Keep reading Hokes, good winter time project till the mining season rolls around.
Page 66 C.M. Hokes states up 8% silver dissolving gold directly in aqua regia is not much of a problem.
under 12% silver can be done with some difficulty.
greater than 12% silver it is best to in-quarter the gold, as the silver chloride becomes so difficult.
much also depends on conditions.
your 13% would be difficult, in-quartering, or upgrading the gold content would be the best choice, in-quartering the gold with silver is the easiest.
Depending on the condition of the mined gold (small flakes) you may be able to remove some silver with nitric, before trying aqua regia, or take previously recovered gold that has had silver removed to upgrade your gold.
Once you understand Hokes, and begin using what she teaches, you will find what works best for you and the material you are working with.
Personally I just in-quarter the placer gold, it is not that much problem for me and the little bit I am able to mine, this way I also upgrade the sterling I collect at the same time and do not worry about fighting the silver chloride.