Rio Tinto signed off on the dumping of Indigenous heritage material at the Darwin tip from at least 14 cultural sites at its Marandoo mine operation in the 1990s, an explosive submission to the Juukan Gorge inquiry has revealed.

Wintawari Guruma Aboriginal Corporation published the submission on Friday, alleging the accidental disposal of excavated material from Manganese Gorge led to Rio Tinto, its contractor Kinhill Engineers and the WA Aboriginal Affairs Department conspiring to dispose of the remaining heritage material and associated records at the same tip.

In 1992, Rio Tinto engaged Kinhill to excavate Eastern Guruma sites as required by its Section 18 consent. Kinhill salvaged just 28 sites out of an estimated 400-plus sites at Marandoo, a 193 square kilometre area.

The bulk of the material was sent to Northern Territory University (NTU), now Charles Darwin University,for analysis.

The Manganese Gorge material was found to date back at least 18,000 years — a significant finding at the time due to the dating technology available — and proved Traditional Owners connection to the Hamersley Ranges prior to the last ice age.

In October 1995, NTU sent a letter to Kinhill delivering the devastating news that the material from Manganese Gorge had accidentally been taken to the Darwin tip.

Eastern Guruma people have long maintained that burial sites and ancestral remains rested at Manganese Gorge and the surrounding sites at Marandoo before it was ravaged by Rio Tinto.

“My Old People were adamant that Manganese Gorge … be preserved. It was too important to destroy,” said a WGAC Board member and Traditional Owner who did not want to be named.

“They told me the places at Marandoo were too important to lose. They spoke and no one listened.”

The letter, seen by NIT, says university staff cleaning the building “removed the archaeological collections to the local dump”.

“All other archaeological materials from Marandoo have not been damaged and are safely housed,” the letter said.

In March 1996, five months later, Kinhill proposed to the WA Aboriginal Affairs Department and Rio Tinto that they return some of the material to the miner and dump the rest at the tip, as the project was taking too long and getting too expensive.

“The other unsorted excavated materials from the ‘minor’ sites probably should be discarded in Darwin,” the proposal said.

The proposal, seen by NIT, then outlines a table of materials to be sorted, analysed, unanalysed or discarded at the tip. More than 10 sites were suggested to be discarded.

In a letter dated January 1997, Rio Tinto (then Hamersley Iron) accepts the proposal.

One month later, NTU issues a report on the “status of archaeological material” from Marandoo.

“The current status (existence) of the material was determined and, under the direction of [Kinhill and NTU] … was returned to Hamersley Iron, sent to … [another university] for analysis or discarded.”

The report notes material from 14 sites were discarded at the Darwin tip.

Almost 30 years later, it remains unknown what was in the discarded material as notes and records appear to have been thrown out as well. Eastern Guruma Traditional Owners were unaware until now that their artefacts and material from these sites had been thrown away.

“Marandoo is a part of our Country, and for all these years we’ve been kept out. We can’t access our Country, and we don’t know what goes on there. Rio said they would look after it, and it is upsetting that they didn’t even do that,” Traditional Owners and the WGAC Board said.

“We are releasing this story because we never want to see this happen again.”

Rio Tinto Iron Ore chief executive Simon Trott told NIT the miner is “not proud of many parts of our history at Marandoo”.

“We reiterate our apology to the Traditional Owners of the land, the Eastern Guruma People, for our past actions. We know we have a lot of work ahead to right some of these historical wrongs which fell well short of the standards we expect today,” Trott said.

Trott said Rio Tinto supports repealing the Marandoo Act and that the miner has been engaging with Traditional Owners and the WA Government on the matter.

But WA Minister for Aboriginal Affairs Stephen Dawson says it is yet to be determined whether the State Government will repeal the controversial Act.

WA Minister for Aboriginal Affairs Stephen Dawson has been contacted for comment.

By Hannah Cross