The BHP Group Ltd (ASX: BHP) share price got off to a shaky start in September.
Very shaky.
Shares in the S&P/ASX 200 Index (ASX: XJO) mining stock closed out August trading for $40.77.
When the closing bell rang on 6 September, shares were trading for $38.45 apiece.
This saw BHP stock down 5.7% in the first week of September trading.
From there, however, things took a sharp turn for the better for stockholders in Australia's biggest miner.
At the close of trade on 30 September, shares were changing hands for $45.96 each.
That put the BHP share price up 12.7% in September and up 19.5% from 6 September levels. This smashed the benchmark returns, with the ASX 200 gaining 2.2% over the month just past.
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What sent the BHP share price on this wild ride?
There were no price-sensitive announcements released by the miner over the month.
But the BHP share price is very sensitive to moves in iron ore and copper prices. Iron ore counts as the miner's top revenue earner, with copper coming in at number two.
So how did these two industrial metals move?
Well, the iron ore price closed out August at just over US$100 per tonne before trending lower for much of the month to dip below US$90 per tonne on 20 September. Then the iron ore price came roaring back to end the month at just over US$107 per tonne.
It was a similar story for copper, BHP's second biggest revenue earner.
According to data from Bloomberg, the red metal was fetching US$9,235 per tonne on 30 August before sliding to a low of US$8,954 per tonne on 3 September. As with iron ore, the copper price then went on a tear to trade for US$9,839 by 30 September.
Clearly, the 7.0% monthly gain in the iron ore price and 6.5% lift in the copper price offered some heady tailwinds for the BHP share price. Even more so, as most of the miner's costs to dig up a tonne of copper or iron ore are fixed. Meaning much of the price boost in the commodities will go straight to the bottom line.
Why did the copper and iron ore price surge?
Both industrial metals received a big boost in the latter half of September following fresh stimulus measures from China.
But investors were already bidding up the BHP share price on 13 September, following a speech by Chinese President Xi Jinping in which he urged China's local and central governments to enact policies required for the government to achieve its 5% growth target.
Investors didn't have to wait long for Chinese officials to respond.
As the Motley Fool reported on 24 September, BHP and other ASX 200 mining stocks enjoyed a big lift after the People's Bank of China (PBoC) announced it was lowering the reserve requirements for Chinese banks by 0.50% and reducing downpayment requirements on existing mortgages by 0.50%.
Later that week, investor sentiment was further roused when Chinese officials flagged more rate cuts and fiscal stimulus measures would be forthcoming.
As we approach the end of the second trading day of October, the BHP share price is down 1.2% in the early stages of this month at $45.43.
