Editing Placer mining (section)

=== Deep leads ===
Deep leads are created when a former stream bed is covered over by later sediments or by igneous rock from a volcanic eruption. Examples existed in the goldfields of [[Gulgong]] and [[Creswick, Victoria|Creswick]] in Australia. The gold bearing gravel is accessed by [[Shaft sinking#Mine shafts|shafts]] and drives similar to underground mining techniques but is typically processed as if alluvial gold.<ref>{{Cite web |title=Gulgong – Gold Rush Heritage |url=http://www.geomaps.com.au/scripts/gulgong.php |access-date=2023-09-05 |website=www.geomaps.com.au}}</ref><ref>{{Cite web|last=Design|first=UBC Web|title=Buried Rivers of Gold Heritage Trail {{!}} Business & Tourism Creswick Inc.|url=https://www.creswick.net/things-to-do/buried-rivers-of-gold-heritage-trail/|access-date=2021-01-22|website=www.creswick.net|language=en}}</ref><ref>{{Cite web |title=Buried Rivers of Gold {{!}} Creswick |url=https://www.buriedriversofgold.org/ |access-date=2023-11-01 |website=Buried Rivers Gold |language=en}}</ref> The heat associated with an igneous lava flow, in some cases, altered the gold bearing gravel so that it needed to be crushed first to extract the gold; an example of this kind of deep lead was found at [[Forest Reefs, New South Wales|Forest Reefs]], also in Australia.<ref>{{Cite news |date=1900-04-21 |title=The Forest Reefs Gold Field. II. |pages=41 |work=Australian Town and Country Journal |url=http://nla.gov.au/nla.news-article71379404 |access-date=2023-05-15}}</ref> If vegetation was buried along with the old stream, by a volcanic eruption, the effect of heat and decay upon that buried vegetation can result in the presence of harmful amounts of [[carbonic acid|carbonic acid gas]] ({{chem2|H2CO3|auto=on}}), as occurred in the deep lead deposit at Creswick.<ref>{{Cite web |title=The Dark Labyrinth |url=https://www.buriedriversofgold.org/the-dark-labyrinth |access-date=2023-11-01 |website=Buried Rivers Gold |language=en}}</ref>