How gold deposits can be formed in Himalayas?
Himalayas represent a huge area with a complex geological history which I suspect has large areas that have never been adequately mapped to allow for an ore deposit evaluation. Nevertheless, placer gold might be found within any region of the  Himalayas.  

Placer gold deposits are an accumulation of gold formed by gravity separation during sedimentary processes of the gold bearing host rock.

Reference: https://www.britannica.com/science/placer-deposit

Many granite bearing mountains containing quartz-feldspar porphyry dykes/veins associated with elevated concentrations of sulfur and arsenic can be found  in many parts of the world. I would assume these deposits would also found in the Himalayas. 

Reference: http://www.minelinks.com/alluvial/goldDeposits.html