatic-hydrothermal tourmaline-cemented breccia. Tourmaline-group minerals have an extensive range of chemical composition that could reflect their host environments, including some of economic interest. To investigate the potential of tourmaline as a geochemical indicator for ore exploration, chemical and boron isotope compositions of tourmaline from the Longtoushan Au deposit have been determined. Tourmaline mainly occurs as orbicular aggregates, clusters and veinlets in rhyolite porphyries, granite porphyries and in spatially associated hydrothermal breccias (Au-mineralized and Au-barren). Three types of tourmaline (I, II and III) are recognized at Longtoushan, most of which being alkali-deficient schorl-dravites. The pre-ore Tourmaline I in the rhyolite porphyry is Fe-rich schorl, contrasting with the dravitic ore-stage Tourmaline II in the mineralized breccia and Tourmaline III in the granite porphyry. Locally, Tourmaline II shows concentric zoning which correlates with core-rim chemical variations (higher Fe, Al in the rim than in the core). Tourmalines from Longtoushan are characterized by a dominant MgFe -1 exchange vector and have relatively low Na contents (avg. 0.66 apfu; formula normalized on 15 total cations), indicating they possibly crystallized in reduced fluids of low-salinity. The δ 11 B values of tourmalines from this study fall in a narrow range of −14.5 to −10.2‰, indicating dominant magmatic-hydrothermal fluids of continent crust origin. The results suggest that tourmaline from Longtoushan may originate from boron-rich vapor fluids unmixed from felsic magmas. Tourmaline breccia was likely formed by hydraulic fracturing, with open spaces infilled by boron-rich boiling fluids released from the magma. The fluid boiling process during brecciation would be critical for Au-bearing pyrite precipitation, which could compete with coexisting tourmaline for Fe, leaving tourmaline Fe-depleted. We propose that the Fe-poor dravitic tourmaline in the hydrothermal breccia could be used as prospecting guide at the Longtoushan Au deposit. Supplementary material: major element and boron isotope composition analyses of tourmaline are available at https://doi.org/10.6084/m9.figshare.c.4468808
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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 source (direct Gemma or distilled Codex), 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".