The Rio Tinto Ltd (ASX: RIO) stock price is hardly making waves today. Yet, plenty of talk about the mining giant is being had regardless.

Shares in the metals and minerals titan are down a smidgen, slipping 0.8% to $122.09 as we waltz into the closing moments of trade. The move contrasts the 0.4% rise in the S&P/ASX 200 Index (ASX: XJO), which is being hoisted higher with the help of tech and real estate shares.

Why are investors fussing over Rio Tinto shares amid the modest move?

Rio Tinto is at risk of losing one of its largest shareholders.

The fifth biggest chunk of Rio Tinto ownership is held by the world's largest sovereign wealth fund, the Government Pension Fund of Norway, held by the Norges Bank Investment Management. The position is valued at A$3.2 billion and accounts for 1.9% of all Rio Tinto shares on issue.

The Norwegian fund operates under strict ethical guardrails, excluding any company that violates its ethical criteria. This examination is conducted by Norway's Council of Ethics, which then provides its recommendation to the investment fund.

According to The Wall Street Journal, the ethics council questions whether Rio Tinto is investable. Reportedly there has been communication between the mining company and the Council of Ethics where environmental damage in the Brazilian Amazon is being raised as a concern.

Moreover, it is believed that the Rio Tinto stock may be tarnished by its partial ownership of Mineração Rio do Norte (MRN). The northern Brazilian bauxite miner is also reportedly bringing forward the same headache for South32 Ltd (ASX: S32), with a 33% stake in MRN on its books.

A recommendation by the Council of Ethics has yet to be made. However, it wouldn't be the first time if Rio Tinto gets booted from the Norway fund.

Rio Tinto was ousted from the fund in 2008 due to the risk of severe environmental damage related to its Grasberg mine in Indonesia. The company was then readmitted to the Government Pension Fund of Norway in 2019 after Rio Tinto agreed to sell its interest in the mine.

Would the Norwegian fund be passing up a top-performing ASX 20 stock if it were to jump ship?

The S&P/ASX 20 Index (ASX: XTL) has climbed 7.4% over the last year. Meanwhile, the Rio Tinto share price has increased by a lesser 3.5%. Only six companies inside the top 20 have delivered a weaker return:

Indeed, Rio Tinto's stock is far from being the top performer in the last year. That title instead goes to James Hardie Industries Plc (ASX: JHX) with its 87.7% gain.