There probably isn't any one pant, Jimmy. The weather in Patagonian Chile is just wildly variable. You may see temps from 40 to 90 degrees, rain, wind, sun, you name it.
The nearest thing to an all-purpose pant likely would be something such as Arc'Teryx's Gamma LT Pant ($169). These are tough, comfortable pants made from a light soft shell material that resist rain and wind well, has enough insulation for cool weather, yet breathes well enough that they're comfortable up into the 60s. They're also fantastic for mobility, so you can hop around boulders and over logs with ease. And if you get wet, they'll dry very quickly.
You might also check out the Patagonia Men's Simple Guide Pants ($99). They're similar to the Arc'Teryx pants, with stretch-woven fabric construction that has some give and offers just enough insulation for cool weather. And they're synthetic, so they'll dry well. Like the Gamma LTs, it's easy to extend the temp rating down by adding some light long underwear. And on their own you'll be reasonably comfortable even if the sun comes out and it warms up a bit.
You always can give up on looking like anything but a tourist, and buy a pair of (gag!) convertibles. They do at least have the advantage of turning into shorts in case it gets hot. Ex Officio's Men's Amphi Convertible Pants ($65) are sort of the gold standard here. They're sturdily made from fast-drying nylon, treated with Teflon to shed light rain and stains, a pleated front that looks pretty good around town. I will say, even the flap over the knee-high zipper is pretty inconspicuous.
Hope you have a great trip! Send us some pictures!