SHANGHAI — The deal could be hard to resist.

A Shanghai investment firm is offering a fat return of up to 10 percent a year, handily beating both the local stock market and the paltry payouts from bank accounts. It requires a minimum deposit of about $15, making it accessible to just about anyone. Investors can pull out in as little as seven days. Best of all, the money is guaranteed.

There is just one catch: Investors know surprisingly little about what they are buying.

The firm, State Gold Treasure, said the money would be plowed into a real estate company building a luxury serviced-apartment complex here in Shanghai. But it will not release details, including the complex’s address.

The offering is just one of a spate of barely regulated, highly opaque investments that are posing a growing threat to the Chinese financial system. As the country’s economy slows, experts increasingly worry that many of the investment funds could fail — and the government may not know how to handle the shock to its financial system.

Over the last five years, Chinese investors have plowed at least $2.8 trillion into buying such funds from banks alone. After quintupling since 2011, these investments, known as wealth management products, now total an amount roughly equal to more than one-third of the country’s annual economic output. Their growth has increased as China’s economy has slowed.