The shop stands in a litter-strewn clear-ing on an over-grown tributary of Venezuela’s Paragua River. The walls are made of sticks, the roof is made of plastic and the whole thing is tied together with string.

On roughhewn shelves, tins of Mexican tuna are stacked next to shiny steel fish hooks, and on the counter there’s a tiny bottle of the espresso-coloured substance the Indians callcomache.

Dispensed in medicinal quantities, this is probably the most vicious condiment on earth. The key ingredients are boiled strychnine – a byproduct of manioc – termites and high-octane jungle chillies, though anaconda bile, snake venom or the leg hairs of a tarantula can be added to spice it up.

But comache isn’t the hottest item in stock here – the clue to that is on a hand-written sign above the counter. It reads: “Bodega El Bucanero. We buy gold and diamonds.” That explains why good-time-girl-turned-shopkeeper Nurys – who sells me a nugget for £15 – is packing a 12-bore.

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It’s a long haul to El Bucanero. First, you fly via Caracas to Puerto Ordaz, on the banks of the Orinoco. Then you make the four-hour drive to the lawless frontier town of La Paragua. Then you take a dugout canoe and dive down the river’s throat, deep into the lungs of the earth. It takes four days if you’ve got an outboard motor and three weeks if you haven’t. By then, you’ll have reached the gold-rich home-lands of the Shirian Indians, near the shores of Lake Manoa and El Dorado.

The problem is, there’s no such place as Lake Manoa or El Dorado. Many, however, say otherwise: miners, treasure-hunters and adventurers have scoured the jungle all over the upper Paragua, seeking the City of Gold, which is always just around the next bend in the river or over the next soaring ridge.

In a bar in Ciudad Bolivar, a French prospector with the soft face of an advertising executive and the scarred hands of a gorilla talks of “the golden place”; in a mosquito-infested clearing, a fever-stricken miner whispers of a reef of pure gold “just two hours away”.

What of the Shirian, who live in this awesome landscape? What do they gain from El Dorado? The answer is little. Outnumbered by their Pemon neighbours, the 600 or so remaining members of the tribe – distant relatives of Brazil’s Yanomami Indians – don’t even exist, according to the Chavez government. Search the Venezuelan land registry and you’ll find their territory, a huge area of emerald forest and tea-brown rivers rich in gold, diamonds and uranium, marked unoccupied.

Now, though, the Shirian have a champion. Agustin Ojeda Mujica is 33, and he’s using his wits to secure his people’s future. “We can’t stop outsiders encroaching on our world,” he says, “but we can try to determine which outsiders come.” Until now, it’s been the missionaries, with their hegemonic brand of cultural genocide, or Brazilian miners, who pollute the Shirian’s waters with mercury and their daughters with Aids.

Working with the veteran British explorer Doug Pridim – one of the handful of outsiders to speak Shirian – and the adventure-tour operator Explore, Agustin sees a third and gentler option. “Tourism is the answer,” he says. “If the world knows about us, we must be recognised by the government. We will stand a chance of survival.” I’m not convinced.

Explore has certainly raised the bar by introducing this extraordinary trip, blurring even further the fuzzy line between adventure tourism and genuine exploration. Yet, as we push past soaring tepuys – the sheer, jungle-covered limestone outcrops that inspired Arthur Conan Doyle’s The Lost World – hauling our boat around waterfalls and heading deeper into territories so remote, my map warns “relief data incomplete”, I wonder if this expedition really is a serious attempt to introduce so-called sustainable tourism as a means of saving the Shirian, or merely the commercial exploitation of a vulnerable society.

Agustin is adamant that his people will benefit, despite the apathy of the tribal elders, but academics at the University of Caracas say otherwise. The anthropologist Francia Medina, a leading authority on the Shirian, says the tourists are carrying on a tradition started by the Spanish: “The conquest continues.”

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