Antofagasta (LSE:ANTO) fell 3.5% on Tuesday as a slide in copper to a seven-week low dragged Mining shares lower and helped push the FTSE 100 to its sharpest one-day fall in a month.

Copper had a bad day on Tuesday 3 November, and few FTSE 100 shares felt it more directly than Antofagasta (LSE:ANTO). The Chile-focused copper producer closed 3.5% lower, according to Reuters, as the red metal slipped to a multiweek low and investors cut exposure to the mining sector across the board.

The wider index did not escape. The FTSE 100 finished at 10,638.43, down 122.90 points or 1.14%, bringing to an end a run of four consecutive daily gains. Reuters described it as the index's steepest one-day decline in a month. Miners and banks did most of the damage, with the FTSE 350 basic resources index losing 2.6%.

There was no company-specific announcement from Antofagasta in the material used for this article. Instead, its decline was part of a metals sell-off that swept through the sector, which makes the move a useful case study in how closely a focused producer's share price can track the Commodity it digs out of the ground.

A seven-week low for copper

Midday reports carried by Share Talk, citing Reuters, said copper had touched seven-week lows. Alliance News reported that the metal fell 2.5% on the day. That weakness pushed industrial metal miners to their lowest level in almost four months, according to those same midday reports.

Why might copper have fallen? The fact sheet for this article does not give a single explanation, and it would be wrong to invent one. What it does record is a nervous backdrop. Oil had been extremely volatile, with crude sliding about 2% on Tuesday after a fall of roughly 7% on Monday, as investors weighed talks aimed at ending the wars in Ukraine and Iran. Elevated oil prices have fuelled Inflation fears, and Reuters noted that many central banks have raised interest rates. The Bank of England is widely expected to lift its own rate by 25 basis points to 4% on Thursday 5 November.

In general terms, industrial metals are often viewed as barometers of economic activity, because they are used in construction, power grids, vehicles and manufacturing. When markets worry about higher borrowing costs or slower growth, Demand expectations for metals such as copper can soften, and prices can fall accordingly. That is a broad pattern rather than a diagnosis of Tuesday's specific move.

Why a focused miner moves more

Antofagasta's 3.5% decline was larger than copper's reported 2.5% fall and larger than the 2.6% drop in the basic resources index. That kind of amplification is common among producers concentrated on one commodity, for a reason sometimes described as operating leverage.

A mine's costs, such as labour, energy, equipment and maintenance, do not fall automatically when the metal price drops. So a given percentage change in the selling price can translate into a larger percentage change in expected profit. investors, who own the residual profit after costs, therefore tend to react more sharply than the commodity price itself.

For illustration, consider a hypothetical miner that sells metal for 100 units and has costs of 60 units, leaving profit of 40. If the price falls 5% to 95 while costs stay at 60, profit drops to 35, a decline of 12.5%. That simple arithmetic shows why focused producers can be more sensitive than the metal they mine, although real companies have hedging, by-products and other factors that change the picture.

Diversified miners have more ways to cushion a fall in a single commodity, because their Earnings come from several sources. On Tuesday, Glencore (LSE:GLEN) fell 2.8% and Anglo American (LSE:AAL) was down about 2.8% at midday. Antofagasta's steeper drop is consistent with its tighter focus on copper.

Tuesday in context

The metals slide was only one part of a broadly negative session. The FTSE 350 banks index fell 1.6% ahead of the Bank of England decision, and HSBC (LSE:HSBA), down 2.0%, was the single biggest approximate drag on the FTSE 100 in points terms. Precious metal miners also fell about 2.7% to a three-month low at midday.

Selling intensified as the day went on. The index was at about 10,705 early in the morning and around 10,687.72 at midday, before closing more than 49 points below that midday level. The FTSE 250 held up better, falling 0.5% to 24,674.91.

The interest-rate debate is worth watching for anyone following metals. Money markets have fully priced four 25bp rises by next September, according to LSEG data cited by Reuters, yet not everyone believes that many will be needed. Alessia Berardi, head of global Macroeconomics at Amundi Investment Institute, said the market was pricing a full hiking cycle but added: "we think that one or two moves will be enough..." How that debate resolves could affect sentiment towards growth-sensitive assets, including industrial metals and the companies that produce them, although no one can know in advance which way it will break.

For illustration of what a 3.5% move means in money terms, a hypothetical holding worth £1,000 at Monday's close would have been worth £965 at Tuesday's close, assuming no other changes.

Key dates to watch

Wed 4 Nov:Eurozone inflation figures, expected at 3.9%.

Thu 5 Nov:Bank of England MPC decision; a 25bp rise to 4% is widely expected.

Fri 6 Nov:US October jobs report, closely watched by the whole market.

Thu 12 Nov:ONS first quarterly GDP estimate for July–September 2026, at 7:00am.

Wed 2 Dec:FTSE Russell December review announcement, after the UK close.

The bottom line

Antofagasta's 3.5% fall on Tuesday was less about the company and more about copper. With the metal at a seven-week low and industrial metal miners at their weakest in almost four months, a producer concentrated on copper was always likely to feel the pressure acutely. The coming days bring a Bank of England decision, US jobs data and UK growth figures, all of which could influence how markets think about demand for industrial metals. For anyone following Antofagasta within the FTSE 100, the copper price remains the variable that matters most.