The theater company, itself rescued from potential bankruptcy by enthusiastic online investors last year, bought 22% of Hycroft on Tuesday—a quickly executed investment that caught analysts and investors off guard and was hailed by Mr. Aron as a “bold diversification move.”

Hycroft landed on AMC’s radar after a surge in interest from individual investors scouring social media for penny stocks that might be poised to benefit from the market volatility spurred by Russia’s invasion of Ukraine. Jason Mudrick, a hedge-fund manager and one of Hycroft’s top investors, saw an opportunity for the company to raise much-needed funding, people familiar with the matter said. It was down to just $8 million in cash.

Mr. Mudrick’s eponymous hedge fund had also invested in AMC when the movie-theater chain was struggling to survive the Covid-19 pandemic, sparking interest among individual investors to do the same.

Mr. Mudrick called Mr. Aron last week and asked if he could offer Hycroft CEO

Diane Garrett

advice on how to tap equity funding through the type of at-the-market share offering that AMC had successfully used, people familiar with the matter said.

Mr. Mudrick’s call to Mr. Aron set in motion the $27.9 million cash infusion by AMC that makes it among the largest stockholders of Hycroft, giving the mining venture years of financial runway, its CEO said. AMC’s investment comes alongside a similar $27.9 million investment by Eric Sprott, a prominent precious-metals investor.

AMC had spent the last nine months evaluating possible acquisition opportunities with a mind to diversifying its business. The company has been experimenting with ideas including starting its own wholesale popcorn line and letting customers pay with cryptocurrencies including dogecoin.

After Mr. Mudrick asked for advice on a Hycroft stock offering, Mr. Aron became intrigued and said that AMC was interested in investing in Hycroft itself, people familiar with the matter said. Mr. Aron had never heard of Hycroft before Mr. Mudrick called him, and soon set up a Zoom call with Ms. Garrett to learn more about the business, according to one of the people.

The cash infusion into Hycroft spotlights the unexpected ripple effects from Russia’s invasion of Ukraine, as well as the enduring influence that individual investors have had in the markets. Before last week, mentions of Hycroft or its ticker HYMC were hardly mentioned on social media, data from global media-intelligence company

Meltwater

show.

But starting last week, as the effects of the war in Ukraine cascaded across commodity markets, chatter about the stock started heating up. Individual investors have been eager to capitalize on the wild swings in stocks and other assets including gold. Hycroft owns a more than 70,000-acre gold and silver mine in northern Nevada.

Before Tuesday’s deal, Hycroft was also in talks with several other potential financing parties, one of the people familiar with the matter said. But the company ended up deciding to do a joint investment deal with AMC and Mr. Sprott, with the idea that AMC’s popularity with individual investors would help propel enthusiasm for its shares.

AMC has weathered its own financial challenges, pulling back from the brink of bankruptcy in late 2020 and then finding a lifeline from retail investors in 2021.

Messrs. Mudrick and Aron, along with AMC’s lead board director, Philip Lader, and AMC general counsel Kevin O’Connor in recent days flew to Nevada, where the AMC officials took a tour of the Hycroft mine in Winnemucca, the people familiar with the matter said. They ended up signing the deal a few minutes before 4 p.m. on Monday, they said.

Shares of Hycroft surged Tuesday morning after the investment was announced, jumping as high as 96% during the session before closing about 9.4% higher at $1.52 a share. AMC shares also saw a bump, finishing 6.8% higher for the day.

Many traders that piled into AMC when it became a meme-stock favorite still haven’t sold, even as the stock has fallen roughly 77% from its closing high of $62.55 last year. Many also on Tuesday cheered the news of AMC’s investment on social media.

Michael Pachter, an analyst who covers the company for Wedbush Securities, said the deal “makes no sense whatsoever” and that “there is no rational argument where AMC is a good strategic investor in a gold-mining company.”

Hycroft on Tuesday indicated it was eager to capitalize on the newfound interest in its stock. It said in a filing with securities regulators that it had entered an agreement with B. Riley Securities Inc. to sell from time to time up to $500 million of its stock in an at-the-market offering program. B. Riley also advised AMC on some of its stock offerings when it was low on liquidity during the pandemic.

When AMC was in the process of raising money from stock offerings in 2020 and 2021, it ended up running out of new shares to sell, and found that its new shareholders, primarily individual investors, wouldn’t let the company sell more because of fears of dilution.

Mindful of this, Hycroft has already sought permission from its owners to issue a substantial number of additional shares should it need to raise cash in the future, people familiar with the matter said.

Write to Alexander Gladstone at alexander.gladstone@wsj.com and Caitlin McCabe at caitlin.mccabe@wsj.com