New Zealand Gold Prospecting & Metal Detecting Forums Archive

Below are some images of electrum (a gold-silver alloy) from epithermal quartz veins of the Hauraki Goldfield


Typically most electrum is extremely fine-grained occuring as sub-microscopic to microscopic specs in quartz. Below are some examples from the Karangahake deposit. Access and collection of these sample was kindly granted by Heritage Gold Ltd.


An example of a crustiform banded / brecciated quartz vein from Karangahake. The minute black blebs consist of silver sulfides with



Example of partially oxidised ore. The dark grey areas seen best in the lower image are rich with silver sulfides / sulfosalts: mainly acanthite with a little electrum


Drill core that insected a rich portion of the Maria vein known as BM37. This has mineralised fragment in crustiform banded vein with visible electrum


A most impressive slab from BM37. The metallic band adjacent the coin is all electrum. The coppery looking band is the sulfide chalcopyrite. Sample Heritage Gold Ltd.


The Hauraki Goldfield is also famous for bonanza-grade veins. Below are a couple of examples that I have been able to study on kind loan from Auckland museum.


Platy like electrum in quartz, Thames (mine unknown)


Fine wire electrum? / gold coating quartz, Thames (mine unknown)


Three samples of bonanza-grade electrum in quartz from the Four in Hand mine near Coromandel township



Mass of electrum plates coating comb quartz, Four in Hand mine near Coromandel

they are really cool shots!!
Cheers

Thanks for showing those MPS Gold Hunter.
Mem

Those bonanza-grade samples held by Auckland museum are certainly impressive. I need to re-take images of the smaller samples and add these to the above. As mentioned in another post, these samples I was able to use tiny chips from for my research but unfortunately none contained measurable fluid inclusions. Nonetheless, though hitting a wall on that front it has provided a unique opportunity to view these amazing samples. Because there's essentially nothing internationally published on them, not all is lost and I'm planning to write a paper on the bonanza's based on as many samples I can view / photograph and combine with historical descriptions by Fraser and Adam 1907; Fraser, 1910; and Downey 1935.

Edited by user Wednesday, 29 June 2011 5:17:01 PM(UTC)  | Reason: Not specified

The international one will be some time away as there are other papers to sort first. However I am working on a smaller one for this year’s AusIMM conference. I'm sure there will be no problem with sending you a copy (no charge).


Fluid inclusions can occur in a number of different minerals and deposit types. Quartz is the most common host mineral, but fluid inclusions can occur in calcite, barite, fluorite, sphalerite, stibnite, enargite, wolframite and occasionally pyrite (in the case of the sulfides need an infrared microscopic / camera system to view). In terms of deposit types, quartz veins of the Coromandel (e.g. Martha-Favona, Karangahake, Golden Cross, Broken Hills) are classed as epithermal and can commonly have fluid inclusions in quartz; particularly in the larger, elongated / needle crystals. Fluid inclusions also occur in quartz of orogenic (Mesothermal) gold deposits like those in the South Island (e.g. Blackwater); the eroded remains and accumulations reported as the source of the gold placers. Inclusions in orogenic gold deposits tend to be smaller, less common and different than those in epithermal; they common contain liquid CO2. They also occur in porphyry Cu, Cu-Au, Cu-Mo deposits which commonly trap hypersaline fluid inclusions that contain minute salt crystals (halite, sylvite) and sometimes chalcopyrite. Mississippi type valley deposits mined for zinc and lead also have fluid inclusions; thought these tend to be a bit cooler. Lastly spectacular multiphase inclusions crowded with daughter crystals occur in emeralds / these would be really nice to study ;)

This is by no means exhaustive as they can occur in many deposit types, but not every deposit will have measureable inclusions. For example in my studies Karangahake, Waiorongomai has exceptional inclusions but Tokatea, Kapanga, Hauraki, Thames so far no good.

Edited by user Friday, 1 July 2011 2:39:35 PM(UTC)  | Reason: Not specified