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Identity HelpQuartz after platy calcite?

24th Sep 2023 21:45 UTCKyle Beucke 🌟

These samples are from an epithermal gold-silver deposit. I suspect these platy textures may represent quartz growing on platy calcite followed by dissolution of the calcite.

The first attached images are from a coarse example (plates >1 inch across). This sample has silver minerals in some of the quartz (the gray patches/bands). Some interesting textural observations:

-There does not appear to be a clear infill of the dissolved calcite plate. The central area within a plate appears to include quartz and clay/mica but it is very uneven.

-The silver minerals (gray patches) mostly appear to occur in the quartz on one side of the "plate" or the other (see closeup image).

--> Based on these two observations, I suspect what may have happened is the following:

1. Platy calcite deposited in open space.

2. One generation of quartz (maybe with clay/mica, which does deposit with the silver minerals in this deposit) deposited on the calcite plates, primarily on one side of most of the plates due to differences in permeability, etc.

3. Calcite dissolves.

4. Possibly after fracturing, quartz deposits on base of previous quartz, leaving a two-stage plate of quartz with no central layer (a sandwich without the filling).

Thoughts?

Thanks,

Kyle

Questions Answered
Can it scratch glass? : Unable to test
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24th Sep 2023 21:46 UTCKyle Beucke 🌟

Here is another sample. This looks a little more typical for quartz after platy calcite.

24th Sep 2023 21:57 UTCKyle Beucke 🌟

Here are some shots of a homemade thin section of this sample (crossed polars).

Observations:

-Some of the plates are asymmetrical, with finer quartz at the base of one layer, followed by coarser quartz, whereas the other layer is entirely coarser quartz.

-Other (most?) of the plates are semi-symmetrical: Finer quartz at base of both layers. However, in this case, there is no clear center line and there is a sort of stitch-like texture.

24th Sep 2023 22:01 UTCKyle Beucke 🌟

In summary, it seems like, if this texture can be safely assumed to represent the original presence of platy calcite, the quartz textures may be the result of 1. heterogeneous coating of the original calcite plates by silica (giving the differences in mineralization seen on either layer in first example), and 2. recrystallization of the silica, resulting in obliteration of the original flat surface/line that was originally present because of the calcite plates (seen in later example).

Thoughts?

Thank you,

Kyle

25th Sep 2023 16:34 UTCUwe Kolitsch Manager

Or former platy baryte?

Pseudomorphs of quartz after baryte are visually similar; they are called 'zerhackter Quarz' in German.

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