ABANDONED. adventure to tropical jungle, no text, clues hidden in illustrations

[Nadia]

Help - I've been wracking my brain to try and remember anything about this book (enough to look it up, anyway!)
It is a grandfather and granddaughter (?) who area off to find some sort of treasure hidden in a temple/cave (?) in a tropical country.
The book has no text, but:
- within the illustrations are clues, and the motif of a mask (green, with orange hair, similar to south american/melanesian indigenous war masks) recurs on every double page
- each page also has torn up pieces of an old letter that you have to piece together by writing down the text from each piece
- the journey begins in an office, I think, or a museum, but eventually makes its way into the jungle and into the temple/cave.
- there's a treasure map involved

I remember it from the late 80s/early 90s I think. The mask motif is featured on the front cover, if I recall correctly.

Any suggestions as to title or author/illustrator are welcome!

[Lobstergirl, au gratin]

Is it for children, YA, or adults?

[Nadia]

Lobstergirl wrote: "Is it for children, YA, or adults?"

Children, not quite young adults but definitely older than 8. It is a picture book after all, despite its complexity.

[Nadia]

BUMP!

Anyone got any ideas?

[Lobstergirl, au gratin]

Nadia, the bump period for this group is 30 days because the group is so large, unless you remember new information which you can add at any time.

[Lobstergirl, au gratin]

Nadia, are you still looking for this or did you find it?

[Kris]

No response, moving to Abandoned folder.

Nadia (OP) was last active on the site in November 2019.