It can be a couple of things..

But as your thread suggests, yellow veining can almost always be linked to poor root function due to excessive watering or cold or hot roots temps making nutrients unavailable..

If it is an iron (Fe) deficiency, as I suspect, it's almost always can be traced to a pH induced situation, (caused by too high a pH level) and not due to the fact there is insufficient Fe in the soil, if I can assume you are using a good fertilizer.

I wish I could pull up the pics, but I am having a problem here at work to do so. But usually if the newer leaves are yellow veining, it is usually a lack of iron, because iron is not mobile in a plant, therfore the the newer leaves can not take it from the older ones..

Using something that acidifies the soil and lowers PH could correct the problem if there is iron already in the soil making it soluble and available, almost immediately..

Maybe the pictures will prove otherwise to someone else.

Thought I would at least help in some way while I had a moment..Goodluck..:-)

Tangelos are noted for their curled leaves. It is
common for the the ends to curl.

My container citrus with yellow main leaf viens have
usually been due to root damage, either rot or sun
burn on the container wall.

And in turn, root damage will absolutely cause an Fe deficiency.

Wish you could take a picture of the tree as a whole.

It also looks like you might have others issues with it too, after looking at the pictures of just the leaves..
I would definately look at your watering practices, the soil that it is in, and sun burn as well...Sun burn can be from a lack of moisture in the leaves, dry air mixed with a hot sun with the inability of the roots to satisfy the need for a drink....

Hopefully many here will see what else could be wrong.

I wish you success..

Mike:-)

Yellow Vein Chlorosis isn't unusual in citrus. Unfortunately, it can be caused by many factors. It may be as simple as cold weather making nitrogen less available for uptake by the plant. Or it might be caused by any number of things causing problems for the vascular system of the plant.

These might include root or soil-borne diseases, damage to the trunk (as in girdling), compaction to root/soil system, etc. Even ants can be a causal agent for YVC.

Truthfully, there are so many factors that can create a problem for the root and vascular systems of our plants that it might be hard to narrow it down for you. But the symptoms surely look like Yellow Vein Chlorosis.