Table 2 Calculation of the amounts of sulphides produced by biogenic activity at Las Cruces.

From: Formation of recent Pb-Ag-Au mineralization by potential sub-surface microbial activity

  

Grade

wt %

Tons

Moles metal

Moles CO2

Tons CO2

Moles H2S

Tons H2S

Gossan

1.7 Mt

        

Black Rock

40%

 

0.7

      

 Calcite

  

60.0

163,428

4.078E+09

4.078E+09

1.794E+05

  

 Galena

 

8.7%Pb

10.0

58,888

2.842E+08

  

2.842E+08

9.680E+03

 Silver sulphides (as Ag2S)

 

109 g/t Ag

0.0

185

1.717E+06

  

8.587E+05

2.925E+01

 Gold

 

3.85 g/t Au

0.0

7

3.322E+04

    

 Iron sulphides (as FeS)

  

30.0

129,551

2.320E+09

  

2.320E+09

7.902E+04

Red Rock

60%

 

1.0

      

 Siderite

  

80.0

326,857

6.779E+09

6.779E+09

2.982E+05

  

 Galena

 

3.9%Pb

4.5

39,750

4.590E+08

  

4.590E+08

1.563E+04

 Haematite

  

15.5

110,596

1.581E+09

    

Total

     

1.086E+10

4.776E+05

3.064E+09

1.044E+05

  1. Galena has 86.6% Pb; thus, the total amount is 113,900 t.
  2. Ag2S has 87.07% Ag;thus, the total amount of silver sulphides as acanthite/argentite is 638 t.
  3. The total moles of calcite and siderite correspond to 1.19 Mt of total carbonates.
  4. The Black and Red rocks host ~1.7 Mt of ore averaging 5.81% Pb, 3.85 g/t Au and 109 g/t Ag ( www.first-quantum.com); visual estimates in the pit and petrographic study suggest that 60% by weight of the capping zone is made of Red Rock (80% siderite, 15.5% haematite, 4.5% galena) and the remainder is Black Rock (60% calcite, 10% galena, 30% iron sulphides).
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