What is the difference between treacle and golden syrup, and why do both have a metallic taste?

Treacle is the British generic name for any syrup made during the refining of sugar cane. Therefore, theoretically, Treacle, Black Treacle, Molasses, Golden Syrup and Blackstrap are ALL treacles. In practice however, there is a technical difference between “treacle” and “Molasses” in that molasses is obtained from the drainings of raw sugar during the refining process and treacle is made from the syrup obtained from the sugar.

Light Treacle is better known as Golden Syrup (aka Light Molasses), is made from the syrup obtained during the first boiling of the sugar cane/beets. About 65% sucrose, it is the lightest in colour and the sweetest of all the treacles and is usually unsulphered.

Black Treacle, (equivalent = dark molasses) is made from the syrup obtained from later boilings and is about 55% sucrose.

(No comment on the metallic taste)

https://www.homebrewtalk.com/threads/treacle-v-molasses.10305/

Black molasses is included in many lists of iron-rich foods. That could account for the metallic taste.

https://en.wikipedia.org/wiki/Molasses#Blackstrap_molasses

In common British usage, syrup is golden and see-through, while treacle is very dark and hard to see through (in the tin, that is). Those are the only two terms in common use, the tins saying Golden Syrup and Black Treacle but common speech dropping the adjective. In practice, Black Treacle may be widely found but rarely used as most people have no use for it. It is used by those making treacle toffee (nice but bad for your teeth - if it doesn't break them, it'll rot them...). Treacle tart and treacle pud contain golden syrup not treacle. Molasses is a term associated with the West Indies. pirates, and large tanks down near the docks. It is used in specialised commercial production, and in specialised and uncommon home use. The majority of people here wouldn't have a clue what to do with it, and think of it as an early stage of sugar production - if they think of it at all. A quick trip into Baloo's Interesting Ingredients cupboard with a teaspoon confirms a slight metallic taste to golden syrup in a steel tin. The black treacle tin got thrown out last year when it reached the tenth anniversary of its sell-by date without having been opened. Source: Baloo's cupboard and culinary experience.