Paragenetic Evolution of the Robertson Deposit: Eocene Reduced Intrusion-Related Gold Deposit in the Northern Shoshone Range, Nevada
Bibliographic record
Abstract
Abstract The Robertson deposit, formerly known as Tenabo, is an Eocene reduced intrusion-related gold deposit located 5 km north of Pipeline, a world-class 20+ Moz Carlin-type gold deposit in the northern Shoshone Range, Nevada. This study is the first in-depth examination of Robertson describing gold mineralization, hydrothermal alteration, petrology of the intrusive rocks, and geochronology of the magmatic and hydrothermal events. We present six new U/Pb zircon ages of intrusive phases, one vein-hosted molybdenite Re/Os age, and one hydrothermal orthoclase 40Ar/39Ar age. Gold mineralization is spatially and temporally related to an intrusive complex referred to as the Tenabo stock, which is composed of successive intrusive phases including an early ilmenite-bearing diorite, andesite dikes, hypabyssal dacitic porphyry dikes, composite ilmenite-bearing granodiorite, granitic porphyry dikes, and hypabyssal rhyolitic dikes. Emplacement of the Tenabo stock resulted in contact metamorphism of the host siliciclastic rocks, yielding quartz, biotite, and calc-silicate hornfels that formed an aureole up to 1 km from the stock. Through petrographic observations and geochemical analyses, distinct alteration assemblages are identified, encompassing sodic-calcic, potassic, calcic, sericitic, and chloritic alteration. Petrographic observations of veins at Robertson indicate the succession of three main stages referred to as (1) pregold, (2) syngold, and (3) postgold. The pregold stage consists of barren quartz veins, potassic alteration veins comprising biotite veinlets and quartz-K-feldspar, quartz-arsenopyrite, quartz-chalcopyrite, and quartz-molybdenite veins. The syngold stage is composed of quartz-gold ± Bi-Te-Se-Pb-Ag–bearing sulfosalt veins containing native visible gold observed as single grains in quartz and/or rimming pyrrhotite, arsenopyrite, chalcopyrite, and loellingite (FeAs2) grains. The postgold veins encompass pyrite ± quartz, quartz-calcite-polymetallic sulfide veins with Pb-Zn-Sb-Ag-Sn base metal sulfides, bull quartz veins, and calcite veins. Crosscutting relationships, along with U/Pb zircon dating of intrusive phases, Re/Os molybdenite dating, and 40Ar/39Ar orthoclase dating, indicate two distinct magmatic phases at Robertson. Phase one is an early phase associated with hydrothermal activity, and phase two is a late phase associated with hypabyssal rhyolitic dikes. The period of gold mineralization at Robertson is linked with the initial phase of magmatism and subsequent hydrothermal activity, which was found to occur between 39.6 and 39.0 Ma. This study demonstrates that gold mineralization is spatially and temporally related to the emplacement of an upper Eocene 39.6 Ma reduced ilmenite-bearing intrusive complex into Paleozoic siliciclastic sedimentary rocks. This produced a high-tonnage, low-grade gold deposit characterized by an Au-As-Bi-Te-Cu metal association. Mineral assemblages observed at Robertson, comprising native bismuth, Bi sulfosalts, arsenopyrite, pyrrhotite, and loellingite (FeAs2), are characteristic of low-sulfidation and oxidation states akin to reduced intrusion-related gold deposits and gold skarns described globally.
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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 | 0.000 |
| 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) | 0.002 | 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".