Ann: Consolidated Tin Mines Monthly Update - June 2015, page-6

I would say it is not a standard practice to produce and stockpile metal simply because of the opex costs not being covered in the short term by revenue from sales.
Especially so for a company with only two quarters as a producer under their belt.
Having said that I believe it is common for the gold miners to hold metal. How much and for how long I don't know. Would depend heavily on their cashflow and overall financial position.