Sweden was the largest source by far of high-grade iron ore in Europe at the time, and the world's second-largest exporter. It had two centers of iron ore production, with some of the largest reserves in Europe; the ore fields of Lapland, with major mines in Kiruna, Svappavaara, Gällivare, Malmberget. Secondary you had the older mines of Bergslagen region around Grängesberg, the single largest mine in that area. The former, making up about 80% of prewar production, were connected by the Ore Line (Malmbanan) running from one ore harbor in Luleå on the Baltic and the other end in Narvik. The Bergslagen ore, making up the remaining 20% was mainly exported through the Baltic port of Oxelösund, via the TGOJ railroad. Luleå is so far north (and the Swedish fleet of icebreakers so small at the time) that that it was often unusable during winters, while Narvik is ice-free year round due to the Gulf Stream. (Although these railroads are connected through the Swedish upper northern mainline, the capacity was low because it couldn't take as heavy trains as these other lines that were built specifically for ore transports. So only a limited amount of Lapland ore could be shipped to ports farther south)

The northern ores made up the larger part of exports are phosphate rich apatite ores, which were however usefulness for munitions production, as higher phosphorus concentrations result in poorer quality steel. However, Germany needed ore. They produced a great deal of iron themselves but mining had not kept pace with the need for steel. Germany's ore production (in terms of tons of iron) was about 4 Mton (iron) in 1938, while imports stood at over 10 Mton. Sweden stood for half of that (5.4 Mton) and the second-largest exporter France made up 14%. Obviously the French exports ceased immediately at the outbreak of the war. So Sweden's exports became all the more important to the Germans. This put Sweden in a rather complicated position; it and the other Nordic countries had declared themselves neutral at the very outbreak of the war, but Germany had always been Sweden's main trading partner. (However, exports to Britain were not insignificant either; in the case of iron ore, it was about 15% of Sweden's ore exports pre-war) The country was still critically dependent on imports of many German goods, such as coal and coke. In December 1939 Sweden therefore reached a War Trade Agreement with Germany and Britain, to be renewed annually and allowing for continued imports and exports with belligerents at certain proportions of pre-war levels.

Anyway, although it had stood for 50% of exports to Germany alone in peacetime, Narvik would not be the most important export port of ore to the Germans; Oxelösund and Luleå took up that role - as the Baltic was more or less out of the reach of the Royal Navy and Air Force, and thus a much safer route. So exports from these harbors increased greatly by the end of 1939 - Luleå was exporting more than twice as much iron as Narvik was, and well over a hundreds of ktons of ore were exported from there in December 1939 (as compared to zero exports the December 1938) With France and others out of the picture, Sweden was make up 90% of Germany's ore imports by the fourth quarter of 1939.

It seems that to some extent, the significance of Narvik was exaggerated by the belligerents, particularly the British, and the ability to ramp up capacity at Luleå underestimated. On the British side, this may have been partly wishful thinking to an extent; the thought being that the Royal Navy would be able to disrupt the traffic from Narvik. But they'd also been told by German industrialist Fritz Thyssen that he'd reported the German government that control over the Lapland ore fields would be critical to the outcome of the war.

The Germans on their hand, wanted to deny the Swedes the ability to export to Britain, which the invasions of Denmark and Norway in April 1940 and subsequent mining of the Skagerrak channel achieved.

However, the Battle of Narvik was not even over by the time the invasion of France and the Low Countries had begun in May-June 1940. Adding France's iron ore production to its own meant an increase in 'domestic' ore production in Germany that was significantly larger than the imports from Sweden, although Swedish imports were still significant (a 'domestic' production of 13 Mton iron in 192 versus 4.2 Mton from Sweden)

So from the German perspective, control over Narvik and the Norwegian coast was important to securing additional iron ore during the period Luleå was closed, but it was only one of a number of reasons for their invasion of Norway. On the other hand, the British plans for invading Norway and possibly Swedish Lapland (Operation Wilfred and R4) around April 1940 to secure the ore fields seems to overstate their importance greatly. Besides the reasons it was rejected at the time (the issues of invading of a neutral country, and the likelihood of a German occupation of Sweden in response which they couldn't stop), the impending occupation of France (and particularly Lorraine) would soon be supplying Germany with more iron than the entire Swedish exports did. With Germany having sizable reserves and the ability to take steel from lower-priority things (such as megalomanial construction projects), even if Britain had taken control over Lapland in March or April 1940, it does not seem plausible it'd have had an effect on the impending Battle of France, at least not through the iron supply.

For the stats above see Martin Fritz, German steel and Swedish iron ore 1939-1945, 1974