BENEFIT PLANS: (Tables)

12 Months Ended

Dec. 31, 2011

Peru Defined Benefit Pension Plans

Components of net periodic benefit costs

Schedule of components of net periodic benefit costs

	Years ended December 31,
(in millions)	2011			2010			2009
Interest cost	$	0.6		$	0.6		$	0.7
Expected return on plan assets	(0.7		)	(0.7		)	(0.6		)
Amortization of net loss	0.1			0.1			0.1
Net periodic benefit cost	$	—		$	—		$	0.2

Schedule of changes in the benefit obligation and plan assets, the funded status of the plans and the amount recognized in balance sheet and accumulated other comprehensive income

	As of December 31,
(in millions)	2011			2010
Change in benefit obligation:
Projected benefit obligation at beginning of year	$	12.5		$	12.1
Interest cost	0.6			0.6
Benefits paid	(1.0		)	(0.9		)
Actuarial gain census	0.2			0.1
Actuarial gain assumption changes	1.3			0.6
Projected benefit obligation at end of year	$	13.6		$	12.5
Change in plan assets:
Fair value of plan assets at beginning of year	$	16.6		$	16.5
Actual return on plan assets	1.1			1.0
Benefits paid	(0.9		)	(0.9		)
Fair value of plan assets at end of year	$	16.8		$	16.6
Funded status at end of year:	$	3.2		$	4.1
ASC-715 amounts recognized in statement of financial position consists of:
Non-current assets	$	3.2		$	4.1
Current liabilities				—
Non-current liabilities	—			—
Total	$	3.2		$	4.1
ASC-715 amounts recognized in accumulated other comprehensive income consists of:
Net loss net of income tax	$	3.1		$	2.5
Total (net of income tax of $1.7 million and $1.3 million in 2011 and 2010, respectively)	$	3.1		$	2.5

Schedule of changes in accumulated other comprehensive income

	As of December 31,
(in millions)	2011			2010
Reconciliation of accumulated other comprehensive income:
Accumulated other comprehensive income at beginning of plan year	$	2.5		$	2.3
Net loss/(gain)amortized during the year	(0.1		)	(*		)
Net loss/(gain)occurring during the year	0.7			0.2
Net adjustment to accumulated other comprehensive income	0.6			0.2
Accumulated other comprehensive income at end of plan year	$	3.1		$	2.5

Summary of the amounts in accumulative other comprehensive income amortized and recognized as a component of net periodic benefit cost, net of tax

	As of December 31,
(in millions)	2011			2010
Net loss / (gain)	$	0.7		$	0.2
Amortization of net (loss) gain	(0.1		)	(*		)
Total amortization expenses	$	0.6		$	0.2

Schedule of assumptions used to determine the pension obligation and seniority premiums as of year-end and the net cost

	2011		2010		2009
Discount rate	3.95	%	5.00	%	5.55	%
Expected long-term rate of return on plan asset	4.50	%	4.50	%	4.50	%
Rate of increase in future compensation level	N/A		N/A		N/A

Scheduled maturities of the benefits expected to be paid in each of the next five years, and thereafter

Years	ExpectedBenefit Payments
	(in millions)
2012	1.0
2013	0.9
2014	1.0
2015	0.9
2016	0.9
2017 to 2021	4.6
Total	$	9.3

Peru Post-retirement Health Care Plan

Components of net periodic benefit costs

Schedule of components of net periodic benefit costs

	Years ended December 31,
(in millions)	2011			2010			2009
Service cost	$	—		$	—		$	—
Amortization of net loss / (gain)	(*		)	(*		)	(*		)
Amortization of prior service cost / (credit)	(*		)	(*		)	(*		)
Interest cost	0.1			0.1			0.1
Net periodic benefit cost	$	0.1		$	0.1		$	0.1

(*)Less than $0.1 million

Schedule of changes in the benefit obligation and plan assets, the funded status of the plans and the amount recognized in balance sheet and accumulated other comprehensive income

	As of December 31,
(in millions)	2011			2010
Change in benefit obligation:
Benefit obligation at beginning of year	$	1.7		$	2.0
Interest cost	0.1			0.1
Actuarial loss / (gain) — claims cost	—			(0.3		)
Benefits paid	(0.1		)	(0.2		)
Actuarial (gain) loss	0.2			0.1
Benefit obligation at end of year	$	1.9		$	1.7
Change in plan assets:
Fair value of plan assets at beginning of year	$	—		$	—
Employer contributions	0.1			0.1
Benefits paid	(0.1		)	(0.1		)
Fair value of plan assets at end of year	$	—		$	—
Funded status at end of year	$	(1.9	)	$	(1.7	)
ASC 715 amounts recognized in statement of financial position consists of:
Non-current assets	$	—		$	—
Current liabilities	(0.1		)	(0.1		)
Non-current liabilities	(1.8		)	(1.6		)
Total	$	(1.9	)	$	(1.7	)
ASC 715 amounts recognized in accumulated other comprehensive income consists of:
Net loss (gain)	$	0.6		$	0.5
Prior service cost (credit)	(0.1		)	(0.1		)
Total (net of income tax)	$	0.5		$	0.4

Schedule of changes in accumulated other comprehensive income

(in millions)	2011			2010
Reconciliation of accumulated other comprehensive income:
Accumulated other comprehensive income at beginning of plan year	$	0.4		$	0.5
Net loss /(gain) occurring during the year	0.1			(0.1		)
Net loss/gain amortized during the year	(*		)	(*		)
Net adjustment to accumulated other comprehensive income	0.1			(0.1		)
Accumulated other comprehensive income at end of plan year	$	0.5		$	0.4

(*) Less than $0.1 million

Summary of the amounts in accumulative other comprehensive income amortized and recognized as a component of net periodic benefit cost, net of tax

	For the year endedDecember 31,
(in millions)	2011			2010
Net loss (gain)	$	(0.1	)	$	0.1
Amortization of prior service cost (credit)	(*		)	(*		)
Amortization of net losses	(*		)	(*		)
Total amortization expenses	$	(0.1	)	$	0.1

Scheduled maturities of the benefits expected to be paid in each of the next five years, and thereafter

Year	ExpectedBenefit Payments
	(in millions)
2012	$	0.1
2013	0.1
2014	0.1
2015	0.1
2016	0.1
2017 to 2021	0.6
Total	$	1.1

Minera Mexico Defined Benefit Pension Plans

Components of net periodic benefit costs

Schedule of components of net periodic benefit costs

	For the years ended December 31,
(in millions)	2011			2010			2009
Interest cost	$	0.6		$	1.6		$	1.5
Service cost	0.9			2.1			1.8
Expected return on plan assets	(2.8		)	(3.0		)	(2.2		)
Amortization of transition assets, net	(0.1		)	—			(0.1		)
Amortization of net actuarial loss	(1.3		)	(1.0		)	(0.1		)
Amortization of prior services cost	—			0.2			0.2
Settlement/curtailment(*)	—			(19.0		)	—
Net periodic benefit cost	$	(2.7	)	$	(19.1	)	$	1.1

Schedule of changes in the benefit obligation and plan assets, the funded status of the plans and the amount recognized in balance sheet and accumulated other comprehensive income

(in millions)	2011			2010
Change in benefit obligation:
Projected benefit obligation at beginning of year	$	12.7		$	32.5
Service cost	0.9			2.1
Interest cost	0.6			1.6
Actuarial (loss) gain, net	(0.4		)	(0.6		)
Benefits paid	(0.9		)	(0.9		)
Settlement/curtailment (*)	—			(23.7		)
Inflation adjustment	(1.3		)	1.7
Projected benefit obligation at end of year	$	11.6		$	12.7

(*)Included in the 2010 settlement/curtailment amount is a payment of $11.9 million severance to Buenavista workers who accepted the Company’s buyout offer.

Change in plan assets:
Fair value of plan assets at beginning of year	$	46.3		$	38.8
Actual return on plan assets	(1.1		)	6.9
Employer contribution	(0.5		)	(0.6		)
Benefits paid	(0.2		)	(0.9		)
Currency exchange rate adjustment	(5.4		)	2.1
Fair value of plan assets at end of year	$	39.1		$	46.3
Funded status at end of year:	$	27.5		$	33.6
ASC 715 amounts recognized in statement of financialposition consists of:
Non-current assets	$	27.5		$	33.6
Total	$	27.5		$	33.6
ASC 715 amounts recognized in accumulated othercomprehensive income consists of:
Net loss (gain)	$	(7.0	)	$	(11.2	)
Transition obligation (asset)	—			(0.1		)
Total (net of income tax of $4.7 million and $7.6 million, respectively)	$	(7.0	)	$	(11.3	)

Schedule of changes in accumulated other comprehensive income

(in millions)	2011			2010
Reconciliation of accumulated other comprehensive income:
Accumulated other comprehensive income at beginning of plan year	$	(11.3	)	$	(6.1	)
Prior services cost amortized during the year	—			(0.1		)
Net loss/gain amortized during the year	0.8			0.6
Net loss (gains) occurring during the year.	2.2			(2.7		)
Settlement/curtailment	—			(2.8		)
Currency exchange rate changes	1.3			(0.2		)
Net adjustment to accumulated other comprehensive income	4.3			(5.2		)
Accumulated other comprehensive income at end of plan year	$	(7.0	)	$	(11.3	)

Summary of the amounts in accumulative other comprehensive income amortized and recognized as a component of net periodic benefit cost, net of tax

	For the year ended  December 31,
(in millions)	2011		2010
Amortization of net losses	$	0.8	$	0.6
Amortization of prior services cost	—		(0.1		)
Total amortization expenses	$	0.8	$	0.5

Schedule of assumptions used to determine the pension obligation and seniority premiums as of year-end and the net cost

	2011		2010		2009
Weighted average discount rate	7.5	%	7.5	%	8.0	%
Expected long-term rate of return on plan asset	7.5	%	7.5	%	8.0	%
Rate of increase in future compensation level	4.5	%	4.0	%	0.0	%

Scheduled maturities of the benefits expected to be paid in each of the next five years, and thereafter

Years	ExpectedBenefit Payments
	(in millions)
2012	$	7.3
2013	0.5
2014	0.5
2015	0.5
2016	0.5
2017 to 2021	3.4
Total	$	12.7

Schedule of asset mix of the investment portfolio

	2011		2010
Asset category:
Equity securities	74	%	71	%
Treasury bills	26	%	29	%
	100	%	100	%

Minera Mexico Post-retirement health care plan

Components of net periodic benefit costs

Schedule of components of net periodic benefit costs

	For the years ended  December 31,
(in millions)	2011			2010		2009
Interest cost	$	3.2		$	4.3	$	4.3
Service cost	—			0.4		0.4
Amortization of net loss (gain)	—			0.1		0.5
Amortization of transition obligation	1.3			1.5		1.5
Amortization of prior service cost	(10.0		)	—		—
Net periodic post-retirement benefit costs	$	(5.5	)	$	6.3	$	6.7

Schedule of changes in the benefit obligation and plan assets, the funded status of the plans and the amount recognized in balance sheet and accumulated other comprehensive income

(in millions)	2011			2010
Change in benefit obligation:
Projected benefit obligation at beginning of year	$	49.7		$	53.0
Service cost	—			0.4
Interest cost	3.2			4.3
Amendments	(24.2		)
Actuarial (loss) gain, net	(3.3		)	(11.1		)
Benefits paid	(1.3		)	—
Currency exchange rate adjustment	(5.7		)	3.1
Projected benefit obligation at end of year	$	18.4		$	49.7
Funded status	(18.4		)	(49.7		)
ASC-715 amounts recognized in statement of financialposition consists of:
Non-current assets	$	—		$	—
Current liabilities	—			—
Non-current liabilities	(18.4		)	(49.7		)
Total	$	(18.4	)	$	(49.7	)
ASC-715 amounts recognized in accumulated other comprehensive income consists of:
Net loss (gain)	$	(4.0	)	$	(2.3	)
Prior service cost (credit)	—			—
Transition obligation (asset)	—			10.5
Total (net of income tax of $(2.7) million and $5.5 million, respectively)	$	(4.0	)	$	8.2

Schedule of changes in accumulated other comprehensive income

(in millions)	2011			2010
Reconciliation of accumulated other comprehensive income:
Accumulated other comprehensive income at beginning of plan year	$	8.2		$	15.5
Amortization of net loss (gain)	—			(0.1		)
Amortization of transition obligation	(0.8		)	(0.9		)
Amortization of prior service cost (credit)	6.0			—
Prior services cost (credit)	(14.5		)	—
Net loss / (gain) occurring during the year.	(2.0		)	(6.7		)
Currency exchange rate changes	(0.9		)	0.4
Net adjustment to accumulated other comprehensive income	(12.2		)	(7.3		)
Accumulated other comprehensive income at end of plan year	$	(4.0	)	$	8.2

Summary of the amounts in accumulative other comprehensive income amortized and recognized as a component of net periodic benefit cost, net of tax

	For the year ended December 31,
(in millions)	2011			2010
Amortization of transation obligation	$	0.8		$	0.9
Amortization of prior service cost (credit)	(6.0		)	—
Amortization of net losses	—			0.1
Total amortization expenses	$	(5.2	)	$	1.0

Scheduled maturities of the benefits expected to be paid in each of the next five years, and thereafter

Years	ExpectedBenefit Payments
	(in millions)
2012	$	1.4
2013	1.5
2014	1.6
2015	1.7
2016	1.8
2017 to 2021	13.0
Total	$	21.0

Schedule of effects of one percentage-point change in assumed other benefits cost trend rates

	One Percentage Point
(in millions)	Increase		Decrease
Effect on total service and interest cost components	$	1.5	$	1.2
Effect on the post-retirement benefit obligation	$	20.2	$	16.8