T217 Magmatism and Accreted Terranes from the North American Cordillera to the Norwegian Caledonides II: a Session in Honor of the Scientific Contributions of Calvin G. Barnes

Tuesday, 24 September 2024
1:30 PM - 5:30 PM
207B

IMPORTANCE OF FLUID-ROCK INTERACTION PROCESSES FOR THE FORMATION OF IGNEOUS HYDROTHERMAL Sn-W±Cu DEPOSITS DURING BATHOLITIC HERCYNIAN MAGMATISM, CORNWAL, UK

Paper number: T217-11
Program: Technical Sessions
Time: 3:55 PM – 4:10 PM

Authors

Abstract

The Land’s End Granitic Complex of the Cornubian Batholith exemplary demonstrates the magmatic-hydrothermal transition and the physical and chemical requirements for the genesis of igneous hydrothermal Sn-W-Cu deposits. The key locality is the Nanjizal Bay hosting previously mined Sn-W deposits, and a detailed record of alteration-patterns associated with the ore-forming events.

Three vein-types, distinguished from their field appearance, were mapped, sampled and analyzed chemically as well as petrographically to characterize fluid-rock interaction processes and their impact on transporting and subsequently trapping ore-forming minerals. Here we applied a combination of alteration processes, fluid inclusion analysis, tourmaline and rutile chemistry to estimate important parameters such as salinity, pH and Eh.

Three hydrothermal episodes are recognized. 1) tourmaline (± quartz)–veins, characterized by a red potassic alteration zone, consisting of newly formed K- feldspar-quartz at 450-500 °C. This episode was associated with hydrofracturing and oxidation of reducing magmatic hydrothermal fluids under the formation of cassiterite. 2) Rutile and hematite formation and Fe3+ rich vein-tourmaline imply later oxidizing fluids at c. 300-400 C. 3) boiling and subsequent phase separation due to decompression yielded a high density, oxidizing fluid phase, resulting in cassiterite formation and tourmaline-rich veins – also at c. 300-400 C. This later hydrothermal event comprises tourmaline (± quartz)–veins, characterized by sericitic alteration of the host rock. Potassic alteration and later hematitization and the formation of cassiterite indicate oxidizing conditions which, later, is overprinted by sericitic alteration of the feldspars by reducing magmatic-hydrothermal fluids where tourmaline, forming from the reducing fluids, obtained Fe and Mg from the breakdown of biotite during sericite-replacement.

In conclusion, our studies of the Nanjizal locality, demonstrates a delicate balance between fluid controlled (reducing) and host-rock controlled (oxidizing) hydrothermal conditions that subsequently control if Sn remain in solution or is precipitated forming potential ore-deposits.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-404349

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