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(2020) Subseafloor Alteration of a Modern Mafic-Volcaniclastic Hosted Seafloor Massive Sulfide Deposit

Anderson M, Hannington M, McConachy T, Jamieson J, Hansteen T & Petersen S

https://doi.org/10.46427/gold2020.62

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08m: Room 3, Tuesday 23rd June 22:03 - 22:06

Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.

Submitted by Daniel Gregory on Thursday 18th June 21:46
Very interesting presentation. I'm especially interested in the anhydrite, if we're to look at ancient deposits how can we determine whether anhydrite used to be there? Do you know what the S-signature of your anhydrite is?
In some cases, the anhydrite may hydrate to gypsum, which has better preservation potential because there it doesn’t have retrograde solubility (locally abundant in Kuroko ores). But in most cases, the only evidence of anhydrite would be the presence of alteration minerals formed at temperatures that would have caused anhydrite precipitation. The S-isotopic siguature of the anhydrite is that of seawater sulfate (~21 per mil) as we expect).

Submitted by Jun-ichiro Ishibashi on Monday 22nd June 15:24
Thank you for the interesting presentation. You suggested lateral fluid flow is related to the alteration zone consists of I/S and Chamosite . Do you have any idea for how the fluid formed (mixture of any end members ?) and for how the fluid migrated laterally ?
To answer the second part of the question first, the main control on fluid migration is the local permeability of the substrate. The volcaniclastic material display facies variations and crude bedding, and in some cases, we see variations in the calculated clay temperatures associated with these variations in substrate permeability. For the nature of the fluid itself, the alteration assemblages indicate a near-neutral or slightly acidic pH, and fluid inclusion salinities isotopic data indicate seawater-dominated fluids, possibly with a component of condensed vapor-phase liquid, and abundant fluid mixing between the hydrothermal fluids and ambient seawater. We have not yet had the opportunity to sample the hydrothermal fluids themselves. For more information, I encourage you to check out our paper in Economic Geology (Anderson et al., 2019).

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