main riff          
G|------4-----------|
D|646--6-6--646-----|
A|-------------4464-|
E|-----------------4|

Here are some variations of that pattern

G|------4------4464-|------------------|------------------|
D|646--6-6--646----6|------------------|-----------------6|
A|------------------|------2------2242-|------2------2242-|
E|------------------|424--4-4--424----4|424--4-4--424-----|

  After about two minutes of the song, it changes to these riffs and variations of
G|--------4-|----------|
D|6646--66-6|----------|
A|----------|--------2-|
E|----------|4424--44-4|