Can I use stocks as margin for commodities futures?

Last week, I bought 4 shares of emicro gold futures (https://www.cmegroup.com/trading/metals/precious/e-micro-gold_contract_specifications.html) worth about 70k. I got a margin call for owning those gold despite having 400k worth of equities and having no other margin debt in my account.

Why did I get this margin call? I had 0 cash, but 400k worth of stocks. Is it because my equities don’t count towards the maintenance margin of the futures contract? As far as I understood the maintenance margin is 1k per future contract in the link above, so I guess I would need 4k of cash (not stocks) in my account at all times to hold those 4 gold futures contracts?

I think the mistake I made was that despite having lots of stocks, I had no cash in my account. Can someone confirm this is what triggered my margin call?