Dear Peter,
 

My little understanding of magnetite is that the polarity of each deposit  
reflects the past magnetic field of the globe. Each deposit is different I 
am  led to believe, maybe due to the magnetic aura of the globe, and a 
compass  varies over each. In the same way that radioactivity varies these days in 
modern  steel and iron (thus why WW1 and prev. warships are sought for 
their metal  distinctly due to low to negative active content for high duty 
sensing eqpt.),  the polarity of the metal - though not relevant in many 
instances - is also  taken into consideration, since, time varies things, and 
sometimes this must be  neutralised. Degaussing of produced steel/oron is regular 
for many purposes, and  is of course an easy process as opposed to what it 
was with ships in WW2.
 

Getting back to Iron Ore deliveries into GB since 1900 (say), the delivery  
records (ref. Min. Stats.), to the NE Ports vary considerably, with the  
predominant source gradually becoming the Spanish mines as that country's 
mines  progressively became cheaper and so voluminous that they overcame all 
other  opposition for quite some while.
 

W.Cumberland deposits holding apx. 60% Fe were paramount of course for many 
 years, and rivaled anything from anywhere with regards SiO4 content, and 
the P  content being very acceptable (the latter makes steel and iron 
brittle).  Likewise the NE deposits were very acceptable for a long while, but 
supply after  WW1 became a slight problem... but the Iron Works foresaw this and 
were buying  in increasingly from numerous overseas sources from the 1880's 
onwards - very  particularly in the NE (who were also buying in what they 
could of any 'surplus'  from W.Cumberland).
 

Magnetite (Fe3O4), varies again deposit by deposit in it's magnetic  
characteristics and intensity: after same has been subjected to the furnace -  and 
any ore mixing that inevitably occurs for various end product  
characteristics required - it alters yet again to such a degree that the ore  origin is 
nigh on impossible to determine. Local ore mixing must also be  considered, 
since by adding ores to ores can make a predominant local ore smelt  
easier/faster - similarly as a hard lead ore mixed with soft lead ore makes all  
smelt better. This situation re iron ores certainly applied to the NE for 
quite  some while: the Loftus and Skinningrove mines were apx. +/- 30 and had to 
 operate on this margin for quite some while until foreign alternatives  
increasingly made their presence felt. Magnetite is mentioned, indeed it  
occurred in the NE (& of course elsewhere in GB - inc. the Shetlands), and  was 
mined in the NE though not in as large quantities as the gnrl. Iron ores  
there. 
 

So, how one could determine the magnetic locality origin - magnetite or  
otherwise - of any pre WW2 slag (say), is I would think damn near impossible - 
 unless an iron/steel producer was dedicated to one particular ore from a  
singular source (which never happened - GB anyway - unless one is talking 
about  W.Cum. or the Barrow area).
 

I will stand corrected to superior tech. knowledge/hypothesis in this  
matter and my/these gnrl. ramblings... it is not my field or area.
 

Regards, Bernard