Minerals paragenetic sequence of Big Gossan skarn ore deposit

 

The Big Gossan skarn ore deposit, located around 1 km south-west of the Ertsberg
skarn ore deposit, and 2 km south of the Grasberg porphyry Cu-Au deposit. The ore
deposits is skarn ore deposit type with very high copper grade. By the end of 2007, ore
reserve of Big Gossan is 52.7 million ton with grade mean of Cu 2.31%, Au 1.1 gr /ton
and Ag 14.75 gr/ ton. The Big Gossan ore deposit form tabular pattern dimension
having length more than 1 km, height more than 500 m and wide variety up to 200m.
The Big Gossan skarn zones successively from boundary of Hornfels of Kembelangan
shale (Kkeh) showing proximal skarn (garnet>clino-pyroxene), intermediate skarn
(garnet=clino-pyroxene), distal skarn (clino-pyroxene >garnet), and marble.
Petrography observation in the drift and also the drilling core, indicaties that proximal
skarn is started at the boundary between carbonate rock of Waripi Formation and
Limestone Member of Ekmai Formation with hornfels of Shale Member of Ekmai
Formation which have gradation to intermediate, distal skarn till marble. Garnet present
abundance and gradually disappear toward marble. On the contrary clino-pyroxene
present insignificant amount around hornfels and gradually more abundance toward
boundaries of marble.
In general, minerals paragenetic sequence of Big Gossan skarn ore deposit is started
from of metamorphism processes, yielding biotite-k.feldspar hornfels and biotitepyroxene
hornfels. Next process is the interaction of hydrothermal fluid with wall rock
and intrusion rock peripheral yield anhydrous prograde skarn which is marked by the
present of garnet minerals (andradite-grossularite) and clinopyroxene (diopsidehedenbergite)
is accompanied by epidote-calcite-quartz-anhydrite. The abundance
tremolite-actinolite which of accompanied by many other minerals such as talcanhydrite-
calcite-epidote-garnet-magnetite-pyrite, marking the existence of the hydrous
retrograde skarn phase, after hydrothermal breccia. Most sulfide minerals such as
magnetite pyrite, chalcopyrite, sphalerite, pyrrhotite, galena are, associated with Cu-
Au, formed after hydrous retrograde

By:

Sutarto*), Sunyoto,W**).,Widodo, S.**), Soebari,L.,**),Sutanto*),Setyadi,H. **) dan Wiguna,P.**)
*)Teknik Geologi Universitas Pembangunan Nasional “Veteran” Yogyakarta. Jl. SWK 104
Condongcatur, Yogyakarta. Email: sutarto_geoupn@yahoo.co.id
**)PT.Freeport Indonesia,. Plaza 89,5th Floor,Jl.Rasuna Said Kav.X7 No.6 Jakarta

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Where are deltas found?

Deltas can occur in lakes and inland seas as well as in the ocean, but they are most important in the open ocean (Boggs). Deltas are particularly common in the modern ocean owing to post-Pleistocene sea-level rise coupled with high sediment loads carried by many rivers. They often contain extensive wetlands, whose high biological productivity makes them vital nursery grounds for fisheries. Huge amounts of coal, oil, and natural gas are found in subsurface ancient deltaic deposits. Deltas seldom form on active, subductingcontinental margins because there is no stable shallow shelf on which sediments can accumulate. Twenty-five (25) of the world’s largest deltas are found on passive margins of continents. They are categorized according to features within the delta plain and by energies acting on the delta (tide, wave, and fluvial). The size of a delta is dependent on the size of the drainage basin, the elevation of the land, and climate.

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