Passing A Variable To Aggregate Function

I have cursor that loops through a table (the table only contains columnnames of several tables) the cursor has a variable declared @columnname. when i run the following it works fine

select @columnname,0,0,0,0
from temp_prt

it gives me my expected output

mtr_5120,0,0,0,0
mtr_3247,0,0,0,0
mtr_5160,0,0,0,0
etc........

now i want to get the min of each column name like so

select @columnname,min(mtr_5120),0,0,0
from temp_prt ------> this works for min(mtr_5120)
mtr_5120,34.5,0,0,0

now I want to generalize so I try to pass in the variable name and I do the following

select @columnname,min(@columnname),0,0,0
from temp_prt
(the columname (@columnname) exists in the table temp_prt)

but now i get an error
Msg 8114, Level 16, State 5, Line 29

Error converting data type varchar to decimal.how can i pass the colunmame into the min and max functions or is that at all ppossible. I also tried the following:

select @columnname,'min(' + @columnname + ')',0,0,0
from temp_prt

but i get the same error
Msg 8114, Level 16, State 5, Line 29
Error converting data type varchar to decimal.