Are there any documented cases where a society was capable of large scale selflessness for the benefit of future generations?

The answer to your question is unequivocally "No", since you clarify in your comments that you reject any policy solution that is imposed on society by a ruling class. I can't think of a single public policy solution that has been submitted to public referendum. You've tagged ancient, medieval and modern, but a public referendum has only been possible in the last 50 or 60 years once quasi-universal suffrage became semi-common, and even then we don't submit public policy to plebiscites; we refer them to a ruling class.

Beyond that, the notion that altruism cannot be exercised by anyone who is invested by their society with power is even more cynical than I am (and that is a very high bar).

The answer is also "no" since "altruism" differs by individuals. For the vast majority of human history "good" was defined as "it benefits my tribe more than it benefits my enemies, and harms my enemies more than it harms my tribe". No society with that precept can qualify as altruistic. Even today there are violent disagreements about what is "altruistic" action. the definition of "altruism" belongs in philosophy stack exchange, not in history.

I'm actually quite happy that the answer is "no" - because I don't want to live in a society that has an unambiguous definition of altruism. They tend to be fascist. I am much more comfortable living in a society where my definition of "altruism" can be different from someone elses'.

Societies have governments, almost by definition. Any group of individuals capable of making a decision to defer consumption has, by definition, a government. Since OP defines altruism as excluding governments, no society can be altruistic. OP's definition is internally inconsistent.

OP's example is difficult to understand, since for most of history the value of gold was luxury, not production. Only an insane society would condemn the next generation to starve against the undefined possibility that some future generation might benefit more from a luxury good.

On the other hand there are examples of altruistic behavior in every society.

• The USA is the largest donor of foreign aid in the world.

• Every church/cathedral/synagogue/house of worship ever built is an investment of community resources in community welfare.

• Every monastic institution is an investment of community resources in common welfare - literally, since most of them have a responsibility for charity. OP asserts that the charitable works of religious institutions are driven by a desire to appease the gods - which shows a complete lack of understanding of charity, of economics, of religion, and a peculiar understanding of altruism.

• Every soldier, sailor or marine who sacrifices their life for the protection of their country is altruistic by the common understanding of the word.

• Multiple acts of legislation are altruistic. The Civil Rights Act, the Americans with Disabilities act. These were passed by people who intended the primary/fundamental benefit to go to others; but they are not altruistic because they did not originate in an atheistic anarchic commune.

• @PietrGeerkens cites another good example - Most of the Netherlands is created by dikes that push the ocean back. Each of those is a vast investment of community resources to benefit future generations. But since I can't prove that the effort wasn't organized by a ruling class, I don't know if it qualifies.

@PieterGeerkens cites Genesis 41: Joseph and Pharoh's Dream, but that is invalid since OP clarified that any act by a ruling elite cannot be altruistic.

There are innumerable examples that can be cited either to prove or disprove your question, but as @semaphore notes, without a definition of terms, this is a pub question - capable of generating endless conflict with no resolution.

Japan around the 1500s and 1600s had major issues with deforestation. They had the usual problems - soil erosion, lack of fuel, unpleasant living environment.

Here's a brief article that introduces the idea. Quoting from that article:

Japan had a serious deforestation problem 300 years ago, a consequence of unsustainable forest use that had been building up for a long time (Totman 1989). As long ago as 600-850 AD, construction booms in Nara and Heian, along with demands of the ruling elite for timber to supply armies and build castles and religious monuments, had caused serious deforestation in the Kinai region. Forest use was "exploitative". Timber and other forest products where taken without regard to replenishing the supply.

With the advent of the Tokugawa shogunate and peace, followed by rapid growth of cities and monumental construction projects for castles, temples, and shrines, logging increased during 1600s to a scale never before experienced in Japan. Conflict between villagers and rulers over the use of forest lands - subsistence products for the villagers vs. timber for the rulers - became more intense. By 1670 the population had increased to nearly thirty million, and with the exception of Hokkaido, the old growth forests had been completely logged. The supply of timber and other forest products was running out. Soil erosion, floods, landslides, and barren lands (genya) were becoming ever more common. Japan was headed for ecological disaster.

The solution was a cultural shift on the part of the peasants, reinforced by the Shogun & nobility:

... it seems to have derived from the centuries-old tradition of cooperation among villagers for protection against bandits, allotting rice fields and irrigation water, and storing rice. Until then, village cooperation had not extended to forest management, but villages started responding to the forest crisis by refining the management of satoyama secondary forests for subsistence needs (McKean 1982, 1986), and for the first time, planting sugi and hinoki plantations to help satisfy timber demands of the rulers.

As with all altruism, there is some level of self-interest. Do parents sacrifice for their kids because of selfless love, or does the sacrifice aggrandize the ego? This debate is not really on topic here on History. But the Japanese in the 1600's could have gone either way: they could have continued to deforest and destroy their living environment, or they could work together to solve the problem.

Incidentally, forests in Japan remain an important issue even now; the legacy of their decision to sustain their forests in the 1670's remains important and with controversial outcomes; check out this article.